Evaluating the GSI Pathway

by Derrick Lingle
Mar 19, 2020

Pore Water Sampling_2020_03

When a contaminated site is located near a surface water body, there is a risk of contaminants in groundwater migrating into a lake or stream. If this is a potential route of conveyance, the Groundwater Surface Water Interface (GSI) pathway is taken into consideration during an exposure pathway evaluation. Since contaminant concentrations in groundwater do not always remain constant when venting into a surface water body, refined site characterization is sometimes necessary to address the GSI pathway. Envirologic utilizes pore water sampling as a pinpoint, cost-effective option when groundwater data indicates that the GSI pathway is potentially complete. Pore water sampling serves as a way to assess whether contaminant concentrations naturally decrease (due to sorption or oxidation) immediately before discharging into a surface water body.

While there are several methods available for evaluating the GSI pathway, pore water sampling is a straightforward tool that provides definitive evidence as to whether contaminants in groundwater are venting into a surface water body. Pore water is drawn from the transition zone of a lake or stream, which is where conditions progress from groundwater to surface water. Before collecting pore water samples, a surface water body should be evaluated for areas of gaining (areas where groundwater flows into the surface water body) and losing (areas where the surface water body loses water to the underlying aquifer) conditions. In gaining sections (as determined where the hydraulic head in the transition zone is greater than the surface of the water body), a sampling port is inserted into the transition zone and a pore water sample is drawn through tubing attached to the port.

blog_pore water diagram

Figure 1 - A pore water sampling device can be used to evaluate whether a stream is gaining or losing.

During an environmental investigation by Envirologic at a former storage facility for coal and coal combustion residuals, heavy metals were identified in monitoring wells near an adjacent stream. After the stream was evaluated for gaining and losing sections, several sampling ports were installed in gaining sections on either side of the stream. Analytical results from multiple pore water sampling events indicated that the metals of concern in groundwater were not migrating into the stream in excess of generic GSI criteria. This information, along with a thorough understanding of the hydrogeology at the site, demonstrated that there was not an unacceptable exposure risk related to the GSI pathway.

Pore water sampling can be used to trace various contaminants, including metals, petroleum-related compounds, and chlorinated solvents. A pore water sampling port can be installed and used for multiple sampling events; however, high discharge events such as during a spring melt can compromise the integrity of the device if not properly installed. Pore water sampling offers a definitive and economical option to evaluate the GSI pathway.

There are often sampling needs as part of an environmental investigation; let Envirologic support your project using our experienced Field Services team. For a complete list of our Field Services, please visit our website or contact a member of our team at (269) 342-1100 or information@envirologic.com.

Figure 1 - EPA Document, SOP #EH-03 Sediment Porewater Sampling using a Micro Push Point Henry, M. 2000. MHE Push Point Sampling Tools. Proceedings of the Ground-Water/Surface-Water Interactions Workshop. July 2000

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gsi  groundwater surface water interface  pathway  contaminant  pore water sampling  environmental investigation  field sampling  groundwater 

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