John R. Eggleston Jeff P. Raffensperger Andrew G. Hunt Gerolamo C. Casile D. C. Andreasen Niel Plummer 2012 <p>Apparent groundwater ages along two flow paths in the upper Patapsco aquifer of the Maryland Atlantic Coastal Plain, USA, were estimated using ¹⁴C, ³⁶Cl and ⁴He data. Most of the ages range from modern to about 500&nbsp;ka, with one sample at 117&nbsp;km downgradient from the recharge area dated by radiogenic ⁴He accumulation at more than one Ma. Last glacial maximum (LGM) water was located about 20&nbsp;km downgradient on the northern flow path, where the radiocarbon age was 21.5&nbsp;ka, paleorecharge temperatures were 0.5–1.5  °C (a maximum cooling of about 12 °C relative to the modern mean annual temperature of 13 °C), and Cl^–, Cl/Br, and stable isotopes of water were minimum. Low recharge temperatures (typically 5–7 °C) indicate that recharge occurred predominantly during glacial periods when coastal heads were lowest due to low sea-level stand. Flow velocities averaged about 1.0 m a^–1 in upgradient parts of the upper Patapsco aquifer and decreased from 0.13 to 0.04 m a^–1 at 40 and 80&nbsp;km further downgradient, respectively. This study demonstrates that most water in the upper Patapsco aquifer is non-renewable on human timescales under natural gradients, thus highlighting the importance of effective water-supply management to prolong the resource.</p> application/pdf 10.1007/s10040-012-0871-1 en Springer Old groundwater in parts of the upper Patapsco aquifer, Atlantic Coastal Plain, Maryland, USA: Evidence from radiocarbon, chlorine-36 and helium-4 article