RELATIONS BETWEEN AGES OF GROUNDWATER SAMPLES AND ELECTRICAL RESISTIVITY ALONG FLOWPATHS IN THE MISSISSIPPI ALLUVIAL PLAIN

Particle tracking (PT) via groundwater flow models and tracer-based lumped parameter models (tracer LPMs) are two tools widely used to estimate groundwater age, but they often disagree. Hydraulic conductivity (K) along flowpaths is one important control of groundwater age. A gridded electrical resistivity (ER) model from an airborne electromagnetic geophysical survey of the Mississippi Alluvial Plain (MAP) provides a spatially continuous proxy for K down to as much as 300 meters depth in the MAP. This gridded ER model was used to investigate similarities and differences between PT-based and tracer LPM-based ages in the MAP. Two principal aquifer systems underlie the MAP: the surficial Mississippi River Valley alluvial aquifer (MRVA) and the underlying Mississippi embayment aquifer system (MEAS), which includes the Claiborne Group (CG), Wilcox Group (WG), and other Tertiary age hydrogeologic units in the region. Sixty-nine MRVA samples, 17 CG samples, and 2 WG samples in the MAP were analyzed for age tracers—¹⁴C, ³H, ³He, and SF₆—and dated using tracer LPMs. Median ages from PTs via a MODFLOW groundwater flow model were 780 years in the MRVA; 7,200 in the CG; and 420,000 in the WG. Tracer LPM median ages were 230 in the MRVA; 10,000 in the CG; and over 52,000 in the WG. The PT and tracer LPM results suggest that the MRVA has younger groundwater than the MEAS. However, 30 MRVA samples were older than the youngest MEAS sample and 13 MEAS samples were younger than the oldest MRVA sample. Therefore, some MEAS wells may be closer to their recharge zones than MRVA wells. A weak inverse relation exists between ER at the well screen and tracer LPM age for the MEAS. However, no relation exists between well-screen ER and tracer LPM age for the MRVA. For MEAS samples, higher resistivity (and possibly higher K) at the well screen is associated with younger ages. The lack of relation between ER and age for the MRVA may result from greater hydrogeologic heterogeneity in the MRVA than the MEAS. To further investigate the relation between K and age, the distributions of ER along flowpaths to each well will be compared to ages. Understanding the relation between K and age will provide more accurate age estimates.