PUMPING AND TRACER TESTS REVEAL LOCAL HYDRAULIC HETEROGENEITY IN A CRYSTALLINE BASEMENT AQUIFER IN SOUTHWESTERN NIGERIA

Groundwater resources in Nigeria have been considered abundantly available over the years, leading to uncontrolled and uninformed drilling of water wells. This may be true in sedimentary environments, but in crystalline terrain, fracture connectivity which is not well understood, controls its availability. The need to provide scientific information to better understand hydraulic heterogeneity in a typical basement complex terrain led to this research at the first Hydro-geophysical Research Site in Nigeria located within University of Ibadan, Ibadan, Nigeria

The study employed a multiple hydraulic testing approach on the four preexisting wells in the study area. Eighty constant rate pumping tests and two step drawdown tests were carried out with the extraction rates of 0.35, 0.65, and 0.88 l/s used for the latter. A forced gradient NaCl tracer with a concentration of 28 mS/cm was injected at a rate of 0.5 l/s for 108 minutes at a depth of 12 m. The tracer test's injection and post-injection phases lasted 10 hours. The pumping and tracer test results were evaluated using Aqutesolve and TRACT software.

Despite the wells being less than 11 m apart, fracture connectivity was diverse, with wells A and C being the most connected and well D behaving as an isolated system. The average fracture hydraulic conductivity, matrix hydraulic conductivity, transmissivity, and specific yield values range from 1.45 x 10^-5 to 7.85 x 10^-7 m/s, 1.00 to 7.69 x 10^-10 m/s, 1.05 x 10^-4 to 5.76 x 10^-5 m²/s, and 0.07 to 0.24 respectively. The difference between matrix and fracture hydraulic conductivity shows that heterogeneity in the matrix has a higher effect on the wells. The tracer test shows a preferential flow, which suggests the dominance of hydrodynamic dispersion, thus confirming the variability in the drawdown response measured across the wells. A low groundwater flow velocity of 0.00044 m/s estimated further confirms that advection transport is less dominant within the study area.

The variations in the hydraulic properties indicate the magnitude of connection contrast between the wells and the rock matrix, which culminated in different responses to pumping and yields of the various wells in the aquifer. This shows that groundwater flow in basement complex terrain is not limited to the fracture connectivity but also the rock matrix variability.