Origin and geochemical evolution of localized, high-ferrous-iron zones in the Upper Castle Hayne Aquifer, Beaufort County, North Carolina

Abstract

The Tertiary Upper Castle Hayne Aquifer (UCHA) is one of the most productive aquifers in North Carolina's Coastal Plain; however, localized zones containing high, dissolved-iron concentrations ([greater than]0.3 mg/L) are present near the recharge area. Iron-rich groundwater is an expensive water quality and infrastructure problem affecting water suppliers in eastern North Carolina but the evolution of high-iron zones in the UCHA is poorly understood. This study integrates geochemistry, mineralogy, sedimentology, and geochemical groundwater modeling to identify likely sources and sinks of dissolved iron near Washington, NC. Two adjacent sediment cores were collected from the Yorktown and surficial aquifers, which overlie the UCHA at the core site in western Beaufort County. Orangish-brown sediments, extracted between 3.7 and 6.1 meters below the land surface (m BLS), have the highest iron concentrations measured in the core sediments (ranging from 2.2 to 9.0 wt. %). Several additional anomalies occur within this depth range including the highest increase in pH (from 4.9 to 8.1), the largest increase in cation-exchange capacity (from 2.3 to 124.6 meq/100 cm3), and the highest mud content (87.1 wt. %). X-ray diffraction, optical microscopy, and scanning electron microscopy indicate that amphiboles, ilmenite, glauconite, iron-oxyhydroxides, and pyrite are important iron-bearing minerals in the coastal plain overburden. Three hydrogeochemical zones, distinguished by variations in sediment composition, depth, and inferred biogeochemical and hydrologic processes, are identified in the sediments overlying the UCHA. The Iron Depletion Zone, extending from the ground surface to approximately 3.8 m BLS, may be characterized by progressive depletion of iron-bearing constituents over time. The Iron Pigmentation Zone (IPZ), extending from the basal portion of the IDZ to approximately 6.4 m BLS, likely transitions from oxidizing conditions to reducing conditions, resulting in iron-oxyhydroxide precipitation in the upper IPZ and the reduction of ferric iron to ferrous iron in the lower IPZ. High-dissolved-iron concentrations in the UCHA are most likely derived from conditions that are similar to those of the lower IPZ, where in the presence of organic matter, microbially catalyzed reduction of abundant iron-oxyhydroxides results in the production of dissolved-ferrous iron. Geochemical groundwater modeling confirms that microbially catalyzed reduction of iron-oxyhydroxides via organic-matter oxidation yields the highest, dissolved-iron concentrations in slightly acidic, surficial-aquifer water. Below 6.4 m BLS, evidence suggests that the dominant electron-accepting process in the Iron Sulfide Zone is microbially catalyzed sulfate reduction, resulting in the attenuation of dissolved-iron concentrations via the formation of iron-sulfide minerals. Iron-oxyhydroxide reduction, proximal to the upper contact of the UCHA, may be essential to the development of high-iron groundwater along the western edge of the UCHA recharge area. Geochemical modeling indicates that cation-exchange reactions between ferrous iron and glauconite may substantially deplete dissolved iron after groundwater enters the UCHA. Future studies integrating contemporaneous investigation of groundwater geochemistry, sediment composition, and redox-related microorganisms are necessary to better elucidate the formation of high-iron zones in the UCHA.