ORIGIN AND GEOCHEMICAL EVOLUTION OF LOCALIZED, HIGH-FERROUS-IRON ZONES IN THE UPPER CASTLE HAYNE AQUIFER, BEAUFORT COUNTY, NORTH CAROLINA

The Tertiary Upper Castle Hayne Aquifer (UCHA) is one of the most productive and most extensively developed aquifers in the North Carolina Coastal Plain; however, localized zones containing high, dissolved-iron concentrations (>0.3 mg/L) have been measured near the recharge area. Although iron-rich groundwater is an expensive water quality and infrastructure problem affecting municipal water suppliers in eastern North Carolina, the evolution of high-iron zones in the UCHA is poorly understood. This preliminary study integrates mineral geochemistry, mineralogy, sedimentology, and geochemical groundwater modeling techniques to identify potential sources and sinks of dissolved iron near Washington, NC in Beaufort County. Three reddish orange sediment samples, extracted from around 13, 17, and 21 feet below the ground surface, have the highest measured iron concentrations (approximately 13.7% for each sample). Several additional anomalies occur within the aforementioned depth range including both the minimum and maximum pH values (4.9 and 8.1, respectively), the largest increase in cation exchange capacity (from 2.3 to 124.6 meq/100 cm³), and the highest concentrations of mud-sized grains (87.1%), fluorine (0.69%), magnesium (1.6%), aluminum (15.2%), phosphorous (11.1%), potassium (2.5%), manganese (0.16%), copper (0.7 mg/dm³), and zinc (4.1 mg/dm³). X-ray fluorescence and grain size data show that major iron concentrations can be correlated with large proportions of mud-sized particles, elevated cation exchange capacities, and relatively high manganese concentrations. Initial results of magnetic susceptibility, x-ray diffraction, and optical microscopy analyses indicate that iron-bearing minerals such as goethite, hematite, magnetite, ilmenite, pyrite, glauconite, nontronite, and jacobsite may be important sources of dissolved iron in the coastal plain overburden. Geochemical groundwater modeling indicates that cation exchange reactions between ferrous iron and smectite group minerals may substantially deplete dissolved iron once groundwater enters the UCHA. Future biogeochemical studies will be necessary to determine the degree to which microbial processes affect the formation and distribution of high-iron microsites.