New Mexico Geological Society Annual Spring Meeting — Abstracts

The Sandia North Groundwater Investigation is currently studying the occurrence of trichloroethene (TCE) in groundwater at concentrations exceeding the EPA's Maximum Contaminant Level (MCL) of 5 ppb. Studies completed include drilling/installing groundwater monitoring wells, soil-vapor surveys, borehole geophysics, colloidal borescope surveys, groundwater age dating, and quarterly monitoring.

Groundwater in the study area occurs both in the regional aquifer and in a localized shallow groundwater system along the eastern margin of the Albuquerque Basin. The regional aquifer is Albuquerque's drinking water source and is 500 to 600 ft deep in unconsolidated sediments of the upper unit of the Tertiary Santa Fe Group. Flow direction in the study area is north-northwest toward municipal well fields.

The shallow system consists of multiple, perched saturated intervals 175 to 275 ft above the regional aquifer. This system has been identified only in the north-central portion of Kirtland Air Force Base and has a flow direction to the southeast. This system probably is controlled by fine-grained sediments within the alluvial fan facies and does not appear to be hydraulically connected to the regional aquifer in the study area. The source of the water in the shallow system is still under investigation, although an anthropogenic origin (such as leaking water-supply infrastructure) is currently suspected.

TCE has been detected in 5 of 12 shallow-system monitoring wells and exceeds the MCL at 3 of the 5 locations. Other chlorinated compounds have been detected in some of the shallow-system wells at concentrations below their respective MCLs. This low-level contamination is laterally widespread yet discontinuous. To date, investigations of surface releases to identify potential groundwater contaminant sources in the study area are inconclusive, but a likely candidate is an extensive underground waste-water system.