As part of characterization activities at the UMTRA Monument Valley Project Site in<br>Arizona, a surface geophysical survey was performed from May 6 to 15, 1997. One of<br>the primary objectives of this survey was to provide a more comprehensive understanding<br>of the extent of nitrate and sulfate contaminant plumes in the alluvial aquifer. An analysis<br>of IP, resistivity, and TEM datasets, in conjunction with lithologic and groundwater<br>sampling data horn existing wells, indicates that the groundwater contamination plume is<br>detectable as a low resistivity zone which is contained predominantly within the alluvial<br>sediments. IP soundings obtained directly over the low resistivity zone indicate that it is<br>a low chargeability horizon with higher chargeability units above and below. In a<br>comparison with data from monitoring wells, simultaneous resistivity/IP and<br>resistivity/IP/TEM modeling indicates the low resistivity/chargeability horizon coincides<br>in depth and thickness with the depth and thickness of saturated alluvial sediments above<br>the bedrock. In addition, the lateral extent of the low resistivity zone is consistent with<br>groundwater contamination observed in monitoring wells screened in the alluvial aquifer.<br>This alluvial groundwater contamination plume appears to extend from the former<br>tailings pile area in a northerly direction for at least 4500 ft, based on contoured apparent<br>resistivity from TEM soundings, using 20 ps sample data (285 Hz base frequency). The<br>width of the groundwater contamination plume 2700 feet down gradient from the former<br>evaporation pond is at least 500 feet wide. The vertical extent of the alluvial groundwater<br>contamination plume appears to be constrained from above by the top of the saturated<br>zone of the water table, and below by the top of the bedrock surface. Bedrock is<br>comprised of the Shinarump Member of the Chinle Formation, composed primarily of<br>firmly cemented conglomerate and sandstone.


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