GROUNDWATER FLOW AND QUALITY OF COASTAL AQUIFERS: CASE STUDY OF MOMBASA NORTH COAST, KENYA

Abstract

Groundwater is the most readily available source of freshwater in the hydrologic cycle, hence the importance of aquifers. However, these groundwater resources are prone to pollution in the wake of anthropogenic activities, over-exploitation and climate change related activities. Coastal aquifers are faced with additional unique problems of seawater intrusion which may be aggravated by climate change related challenges like sea level rise and coastal flooding. The aquifer of Mombasa, a coastal city, home to East Africa’s busiest seaport is not an exception to these challenges. This study investigated the groundwater flow, quality and vulnerability to seawater intrusion of the coastal aquifer of Mombasa North Coast. This is a 74.2 km2 region bounded by the Indian Ocean on the East, by creeks on the North and South and high elevated hills on the West. The hydrogeological characteristics, salinity and extent of seawater intrusion in the study area were assessed using statistical and geospatial methods. The statistical methods include the use of correlation coefficients, cross plots and piper plots. GALDIT overlay index was used to assess the vulnerability of the study area with the aid of ArcGIS while groundwater flow and solute transport were simulated with the aid of MODFLOW, MT3D and SEAWAT packages. In addition to secondary data obtained, three phases of field data such as static water levels and water quality parameters from boreholes/shallow wells were collected at pre-monsoon, the peak of rainy season, and post-monsoon in 2016. This was followed by laboratory analysis for Na, K, Mg, Ca, Cl, HCO3 and SO4 concentrations in the water samples obtained from the field. The results show that the study area is characterised by shallow unconfined aquifer with groundwater heads ranging from -1 to 33m above mean sea level irrespective of the seasons. The EC and TDS values were observed to have near perfect correlations with each other and generally high, as over 94% of the water samples exceeded WHO drinking water limit of 750 μS/cm and 500mg/l respectively. The pH of the groundwater was slightly alkaline but could be slightly acidic in the rainy season. Over 90% of the water samples had pH values within 6.5 and 8.5 the acceptable limit of WHO guidelines. The groundwater is generally marine in nature while there is a wide variation of the EC, TDS, NaCl, Na, K and Cl values within the aquifer. Over 50% of the water samples also had Na concentrations exceeding the WHO and Kenya drinking water limits of 200mg/l. Hence the groundwater is generally unfit for direct drinking. The salinity of the groundwater varies with seasons and groundwater recharge heavily influences the salinity of the groundwater.The aquifer is largely experiencing a moderate impact of seawater intrusion depending on the season with only xiv a few regions experiencing injurious impacts. This might be because of the current groundwater abstraction rates which are easily compensated by high annual rainfall above 1000mm. Over 50% of aquifer’s coverage experiences moderate vulnerability to seawater intrusion, however, the aquifer is more vulnerable in the dry season than the wet season. The direction of groundwater flow is predominantly towards the north-eastern and southern part of the study area. Finally, solute transport simulation shows that concentrations of NaCl in the groundwater slightly reduced from the centre of the study area outwards from June (peak of rainy season) to September (post-monsoon). In view of the findings made, management and modification of the pumping scheme are recommended. This includes encouraging the digging of wells and boreholes to be as shallow as possible. The recommendations will help sustain the current quality of the groundwater and prevent further pollution of the groundwater.