26 Geology in the Public Interest

Friday, 30 March 2007
8:00 AM - 12:00 PM

ARSENIC GROUNDWATER INVESTIGATIONS - EASTERN SLATE BELT (ESB): NASH AND HALIFAX COUNTIES, NORTH CAROLINA

Paper number: 26-4
Program: THEME SESSIONS
Time: 9:00 AM – 9:20 AM

Authors

Abstract

The presence of naturally occurring arsenic-contaminated groundwater is a potential health risk to users of domestic water supply wells in the Eastern slate belt of North Carolina. Integrated geological and geochemical investigations have determined that ambient arsenic is present in groundwater in two discrete geologic settings: molybdenum porphyry and volcanogenic-gold deposits.

The presence of arsenic was determined by analyzing precipitates from first, second, and third order streams under base flow conditions. When groundwater discharges into surface water, arsenic and other metals are precipitated out of solution, because of REDOX changes between the subsurface and surface environments. These oxides and are then deposited as coatings on streambed materials.

Analyses (As, base metals, Fe and Mn) were determined following chemical extraction of naturally occurring manganese-iron oxide-coatings, which had precipitated from solution onto streambed cobbles. Additionally, researchers produced an artificial REDOX front by placing ceramic tiles in streambeds to collect and analyze oxide precipitates. These samples were collected roughly every quarter, for one year. Thermochemical plots based on these data, as well as information from respective stream water measurements (pH, EH, SPC, DO, and temperature), water sampling, and rock chemical analyses indicate mobile arsenic in predicted stability fields. Initial results show that naturally occurring arsenic-contaminated groundwater is present within the study area. However, the resulting oxidation and precipitation within streams appreciably removes this contaminant from surface water solution.