Using AquaBupH to adjust aquifer PH: ESTCP-funded project at Charleston Naval weapons station - Case Study

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Courtesy of EOS Remediation, LLC

Elevated levels of trichloroethene (TCE) were found in groundwater at a Department of De-fense (DoD) facility in Charleston, SC. Under an ESTCP-funded in situ technology demonstra-tion project , the Navy had been effectively remediating contamination until a drop in aquifer pH caused dechlorination rates to decline. Solutions-IES responded with a study and applica-tion of buffered substrate to adjust the aquifer pH and restart bioremediation.

The Challenge
The DoD’s Charleston Naval Weapons Station (NWS) found up to 18,000 ìg/L TCE in groundwater beneath an area used for surface disposal of solid waste, oils and missile components between 1950 and 1978. The 180ft X 90ft rectangular-shaped source is located in a remote wooded area near a power line easement over a shallow, but relatively tight silty clay formation. The groundwa-ter potentiometric surface is flat with minimal tidal influence. Depth to water table varies season-ally between 0.5ft and 6ft below ground surface (bgs). Hydraulic conductivity of the surficial aqui-fer is on the order of 1 to 10 ft/d. Groundwater flow velocity is only 1 to 5 ft/yr. Although tight silty clays hampered proper distribution, emulsified oil substrate proved effective to stimulate removal of TCE and formation of cis-1,2-dichloroethene (cDCE) until a drop in aquifer pH below 6 caused dechlorination rates to decline.

The Strategy
Solutions-IES selected emulsified oil substrate, EOSPRO (formerly EOS 598B42), because of its proven track record for promoting in situ reductive dechlorination in groundwater. With approval from South Carolina Department of Health and Environmental Control (SCDHEC), a 20x20 ft. pilot-study treatment grid was established to observe EOSPRO, first unbuffered in a paired well recirculation system, followed by a second phase with AquaBupH (buffered substrate) using push injection tech-nology. Solutions-IES monitored performance over 42 months comparing biodegradation, geo-chemical and microbial performance parameters.

The Design
Phase I: Sixteen 18-ft deep injec-tion wells, 5-ft on center (FIG. 1), were paired to inject and recirculate; then pairs reversed for more injection and re-circulation (approx. 84 hrs). Total injected = 165 gal EOSPRO (1260 lbs) diluted in water 1:4. Post-injection perfor-mance monitoring for 29 months.
Phase II: Injected 326 gal (3030 lbs) of AquaBupH via 20 direct push points 5-ft. on center within the existing grid. Post-injection performance monitoring for 13 months.

Results
Substrate Injection: The Phase I approach of recircula-tion via injection wells in the low permeability environ-ment was complicated and time consuming (FIG. 2), yet successfully distributed substrate throughout the treat-ment grid. Low pressure direct injection through the Ge-oprobe® injection tool during Phase II (FIG. 3) overcame the challenges of injecting into the relatively low permea-bility silty clay, although some groundwater mounding and substrate breakout occurred.

Geochemical Changes: Adding substrate during Phase I resulted in dissolved oxygen (DO) removal, decrease in oxidation-reduction potential (ORP) and ferrous iron (Fe+2) production. With the addition of AquaBupH in Phase II, ORP decreased further and methane was produced; Fe+3 complexes formed after pH increased.

Electron Donor Supply: Within 20 days of Phase I injec-tions, total organic carbon (TOC) and volatile fatty acids (VFAs) increased with effective distribution of the sub-strate’s more soluble components. TOC increases occurred again shortly after Phase II buffered injections with pH improvements from 4.9-5.3 to 6.4-7.7 in soil samples . Af-ter three months, injection wells and monitor wells showed increases to pH 6.2 and 8.5, respectively. After one year, these wells remained close to pH 6.0 and 7.5, respectively.

Biodegradation: TCE was reduced by 86% and 99% in injection wells and monitor-ing wells, respectively, over 29 months following Phase I with cDCE concentration in-creases 11-fold and 9-fold in the same wells. However, there was relatively little vinyl chloride (VC) or ethene formation. After Phase II in-jections with AquaBupH , pro-nounced stimulation of the reductive dechlorination pro-cess occurred with substan-tial increases in VC and eth-ene concentrations (FIG. 4). Five years after original in-jection, TCE and cDCE were undetectable in injection zone and VC was trending downward.

Mass Flux: Prior to treatment, the total mass flux through the pilot test area was 0.63 kg/yr (4.76 mole/yr) of TCE and 0.02 kg/yr (0.17 mole/yr) of cDCE. Following treatment, the total mass flux was reduced to 0.01 kg/yr (0.055 mole/yr) of TCE and below detection for cDCE.

Customer comments

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