Hydrogeochemical characterization of a shallow groundwater system in the weathered basement aquifer of Ilesha area, southwestern Nigeria

Hydrogeochemical characterization of a weathered basement aquifer in Ilesha area, southwest Nigeria, was carried out with respect to geogenic and anthropogenic influences on the shallow groundwater system. Physico-chemical parameters revealed a pH of 6.4–8.4 and EC of 22–825 μs/cm in the urban areas compared to a pH of 7.3–10.5 and relatively higher EC of 126–1027 μs/cm in the peri-urban area. The concentrations of major cations (Ca, Na, K, Mg) in the urban areas revealed relatively lower average concentrations of 28.4, 16.7, 8.4 and 5.0 mg/L, respectively, compared to 82.5, 33.4, 19.3 and 12.4 mg/L, respectively, for the peri-urban areas. The low concentrations of major cations in the urban areas can be attributed to low mineral dissolution of quartzite and muscovite quartz-schist bedrocks compared to the weathered granitic, amphibolite and biotite schist in the peri-urban areas. Hydrochemical characterization revealed two main water types; namely Ca-Mg-(Na)-HCO3 mostly in the urban areas suggesting CO2-charged infiltrating recharge rainwater, and Ca-Na-(K)-SO4-Cl type in the peri-urban areas as products of water–rock interactions.


INTRODUCTION
There is no doubt as to the fact that shallow groundwater plays an important role in domestic, agricultural and industrial water supplies in developing countries like Nigeria.This is consequent to the fact that such shallow groundwater systems, especially in the crystalline basement terrains, can be relatively easily tapped with minimal cost, and are a timely assured source compared to surface water development, especially in rural communities (Rao 2006, Fashae et al. 2013).
However, the quality of such groundwater systems is increasingly being endangered by urbanization and industrial activities, along with the emerging threat of impacts of climatic changes.This can be partly attributed to uncontrolled population growth, poor urban planning and associated poor sanitary infrastructure, as found in many developing nations, like Nigeria.In addition, due to the physico-chemical and biochemical interaction between groundwater and anthropogenic contaminants along geogenic water-rock interactions, there is the need for proper understanding of the nature of both anthropogenic and geogenic drivers of groundwater quality, especially under a shallow weathered basement aquifer setting.
Consequently, this study focuses on the hydrogeochemical characterization of shallow groundwater in the weathered basement aquifer of Ilesha Area, south-western, Nigeria, with reference to impacts of urbanization and geogenic water-rock interactions.In addition, the erratic nature of groundwater occurrence in crystalline rocks of the study area also warrants proper understanding of the hydrogeochemical characterization of the associated shallow groundwater system.

LOCATION AND HYDROGEOLOGIC SETTING OF THE STUDY AREA
The study area encompasses Ilesha township and its environs between latitudes 7°25′ and 7°45′N and longitudes 4°35′ and 4°5′E, respectively, and covers approximately 900 km 2 (Fig. 1).The study area, situated in southwestern Nigeria included Ilesha town as the urban / commercial centre and the adjoining villages and smaller settlements, as peri-urban areas.The area falls in the tropical rainforest zone, with distinct wet and dry climatic seasons characterized by mean annual rainfall of 1000-1500 mm, mean annual temperature of 26-31ºC and humidity of 75-95%.
Geologically, the study area lies within the so-called Ilesha Schist Belts Complex of southwest Nigeria, characterized by gneiss-migmatite complex, the metasedimentary assemblages, the amphibolite complex and the granitic rocks units (Elueze 1982).The rock units in Ilesha urban centre include muscovite schist and quartzite schist, while the peri-urban areas are characterized by granitegneiss, hornblende gneiss and biotite-schist (Fig. 1).Hydrogeologically, the groundwater occurrences are in localized weathered regolith aquifers, which are generally discontinuous and essentially under phreatic unconfined to semi-confined conditions, as in the case of most crystalline bedrock settings in Nigeria (Tijani 1994).

METHODOLOGY
A total of 21 representative water samples were collected from shallow hand-dug wells following standard procedures; including 10 samples from the urban area of Ilesha and 11 samples from the surrounding peri-urban areas.The sampling was done across the different rock types of the study area.In addition to the well inventories (e.g.well depth, water level, etc.), sensitive physicochemical parameters such as electrical conductivity (EC), pH, temperature and total dissolves solids (TDS) were measured in situ in the field.
Laboratory analyses include measurements of cations using inductive couple plasma mass spectrometers (ICP/MS), while anions were measured using titrimetric methods.Subsequent data evaluation involved quality assessment with respect to drinking water standards, as well as statistical evaluation involving correlation and hydrochemical characterization using Piper (1944) trilinear plots and Schoeller (1962) diagrams.

Field measurements and hydrochemical analyses
The inventory of the study shallow-dug wells revealed a depth range of 3.6-13.8m (average 9.0 m) and water level depth of 1.4-11.3m characterized by small water columns typical of dug-wells in basement terrain.The field in situ measurements revealed EC of 22-825 µs/cm (average 212 µs/cm) and pH 6.4-8.4 (average 7.3) suggesting a neutral to slight alkaline waters for the Ilesha urban areas compared to moderately high EC of 126-1027 µs/cm (average 504 µs/cm) and pH of 7.3-10.5 (average 8.2) for the peri-urban areas suggesting moderately alkaline water.
Major cations concentrations are in the order of Ca>Na>K>Mg with average values of 28.4,16.7, 8.4 and 5.0 mg/L, respectively, for the urban areas and 82.5, 33.0, 19.3 and 12.4 mg/L, respectively, for the peri-urban areas (Table 1).Bicarbonate and chloride are the dominant anions with average concentrations of 33.6 and 18.9 mg/L, respectively, for the urban areas and 60.0 and 39.0 mg/L, respectively, for the peri-urban areas, while other anions like NO3 and SO4 are generally less than 2 mg/L in both settings.
The concentrations of trace metals (Cd, Co, Cr, Cu, Ni and Pb) are generally low, with values of less than 10 µg/L in both urban and peri-urban areas.However, with the exception of Zn, the trace metal profiles exhibit slight enrichment with average values of 1.5-8.5 µg/L for the water samples from the urban setting, suggesting anthropogenic impacts, compared to average values of 0.6-4.5 µg/L for the peri-urban areas.
Further evaluation of the results revealed lower concentrations of the cations and anions in the urban areas with corresponding TDS of 16.5-619 mg/L compared to the relatively higher TDS of 94.5-770 mg/L for the peri-urban areas.These can be partly attributed to the slightly alkaline to moderately alkaline pH of groundwater from the peri-urban areas while the lower mineralization of the major ions in the urban areas can be attributed to low mineral dissolution of quartzite and muscovite quartz-schist bedrocks to weathering compared to the weathered granitic, amphibolite and biotite schist units in the peri-urban areas.
Furthermore, a plot of the major ions against the measured electrical conductivity (EC) also confirmed a relatively lower mineralization of the water from the urban areas compared to those of the peri-urban while the linear relationship as shown in Fig. 2 is a clear indication of the contribution of these ions to the overall total dissolved solids of the groundwater system.

Quality and usability assessments
The concentrations of the analysed major ions are within the recommended permissible level of both WHO and SON standards for drinking water.For the trace elements, with the exception of Zn, all the analysed trace metals are within the permissible levels of WHO and SON.The overall implication is that the shallow groundwater system in the study area can be said to be chemically potable and suitable for domestic and household uses.However, the microbiological quality cannot be said to be totally satisfactory, due to the observed proximity of some dug-wells to pit latrines and septic tanks in some households, especially in the urban areas.
In terms of agricultural usage, the estimated sodium absorption ratio (SAR) of 0.31-2.1 meq/L falls within the range of 0-10 meq/L recommended for irrigation on most agricultural soils (USDA 1954, Sawyer andMcCarthy 1967).In addition, the salinity hazard as well as the electrical conductivity (EC) and total dissolved solids (TDS) of the analysed groundwater satisfied the criteria for good irrigation water.

Groundwater characterization
The concept of hydrochemical facies was developed in order to understand and identify the hydrochemical evolution of water types using dominant ions in groundwater system (Freeze andCherry 1979, Domenico andSchwartz 1990).Using Piper and Schoeller plots (Figs 3 and 4) shallow groundwater in the study area revealed two main water types; namely Ca-Mg-(Na)-HCO3 type mostly in the urban areas and Ca-Na-(K)-SO4-Cl type with subordinate Na-HCO3 type in the periurban areas.
Ca-Mg-(Na)-HCO3 water type is a reflection of CO2-charged infiltrating recharge rainwater characterized by low mineralization due to limited migratory history, while Ca-Na-(K)-SO4-Cl and Na-HCO3 water types are products of water-rock interactions and cation-exchange process within the weathered bedrock units of the peri-urban, which are characterized by varied weathered bedrock units.This agrees with other previous studies (Tijani 1994, Tijani and Abimbola 2003, Elueze et al. 2004) and it is believed to be common of water types in the basement aquifer of southwestern Nigeria.

SUMMARY AND CONCLUSIONS
The concentrations of major ions and trace metals in the shallow groundwater system of Ilesha area, southwest Nigeria were evaluated and discussed with respect to anthropogenic and geogenic impacts in a typical weathered crystalline basement terrain.The physico-chemical parameters revealed slight to moderate alkaline waters in the peri-urban area, unlike the neutral to slight alkaline waters of the urban areas.Generally, the EC and TDS in the peri-urban areas are moderately higher compared to the urban areas, a situation attributed to the low weathering potential of the muscovite schist and quartzite schist in the urban areas of Ilesha compared to the relatively weathered biotite schist, amphibolite and granites that characterize the peri-urban areas.
Major cation concentrations are in the order of Ca>Na>K>Mg with average values of 28.4,16.7, 8.4 and 5.0 mg/L, respectively, for the urban areas and 82.5, 33.0, 19.3 and 12.4 mg/L, respectively, for the peri-urban areas.Bicarbonate and chloride are the dominant anions with average concentrations of 33.6 and 18.9 mg/L, respectively, for the urban areas and 60.0 and 39.0 mg/L, respectively, for the peri-urban areas.Trace metals (Cd, Co, Cr, Cu, Ni and Pb) are generally low, with values of less than 10 µg/L, with the exception of Zn, which can be attributed to zinc-coated roofs.Nonetheless, slight enrichment with average values of 1.5-8.5 µg/L for the water samples from the urban setting is an indication of anthropogenic impacts, compared to average values of 0.6-4.5 µg/L for the peri-urban areas.In terms of usability, the analysed major ions and trace metals are within the recommended permissible level of both WHO and SON standards for drinking water, suggesting a chemically potable shallow groundwater system suitable for domestic and household uses.In addition, the estimated sodium absorption ratio (SAR) of 0.31-2.1 meq/L falls within the range of 0-10 meq/L recommended for irrigation on most agricultural soils.Nonetheless, a follow-up microbiological analysis is recommended to assess possible contamination of the dug-wells that are in close proximity of pit latrines and septic tanks in some households, as observed during the field sampling operation.
Water characterization revealed geogenic controls on the water chemistry with Ca-Mg-(Na)-HCO3 water type in urban areas suggesting CO2-charged infiltrating rain water characterized by low mineralization and Ca-Na-(K)-SO4-Cl and Na-HCO3 water types as products of water-rock interactions and cation-exchange process within the weathered bedrock units of the peri-urban, which are characterized by varied weathered bedrock units.

Fig. 1
Fig. 1 Geological map of the Ilesha area showing sampling points.

Fig. 2
Fig. 2 Cross plots of the major ions against electrical conductivity measurements.

Fig. 3
Fig. 3 Piper plots of water samples from the study area.

Fig. 4
Fig. 4 Schoeller diagram for samples from urban (a) and peri-urban areas (b).

Table 1
Summary of well inventory and hydrochemical analyses results of water samples from the study area.