Quality of Shallow Groundwater and Drinking Water in the Mississippi Embayment-Texas Coastal Uplands Aquifer System and the Mississippi River Valley Alluvial Aquifer, South-Central United States, 1994-2004

Abstract

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Processes affecting geochemistry and contaminant movement in the middle Claiborne aquifer of the Mississippi embayment aquifer system

“…Agricultural land use in the 500-m buffer areas for the other five wells ranged from 31-75%. Pesticides were more commonly detected in groundwater where soils had high infiltration rates and low organic matter content in a previous MEAS study (Welch et al 2009). No relation was observed between pesticide detections and soil properties in 500-m buffer areas around wells in the present MCA study area.…”

“…These herbicides also were detected in water samples from Pliocene(? )/Pleistoceneage terrace deposits and shallow Tertiary-age wells in a previous study, with the largest number of different pesticides detected in wells in the Memphis area (Welch et al 2009). The higher number of pesticide detections in a developed area could originate from their use for weed control along rights-of-way in urban areas in the study area.…”

“…Fertilizers likely are the dominant source of nitrogen to the MEAS (Welch et al 2009). Nitrate-N concentrations in water from the MCA plotted by apparent recharge dates for age-dated samples generally follow nitrogen inputs to groundwater inferred from county fertilizer sales data for the study area (Fig.…”

“…In parts of northwest Mississippi and west Tennessee, however, this confining unit locally may be absent or may consist of fine sand, conditions that create the potential for downward movement of contaminants from shallow units that overlie the MCCU into the underlying MCA. Furthermore, groundwater withdrawals throughout the MCA have lowered water levels, enhancing the movement of shallow groundwater into the deep aquifers of the MEAS (Graham and Parks 1986;Welch et al 2009). The potential for downward leakage into the MCA in the Memphis, Tennessee, area has been known for some time (Graham and Parks 1986;Parks 1990;Parks et al 1995).…”

“…Larsen et al (2003) found that modern water from shallow units and a nearby stream in an area in Memphis was leaking through the MCCU into the MCA. Previous studies of various aquifers in the MEAS have found that the occurrence of elevated concentrations of nitrate, pesticides, and volatile organic compounds (VOCs) was related to redox conditions, degree of aquifer confinement, and the intensity of agricultural activities (Larsen et al 2003;Welch et al 2009). The geochemical processes affecting the regional groundwater quality in shallow and deep parts of the MCA are not well understood, with the exception of a few localscale studies (e.g.…”