In the West African country of Burkina Faso water scarcity during the annual dry season is a problem for local vegetable farmers. In an effort to provide irrigation water to these farmers, development agencies have invested in the construction of small reservoirs which retain surface water for much of the year. The Nariarlé basin, a sub-unit of the Nakambé River basin, has been the site of reservoir construction over the last thirty years. Although the economic benefit of the reservoirs has been substantial, little is known about their effect on groundwater flowpaths and their potential to allow pesticides to reach groundwater resources. This project will investigate the exchanges between small reservoirs and the local aquifers in an attempt to better answer these questions.

At four study reservoirs within the basin, transects of mini piezometers and seepage meters will be installed and monitored over a period from August to December. These observations will provide a record of the change in hydraulic gradients and seepage fluxes during the transition from the rainy season to the dry season. Water level measurements at monitoring wells adjacent to the four study reservoirs and analysis of stable isotopes (18O and 2H) in water samples from both surface and groundwater sources will highlight the effect of reservoir seepage on regional flowpaths.

An appropriate computer code will be chosen based on its ability to describe both the regional distribution of groundwater head and the transient exchanges occurring between reservoirs and the aquifer. This code will then be used to developing a basin-scale groundwater model using available information on local geology, recharge rates and aquifer properties. After calibration to measured values of head, seepage and flow throughout the basin, the model will be used to simulate the effect of reservoir presence on the distribution and quality of groundwater resources.

Drew Gower