FIELD MEASUREMENTS OF BACK DIFFUSION USING EXISTING WELL BOREHOLES: WINDOWS INTO REMEDIAL OUTCOMES

The short and long-term consequence of contaminant diffusion from a rock matrix can have significant impact on the remediation of dissolved contaminant plumes. Cost effective methods to measure back diffusion are useful tools in helping to predict remedial time frames. Our method utilizes existing open boreholes in fractured rock to conduct backward diffusion experiments by isolating minimally “non” fractured zones of the borehole. These non-fractured zones (called target zones) are identified with borehole geophysical logging and chemical profiling of contaminants of concern (COC). Our testing occurred in a fractured metasedimentary rock on the New Hampshire coast where we investigated back diffusion of cis-1,2-dichloroethylene (cis-1,2, DCE) . Target zones are sealed over short intervals with straddle packers. Hydraulic isolation is tested by pumping adjacent zones and measuring the response in the target zone. In order to initiate back diffusion, we strip cis-1,2, DCE by injecting argon gas into the target zone. Backward diffusion was measured by periodic passive-sampling of the target zone to identify rebound. The passive (non-hydraulically stressed) sampling negates the need to extract water and potentially dewater the target zone. We also monitor active flowing zones of the borehole to assess trends in concentrations in other parts of the fractured rock.

We feel our method provides a unique window into gauging the significance of back diffusion. For example, the size of the sampled zone using our method covers a relatively large interval; therefore, potentially fewer samples are required. Additionally, since new boreholes are not required in most cases, considerable cost advantages may be realized. Finally, the date of exposure of COC’s to potentially previously unexposed rock matrix sections of the borehole is known, which will help constrain forward diffusion models of COC’s.