Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4g. Evaluating Surface Water/Groundwater Interactions: Innovative Monitoring Approaches and Modeling Applications

Formatted Title:

High Resolution Groundwater and Surface Water Characterization: Advanced Techniques to Improve Conceptual Site Models

Background/Objectives:

Metals contamination including zinc and cadmium have impacted groundwater and surface water at the Bonita Peak Mining District Superfund Site. One area of concern is along the Animas River near Silverton, Colorado, where mining occurred from the late 1800s through 1997 and unknown sources contribute metals to the stream. This contamination has multiple potential sources including four large waste repositories, two mine portals, three historic milling locations, and potential for high background metal load due to regional geologic features. Groundwater and surface water monitoring data suggested that groundwater and surface water interactions were likely to be significant. The objective of this study was to identify the sources of metal loading, quantify the unknown surface water load, and evaluate the likelihood of background contamination. The stream reach of interest required techniques capable of rapidly assessing a large area.

Approach/Activities:

Advanced characterization techniques were utilized to perform reconnaissance and quantify surface water impacts across a 4 km reach of the Animas River. Surface geophysical methods such as electromagnetic induction (EMI), magnetometry, and fiber optic distributed temperature sensing (FODTS) were used to rapidly survey and identify groundwater discharge zones for refined sampling. A surface water tracer coupled with high-resolution surface water sampling were used to quantify the metal load in surface water, estimate the groundwater contribution to load, and investigate potential source areas.

Results/Lessons Learned:

Surface geophysical methods such as EMI, magnetometry, and FODTS were used to identify areas of groundwater seepage to surface water, evaluate the relative magnitude of seepage, and inform the potential sources of impacted groundwaters. The applied tracer and high-resolution surface water sampling enabled the project team to quantify the surface water metal load, estimate the groundwater contribution to metals load, and evaluate sources on both banks of the river.

The successful application of these techniques allowed the project team to rapidly assess the stream reach of interest using multiple lines of evidence. The combined interpretation suggests much of the surface water load enters as groundwater from the north bank of the river and that potential sources on the south bank are less significant. At least four areas of likely high metal loading were also identified for further investigation and represent possible targets for remedial action.

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Neil Terry