Groundwater flow modelling of micro-catchments in Tilenga Area, Western Uganda

Abstract

Groundwater resources in the Tilenga area in western Uganda are anticipated to face unprecedented stress from oil and gas projects due to increased abstractions alongside projected climate change and population growth. This study addresses the critical gap left by basic water balance models in use by developing a robust numerical groundwater flow model. Hydrostratigraphic analysis of 42 boreholes delineated a semi-confined aquifer system with thickness raging 5-117 m (mean of 48 m), overlain by an aquitard of thickness ranging 3 to 68 m (mean of 22.5 m). The hydraulic properties were; transmissivity in range of 32.5-361 m2/d (mean of 111 m²/d), storativity of 1.4x10-8 to 3.0x10-1 (mean of 3.3x10-2), hydraulic conductivity of 0.56-3.88 m/d (mean of 1.5 m/d), specific yield of 0.026 -0.046 (mean of 0.035) and specific storage of 1.5x10-10 - 4.0x10-3 1/m (mean of 4.4 x 10-4 1/m). The static water levels range from 617 to 681 masl (mean of 623.2 masl) with flow in the north-westerly direction. Recharge rates ranged from 49 to 435 mm/a (mean of 203.5 mm/a) with 6-37.8% of annual precipitation (mean of 20%) with short time lag of 1-2 days between heavy rainfall events and water-level rises. Recharge rate was estimated using the water table fluctuation method. A 100 m thick single layer model with grids of 150 x 150 m was generated using MODFLOW- 2005 code with Lake Albert as a constant head boundary and the rest as no-flow boundaries. It was calibrated in steady state conditions with 53 wells converging with a mean absolute error of 0.167 m and the root-mean-square error of 1.180 m against a target of ±2 m with a coefficient of regression of 0.9929. Transient calibration was done by history matching of 16 daily water level measurements for close to 2 years with a coefficient of regression of 0.539. The model was highly sensitive to recharge and hydraulic conductivity in the eastern part of the area causing 0.15% and 0.065% change in model outputs, respectively. The model water budget revealed inflows of 21,356.8 m3/d from rainfall recharge and the lake with outflows of 21,356.6 m3/d from the abstraction wells and the lake. Four stressing scenarios were projected from 2024 to 2050. Projecting baseline conditions revealed a decline of groundwater table in range of 0.21 m (near the lake) to 2.52 m (at the CPF). Population growth scenario showed a rise of 1.0 m near the lake and 9.78 m at the CPF. For the abstraction rate increase scenario, a decline of 0.27 m was observed at 50% increment while over 20.5 m decline was observed at 200%, a 300% increment caused mining out of aquifers near the CPF. For recharge rate reduction scenario, an average decline of 0.4 m was observed at 5% reduction with 4 m decline at 50% reduction. Key recommendations include establishing weather stations and a consistent and automated groundwater monitoring network.