What Pore Water Testing Tells Us about Groundwater-Surface Water Exchange Effects

What Pore Water Testing Tells Us about Groundwater-Surface Water Exchange Effects

Cleaning up a contaminated site begins with a holistic investigation process to determine sources responsible for groundwater impacts. Depending on release history, suspected release mechanism, and the hydrogeological properties of the impacted aquifer materials, investigation efforts often extend beyond the source area as the leading edge of the contaminant plume reaches equilibrium. Add in historic development patterns in relation to surface water, and it’s no coincidence that many sites present well-defined source areas, transitional plume zones, and a plume edge with an affinity to discharge into environmentally-sensitive areas, where surface water is often present. 

Under these circumstances and based on our experiences, we recommend incorporating steps to evaluate shallow groundwater and surface water exchange effects by assessing pore water to understand plume dynamics. In this case, pore water is generally defined as the volume of water which occupies the interstitial pore space beneath the sediment bed. To accomplish this, you need to implement an effective pore water sampling program. This program begins with a fundamental understanding of the corresponding groundwater plume; groundwater flow direction, rate, and gradient; and hydraulic influences exerted by nearby surface water or wetlands. These efforts will hopefully result in refinement of plume discharge zones to support conceptual site model (CSM) enhancements, refinement of remedial action alternatives, and more effective remedy evaluations.

While there are a variety of factors to consider when evaluating pore water, here is an overview of three key components to any sampling program:

Sampling Basics

There are many sampling tools and devices to consider when designing a pore water assessment program. These tools are subdivided into two broad categories: diffusion and direct samplers, whose use is dependent on contaminant type and the grain size distribution of the sediment. 

Diffusion samplers rely on a permeable membrane for the contaminant to reach equilibrium with water in very close proximity to the sampler. These types of samplers include water diffusion bags, equilibrium dialysis samplers (peepers) buried in sediment, ion exchange resins, and activated carbon. 

Direct samplers involve placement of the device in sediment and artificial means to extract pore water for sample collection. We have been successful using a push-point sampler inserted to the desired depth with a pump or syringe to extract and sample pore water. Another useful direct sampler is a piezometer, which also supports hydraulic head measurements. 

In some cases, the physical location may be the deciding factor regarding the type of tool to use. Locations in shallow, slow-moving water where a sample can be collected via wading are the simplest and most cost effective, though care must be taken to minimize disruption of the underlying sediment bed. For deeper applications, a boat or even SCUBA diving may be the best approach. Selecting the right method and understanding field limitations go a long way toward maximizing sample integrity in accordance with data quality objectives.

More is Better

Pore water sampling devices and techniques facilitate high-volume and high-resolution collection efforts in a relatively short amount of time. Sample density and the number of transects used vary depending on site conditions, the type of sampler used, and the suite of analytes. A notable upside of pore water sampling is the ability for study area boundaries to encompass suspected contaminant-free zones in support of nature and extent objectives. Increasing the number of pore water samples collected on short order also affords the opportunity to map detached plumes and impacted groundwater located beneath the entirety of a channeled waterway. Vertically dependent samples may also be collected to support profiling objectives as long as the sediment bed physical characteristics align with the depth range of the sampling device. During sampling, stations are easily geo-mapped using conventional hand-held GPS technology to support data analysis.

Sampling Scale

A successful pore water assessment program should inherently rely on steps to maintain quality control and minimize the potential for cross-contamination. For smaller sites, assessments can be scaled down with no laboratory analytical samples collected and field activities may simply involve direct collection of water quality parameters for comparison with monitoring well data. For more complex sites, surface water should be incorporated into the investigation to estimate contaminant flux rates from shallow groundwater. Co-located surface water data may also be a valuable tool used to identify preferential pathways or isolated discharge zones.

Pore water investigations can provide valuable, cost-effective analysis of groundwater-surface water interaction as part of contaminant assessments. If your site is confronted by a dynamic groundwater plume in proximity to surface water, these tips can help you bridge the potential data gap. 

Author

Technical Manager
Remediation

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