Evaluation of groundwater resource potential of Pallisa district in eastern Uganda
This study aimed at assessing the ground water resources potential in Pallisa district, which is found in the eastern part of Uganda. The aims of the investigation can be Iisted as follows: ~ To determine the reliability and applicability ofVES in Pallisa district; ~ To evaluate the potential for sustainable harvesting of groundwater resource; ~ To determine the hydrogeological properties of the aquifer and make recommendations; ~ To suggest improved management strategies of water resource in Pallisa district in order to provide basic water services to the rural population and; ~ To propose a monitoring programme in the area that will include boreholes to determine the variation of water quality and quantity with time. The majority of the population of Pallisa district mainly uses groundwater as a source of supply for potable water. Groundwater is increasingly on demand in Pallisa district due to: (i) Population growth, (ii) Modem agricultural practices (iii) Livestock demands. Though springs and shallow wells are a source of groundwater in Pallisa, many of them have not been developed. There is need to have many of them developed to provide clean water and increase in the number of deep wells also in case of consistent droughts. The geological environment covered by this study consists of a Gneissic Complex Formation or Gneiss and granitic formations of the Pre-Cambrian origins. The study shows that Gneiss complex forms the principal source of ground water supplies while fresh metamorphic rocks are impermeable. From the hydrogeological investigations, it is clear that among the three water bearing zones (the upper, the middle and the lower), the metamorphic formations are widely weathered as you go deeper the earth, meaning that the lower zone of fresh geological granitic material are of a fractured bedrock. The study further revealed that the fracture zones have higher hydraulic conductivity with low storage coefficients and are generally confined. The hydrogeological investigation shows that groundwater in Pallisa district lies right below 30 - 70 m from the surface topography. This was shown by the hydrogeological profiles constructed through the underlying aquifer of the wells around Pallisa town. Higher transmissivity values were observed in the crystalline rock aquifer which related to the western and north - western of Pallisa district, with an average of 13.5 m2/d, due to the presence of open waters of Lake Kyoga, Mpologoma and Dodoi rivers. Such transmissivity values typically corresponded to drawdowns in order of a few meters at pumping yields of about 1 m3/hr. The recharge rate of 110 mm/a, taken as uniform throughout Pallisa district, had the total amount of groundwater recharged being l.956*109*0.11 m3/a = 215.2 million m3/a. The amount of water demanded by both the rural and urban is 35.7-million m3/a, which represents 17% of the groundwater currently used. The rest 83% of groundwater is unutilised. The hydro-chemical characterisation of waters in the study area show that the groundwater from the bedrock aquifers of Pallisa district represent regimes where there are predominantly calcium enrichment, which is typical of lime dosing to neutralize acid waters and both the regolith and bedrock groundwaters are dominated by the carbonate ions. This diversity of bedrock types is as a result of weathering of the underlying material. However, groundwater from many of the boreholes in Pallisa district is generally acceptable for human and livestock consumption. Stable isotopes of oxygen and hydrogen has been extensively used to investigate the rainfall and seasonal patterns of deuterium and 180 content of rainfall in the study area i.e. Pallisa district. This was inferred from historical observations at Entebbe, approximately 180 km to the south. The average monthly rainfall and 018 clearly demonstrates a relationship between the amount of rainfall and depletion in 180. As. part of the present study, monitoring of the groundwater resource in terms of yield aquifers with respect to drawdowns and water quality, consumption at water using production boreholes and management of these boreholes play important roles that can never be underestimated as per the recommendations made.