EVOLUTION OF OXYGEN ISOTOPE AND MAJOR ELEMENT GEOCHEMISTRY OF GROUNDWATER FROM THE SHALLOW AQUIFERS OF EASTERN NORTH CAROLINA

Samples of groundwater and surface waters in southeastern North Carolina were collected in July 2000 and April 2001 and added to a growing database as part of an effort to determine the reactions controlling groundwater chemistry and evolution along chosen flowpaths. Urban development along the coastline relies heavily on groundwater from shallow limestone and calcareous sandstone aquifers including the Tertiary Castle Hayne (CH) and Cretaceous Peedee (PD), which dip and thicken eastward. In the Northern Coastal Plain (NCP), these aquifers are confined, deeper, and distal flow paths are farther from the recharge area, whereas these aquifers are often unconfined and shallower in the Southern Coastal Plain (SCP). d18O values for water from the Surficial Aquifer, CH, and PD in the SCP range from -5.37 to -4.01, -5.22 to -3.52, and -3.95 to -2.68, respectively. These CH values fall nearer the average d18O of modern precipitation than NCP values - consistent with relatively short groundwater flowpaths in the SCP. Groundwater d18O for the easternmost CH in the NCP is more positive (up to -3.12). As for the Floridan Aquifer in South Carolina and Georgia (Plummer, 1993), more positive values downdip are consistent with the idea that CH groundwater from the NCP preserves a record of d18O of meteoric water recharging the aquifer in the past. This inference depends on there having been no significant leakage from surface waters or other groundwater. Artesian spring d18O values from the SCP (-4.75 to -2.96) are consistent with a CH groundwater source. However they are also similar to other surface waters in the region, including the Northeast Cape Fear River and tidal creeks (-3.64 to -2.5, -4.36 to 0.94, respectively). Use of multiple geochemical tracers will be necessary to sort out groundwater – surface water interactions, and exchange between aquifers.