Evaluation of groundwater resource potential of Pallisa district in eastern Uganda

[摘要] This study aimed at assessing the ground water resources potential in Pallisa district, whichis 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 makerecommendations;~ To suggest improved management strategies of water resource in Pallisa district inorder to provide basic water services to the rural population and;~ To propose a monitoring programme in the area that will include boreholes todetermine the variation of water quality and quantity with time.The majority of the population of Pallisa district mainly uses groundwater as a source ofsupply for potable water. Groundwater is increasingly on demand in Pallisa district due to:(i) Population growth, (ii) Modem agricultural practices (iii) Livestock demands. Thoughsprings and shallow wells are a source of groundwater in Pallisa, many of them have notbeen developed. There is need to have many of them developed to provide clean water andincrease in the number of deep wells also in case of consistent droughts.The geological environment covered by this study consists of a Gneissic ComplexFormation or Gneiss and granitic formations of the Pre-Cambrian origins. The study showsthat Gneiss complex forms the principal source of ground water supplies while freshmetamorphic rocks are impermeable.From the hydrogeological investigations, it is clear that among the three water bearingzones (the upper, the middle and the lower), the metamorphic formations are widelyweathered as you go deeper the earth, meaning that the lower zone of fresh geologicalgranitic material are of a fractured bedrock. The study further revealed that the fracturezones have higher hydraulic conductivity with low storage coefficients and are generallyconfined. The hydrogeological investigation shows that groundwater in Pallisa district lies rightbelow 30 - 70 m from the surface topography. This was shown by the hydrogeologicalprofiles constructed through the underlying aquifer of the wells around Pallisa town.Higher transmissivity values were observed in the crystalline rock aquifer which related tothe western and north - western of Pallisa district, with an average of 13.5 m2/d, due to thepresence of open waters of Lake Kyoga, Mpologoma and Dodoi rivers. Such transmissivityvalues typically corresponded to drawdowns in order of a few meters at pumping yields ofabout 1 m3/hr.The recharge rate of 110 mm/a, taken as uniform throughout Pallisa district, had the totalamount of groundwater recharged being l.956*109*0.11 m3/a = 215.2 million m3/a. Theamount of water demanded by both the rural and urban is 35.7-million m3/a, whichrepresents 17% of the groundwater currently used. The rest 83% of groundwater isunutilised.The hydro-chemical characterisation of waters in the study area show that the groundwaterfrom the bedrock aquifers of Pallisa district represent regimes where there arepredominantly calcium enrichment, which is typical of lime dosing to neutralize acidwaters and both the regolith and bedrock groundwaters are dominated by the carbonateions. 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 generallyacceptable for human and livestock consumption.Stable isotopes of oxygen and hydrogen has been extensively used to investigate therainfall 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, approximately180 km to the south. The average monthly rainfall and 018 clearly demonstrates arelationship between the amount of rainfall and depletion in 180. As. part of the present study, monitoring of the groundwater resource in terms of yieldaquifers with respect to drawdowns and water quality, consumption at water usingproduction boreholes and management of these boreholes play important roles that cannever be underestimated as per the recommendations made.

[发布机构] University of the Free State