Vertical Extent of Ground-Water Contamination with Organic Solvents in Grants, New Mexico

Science Center Objects

The City of Grants is located in Cibola County within western central New Mexico and is included within the Bluewater Underground Water Basin. Major aquifers within the Grants area include the Alluvium and basalt flows of Quaternary age and the San Andres Limestone and the Glorietta Sandstone of Permian age. Contamination of shallow ground water with volatile organic carbon compounds including chlorinated organic solvents and BTEX compounds has been documented near Grants. The areal extent of the contamination has been defined but information is lacking on the vertical extent of the contamination. The objectives of the proposed work are to collect and analyze ground water samples to define the vertical extent of ground water contamination, to analyze soil gas samples to better define the potential sources of ground-water contamination, and to collect basic hydrologic information from the aquifer in and near the contaminated area.

BACKGROUND AND PROBLEM

The City of Grants is located in Cibola County within west central New Mexico and is included within the Bluewater Underground Water Basin. Major aquifers within the Grants area include the Alluvium and basalt flow of Quaternary age and the San Andres Limestone and the Glorietta Sandstone of Permian age. The San Andres Limestone and the Glorietta Sandstone are considered to be one aquifer in the Grants area. Historical water levels are shown for the basalt of Quaternary age near the location of well 115 and for the San Andres-Glorietta aquifer at location 112. Deposits of Quaternary age near Grants are about 95 feet thick and the San Andres-Glorietta aquifer is about 189 feet thick near Grants. Deposits of Quaternary age and the San Andres-Glorietta aquifer near the location of well 116 are separated by about 167 feet of Chinle Formation deposits of Triassic age.

Contamination of shallow ground water with dissolved chlorinated organic solvents and BTEX compounds has been documented near Grants (Appaji Sairam, U.S. Environmental Protection Agency, written commun., 2004). The areal extent of the contamination has been defined but information is lacking on the vertical extent of the contamination. Also, some uncertainty remains as to the suspected source locations of ground-water contamination. In addition, basic hydrologic information such as water levels and hydraulic conductivities in the aquifer contained in rocks of Quaternary age are largely unavailable.

OBJECTIVES AND SCOPE

The objectives of the proposed work are to collect and analyze ground water samples to define the vertical extent of ground water contamination, to analyze soil gas samples to better define the potential sources of ground-water contamination, and to collect basic hydrologic information from the aquifer in and near the contaminated area. The scope of the project includes the area including and immediately surrounding the contaminated area.

RELEVANCE AND BENEFITS

Completion of the proposed work will provide the U.S. Geological Survey (USGS) with additional information concerning the fate and transport of chlorinated hydrocarbons and BTEX compounds in a shallow aquifer system in a southwestern urban setting. The data will add to the USGS database and will assist in understanding and describing the Nation’s water resources. In addition, the USGS will benefit from the development and refinement of methods to determine the vertical extent of ground-water contamination. These methods could be applied to similar sites elsewhere in the nation.

The findings of the study will provide U.S. Environmental Protection Agency (USEPA) and the New Mexico Environment Department with reliable and impartial hydrologic data that contributes to protection of life and property at Grants. Specifically, the work will better define the vertical extent of chlorinated hydrocarbon and BTEX compound contamination and refine determinations as to the source or sources of the contaminants. The public will benefit through the use of information generated by this and other studies of the Grants site to remediate the contaminated ground water. Information from this and other studies can be used to raise the awareness of the general public about the source of their water supply and the vulnerability of shallow aquifers to contamination.

APPROACH

Direct-push technology will be used to collect ground-water samples to determine the vertical extent of contamination in and near the mapped areal extent of contamination. Eleven areas represent the desired sampling locations. Exact sampling locations will be subject to site accessibility. Ground-water samples will be collected approximately every 5 feet between the water table and 85 feet total depth or the depth of rejection using direct-push technology. Sample collection will involve use of a screened ground water sampler covered by a retractable sheath. The sampler will be inserted into the subsurface following direct-push penetration to the desired depth. Ground water will be drawn into the unsheathed sampler and brought to the surface using a vacuum pump. Bore holes generated by direct-push methods will be plugged by filling the hole with sand to within approximately six inches of land surface. The top six inches of the hole will be filled with bentonite pellets. Any asphalt penetrated during direct-push operations will be sealed with an asphalt patching material.

Ground-water samples will be analyzed for organic compounds in the field using a Photovac Voyager portable gas chromatograph (GC). Ground-water samples will be collected in 40-milliliter septated glass containers that are certified organic free. Five milliliters of fluid will be removed from the sample and replaced with ambient air. The sample will be agitated for about one minute, allowed to settle for a few seconds, then a sample of the headspace will be withdrawn through the septum using a clean gas-tight syringe. A specific volume of sample will be injected from the gas-tight syringe directly into the GC for analysis. Specific analytes eluted by the GC and analyzed on site include BTEX compounds (including benzene, toluene, ethylbenzene, meta- and para- xylenes, ortho-xylene, and naphthalene) and chlorindated ethenes (include perchloroethene, trichloroethene, dichloroethene, and vinly chloride). Specific compounds will be identified by retention time and comparison with a calibration standard. Concentrations will also be determined by comparison with a calibration standard using a two- or three- point calibration curve. Quantitation is expected to be + 10% or better. Split samples will be sent to an Environmental Protection Agency Laboratory to validate selected field analyses and identify any other volatile and semi-volatile contaminants beyond the capability of the field GC.

Five to six soil gas samples will be collected near the suspected source areas of ground-water contamination. Sampling locations generally will bracket the facilities thought to be the source of ground-water contamination, but also will be determined by accessibility. Soil gas samples will be analyzed using the GC in the same manner as headspace samples.

Up to 30 existing wells open to the aquifer in rocks of Quaternary age within and near the contaminated area will be sampled for volatile organic compounds including chlorinated solvents and BTEX compounds. Ground water samples will be collected from the discharge of a submersible pump lowered into a well. Samples will be collected after measured values of pH, temperature, and specific conductance reach stable values in the discharging water. Discharge rates will be limited to no more than one liter per minute to minimize loss of volatile organic compounds during sample collection. Waste water collected during well sampling will be disposed of at a nearby public liquid waste disposal facility.

Water-level measurements and slug tests will be performed on existing wells following sampling. Water levels will be measured using a steel tape or an electric tape. Ground-surface elevation will be estimated using topographic maps of the appropriate areas. Slug tests will be performed by monitoring changes in water levels on displacement of a known volume of water from the well bore. Water will be displaced using a solid object of know volume thereby eliminating the need for waste water disposal. Water levels will be measured during the slug test by an electronic sensing device and recorded electronically. Hydraulic conductivities of the rocks of Quaternary age will be estimated for slug test results using the Bouwer-Rice or a similar method at a future date depending on new or continued project funding.