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Effect of environmental pollution on susceptibility of sesquioxide-rich soils to water erosion

Effect of environmental pollution on susceptibility of sesquioxide-rich soils to water erosion GeoloGy, ecoloGy, and landscapes, 2018 Vol . 2, no . 2, 115– 126 https://doi.org/10.1080/24749508.2018.1452484 INWASCON OPEN ACCESS Eec ff t of environmental pollution on susceptibility of sesquioxide-rich soils to water erosion Chukwuebuka Emeh and Ogbonnaya Igwe d epartment of Geology, University of n igeria, nsukka, n igeria ABSTRACT ARTICLE HISTORY Received 29 d ecember 2017 This work assessed the impact of environmental pollution on soils susceptibility to water a ccepted 2 March 2018 erosion within South-eastern Nigeria. Sources of pollutants which could possibly affect the chemical composition of runoff; hence its pH, were first determined by remote sensing and KEYWORDS field observations. Rain and runoff water samples collected within the study area were analysed erosion; sesquioxide; for its physicochemical compositions. Soil samples were also collected and analysed for their environmental pollution; pH; geotechnical properties, and chemical composition of their fine fractions. An empirical method dispersion; urban run-off was then employed to determine the effect of change in chemical composition of runoff on the susceptibility of the studied soil to water erosion. This was achieved by conducting soil aggregate slaking, dispersion and dissolution tests on aqueous solutions of varying pH. Results from the experiment shows that the fine particle fractions of the soils are chiefly composed of sesquioxides. The slaking of these sesquioxide-cemented soils is not affected by the variations in pH of the solutions, but rather by the plasticity index of the soils. However, dispersion and dissolution of the soil samples where dependent on variations in the pH of the solutions. It was therefore concluded that environmental pollution has the potential of increasing runoff erosivity. Introduction Simpson, Akpokodje, & Umenweke, 2005; Igwe & Fukuoka, 2015; Emeh & Igwe, 2017). These low- moderate Erosion in South-eastern Nigeria is a devastating nat- plastic fines are chiefly composed of Aluminium and ural geologic hazard causing loss of arable lands, dam- iron oxides (sesquioxides) which serves as cement age to civil engineering constructions, and severing binding the individual soil matrix into bulk aggregates of underground utilities such as pipelines and cables (Townsend  & Reeds, 1971; Smith, 1990; Cornell & which are exposed by deep gully erosions. The fre- Schwertmann, 2003; Igwe et al., 2009; Peng & Sun, quency of this gullies are common within the edges of 2015). However, Oti (2002) reported that the result- urban areas, especial in unpaved surfaces which serves ant aggregates are relatively unstable when exposed to as flow channels for urban run-offs. Formation of these water; thus, leading to slaking and dispersion of the soil gullies has been previously attributed to the geology, aggregate. This soil aggregate behaviour is attributed to geotechnical properties and geochemical composition its low exchangeable sodium percentage (ESP), reduced of the underlying soils. Geologically, three formation calcium-magnesium ratio due to its relatively high con- which includes Ajali formation (cretaceous), Nsukka tent of aluminium and iron, and low content of soil formation (cretaceous) and Nanka Sands (Eocene) essential nutrients such as Na, Ca, and Mg, which helps have been identified to be more susceptible to erosion in formation of water stable soil aggregates (Mbagwu & (Nwajide, 1979, 1992; Obi & Asiegbu, 1980; Okagbue, Auerswald, 1999; Oti, 2002). 1988; Akpokodje, Tse, & Ekeocha, 2010). These rock Soil aggregate stability in water has been described Formations have undergone intensive tropical weath- as one of the major factor controlling soils suscepti- ering, thereby leaving the resultant soils acidic (oxisols) bility to water erosion (Egashlra, Kaetsu, & Takuma, (Igwe, Zarei, & Stahr, 2009). The Particle size distribu- 1983; Igwe, 2005). However, this soil–water interaction tion (PSD) of these soils comprises mostly of medium- is not only dependent of the chemical composition of fine-grained sands that are uniformly graded, with small the soil aggregate, but also on the chemical composi- amount of low-moderate plastic fines (Akpokodje, tion of the prevailing aqueous solution. Prove to this assertion was provided by Arlauckas, Hurowitz, Tosca, Olurunfemi, & EtuEfeotor, 1986; Okagbue, 1988; Hudec, CONTACT chukwuebuka emeh chukwuebuka.emeh.pg79017@unn.edu.ng © 2018 The a uthor(s). published by Informa UK limited, trading as Taylor & Francis Group. This is an open a ccess article distributed under the terms of the creative c ommons a ttribution license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 116 C. EMEH AND O. IGWE and McLennan (2004) on the dissolution of iron oxide run-off on slaking, dispersion and dissolution of the which was revealed to be dependent on the hydrogen soil aggregates. ion concentration of the dissolving solution. Similarly Frenkel, Levy, and Fey (1992) suggested that the addition Study area of humic acid to soil aggregate may be detrimental to e s Th tudy area lie within longitude 7° 1′ 8.0″ –7° 27′ its physical properties, as this acid promotes dispersion. 44.1″ E and latitude 6° 52′ 7.7″–6° 2′ 5.1″ N in South- Also Park, Seong, and Baik (2001) and Abdalla, Jaafar, eastern part of Nigeria (Figure 1). It is been underlain Al-Othman, Alfadul, and Ali Khan (2011) demonstrated by cretaceous to tertiary sediments within the Anambra that iron hydroxides are readily precipitated out of Iron Basin (Table 1). Three formations within this basin have oxide bearing mineral in aqueous solution of ammo- been identified to be relatively more susceptible to water nium hydroxide and ammonium acetate, while Mattson erosion. They include the Nanka sands in Ameki forma - (1927) concluded that flocculation and dispersion of soil tion (Eocene), Ajali formation (cretaceous), and Nsukka aggregate is controlled by the pH of the surrounding formation (cretaceous) (Nwajide, 1979, 1992; Obi & medium. Asiegbu, 1980; Akpokodje et al., 1986; Okagbue, 1988; Meanwhile, several researches which focused on Akpokodje et al., 2010). The Ajali formation comprises environmental pollution, such as works of Kjeldsen et about 400 m thick fine, medium to coarse-grained sub- al. (2002); Aluko, Sridhar, and Oluwande (2003); and angular to subrounded quartz arenites with occasionally Tiwari, Manoj, and Bisht (2007) have revealed the pres- thin layer of grey-coloured silty shale (Reyment, 1965; ence of these chemical compounds which causes defloc - Hoque & Ezepue, 1977). Ibe and Akaolisa (2010) follow- culation, dispersion, and dissolution of soil aggregate in ing Reyment (1965) suggestion, subdivides this forma- rain and run-off samples. The chemical composition of tion into two; the iron-rich and the Akorsic variety. The rain and storm water is greatly influenced by anthropo- Ajali Formation is conformably overlain by the Nsukka genic activities. These include emissions from hydrocar - formation, which comprises successive layers of shales bon production and usage which generates gases such and sandstone units, with occasional occurrences of coal as SO4 and NOx that reacts with water vapour in the seams (Reyment, 1965; Nwajide, 1979). e Th Nanka sands atmosphere, which subsequently results to acidic precip- which is a member of the Ameki Formation lies within itation (Hill, 2010; Efe & Mogborukor, 2012). Similarly, the adjacent Niger delta basin that was deposited during agricultural wastes, industrial effluents, dumpsite lea- the Eocene. The Eocene Ameki Formation comprises a chates, and other non point source of environmental succession of fine to coarse-grained tidally influenced pollutants have been found to be a major determinant u fl vial and fluvial sandstones at the basal part. This is of chemical composition of run-off (Horner, Skupien, successfully overlain by intercalations of clay, shale and Livingston, & Shaver, 1994; Hall & Anderson, 1988). i Th s limestone, with coarse-grained cross-bedded sand- phenomenon is common in industrial and commercial stones and clays at the uppermost part (Reyment, 1965; areas and is particularly higher in areas where theses Nwajide, 1979; Arua & Rao, 1987). wastes are improperly disposed to the environment, However, these rock Formations have so far under- thereby exposing them to direct contact with rain and gone severe tropical weathering which results in forma- storm water which dissolves them; hence ae ff cting the tion of dark-red coloured soils (oxisols), with occasional chemical composition of the resultant run-off (Horner thick layers of laterites. The oxisols are relatively less et al., 1994). resistant to water erosion than the laterites. This is as a However, the effect of this chemically modified run- result of their PSD which mostly comprises uniformly off on its erosivity to the underlying soils has not been graded fine-silt sands, with average coefficient of uni- previously considered in soil erodibility studies. Since formity (cu) of about 3.2 (Okagbue & Ezechi, 1988; increase in population and urbanization is associated Emeh & Igwe, 2017), compared with the laterites which with increase in environmental pollution especially in are composed of concretions ranging from boulder to developing countries such as Nigeria (Hill, 2010; Ubani & clay sized particles. The groundwater level within the Onyejekwe, 2013), it is imperative to investigate its studied areas varied greatly. From 7 m within some area contribution to the chemical composition of run-offs; directly underlain by the Ajali Fm (Onwuka, Uma, & hence its effect on the rapid degradation of land by Ezeigbo, 2004), to about 160  m in areas underlain by water erosion within South-eastern Nigeria. Thus, the the Nsukka Fm (Offodile, 2002). objectives of this work are; (i) to determine the geo- e a Th rea is characterized by a tropical climate with technical properties of the prevailing eroding soils, (ii) maximum temperature of 30.64  °C in the month of March and a minimum of 15.86  °C in the month of ascertain the chemical composition of fine particle por - December. e Th total annual rainfall is 1580.66  mm tion of the soils, (iii) determine some of the chemical with the lowest rainfall of about 16  mm occurring in compositions and physicochemical properties of the February and the highest about 350  mm in July (Eze, prevailing rainwater and run-offs, and then (iv) eval- 2007). The vegetation ranges from tropical rain forest uate the effect of change in chemical composition of GEOLOGY, ECOLOGY, AND LANDSCAPES 117 Figure 1. Geologic map of the study area. s ource: a uthor. Table 1. lithostratigraphy of the study area (modified after n wajide, 1990). Age Formation Members Lithologies and facies eocene ameki Group nanka s ands, nsugbe s ands Fine to coarse-grained tidally influenced fluvial and fluvial sandstones. Intercalations of clay, shale and limestone. c oarse-grained cross-bedded sandstones and clays. paleocene Imo shale clays, shales & siltstone Mainly bluish-grey shales and black shales with thin interbeds and nodules of coquinas, limestone and sharp-based micaceous siltstones. paleocene nsukka Formation   dark grey shales, intercalations of fine-grained sandstone and sandy shale. White sandstone, sandy shales and fine-grained clean sandstone. Thick fine-grained sandstone, medium-grained sandstone, grey to brick red coloured coarse-grained sandstone with calcareous cement. Fine to medium-grained sandstone, flat bedded medium-grained sandstone and shale. Maastrichtian a jali Formation s andstone Fine, medium to coarse-grained subangular to subrounded quartzarenites. d ominant sands with grey-coloured silty shale. Maastrichtian Mamu Formation shale, s andstone Fine to coarse-grained sandstone that is locally coally. alternating black carbonaceous black characteristic ooids and shell debris. Fine to medium-grained sandstones and siltstones. c ampanian n kporo Group n kporo shale, o welli s andstone c arbonaceous shale and concretional siltstone. enugu shale Rapid alternation of shale, siltstone and fine-grained sandstone. Micaceous sandstone with thin shale layers. Medium to coarse-grained quartz sandstone. dark shale and mudstone with occasional thin beds of sandy shale and sand- stone and shelly limestone. in the southern part to tropical savannah with numer- Methods ous scrubs in the northern part. However, most of these e s Th tudy combines field observations and laboratory vegetations have been removed as a result of urbaniza- experiments. Though prior to the field survey and obser - tion and agricultural activities, thus exposing the soils vations, a digital elevation model (DEM) was acquired to water erosion (Figure 2a and b). The studied areas from the United States geological survey (USGS) online are mostly densely populated with slums, markets, and archives. Topographical map of the areas were thus many small-scale industries. e Th towns lack proper land - generated from the DEM, and was analysed with the fill disposal system; hence wastes are indiscriminately aid of Surfer 11; a geospatial information system (GIS) disposed in urban drainage channels, open empty lands, software. This helped in determining different areas and inside the nearby gullies (Figure 3a and b). with varying gully erosion density and urban run-o ff 118 C. EMEH AND O. IGWE Figure 2. (a) and (b) photographs of severely eroding soils within the study area. Figure 3. (a) and (b) photographs of indiscriminate dumpsites within the study area. channels; hence guiding the choice of sites visited during for sampling of water and wastewater of the American actual field study. public health association (APHA, 1998). Field sampling Sample preparation Eight disturbed soil samples were collected from each Soil aggregate preparation of the three geological formations (Ameki, Ajali, and About 90 gram of the air dried soil sample passing Nsukka) that are easily susceptible to water erosion. The through ASTM sieve number 60 was thoroughly mixed sampling points were selected considering the erosion with tap water using a spatula until the soil is slightly density within a giving area, and the proximity to sources above its plastic limit. The mixture was then divided of pollutants which could possibly ae ff ct the chemical into three equal parts. Each portion was moulded into composition of the prevailing run-o. Th ff e samples were a ball-shaped soil aggregate of about 3 cm in diameter. collected at approximately 2 m away from the gully walls e r Th esultant soil aggregate samples were then allowed in a hand dug hole of about 2  m deep within an area to air dry for about 80 days. While drying the samples, it which have not been previously cultivated. This was was periodically moistened by spraying water on it using to ensure that relatively fresh representative soil sam- a small hand-held aerosol can sprayer. This is to pre- ples devoid of humus and/or artificial fertilizer were vent the soil samples from desiccation and to allow the obtained. e s Th oil samples were then bagged in water - sample to attain near field condition of about 3 months proof bags to keep moisture from entering it before sub- dry period. jecting it to laboratory analysis. Aqueous solution preparation To determine the chemical composition of the pre- Three aqueous solutions labelled A, B, and N was pre- vailing run-offs, 20 water samples were collected from pared using Nitric acid (HNO ), Sulphuric acid (H SO ), stagnant water in drainage channels of urban commer- 3 2 4 Ammonium hydroxide (NH OH), and deionized water. cial area and urban residential area, run-offs from agri- Acidic solution (A) was prepared using a mixture of con- cultural farmlands, and run-offs from dumpsites aer ft a centrated HNO and H SO acids which was diluted with precipitation event. Similarly, to determine the chemical 3 2 4 deionized water till a concentration of about 0.001  M composition of the prevailing rainfall, 20 rainwater sam- and a pH of 3.5 is attained. Similarly, a basic solution ples were collected during the beginning of the rainy (B) was prepared by diluting concentrated NH OH with season (March-May). Water samples collection was done deionized water till a concentration of about 0.01 M and following the methods outlined in standard methods GEOLOGY, ECOLOGY, AND LANDSCAPES 119 a pH of 10 was attained. Deionized water with pH of than 63 μm) of six soil samples, two representative sam- 7.0 was used as a neutral (N) solution. These solutions ples each from the three different geologic Formations, were carefully labelled and stored prior to its use for the were further subjected to pH test following the method slaking, dispersion and dissolution experiment. described by Hendershot, Lalande, and Duguette (1993), and also to X-ray fluorescence (XRF) analysis to deter - mine their chemical composition. Labouratory tests and experiments Rain and run-off chemical composition Slaking, dispersion and dissolution experiment Chemical compositions of the rain and run-off water Three 250 ml glass beaker were filled up to 200 ml point 2− − samples such as sulphate (SO ), nitrate (NO ), car- with the prepared aqueous solution A, B, and N; each 4 3 bonate (HCO ) and ammonium (NH ) were deter- solution in a separate beaker. The prepared soil sam- 3 4 mined following the methods outlined the in standard ples were then soaked in these solutions; each solution methods for the examination of water and wastewater of containing a replicate of the same soil sample. The sam- the American public health association (APHA, 1998) ples were observed every 0, 10, 30, and 60 min for the unless otherwise stated. pH and Electrical conductivity degree of slaking and dispersion. The degree of slaking (EC) were tested immediately in the field using a hand- was determined similarly to that of Emerson (1967); by held pH-meter and EC-metre, respectively. Total dis- recording the quantity of soil particles (crumbs) that solved solute (TDS) was calculated from the theoretical fail off from the soil aggregate in a given time, and the relationship between TDS and EC (Lloyd & Heathcote, time it took the aggregate to completely crumble. Aer ft 1985) 60 min, the samples were thoroughly stirred for about 2 min using a glass rod. The resultant suspensions were Soil geotechnical and chemical tests then allowed to stand undisturbed for 48 h aer s ft tirring. In order to determine the PSD of the soils, air dried sam- e deg Th ree of dispersion of the soil samples were deter - ple were subjected to sieve analysis using the American mined by noting the following: (i) the time it took the standard of testing materials (ASTM) sieve mesh num- supernatant to settle; hence the suspension becoming bers 5, 10, 18, 40, 60, 100 and 230 stacked on an electri- clear again aer ft stirring, and (ii) the crumb size of the cally controlled sieve shaker and was allowed to vibrate settled soil aer s ft tirring. The clarity of the suspension for about 25–30  min. Particles passing sieve number is determined when a dark round dot of about 1 cm in 40 were used to determine the soils liquid and plastic diameter pasted on the glass beaker can be clearly seen limit; hence its plasticity index following ASTM stand- through the suspension (Figure 4a and b). Aer 48  ft h, ard procedures. The fine particle fractions (particles less the solutions were carefully filtered through a 63  μm sieve into another container, ensuring that larger par- ticles do not pass through. The resultant solutions were then analysed for dissolved cations such as; Calcium (Ca), potassium (K), Sodium (Na), Magnesium (Mg), Aluminium (Al) and Iron (Fe), using atomic absorption spectroscopy (AAS) chemical analytical method. Solute dissolution was determined by summation of Al, Fe, K, Ca, Mg, and Na ions present in the resultant solution. Results Physicochemical properties of rainwater and run- off e Th rainwater samples were acidic with an average pH value of 4.99 (Table 2). The minimum pH of 3.3 was recorded at location 9 which is at the southernmost part of the study area, while the maximum pH of 6.1 was recorded at location 14 in the northernmost part of the study area (Figure 1); noting that the southern part is more urbanized and industrialized than the northern part. The run-off water samples were generally basic with an average pH value of 8.94 (Table 2). The maxi- mum pH of 10.2 was recorded in run-off from an urban dumpsite, while the minimum pH of 7.4 was recorded in Figure 4.  (a) and (b) photographs of the samples in solution run-off from urban residential areas. Generally, the pH 10 min after soaking and 1 h after stirring. 120 C. EMEH AND O. IGWE of run-off water samples from dumpsites, agricultural Geochemical composition of the fine fraction of farms, and stagnant water in urban drainage channels the soil samples were relatively higher than pH of run-offs from urban e pH a Th nd chemical compositions of the fine fraction of residential areas. The EC values of run-off water samples the soils were summarized in Table 4. pH of the soil sam- were very much higher than the EC values of rainwater ples were all acidic with an average value of 5.1. Apart (Table 2). Figure 5 revealed that the EC values of both from silicon oxide (SiO ) which contributes about 51% the rainwater and run-off samples tends to increase away of the total oxide in the soil’s fine fraction, iron oxide from the Neutral pH in both direction, with R-squared (Fe O ) is the next most abundant oxide, contributing an 2 3 (R ) of −0.702 and 0.703, respectively. This trend in average of 36.30%, followed by aluminium oxide (Al O ) 2 3 EC values were also observed in the values of the TDS with an average of 7.15%. The percentages of all other (Figure 6); hence dissolution of solutes appears to be oxides are in insignificant quantity with respect to their dependent on the pH with R  = −0.702 and 0.703, for contribution as a cementing material. This result of the rainwater and run-offs, respectively. chemical composition of the soils is similar with that of 2− − e a Th verage concentration values of SO , NO , 4 3 Ibe and Akaolisa (2010) and Igwe et al. (2009) on the HCO and NH in the rainwater samples were 38.1, 3 4 same study area. However, the percentage of SiO is rel- 19.8, 2.98, and 2.63 ppm, respectively, while their val- atively higher, and that of Fe O is relatively lower in the 2 3 ues in run-off samples were 283, 317, 68, and 735 ppm above author’s results compared with the result of this (Table 2). Generally, the acidic nature of the rain could work. The reason for these variations in SiO and Fe O 2− 2 2 3 be attributed to its relatively high concentration of SO composition could be attributed to the fraction of the and NO which could react with hydrogen ion (H ) in soil sample analysed. While the other authors analysed water to form acids, while the basic nature of the run-o ff the bulk aggregate sample of the soil, this work focused could be attribute to its relatively high concentrations on the fine particle fraction of the soil. Noting that SiO + − of NH which could react with hydroxyl ion (OH ) in is relatively stable to weathering, and will constitute the water to form a base. majority of the coarse grains in the bulk aggregate; hence the observed variation. Geotechnical properties of soil e Th structures of the in situ soils were generally homog- Slaking, dispersion, and dissolution enous and are reddish in colour. They appear cohesive in e deg Th ree of slaking and dispersion as determined from nature under its dry condition and were relatively stiff the experiment were scored from 1 to 5, and were tab- that it cannot be indented by the thumb. The geotech- ulated in Tables 5 and 6. The mean value of about 3.5 nical properties of the soil such as PSD, coefficient of of the slaking score (Table 7) revealed that most of the uniformity (Cu), and consistency limits were summa- samples were moderately-strongly slaked. However, a rized in Table 3. The PSD (Figure 7) revealed that the careful observation revealed that the degree of slaking soils are coarse-grained with coefficient of uniformity was not determined by variations in pH of the solution, ranging from 2.74 to 5.99, with an average value of 3.98; but rather by the plasticity index of the soil samples hence the soils are uniformly graded. The plasticity index (Figure 8), with (R ) value of −0.907. The degrees of slak - (PI) of the soils in the area underlain by Nsukka Fm and ing of soils within the area underlain by Ajali Fm were Nanka sands with an average value of 19.11 indicates severe, while that of the ones within the areas under- that they are moderately plastic, while those underlain lain by Nsukka Fm and Nanka sands shows slight-strong by Ajali Formation are of low plasticity with average PI of degree of slaking. This was because the plasticity of soils 3.59. In general, the soils could be classified as silty sands within Ajali Fm are relatively low compared with the one (SM) of low-medium plasticity according to unified soil within Nsukka Fm and Nanka sands which are moder- classification system (USCS). ately plastic (Table 7). Furthermore, the mean of the dispersion score was plotted against the pH of the dispersing solutions. This Table 2. s ome physicochemical properties of the studied run- reveals that the dispersion of the soil samples were off and rainwater. dependent on the pH of the solutions. Most of the sam- Source ples in neutral solutions (N ) did not show any sign of Runoff Rainwater dispersion, the ones in acidic solution (A ) were mod- Parameter Mean Max Min Mean Max Min erately dispersed, while samples in the basic solutions 2− 282.65 501 19 38.1 63 27 SO (ppm) (B ) were all strongly dispersed (Figure 9). The pattern NO (ppm) 316.7 978 109 19.1 48 7 3 D HCO (ppm) 67.6 167 13 2.98 5 0.8 3 that exists in the soils dispersion was also observed in NH nH 734.5 1567 205 2.63 4 0.9 3 4 the dissolution of the soil samples in aqueous solutions (ppm) pH 8.94 10.2 7.4 4.98 6.1 3.3 of varying pH. Basic solutions (B ) has more dissolved Di ec (μs/cm) 568.8 890 89 82.6 186 21 cations with an average value of 40  ppm, followed by Tds (ppm) 387 605 61 56 126 14 GEOLOGY, ECOLOGY, AND LANDSCAPES 121 the acidic solutions (A ) with an average of 20.31 ppm, Di y = 262.38x- 1775.6 R² = 0.7028 and neutral solution (N ) has the least dissolved cations Di with an average value of 11.59 ppm (Figure 10). It was observed that iron contributes about 60% to the quantity Ecro vs pH of the dissolved cations in the basic solution, while con- Ecrw vs pH 400 tributing just about 11 and 4% in the Acidic and Neutral Linear (Ecrw vs pH) y = -37.503x + 269.18 Linear (Ecrw vs pH) solution, respectively (Figure 11). R² = 0.7019 Discussion 0246 81012 pH e s Th tudied soils are majorly composed of medium-fine- grained sands with about 6% of low-medium plastic Figure 5.  Relationship between ec and pH of run-off and fines. About 43% of the fine particle fractions of these rainwater samples. soils are chiefly composed of Iron and Aluminium oxide (sesquioxides). These oxides have been attributed to be the major cement binding the individual soil particles y = 178.42x-1207.4 600 into larger aggregates (Cornell & Schwertmann, 2003; R² = 0.7028 Peng & Sun, 2015). Valentin (1986b), and Van der Walt and Valentin (1992) also suggested that these oxides are TDSro vs pH y = -37.503x + 269.18 responsible for encrusting of soil surfaces; hence reduc- TDSrw vs pH R² = 0.7019 Linear (TDSrw vs pH) ing infiltration and promoting run-o. H ff owever, the instability of these soils to water erosion has been seen to be dependent not only on the chemical composition 0 246 81012 pH of the soils, but also on the chemical composition of the prevailing aqueous environment. While these soil Figure 6.  Relationship between Tds and pH of run-off and aggregates maybe relatively stable in aqueous solution rainwater samples. of neutral pH – which may not be naturally attainable, Table 3. s ome geotechnical properties of the studied soils. G CS-MS FS FINES Cu Consistency limits Sample location (>2 mm) (<2–0.25 mm) (<0.25–0.063 mm) (<0.063 mm)   LL PL PI 1* 0 37.06 58.51 4.25 3.43 30.4 10.6 19.80 2* 0 36.07 60.20 3.86 3.42 30.00 12.80 17.20 3* 0 40.45 54.66 5.30 3.49 34.6 15.9 18.70 4* 0 38.40 61.30 6.10 3.50 32 15.1 16.90 5* 0 33.24 57.93 4.34 3.38 40 19.9 20.10 6* 0 41.22 59.10 4.26 3.34 41.8 22 19.80 7* 0 36.89 58.56 3.48 3.41 38.3 20.9 17.40 8* 0 39.87 63.89 4.93 3.47 42.3 23.5 18.80 9** 0 32.73 57.94 8.38 5.82 45 21.7 23.30 10** 0 32.90 61.56 7.58 5.20 42.5 26.1 16.40 11** 0 29.23 64.12 9.45 5.99 38.7 17.1 21.60 12** 0 26.95 59.22 9.00 5.98 37.8 18.3 19.50 13** 0 28.22 56.97 6.90 4.80 43.4 24.7 18.70 14** 0 36.23 60.85 7.89 5.20 31.2 13.3 17.90 15** 0 33.14 62.78 7.23 5.46 40 19.7 20.30 16** 0 29.99 60.12 8.84 5.63 36.8 17.4 19.40 17*** 0 29.61 65.08 4.61 3.2 26 22.4 3.60 18*** 0 32.12 72.80 3.78 2.90 32.8 29.2 3.60 19*** 0 27.84 63.39 4.20 3.00 28.2 25.4 2.80 20*** 0 28.34 69.20 4.32 3.11 25.7 21.5 4.20 21*** 0 30.45 66.80 5.23 3.30 29.1 24.3 4.80 22*** 0 30.56 71.12 3.67 2.78 25.4 21.5 3.90 23*** 0 23.98 59.55 3.33 2.74 26.7 24 2.70 24*** 0 31.34 68.20 4.86 2.89 30.6 27.3 3.30 summary statistics Mean 0 32.78 62.24 5.66 3.98 34.55 20.61 13.95 Max 0 41.22 72.8 9.45 5.99 45 29.2 23.3 Min 0 23.98 54.66 3.33 2.74 25.4 10.6 2.7 notes: G = Gravel, cs -Ms = c oarse to medium grain sand, Fs = Fine sand, FInes = silt and clay size particles, c u = c oefficient of uniformity, ll = liquid limit, pl = plastic limit, pI = plasticity index. *nanka Fm.; **nsukka Fm.; ***a jali Fm. TDS (ppm) EC (uS/cm) 122 C. EMEH AND O. IGWE 0.01 0.11 10 Particle size (mm) Figure 7. Mean psd of the soil samples. Table 4. chemical composition of some of the studied soil e r Th esults from this research have shown that the samples. chemical composition of the prevailing run-off is deter - mined by the type and amount of pollutant which are Concentration values (%) present in rainwater, dumpsites, sewages, and agricul- Parameter Mean Max Min tural wastes. Amongst these chemical pollutants are pH* 5.12 5.6 4.8 sio 51.22 66.4 34.8 2 sulphuric acid, nitric acid, and ammonium hydroxide. Tio2 0.44 0.83 0.05 es Th e chemical compounds appear to be the major al o 7.15 20.32 1.39 2 3 Fe o 36.3 50.97 18.86 compounds ae ff cting the chemical composition of the 2 3 Mno 0.03 0.05 0.01 run-off since they are found in appreciable percentage Mgo 0.36 0.5 0.1 ca o 0.5 0.88 0.1 in the run-off samples that was analysed. This result is na o 0.04 0.07 0.02 consistent with the work of other researchers working K2o 0.05 0.07 0.04 p o 0.44 0.97 0.05 on environmental pollution (Hall & Anderson, 1988; 2 5 lo I 3.38 7.07 2.1 Sumner & McLaughlin, 1996; Aluko et al., 2003; Tiwari et al., 2007). Run-off generated from agricultural active areas and those from industrial/commercial areas, espe- they disperse and dissolve in acidic and/or basic aque- cially those sourced from urban dumpsites appears to ous solutions which are typical of run-offs within the have higher concentration of these chemicals, compared study area. with those from residential and economic less active Table 5. laboratory observation results of slaking of the soil samples. Parameters used in determining the degree of slaking Slaking score Degree of slaking s oil aggregate remains intact after 60 min with no crumbs falling off it 1 non Few crumbs fall off from the soil aggregate, but aggregate remains spherical after 30 min 2 slight about 40% of crumbs fall off from the soil aggregate after 10 min, but the aggregate still partly maintained its 3 Moderate spherical shape after 30 min about 75% of crumbs fall off from the soil aggregate after 10 min, forming a conical shape, and soil lump com- 4 s trong pletely collapsed into crumbs after 30 min. 100% of the soil aggregate completely collapsed into crumbs immediately it is been soaked in the solution 5 s evere Table 6. laboratory observation results of dispersion of the soil samples. Parameters used in determining the degree of dispersion Dispersion score Degree of dispersion s olution remains clear after soaking the sample, and reappears clear within 10 min after stirring; crumb 1 non size appears larger 2 mm. s olution remains clear after soaking the sample, but took about 30 min for solution to clear out after 2 slight stirring; crumb size appears less than 2 mm but not powdered. s olution remains clear after soaking the sample, but took about 1 h for solution to clear out after stirring; 3 Moderate crumb size appears less than 2 mm, and about 30% appears powdered. s olution remains clear after soaking the sample, but took about 5hours for solution to clear out after 4 s trong stirring; crumb size appears less than 2 mm and about 50% appears powdered. s olution becomes unclear immediately the sample is soaked in it, and remains unclear after 48 h; crumb 5 s evere size cannot be determined because the solution appears very cloudy (muddy). Percentage finer GEOLOGY, ECOLOGY, AND LANDSCAPES 123 Table 7. experimental results of slaking, dispersion, and dissolution of the soil samples. Slaking (S) Dispersion (D) Dissolution (Di) (ppm) Dissolved iron (Fe) (ppm) Sample location A B N A B N A B N A B N s s s D D D Di Di Di Fe Fe Fe 1* 2 3 2 3 5 1 25.33 41.01 14.45 3.79 25.55 0.73 2* 2 3 3 3 5 2 22.14 51.22 10.40 2.86 32.65 0.64 3* 2 3 2 4 5 1 23.16 43.76 13.20 4.27 28.22 0.78 4* 3 3 3 4 5 1 17.50 54.98 9.34 3.27 34.22 0.67 5* 2 2 2 3 5 1 19.64 39.27 11.10 2.10 22.87 0.23 6* 2 3 2 3 5 2 24.34 51.33 11.45 4.73 29.88 0.94 7* 3 4 3 3 5 1 18.75 58.19 7.54 3.89 37.82 0.12 8* 3 3 2 3 5 1 20.48 44.56 11.20 5.30 31.64 0.72 9** 2 2 2 2 5 1 21.26 26.52 14.89 0.86 8.19 1.35 10** 3 4 3 2 5 1 22.12 29.17 13.88 0.56 10.23 0.86 11** 2 2 2 2 5 1 18.39 24.98 14.22 1.37 9.22 0.03 12** 2 3 2 2 5 1 19.98 24.78 15.90 0.68 8.83 0.02 13** 3 2 2 2 5 1 22.98 27.74 10.89 0.96 11.60 1.30 14** 2 3 2 3 5 1 19.70 23.80 16.30 1.22 8.46 0.07 15** 2 2 3 2 5 1 16.43 28.12 14.60 0.79 8.97 0.78 16** 2 3 2 2 5 1 19.49 26.72 10.56 0.56 12.40 0.14 17*** 5 5 5 3 5 1 22.77 42.20 12.18 1.83 30.81 0.49 18*** 5 5 5 3 5 1 18.56 49.33 17.40 2.05 34.89 0.34 19*** 5 5 5 3 5 1 19.45 33.37 10.63 1.89 26.99 0.16 20*** 4 5 5 3 5 1 18.90 44.57 7.98 2.22 29.42 0.09 21*** 5 5 4 4 5 2 22.67 56.42 4.67 3.40 39.12 0.56 22*** 5 5 4 3 5 1 11.89 42.73 4.46 1.78 27.70 0.48 23*** 5 5 5 3 5 1 17.56 39.12 12.11 2.29 22.96 0.14 24*** 5 5 5 3 5 1 23.89 56.23 8.89 1.90 40.12 0.04 summary statistics sum 76.00 85.00 75.00 68.00 120 27.00 487.38 960.11 278.24 54.56 572.77 11.68 Mean 3.17 3.54 3.13 2.83 5 1.13 20.31 40.00 11.59 2.27 23.87 0.49 Max 5.00 5.00 5.00 4.00 5 2.00 25.33 58.19 17.40 5.30 40.12 1.35 Min 2.00 2.00 2.00 2.00 5 1.00 11.89 23.80 4.46 0.56 8.19 0.02 δ 1.28 1.15 1.24 0.62 0 0.33 2.91 11.24 3.31 1.35 10.87 0.39 notes: a = a cidic solution, B = Basic solution, n = neutral solution, δ = s tandard deviation. *nanka Fm.; **nsukka Fm.; ***a jali Fm. areas – which reflected in the values of their pH, EC, and TDS. e deg Th ree of dispersion and dissolution of soil aggregates and its constituent elements, respectively, are found to be higher in basic aqueous solution of ammo- Ns vs PI nium hydroxide compared with acidic aqueous solu- y = -0.1576x + 5.3216 tion of sulphuric and nitric acid mixture. The reason for R² = 0.9067 this could be as a result of the higher concentration of NH OH which is about 0.01 M in the basic solution that was used compared with 0.001 M of the acid mixture that 05 10 15 20 25 Plasticity index (PI) was used. NH OH of 0.01 M was used because similar concentration was observed in most of the sampled run- Figure 8. Relationship between slaking and plasticity of the soil. o. Th ff e same reason was used to condition the aqueous mixture of H SO and HNO at a concentration level of 2 4 3 0.001 M. Though this experiment was conditioned at a constant hydrogen ion concentration, solubility of metal- lic oxides such as iron oxide have been found to increase 4 with increase in hydrogen ion concentration in aqueous acidic solution (Arlauckas et al., 2004). Thus, increase in the molar concentration of the acidic mixture used may D vs pH subsequently increase the solubility of soil constituent ele- ments; hence leading to aggregate instability. Another rea- son for this observed variation as discussed by Park et al. 0246 81012 (2001) is that hydroxides of Fe and Al in sesquioxide-rich pH soils are readily precipitated out in basic aqueous solution of ammonium compounds, such as ammonium hydrox- Figure 9.  Relationship between dispersion and pH of the solutions. ide and ammonium acetate, while iron and aluminium Slaking Dispersion score 124 C. EMEH AND O. IGWE e deg Th ree of dissolution of other non sesquioxides 40.00 such as NaO , CaO, and MgO appears to be higher in the aqueous acidic solution. However, in the case of the 30.00 studied soil, it may not be a major concern as these metal 20.00 oxides constitutes very little fraction in the amount of Di vs pH cement within the soil aggregates which are predomi- 10.00 nantly sesquioxides. Moreover, the experiment reveals that sesquioxides of iron and aluminium are sparingly 0.00 02468 10 12 soluble in acidic solution of relatively low hydrogen ion pH concentration of about 0.001 M. However, there solubil- ity may increase with increase in the concentration level Figure 10.  Relationship between dissolved cations and pH of of the aqueous solution (Arlauckas et al., 2004). Thus, the solutions. increase in the concentration level of these compounds in the run-off may result to increase in the solubility of metallic oxides in the soil; hence leading to soil aggregate instability. Another important component of run-off which 59.66% 25 could ae ff ct the deflocculation and dispersion of soil aggregate is the humic acid. This acid has been reported by (Frenkel et al., 1992) as a major chemical compound causing dispersion of soil aggregate. Though this com- 5 11.20% 4.20% pound was not analysed for in the run-off samples used ADiBDi NDi for this experiment, its presence in dumpsites leachate, TDC DFe sewage, and composts have been reported by several authors. Figure 11. percentage contribution of iron in the total dissolved cation. Conclusion have been found by (Arlauckas et al., 2004; Lewis, 2007) This work has revealed that the studied soil is mainly to be sparingly soluble in acidic solution of relatively low composed of medium-fine-grained sands with little hydrogen ion concentration of about 0.001 M. amount of low-moderate plastic fines which are chiefly However, dispersion of the soil aggregates appears composed of iron and aluminium oxides that serve as to be more closely associated with soils susceptibility cement in the soil matrix. to erosion than dissolution of its constituent minerals. es Th e sesquioxide cements disperses and dissolves in e Th degree of dispersion which was determined by the basic aqueous solution of ammonium hydroxide and in cloudiness of the solution and the settling time of the acidic aqueous solution of sulphuric and nitric acid mix- particles is a result of the reaction between ammonium ture. However, the degree of dissolution and dispersion hydroxide and the sesquioxides in the soil aggregate – of iron oxide is more compared to that of aluminium especially iron oxides which contributed more in the oxide. total amount of dissolved solute. Following the work of e deg Th ree of dispersion and dissolution of soil aggre - Park et al. (2001) and Abdalla et al. (2011), the mecha- gate and its constituent element, respectively, is more in nism for this reaction could be stated as follows; basic aqueous solution of ammonium hydroxide than in acidic aqueous solution of sulphuric and nitric acid 3+ + Fe aq + 3NH aq + 3H O ←→ Fe(OH) (s) + 3NH aq 3 2 3 4 mixture. Precipitate (Iron bearing mineral) (Dumpsite runoff) ( ) Ammonium hydroxide, Sulphuric acid, and Nitric (1) acid are found in appreciable quantity in the prevailing e in Th soluble brick red precipitate forms a colloidal sus - rainwater and run-offs. However, the concentration level pension (Park et al., 2001), which are then transported of this chemical compounds are higher in run-offs that away as suspended solids by run-o, le ff aving behind were generated from urban dumpsites and agricultural disaggregated soil particles with little or no cements. farmlands. Since this sesquioxides are considered as flocculating er Th efore, in order to prevent the susceptibility of iron agent binding individual soil grains into bulk aggregates oxide-cemented soils to water erosion, it is imperative (Townsend & Reeds, 1971), there removal in the soil to determine the sources and chemical composition of matrix will lead to the reduction in the cohesion of the the prevailing run-offs. This will help in proper run- soil. Lack of cohesion in soils has been associated with off treatment and drainage system control to avoid the increase in its instability, thus resulting in erosion and contact of chemically erosive run-offs with iron-oxide- land sliding (Igwe & Fukuoka, 2015). cemented soils. Also it is important that urban dumpsites Dissolved cations (ppm) GEOLOGY, ECOLOGY, AND LANDSCAPES 125 Frenkel, H., Levy, G. J., & Fey, M. V. (1992). Clay dispersion are properly designed, and indiscriminate disposal of and hydraulic conductivity of clay-sand mixtures as both liquid and solid wastes should be highly discour- ae ff cted by the addition of various anions. Clays and Clay aged in order to avoid direct contact with rainfall, which Minerals, 40, 515–521. could generate chemically modified erosive run-offs. Hall, K.J., & Anderson, B. (1988). The toxicity and chemical composition of urban stormwater runo. ff Canadian Journal of Civil Engineering, 15, 98–106. 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Effect of environmental pollution on susceptibility of sesquioxide-rich soils to water erosion

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GeoloGy, ecoloGy, and landscapes, 2018 Vol . 2, no . 2, 115– 126 https://doi.org/10.1080/24749508.2018.1452484 INWASCON OPEN ACCESS Eec ff t of environmental pollution on susceptibility of sesquioxide-rich soils to water erosion Chukwuebuka Emeh and Ogbonnaya Igwe d epartment of Geology, University of n igeria, nsukka, n igeria ABSTRACT ARTICLE HISTORY Received 29 d ecember 2017 This work assessed the impact of environmental pollution on soils susceptibility to water a ccepted 2 March 2018 erosion within South-eastern Nigeria. Sources of pollutants which could possibly affect the chemical composition of runoff; hence its pH, were first determined by remote sensing and KEYWORDS field observations. Rain and runoff water samples collected within the study area were analysed erosion; sesquioxide; for its physicochemical compositions. Soil samples were also collected and analysed for their environmental pollution; pH; geotechnical properties, and chemical composition of their fine fractions. An empirical method dispersion; urban run-off was then employed to determine the effect of change in chemical composition of runoff on the susceptibility of the studied soil to water erosion. This was achieved by conducting soil aggregate slaking, dispersion and dissolution tests on aqueous solutions of varying pH. Results from the experiment shows that the fine particle fractions of the soils are chiefly composed of sesquioxides. The slaking of these sesquioxide-cemented soils is not affected by the variations in pH of the solutions, but rather by the plasticity index of the soils. However, dispersion and dissolution of the soil samples where dependent on variations in the pH of the solutions. It was therefore concluded that environmental pollution has the potential of increasing runoff erosivity. Introduction Simpson, Akpokodje, & Umenweke, 2005; Igwe & Fukuoka, 2015; Emeh & Igwe, 2017). These low- moderate Erosion in South-eastern Nigeria is a devastating nat- plastic fines are chiefly composed of Aluminium and ural geologic hazard causing loss of arable lands, dam- iron oxides (sesquioxides) which serves as cement age to civil engineering constructions, and severing binding the individual soil matrix into bulk aggregates of underground utilities such as pipelines and cables (Townsend  & Reeds, 1971; Smith, 1990; Cornell & which are exposed by deep gully erosions. The fre- Schwertmann, 2003; Igwe et al., 2009; Peng & Sun, quency of this gullies are common within the edges of 2015). However, Oti (2002) reported that the result- urban areas, especial in unpaved surfaces which serves ant aggregates are relatively unstable when exposed to as flow channels for urban run-offs. Formation of these water; thus, leading to slaking and dispersion of the soil gullies has been previously attributed to the geology, aggregate. This soil aggregate behaviour is attributed to geotechnical properties and geochemical composition its low exchangeable sodium percentage (ESP), reduced of the underlying soils. Geologically, three formation calcium-magnesium ratio due to its relatively high con- which includes Ajali formation (cretaceous), Nsukka tent of aluminium and iron, and low content of soil formation (cretaceous) and Nanka Sands (Eocene) essential nutrients such as Na, Ca, and Mg, which helps have been identified to be more susceptible to erosion in formation of water stable soil aggregates (Mbagwu & (Nwajide, 1979, 1992; Obi & Asiegbu, 1980; Okagbue, Auerswald, 1999; Oti, 2002). 1988; Akpokodje, Tse, & Ekeocha, 2010). These rock Soil aggregate stability in water has been described Formations have undergone intensive tropical weath- as one of the major factor controlling soils suscepti- ering, thereby leaving the resultant soils acidic (oxisols) bility to water erosion (Egashlra, Kaetsu, & Takuma, (Igwe, Zarei, & Stahr, 2009). The Particle size distribu- 1983; Igwe, 2005). However, this soil–water interaction tion (PSD) of these soils comprises mostly of medium- is not only dependent of the chemical composition of fine-grained sands that are uniformly graded, with small the soil aggregate, but also on the chemical composi- amount of low-moderate plastic fines (Akpokodje, tion of the prevailing aqueous solution. Prove to this assertion was provided by Arlauckas, Hurowitz, Tosca, Olurunfemi, & EtuEfeotor, 1986; Okagbue, 1988; Hudec, CONTACT chukwuebuka emeh chukwuebuka.emeh.pg79017@unn.edu.ng © 2018 The a uthor(s). published by Informa UK limited, trading as Taylor & Francis Group. This is an open a ccess article distributed under the terms of the creative c ommons a ttribution license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 116 C. EMEH AND O. IGWE and McLennan (2004) on the dissolution of iron oxide run-off on slaking, dispersion and dissolution of the which was revealed to be dependent on the hydrogen soil aggregates. ion concentration of the dissolving solution. Similarly Frenkel, Levy, and Fey (1992) suggested that the addition Study area of humic acid to soil aggregate may be detrimental to e s Th tudy area lie within longitude 7° 1′ 8.0″ –7° 27′ its physical properties, as this acid promotes dispersion. 44.1″ E and latitude 6° 52′ 7.7″–6° 2′ 5.1″ N in South- Also Park, Seong, and Baik (2001) and Abdalla, Jaafar, eastern part of Nigeria (Figure 1). It is been underlain Al-Othman, Alfadul, and Ali Khan (2011) demonstrated by cretaceous to tertiary sediments within the Anambra that iron hydroxides are readily precipitated out of Iron Basin (Table 1). Three formations within this basin have oxide bearing mineral in aqueous solution of ammo- been identified to be relatively more susceptible to water nium hydroxide and ammonium acetate, while Mattson erosion. They include the Nanka sands in Ameki forma - (1927) concluded that flocculation and dispersion of soil tion (Eocene), Ajali formation (cretaceous), and Nsukka aggregate is controlled by the pH of the surrounding formation (cretaceous) (Nwajide, 1979, 1992; Obi & medium. Asiegbu, 1980; Akpokodje et al., 1986; Okagbue, 1988; Meanwhile, several researches which focused on Akpokodje et al., 2010). The Ajali formation comprises environmental pollution, such as works of Kjeldsen et about 400 m thick fine, medium to coarse-grained sub- al. (2002); Aluko, Sridhar, and Oluwande (2003); and angular to subrounded quartz arenites with occasionally Tiwari, Manoj, and Bisht (2007) have revealed the pres- thin layer of grey-coloured silty shale (Reyment, 1965; ence of these chemical compounds which causes defloc - Hoque & Ezepue, 1977). Ibe and Akaolisa (2010) follow- culation, dispersion, and dissolution of soil aggregate in ing Reyment (1965) suggestion, subdivides this forma- rain and run-off samples. The chemical composition of tion into two; the iron-rich and the Akorsic variety. The rain and storm water is greatly influenced by anthropo- Ajali Formation is conformably overlain by the Nsukka genic activities. These include emissions from hydrocar - formation, which comprises successive layers of shales bon production and usage which generates gases such and sandstone units, with occasional occurrences of coal as SO4 and NOx that reacts with water vapour in the seams (Reyment, 1965; Nwajide, 1979). e Th Nanka sands atmosphere, which subsequently results to acidic precip- which is a member of the Ameki Formation lies within itation (Hill, 2010; Efe & Mogborukor, 2012). Similarly, the adjacent Niger delta basin that was deposited during agricultural wastes, industrial effluents, dumpsite lea- the Eocene. The Eocene Ameki Formation comprises a chates, and other non point source of environmental succession of fine to coarse-grained tidally influenced pollutants have been found to be a major determinant u fl vial and fluvial sandstones at the basal part. This is of chemical composition of run-off (Horner, Skupien, successfully overlain by intercalations of clay, shale and Livingston, & Shaver, 1994; Hall & Anderson, 1988). i Th s limestone, with coarse-grained cross-bedded sand- phenomenon is common in industrial and commercial stones and clays at the uppermost part (Reyment, 1965; areas and is particularly higher in areas where theses Nwajide, 1979; Arua & Rao, 1987). wastes are improperly disposed to the environment, However, these rock Formations have so far under- thereby exposing them to direct contact with rain and gone severe tropical weathering which results in forma- storm water which dissolves them; hence ae ff cting the tion of dark-red coloured soils (oxisols), with occasional chemical composition of the resultant run-off (Horner thick layers of laterites. The oxisols are relatively less et al., 1994). resistant to water erosion than the laterites. This is as a However, the effect of this chemically modified run- result of their PSD which mostly comprises uniformly off on its erosivity to the underlying soils has not been graded fine-silt sands, with average coefficient of uni- previously considered in soil erodibility studies. Since formity (cu) of about 3.2 (Okagbue & Ezechi, 1988; increase in population and urbanization is associated Emeh & Igwe, 2017), compared with the laterites which with increase in environmental pollution especially in are composed of concretions ranging from boulder to developing countries such as Nigeria (Hill, 2010; Ubani & clay sized particles. The groundwater level within the Onyejekwe, 2013), it is imperative to investigate its studied areas varied greatly. From 7 m within some area contribution to the chemical composition of run-offs; directly underlain by the Ajali Fm (Onwuka, Uma, & hence its effect on the rapid degradation of land by Ezeigbo, 2004), to about 160  m in areas underlain by water erosion within South-eastern Nigeria. Thus, the the Nsukka Fm (Offodile, 2002). objectives of this work are; (i) to determine the geo- e a Th rea is characterized by a tropical climate with technical properties of the prevailing eroding soils, (ii) maximum temperature of 30.64  °C in the month of March and a minimum of 15.86  °C in the month of ascertain the chemical composition of fine particle por - December. e Th total annual rainfall is 1580.66  mm tion of the soils, (iii) determine some of the chemical with the lowest rainfall of about 16  mm occurring in compositions and physicochemical properties of the February and the highest about 350  mm in July (Eze, prevailing rainwater and run-offs, and then (iv) eval- 2007). The vegetation ranges from tropical rain forest uate the effect of change in chemical composition of GEOLOGY, ECOLOGY, AND LANDSCAPES 117 Figure 1. Geologic map of the study area. s ource: a uthor. Table 1. lithostratigraphy of the study area (modified after n wajide, 1990). Age Formation Members Lithologies and facies eocene ameki Group nanka s ands, nsugbe s ands Fine to coarse-grained tidally influenced fluvial and fluvial sandstones. Intercalations of clay, shale and limestone. c oarse-grained cross-bedded sandstones and clays. paleocene Imo shale clays, shales & siltstone Mainly bluish-grey shales and black shales with thin interbeds and nodules of coquinas, limestone and sharp-based micaceous siltstones. paleocene nsukka Formation   dark grey shales, intercalations of fine-grained sandstone and sandy shale. White sandstone, sandy shales and fine-grained clean sandstone. Thick fine-grained sandstone, medium-grained sandstone, grey to brick red coloured coarse-grained sandstone with calcareous cement. Fine to medium-grained sandstone, flat bedded medium-grained sandstone and shale. Maastrichtian a jali Formation s andstone Fine, medium to coarse-grained subangular to subrounded quartzarenites. d ominant sands with grey-coloured silty shale. Maastrichtian Mamu Formation shale, s andstone Fine to coarse-grained sandstone that is locally coally. alternating black carbonaceous black characteristic ooids and shell debris. Fine to medium-grained sandstones and siltstones. c ampanian n kporo Group n kporo shale, o welli s andstone c arbonaceous shale and concretional siltstone. enugu shale Rapid alternation of shale, siltstone and fine-grained sandstone. Micaceous sandstone with thin shale layers. Medium to coarse-grained quartz sandstone. dark shale and mudstone with occasional thin beds of sandy shale and sand- stone and shelly limestone. in the southern part to tropical savannah with numer- Methods ous scrubs in the northern part. However, most of these e s Th tudy combines field observations and laboratory vegetations have been removed as a result of urbaniza- experiments. Though prior to the field survey and obser - tion and agricultural activities, thus exposing the soils vations, a digital elevation model (DEM) was acquired to water erosion (Figure 2a and b). The studied areas from the United States geological survey (USGS) online are mostly densely populated with slums, markets, and archives. Topographical map of the areas were thus many small-scale industries. e Th towns lack proper land - generated from the DEM, and was analysed with the fill disposal system; hence wastes are indiscriminately aid of Surfer 11; a geospatial information system (GIS) disposed in urban drainage channels, open empty lands, software. This helped in determining different areas and inside the nearby gullies (Figure 3a and b). with varying gully erosion density and urban run-o ff 118 C. EMEH AND O. IGWE Figure 2. (a) and (b) photographs of severely eroding soils within the study area. Figure 3. (a) and (b) photographs of indiscriminate dumpsites within the study area. channels; hence guiding the choice of sites visited during for sampling of water and wastewater of the American actual field study. public health association (APHA, 1998). Field sampling Sample preparation Eight disturbed soil samples were collected from each Soil aggregate preparation of the three geological formations (Ameki, Ajali, and About 90 gram of the air dried soil sample passing Nsukka) that are easily susceptible to water erosion. The through ASTM sieve number 60 was thoroughly mixed sampling points were selected considering the erosion with tap water using a spatula until the soil is slightly density within a giving area, and the proximity to sources above its plastic limit. The mixture was then divided of pollutants which could possibly ae ff ct the chemical into three equal parts. Each portion was moulded into composition of the prevailing run-o. Th ff e samples were a ball-shaped soil aggregate of about 3 cm in diameter. collected at approximately 2 m away from the gully walls e r Th esultant soil aggregate samples were then allowed in a hand dug hole of about 2  m deep within an area to air dry for about 80 days. While drying the samples, it which have not been previously cultivated. This was was periodically moistened by spraying water on it using to ensure that relatively fresh representative soil sam- a small hand-held aerosol can sprayer. This is to pre- ples devoid of humus and/or artificial fertilizer were vent the soil samples from desiccation and to allow the obtained. e s Th oil samples were then bagged in water - sample to attain near field condition of about 3 months proof bags to keep moisture from entering it before sub- dry period. jecting it to laboratory analysis. Aqueous solution preparation To determine the chemical composition of the pre- Three aqueous solutions labelled A, B, and N was pre- vailing run-offs, 20 water samples were collected from pared using Nitric acid (HNO ), Sulphuric acid (H SO ), stagnant water in drainage channels of urban commer- 3 2 4 Ammonium hydroxide (NH OH), and deionized water. cial area and urban residential area, run-offs from agri- Acidic solution (A) was prepared using a mixture of con- cultural farmlands, and run-offs from dumpsites aer ft a centrated HNO and H SO acids which was diluted with precipitation event. Similarly, to determine the chemical 3 2 4 deionized water till a concentration of about 0.001  M composition of the prevailing rainfall, 20 rainwater sam- and a pH of 3.5 is attained. Similarly, a basic solution ples were collected during the beginning of the rainy (B) was prepared by diluting concentrated NH OH with season (March-May). Water samples collection was done deionized water till a concentration of about 0.01 M and following the methods outlined in standard methods GEOLOGY, ECOLOGY, AND LANDSCAPES 119 a pH of 10 was attained. Deionized water with pH of than 63 μm) of six soil samples, two representative sam- 7.0 was used as a neutral (N) solution. These solutions ples each from the three different geologic Formations, were carefully labelled and stored prior to its use for the were further subjected to pH test following the method slaking, dispersion and dissolution experiment. described by Hendershot, Lalande, and Duguette (1993), and also to X-ray fluorescence (XRF) analysis to deter - mine their chemical composition. Labouratory tests and experiments Rain and run-off chemical composition Slaking, dispersion and dissolution experiment Chemical compositions of the rain and run-off water Three 250 ml glass beaker were filled up to 200 ml point 2− − samples such as sulphate (SO ), nitrate (NO ), car- with the prepared aqueous solution A, B, and N; each 4 3 bonate (HCO ) and ammonium (NH ) were deter- solution in a separate beaker. The prepared soil sam- 3 4 mined following the methods outlined the in standard ples were then soaked in these solutions; each solution methods for the examination of water and wastewater of containing a replicate of the same soil sample. The sam- the American public health association (APHA, 1998) ples were observed every 0, 10, 30, and 60 min for the unless otherwise stated. pH and Electrical conductivity degree of slaking and dispersion. The degree of slaking (EC) were tested immediately in the field using a hand- was determined similarly to that of Emerson (1967); by held pH-meter and EC-metre, respectively. Total dis- recording the quantity of soil particles (crumbs) that solved solute (TDS) was calculated from the theoretical fail off from the soil aggregate in a given time, and the relationship between TDS and EC (Lloyd & Heathcote, time it took the aggregate to completely crumble. Aer ft 1985) 60 min, the samples were thoroughly stirred for about 2 min using a glass rod. The resultant suspensions were Soil geotechnical and chemical tests then allowed to stand undisturbed for 48 h aer s ft tirring. In order to determine the PSD of the soils, air dried sam- e deg Th ree of dispersion of the soil samples were deter - ple were subjected to sieve analysis using the American mined by noting the following: (i) the time it took the standard of testing materials (ASTM) sieve mesh num- supernatant to settle; hence the suspension becoming bers 5, 10, 18, 40, 60, 100 and 230 stacked on an electri- clear again aer ft stirring, and (ii) the crumb size of the cally controlled sieve shaker and was allowed to vibrate settled soil aer s ft tirring. The clarity of the suspension for about 25–30  min. Particles passing sieve number is determined when a dark round dot of about 1 cm in 40 were used to determine the soils liquid and plastic diameter pasted on the glass beaker can be clearly seen limit; hence its plasticity index following ASTM stand- through the suspension (Figure 4a and b). Aer 48  ft h, ard procedures. The fine particle fractions (particles less the solutions were carefully filtered through a 63  μm sieve into another container, ensuring that larger par- ticles do not pass through. The resultant solutions were then analysed for dissolved cations such as; Calcium (Ca), potassium (K), Sodium (Na), Magnesium (Mg), Aluminium (Al) and Iron (Fe), using atomic absorption spectroscopy (AAS) chemical analytical method. Solute dissolution was determined by summation of Al, Fe, K, Ca, Mg, and Na ions present in the resultant solution. Results Physicochemical properties of rainwater and run- off e Th rainwater samples were acidic with an average pH value of 4.99 (Table 2). The minimum pH of 3.3 was recorded at location 9 which is at the southernmost part of the study area, while the maximum pH of 6.1 was recorded at location 14 in the northernmost part of the study area (Figure 1); noting that the southern part is more urbanized and industrialized than the northern part. The run-off water samples were generally basic with an average pH value of 8.94 (Table 2). The maxi- mum pH of 10.2 was recorded in run-off from an urban dumpsite, while the minimum pH of 7.4 was recorded in Figure 4.  (a) and (b) photographs of the samples in solution run-off from urban residential areas. Generally, the pH 10 min after soaking and 1 h after stirring. 120 C. EMEH AND O. IGWE of run-off water samples from dumpsites, agricultural Geochemical composition of the fine fraction of farms, and stagnant water in urban drainage channels the soil samples were relatively higher than pH of run-offs from urban e pH a Th nd chemical compositions of the fine fraction of residential areas. The EC values of run-off water samples the soils were summarized in Table 4. pH of the soil sam- were very much higher than the EC values of rainwater ples were all acidic with an average value of 5.1. Apart (Table 2). Figure 5 revealed that the EC values of both from silicon oxide (SiO ) which contributes about 51% the rainwater and run-off samples tends to increase away of the total oxide in the soil’s fine fraction, iron oxide from the Neutral pH in both direction, with R-squared (Fe O ) is the next most abundant oxide, contributing an 2 3 (R ) of −0.702 and 0.703, respectively. This trend in average of 36.30%, followed by aluminium oxide (Al O ) 2 3 EC values were also observed in the values of the TDS with an average of 7.15%. The percentages of all other (Figure 6); hence dissolution of solutes appears to be oxides are in insignificant quantity with respect to their dependent on the pH with R  = −0.702 and 0.703, for contribution as a cementing material. This result of the rainwater and run-offs, respectively. chemical composition of the soils is similar with that of 2− − e a Th verage concentration values of SO , NO , 4 3 Ibe and Akaolisa (2010) and Igwe et al. (2009) on the HCO and NH in the rainwater samples were 38.1, 3 4 same study area. However, the percentage of SiO is rel- 19.8, 2.98, and 2.63 ppm, respectively, while their val- atively higher, and that of Fe O is relatively lower in the 2 3 ues in run-off samples were 283, 317, 68, and 735 ppm above author’s results compared with the result of this (Table 2). Generally, the acidic nature of the rain could work. The reason for these variations in SiO and Fe O 2− 2 2 3 be attributed to its relatively high concentration of SO composition could be attributed to the fraction of the and NO which could react with hydrogen ion (H ) in soil sample analysed. While the other authors analysed water to form acids, while the basic nature of the run-o ff the bulk aggregate sample of the soil, this work focused could be attribute to its relatively high concentrations on the fine particle fraction of the soil. Noting that SiO + − of NH which could react with hydroxyl ion (OH ) in is relatively stable to weathering, and will constitute the water to form a base. majority of the coarse grains in the bulk aggregate; hence the observed variation. Geotechnical properties of soil e Th structures of the in situ soils were generally homog- Slaking, dispersion, and dissolution enous and are reddish in colour. They appear cohesive in e deg Th ree of slaking and dispersion as determined from nature under its dry condition and were relatively stiff the experiment were scored from 1 to 5, and were tab- that it cannot be indented by the thumb. The geotech- ulated in Tables 5 and 6. The mean value of about 3.5 nical properties of the soil such as PSD, coefficient of of the slaking score (Table 7) revealed that most of the uniformity (Cu), and consistency limits were summa- samples were moderately-strongly slaked. However, a rized in Table 3. The PSD (Figure 7) revealed that the careful observation revealed that the degree of slaking soils are coarse-grained with coefficient of uniformity was not determined by variations in pH of the solution, ranging from 2.74 to 5.99, with an average value of 3.98; but rather by the plasticity index of the soil samples hence the soils are uniformly graded. The plasticity index (Figure 8), with (R ) value of −0.907. The degrees of slak - (PI) of the soils in the area underlain by Nsukka Fm and ing of soils within the area underlain by Ajali Fm were Nanka sands with an average value of 19.11 indicates severe, while that of the ones within the areas under- that they are moderately plastic, while those underlain lain by Nsukka Fm and Nanka sands shows slight-strong by Ajali Formation are of low plasticity with average PI of degree of slaking. This was because the plasticity of soils 3.59. In general, the soils could be classified as silty sands within Ajali Fm are relatively low compared with the one (SM) of low-medium plasticity according to unified soil within Nsukka Fm and Nanka sands which are moder- classification system (USCS). ately plastic (Table 7). Furthermore, the mean of the dispersion score was plotted against the pH of the dispersing solutions. This Table 2. s ome physicochemical properties of the studied run- reveals that the dispersion of the soil samples were off and rainwater. dependent on the pH of the solutions. Most of the sam- Source ples in neutral solutions (N ) did not show any sign of Runoff Rainwater dispersion, the ones in acidic solution (A ) were mod- Parameter Mean Max Min Mean Max Min erately dispersed, while samples in the basic solutions 2− 282.65 501 19 38.1 63 27 SO (ppm) (B ) were all strongly dispersed (Figure 9). The pattern NO (ppm) 316.7 978 109 19.1 48 7 3 D HCO (ppm) 67.6 167 13 2.98 5 0.8 3 that exists in the soils dispersion was also observed in NH nH 734.5 1567 205 2.63 4 0.9 3 4 the dissolution of the soil samples in aqueous solutions (ppm) pH 8.94 10.2 7.4 4.98 6.1 3.3 of varying pH. Basic solutions (B ) has more dissolved Di ec (μs/cm) 568.8 890 89 82.6 186 21 cations with an average value of 40  ppm, followed by Tds (ppm) 387 605 61 56 126 14 GEOLOGY, ECOLOGY, AND LANDSCAPES 121 the acidic solutions (A ) with an average of 20.31 ppm, Di y = 262.38x- 1775.6 R² = 0.7028 and neutral solution (N ) has the least dissolved cations Di with an average value of 11.59 ppm (Figure 10). It was observed that iron contributes about 60% to the quantity Ecro vs pH of the dissolved cations in the basic solution, while con- Ecrw vs pH 400 tributing just about 11 and 4% in the Acidic and Neutral Linear (Ecrw vs pH) y = -37.503x + 269.18 Linear (Ecrw vs pH) solution, respectively (Figure 11). R² = 0.7019 Discussion 0246 81012 pH e s Th tudied soils are majorly composed of medium-fine- grained sands with about 6% of low-medium plastic Figure 5.  Relationship between ec and pH of run-off and fines. About 43% of the fine particle fractions of these rainwater samples. soils are chiefly composed of Iron and Aluminium oxide (sesquioxides). These oxides have been attributed to be the major cement binding the individual soil particles y = 178.42x-1207.4 600 into larger aggregates (Cornell & Schwertmann, 2003; R² = 0.7028 Peng & Sun, 2015). Valentin (1986b), and Van der Walt and Valentin (1992) also suggested that these oxides are TDSro vs pH y = -37.503x + 269.18 responsible for encrusting of soil surfaces; hence reduc- TDSrw vs pH R² = 0.7019 Linear (TDSrw vs pH) ing infiltration and promoting run-o. H ff owever, the instability of these soils to water erosion has been seen to be dependent not only on the chemical composition 0 246 81012 pH of the soils, but also on the chemical composition of the prevailing aqueous environment. While these soil Figure 6.  Relationship between Tds and pH of run-off and aggregates maybe relatively stable in aqueous solution rainwater samples. of neutral pH – which may not be naturally attainable, Table 3. s ome geotechnical properties of the studied soils. G CS-MS FS FINES Cu Consistency limits Sample location (>2 mm) (<2–0.25 mm) (<0.25–0.063 mm) (<0.063 mm)   LL PL PI 1* 0 37.06 58.51 4.25 3.43 30.4 10.6 19.80 2* 0 36.07 60.20 3.86 3.42 30.00 12.80 17.20 3* 0 40.45 54.66 5.30 3.49 34.6 15.9 18.70 4* 0 38.40 61.30 6.10 3.50 32 15.1 16.90 5* 0 33.24 57.93 4.34 3.38 40 19.9 20.10 6* 0 41.22 59.10 4.26 3.34 41.8 22 19.80 7* 0 36.89 58.56 3.48 3.41 38.3 20.9 17.40 8* 0 39.87 63.89 4.93 3.47 42.3 23.5 18.80 9** 0 32.73 57.94 8.38 5.82 45 21.7 23.30 10** 0 32.90 61.56 7.58 5.20 42.5 26.1 16.40 11** 0 29.23 64.12 9.45 5.99 38.7 17.1 21.60 12** 0 26.95 59.22 9.00 5.98 37.8 18.3 19.50 13** 0 28.22 56.97 6.90 4.80 43.4 24.7 18.70 14** 0 36.23 60.85 7.89 5.20 31.2 13.3 17.90 15** 0 33.14 62.78 7.23 5.46 40 19.7 20.30 16** 0 29.99 60.12 8.84 5.63 36.8 17.4 19.40 17*** 0 29.61 65.08 4.61 3.2 26 22.4 3.60 18*** 0 32.12 72.80 3.78 2.90 32.8 29.2 3.60 19*** 0 27.84 63.39 4.20 3.00 28.2 25.4 2.80 20*** 0 28.34 69.20 4.32 3.11 25.7 21.5 4.20 21*** 0 30.45 66.80 5.23 3.30 29.1 24.3 4.80 22*** 0 30.56 71.12 3.67 2.78 25.4 21.5 3.90 23*** 0 23.98 59.55 3.33 2.74 26.7 24 2.70 24*** 0 31.34 68.20 4.86 2.89 30.6 27.3 3.30 summary statistics Mean 0 32.78 62.24 5.66 3.98 34.55 20.61 13.95 Max 0 41.22 72.8 9.45 5.99 45 29.2 23.3 Min 0 23.98 54.66 3.33 2.74 25.4 10.6 2.7 notes: G = Gravel, cs -Ms = c oarse to medium grain sand, Fs = Fine sand, FInes = silt and clay size particles, c u = c oefficient of uniformity, ll = liquid limit, pl = plastic limit, pI = plasticity index. *nanka Fm.; **nsukka Fm.; ***a jali Fm. TDS (ppm) EC (uS/cm) 122 C. EMEH AND O. IGWE 0.01 0.11 10 Particle size (mm) Figure 7. Mean psd of the soil samples. Table 4. chemical composition of some of the studied soil e r Th esults from this research have shown that the samples. chemical composition of the prevailing run-off is deter - mined by the type and amount of pollutant which are Concentration values (%) present in rainwater, dumpsites, sewages, and agricul- Parameter Mean Max Min tural wastes. Amongst these chemical pollutants are pH* 5.12 5.6 4.8 sio 51.22 66.4 34.8 2 sulphuric acid, nitric acid, and ammonium hydroxide. Tio2 0.44 0.83 0.05 es Th e chemical compounds appear to be the major al o 7.15 20.32 1.39 2 3 Fe o 36.3 50.97 18.86 compounds ae ff cting the chemical composition of the 2 3 Mno 0.03 0.05 0.01 run-off since they are found in appreciable percentage Mgo 0.36 0.5 0.1 ca o 0.5 0.88 0.1 in the run-off samples that was analysed. This result is na o 0.04 0.07 0.02 consistent with the work of other researchers working K2o 0.05 0.07 0.04 p o 0.44 0.97 0.05 on environmental pollution (Hall & Anderson, 1988; 2 5 lo I 3.38 7.07 2.1 Sumner & McLaughlin, 1996; Aluko et al., 2003; Tiwari et al., 2007). Run-off generated from agricultural active areas and those from industrial/commercial areas, espe- they disperse and dissolve in acidic and/or basic aque- cially those sourced from urban dumpsites appears to ous solutions which are typical of run-offs within the have higher concentration of these chemicals, compared study area. with those from residential and economic less active Table 5. laboratory observation results of slaking of the soil samples. Parameters used in determining the degree of slaking Slaking score Degree of slaking s oil aggregate remains intact after 60 min with no crumbs falling off it 1 non Few crumbs fall off from the soil aggregate, but aggregate remains spherical after 30 min 2 slight about 40% of crumbs fall off from the soil aggregate after 10 min, but the aggregate still partly maintained its 3 Moderate spherical shape after 30 min about 75% of crumbs fall off from the soil aggregate after 10 min, forming a conical shape, and soil lump com- 4 s trong pletely collapsed into crumbs after 30 min. 100% of the soil aggregate completely collapsed into crumbs immediately it is been soaked in the solution 5 s evere Table 6. laboratory observation results of dispersion of the soil samples. Parameters used in determining the degree of dispersion Dispersion score Degree of dispersion s olution remains clear after soaking the sample, and reappears clear within 10 min after stirring; crumb 1 non size appears larger 2 mm. s olution remains clear after soaking the sample, but took about 30 min for solution to clear out after 2 slight stirring; crumb size appears less than 2 mm but not powdered. s olution remains clear after soaking the sample, but took about 1 h for solution to clear out after stirring; 3 Moderate crumb size appears less than 2 mm, and about 30% appears powdered. s olution remains clear after soaking the sample, but took about 5hours for solution to clear out after 4 s trong stirring; crumb size appears less than 2 mm and about 50% appears powdered. s olution becomes unclear immediately the sample is soaked in it, and remains unclear after 48 h; crumb 5 s evere size cannot be determined because the solution appears very cloudy (muddy). Percentage finer GEOLOGY, ECOLOGY, AND LANDSCAPES 123 Table 7. experimental results of slaking, dispersion, and dissolution of the soil samples. Slaking (S) Dispersion (D) Dissolution (Di) (ppm) Dissolved iron (Fe) (ppm) Sample location A B N A B N A B N A B N s s s D D D Di Di Di Fe Fe Fe 1* 2 3 2 3 5 1 25.33 41.01 14.45 3.79 25.55 0.73 2* 2 3 3 3 5 2 22.14 51.22 10.40 2.86 32.65 0.64 3* 2 3 2 4 5 1 23.16 43.76 13.20 4.27 28.22 0.78 4* 3 3 3 4 5 1 17.50 54.98 9.34 3.27 34.22 0.67 5* 2 2 2 3 5 1 19.64 39.27 11.10 2.10 22.87 0.23 6* 2 3 2 3 5 2 24.34 51.33 11.45 4.73 29.88 0.94 7* 3 4 3 3 5 1 18.75 58.19 7.54 3.89 37.82 0.12 8* 3 3 2 3 5 1 20.48 44.56 11.20 5.30 31.64 0.72 9** 2 2 2 2 5 1 21.26 26.52 14.89 0.86 8.19 1.35 10** 3 4 3 2 5 1 22.12 29.17 13.88 0.56 10.23 0.86 11** 2 2 2 2 5 1 18.39 24.98 14.22 1.37 9.22 0.03 12** 2 3 2 2 5 1 19.98 24.78 15.90 0.68 8.83 0.02 13** 3 2 2 2 5 1 22.98 27.74 10.89 0.96 11.60 1.30 14** 2 3 2 3 5 1 19.70 23.80 16.30 1.22 8.46 0.07 15** 2 2 3 2 5 1 16.43 28.12 14.60 0.79 8.97 0.78 16** 2 3 2 2 5 1 19.49 26.72 10.56 0.56 12.40 0.14 17*** 5 5 5 3 5 1 22.77 42.20 12.18 1.83 30.81 0.49 18*** 5 5 5 3 5 1 18.56 49.33 17.40 2.05 34.89 0.34 19*** 5 5 5 3 5 1 19.45 33.37 10.63 1.89 26.99 0.16 20*** 4 5 5 3 5 1 18.90 44.57 7.98 2.22 29.42 0.09 21*** 5 5 4 4 5 2 22.67 56.42 4.67 3.40 39.12 0.56 22*** 5 5 4 3 5 1 11.89 42.73 4.46 1.78 27.70 0.48 23*** 5 5 5 3 5 1 17.56 39.12 12.11 2.29 22.96 0.14 24*** 5 5 5 3 5 1 23.89 56.23 8.89 1.90 40.12 0.04 summary statistics sum 76.00 85.00 75.00 68.00 120 27.00 487.38 960.11 278.24 54.56 572.77 11.68 Mean 3.17 3.54 3.13 2.83 5 1.13 20.31 40.00 11.59 2.27 23.87 0.49 Max 5.00 5.00 5.00 4.00 5 2.00 25.33 58.19 17.40 5.30 40.12 1.35 Min 2.00 2.00 2.00 2.00 5 1.00 11.89 23.80 4.46 0.56 8.19 0.02 δ 1.28 1.15 1.24 0.62 0 0.33 2.91 11.24 3.31 1.35 10.87 0.39 notes: a = a cidic solution, B = Basic solution, n = neutral solution, δ = s tandard deviation. *nanka Fm.; **nsukka Fm.; ***a jali Fm. areas – which reflected in the values of their pH, EC, and TDS. e deg Th ree of dispersion and dissolution of soil aggregates and its constituent elements, respectively, are found to be higher in basic aqueous solution of ammo- Ns vs PI nium hydroxide compared with acidic aqueous solu- y = -0.1576x + 5.3216 tion of sulphuric and nitric acid mixture. The reason for R² = 0.9067 this could be as a result of the higher concentration of NH OH which is about 0.01 M in the basic solution that was used compared with 0.001 M of the acid mixture that 05 10 15 20 25 Plasticity index (PI) was used. NH OH of 0.01 M was used because similar concentration was observed in most of the sampled run- Figure 8. Relationship between slaking and plasticity of the soil. o. Th ff e same reason was used to condition the aqueous mixture of H SO and HNO at a concentration level of 2 4 3 0.001 M. Though this experiment was conditioned at a constant hydrogen ion concentration, solubility of metal- lic oxides such as iron oxide have been found to increase 4 with increase in hydrogen ion concentration in aqueous acidic solution (Arlauckas et al., 2004). Thus, increase in the molar concentration of the acidic mixture used may D vs pH subsequently increase the solubility of soil constituent ele- ments; hence leading to aggregate instability. Another rea- son for this observed variation as discussed by Park et al. 0246 81012 (2001) is that hydroxides of Fe and Al in sesquioxide-rich pH soils are readily precipitated out in basic aqueous solution of ammonium compounds, such as ammonium hydrox- Figure 9.  Relationship between dispersion and pH of the solutions. ide and ammonium acetate, while iron and aluminium Slaking Dispersion score 124 C. EMEH AND O. IGWE e deg Th ree of dissolution of other non sesquioxides 40.00 such as NaO , CaO, and MgO appears to be higher in the aqueous acidic solution. However, in the case of the 30.00 studied soil, it may not be a major concern as these metal 20.00 oxides constitutes very little fraction in the amount of Di vs pH cement within the soil aggregates which are predomi- 10.00 nantly sesquioxides. Moreover, the experiment reveals that sesquioxides of iron and aluminium are sparingly 0.00 02468 10 12 soluble in acidic solution of relatively low hydrogen ion pH concentration of about 0.001 M. However, there solubil- ity may increase with increase in the concentration level Figure 10.  Relationship between dissolved cations and pH of of the aqueous solution (Arlauckas et al., 2004). Thus, the solutions. increase in the concentration level of these compounds in the run-off may result to increase in the solubility of metallic oxides in the soil; hence leading to soil aggregate instability. Another important component of run-off which 59.66% 25 could ae ff ct the deflocculation and dispersion of soil aggregate is the humic acid. This acid has been reported by (Frenkel et al., 1992) as a major chemical compound causing dispersion of soil aggregate. Though this com- 5 11.20% 4.20% pound was not analysed for in the run-off samples used ADiBDi NDi for this experiment, its presence in dumpsites leachate, TDC DFe sewage, and composts have been reported by several authors. Figure 11. percentage contribution of iron in the total dissolved cation. Conclusion have been found by (Arlauckas et al., 2004; Lewis, 2007) This work has revealed that the studied soil is mainly to be sparingly soluble in acidic solution of relatively low composed of medium-fine-grained sands with little hydrogen ion concentration of about 0.001 M. amount of low-moderate plastic fines which are chiefly However, dispersion of the soil aggregates appears composed of iron and aluminium oxides that serve as to be more closely associated with soils susceptibility cement in the soil matrix. to erosion than dissolution of its constituent minerals. es Th e sesquioxide cements disperses and dissolves in e Th degree of dispersion which was determined by the basic aqueous solution of ammonium hydroxide and in cloudiness of the solution and the settling time of the acidic aqueous solution of sulphuric and nitric acid mix- particles is a result of the reaction between ammonium ture. However, the degree of dissolution and dispersion hydroxide and the sesquioxides in the soil aggregate – of iron oxide is more compared to that of aluminium especially iron oxides which contributed more in the oxide. total amount of dissolved solute. Following the work of e deg Th ree of dispersion and dissolution of soil aggre - Park et al. (2001) and Abdalla et al. (2011), the mecha- gate and its constituent element, respectively, is more in nism for this reaction could be stated as follows; basic aqueous solution of ammonium hydroxide than in acidic aqueous solution of sulphuric and nitric acid 3+ + Fe aq + 3NH aq + 3H O ←→ Fe(OH) (s) + 3NH aq 3 2 3 4 mixture. Precipitate (Iron bearing mineral) (Dumpsite runoff) ( ) Ammonium hydroxide, Sulphuric acid, and Nitric (1) acid are found in appreciable quantity in the prevailing e in Th soluble brick red precipitate forms a colloidal sus - rainwater and run-offs. However, the concentration level pension (Park et al., 2001), which are then transported of this chemical compounds are higher in run-offs that away as suspended solids by run-o, le ff aving behind were generated from urban dumpsites and agricultural disaggregated soil particles with little or no cements. farmlands. Since this sesquioxides are considered as flocculating er Th efore, in order to prevent the susceptibility of iron agent binding individual soil grains into bulk aggregates oxide-cemented soils to water erosion, it is imperative (Townsend & Reeds, 1971), there removal in the soil to determine the sources and chemical composition of matrix will lead to the reduction in the cohesion of the the prevailing run-offs. This will help in proper run- soil. Lack of cohesion in soils has been associated with off treatment and drainage system control to avoid the increase in its instability, thus resulting in erosion and contact of chemically erosive run-offs with iron-oxide- land sliding (Igwe & Fukuoka, 2015). cemented soils. Also it is important that urban dumpsites Dissolved cations (ppm) GEOLOGY, ECOLOGY, AND LANDSCAPES 125 Frenkel, H., Levy, G. J., & Fey, M. V. (1992). Clay dispersion are properly designed, and indiscriminate disposal of and hydraulic conductivity of clay-sand mixtures as both liquid and solid wastes should be highly discour- ae ff cted by the addition of various anions. Clays and Clay aged in order to avoid direct contact with rainfall, which Minerals, 40, 515–521. could generate chemically modified erosive run-offs. Hall, K.J., & Anderson, B. (1988). The toxicity and chemical composition of urban stormwater runo. ff Canadian Journal of Civil Engineering, 15, 98–106. 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Journal

Geology Ecology and Landscapes – Taylor & Francis

Published: Apr 3, 2018

Keywords: Erosion; sesquioxide; environmental pollution; pH; dispersion; urban run-off

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