By Mark Reinsel and Bill Thompson
Alpha Explosives (Alpha) operates an explosives manufacturing and distribution business five miles north of Lincoln, Calif. Nitrate and perchlorate were used in explosives manufacturing at the site, and are found in groundwater beneath the site at concentrations above state water quality objectives. These constituents are the focus of ongoing site remediation and monitoring. Due to their release to the environment, groundwater monitoring is required by the Regional Water Quality Control Board (RWQCB) of the State of California.
|GeoProbe rig injecting reagents into boreholes.|
In situ biological treatment at the Alpha site appears promising for remediation of nitrate and perchlorate. Periodic reagent addition to injection wells in two source areas over the past six years has removed up to 48% and 75% of the nitrate in the source areas, along with 61% and 82% of the perchlorate.
A series of field investigations has been completed on the Alpha site since 1995 to investigate the source and extent of nitrate and perchlorate in groundwater, and to evaluate potential remediation alternatives. Hydrometrics Inc., Helena, MT, and Apex Engineering, Missoula, MT, have worked with Alpha since 2000, directing site monitoring and remediation activities that have included:
- Bench testing in 2000, 2002 and 2006.
- Four pilot tests between 2002 and 2005.
- Several phases of full-scale remediation in the source areas beginning in 2006.
Overall objectives of these remedial activities have been to:
1. Eliminate the former Evaporation Pond (EP), which is one source area where solutions containing nitrate and perchlorate were historically discharged, as a direct pathway for infiltration to groundwater;
2. Significantly reduce concentrations of nitrate and perchlorate in the EP area and in the Mix Building (MB) area, which is the other source area where nitrate-containing solutions are currently mixed and stored; and
3. Establish a trend of decreasing nitrate and perchlorate concentrations at downgradient monitoring wells.
Previous drilling programs have documented a mixed sequence of fine-grained sediments beneath the site that belong to the Pleistocene Riverbank formation. These units are predominantly clayey silt, sandy silt, and clayey sand and gravel. Groundwater velocities are very low, with estimates in the range of 20 to 50 ft/yr. Groundwater flow is generally west to northwest at the site but appears to vary locally. Most monitoring wells at the site (4 inch in diameter) produce less than two gallons per minute.
Groundwater remediation goals at the site are 10 mg/L of nitrate-N and 6 ug/L of perchlorate. Before commencing remediation, groundwater concentrations in the source areas were as high as 2,200 mg/L of nitrate-N and 110,000 ug/L of perchlorate.
Bench and Pilot Testing
In the earliest bench testing with Alpha groundwater and native bacteria, acetic acid and molasses were added to parallel continuous-flow columns. Both carbon sources were effective for nitrate removal and molasses was also effective for perchlorate removal. However, fermentation of molasses produced a disagreeable odor and color; therefore, molasses was not further evaluated. In 2006, bench testing showed that almost 100% nitrate and perchlorate reduction was possible in "worst-case" source area waters, at least under controlled conditions.
Four pilot tests were completed since 2002 to assess the feasibility of carbon and nutrient addition to groundwater to enhance in situ biodegradation of nitrate and perchlorate in groundwater. Ethanol and sodium acetate (a neutralized version of acetic acid) were the carbon sources added. Tracer studies in two pilot tests demonstrated the limited dispersion of reagents in this low-permeability groundwater system, with water quality at monitoring wells not changing substantially over several months. These tests also showed that ethanol may ferment to organic acids at the site, especially when added at high concentrations; sodium acetate did not ferment. In addition to molasses, ethanol was removed from further consideration as a carbon source.
Full-Scale Remedial Activities
Before commencing full-scale activities, Alpha personnel backfilled the former Evaporation Pond (EP) in October 2006. The final surface was sloped to direct runoff away from the area.
Work then focused on two source areas—the EP area and the Mix Building (MB) area. Phase 1A work was initiated in October 2006, with sodium acetate added to the groundwater system using direct push injection techniques in multiple boreholes at the EP and MB areas. A second round of injections (Phase 1B) was conducted in June 2008 using sodium acetate at injection wells rather than direct push injection technology. Because of fermentation concerns, neither ethanol nor molasses have been used in full-scale remedial activities.
Phase 1C groundwater remediation activities were conducted in October-November 2009. Carbon was again introduced to groundwater in the MB and EP areas through existing injection wells. A reagent known as Enhanced Oil Substrate (EOS) 450, a food-grade emulsified vegetable oil, was tested in two wells in the EP area as an alternative carbon source to sodium acetate. Sodium acetate and neutralized acetic acid were added to the other three injection wells in the EP area. A sodium acetate solution was added to the MB area injection wells.
Phase 1D groundwater remediation activities were conducted in May 2011, with an EOS solution gravity-fed to all injection wells in the MB and EP areas. No sodium acetate was added during this phase. Sodium trimetaphosphate was added (25 mg/L as P) with all carbon additions. Phase 2 of full-scale remediation began in 2012, with Alpha personnel taking the lead on reagent addition with limited oversight from technical consultants.
Nitrate and perchlorate concentrations have been substantially reduced in the two source areas. Nitrate concentrations have decreased by an average of 48% in the EP area since 2006. Perchlorate has been reduced by 61% since 2006 and by 76% since 2003. Nitrate concentrations have decreased by an average of 75% in the MB area since 2006. Perchlorate has been reduced by 82% since 2006.
Sodium acetate and EOS have been used in pilot tests and full-scale injection rounds at the Alpha site, with a relatively high degree of success considering the limited groundwater flow. Sodium acetate is now being used exclusively because:
1. EOS has emulsified and coated several of the injection wells, particularly where permeability is very low, and has made water sampling and reagent injections difficult. It is believed that addition of sodium acetate solution to these wells, if permeability will allow, will flush EOS into the groundwater system, and will make subsequent sampling simpler and more representative.
2. In the MB area, higher TOC levels were seen downgradient after sodium acetate injection than after EOS injection. TOC levels at these wells were about 20-30 mg/L after acetate injection but only about 5 mg/L after EOS injection. This is probably due to the EOS dissolving at a slower rate and its movement being retarded in groundwater, which are both objectives of a product like EOS but are less essential in a low-permeability groundwater system. It is believed that more rapid remediation may be achieved if a more soluble product such as acetate is used.
3. Sodium acetate is less expensive. This is important because of the significant quantities of carbon still required to meet remediation goals.
During Phase 1B and Phase 1C carbon injections, about 2400 pounds of sodium acetate were added to the MB area in each injection round. This quantity was based on conservative estimates of the amount of contaminants (primarily nitrate) present in groundwater. About 1600 pounds of sodium acetate were added to the EP area in each injection round. On a carbon basis, an equivalent amount of EOS was added to both source areas in Phase 1D.
To speed up remediation of the Alpha source areas, quarterly (rather than yearly) carbon injection commenced in June 2012, which is Phase 2 of the site remediation. Three hundred gallons of reagent solution (containing 30,000 mg/L of total organic carbon) are added to each of four to six injection wells in each source area. Sodium trimetaphosphate (25 mg/L as P) continues to be added.
Future Plans and Conclusions
Unless excessive carbon concentrations (signifying nitrate and perchlorate depletion) or measurable methane concentrations (signifying fermentation) are noted at downgradient wells, quarterly addition of sodium acetate and phosphate will continue at the Alpha site. Water quality at wells in the source areas and downgradient locations is monitored semi-annually.
Pumping groundwater from adjacent wells in the EP area during reagent injection was implemented during the 2009 and 2011 injections, and may have been responsible for the greater remediation effects compared to 2012 (when this "forced-gradient" injection technique was discontinued). During the next several injection rounds, Alpha will again use forced-gradient injection, pumping from adjacent wells to facilitate reagent dispersal in the groundwater system.
Because of very limited hydraulic conductivity throughout the site, pumping groundwater for above-ground treatment has not been pursued at the site and will probably not be implemented in the future. The client and the California RWQCB are pleased with the promising results obtained at the source areas with in situ bioremediation for nitrate and perchlorate to date, which will continue. Concentrations in downgradient locations have not yet been reduced, possibly because of low groundwater velocities.
A long-term view of remediation results shows that substantial progress has been made in the past five to 10 years. After a long period of historical releases, the contaminant plume is large yet stable, and there are no downgradient groundwater receptors. It is anticipated to take several more decades before groundwater in the source areas and downgradient locations meets the current state water quality objectives for nitrate and perchlorate. However, the plume stability in combination with the relatively low cost of this treatment approach relative to other potential options make in situ bioremediation at the Alpha Explosives site a practicable solution.
About the Authors: Mark Reinsel is president of Apex Engineering, PLLC, in Missoula, MT, and has nearly 30 years of experience in consulting, industry and academia. His consulting work focuses on treating mining and other industrial wastewaters through biological, chemical and physical methods. He may be contacted at email@example.com. Bill Thompson, RG, is Vice President, Sciences Manager, at Hydrometrics Inc., based in Helena, MT. The company provides an expansive range of science, engineering and construction services.