Background

The town of Artesia is located in the southeast corner of New Mexico (Fig. 1). The Artesia Yard, which is used for a variety of road repair and maintenance activities, is located along the west side of town on flat terrain that slopes from west to east toward the Pecos River.

On May 17, 1993, two underground storage tanks (USTs) at the Artesia Yard failed tank-tightness tests. A representative of the New Mexico Environment Department (NMED) confirmed the release on August 2, 1993, when the tanks were removed (Fig. 2). In addition to removing the tanks and associated piping, which were the primary sources of contamination, the excavation contractor removed approximately 100 yards of hydrocarbon-contaminated soil from the former tankhold.

The initial consultant for the NMSHTD conducted an investigation in the fall of 1993, installing seven soil borings to determine the extent of contamination (INTERA/BAI 1994a). The supervising geologist described the sediments as massive to poorly stratified, silty clay/clayey silt resting on a thick, hard clay that occurred between about 85 and 96 feet below ground surface (bgs). The borings intersected a zone of perched groundwater extending from about 60 feet to about 80 feet bgs, and three of the borings were converted to perched-zone monitor wells. The supervising geologist also observed occasional caliche layers between the surface and the perched groundwater table.

After the primary source (the underground tanks and piping) and accessible contaminated soil were removed, recovery of the PSH that had collected in the perched-zone wells was initiated. Due to the fortuitous presence of the perched aquifer (the regional groundwater occurs at about 150 feet bgs), the PSH pool occurred between 55 and 60 feet bgs in the alluvial sediments. Between November 1993 and February 1994, NMSHTD personnel removed PSH by hand bailing the three monitor wells, recovering about 400 gallons of PSH from the perched groundwater surface.

\ I Monitor well i| Cros? sattion location

Figure 2. Site layout

\ I Monitor well i| Cros? sattion location

Figure 2. Site layout

In January 1994, personnel from INTERA/BAI installed five additional monitor wells to further define the extent of soil and perched groundwater contamination (one well installed below the hard clay perching layer remained dry and was abandoned). They discovered that the petroleum hydrocarbons had penetrated the vadose zone down to the perched water table, where the PSH then spread laterally (Figs. 3 and 4). The results of four slug tests suggested that the hydraulic conductivity ranged from about centimeters per second. The actual values are probably lower, given the fine-grained nature of the alluvial deposit, as described by the field geologist.

In late April 1994, one additional monitor well (MW-9) was installed in an attempt to define the southern boundary of the contaminant plume. The consultant also conducted a pilot test to determine the design parameters for an SVE system (INTERA/BAI 1994b). The pilot test was done using 50 inches water column (wc) vacuum, resulting in a well flow rate of 3 cubic feet per minute (cfm) and extracted vapor concentrations of less than 1,000 parts per million volume (ppmv). The radius of influence (ROI) suggested by the test results ranged from about 6.8 to 8.6 feet, which would result in an extraction well spacing of 10 to 15 feet.

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