Sustainable Remediation of a Nitrate Plume:
It’s all About the Bugs
A release of approximately 15,000 gallons of ammonia nitrate and urea (liquid fertilizer) occurred in 2003 next to a railroad track. Initial response activities included the excavation of two recovery trenches, soil sampling from the two trenches, installation of slotted polyvinyl chloride (PVC) pipe at the base of the trenches, excavation of test pits within and downgradient of the spill area to capture liquid fertilizer in the subsurface, and the installation and sampling of three recovery wells. During the initial response, approximately 600 gallons of liquid fertilizer was recovered. Initial response data indicated elevated concentrations of nitrate and ammonia in the subsurface soils and groundwater in the area of the release. Groundwater was monitored periodically between 2003 and 2012 with minimal natural attenuation of nitrate in groundwater. Excavation of source area soils under the tracks was not practicable, and the agency agreed that the remediation should focus on groundwater.
Basis of Remediation
Carbstrate™, a dextrose and micronutrients (potassium phosphate and yeast extract) amendment produced by ETEC, LLC, is used to enhance biodegradation of nitrates in soil and groundwater by naturally-occurring denitrifying bacteria (the “bugs”). The bacteria utilize the carbon substrate as a food source, converting the nitrate to nitrogen gas.
Additionally, the carbon substrate and water mixture can be applied via gravity injection to existing site infrastructure. This limits the energy used and additional infrastructure required.
In 2012, KJ Consultants conducted a pilot study as part of an Interim Remedial Measure (IRM) to stimulate the growth of denitrifying bacteria throughout the vadose and saturated zone. The IRM involved gravity injection of Carbstrate™ via an existing recovery trench adjacent to the railroad tracks, followed by quarterly groundwater sampling events (one pre-injection and four post-injection). The existing trench was approximately four feet deep and 10 feet above the groundwater table, which allowed for flushing and treatment of the source area soils. The sampling results indicated that the pilot IRM was reducing nitrate concentrations in vadose zone soils and groundwater.
In March 2014, an additional IRM injection was performed using the recovery trench. Carbstrate™ was also injected into three site monitoring wells in an effort to introduce the substrate directly into groundwater near the original source area. Quarterly groundwater sampling events (one pre-injection and three post-injection) began in March 2014 through April 2015, with additional semi-annual events in October 2015 and March 2016.
Since implementing the IRM, the analytical results have indicated a significant reduction in groundwater nitrate concentrations:
- Prior to the IRM, nitrate concentrations exceeded the groundwater standard (10 milligrams per liter) in six site wells, with maximum concentrations of nitrates exceeding 500 milligrams per liter.
- Currently, nitrate concentrations are below the groundwater standard in all but two site wells. Concentrations of nitrate in the downgradient monitoring point have been reduced by over 65% following the 2014 injection. An IRM Injection was completed in June 2016 in the remaining two site wells that exceeded the groundwater standard. Results of this injection are pending, however, it is likely that only two years of monitoring will be required to close the site.
Green and Sustainable Remediation
This technology was applied using existing infrastructure (recovery trench and monitoring wells) with reliance on biological remediation processes.
According to the United States Environmental Protection Agency (EPA), Green remediation Best Management Practices (BMPs) focus on the core elements of greener cleanups:
- Minimize total energy use and increase the percentage from renewable energy.
- Minimize emission of air pollutants and greenhouse gases.
- Minimize water use and preserve water quality.
- Conserve material resources and minimize waste.
- Protect land and ecosystem services.
This project utilized green remediation BMPs in the following ways:
- Minimized the total energy consumed by gravity injection of the carbon substrate and water mixture.
- Minimized emissions of air pollutants by only using gasoline-powered pumps during mixing of the substrate.
- Minimized water use by batch injecting the substrate at a high concentration.
- Used in-situ bioremediation, therefore did not generate any wastewater.
- Additional infrastructure was not needed, leading to minimal land disturbance.
- Groundwater samples are collected from monitoring wells using GeoInsights’ Hydrasleeve® sampling device. Purge water is not generated with the Hydrasleeve®; therefore, storage and disposal of purge water is not required.
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