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

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Mark J Akland (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

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.

Additional Information

Publication

Thesis

Language: English

Date: 2017

Keywords

Castle Hayne Aquifer, ferrous iron, redox, iron oxyhydroxides

Subjects

Email this document to