Fraction of Young Water as an Indicator of Aquifer Vulnerability Along Two Regional Flow Paths in the Memphis Aquifer

Wells along two flow paths beginning in the outcrop area and ending at public-supply wells in the confined parts of the Memphis aquifer were sampled for age-date tracers, inorganic and organic constituents to characterize the sources of water and the vulnerability of the Memphis aquifer to contamination across a range of hydrologic and land-use conditions in southwest Tennessee. Young groundwater (< 60 years) was present at 13 of the 16 wells sampled in both unconfined and confined parts of the aquifer. These age-dating results generally were consistent with previous studies at Memphis, Tennessee, that documented leakage of shallow water into the Memphis aquifer locally where the overlying confining unit is thin or absent. Mixtures of young and old water were most prevalent where long-term pumping for public supply has lowered groundwater levels and induced downward movement of young water. The fraction of young water was correlated to the occurrence of anthropogenic organic contaminants, but relatively few contaminants were detected and concentrations were low because of the large fraction of old water at most locations. Long-term water-quality data collected from 1990 to 2015 for one of the flow-path wells in the confined part of the aquifer indicate that the fraction of young water is increasing over time as withdrawals for public supply are increasing. End member mixing models of shallow and deep groundwater based on the fraction of young water from age-tracer results suggest that over time the young fraction has increased from about 5 percent in 1990 to almost 20 percent in 2015. In general the vulnerability of this aquifer is low because of the predominance of old water at most locations, but as demand for water for various uses increases over time, the vulnerability of the aquifer is likely to increase.