An instrument that has been deployed more than 10 times at DOE and other sites is having a beneficial effect on the cost and ease of obtaining reliable measurements of groundwater rates and velocities. The Colloidal Borescope (OST/TMS ID 465), one of the technologies in the inventory of the Subsurface Contaminants Focus Area, was developed by Oak Ridge National Laboratory and R. J. Electronics Inc. and is commercially available through AquaVISION Environmental LLC of Palisade, Colorado. The data provided by the Colloidal Borescope are augmenting site characterizations and helping confirm the paths of contaminants migrating through the subsurface. The Colloidal Borescope is also being used at commercial and departments of Energy and Defense sites to provide corroborative evidence in assessing the effectiveness of remediation systems.

As a method for directly measuring groundwater flow rates and directions, the Colloidal Borescope is an alternative to tracer tests, which can be difficult, time intensive, and expensive to perform. When the Colloidal Borescope is inserted into a monitoring well, it directly measures the movement of naturally occurring particles in groundwater, or colloids, as they flow through a well’s screened interval, which corresponds to a distinct lithographic unit in the subsurface. Various depths within a single screened interval can be evaluated. The resulting data provide a vertical distribution of groundwater flow directions and velocities, which is useful in helping map the hydrogeologic composition of a site, as well as interpreting contaminant migration in complex flow systems. The instrument’s data can also be used to indicate the effects on groundwater flow rates of man-made intrusions, such as drainage ditches, outfalls, and leaking utilities.

The Colloidal Borescope consists of a CCD (charge-coupled device) camera, a flux-gate compass, an optical magnification lens, an illumination source, and stainless steel housing. The device is approximately 89 cm long and has a diameter of 44 mm, so it can be easily inserted into a 5-cm-diameter monitoring well. Upon insertion into a well, the device produces an electronic image magnified 140X, which is transmitted to the surface, where it is viewed and analyzed. The flux-gate compass is used to align the borescope in the well. As colloids pass beneath the lens, the back lighting source illuminates them in a manner similar to a conventional microscope with a lighted stage. A video frame grabber digitizes individual video frames at intervals selected by the operator.

A software package developed by Oak Ridge National Laboratory compares two digitized video frames, matches colloids from the two images, and assigns pixel addresses to the particles. Using this information, the software program computes and records the average particle size, number of particles, and their speed and direction. A computer analyzes flow measurements every four seconds, resulting in a large database after only a few minutes of observations. Since standard VHS video uses 30 frames per second, a particle that moves 1 mm across the field of view can be captured in subsequent frames 1/30 of a second apart, resulting in an upper measurement velocity range of 3 cm/s. For low-flow conditions, the operator can set longer intervals of time between frames, resulting in a lower velocity range for stagnant-flow conditions.

Plotting the movement of a plume
In a project conducted at a site in the Sandia Mountains east of Albuquerque, New Mexico, the Colloidal Borescope was used to supplement an ongoing investigation into the location and size of a hydrocarbon plume. A service station’s underground storage tanks had leaked, resulting in the contamination of several residential wells. The use of the Colloidal Borescope was planned to provide more evidence of the actual direction taken by the plume in an area where the aquifer is characterized by faults and porous blocks.

Groundwater flow and contaminant transport in fractured, porous aquifers is complicated by flow in the porous blocks. Flow in sandstone or siltstone blocks may be in the direction of the hydraulic gradient while groundwater in the fractures will migrate in a direction parallel to the fractures. It is a combination of these characteristic flows that ultimately determines the actual direction of contaminant migration in fractured porous systems. In the East Mountain site, the hydraulic gradient trended in a northwest direction, while the faults ran in a north-south direction.

Results from the Colloidal Borescope indicated that groundwater velocities in the fracture zones are approximately one order of magnitude higher than those in the unfractured zones. This evidence suggests that the faults are acting as preferential flow zones, which dominate the groundwater flow component and redirect the hydrocarbon contamination from the northwesterly regional flow direction to a southern direction parallel to the fault trend.

Results from the field investigation at the Sandia site have shown that the Colloidal Borescope is an effective tool for providing groundwater flow information in complex, fractured aquifers. The Colloidal Borescope provides direct measurements at specific locations and, when combined with other site information, provides a comprehensive understanding of groundwater flow and contaminant migration.

Monitoring cleanups
The newest application of the Colloidal Borescope has been monitoring the effectiveness of remediation—specifically, determining the radius of influence of pump-and-treat systems and assessing groundwater flow through permeable reactive barriers. Nic Korte, ORNL’s principal investigator for a project evaluating the effectiveness of reactive barriers, says, “The borescope has a significant advantage over other single borehole methods for evaluating flow direction and velocity because you can see the colloids on a video screen. The direct observation is very useful for demonstrating subsurface heterogeneity to site owner/operators and regulators.” In June 1999, the Colloidal Borescope helped determine, along with other tools and techniques, that part of the plume at DOE’s Kansas City Environmental Restoration Project was not being addressed by the site’s permeable reactive barrier. Unexpected complexities in the site’s hydrogeology or the effect of industrial discharges may be causing a portion of the plume to bypass the barrier. The instrument was also used in December 1998 and 1999 at DOE’s Y-12 Plant in Oak Ridge, Tennessee to evaluate flow directions and velocities near the barrier at S-3 Pond (see Initiatives, Winter 1999). And DOD sites at Lowery and Dover Air Force bases have checked their barriers by using the instrument to determine flow directions and velocities along the length of the barriers. ORNL is using the Colloidal Borescope to evaluate the effectiveness of natural flushing in cleaning the contaminated aquifer underlying DOE’s Grand Junction Project Office.

As DOE moves closer to the day when its sites are turned over to the public, instruments like the Colloidal Borescope, which can be used to monitor the status of permeable reactive barriers and other long-term solutions to environmental problems, will play a major role in DOE’s continuing stewardship responsibilities. For more information about DOE’s investigations of the Colloidal Borescope, contact Nic Korte at the Oak Ridge National Laboratory/Grand Junction Office at (970) 248-6210. The vendor’s (AquaVISION) Web site at http://www.aquavisionenv.com links to technical papers.