DELAWARE GROUNDWATER MONITORING NETWORK: GEOCHEMICAL ANALYSES SUPPORTING SUSTAINABLE RESOURCE MANAGEMENT

The Delaware Groundwater Monitoring Network has collected detailed hydrogeologic information from new wells constructed in confined aquifers (Rancocas, Mt. Laurel, and Magothy) that address near-term critical water resource management issues in southern New Castle and northern Kent Counties. The results of this research will improve our predictive capabilities of how changes in environmental conditions and a growing population will affect future water availability.

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.