T283 Advances in Characterizing Sources and Release of Naturally Occurring Trace Elements to Aquatic Systems and Groundwater

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room M100FG

CHARACTERIZING THE ROLE OF WELL FIELDS IN DISTRIBUTION OF ARSENIC IN GROUNDWATER SYSTEMS

Paper number: T283-5
Program: Technical Sessions
Time: 2:35 PM – 2:55 PM

Authors

Abstract

Chemical and biological reactions play an important role in controlling the distribution and cycling of arsenic in groundwater systems. Water management activities that alter the biogeochemical conditions of wellbores and aquifers can thus have an effect on local and regional aqueous arsenic concentrations. Wells provide a conduit for air to reach the rock/water interface, while pumping and dewatering can introduce oxygen to aquifer solids. Routine pumping can impose rapid change within a well, from strongly reducing to oxidizing conditions. Chlorine disinfection of wells to control nuisance or pathogenic bacteria also change the redox state in the borehole. Similarly, aquifer storage and recovery programs have the potential to alter aquifer redox conditions by introducing treated or oxygenated surface water. All of these activites impact subsurface biogeochemical conditions and have the potential to mobilize trace metals.

In many regions of Wisconsin, elevated levels of arsenic impact unlithified and bedrock aquifers. In glacial deposits, arsenic is typically associated with Fe-(hydr)oxides and is mobilized under reducing conditions generated by the microbial oxidation of organic matter. In sandstone and carbonate aquifers, sulfide oxidation controls arsenic release from sulfide minerals and sulfide-bearing cements. In all settings, a geologic sink of arsenic can become a source of mobile arsenic given a change in redox and pH conditions, complicating efforts to identify, test and effectively treat water supplies.

Some densely populated areas in Wisconsin have benefitted from a recent switch to public water supply systems. These well fields are designed and managed using practices consistent with our understanding of arsenic sources and mobility. Additionally, public water supplies benefit from routine testing, resulting in reduced arsenic exposure for thousands of individuals. Successful public health initiatives are more elusive in rural areas of the state where public water supply is expensive, and the complexity of arsenic sources and geochemical conditions pose challenges to adequate well testing or design of alternative well construction.