The ability to predict contaminant (e.g., chlorinated volatile organic compounds [CVOCs]) fate and transport in aquifers is often limited by the intrinsic heterogeneity associated with the flow field, contaminant distribution, and coupled biotic and abiotic reactions. Processes occurring in low permeability zones are particularly important, as studies have demonstrated that contaminants residing in such materials can sustain groundwater plumes and impede overall contaminant attenuation. While the importance of identifying these processes in heterogeneous media has been well documented, previously there has been no cost effective tool for providing high resolution profiling of coupled contaminant, biogeochemical, and microbiological characteristics at the cm-scale. The primary objective of this research was to develop and demonstrate a High-Resolution Passive Profiler (HRPP) as a fine-scale delineation tool for the saturated subsurface. Focus was placed on discerning contaminant, microbiological, and biogeochemical differences between low permeability and high permeability zones within heterogeneous or stratified media.This project will modify the design of equilibrium diffusion samplers (peepers) to provide high resolution delineation of the saturated subsurface with respect to distribution of contaminants (particularly chlorinated solvents), hydrogeologic conditions, geochemistry, and microbial community structure and activity. The primary objective is to develop and demonstrate a High Resolution Passive Profiler (HRPP) as a fine-scale delineation tool for the saturated subsurface. Focus will be placed on discerning contaminant, microbiological, and biogeochemical differences between low permeability and high permeability zones within heterogeneous or stratified media.

The project's approach consisted of developing a direct-drive HRPP designed to determine contaminant concentration, groundwater velocity, microbial community structure, and potential for abiotic/biotic contaminant degradation in situ at the cm-scale along a vertical profile. Laboratory studies were used to develop a new method to measure pore velocity and to demonstrate the ability of using micro-biotraps to measure the microbial community and perform compound specific isotope analysis on adsorbed CVOCs. Based on laboratory results, a full scale HRPP was designed, manufactured, and tested in the field. Based on initial field tests, new generations of HRPP were manufactured and field tested. A second field test evaluated HRPP performance against other methods of sub-surface site evaluation (vertical discrete wells, cores, membrane interface and hydraulic profiling tools (membrane interface probe/hydraulic profiling tool), and standard monitoring wells. A third field test was used to contrast site assessment results using standard monitoring wells and HRPP deployed along a contaminated groundwater transect at a site using a mulch bio-wall for treatment.

A direct drive HRPP was developed that can be coupled together to evaluate vertical lengths ranging from 1.2 to 3.6 meter (m) and can be deployed down to ~9 m using direct drive devices. It produces co‐located concentration profiles (~20 cm resolution) of CVOCs, geochemical indicators (e.g. Cl^‐, NO₃^‐, SO₄^2-, Fe, CH₄, ethene), microbial community, compound specific isotope analysis of CVOCs, and pore velocity (0‐100 cm/d) even in low hydraulic conductivity media (e.g. clay). HRPP concentration profiles of these parameters were comparable to other traditional site assessment methods. However, the resolution achieved by the HRPP provides information on contaminant fate and transport that cannot be obtained by other methods.

During this SERDP project, the project team developed and validated a modified peeper design (HRPP) capable of providing information far beyond concentration data, including microbial numbers and activity, groundwater and contaminant flux, and contaminant degradation at cm-scale resolution. Samplers capable of producing such a holistic set of characterization parameters with this level of resolution will be an enormous advantage over existing methods and should lead to higher fidelity site models, more tailored design of remediation activities, and improved remedial performance evaluations. Further, the tool allows monitoring and assessment of highly heterogenous contaminated formations that are presently hard to evaluate, particularly with respect to processes occurring in low permeability regions. The new tool can be deployed relatively easily, similar to other direct drive tools and can provide data to guide source zone assessment, well placement, rebound potential from low permeability zones, homogeneity and extent of bioaugmentation/stimulation efforts, or other remedial activities. (Project Completion - 2020)

Schneider, H., W.A. Jackson, K. Rainwater, D. Reible, S. Morse, P.B. Hatzinger, U. Rubalcava. 2019. Estimation of Interstitial Velocity Using a Direct Drive High Resolution Passive Profiler. Groundwater, 57:915-924.

Schneider, H., W.A. Jackson, P.B. Hatzinger, and C. Schaefer. 2020. High-Resolution Characterization of a Chlorinated Solvent Impacted Aquifer Using a Passive Profiler. Groundwater Monitoring and Remediation. dx.doi.org/10.1111%2Fgwmr.12409.