A geotechnical assessment was made of the Northwest Boundary Containment System (NWBCS) at the Rocky<br>Mountain Arsenal near Denver, Colorado. This assessment was conducted to support the Interim Response<br>Action (IRA) Alternative Assessment addressing the NWBCS Long-Term Improvements IRA, and was designed<br>to evaluate the present groundwater and recharge system, assess potential contaminant bypass of the system, and<br>identify remediation requirements of the system. Geophysical techniques were used as part of the geotechnical<br>assessment. Three surface geophysical techniques were used including seismic refraction, electromagnetics (EM),<br>and electrical resistivity soundings. Additionally, limited downhole geophysical logging was conducted for<br>correlation with surface geophysical data. The field program included approximately 32,000 linear feet of seismic<br>refraction data, 52,000 linear feet of EM data, 13 resistivity soundings using the Schlumberger electrode array,<br>and downhole geophysical logging in two boreholes. Seismic refraction data were combined with lithologic log<br>data to produce a map of the top of a sand and gravel unit that overlies the bedrock Denver Formation. Data<br>from the EM survey were combined with lithologic log data to produce a map of the top of the Denver<br>Formation. The resistivity and downhole results were used to augment the seismic and EM data. The results<br>of the geophysical investigation provided additional information for defining the location and geometry of<br>paleochannels and aided characterization of the interface between the Denver Formation and overlying alluvium.<br>EM results were particularly useful in providing information about the paleotopography of the Denver Formation<br>in areas where borehole data were sparse, allowing the detection of several previously unknown small<br>paleochannels in the Northwest Boundary area. Geophysical results allowed further refinement of the main<br>paleochannel and its orientation at the southwest end of the boundary system and confirmed the presence of a<br>suspected paleochamtel northeast of the system. All geophysical results, along with drilhng data, were then<br>incorporated into the hydrogeologic and contaminant transport models of the boundary system to aid evaluation<br>of its present and future effectiveness.


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