Abstract

At a natural gas plant and pipeline facility where BTEX had affected the unconfined aquifer beneath it, biosparging appeared to be the method most likely to provide a timely, effective restoration to the applicable regulatory standards. The affected area was relatively large (approximately 80 acres) and heterogeneous with respect to subsurface soil stratigraphy. In view of these site characteristics, the remedial design process included preliminary tests to collect information on which the final biosparge approach was selected and developed into a conceptual design, which was approved by the overseeing regulatory agency. It is the objective of this paper to describe the nature of these preliminary tests and to identify their role in the design process.

The site geology consists of alluvium overlying a buried bedrock surface, which occurs at depths of 40–60 feet below ground surface. The composition of the bedrock is fine-grained sandstone, shale, gypsum and dolomite. Soils consist of a layer of basal sand on top of the bedrock overlain by a zone of interbedded lenticular sands within discontinuous sands, silts and clays, those in turn overlain by less permeable, discontinuous silty and sandy clays, and finally caped by eolian-silt deposits at the surface. Groundwater occurs at depths of 10–20 feet below ground surface.

Preliminary testing included monitoring for the occurrence of intrinsic biodegradation, pilot testing of biosparging using vertical and horizontal air-sparge wells and air-sparging tests using piezometers installed with the cone-penetrometer technique (CPT). The monitoring for intrinsic biodegradation demonstrated its occurrence in the affected area of groundwater thereby confirming the feasibility of biosparging as an enhancement of the naturally occurring process. The pilot testing of vertically and horizontally-drilled wells found that both configurations were equally effective in reducing plume constituents to a degree that would satisfy the remedial goals for the site. In addition, the results of this pilot test developed a design basis for spacing of vertical wells and air-injection flow rates. Finally, the testing of air-injection piezometers installed with CPT confirmed that their performances were comparable to vertically drilled wells with respect to radius of influence.

Introduction

An environmental assessment of a natural gas plant and pipeline compression facility with a combined property of approximately 80 acres indicated that the unconfined aquifer beneath it had been affected by hydrocarbons, primarily benzene, toluene, ethylbenzene and xylene (BTEX). Figure 1 is a schematic diagram of the site showing the approximate extent of the plume. Based on the nature of the groundwater constituents and the characteristics of the site, in-situ bioremediation appeared to be the method most likely to provide for a timely, effective restoration of this aquifer. Of the various in-situ methods, biosparging appeared to be the most applicable. As used here, biosparging means injection of air into an unconfined aquifer through an air-sparging well at a rate sufficient to stimulate biodegradation of organic constituents, but sufficiently limited to prevent significant air stripping of volatile constituents to the atmosphere.

The geology of the site consists of alluvium overlying a buried erosional bedrock surface. The bedrock, occurring at depths of 40-60 feet below ground surface (bgs), is composed of fine-grained sandstone, shale, gypsum and dolomite. The soils consist of a layer of basal sand on top of the bedrock overlain by a zone of interbedded lenticular sands within discontinuous sands, silts and clays which is in turn overlain by less permeable, discontinuous silty and sandy clays, and finally capped by eolian-silt deposits at the surface. Groundwater occurs at depths of 10–20 feet bgs.

As a next step in restoration of the site, tests were performed to evaluate the effectiveness of the selected biosparging approach. This paper presents a description of the methods and findings of these preliminary tests. P. 283^

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