Complex hydrogeologic conditions such as fractured and karst bedrock settings pose substantial economic and technical challenges both to the characterization and remediation of dense nonaqueous phase liquid (DNAPL) source zones. To reduce the cost of characterization and remediation of fractured rock sites, it is critical to identify candidate sites for monitored natural attenuation (MNA) and prioritize the remaining sites for remediation. To assist in this endeavor, cost-effective monitoring tools are needed that can be used in concert with existing borehole technologies to directly measure groundwater and contaminant flux in fractured rock. These flux measurements combined with data gathered from other available borehole technologies will bring the Department of Defense (DoD) much closer to estimating contaminant mass discharge from source zones and in turn expedite assessments of environmental risks and benefits associated with natural attenuation, source removal, or remediation at complex sites.
The overall objective of this project was to demonstrate and validate a new closed-hole passive sensing technology for fractured media: the Fractured Rock Passive Fluxmeter (FRPFM). The FRPFM provides simultaneous measurement of (1) the presence of flowing fractures; (2) the location of active or flowing fractures; (3) active fracture orientation, i.e., dip and azimuth; (4) direction of groundwater flow in each fracture; (5) cumulative magnitude of groundwater flux in each fracture; and (6) cumulative magnitude of contaminant flux in each fracture. Various technologies exist to measure (1), (2), and (3) above; however, the FRPFM is the only technology that also measures (4), (5), and (6).
The specific objectives of this demonstration were to:
- Demonstrate and validate an innovative technology for the direct in situ measurement of cumulative water and contaminant fluxes in fractured media.
- Formulate and demonstrate methodologies for interpreting contaminant discharge from point-wise measurements of cumulative contaminant flux in fractured rock.
- Enable the technology to receive regulatory and end user acceptance.
The FRPFM is designed with an inflatable core and separate upper and lower end packers. The core is simply a packer (or flexible inflatable liner) covered with an internal nonreactive layer of permeable mesh that is then wrapped in a permeable layer of material derived from activated carbon, ion exchange resin, or similar sorbent material impregnated with tracers, and then all of this is encased in a thin external permeable layer of cloth material impregnated with a visible dye. The core inflates separately from the two end packers to provide a mechanism for holding the one or more reactive fabrics against the face of the borehole and any fracture intersecting that borehole, while the end packers isolate the zone of interest from vertical hydraulic gradients within the borehole. As currently designed, the FRPFM provides high resolution measurements over a specified interrogation zone (typically 1 meter).
Deploying the FRPFM in a borehole and exposing it to flowing groundwater for duration t [T] gradually leaches visible dyes and tracers from the internal and external sorbent layers and produces residual dye and tracer distributions. Visual inspection of the external layer impregnated with a visible dye leads to estimates of the following for active or flowing fractures alone: (1) locations along the borehole; (2) number; (3) individual fracture orientations in terms of strike, dip, and orientation of dip (direction of falling dip, e.g., SW); (4) cumulative groundwater flux; and (5) groundwater flow direction. Fracture characteristics (1) through (3) can be obtained through existing borehole imaging technologies as long as those fractures possess apertures ≥1mm; however, these commercially available technologies cannot distinguish active from inactive fractures or measure the magnitude or direction of fracture flow. Further analytical analysis of the FRPFM internal sorbent layer at indicated locations of active fractures yields: (1) additional estimates of cumulative groundwater flux in fractures and (2) cumulative contaminant flux in those fractures. Thus, the in situ measurements of direction and magnitude of water and contaminant fluxes in active fractures are innovations given by the FRPFM alone.
In support of the first demonstration objective, the project defined six specific technology performance objectives and established metrics to compare FRPFM measures (contaminant and groundwater fluxes, flow direction, detection of active flowing fractures, fracture location and orientation) to those obtained from five different competing/comparative technologies: High Resolution Temperature Profiling (HRTP), Acoustic Televiewer (ATV), Optical Televiewer (OTV), Temperature Vector Probe (TVP), and Borehole Dilution (BHD). Field tests were conducted at two chlorinated solvent contaminated fractured rock sites. The Cost & Performance Report presents 16 separate field tests and their results. A total of nine down-hole tests were executed in 4- and 6-inch rock wells at the Guelph Tool Site in Ontario, Canada, and another seven tests were conducted in one 6-inch rock well located on the premises of the former Naval Air Warfare Center in West Trenton, New Jersey. Based on the results of the 16 field tests, the FRPFM achieved the standard in each of the six quantitative performance objectives.
In support of the second demonstration objective, methodologies were formulated and demonstrated for interpreting contaminant discharge from point-wise measurements of cumulative contaminant flux in fractured rock. Those methods were published in the peer-reviewed journal Water Resources Research (Acar et al. 2013).
In support of the third project objective, Enviroflux Inc. assumed exclusive rights to commercialize the FRPFM technology (patented in 2008). At this time, Enviroflux Inc. is engaged in discussions to deploy FRPFMs for a major client of a large environmental firm. The U.S. Environmental Protection Agency has also shown interest in continued field testing and site selection is under way.
The FRPFM technology currently functions through deployment of custom-built prototypes designed with a specified interrogation zone (typically 1 meter). Currently prototypes exist for application in 4-inch and 6-inch fractured rock wells. Deployment, retrieval, and sampling are straightforward and have been demonstrated to field technicians from the University of Guelph and U.S. Geological Survey who experienced minimal issues with methodology transfer.
Depending on site conditions, permits may be required for permission to release small quantities of food-grade tracers into the aquifer. A standard list of tracers is available, and no issues have been experienced with previous permit requests.
As technology development continues, refinements will be made and applied to future prototypes (such as expanded interrogation zone). Site-specific refinements can be made as needed.