Grantee Research Project Results

Final Report: Subsurface Treatment for Arsenic Removal

EPA Contract Number: 68D02099
Title: Subsurface Treatment for Arsenic Removal
Investigators: Wolf, Christopher
Small Business: Daniel B. Stephens and Associates Inc.
EPA Contact: Richards, April
Phase: I
Project Period: October 1, 2002 through July 31, 2003
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2002) RFA Text |  Recipients Lists
Research Category: SBIR - Water and Wastewater , Watersheds , Small Business Innovation Research (SBIR)

Description:

Subsurface Treatment for Arsenic Removal (STAR) is an innovative technology for the treatment of arsenic in groundwater within the aquifer. The STAR technology can result in very large cost savings when compared with conventional aboveground treatment methods. The concept is to create a subsurface biogeochemical barrier composed of reactive iron hydroxide minerals and iron bacteria within the aquifer surrounding a water supply well. Following emplacement of the geochemical barrier, the well will yield water that meets the arsenic drinking water standard of 0.010 mg/L.

Daniel B. Stephens & Associates, Inc. (DBS&A) used a STAR system installed at a supply well in San Antonio, NM (San Antonio No. 3), that is operated by the San Antonio Mutual Domestic Water Consumers Association. In conjunction with Subsurface Technologies, Inc. (STI), DBS&A evaluated the physical and chemical nature of groundwater in the Santa Fe Formation Aquifer. Using aquifer parameters and groundwater chemistry, STI calculated the optimum spacing for aeration wells. DBS&A provided oversight during drilling and installation of nine aeration wells. Following well construction and development, DBS&A installed and operated the air handling system. During air injection, water quality was closely monitored to observe the effects of metal precipitation in the subsurface.

The key technical objectives for Phase I were: (1) demonstration of the feasibility of in situ arsenic treatment, and (2) development of methodologies to efficiently identify system performance limitations in field settings. To demonstrate the feasibility of in situ arsenic treatment, DBS&A completed three major tasks: system construction, system startup and monitoring, and system tuning. Data were collected to evaluate rate, length, and velocity limitations on treatment efficacy. Three major testing components have been completed: (1) pumping tests on the production well to obtain aquifer hydraulic parameters; (2) dissolved oxygen monitoring in groundwater within the subsurface treatment zone; and (3) monitoring of arsenic and iron concentrations in groundwater before, during, and following implementation of the STAR treatment technology. Two limiting factors have been identified—air delivery and iron concentration—and DBS&A is proposing tests to overcome these limitations in Phase II. The concept is proven in principle, a provisional patent for the technology has been filed, and project goals have been met.

Dissolved oxygen concentrations and redox potential were elevated in the treatment zone as compared to pre- and post-test conditions. The air delivery system design and equipment have been shown to be capable of favorably altering subsurface chemistry in a sustained manner. The system was able to achieve significant iron removal during initial tests, and a reduction in arsenic concentration was confirmed.

Summary/Accomplishments (Outputs/Outcomes):

Although some reduction in arsenic was achieved, test results indicated that the system as currently configured probably will not be able to reduce arsenic concentrations in the produced water to below the 0.010 mg/L maximum contaminant level (MCL). However, surface complexation modeling has indicated that if all iron was removed from the water in the treatment zone (assuming 0.26 mg/L iron as Fe(II)), the reduction in arsenic concentration should have been nearer to 0.010 mg/L than the 0.005 mg/L observed. Modeling indicates that increasing the available iron concentration will result in increased arsenic removal and should achieve produced water that will have arsenic concentrations below the MCL.

Conclusions:

Testing results indicated that the STAR technology can be very effective in removing arsenic to acceptable levels with lower initial construction cost, lower operation cost per unit volume of water, and lower total costs than conventional treatment. The potential for complete automation and very low maintenance makes the STAR technology ideal for small systems. In aquifers with high ambient iron conditions, the system can work completely without chemical additives, using natural processes of maintaining or creating oxidized conditions in the aquifer to remove arsenic before the water is pumped from the ground. When insufficient iron is present in the aquifer, iron solutions can be added and then precipitated, achieving successful arsenic removal. STAR produces only a very limited waste stream associated with compressor operation and maintenance, and produces no arsenic-bearing waste stream whatsoever. All of these factors greatly lower the capital cost of STAR and make operation simpler and less costly as compared with conventional treatment. A patent application (Provisional Patent Application No. 60/456,867) was submitted to the U.S. Patent Office.

Supplemental Keywords:

Subsurface Treatment for Arsenic Removal, STAR, groundwater, aquifer, subsurface biogeochemical barrier, iron hydroxide, bacteria, drinking water, aeration well, in situ arsenic treatment, dissolved oxygen, maximum contaminant level, MCL, SBIR, small business., RFA, Scientific Discipline, Water, Environmental Chemistry, Analytical Chemistry, Arsenic, Drinking Water, Environmental Engineering, Environmental Monitoring, community water system, Safe Drinking Water, treatment, iron hydroxide, chemical contaminants, monitoring, contaminant removal, drinking water system, drinking water contaminants, water treatment, drinking water treatment, subsurface treatment, public water systems, risk management, arsenic exposure