PROJECT DATE: November 2001 to present
CLIENT: Air Force Center for Environmental Excellence (AFCEE)
BACKGROUND: TCE contamination of the groundwater was found at the Altus AFB Site SS-17 at a depth of 7 to 20 feet below ground surface. The aquifer at Altus AFB consists unconsolidated fine-grained alluvial deposits, primarily clay, overlying a fractured weathered shale with beds of gypsum. The soil contains high levels of both iron and sulfate. A nearby fuel release has impacted this area of the aquifer. Biodegradation of fuel components has resulted in a reduction in the sulfate levels and the development of an microbial population capable of complete dechlorination of TCE to ethene and ethane. However, under the prevailing low soluble carbon conditions, ethene and ethane concentrations are low. AFCEE funded TSI and Solutions-IES to install an interceptor barrier in a portion of the SS-17 plume. A second smaller scale pilot is being conducted in the OU-1 plume that originated in a landfill. Very high sulfate levels (about 1,700 mg/L) and higher TCE levels (4.5 mg/L) were seen in two injection wells. The objectives of this project were to: 1) evaluate the distribution of the emulsified substrate in another geological setting, 2) determine the effectiveness of the emulsified substrate in promoting biological dechlorination in an aquifer with high ferrous iron and sulfate levels, and 3) evaluate the potential for ferrous sulfide, produced from the biodegradation of the emulsion with sulfate and ferric iron as electron acceptors, to promote the abiotic degradation of TCE to acetylene.
ACTION TAKEN: For the SS-17 pilot, six injection wells screened from 8 to 18 feet below land surface were installed using a hollow stem auger drilling rig. Four injection wells and six monitoring wells were sampled in November and December 2001 and in April 2002. Two injection wells were installed for the OU-1 pilot. One of the wells was bioaugmented with groundwater from the SS-17 pilot area thought to contain a dechlorinating enrichment. The emulsified substrate package was injected into the injection wells in December 2001.
EFFECTIVENESS: In the SS-17 pilot, the emulsified substrate was distributed up to 25 feet away from the barrier. Over the first four months of monitoring, the emulsion has enhanced dechlorination of TCE and cDCE to VC, ethene, and ethane. For example, well NB had 0.15 mM TCE, 1.0 mM cDCE, 0.90 mM tDCE, 0.83 mM VC, 0.11 mM ethene, and 0.057 mM ethane in November 2001. In April 2002, this well contained <0.043 mM TCE, 0.065 mM cDCE, 0.10 mM tDCE, 0.19 mM VC, 0.39 mM ethene, and 0.13 mM ethane. The percent ethene and ethane relative to total chlorinated ethenes increased from 5.8 to 59% in well NB. The percent ethene + ethane in all wells in the SS-17 pilot ranged from 9 to 99%.
The OU-1 injection wells also showed large increases in the ethene and ethane concentrations between November 2001 and April 2002. The percent ethene + ethane increased from 0 to between 20 and 46% in the two injection wells. Bioaugmentation with the groundwater from the SS-17 plume did not improve the extent of dechlorination. Because neither pilot has shown appreciable concentrations of acetylene, the abiotic reaction of TCE with ferrous sulfide does not appear to be occurring.