An assessment of groundwater potential and vulnerability in The Upper Manyame Sub-Catchment of Zimbabwe.

Abstract

Groundwater plays a pivotal role in meeting potable water needs. However, significant stresses generated from anthropogenic activities have affected its safe use. In-order to enable appropriate risk communication and informed decision making on groundwater use and management, full knowledge of its quality, quantity and vulnerability is fundamental. The main objective of this study was to assess Groundwater Potential (GWP) and vulnerability in the Upper Manyame Sub-catchment (UMSC). GWP was mapped through the use of Geographic Information Systems and remote sensing-based multi-criteria analysis. Spatial thematic layers; viz., geology, slope, land-use, drainage density, recharge, topographic index, altitude and rainfall were developed and weighted using Saaty’s Analytical Hierarchy Process (AHP) and the Quantile Method. Layers were subsequently aggregated using the Weighted Linear Combination and Index Overlay methods, respectively, to develop two Groundwater Potential Index (GWPI) maps. Indices from each map were correlated with borehole yield data to select the best method from which GWP zones were then developed. Given the widespread use of groundwater for domestic purposes in the study area, its quality was also analysed. Groundwater samples from 15 sampling sites were analysed for selected physico-chemical and biological parameters as recommended by the World Health Organization. Results were then subjected to descriptive statistical analysis and Principal Component Analysis (to identify key parameters). Repeated Measures ANOVA (RMA) was used to analyse if there were any significant variations in mean groundwater parameter levels. Groundwater vulnerability was determined using the GOD Model with key input parameters being groundwater occurrence, overlaying lithology and depth to water table. The results for GWP mapping showed that AHP-based GWP index map exhibited a stronger correlation with borehole yield data (r=0.65, n=120), indicating the robustness of the AHP as a factor rating method. About 72 % (2725.9 km2) of the UMSC was noted to be of moderate GWP (10-100m3/day), while 19 % (719.3 km2) and 9 % (340.7 km2) exhibited high and low GWP, respectively. Groundwater quality results indicated that: pH, coliforms, TDS, EC, total hardness, Fe, NH4+ and turbidity exceed SAZ/WHO drinking water limits in most cases. However, Fl-, Zn, Pb, Cu and Cl- were within acceptable limits. Four Principal Components (PCs) representing 84 % of the cumulative variance were extracted. PC1 was characterized by high dominance of pH, TDS, EC and total hardness. PC2's variance was found to be associated with elevated levels of Cl-, Zn and Cu. On the other hand, PC3 had high loadings of total and faecal coliforms, Fl-.and turbidity. PC4 was characterized by high loadings of Pb, Fe, ammonia and turbidity. Overall, PCA showed that most of the variation in the water quality was accounted for by pH, Zn, Cl-, TDS, ammonia, Fl-, Cu, turbidity, Fe, Pb and faecal. The results of RMA indicated that there were significant differences in mean parameter levels across sampling sites and within subsequent campaigns (p<0.05), for parameters DO, total coliforms and faecal coliforms. The study area was found to be largely of moderate groundwater vulnerability (77.3 % of area). A moderate correlation of 0.47 was exhibited between the measured ammonia levels and groundwater vulnerability indices. The correlation indicate instances of ammonia contamination, hence it can be concluded that groundwater in the UMSC is being polluted by anthropogenic activities. Regular monitoring is therefore recommended to safeguard public health and prevent further deterioration of groundwater.