DELAWARE GROUNDWATER MONITORING NETWORK: GEOCHEMICAL ANALYSES SUPPORTING SUSTAINABLE RESOURCE MANAGEMENT
Groundwater 18O and 2H values indicate a colder recharge temperature, correlating to an age of approximately 15,000 years old. In comparison, concurrently collected samples yield conventional 14C ages of 6,500 – 16,200 years old. The 18O and 2H values plot below the Global Meteoric Water Line, indicating slight evaporative affects prior to infiltration and potentially smaller paleo-recharge areas. Areas and rates of recharge were reduced due to the presence of discontinuous permafrost. 18O-enriched values correlate with elevated total dissolved solids (TDS) caused by pumping-induced saltwater intrusion or the dissolution of Na (smectite and feldspar group clays) and Cl (chloride) via the percolation of meteoric water through the subsurface. The model indicates that the Magothy aquifer has elevated salinity values due to pumping-induced landward flow beneath the Delaware Bay.
TDS does not increase along flow paths determined by a flow-model particle track simulation of the Mount Laurel and Magothy aquifers, even though shallow upgradient aquifer units are composed of resistant quartz sand and more soluble calcite- and glauconite-rich sands are present downgradient. Rather, local hydrogeologic factors and leaching of agricultural contaminants run-off upgradient cause elevated TDS in shallow portions of flow paths. TDS within the Rancocas aquifer appears to increase with increasing flow distance; however, the R2-value is not significant. TDS increases with decreasing elevation indicating that relative groundwater age can be determined from TDS values.