Theme: 02. Assessing Remediation Effectiveness

Topic Area: 02b. Remedy Implementation: Assessing Performance and Costs

Formatted Title:

11-Year Performance Monitoring of Enhanced Reductive Dechlorination within Fractured Bedrock at a Densely Instrumented Site

Background/Objectives:

Chlorinated solvent use at an historical tannery in the UK has resulted in impacts to groundwater within both superficial deposits (River Terrace Deposits) and the underlying fractured limestone bedrock (Holywell Nodular Chalk/Zig Zag Chalk). Dissolved-phase chlorinated ethenes in groundwater extend greater than 2 kilometres northwest from the former tannery and are causing failure of water quality objectives at a disused public groundwater supply and nearby surface water features. Partial reductive dechlorination of parent solvents tetrachloroethene and trichloroethene is naturally occurring with the presence of daughter product cis-1,2-dichloroethene and lower concentrations of chloroethene. Enhanced reductive dechlorination (ERD) was identified as a potential remediation technique for managing the dissolved-phase migration from the former tannery but required pilot testing to evaluate efficacy and longevity in fractured bedrock.

Approach/Activities:

A pilot trial of ERD remediation was completed to treat part of the dissolved-phase chlorinated ethene plumes present in both the superficial and bedrock aquifers. The trial was completed in an amenity field immediately down-gradient of the historical tannery. The objective of the trial was to stimulate biological and/or abiotic reductive dechlorination of the chlorinated ethene contamination through the injection of a carbon source (electron donor) either in isolation (ELS® shallow bedrock) or combined with an iron component (EHC® (superficial aquifer) and EHC®-L (intermediate bedrock). The trial commenced in December 2011 with a single injection event, with a further injection event targeting shallow bedrock, that had not been previously targeted, in March 2013. Bioaugmentation was also completed in both the superficial and bedrock aquifers in March 2012 through injection of Dehalococcoides bacteria.

Thirteen rounds of performance monitoring were completed over an 11-year period. Monitoring infrastructure was installed across a 0.5-hectare area and comprised two bedrock reagent injection wells (one shallow, one intermediate depth), thirteen 7-port and five 3-port CMT multi-level systems to depths of up to 55 metres (m) with a further nine groundwater and three vapour wells installed within the superficial aquifer. Together with existing bedrock wells, information on groundwater level and quality was available at 120 discrete monitoring points across four transects: up-gradient of the pilot trial (groundwater leaving the former tannery); 5-10 m down-gradient; 40 m down-gradient; and 85 m down-gradient of reagent injection.

Results/Lessons Learned:

The additional plume delineation afforded by the monitoring infrastructure indicated that over 93-percent of the chlorinated ethene mass discharge occurred within the upper 21 m of bedrock. The total chlorinated ethene mass discharge was calculated to be 0.7 kg/day, compared to an historical mass discharge of 1.1 kg/day based on impact at receptors (1983-1994).

Within the superficial aquifer down-gradient of the EHC® injection conditions were not conducive to complete dechlorination. The addition of further organic carbon, as a side-effect of the ELS® injection into shallow bedrock in March 2013, indicated that complete dechlorination was achievable, but high sulphate concentrations provided a strong competing hydrogen demand.

Based on ethene concentrations detected within bedrock in June 2013 5-10 m down-gradient of the ERD reagent injection (EHC-L® and ELS®) the pilot trial had completely dechlorinated approximately 12 percent of the total chlorinated ethene mass discharge from a single injection of each reagent.

Over the subsequent 9.5 years the degree of complete dechlorination declined away from the injection centreline, but sampling locations on the injection centreline generally maintained a high degree of complete dechlorination. It is concluded that the pilot trial has successfully demonstrated the efficacy and longevity of ERD within fractured bedrock and potential next steps will be outlined.

David Granger

Affiliation

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(Group 1, Poster Board #43) 11-Year Performance Monitoring of Enhanced Reductive Dechlorination within Fractured Bedrock at a Densely Instrumented Site