DNAPLs Drew Lonigro

Remediation In order to have a successful remediation, it is necessary to first isolate or remove the source of the contamination. The ability to identify the location of and remediate DNAPLs is the subject of much debate. It was previously thought that the pump and treat technology could be used for DNAPL remediation. It is now widely accepted that pump and treat is not an effective remediation technology for DNAPL, but can provide contaminant plume control.

Pump and Treat Systems Advantages: non-NAPL-contaminated ground water can be brought to the surface and treated with conventional methods. LNAPLs are somewhat easy to locate and remove. Disadvantages: 1) if NAPLs are present they will partition themselves between the NAPL phase and the dissolved phase. 2) Clean water that is drawn in will become contaminated. 3) DNAPLs are hard enough to locate, especially in fractured rock aquifers, much less recover. 4) Many, many permits are often required.

Pump and Treat Systems Flushing: “[As a] DNAPL will tend to form pools on the top of low permeability layers, the flushing liquid will only come in contact with the top of the DNAPL pool and little dissolution will actually occur. As a result, pump and treat will be inefficient even for mass removal in the case of DNAPLs. However, pumping still is viable as a means of plume stabilization.” (Fetter)

Bioremediation With DNAPLs, physical removal is most often not feasible. Bioremediation can effectively renovate soil and groundwater containing small amounts of dissolved organic molecules, even those trapped in low permeability zones and those clinging to soil particles.

Bioremediation Microbes that preexist in soils that have been contaminated with synthetic organic compounds have been found to be capable of degrading these compounds. In some cases the microbes take a while to become acclimated to the contamination. The time this takes is not known.

Bioremediation Environmental Factors: pH, temp., moisture, carbon substrate, amount of nutrients (nitrogen, potassium and phosphorous) and the amount of electron acceptors (oxygen, ferric iron, nitrate, sulfate, and carbon dioxide). Nutrient levels and microbial effectiveness are evaluated in a lab. Nutrients are often injected (oxygen is key)…the geologic medium must be permeable enough for the nutrients to reach the zone of contamination.

Bioremediation of chlorinated organic compounds Reductive Dechlorination – chlorinated compound acts as an electron acceptor so that a chloride atom is removed and replaced with a hydrogen atom. Microbes that accomplish this need a source of carbon and operate under anaerobic conditions. Trichloro’s become Dichloro’s Also get vinyl chloride (VC) and chloroethane (CA) which actually need aerobic conditions in order to degrade to ethylene, ethane or ethanol.

Bioremediation of chlorinated organic compounds Nyer and Duffin (1997), three situations… 1) Anthropogenic source of carbon such as a BTEX spill, which consumes oxygen (aerobic condition). Once conditions in the plume become anaerobic, the chloronated volatiles become electron acceptors and degrade. This process is quick. 2) Natural source of carbon in a reducing environment. Chloronated volatiles become electron acceptors for microbes consuming the soil organic carbon. VC and CA are destroyed. This is also quick. 3) Carbon source is added to remove oxygen. Methanol and molasses are possibilities….depends on local regulations.

Case Study-1 Field experiment at Moffett Naval Air Station, Mountain View, California to see if biostimulation could be used to enhance the in situ degradation of chlorinated ethenes. Confined sand aquifer 1.5m thick. Injection and extraction wells located 6m apart and aligned parallel to regional hydraulic gradient. Intermediate sampling wells placed at distances of 1, 2.2, and 4m from injection wells. Travel times from the injection wells to the withdrawal wells were from 20 to 42hr.

Case Study-1 Before contamination and biostimulation, oxygenated water was injected into the aquifer and extracted…with little loss of oxygen. Compounds selected for study were VC, DCE and TCE. It was found that with a long period of injection prior to biostimulation, the sorption capacity of the aquifer could be saturated with respect to those compounds. After 1,000hrs. of injection, the concentrations 1m away were 90-95% of the injected concentration.

Case Study-1 After the aquifer reached steady-state concentrations of the organic halides, it was biostimulated by injecting alternating pulses of dissolved oxygen and methane, along with continuous injection of the organic halides. The methane acted as the primary electron donor for the growth of indigenous methane- utilizing bacteria, while the oxygen was the electron acceptor. The halides were degraded. Within 2m of travel, VC was reduced 90-95%, t-DCE by 80-90%, c- DCE by 45-55%, and TCE by 20-30%.

Combination Methods Ardito and Billings (1990) described a method called the subsurface volatilization and ventilation system. The system consists of a number of well nests. Each nest consists of an air-injection well, which is screened below the water table, and a vapor-extraction well, which is screened in the vadose zone. This combines physical removal with bioremediation.

Combination Methods This is particularly effective for chlorinated solvents, which are not as biodegradable as hydrocarbons. The air sparging provides an in situ air-stripping system, with the stripped material being captured by the vapor-extraction wells.

Case Study-2 Remediation of soil and ground water at the site of an automotive manufacturer’s tank farm. 10 tanks at one assembly plant. Geology was heterogeneous with alternating layers of non-cohesive and cohesive sediments. There was a clay layer at a depth of about 16 ft. below grade with a shallower clay layer acroos part of the site. There was a perched aquifer where the upper clay layer was present, and a lower saturated zone which went across the entire site.

Case Study-2 The tank farm was a ground water recharge area with radial ground-water flow away in three directions…an iron retaining wall in the fourth direction. During site investigations the soil was tested for BTEX and total petroleum hydrocarbons (TPH) and the ground water was tested for BTEX and total organic carbon (TOC). Free product was found in several of the monitoring wells within the tank farm area. There was estimated 4800 gal of product in the tank area, with 72% in the soil, 2% dissolved in ground water and 25% as free product.

Case Study-2 A fairly significant drop off in contaminant concentration (mg/L) occurred outside the tank farm area with the only VOC above MCLs being benzene. The remediation system consisted of a series of duel extraction wells. Each well had a pump to extract ground water and free product, if present. Each well also was sealed with a vapor extraction system to remove soil vapor and promote air movement through the soil zone.

Case Study-2 The lowered water table caused by the extraction wells created a larger vadose zone for vapor extraction. The air flow promoted bioremediation. There were two layers of wells. One in the perched zone above the upper clay and one in the main saturated zone below the upper clay. Upon system startup, only the extraction pumps were used. The vacuum was not applied for the first 18 days.

Case Study-2 The initial pumping rate for 13 duel extraction wells was 5500 gal/day. After 18 days this had dropped to 3000 gal/day as the water levels in the aquifer declined. After the vacuum system was turned on the collective extraction rate rose to 5000 gal/day. The 13 wells were operated for 16 months. At that time an additional 9 wells were added and the total of 22 wells operated for an additional 5 months.

Case Study-2 During the course of the remediation a total of 4414 gal. of product were recovered. That’s 92% of the initial 4820 gal. The bottom line in this case is that vapor extraction was the most effective removal mechanism followed by biodegradation and then pump and treat.

Other Methods Cosolvent Flushing – chemically enhanced pump-and-treat technique…uses alcohol to increase solubility, desorption, and mass transfer of NAPLs. Six-Phase Heating (SPH) - The in-situ steam created by SPH rapidly vaporizes DNAPLs above and below the ground-water table. SPH provides controlled heating that can be applied directly to suspected DNAPL contaminant zones including those along the bottom of aquifers. DNAPLs with boiling points near that of water will be directly remediated by bulk DNAPL pool boiling.

Other Methods (SPH) is a patented, multi-phase electrical technique that uses readily available 60 Hz electricity to resistively heat soil and groundwater. Heating between the SPH electrodes creates an in-situ source of steam to strip volatile and semi-volatile contaminants from the subsurface. Soil Vapor Extraction (SVE) is then used to capture the off-gases for above- ground treatment. The SPH technology is not only effective in treating difficult sites with DNAPL and low permeability soil, but can also be used to heat soil to enhance biodegradation.

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