While previous intrusive remediation technologies have failed to remediate contaminated groundwater at a site near the city of Odessa, TX, natural processes are showing promising results.
This property’s previous use as an oilfield services facility impacted the soil and groundwater. To date, remediation measures centered on removing the impacted soil from the site. However, the groundwater under the property still contained chlorinated solvents such as trichlorethylene (TCE), perchlorethylene (PCE), cis-1,2-dichloroethene (cis-1,2-DCE), trans-1,2-DCE, as well as vinyl chloride. The chemicals posed a danger to anyone drinking groundwater from the site, as well as a health threat by vapour emitted by the soil and groundwater.
A previously-installed groundwater treatment system employed a packed air stripper and included the re-injection of aerated groundwater. The aerobic conditions created by the system are not conducive to reductive dehalogenation of chlorinated ethenes and failed to reduce their concentrations in the groundwater to less than the drinking water standards.
Golder* took a different approach. We determined that despite concentrations remaining above standards, the groundwater plume was no longer expanding because the source of contamination was removed when the impacted soils at the site were excavated. So, if we could find a way to protect the general population from the groundwater while remediation was carried out, the public’s interest would be served and health would be protected.
Our approach involved two aspects: technical and legislative.
Giving natural processes a chance to work
The technical aspect began with shutting down the ineffective aeration system. Aeration was actually counter-productive, because for chlorinated solvents to be broken down by bacteria in the soil, natural anaerobic conditions in the soil are required. Termination of the aeration system also had the effect of an immediate reduction in costs at the site since operations and maintenance of the system was no longer needed.
We considered drilling injection wells to add bacteria to those already present in the soil but concluded it would be impractical due to the low permeability of the bedrock, and the scant amount of groundwater flow. This environmental condition has the benefit of preventing the plume from migrating, but also means that any bacteria injected into the formation would not spread very far from the injection point. Adding enough wells to populate the soil with supplementary bacteria would have been cost-prohibitive, and the drilling would have added to the carbon-emissions impact of the project.
Since 2008, semi-annual groundwater monitoring has been performed to evaluate the restoration of anaerobic conditions. Monitoring indicated a stable plume and significant attenuation of chlorinated ethene concentrations in site groundwater and continues to confirm the continued attenuation of chemicals of concern. Dissolved oxygen and oxidation reduction potential concentrations have also shown a continuing trend toward favorable oxidation-reduction conditions since termination of the operation of the groundwater remediation system. To date, we have noted a decline in contaminant levels of PCE from an average of 72 parts per million to an average of 20, an approximately 70 percent decrease.
Finding legislative ways to protect the public
Evaluations of the chlorinated ethene trends indicated COC concentrations would attenuate to below their respective PCLs within the TCEQ-defined reasonable time period of 15 years. However, while significant attenuation has occurred at the site, concentrations above PCLs continue to be detected in some wells. As a result, a Municipal Settling Designation (MSD) is being sought. By restricting the use of groundwater, human health and the environment are protected while also promoting the continued economic development of the area.
We are currently working with the City of Odessa to implement a Municipal Settling Designation (MSD), which is an official state designation that certifies that groundwater at the property is not used as potable water because that groundwater is contaminated in excess of the applicable drinking water protective concentrations. As part of the MSD process, the City also agrees to prohibit the drilling of potable use wells in places that would draw water from the impacted property. Our obligations included notifying all the owners of wells – about 2000 in total – within five miles of the property limits.
This case study demonstrates how site management based on monitored natural processes and regulatory covenants can protect human health and the environment in a cost-effective manner.
* At the time of this assignment Pastor, Behling & Wheeler, LLC (PBW) was the contract entity. Golder acquired PBW in May 2018.
