OLD GROUNDWATER IN PARTS OF THE UPPER PATAPSCO AQUIFER, ATLANTIC COASTAL PLAIN, MARYLAND, USA: EVIDENCE FROM RADIOCARBON, CHLORINE-36 AND HELIUM-4

Water from 16 wells located along two flow paths in the upper Patapsco aquifer of southern Maryland and the Delmarva Peninsula was sampled for a suite of chemical, isotopic and age tracers. The two flow paths extend from the outcrop of the Lower Cretaceous-age sediments to approximately 59 and 117 km, respectively, to the southeast, reaching a maximum depth of 378 m beneath the Delmarva Peninsula east of Chesapeake Bay. Most of the samples are beyond the radiocarbon timescale in age (>30 ka). Radiogenic ⁴He accumulation rates calculated from measurements of U and Th in samples from six cores in the upper Patapsco were higher along the northern path than the southern path and in good agreement with radiogenic ⁴He accumulation rates calibrated using ¹⁴C ages, and ³⁶Cl ages. The apparent tracer ages range from modern to about 600 ka, except for the most downgradient sample that has a ⁴He accumulation age of more than one Ma. Last Glacial Maximum (LGM) water was located about 20 km downgradient on the northern flow path where paleorecharge temperatures were 0.5 to 1.5°C; a maximum cooling of about 12°C relative to the modern mean annual temperature of 13°C. Cl^-, Cl/Br, and stable isotopes of water all were a minimum at the LGM where the radiocarbon age of DIC was 21.5 ka. Because all samples have low recharge temperatures, the upper Patapsco was recharged primarily during glacial periods when coastal heads were lowest due to low sea level stand. Apparent ages range from 0-100 ka in the upper 1/3 of the flow system indicating that the upgradient parts of the aquifer were flushed relatively rapidly during glacial periods, as originally proposed by Purdy et al. (1996) for the Aquia aquifer, which is located above the Patapsco. Downgradient, the upper Patapsco aquifer discharges very slowly by upward leakage through multiple confining layers and stagnates along a freshwater-saltwater interface. The stable isotope data indicate that the southern flow path was recharged by Potomac River water that has an evaporation signal. Groundwater is the primary water supply in most parts of the Atlantic Coastal Plain of Maryland, USA, and deeper aquifers, such as the upper and lower Patapsco, show large drawdowns in many areas. This study demonstrates that most water in the upper Patapsco is fossil water and irreplaceable on human timescales under natural gradients.