USE OF A UNIQUE BIOBARRIER TO REMEDIATE NITRATE AND PERCHLORATE IN GROUNDWATER
- Elizabeth A.
- Melissa L.
- Joshua D.
- Patricia A.
- Evangeline M.
Research was conducted to evaluate a multiple-layer system of volcanic rock, limestone, Apatite mineral and a 'biobarrier' to impede migration of radionuclides, metals and colloids through shallow alluvial groundwater, while simultaneously destroying contaminants such as nitrate and perchlorate. The 'bio' portion of this Multi-Barrier system uses highly porous, slowly degradable, carbon-based material (pecan shells) that serves as an energy source and supports the growth of indigenous microbial populations capable of destroying biodegradable compounds. The studies, using elevated nitrate concentrations in groundwater, have demonstrated reduction from levels of 6.5-9.7 mM nitrate (400-600 mg/L) to below discharge limits (0.16 mM nitrate). Perchlorate levels of 4.3 {micro}M (350 {micro}g/L) were also greatly reduced. Elevated levels of nitrate in drinking water are a public health concern, particularly for infants and adults susceptible to gastric cancer. Primary sources of contamination include feedlots, agriculture (fertilization), septic systems, mining and nuclear operations. A major source of perchlorate contamination in water is ammonium perchlorate from manufacture/use of rocket propellants. Perchlorate, recently identified as an EPA contaminant of concern, may affect thyroid function and cause tumor formation. A biobarrier used to support the growth of microbial populations (i.e. a biofilm) is a viable and inexpensive tool for cleaning contaminated groundwater. Aquatic ecosystems and human populations worldwide are affected by contaminated water supplies. One of the most frequent contaminants is nitrate. Remediation of nitrate in groundwater and drinking water by biodegradation is a natural solution to this problem. Microbial processes play an extremely important role in in situ groundwater treatment technologies. The assumption of carbon limitation is the basis for addition of carbon-based substrates to a system in the development of bioremediation schemes for nitrate-contaminated groundwater. The biobarrier concept typically involves construction of a wall of porous carbon-based material that is placed in a trench perpendicular to the direction of groundwater flow that extends at least the width and depth of the contaminant plume. A biobarrier can be used as a stand-alone system when biodegradable materials are the only contaminants, or it can be used along with other barriers, as has been done in the LANL Multi-Barrier system, designed to remediate multiple contaminants. The groundwater system must be reasonably well characterized in terms of direction of flow, width and depth of plume, concentrations along the plume, flow velocity and hydraulic conductivity. Barrier technology is largely applicable to shallow, alluvial plumes (less than 20 feet deep), although permeable reactive barriers (PRBs) have been placed at much greater depths, up to 70 ft. deep. Under these conditions, a barrier could be placed across the plume downstream from the source to prevent migration from a controlled site. The most effective barrier materials are natural waste materials of high porosity, resistant to degradation, that will not require removal or replacement with time. Pecan shells are a significant waste problem in pecan-growing areas. The most commonly used solution is land disposal. Use in biobarriers provides a desirable alternative. Pecan shells are composed of cellulose and lignin, and they degrade very slowly, providing a 'time-release' carbon source. If left uncrushed, they provide a high porosity material. Fishbone is a waste product made of calcium phosphate, or hydroxyapatite, which is very resistant to deterioration. Apatite-II effectively removes dissolved metals and radionuclides from groundwater. The precipitates formed with metals and radionuclides are highly insoluble and very unlikely to leach subsequently from the barrier. The residual tissue associated with the fishbones provides nutrient materials that contribute to formation of a microbial population as an additional benefit. We have investigated denitrification and perchlorate reduction processes within a biobarrier. The batch studies involved comparison of two potential carbon-based biobarrier support materials: pecan shells alone and pecan shells mixed with dog food in a 10:1 ratio. The results of the column studies have been reported elsewhere. The column studies were used to confirm that the microbial reactions occurring under denitrifying conditions in the batch studies are not altered dramatically under flowing conditions with the introduction of oxygenated groundwater. The objectives of these studies were to: (1) determine the effectiveness of the materials in development of a biofilm, and in destruction of nitrate, (2) quantify microbial populations present in batch systems, (3) determine amounts of nitrite and ammonia produced, and (4) determine pH conditions produced by the microbial activity in the biobarrier.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 975299
- Report Number(s):
- LA-UR-01-2122; TRN: US201008%%40
- Resource Relation:
- Conference: "Submitted to: 2001 International Containment and Remediation Technology Conference."
- Country of Publication:
- United States
- Language:
- English
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