--> Abstract: Enhancement of In Situ Natural Bioremediation with Oxygen Release Compound®, by J. S. Roemmel and K. S. Burgess; #90937 (1998).

Datapages, Inc.Print this page

Abstract: Enhancement of In Situ Natural Bioremediation with Oxygen Release Compound®

ROEMMEL, JANET S., C.P.G.; and KRISTINE S. BURGESS, P.E., SECOR International Incorporated

Introduction

A release of gasoline of an unknown volume from an underground storage tank (UST) system resulted in the contamination of soil and groundwater. Three 12,000-gallon gasoline USTs were permanently closed by removal from a former convenience store in northern Utah in 1991. A “thin, inconsistent layer” of free-phase gasoline was observed on the groundwater in the UST excavation. The excavation was backfilled with gasoline-contaminated soils, in accordance with the standard practice of the day.

The Utah Department of Environmental Quality (DEQ) Leaking Underground Storage Tank Program required the convenience-store operator to remediate the contamination. Historically, benzene was the only compound that exceeded the clean-up level in groundwater; the benzene-contaminated groundwater extended off site beneath the adjacent road. Benzene and gasoline-range total petroleum hydrocarbons (TPH) exceeded soil clean-up levels in an area on site limited to the former tank and dispenser area. No free-phase petroleum has been observed in the groundwater monitoring wells.

Regulatory Framework

The Utah DEQ has set two types of clean-up standards:

1) Risk-Based Corrective Action (RBCA) Tier I Screening Levels, and

2) The more stringent water quality standards and Recommended Contaminant Levels (RCLs).

RBCA Tier 1 Screening Levels apply for soil and groundwater at sites where there are no sensitive receptors within 30 feet of the source area. All other sites are subject to Utah water quality standards, which include Federal drinking water Maximum Contaminant Levels for groundwater, and RCLs for soil.

RBCA Tier 1 Screening Levels do not apply at this site because there are several sensitive receptors within 30 feet of the source area. By DEQ definition, these receptors include property lines and several underground utilities. Thus, the operator must remediate the soil and groundwater to the more stringent DEQ standards. The benzene clean-up level is 5 micrograms per liter (µg/L) for groundwater and is 0.200 milligrams per kilogram (mg/Kg) for soil. A gasoline-range TPH clean-up level for groundwater has not been established. The gasoline-range TPH clean-up level for soil is 30 mg/Kg.

This site is typical in that the clean up effort is focused on the ability to reduce the concentration of benzene, a carcinogen, in the soil and groundwater. To measure concentrations of benzene, toluene, ethylbenzene, and total xylenes (BTEX) the DEQ requires laboratory analysis using EPA Method 8020 for soil and EPA Method 602 for water. Remedial efforts are also often driven by reduction of gasoline-range TPH. The DEQ requires analysis of soil or groundwater for gasoline-range TPH by EPA Method 8015 modified.

The DEQ established the Petroleum Storage Tank Fund to reimburse UST owners for costs incurred to conduct the site assessment and remediation of eligible releases from USTs. The operator at this site cannot receive reimbursement from the Fund because the release was discovered in the spring of 1991 before the Fund was established. Cost-effective alternatives for clean up were emphasized because the UST operator is responsible for the entire assessment and remediation cost.

Environmental Setting

Groundwater at the site flows from northeast to southwest at an average hydraulic gradient of 0.05 feet per foot, with an observed 4-foot groundwater table fluctuation over 4 years. At different locations on the site, the depth to groundwater has varied between 1 and 9 feet below ground surface (bgs). The aquifer is unconfined and has an estimated hydraulic conductivity of 0.86 feet per day. The soils are relatively continuous sandy silt and clayey silt (ML), minor silty clay (CL), and silty sand with gravel and clay (SM).

Concentrations of BTEX and TPH in groundwater have remained relatively constant since regular groundwater monitoring began in 1992. Total BTEX concentrations in groundwater in February 1996 are shown contoured on Figure 1 with the highest BTEX at 1,339 µg/L. The highest benzene was 97 µg/L and the highest gasoline-range TPH in groundwater was 3,600 µg/L.

The areal extent of the contaminated soil above clean-up standards in the UST area has decreased from 1,500 square feet in 1992 to 720 square feet in 1996. The highest BTEX concentration in soil in 1996 was 21.78 mg/Kg, the benzene in soil was less than 0.25 mg/Kg, and the highest TPH was 200 mg/Kg.

The relatively constant size and shape of the groundwater BTEX plume also suggests that a soil source has likely continued to add contaminant mass from the smear zone to the dissolved phase.

Conditions Favorable for Natural Bioremediation

Evaluation of site physical and chemical parameters and trends in groundwater analytical data suggest that site conditions are favorable for natural bioremediation.

^bull Laboratory analyses of groundwater samples showed that benzene concentrations had passively decreased by 80 percent from 1992 to 1996,

^bull The areal extent of groundwater contamination has remained relatively constant, suggesting that the centroid of the plume is not moving downgradient by advection or laterally by dispersion.

^bull Petroleum-hydrocarbon concentrations were less than toxic levels for indigenous microbes.

^bull Depleted dissolved oxygen and low oxidation-reduction potential suggested that natural aerobic biodegradation consumed available oxygen and caused reducing, anaerobic conditions.

^bull Mean and maximum temperatures and precipitation at the site occur within typical ranges in which natural bioremediation occurs.

The BIOSCREEN model was used to assess if natural bioremediation was a likely contributor to the degradation observed at the site over the extended monitoring period. BIOSCREEN is a groundwater contaminant fate and transport model that includes biodegradation. Results suggested that the observed contaminant concentrations and distribution were likely to be due to a combination of both aerobic and anaerobic biodegradation processes.

The BIOSCREEN predictions indicated that the BTEX plume would have been about 15 times larger in area than observed in 1996 (Figure 2) if no biodegradation had occurred. The BTEX plume would have been five times larger in area than observed in 1996 if only aerobic degradation had occurred (utilizing the limited naturally occurring background dissolved oxygen of about 1 milligrams per liter). When both aerobic and anaerobic degradation processes were included, a stable plume size was simulated, similar to that observed in 1996.

Remedial Alternative

Localized concentrations of petroleum constituents in the groundwater at the site, although relatively low, still exceeded clean-up levels, and the rate of natural attenuation appeared slow.

Natural attenuation enhanced by Oxygen Release Compound® (ORC®) was selected as the remedial alternative for the following reasons:

1) There was no immediate threat to public health or the environment.

2) The alternative causes minimal disruption to the on-site commercial business.

3) Groundwater and soil contained relatively low concentrations of benzene that did not justify an active engineered remediation system.

4) Remediation duration would be shorter than natural attenuation alone, and

5) Contaminant mass is reduced, not simply transferred to another phase.

ORC® was included to increase the rate of natural biodegradation by increasing the amount of aerobic activity. Since oxygen is typically the limiting factor in aerobic biodegradation, ORC® was employed to augment in situ bioremediation by providing a slow, steady supply of oxygen into the groundwater. Aerobic biodegradation rates are generally faster than anaerobic processes; therefore the increasing aerobic activity is believed to speed up the bioremediation process.

ORC® is a proprietary mixture of magnesium peroxide (MgO2), the active agent, magnesia (MgO), and a few percent of food grade potassium phosphate (KH2PO4 or K2HPO4). When ORC® is hydrated, oxygen is introduced into the groundwater from the chemical reaction:

In this case, oxygen is delivered to the saturated zone using ORC® socks. The socks are filled with the powdered mixture, laced together, and placed in a string throughout the 10-foot-long water column in each of two existing groundwater monitoring wells. The two ORC® are located in the part of the plume with the greatest concentrations of dissolved petroleum compounds.

Monitoring for Success

A key component of this remedial alternative is the on-going, frequent monitoring of remediation progress. The efficacy of the enhanced degradation of contaminants is dependent upon the distribution of oxygen in the groundwater. Remediation progress is assessed by monitoring the groundwater at surrounding and downgradient monitoring points. Monitoring points were installed 5 feet upgradient and 5 feet downgradient of the ORC® wells. Existing groundwater monitoring wells were also used for remediation-progress monitoring. Dissolved oxygen and oxidation-reduction potential are measured each month using field meters. Biannual groundwater sampling and analysis for BTEX and TPH is also conducted. Confirmation soil samples for this site are collected at one event per year.

Since installation of ORC® in two wells, field values of dissolved oxygen (Figures 3a and 3b) and oxidation-reduction potential (Figures 4a and 4b) showed significant increases downgradient of the two ORC® wells. In general, the ORC® was successful in introducing oxygen into the groundwater surrounding the ORC® wells. Benzene concentrations dropped at all monitoring locations.

The duration of the ORC® sock life depends upon site-specific conditions. In this case, monitoring data indicated that ORC® replacement was necessary after approximately 6 months. Based upon monthly monitoring data, the ORC® was replaced in both wells when groundwater conditions reversed to oxygen-depleted and reducing.

Conclusions

Several tools were utilized to evaluate the potential for in situ bioremediation.

^bull Observation of the size, shape, and concentrations of the dissolved BTEX groundwater plume;

^bull BIOSCREEN modeling calibrated by analytical data and known site hydrogeologic conditions; and

^bull Field measurements of the dissolved oxygen and oxidation-reduction potential in site groundwater.

Site conditions were favorable for natural bioremediation of dissolved petroleum contaminants. BIOSCREEN modeling results indicated that a much larger BTEX plume would have been present if natural biodegradation had not occurred. Background concentrations of dissolved oxygen at the site are low; therefore, a significant portion of the biodegradation was attributed to anaerobic processes. Because aerobic biodegradation processes are believed to be more rapid than anaerobic processes, ORC® was added to increase dissolved oxygen concentrations in the petroleum hydrocarbon-contaminated groundwater, thereby enhancing natural aerobic biodegradation. With the installation of ORC® in two wells, subsurface conditions are conducive to more rapid aerobic bioremediation.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah