STRONTIUM ISOTOPES AND MAJOR/MINOR ELEMENTS IN SHALLOW GROUNDWATER AND COASTAL SURFACE WATER FROM SOUTHEASTERN NORTH CAROLINA

Following sampling in July, 2000 and April, 2001 analyses of water samples from aquifers, tidal creeks, rivers, and springs in southeastern NC were added to a growing database needed to develop geochemical methods for: 1) characterizing water from various groundwater and surface water reservoirs, 2) quantifying groundwater discharge into estuaries and the coastal ocean, and 3) investigating saltwater intrusion into coastal aquifers. Communities in the rapidly developing region rely heavily on groundwater from shallow limestone and calcareous sandstone aquifers like the Tertiary Castle Hayne (CH) and Cretaceous Peedee (PD). Groundwater in both aquifers is Ca- and HCO3-rich. Although major element chemistries of groundwater from these aquifers also vary, strontium isotopic signatures, which change systematically with the age of aquifer materials, may prove particularly distinctive. Because the CH and PD are close to the surface and poorly confined in places, considerable leakage from the Surficial Aquifer (SA) and surface waters may occur. 87Sr/86Sr values for water from the CH, PD and SA in the study area range from 0.70874-0.70911, 0.70806 - 0.70896, and 0.70844-0.71049, respectively. Waters from the Cape Fear and Northeast Cape Fear Rivers are generally characterized by decreasing 87Sr/86Sr ratios (0.70915-0.70984) and increasing strontium concentrations (0.0360-2.676) downstream. Ratios of spring waters (0.70878-0.70887) suggest flow through limestones. Downgradient changes in Sr-isotopes in groundwater in the study area are not yet well-constrained. However, north of the study area data indicate that 87Sr/86Sr for CH groundwater decreases downdip. Deviations from this pattern seem to indicate intermixing of surface water or groundwater from other aquifers. Chemical characterization of water from various sources, combined with geochemical modeling and statistical analysis should permit estimation of volumes of water moving in and out of important aquifers.