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Using Tritium and General Geochemistry to Constrain Recharge Estimates within the Mississippi River Valley Alluvial Aquifer
Proceedings of the 2020 Mississippi Water Resources Conference

Year: 2020 Authors: Wacaster S.R., Knierim K.J., O'Reilly A.M.

The Mississippi River Valley Alluvial Aquifer (MRVAA), one of the regional aquifers within the Mississippi embayment, is a major source of groundwater for irrigation as well as some public supply use within the Mississippi Alluvial Plain (MAP). An aquifer-scale assessment of recharge processes (where water availability is significantly affected) is critical for the economic welfare of Mississippi, Louisiana, Arkansas, Missouri, Kentucky, and Tennessee. Using readily available recharge estimates and groundwater age will allow water quality and availability to be related.

Groundwater age represents the time required for water to travel from the point where infiltration reaches the saturated zone to a discharge point in an aquifer such as a water well. Groundwater age can be simulated with numerical groundwater flow models and characterized using environmental tracers. Tritium (a radioactive isotope of hydrogen) is one of several age-date tracers and can be used to qualitatively date groundwater—its presence in groundwater indicates that a component of young water (less than 60 years old) is present in an aquifer. This research compares various several regional estimates of recharge (from available datasets (soil water balance (SWB), empirical water balance (EWB)) and calculated estimates such as chloride mass balance (CMB)); recharge estimates will also be compared to groundwater geochemistry (major/minor ions, field parameters, and trace metals) and qualitative age from tritium dating. Results indicate that areas with higher EWB recharge rates were associated with the distribution of alluvial geomorphology. Pleistocene valley trains had higher EWB recharge rates and correspondingly higher tritium. This is likely due to differences in infiltration, where water is likely recharged more readily in the coarser-grained Pleistocene features. The CMB results indicated, however, that slightly greater recharge occurs in the Holocene alluvium. This was the opposite of what other studies found where CMB recharge was limited to the finer-grained sediments of the Holocene due to elevated chloride concentrations from little to no flushing of salts concentrated during evapotranspiration. The SWB will be incorporated to the overall assessment of recharge estimates and CMB estimates will be improved by using a continuous chloride grid to be included in the calculation. This will likely provide insight where high and low recharge values exist spatially, and which method provides the best overview of recharge in the MRVAA. This will improve confidence where water availability will be an issue—where recharge is relatively low in portions of the MRVAA.

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