Hydrogeochemical evolution, quality appraisal and risk assessment of shallow groundwater aquifers within the Atankwidi Basin of Ghana

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September, 2019
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Groundwater in the last five decades has become the primary resource of freshwater for almost all arid and semi-arid regions as well as most developing countries. Within the semi-arid Atankwidi basin of Ghana, groundwater from shallow aquifers has been the main source of potable water for the past two decades. The unreliablity of surface water resources for irrigation had resulted in the identification of some shallow groundwaters as a viable alternative in terms of quantity to support upscaling of irrigation farming. To ensure sustainable utilization of groundwater to meet domestic and irrigation demands within the area, this study seeks to provide comprehensive appraisal on the hydrogeochemical evolution, quality for potability and irrigation, risks to human health and potential contamination of shallow groundwater aquifers within the Atankwidi basin of Ghana. Hydrochemical models and a statistical approach- principal component analysis (PCA) were employed to identify possible sources and processes controlling groundwater. Water quality index was utilized to evaluate the overall potability whilst chlorine index, salinity index, permeability index, sodium aborption ration, residual sodium bicarbonate, percent sodium and magnesium hardness were evaluated to assess the groundwater suitability for irrigation. Hazard quotients (HQ), hazard index (HI) and cancer risk (CR) of heavy metals were estimated to assess carcinogenic and non-carcinogenic risk to human health. DRASTIC index combined with ArcGIS tools were utilized to assess the risk of shallow aquifers to contamination. All parameters fell within acceptable limits for drinking water except in 15%, 19%, 19%, 35%, 15% and 46% of groundwater where fluoride (F), conductivity (EC), total hardness, lead, arsenic and zinc respectively, exceeded their limits for potability. Groundwater facies were Ca-Na-Mg-HCO3, Na-Ca-Mg-HCO3, Na-Ca-HCO3 and Ca-Na-HCO3. The major source of chemical evolution in shallow GW within the Atankwidi basin could be water-rock interaction whilst the mechanism (chemical process) of chemical mobilisation could mainly be from the weathering of silicate minerals with acid as the prime agent, resulting in the release of major ions (Na, Ca, K, and HCO3) with minor contribution from cationic exchange reaction resukting in the consumption of Na at favourable sites (clay surfaces). PCA revealed Ca and HCO3 originating from a possible common source (anorthite) appear to control the general salinity of groundwater. K and F originated from the microcline and hornblende contained in the Bongo granitoids. About 97% of groundwater had good or better drinking water quality (WQI< 100) whilst 3% had poor water quality (WQI = 100-200). HIing-Pb for adults and children were 1.136 and 4.4407 respectively, indicating the existence of potential risk to non-carcinogenic effects. Estimated CR of Pb and As for adults and children were all greater than 1E-06 (i.e. CRadults-Pb=3.4E-05; CRadults-As = 9.3E-05; CRchildren-Pb = 1.3E04; CRchildren-Pb=4.6E-04). These imply that both adults and children were potentially at risk to carcinogenic effects due to Pb and As. Risks of exposures in children were thrice that of adults. Groundwater was generally suitable for irrigation, especially for moderate salt-tolerant crops. Analysis of the DRASTIC model showed that on the average, about 18 %, 49 % and 33 % areas within the basin had, respectively, low, moderate and high risks to contamination. The spatial distribution of lead, arsenic, zinc and iron, which are common traces in agro-chemicals, revealed that elevated levels of heavy metals were found within the high-risk areas and viceversa. Groundwater is generally suitable for domestic and irrigational use in terms of quality. However, the presence of elevated levels of harmful trace metals in moderate to high vulnerable areas in the area when farming is at subsistent levels requires that pragmatic policies be put in place to minimize potential contamination and preserve the groundwater quality during the upscaling of irrigational farming.
A thesis submitted to the Department of Civil Engineering, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfillment of the requirements for the award degree of Doctor of Philosophy in Water Resources Engineering.
Hydrogeochemica, Groundwater aquifers, Atankwidi Basin, Ghana, DRASTIC model, Drinking water quality