Old groundwater in parts of the upper Patapsco aquifer, Atlantic Coastal Plain, Maryland, USA: evidence from radiocarbon, chlorine-36 and helium-4

Plummer, L. N.; Eggleston, J. R.; Andreasen, D. C.; Raffensperger, J. P.; Hunt, A. G.; Casile, G. C.

Old groundwater in parts of the upper Patapsco aquifer, Atlantic Coastal Plain, Maryland, USA: evidence from radiocarbon, chlorine-36 and helium-4 | Eaux souterraines anciennes dans certaines parties de l’aquifère supérieur de Patapsco, Plaine Atlantique côtière, Maryland, Etats-Unis d’Amérique, à partir des données isotopiques du Carbone 14, Chlore 36 et Hélium 4 Agua subterránea antigua en partes del acuífero del Patapsco superior, planicie costera Atlántica, Maryland, EEUU: evidencias a partir de radiocarbono, cloro-36 y helio-4 美国马里兰州大西洋滨海平原中赋存在Patapsco上段含水层中的古老地下水:来自于放射性碳,36Cl和 4He的证据 Água subterrânea antiga em zonas do aquífero superior de Patapsco, Planície Costeira Atlântica, Maryland, EUA: evidências a partir de radiocarbono, cloro-36 e hélio-4

2012

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 ka, with one sample at 117 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 km downgradient on the northern flow path, where the radiocarbon age was 21.5 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 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.

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