Reservoir Contribution to Groundwater Flowpaths in the Nariarlé Basin of Burkina Faso

A study of interactions between seasonal irrigation reservoirs and groundwater within the Nariarlé basin in Central Burkina Faso was undertaken in order to assess potential for groundwater contamination from agricultural activities. Seepage rates at three reservoirs of varying size were measured at the beginning of the dry season from October 2007 to December 2007 in order to determine the direction and magnitude of aquifer/reservoir exchange. The elevation of the regional water table was monitored over a similar period using measurements from large-diameter wells. Samples of reservoir, well and borehole water were taken from points in ten clusters throughout the basin. These samples were then analyzed for geochemical and isotopic signatures. Well logs and pumping test results of water supply boreholes were used to characterize the aquifer dimensions and hydraulic parameters at points in and around the basin. This information was combined with a GIS topographic profile to determine basin boundaries as well as potential recharge and discharge zones.

Seepage meter measurements show that larger, more central reservoirs generally act as groundwater discharge zones, with seepage losses to groundwater occurring only in regions close to the dam. Smaller hillslope reservoirs generally act as zones of focused infiltration throughout, with few areas of groundwater discharge. Isotopic analyses show that water taken from both wells and boreholes downstream of reservoirs is enriched in heavier isotopes while samples taken at points upgradient of reservoirs have the same signature as rainwater. These results indicate that hillslope reservoirs act as zones of focused recharge and thus could affect downgradient groundwater quality. A numerical model of groundwater flow, calibrated using observed water levels and transmissivity estimates, provides further constraints on the magnitude of reservoir/groundwater exchange.