Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7m. PFAS Site Characterization

Formatted Title:

PFAS Surface Water Investigation Using Flumes and Mass Discharge Estimates

Background/Objectives:

A facility within a small community historically used per- and polyfluoroalkyl substances (PFAS) containing materials in its manufacturing processes beginning in the 1970s and is currently under investigation for releases under the State’s remedial investigation program. The conceptual site model based on previous investigations indicated that surface runoff and shallow groundwater seepage from the Site potentially contribute PFAS concentrations to a stream that travels through the Site. This stream originates off-Site and receives contributions from several off-Site tributaries. Previous investigations indicated PFAS concentrations in the main channel upstream of the Site and from the off-Site tributaries. Existing data suggested that there may be specific conditions (e.g., high groundwater, following heavy rainfall, etc.) where discharges from the Site may result in measurable changes to PFAS concentrations in the stream.

The objective of this work was to generate data to allow for the estimation of PFAS mass discharges under various flow conditions, the comparison of relative PFAS contributions to this stream from on-Site and off-Site areas, and the further evaluation of on-Site groundwater seepages contributing to PFAS concentrations in the stream.

Approach/Activities:

The investigation consisted of six consecutive bimonthly monitoring events (one calendar year) conducted at six different surface water locations at the Site. One event was prompted by a high (i.e., within 24 hours of a 1-inch rainfall event) and a low (i.e., baseflow) streamflow condition. Each monitoring event included surface water sampling for laboratory analysis of PFAS, surface water elevation gauging, and measurement of streamflow using cutthroat flumes (i.e., discharge in gallons per minute). Where conditions did not allow for a flume to be installed, streamflow was measured using the velocity-area method. Each monitoring event also included sitewide shallow groundwater elevation gauging and sampling of a shallow groundwater monitoring well upgradient of potential seepage and nearest to the stream. The data collected was used to estimate the PFAS mass discharge (i.e., mass unit of PFAS per unit time) at each location under the variable streamflow conditions and detail the potential relationship to seasonal and meteorologic conditions.

Results/Lessons Learned:

The results of the investigation demonstrated that little to no groundwater discharges to the surface water features at the Site based on observed dry conditions at on-Site drainages and groundwater/surface water elevation data. Despite higher concentrations, PFAS mass discharge in surface water from the Site was estimated to be as much as 200 times lower than from individual off-Site tributaries. The data generated served to delineate Site impacts and place them in context for potential future evaluations of remedial alternatives.

With the potential influence of other sources and background, evaluation of PFAS concentrations alone may not be sufficient for many remedial investigations. Estimation of mass discharge, using streamflow and analytical results, may prove useful in many PFAS investigations. Measuring streamflow in small flashy streams is difficult but possible with portable cutthroat flumes. Practical information about streamflow monitoring methods using cutthroat flumes will be shared.

* Barr employees performed work while seconded to BEC Engineering and Geology, PC.

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Melissa Nabors