Characterizing the GSI Pathway With Pore Water Samples

by Derrick Lingle
Feb 19, 2016

Pore Water Sampling

When a contaminated site is located near a surface water body, there is a risk of groundwater contaminants migrating into a lake or stream. If this situation occurs, 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 has recently utilized pore water sampling as a pin-point, cost-effective, option when groundwater data indicates that the GSI pathway is potentially complete. (Pore water is the water that fills the void space between sand grains.) Pore water sampling serves as a way to assess whether contaminant concentrations are naturally reduced (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 groundwater contaminants 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 gaining (groundwater flows into the surface water body) and losing (the surface water body loses water to the underlying aquifer) sections. 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 sample is drawn through tubing attached to the port.

blog_pore water diagram

A pore water sampling device can be used to evaluate whether a stream is gaining or losing. *Image: EPA Document, SOP #EH-03 Sediment Porewater Sampling using a Micro Push Point

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 indicate that the metals of concern in groundwater are not migrating into the stream in excess of generic GSI criteria. This information, along with a thorough understanding of the hydrogeology at the site, demonstrates that there is not an unacceptable level of exposure related to the GSI pathway.

Pore water sampling can be used to trace various contaminants, including metals, petroleum related compounds, and chlorinated solvents. Before committing to a pore water sampling plan, it is important to understand the type of sediment in the bed of the surface water body; clay or silt-rich bottoms can easily clog the intake screen on the sampling device. 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. When compared to other tools that can be cumbersome, pore water sampling offers a definitive and economical option to evaluate the GSI pathway.

Derrick Lingle

If you have questions about Pore Water Sampling (or any of our other sampling methods) please contact Project Geologist, Derrick Lingle.

*Henry, M. 2000. MHE Push Point Sampling Tools. Proceedings of the Ground-Water/Surface-Water Interactions Workshop. July 2000

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pore water sampling  water sampling  water contamination  contaminants  heavy metals  contaminated site  groundwater  gsi pathway  site characterization 

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