Remediation of BTEX in Groundwater with LNAPL Using Oxygen Releasing Materials (ORM).

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Abstract: A field scale study was conducted at a former gasoline service station to evaluate the remediation of groundwater contaminated with dissolved benzene, toluene, ethylbenzene, and xylenes (BTEX) using oxygen-releasing materials (ORM). The injection of oxygen through ORM slurry was the remedial alternative chosen to enhance the in-situ microbial degradation of the BTEX in groundwater. Direct-push drilling methods were used to inject the ORM into the source area of impacted groundwater in a grid pattern with five-foot centers. The remediation of residual pockets of light, non-aqueous phase liquids (LNAPL) was enhanced by the injection of the ORM slurry, which forced the LNAPL to flow into existing monitoring wells where it could be collected with a bailer. The results of groundwater sampling show increased post-injection dissolved oxygen (DO) concentrations and a 16% to 87% decrease in BTEX concentrations in the wells located within the injection grid. The disappearance of dissolved iron (Fe2+) in groundwater within the injection grid indicates that there is sufficient oxygen to oxidize iron. The analytical data shows that the groundwater has been enriched in dissolved oxygen and BTEX concentrations have decreased significantly within the area treated with ORM.

INTRODUCTION

A field study was conducted at a former gasoline service station in Cincinnati, Ohio to remediate groundwater contaminated with dissolved benzene, toluene, ethylbenzene, and xylenes (BTEX). The site was operated as a gasoline service station until operations ceased in the late 1970’s. The underground storage tanks (USTs) were removed in 1981. The former USTs were located near the center of the site, and a former gasoline dispenser was located northeast of the USTs (Figure 1). A total of seven groundwater monitoring wells, MW-1 to

MW-7, were installed at the site between May and October 1996 for site assessment activities. Soil samples were collected during the installation of the wells, and groundwater samples have been collected from each well.

The geology of the site consists of silty clay from surface grade to depths ranging from 8 to 12 feet (2.4 to 3.7 m) below ground surface. The soil grades into silt with variable amounts of sand below the silty clay from 12 to 29 feet (3.7 to 8.8 m), then the soil grades back into silty clay from 29 feet to 31 feet (8.8 to 9.5 m), the greatest depth sampled at the site. The former UST pit consists of silty clay fill material with some gravel and large concrete fragments. The monitoring wells are screened through the bottom of the silty clay layer into the saturated silt and sand layers. Groundwater generally flows to the southwest, parallel to the road, with a hydraulic gradient of 0.05 feet per foot (Figure 1).

FIGURE 1. Site Plan showing site features and monitoring well locations.

 

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