USING TRITIUM AND SOIL CHARACTERISTICS TO CONSTRAIN RECHARGE ESTIMATES FOR THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER

The Mississippi River Valley alluvial aquifer (MRVA), a surficial aquifer of the Mississippi Alluvial Plain (MAP), is a major source of groundwater for irrigation in the central United States. Groundwater withdrawals exceed water input in some areas of the MRVA, leading to declining water levels. Recharge is not well constrained and is spatially variable as a result of heterogeneity within depositional environments during the Holocene and Pleistocene epochs. Tritium was used to qualitatively date groundwater as modern, mixed, or premodern, where the presence of tritium indicates a component of modern water (recharged after 1950) in groundwater. Two regional models of recharge—soil water balance (SWB) and empirical water balance (EWB)—were compared to qualitative groundwater age and geomorphology (depositional age and sediment size). Groundwater samples from 156 wells in the MRVA were characterized as primarily modern (41%) and mixed (43%), with 16% being premodern. Within Pleistocene sediments, 10% of samples were premodern compared to 20% for Holocene. These results are consistent with a surficial aquifer, but the premodern water indicates that there are areas where either recharge is limited or other (older) sources of water contribute to the MRVA. SWB recharge ranged from 0 to 37 in/year (median=3.7 in/year) and was lower than EWB recharge which ranged from 0 to 28 in/year (median=3.5 in/year). Both EWB and SWB models had higher recharge in coarse-grained deposits of the Pleistocene and Holocene. Water infiltrates more readily in coarser-grained deposits regardless of depositional age. No statistical difference in SWB recharge rates was observed among relative groundwater age categories, but recharge was significantly higher in the modern grouping for the EWB. Estimates of recharge from the EWB model overall tends to better fit the conceptual model for the MRVA, where more modern water is associated with higher recharge rates through coarse-grained sediments. Higher recharge rates from EWB corresponded to younger groundwater, highlighting that the EWB model, which considers subsurface geology, may better estimate groundwater recharge compared to SWB which models potential recharge from the root zone.