Abstract

Two separate hydrocarbon plumes located along the Zuzax fault near Tijeras, New Mexico have migrated into both unconfined and semiconfined aquifers in the Paleozoic Abo and Madera Formations and have contaminated several domestic and commercial wells in the area. Groundwater flow beneath the site occurs in fractured sandstone and limestone beds which are separated by mudstone and shale aquitards. An extensive benzene, toluene, ethylbenzene, xylenes (BTEX) and methyl-tert-butyl ether (MTBE) plume and a smaller BTEX/EDC (dichloroethane) plume originate from separate locations. In both source areas, phase separated hydrocarbons (PSH) are present in perched water zones within fractured siltstone or sandstone beds. In the BTEX/MTBE source area, the perched water zone leaks into a deeper aquifer utilized by domestic and commercial water users in the area. Under the influence of down gradient pumping stresses, hydrocarbon contamination of the aquifer located below the BTEX/MTBE contaminant source at a depth of 60 to 80 feet has migrated vertically along a splay of the Zuzax fault into a semiconfined aquifer located at a depth of 110 to 190 feet below ground surface. The BTEX/MTBE contaminant plume has subsequently migrated approximately one-half mile down gradient along the northeast trending Juniper Ridge half-graben which parallels the Zuzax fault. Pumping test data and contaminant plume geometry indicate that graben bounding faults are low-permeability barriers to horizontal ground water flow and contaminant migration, but likely act as pathways for vertical ground water flow and contaminant migration. Pumping tests conducted on wells completed into each aquifer show strong boundary effects from graben bounding faults, exhibit dual porosity characteristics, and demonstrate communication between the upper two aquifers. Hydraulic conductivity (k) values for the upper two aquifers range from fracture k values of 110 ft/day and matrix k of 12 ft/day for upper Aquifer A to a fracture k of 40 ft/day and a matrix k of 1 ft/day for underlying Aquifer B. Fracture k of lowermost Aquifer C is 15 ft/day; matrix k could not be determined from the Aquifer C pumping test. The aquifer test data demonstrate a decrease in fracture and matrix k with depth. Estimated solute transport rates are 3 ft/day for Aquifer A and 1.2 ft/ day for Aquifer B.