INNOVATIVE TECHNIQUES TO INVESTIGATE CONTAMINATION IN FRACTURED BEDROCK

Transcription

1 INNOVATIVE TECHNIQUES TO INVESTIGATE CONTAMINATION IN FRACTURED BEDROCK Abstract Christopher Gaule 1, Kenneth Goldstein 2, Grant Anderson 3 Watervliet Arsenal, located in Watervliet, New York, is the oldest continuously operating, cannon manufacturing facility in the United States. The Arsenal is near the eastern boundary of Albany County, New York, along the west bank of the Hudson River, the regional discharge area. The Main Manufacturing Area of the Arsenal is a 125-acre tract where the manufacturing and administrative operations occur. A second area, known as the Siberia Area, as a shipping yard used for the interim storage of raw and hazardous materials, finished goods, and supplies for the Arsenal. The extent of groundwater contamination at the Arsenal was investigated through the installation of traditional groundwater monitoring wells along with innovative investigative techniques. During the first two phases of the investigation, groundwater monitoring wells were installed at the watertable and the intermediate bedrock. The highest concentrations, of Dense Non-Aqueous Phase Liquids (DNAPL), were detected at the regional discharge boundary approximately 110 feet below the ground surface. The later phases of well installation were focused through the use of borehole geophysical techniques such as temperature and resistivity logging, traditional video logging and the relatively new technique of enhanced digital Borehole Imaging Processing System (BIPs). The use of BIPs allowed for the determination of fracture orientation, fracture aperture width and frequency which focused the placement of the third phase of wells. The results of the third phase of the investigation indicated that the contaminants extended to a depth in excess of 160 feet below the ground surface. Additional borehole geophysical logging techniques were conducted to further define the high flow zones in the contaminated bedrock section. These techniques included acoustic televiewer, electro-magnetic (EM) and heat pulse flowmeter under pumping conditions. This last method, heat pulse flowmeter is very accurate and is capable of defining the active fractures within the system to 0.01 gallons per minute. These methods were combined in the field in a cross borehole configuration to determine the degree of interconnection between wells this method replaced conventional long-term pumping tests. INVESTIGATION METHODS The Watervliet Arsenal is under an Order on Consent for the investigation and potential remediation of site contaminants. The site is a RCRA facility and has been investigated through a series of Facility Assessments and Investigations. The initial investigation phase completed by USACE and Malcolm Pirnie was completed in the shallow and intermediate groundwater flow system, using traditional overburden and bedrock monitoring wells. The purpose of this initial phase was to characterize the groundwater flow regime throughout the facility and to determine if a release had occurred from the identified Solid Waste Management Units (SWMUs). The distribution of the monitoring wells installed at the site is presented in Figure 1. The initial phase of the RFI identified the presence of chlorinated organics in the bedrock at three main locations in the Main Manufacturing Area. The source of the chlorinated organics at two of the locations was not identified in the initial phase of the investigations, neither was the vertical or lateral extent of the contamination defined. The later phases of the RFI defined the vertical and lateral extent of the chlorinated organic contamination through the use of discrete zone bedrock packer sampling in combination with emerging and traditional geophysical techniques. 1 Senior Project Hydrogeologist, Malcolm Pirnie, Inc., 15 Cornell Road, Latham, NY 12110, USA, Ph , Fx , cgaule@pirnie.com (corresponding author) 2 Senior Associate, Malcolm Pirnie, Inc., 104 Corporate Park Dr., White Plains, NY , USA, Ph , Fx , kgoldstein@pirnie.com 3 United States Army Corps of Engineers, Baltimore District, 10 South Howard Street, Baltimore, MD 21201, USA, Ph , Fx , grant.a.anderson@usace.army.mil

2 Figure 1 Monitoring Well and Groundwater Chlorinated Organic Compound Contamination Based on the results of the initial phase of drilling and sampling, additional monitoring wells were installed along what was believed to be the primary flow path from the recharge area located in the central portion of the site to the eastern site boundary. The second phase of monitoring well installation investigated the intermediate groundwater flow zone. During this second phase of drilling a borehole geophysical program was also implemented to aid in the definition of the fracture network at the site and to aid in the definition of potential source areas. Techniques used during this phase of the investigation included the use of traditional logging methods such as temperature, resistivity logging, and traditional video logging. In addition to these methods, the relatively new technique of enhanced digital Borehole Imaging Processing System (BIPs) was used. The use of the BIPs system offered several advantages over the traditional video logs, including the ability to determine strike and dips for linear features, including bedding planes and fractures, encountered in the borehole, and allows the geologist the ability to differentiate between drilling induced breaks in the bedrock and true fracturing and bed surfaces. Using this system feature apertures may also be calculated for the portion of the feature which intersects the borehole. Examples of the images and data collected using the BIPs system are presented in Figure 2. One of the unique images that can be generated using the BIPs data is a virtual core, Figure 3. The data generated during the logging process is wrapped on a cylinder, thus creating a virtual core. The advantage to this image is it allows the geologist to visualize the bedrock in an undisturbed manner. The data collected during this phase of the investigation was used to further delineate the areas of investigation through the generation of stereo net diagrams to define the fracture and lineament network. Using the stereo net projections it was possible to determine the potential direction of contaminant flow and allowed for the determination of depth for the third phase of wells. Figure 4 shows the stereo net diagrams prepared for the eastern property boundary where the highest levels of chlorinated organics were detected. Figure 5 shows the vertical extent of chlorinated organic contamination at the eastern property boundary. The cross-section parallels the eastern property boundary in a north-south direction.

3 Figure 2 Borehole Imaging Process System (BIPs) Data Figure 3- Virtual BIPs Core Figure 4 Stereo net Projections, Eastern Property Boundary Figure 4 Stereo Net Projections, Eastern Boundary

4 Figure 5 Vertical and Lateral Extent of Contamination, Eastern Boundary The third and final phase of the investigations involved the further delineation of the contamination along the eastern property boundary through the installation of permanent discrete zone multi-port monitoring systems. The location of the monitoring ports were based on the analytical data collected during drilling operations and the results of the cross-borehole geophysical flow meter testing. The cross-borehole flow meter testing was also used as an alternative method to traditional pumping tests in the highly contaminated zones of the bedrock, eliminating the need for water disposal. There were several advantages to completing these cross-borehole tests, which included the ability to calculate transmissivities for individual fractures, and determine the degree of connectivity of fractures across the site. Figure 6 presents a typical data set generate during the injection tests. Figure 6 Cross-Borehole Data Set

5 The transmissivities of the fractures along the eastern property boundary were found to range from 0.1 to 260 ft 2 /d, based on single borehole tests, and ranged from 100 to 250 ft 2 /d based on the cross-borehole tests. The results of the cross-borehole testing determined that the shallow fracture network in the vicinity of the eastern property boundary is well connected with monitoring wells/fractures in the intermediate system (approximately 90 feet) responding to the injection of water at a depth of approximately 25 feet. The resulting flow and head change in the intermediate system stabilized in 15 minutes from the start of injection, Figure 7 presents the results of the cross-borehole head changes due to the injection of water in shallow monitoring well MW WATER-LEVEL ELEVATION (FT) ELAPSED TIME(MIN) MW65 MW71 MW59 MW51 MW68 Figure 7 Cross-Borehole Test Results, Injection at MW-34 (shallow zone)