New Mexico Geological Society Annual Spring Meeting — Abstracts

A contaminant plume has been identified in the floodplain below the U.S. Department of Energy's Uranium Mill Tailing Remedial Action at Shiprock, New Mexico. Movement the plume is controlled by behavior of the unconfined aquifer within the floodplain. To characterize the unconfined aquifer we used geologic data from monitoring well water level measurements, electrical conductivity (EM), and reflection seismic data. Lithologies from monitoring well logs and seismic reflection were used to define the floodplain stratigraphy and the paleotopgraphy map. Water level measurements were collected on a monthly basis, to detennine the interaction of the San Juan River on the floodplain. A seismic reflection survey defined the alluvium, gravel, and shale interfaces, fractures and offsets within in the floodplain stratigraphy. The stratigraphy consists of alluvial gravels overlying coarser outwash gravels which are deposited an erosional terrace cut into Mancos shale. Paleo-channels can be identified by fluctuations in lithology elevations from the monitoring well log data. The larger outwash gravels currently a major factor in controlling the water and contaminate flow directions in the floodplain. The outwash-gravels contain larger pore space than the alluvium. An the preferential flow pattern is in the outwash gravels. Movement of the contaminant plume was determined from chemical water analysis from existing wells over several years. EM-31 and EM-38 surveys traversed the floodplain to determine the vertical and horizontal extent of a salt contaminant plume. Comparison of chemical analysis and EM-31/38 readings were used to determine if movement ofthe plume varies with flows in the San Juan River. Correlation of all four results indicates the general direction of flow of groundwater and how the lithology influences the groundwater and contaminant movements within the floodplain.