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Groundwater age dating with 14C, 3H, and SF6 to investigate the spatial distribution of recharge rates to the Mississippi River Valley alluvial aquifer
Proceedings of the 2020 Mississippi Water Resources Conference
Year: 2020 Authors: Gratzer M., Knierim K., Kingsbury J., Wacaster S.
The Mississippi Alluvial Plain (MAP) is the lower part of the Mississippi River floodplain; it overlies the Mississippi River Valley alluvial (MRVA) aquifer. Water-level declines in the MRVA aquifer are of concern. To better characterize the water availability of the aquifer, the spatial distribution of recharge must be well constrained. An approach to investigating the spatial distribution of recharge rates is groundwater age dating: determining the amount of time that has passed since the water became isolated from the unsaturated zone. The U.S. Geological Survey Lower Mississippi-Gulf Water Science Center collected groundwater samples from the MRVA aquifer, which were analyzed for age tracers (14C, 3H, and SF6); samples were also collected from the underlying Claiborne aquifer to investigate areas where water may be moving upward into the MRVA aquifer. These age tracer concentrations are used to estimate the distribution of groundwater age in the aquifer at each sample location using lumped parameter models. Multiple age tracers are used because a groundwater sample tends to contain a mixture of water of various ages and each tracer is appropriate for a different time range: up to 30,000 years before present for 14C, up to 60 years before present for 3H, and up to 67 years before 2020 for SF6. Historical atmospheric 14C activities range from 97 to 140 pmC. Groundwaters typically have 3H concentrations less than 10 TU. Historical atmospheric SF6 concentrations range from 0 to 8.5 parts per trillion (ppt). Generally, the higher the concentration of a given tracer, the younger the water. However, atmospheric 3H activity varies spatially and temporally, so groundwater 3H activities must be evaluated in terms of the historical atmospheric 3H activity of the recharge zone in order to estimate groundwater ages. Based on preliminary analysis, 14C activities (not corrected for dilution) in the MRVA aquifer range from 0.7 to 109.3 pmC; most of the MRVA aquifer has 14C activities above 60 pmC. Tritium concentrations in the MRVA aquifer range from 0 to 17 TU; most of the MRVA aquifer has 3H concentrations above 3.5 TU. Sulfur hexafluoride concentrations in the MRVA aquifer range from 0.4 to 9.4 parts per trillion by volume (pptv); most of the MRVA aquifer has SF6 concentrations above 2.4 pptv. The 14C activity in the MRVA aquifer is lowest in Mississippi County, AR, and highest in Butler County, MO, and Iberville Parish, LA. The spatial distributions of 3H and SF6 are very similar to each other. The spatial distribution of 14C is similar to 3H and SF6 except for a group of well locations, in a north-south orientation from the bottom of the Grand Prairie region of Arkansas to the top of Louisiana, where 14C is high and 3H and SF6 are low. The lowest (oldest) and highest (youngest) SF6 concentrations lie in areas with similar, relatively high (young) 14C activities, probably due to the finer temporal resolution of SF6 concentrations for young groundwater than that of 14C activities. The highest 3H activity lies in an area with high (young) 14C activity, and the lowest 3H activity is the same location as the lowest (oldest) 14C activity. The lowest (oldest) SF6 concentration was collected from a location with a low 3H activity and the highest (youngest) SF6 concentration was collected at a location with a high 3H activity. Results of the lumped parameter models will be presented.