Theme: 08. Metals

Topic Area: 8b. Mining and Uranium Site Restoration

Formatted Title:

Using 3D Visualization to Calculate Bulk Plume Metrics for Former Uranium-Ore Mill and Disposal Sites

Background/Objectives:

The U.S. Department of Energy Office of Legacy Management (LM) is responsible for the long-term surveillance and monitoring of former uranium-ore mill and disposal sites. A primary objective of long-term groundwater monitoring at these sites is tracking the behavior of dissolved contaminant plumes over time relative to each site’s compliance requirements. Bulk plume metrics including the overall plume mass, average concentration, and volume are calculated from three-dimensional (3D) interpolations of dissolved plume volumes generated in Earth Volumetric Studio (EVS). Evaluating plume behavior over time using bulk plume metrics is a valuable tool used in groundwater assessments of numerous LM sites and has wide applications including general site characterization, assessment of remediation strategies, updating conceptual site models, and annual reporting to regulators and stakeholders.

Approach/Activities:

Temporal 3D dissolved contaminant plumes for constituents of concern at a site are created by interpolating temporal concentration data in EVS. The spatial extent of each constituent for a given plume is specified in accordance with the established regulatory or background thresholds and physical boundaries, such as the water table or impermeable geologic layers. The interpolated plumes are assessed and interpreted visually before calculating plume metrics. A function written using the python module of EVS provides temporal estimates of the plume footprint area, volume, center of mass, average contaminant concentrations, and corresponding dissolved plume mass. These estimates are then plotted over the analyzed time period with a best-fit locally estimated scatterplot smoothing (LOESS) regression line. When warranted, a Mann Kendall trend analysis is also performed on the dataset, allowing for a more comprehensive understanding of dissolved plume behavior.

Results/Lessons Learned:

Bulk plume metrics calculated from temporal 3D contaminant plume volumes have proven to have many benefits and applications for tracking plume behavior at a wide variety of sites. Collectively, bulk plume metrics provide quantitative measures of the overall temporal trends and stability of dissolved contaminants. By combining bulk plume metrics with statistical testing, site specific examples have shown their utility in the following regards: (1) evidence regarding the efficacy of a treatment system in reducing plume volume; (2) long-term plume stability; (3) plume expansion that triggered the need for additional active remediation; and (4) calculating the volume of saturated tailings within an engineered disposal cell that guided where to conduct further investigations. The bulk plume metric approach has also shown to be particularly useful at sites with extensive well networks, where a more comprehensive analysis of the collective well population is necessary for characterizing overall contaminant plume behavior through time. The incorporation of bulk plume metrics into groundwater evaluations at a variety of former uranium mill and disposal sites has helped strengthen ongoing characterization efforts as well as aided in better communication of plume conditions to stakeholders and regulators. The utilization of bulk plume metrics has thus become a commonly used analysis at the Office of Legacy Management for understanding contaminant plume behavior through time.

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Stacy Trowbridge