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Use of Fracture Fabric Analysis Facilitates Well Siting and Assessment of Contaminant Distribution in Bedrock

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Use of Fracture Fabric Analysis Facilitates Well Siting and Assessment of Contaminant Distribution in Bedrock Use of Fracture Fabric Analysis Facilitates Well Siting and Assessment of Contaminant Distribution in Bedrock Daniel Folan, PhD, PG; Dennis Albaugh, PG; A. Curtis Weeden, Jr., PG; and Mark Gerath, ENSR International Abstract A supplemental water supply from a granitic bedrock aquifer in New England that has a potential maximum sustainable yield of 0.25 million gallons per day (MGD) was developed using fracture fabric analysis. The project site was highly constrained due to its relatively small size, the presence of potential contaminant threats in the area, and its proximity to other water resources. Success of this program was due to careful execution of bedrock fracture fabric analysis during the planning phase as well as refinement of the analysis during the subsequent investigation phase. Not only did the fracture fabric analysis optimize well siting from a water yield perspective but it also greatly facilitated understanding and evaluation of contaminant threats and potential impacts to other water resources. The approach to, and data from, three phases of this effort (fracture trace and fracture fabric analyses; well drilling and logging; and aquifer pumping test and water quality investigation) will be presented. The fracture fabric analysis was developed from three components that involved: (1) a review of published literature and inventory of mapped faults and fracture networks in the area; (2) inspection of air photos for lineaments indicative of bedrock structure; and (3) mapping the orientation of several hundred fractures at numerous bedrock outcrops near the project site. While these components were independently assessed, correlation of findings between the methods was taken as support for the presence of fractures with potential hydrogeologic significance. Based on the fracture fabric analysis (as summarized by rose diagrams of fracture strike azimuth), a geophysical survey utilizing very-low-frequency electromagnetic and total-field-intensity geomagnetic methods was used to pinpoint the location of an inferred bedrock fracture on the site. The geophysical survey resulted in the detection of subtle anomalies that, in terms of orientation, were well correlated with slickensided fractures mapped elsewhere within the study area. To intercept the inferred fracture zone at a depth of several hundred feet and thereby maximize available drawdown for water supply development, test wells were installed to the east of the anomalies based on the observed dip of correlative near- surface fractures. Three of the four test wells intersected the fracture zone as evidenced by significant increases in water yield at target depths and pronounced interference effects between wells observed during preliminary air-lift tests. The air-lift capacities of the three interconnected test wells were 40, 70 and 100 gallons per minute (gpm). An extensive pumping test program was executed to evaluate sustainable yield as well as potential impacts to nearby water resources. Step-rate tests and long-term constant rate tests were performed on the test wells while 28 water level observation points were monitored. Observation wells were located in the overburden aquifer (including a nearby municipal wellfield), the bedrock aquifer (both within and outside the major fracture zone), and in the adjacent river and wetlands systems. Test results were consistent with the conceptual model developed from a knowledge of overburden stratigraphy and fracture network geometry. The bedrock production wells are located in an urban area and are proximal to several potential sources of groundwater contamination. A review of state records indicated petroleum hydrocarbon contamination in some nearby surficial aquifer monitoring wells. Water quality testing during bedrock well development indicated that methyl tertiary butyl ether (MTBE) was present in various bedrock wells. As confirmed by an extensive data set, the concentration of MTBE in the various wells changed in response to pumping in a way that was consistent with fracture fabric geometry and hydraulic gradients induced by pumping. These trends were used to develop a site conceptual model, to demonstrate to public officials that the contaminants do not represent a threat to the public water supply and to design a groundwater quality monitoring network. INTRODUCTION 508 In late 2001, severe drought conditions in the New England area prompted a local private industrial entity to re- examine their water supply alternatives. As a result, the potential for development of a supplemental bedrock groundwater supply on the client’s property was evaluated. The results of a preliminary assessment using fracture fabric analysis indicated the presence of potential water-bearing bedrock fracture zones. A bedrock test well investigation suggested that several wells could produce meaningful amounts of water. During the water supply investigation, the gasoline additive MTBE was observed in groundwater from the bedrock system. In addition to aiding in the identification of potential water-bearing zones, fracture fabric analysis was useful in evaluating the likely source and migration pathways of MTBE in the fractured bedrock aquifer. Project Setting and Site Hydrogeology The study area is located within a typical New England river valley. The area is within an urbanized setting and the surrounding land is zoned for residential, commercial and industrial uses. The study site abuts the bank of a local river several miles downstream of its headwaters. Other than riparian wetlands associated with the river, no surface water features exist within the site boundary. Uplands are located on each side of the valley where bedrock outcrops are visible. A conceptual block diagram of the study site is provided in Figure 1. The overburden aquifer of the site consists of a thin layer of alluvium associated with the small river. These deposits are comprised primarily of fine-grained, low-permeability material with occasional lenses of sand and gravel. The latter, coarser-grained material, however, failed to produce significant amounts of water and is consistent with the notion that these channel lag deposits are interbedded with, and enveloped by, fine to very fine alluvial sand, silt, and clay. A two- to eight-foot thick layer of basal till was observed at several locations directly overlying the fractured bedrock aquifer, which is approximately 20 feet below land surface (bls) in the vicinity of the site. The fractured bedrock aquifer beneath the site consists of highly deformed, Precambrian metamorphic rocks. Lithologies found in the study area include a granitic gneiss as well as metasedimentary and metavolcanic rocks consisting primarily of quartzite, biotite schist, and amphibolite. The rocks of the study area are complexly deformed by multiple generations of folds. In some areas, this polyphase deformation is evidenced by dome- and-basin interference structures although there is no discernible regional-scale fold pattern developed in the Precambrian rocks in the vicinity of the project site. The primary recharge mechanisms to the fractured bedrock aquifer are infiltration directly into bedrock in the uplands that border the river valley, and infiltration through the overburden. This paper will review the methods used to develop a cost-effective water supply using a variety of different investigation techniques that together yield a coherent conceptual model of the bedrock system. Application of these methods facilitated water supply development in a highly constrained setting. This included development of test wells that intercepted a large, water-bearing fracture zone as well as development of a supply that did not unduly influence local water resources such as the river and its wetlands. Contaminant threats were identified and managed within the context of site data and the conceptual model. We believe that application of such techniques is appropriate for water supply projects as well as investigation of groundwater contamination in bedrock. METHODS A variety of investigative methods were used in an effort to generate a conceptual hydrogeologic model of the study area and identify test drilling locations as part of the groundwater resource exploration and development program. Some of these methods have been applied routinely in groundwater exploration programs involving fractured rock aquifers while others have probably received less attention in the literature than they deserve and consequently their scope of application has been somewhat limited within the consulting community. In this particular case, three different yet complimentary fracture mapping techniques were used to successfully identify test drilling sites for high-yield bedrock wells. Each is briefly described below. These methods were supplemented by more routine aquifer pumping tests and groundwater quality investigations. 509 Fracture Trace Analysis Before initiating field work, a fracture trace analysis of the exploration area was performed in an effort to identify lineaments of potential hydrogeologic significance. This entailed the analysis of three sets of aerial photographs of differing scales, acquisition dates, and film type (black-and-white and color infrared). The air photos were examined using a variable-power mirror stereoscope and multiple observers were employed in an attempt to reduce observer bias. Lineaments were marked on clear mylar overlays affixed to the photographs and classified according to a subjective evaluation of their strength
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