INSIGHTS FROM GROUNDWATER AGE TRACERS INTO SOURCES OF WATER TO THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER

This study investigated the spatial distribution of groundwater age in the Mississippi River Valley alluvial aquifer (MRVA) to better understand recharge pathways to this highly productive aquifer. Groundwater pumping has created long-term cones of depression in the MRVA in Arkansas and Mississippi with decreases in saturated thickness of up to 100 ft since predevelopment. Groundwater samples from 79 wells (55 in the MRVA, 19 in underlying Tertiary units) across the MRVA extent were analyzed for a suite of environmental age tracers including ¹⁴C and ³H. Older groundwater tends to have lower ¹⁴C. The atmospheric concentration of ³H has varied spatially and temporally, so the relationship between ³H concentration and age is nonlinear. Tritium results were used to classify samples as modern (< 70 years old), premodern (> several hundred years old), or mixed (a mixture of modern and premodern water). Age distributions of samples were calculated using lumped parameter models that can account for mixing of water from different flowpaths in a well and dispersion of tracers along flowpaths. Most (82%) MRVA samples were modern or mixed. Based on ¹⁴C results, premodern samples were about 3,000 to 7,000 years old. The spatial distribution of ages was compared to major hydrologic features of the aquifer. Corrected ¹⁴C activities decreased with distance from drainage divides and with depth to water table, suggesting older water farther from streams and in cones of depression. Qualitative ³H ages increased with distance from drainage divides. To further investigate the causes of differences in age, wells near each other that had substantial differences in age were evaluated. The steepest gradient in corrected ¹⁴C activities existed near the cone of depression in the Arkansas Grand Prairie region. Moving from higher to lower water level, ¹⁴C decreased and the ³H category changed from modern to mixed. The lower mixed water was closer to an upgradient subcrop of a Tertiary aquifer than the modern water. Chloride concentrations in the low-¹⁴C MRVA wells were similar to nearby Tertiary wells. These results suggest inflow to the MRVA from the underlying aquifers. Further analysis of water chemistry, water levels, and other geological data will help determine the likelihood of this connection.