Assessment and Mapping of Groundwater Quality in the Thiririka Sub Catchment Kiambu County, Kenya
Groundwater is an important natural resource used for drinking by many people globally, especially in rural and peri-urban areas, but this resource cannot be sustainably used unless its quality is ascertained. Urbanization and the increasing population growth in the Thiririka Sub Catchment has put increasing pressure on natural resources and social services such as housing, water supply and sanitation, education and health facilities in the Sub Catchment. This increased in demand, coupled with the inadequate or lack of supply of potable water and sanitary services has consequently made the inhabitants to increasingly rely on groundwater as their sole or supplementary source of water, however, groundwater in the Sub Catchment is under threat of contamination due to the utilization of on-site sanitation systems, dominated by pit latrines and agricultural pollution originating mainly from run off water after rains, carrying fertilizers, pesticides, herbicide and faecal matter, hence this study was conducted in the Thiririka Sub Catchment, Kiambu County, Kenya, to determine the safety of groundwater sources and to examine the factors influencing groundwater quality. The study sought to assess the groundwater quality and to map its spatial distribution in terms of suitability for domestic purposes. Groundwater samples from 19 boreholes and 17 shallow wells were sampled from during the months of April to June 2013 and analysed for selected physico - chemical and microbial parameters. Standard methods were used for the analysis of groundwater samples in the laboratory. The results obtained were compared to guideline values of the NEMA, USEPA and WHO to establish the potability of groundwater. The the concentration of nitrates and turbidity in some of the samples exceeded the prescribed NEMA, USEPA and WHO standards of 10 mg/L and 5 NTU respectively. The concentration of iron and maganese also exceeded the established standards of 0.3 mg/L and 0.4 mg/L while 15 of the sampled boreholes and all of the 17 sampled shallow wells had faecal coliform contamination and did not conform to the NEMA, USEPA and WHO guideline value of 0 cfu/100 ml. The student‟s t-test performed at 95% confidence interval showed a significant difference between the means for the sampled boreholes and shallow wells for the following parameters; nitrates, faecal coliform count and turbidity. The Inverse distance weighting methodology in the 3D Analyst module of ArcGIS 9.3 was used to generate a thematic map for each of the tested parameters. It was observed from the calculated Water Quality Index (WQI) that, 21 of the samples were in the 100 - 200 range indicating poor quality, 11 in the 200 - 300 range, indicating very poor quality and 4 samples in the above 300 range indicating unsuitable for drinking purposes. This was due to the high concentration iron, zinc and manganese present in groundwater samples in the Sub Catchment. The close proximity of cattle kraals, pit laterines and domestic sewage effluents were found to be the factors that influence and degrade the quality of groundwater. The sanitary survey also revealed that the risk to pollution for the boreholes ranged from low to medium but all the shallow wells were in the high risk category due to the proximity of cattle kraals, pit latrines and domestic waste dumps to these wells. Groundwater in the Thirirka Sub Catchment should therefore be treated before used. The construction of wells with cement blocks with an apron and a well drainage system around these wells should be encouraged. Inhabitants of the Sub Catchment should be informed by officials of NEMA and WRMA about the status of groundwater in the Sub Catchment.