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Rocky Mountain (53rd) and South-Central (35th) Sections, GSA, Joint Annual Meeting (April 29–May 2, 2001)

Paper No. 0
Presentation Time: 10:10 AM

HYDROGEOLOGIC CONTROLS ON VOLATILE ORGANIC COMPOUND CONTAMINATION IN A SHALLOW GROUNDWATER SYSTEM WITHIN THE ALBUQUERQUE BASIN, AT TIJERAS ARROYO, NEW MEXICO


SKELLY, Michael F.1, COLLINS, Sue S.2, COPLAND, John R.3 and THARP, Tommy L.1, (1)Roy F Weston, Inc, 6501 Americas Pkwy NE Ste 800, Albuquerque, NM 87110-8146, (2)Department 6133, Sandia National Lab, MS-1087, PO Box 5800, Albuquerque, 87185, (3)Sci Applications Int'l Corporation, 2109 Air Park Road SE, Albuquerque, NM 87106, mfskell@sandia.gov

The Tijeras Arroyo Groundwater Investigation is currently studying the distribution of trichloroethene (TCE) in groundwater at SNL/NM. Groundwater occurs in both the regional aquifer and in a localized perched aquifer. The regional aquifer is Albuquerque's drinking water source and is at a depth of 500 to 600 ft in unconsolidated sediments of the upper unit of the Tertiary Santa Fe Group. Flow direction in the study area is north-northwest toward production wells. Groundwater travels preferentially through highly conductive sediments associated with the ancestral Rio Grande.

The perched aquifer consists of multiple, saturated intervals 175 to 275 ft above the regional aquifer. This system has only been identified in the north-central portion of SNL/NM and does not appear to be hydraulically connected to the regional aquifer. Flow in this system is probably to the southeast and is likely controlled by gently dipping beds of fine-grained alluvial fan sediments.

The two aquifers are geochemically distinct, with the regional aquifer exhibiting more consistent geochemistry both over time and between wells. The perched water is more geochemically variable and has concentrations of calcium, chloride and sulfate generally greater than found in the regional aquifer.

Water levels in the two aquifers currently represent transient conditions possibly due to cessation of anthropogenic recharge. Over time, water levels in both the regional and perched aquifers are increasing in elevation in the southeastern portion of the study area (toward the mountain front), and are decreasing in the northwestern portion of the study area (toward water supply wells).

TCE has been detected in 5 of 12 shallow-system monitoring wells. Concentrations exceed the regulatory limit of 5 mg/L at two of the five locations. Vapor-phase source(s) of TCE are currently suspected; the fine-grained alluvial sediments and lack of recharge hinder downward transport of the vapor-phase TCE.

[Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.]

Session No. 1

S4. Hydrogeology of the Middle Rio Grande Basin
Monday, 30 April 2001: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 33, No. 5, April 2001, p.2
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