An integrated approach to estimating groundwater recharge and storage variability in southern Mali, Africa

Groundwater recharge in southern Mali is investigated using a variety of methods. The aquifer system comprises a surficial unconfined aquifer in laterite that is hydraulically connected by vertical fractures through a sedimentary rock layer to a deep fractured semi-confined aquifer. Observed groundwater storage fluctuations from historical water level data correlate with GRACE satellite terrestrial water storage (TWS) variations, with peaks in September and lows in May; however, soil-moisture corrected GRACE data peaked in November due to the GLDAS model poorly predicting the timing of soil-water storage changes. Recharge modeling using HELP gave an average annual net recharge of 132.2 mm (12.6% of rainfall), comparing well with estimates from historical water level (149.1 mm; 16.4%) and GRACE (149.7 mm; 14.8%) data. Major ion chemistry suggests groundwater is fresh (average TDS of 205 mg/L) and rapidly recharged. 18O and 2H concentrations in groundwater and precipitation indicate July-September rainfall as the recharge source