Formatted Title
Practical Applications of Lysimeters within a Regulatory Framework and Lessons Learned
Background/Objectives
As site characterization techniques mature, increased focus has been placed on the leaching of contaminants from surface soil through the vadose zone to groundwater at impacted sites; however, collection of data for fate and transport understanding and interpretation for decisive action remains a challenge for the site characterization community. Porous cup suction lysimeters provide a method for direct collection of contaminants that leach from soil and are subsequently transported in soil porewater, representing the mobile mass discharging from the source. As mobile mass is leached and transported to groundwater, lysimeter porewater samples provide insight into source strength, contaminant pathways, and mass flux/mass discharge, which in turn directs risk management discussions within regulatory frameworks and influences effective remediation strategies. While lysimetry provides essential leaching information, increased standardization for field methods is necessary to support defensible data interpretation and decision making.
Approach/Activities
Case studies are presented showing field examples of soil porewater sample collection from a variety of sites including per- and polyfluoroalkyl substances (PFAS) containing aqueous film-forming foam (AFFF) impacted sites and a chlorinated solvent site. These case studies demonstrate the use of porewater samples for data-driven decision making and follow-up site characterization and/or risk management action with regulatory approval. Lessons learned from design, installation, sampling, evaluation, and troubleshooting field activities are shared and improved methods presented.
Results/Lessons Learned
The use of lysimetry is an established method for evaluating contaminant leaching at a variety of impacted sites, and it can provide insight into fate and transport of contaminant compounds and the mobile mass contributing to groundwater plumes. Case studies show that direct measurement of soil porewater concentrations from porous suction cup lysimeters can be used within regulatory frameworks and is defensible. With reasonable repeatability (within a factor of 2-3) among lysimeters between sampling events, lysimeter data can be used both qualitatively and quantitatively for site decision making. Pre-purging of newly installed lysimeters as well as accounting for water used during installation is crucial to collecting and understanding samples. When paired with moisture content data, lysimeter samples (or the lack of sample from a properly functioning lysimeter) is valuable information during regulatory discussions. These data support site characterization conclusions and activities, robust remedial strategies, and risk management plans by allowing project teams to assess retention of contaminants in vadose zone soils. While conclusions about source strength and mobile mass were able to be made at the sites, and field data supported next steps in each of these case studies, the need for standardization was identified as an area for improvement that would increase the defensibility of data collected from lysimeters during discussions with regulators. The utilization of lysimeter samples helps meet the need of understanding fate and transport at source zones, and with increased technical guidance documents and standardization based on lessons learned in the field, lysimeter data interpretation within a regulatory framework can be turned into site action for risk mitigation.