Remedial Investigation of Large Scale Karstic Flow Conduits with Brine-Enhanced Resistivity Imaging and Downhole Colloidal Borescope Methods

Locating conduit-type permeability in karstified limestone and dolomite and measurement of flow velocity and flow direction are key requirements for remediation of groundwater contamination, and sealing of massive inflows to pits, quarries, and underground mines. The massive rates of water inflow to a mine can cause catastrophic flooding in days to weeks, requiring a rapid reconnaissance method that can locate deep karstic permeability in a shortened timeframe as compared to traditional geophysical and drilling methods. Off-site migration of groundwater contaminants can occur by conduit flow in days to weeks as compared to much longer timeframes for diffuse flow permeability, and remediation often requires rapid remedial planning, and strategic grouting of selected conduits to eliminate the major transport pathways. Resistivity profiling was used in conjunction with salt brine enhancement of the conduit flow to locate and image deep-lying karst flow conduit features using the Advanced Geosciences, Inc. (AGI) SuperSting R8 earth resistivity geophysical equipment. The equipment consists of the meter, external power supply, and 112 electrodes at 3 m spacings, for a surveyed length of 333 m (1,092 ft). Ambient resistivity conditions were measured throughout the surveyed area, and the average depth to karst bedrock was determined. Constant-rate injection of saturated NaCl brine solution into the flow conduit was commenced after the background survey was completed, and the conductivity of the conduit water was increased by roughly 10 times background. A second resistivity survey was conducted in which the resistivity readings above the top of bedrock were not collected to reduce the duration of data collection, and to reduce the brine volume requirement. The volume of saturated brine that is required is typically about 57,000 L (15,000 gallons) at a continuous pumping rate of 757 L/min (200 GPM). The 2-D models of subsurface resistivity from the pre-brine, and brine-enhanced surveys are subtracted to produce a conductive anomaly map. For a project site in Hagerstown, Washington County, Maryland, a 2-D anomaly of roughly -500 Ohm-m, with a cross-sectional area of about 1,860 m2 (20,000 ft2), was detected. The total survey area was roughly 12X this size. The conductive anomaly was drilled with multiple boreholes, and conduit flow to a nearby mine was identified within weeks. The brine-enhanced 2-D electrical resistivity survey is believed to be the first application of its kind at this scale. The preliminary methodology and initial results from two case studies have been presented in Lolcama and Stuby (2015). The colloidal borescope instrumentation by AquaVision utilizes natural colloidal particles which are suspended in the conduit flow past a video camera to determine groundwater flow velocity and direction. The system is capable of analyzing flow measurements every 4 seconds, and produces a very large dataset of several thousand particles tracked during an hour long test. The borescope can measure groundwater velocities from near zero up to 1.6 m/min (5 fpm). Testing of deep conduit flow that was encountered by drill holes was completed at a site in south-central Pennsylvania, which is different from the Hagerstown, MD site. Testing showed that the average direction of horizontal flow through the karst conduit features lies at about azimuth 64 degrees, or towards the east-north-east, with an average groundwater velocity of roughly 0.002 m/s (0.3 fpm), with occasional spikes in velocity to 0.01 m/s (1.7 fpm). The velocity is similar in magnitude to point dilution testing results for the same karstic flow field. The results were instrumental in achieving a better understanding of the locations of conduit flow features beneath the site, and the flow velocities and directions which were needed for remedial planning purposes