Mineral transformation and release of arsenic to solution under the oxidizing conditions of well disinfection

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Project Number:

DNR-192

Funding Year:

2006

Contract Period:

7/1/2006 - 6/30/2007

Funding Source:

DNR

Investigator(s):
PIs:
  • Madeline Gotkowitz, Wisconsin Geological and Natural History Survey
  • Eric Roden, UW-Madison
  • Madeline Schreiber, Virginia Tech University
  • Evgenya Shelobolina, UW-Madison
Abstract:

Background/Need

Guidance for disinfection of domestic wells in arsenic-sensitive areas of Wisconsin calls for a treatment at 20 percent of the chlorine strength and 10 percent of the contact time recommended for non-arsenic impacted wells. This guidance is based upon an assumption that the oxidizing strength of chlorine causes chemical oxidation of arsenic-rich sulfide minerals. However, microbially-mediated reductive dissolution of arsenic-bearing iron (hydr)oxides also contributes arsenic to groundwater in some areas of Wisconsin. Therefore, a low-dose chlorination treatment may not be a preferred method of disinfection in all settings where groundwater and wells are arsenic impacted.

Objectives

Investigate geochemical and biogeochemical mechanisms of arsenic release to well water from arsenic-rich sulfide minerals and iron (hydr)oxides during in situ disinfection with chlorine

Methods

In laboratory batch experiments, arsenic-rich aquifer sediments were subjected to deionized water with 8 mg/L O2 or chlorine disinfection solution at 1200 mg/L Cl- . Solution chemistry was monitored during 24 hours of reaction. The second portion of this project utilized a field setting to monitor microbial populations and water chemistry in a test well completed in the St. Peter sandstone. The well was subjected to periods of routine pumping, periods of non-use, and treatment with a low-dose chlorine disinfection.

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