Developing characteristic regional groundwater yield curves in different geological settings of Pader, Agago and Kitgum districts in northern Uganda
The main objective of this study was to develop characteristic regional groundwater yield curves in different geological settings of Pader, Agago and Kitgum Districts in Northern Uganda to act as references and guides during ground water prospecting. The specific research objectives were to: (i) identify the Vertical Electrical Soundings (VESes) for forty selected wells(Deep boreholes) drilled in Agago, Pader and Kitgum Districts and characterise the wells yields ranges, (ii)analyse geological and hydrogeological conditions for ground water exploration in the Districts and (iii) develop the characteristic regional groundwater yield curves that will be referred to during ground water investigations in the Districts. The identified VESes for forty selected wells drilled in Agago (08), Pader (04) and Kitgum (28) Districts under Orom Water Supply project respectively were used in this study. Resistivity curves (VES) used for each of the forty selected well sites was based on availability of supervised drilling data (lithological logs and Test pumping data) from the onsite supervisor and drilling contractor. The Total Depth of boreholes drilled in the study area was found to be highest with a mean of (130.6±3.0) m in Agago District, followed by (126.9±1.3) m in Kitgum district and lowest in Pader District with (121.0±0.5) m. On the other hand, mean water strike depth was found to be deepest in Kitgum (64.6±3.1) mbgl, followed by Pader (55.9±9.2) mbgl and shallowest in Agago District with (54.7±7.1) mbgl. However, the mean aquifer yield was highest in Pader district (19.0±4.1) m3/hr, (17.8±2.5) m3/hr in Kitgum district and (11.9±1.2) m3/hr in Agago District. The mean Swing Angle of the resistivity curves in the three districts of Agago, Kitgum and Pader were respectively (4.9±5.6)o, (17.8±2.5)o, and (19.0±4.1)o. Over all, as the distance within the region increases in the Eastern direction, the yield decreases from as high as over 26.0 m3/hr to as low as below 2.0 m3/hr. From a distance of 520000 mE to over 570000 mE, the yield is not spatially affected by shifting in the Northern direction, i.e., generally no change in yield from 340000 mN to 390000 mN. For distances between 335,000 mN to 360,000 mN, the Water Strikes are generally experienced at depths less than 60 mbgl, regardless of the eastern distance. This is a similar situation for distances between 490,000 mE to 530,000 mE where Water Strikes are achieved at depths less than 60mbgl, regardless of the distance along the Northern direction. The Total Depth (TD) of wells vary from 102 m to as high as 156 m, with half of the area having TD of less than 120 m and the other half having depths of 120 to 156 m. Spatially, there is a general similar TD of wells within the region, indicating that there are aquifers within the region at depths of 102 m to 156 m with an average value of 129 m. The Average Swinge Angle (ASA) of the Resistivity Curves typical of the area varies from as low as (-35)o to as high as 40o. Generally, the ASA varies relatively uniform over the area and therefore has no much effect on the groundwater exploration within the area. Below Total Depth (TD) of 115 m, regardless of the Water Strike depths, the Pump Test yield varies from as low as 18.0 m3/hr to as high as greater than 38.0 m3/hr. Above Total Depths of greater 140 m, the Pump Test Yield decreases from as high as 22.0 m3/hr to as low as below 2.0 m3/hr, regardless of the Water Strike depths. Between Total Depths of 120 m and 140 m, the Pump Test Yields are relatively higher, ranging from 18.0m3/hr to as high as 30.0 m3/hr. The best predictive model for Aquifer Yield (Y) within the region based on Total Depth (TD) and Water Strike Depths (WSD) for each lithological (geology)feature was found to have satisfactory Coefficient of Determination (R2) of 66.8% and is of the form, Y_i=β_0+β_1 ln(TD)+β_2 ln(WSD) Where Y_i= The Pump Test Yield for a borehole of Lithology ‘i’ in m3/h; TD =Total Depth of the borehole in m; WSD= Water Strike Depth in m; β_0, β_1 and β_2 are the Regression coefficients determinable by Multiple Linear Regression method. These coefficients were determined for each of the eight (8) Lithologies in the region and summarized using matrix notation as: (█(■(Y_AGG@Y_ASLD@Y_BGC )@Y_IGSVDP@Y_KG@Y_MG@Y_MGG@Y_VDG ))=(█(■(email@example.comfirstname.lastname@example.org&-32.67&0.712)@■(email@example.comfirstname.lastname@example.org&-32.67&0.712)@■(167.5&-32.67&0.712)@■(165.8&-32.67&0.712)))(■(1@ln(TD)@ln(WSD))) Therefore, this study major finding will guide groundwater exploration within the region by identifying and determining the Yield based on lithology. The approach used in the study can be applied during groundwater investigations to identify potential drilling points for groundwater supplies while minimizing cases of dry or low yielding wells. Application of this approach to groundwater investigation should take into account the factors that influence the occurrence and movement of groundwater in the study area. Geology and geological structures should be comprehensively investigated prior to application of this method. This study has also revealed that the North Western (extended network of geological structures) and North Eastern regions (with direct connectivity with the main aquifer/alluvial sediments) specifically in Kitgum District constitute good groundwater potential for prospecting, while based on the data density, the Northern part of Pader and Agago constitute good potential areas.