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Ip And Tem For Discrimination And Resolution In Mapping Groundwater Contamination At Monument ValleyNormal access

Authors: S.K. Sandberg, N.T. Rogers, K.E. Karp, C.S. Goodknight and L.F. Spencer
Event name: 11th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Methods Emphasis: Use Of The Induced Polarization Method
Publication date: 22 March 1998
Organisations: EEGS
Language: English
Info: Extended abstract, PDF ( 698.34Kb )

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

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