SCALE-DEPENDENT HYROGEOLOGIC CONTROLS ON GROUNDWATER CONTAMINATION AT AN INDUSTRIAL SITE IN THE RIO GRANDE VALLEY AREA OF NORTHWESTERN ALBUQUERQUE, NEW MEXICO

Scale-dependent hydrogeologic controls on shallow groundwater contamination have been investigated during the past 17 years at a former industrial site in a valley-border area of NW Albuquerque adjacent to the Rio Grande floodplain. The site is on a low alluvial terrace about 3,000 ft WNW of the river channel and 1,000 ft SW of lower Arroyo de las Calabacillas. The main shallow aquifer affected comprises thin river-valley fill (latest Quaternary) and underlying basin fill (upper Santa Fe Group--mainly Pliocene). Depth to the water table at the site is about 75 ft and flow gradient is westward. Major contaminants in the vadose and phreatic zones are dense industrial solvents (DNAPLs), including TCE, used in production of electronic equipment. Merging two hydrogeologic databases of significantly different scales was the major goal of our study. This presentation illustrates integration of a ‘macro-scale’ (basin-wide) conceptual model, recently developed for groundwater-resource management in the Albuquerque Basin, with a ‘micro- to meso-scale’ model used for characterization of the industrial site. Detailed subsurface studies and mapping of surficial deposits in a six square-mile area surrounding the site have facilitated construction of a 3-D physical model of basin and valley fills potentially affected by groundwater and vadose-zone contamination. The model-base elevation is 3,400 ft, or about 1,600 ft below the water table. Six hydrogeologic cross sections, which can be arranged as a fence diagram, illustrate the detailed (shallow) and general (deep) distribution patterns of six major lithofacies assemblages. These compositional and genetic classes are grouped into six hydostratigrphic units comprising two (informal) upper and middle Santa Fe Group subdivisions, and four valley-fill units (including fluvial and arroyo facies). Detailed borehole and piezometer data from the site area document the importance of ‘micro- and meso-scale’ features, primarily fine-grained floodplain facies, in restricting downward and directing lateral (westward) contaminant movement.