Arsenic Remediation in Douglas County

Dissolved arsenic concentrations in ground water pumped by municipal wells throughout much of Nevada exceed the new 10 µg/L US Environmental Protection Agency drinking-water standard. A potentially cost-effective alternative to removing arsenic using traditional above-ground treatment is reducing arsenic concentrations within the aquifer. In this process, which is commonly termed in-situ removal, arsenic is retained within the aquifer through the coprecipitation or adsorption of arsenic with iron oxide in the vicinity of a production well. This retention can be enhanced by increasing the adsorption capacity through an increase in the iron oxide content of an aquifer or the adsorption efficiency by adjustment of pH, or both.

Phase I sampling in 2005 and experiments for in-situ arsenic remediation in 2006 were conducted using existing wells to determine the potential for in-situ arsenic remediation. Sampling of 8 wells of varying depth near the remediation site showed arsenic concentrations ranged from 30 to 38 µg/L and that the predominant arsenic species is arsenate [(As(V)] above about 150 feet below land surface, and arsenite [(As(III)] below about 300 feet. Experiments involved piping water from an existing production well; treating the water with dissolved oxygen, iron, and reducing pH; and injecting the treated water into two wells in a nested piezometer; one screened above 150 feet and the other screened below 300 feet. Injected water was withdrawn and sampled after 1-2 hours of reaction time. Arsenic concentrations were as low as 1 µg/L in water withdrawn from the shallow well and as low as 5.7 µg/L from the deep well. However, injected iron concentrations of 4-6 mg/L were required to achieve such removal resulting in iron concentrations above drinking-water standards for short periods during withdrawal. Additional work is needed to refine the remediation process that would involve the installation of additional injection and withdrawal wells, obtaining core materials for laboratory testing, and conducting cross-flow tests between injection and withdrawal wells screened in the lower part of the aquifer.

Objectives

Develop and test methods for in-situ remediation of arsenic in alkaline ground water, Douglas County, Nevada.

Strategy and Approach

In 2007, additional wells will be installed near the site where remediation experiments were performed in 2006, and will be screened in the lower part of the aquifer. Core material will be collected and tested in the laboratory to guide continued experiments. Cross-flow experiments simulating a production well and surrounding injection wells will be conducted in 2008.

Relevance and Benefits

 This cooperative project provides the USGS an opportunity to develop and test methods for in-situ remediation of arsenic in alkaline ground water that builds on previous work (Welch and others, 2003). This research may provide information that could be used to develop methods for removal of other inorganic constituents, such as uranium and chromium.  Such methods have not been developed for routine application to different geochemical and hydrologic settings in the United States. The work will develop and test methods for remediation that may be used by water purveyors throughout the western United States where existing concentrations of arsenic often exceed the new arsenic standard.

The work is consistent with USGS Water Resources Discipline Strategic Directions, Issue 3, maintaining a safe supply of drinking water, and drinking-water treatment, and is consistent with USGS Strategic Directions to develop a better knowledge base for the sustained development of the Nation’s ground-water supplies. The work is also consistent with the USGS Strategic Plan to work with local agencies in the mitigation of problems and development of new technologies.