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.

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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 of expression. A three-tiered classification scheme was used to differentiate subtle or questionable lineaments from those with moderate or strong expression (the highest level of confidence being associated with the latter class).

After interpreting each set of photographs, the lineaments drawn on the mylar overlays were digitized and stored as a separate Geographic Information System (GIS) map layer. A composite lineament map was then compiled by plotting the lineaments from each set of photos on the base map of the project site.

Fracture Fabric Analysis

Only by comparing lineament trends and fracture fabric data from outcrops in the study area is it possible to evaluate the potential hydrogeologic significance of a given lineament that has been identified in air photos and separate features that are probably attributable to bedrock structure from those associated with other, unrelated phenomena. The basic premise of lineament verification is that an actual fracture trace seen in aerial photos should have some counterpart in the fracture fabric(s) developed in nearby outcrops on the ground.

Beyond lineament verification, the ultimate goal of this task is a preliminary hydrogeologic characterization of the fractured bedrock environment in the study area. Toward this end, fracture fabric analysis involves the geometrical and statistical characterization of the fracture network(s) in a rock mass as well as a detailed description of relevant fracture attributes as they pertain to the circulation and storage of fluids such as groundwater or liquid chemical contaminants. Other important fracture characteristics of potential hydraulic significance include, for example, fracture spacing, persistency (maximum and average fracture dimensions for fractures in a given set), aperture (dilation or separation of fracture walls), abutting relationships with other fracture sets (interconnectivity), spatial variations in fracture intensity (fracture surface area per unit volume of rock), types of vein infillings and fracture surface encrustations (e.g. iron oxide or manganese oxide staining), fracture surface planarity and roughness, and any evidence of faulting or shear displacement (Gale, 1982; Makurat, 1985). Observations regarding such attributes as well as fracture network geometry were used to assess the water-bearing potential of individual fracture sets with respect to groundwater flow through bedrock and fracture network permeability anisotropy.

Geophysical Surveys

As a result of the fracture trace analysis, five different lineaments were identified in close proximity to the client’s property. One of these features is situated on the eastern side of the property. The remaining four lineaments were identified in off-site locations, but their projected extensions transect the land parcel under client-ownership control. All five lineaments exhibited a strong azimuthal correlation with fracture sets known to occur in the study area, although one was found to coincide with a property boundary (and related surface features) and its hydrogeological significance was therefore considered suspect.

Geophysical data were collected along two sets of parallel survey lines; one group of lines was oriented essentially in an east-west direction, the other roughly northwest to southeast. The orientations of the survey lines were determined by the trends of potentially significant lineaments, mapped faults, and systematic bedrock fracture sets identified near the site. The orientation of the survey grid was also constrained by the presence of high-voltage power lines and a nearby railroad. Very-low-frequency electromagnetic (VLF-EM) data were collected using a multi-frequency T-VLF unit with a data stacking capability manufactured by Iris Instruments. Total-magnetic-field-intensity data were procured with a Geometrics model G858 cesium vapor magnetometer.

510 Geophysical data were collected along 12 survey lines totaling 5,590 feet in length. The station spacing along each line (i.e., the distance between successive instrument readings) was 20 feet.

The fracture fabric data and their statistical derivatives were used to constrain the interpretation of high- resolution geophysical surveys and determine the direction and distance of drilling offsets relative to the locations of geophysical anomalies. This involved correlating specific geophysical anomalies with one or more fracture sets mapped in the study area. The geophysical data provided the basis for selecting the test well drilling locations that were most likely to intercept significant water-bearing fractures at depth. Four six-inch bedrock test wells were installed on the land parcel situated east of the railroad tracks. The test wells were positioned to intersect water-bearing fractures inferred from the geophysical surveys. These wells have been designated BTW-1, BTW-2, BTW-3, and BTW-5 (Figure 2). Other, “wildcat” wells (BTW-6 and BTW-7) were installed at the client’s request but failed to intercept major fractures or yield significant water.

Pumping Tests

In May of 2002, six six-inch-diameter bedrock water supply test wells were installed at the study site to an average completion depth of 600 feet below grade. These included wells BTW-1, BTW-2, BTW-3, BTW-5, BTW-6, and BTW-7.

Three different types of pumping test were performed at the study site between August 2002 and January 2003. These tests included: 1) Individual step-rate tests in 5 bedrock water supply wells (BTW-2, BTW-3, BTW-5, BTW-6, and BTW-7); 2) Three separate constant-rate tests that consisted of simultaneous pumping of 2 to 5 wells; and 3) A long-term pumping test of BTW-2 at a rate just below 69 gpm.

In August and September of 2002, pumping tests were performed on the five highest yielding wells (BTW-2, BTW-3, BTW-5, BTW-6, and BTW-7). The pumping tests were conducted during a period of minimal groundwater recharge and under severe drought conditions that prevailed during the summer and early fall months of 2002. Therefore, in terms of assessing potential impacts attributable to pumping and evaluating well yield, the adverse hydrologic conditions that existed at the time of testing represent a near-worst-case scenario.

Between September 2002 and January 2003, a long-term (123-day) pumping test was conducted in BTW-2.

Twenty-eight observation stations were monitored during the tests. The network of observation points included: the six on-site bedrock test wells; six on-site overburden monitoring wells; and three piezometer couplets (two of which were installed in bordering vegetated wetlands adjacent to the river); two staff gages in the adjacent river (one upstream and one downstream of the study site); and two on-site rain gages. Additional overburden observation wells in the nearby municipal wellfield were also monitored with the permission of the town water company.

In each case, quantification of sustainable yield was predicated on attainment of the water level stabilization criterion. For purposes of well performance evaluation, the stabilization criterion was satisfied if on semi- logarithmic plots of drawdown versus time, the drawdown curve remained above the highest, major water- producing fracture in the well when extrapolated out to 180 days, thereby simulating a prolonged, no-recharge scenario.

Following completion of the step-rate tests, three separate constant-rate pumping tests were run. Each constant- rate test was of five day’s duration and involved simultaneous pumping of two to five wells. In each constant- rate test, all pumping wells achieved stabilization under the criterion defined above. In most cases, the pumping wells achieved 96 to 98 percent recovery within 24 hours of pump shut down.

The third constant-rate test involved pumping all five water supply wells at near their maximum combined capacity. The combined yield, which was sustained over a five-day period, was 154 gpm or 221,760 gallons per day (gpd).

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In an attempt to determine what affects, if any, prolonged pumping has on measured concentrations of MTBE in groundwater in the vicinity of the site, well BTW-2 was pumped at approximately 69 gpm (99,360 gpd) for a 123-day period. During the long-term pumping test, groundwater samples were collected from the pumping well every three to four days and submitted for analysis of volatile organic compounds (VOCs) and MTBE using USEPA Method 8260B.

RESULTS

Part of the Phase-One preliminary feasibility assessment that was conducted in the late fall of 2001 involved a detailed bedrock fracture fabric analysis of the project site and surrounding environs. Pertinent aspects of that analysis, as they relate to test well siting and the development of a conceptual hydrogeologic model of the study area, are summarized herein.

Fracture Fabric Analysis

The outcrops examined for the fracture fabric analysis were situated between 1,700 and 4,600 feet from the test drilling site and consisted of natural exposures, road cuts in housing developments, and cuts along an abandoned railroad track. (No bedrock exposures were found within 1,700 feet of the facility.) All outcrops consisted of Precambrian metamorphic rock including granitic gneiss, schist, and quartzite. At each outcrop the orientations (strike and dip) of steeply dipping, systematic fractures were recorded. A total of 189 separate readings were made. During the collection of fracture fabric data, three different stations were established in the study area. Each station was defined by a group of several outcrops located within a relatively small sub- area. It should be noted that the outcrop groupings do not imply, or necessarily coincide with, any structural domains.1 They have been defined solely on the basis of map location for purposes of facilitating data reduction and statistical analysis. Between 44 and 82 strike-and-dip measurements were recorded at each station; an average of 63 measurements were made within each of the three sub-areas.

Digitally filtered fracture trend histograms and rose diagrams2 were generated for each sub-area as well as for all three stations combined. Corresponding pi diagrams (showing poles to fracture planes) and contoured stereonet plots were also compiled. Cursory inspection of the histograms and rose diagrams shows that the bedrock fracture fabrics present in the vicinity of the facility are relatively complex and highly variable in character. A visual comparison of the rose diagrams from the individual stations suggests that each sub-area falls within a separate fracture fabric domain (i.e., the fracture fabrics seen in each group of outcrops are significantly dissimilar). When the fracture fabric data from all three stations are viewed in aggregate, it is apparent that there are at least five, and perhaps as many as seven distinct fracture sets developed in outcrops located within one mile of the facility.

Character of the Major Water-Bearing Fracture Zone

Pertinent results of the test-drilling program are summarized in Table 1 below. Well yields, the character of mineralization in water-bearing fracture zones, and pronounced interference effects observed during drilling indicate that the three most productive test wells, BTW-2, BTW-3, and BTW-5 penetrate the same fracture zone. The fracture zone is characterized by extensive chloritization and pyritization. The presence of chlorite is attributed to retrograde metamorphism (biotite → chlorite) that probably occurred due to hydrothermal fluid

1 For the purposes of this analysis, a domain is defined as an area within which fracture fabric geometry is fairly consistent, both in terms of the number of fracture sets present, their orientations, and the relative intensity of development of each fracture set. Rose diagrams from the same domain should be similar in appearance. Conversely, two rose diagrams that are markedly different in their shape and appearance indicate that the fracture fabric data used to generate the diagrams belong to different fracture fabric domains. 2 The filtering algorithm used to generate the histograms and rose diagrams involves calculating a running average at one- degree increments using a counting window 10 degrees wide. The filter is initially applied to the raw frequency data; the resultant smoothed frequency distribution is then filtered a second time using the same algorithm.

512 circulation within the fracture zone. Locally, calcite (CaCO3) veining is also evident within this water- producing zone, which probably accounts for the moderate hardness of the well water.

Based on the surveyed test well locations (x, y, and z for BTW-2, BTW-3, and BTW-5) and the depths within each well where the water-bearing fracture zone was intercepted (Table 1), the overall orientation of the fracture zone is calculated to be roughly 332o, 69o. This compares favorably with the generalized trend of the correlative geophysical anomalies defined by the VLF-EM and geomagnetic data (336o to 344o).

Table 1: Summary of Test Well Characteristics

Depth of Casing Major Static Total Depth to Bottom of Approx. Stick Up, Test Well Water- Water Depth Bedrock Casing Well Yield (ft above No. 1 Producing 3 Level (ft bls ) (ft bls) (ft bls) (gpm) 4 Fractures (ft btoc ) land (ft bls)2 surface) 100 & 127- 5.25 (5.51) BTW-1 531 21 29.5 4 0.70 128 [7.16]

3.65 (4.05) BTW-2 502 19 30 260-266 100 1.30 [5.67]

262-270, 2.69 (2.97) BTW-3 600 17 29 46 1.50 280, & 285 [4.58]

195-215, 4.86 (5.21) BTW-5 600 24 34 308-312, 70 1.50 [6.84] 375, & 568

1 below land surface (bls) 2 fracture depths shown in bold typeface are correlated with the main, water-producing fracture zone encountered in each well 3 yield, in gallons per minute (gpm), as determined from step-rate pumping tests with the exception of BTW-1 which was determined from a sustained air-lift test at the time of well completion 4 below top of casing (btoc), as measured May 20, 2002; number in parentheses is the static water level measured on June 7, 2002; number in brackets is the static water level measured on July 2, 2002

Comparison Between Bedrock Fracture Fabrics and the Orientation of the Water-Bearing Zone

The inferred trend of the main water-bearing fracture zone is sub-parallel to a system of north-northwest trending, high-angle faults and fractures originally noted by Volckmann (1977). The calculated strike and dip of the water-bearing zone are also remarkably consistent with fracture fabric data collected from outcrops located within 4,000 feet of the test-drilling area. In fact, the second most common fracture set developed in the immediate vicinity of the wellfield strikes north-northwest/south-southeast and is denoted by the frequency peak at 140o/320o in the fracture trend histogram for this sub-area (Fig. 3).

Of all the fractures measured in outcrops near the site, over one third (37 percent of the sample population) belong to this set. Moreover, of all the north-northwest trending fractures measured within 4,000 feet of the wellfield, 93 percent dip to the northeast (i.e., in the same direction as the water-bearing fracture zone in wells BTW-2, BTW-3, and BTW-5). Of all the north-northwest trending fractures that dip to the northeast, the average dip is about 71o which agrees well with the inferred dip of the water-bearing fracture zone (approximately 69o).

513 Fracture fabric data from Station C (Fig. 4) exhibit what is perhaps the best correlation with the inferred trend of the water-producing fracture zone and its associated geophysical anomalies. At Station C, located approximately 4,400 feet south-southwest of test wells BTW-2, BTW-3, and BTW-5, north-northwest trending fractures comprise roughly 43 percent of the sample population (n = 63); they constitute the most intensely developed fracture set in this sub-area. In the group of outcrops examined at Station C, the observed range of strike azimuth for north-northwest trending fractures is 140o to 179o (320o to 359o). Of the 27 north-northwest trending fractures measured at Station C, 93 percent dip to the northeast at an average angle of about 61o as shown by the contoured stereonet plot for poles to fracture planes in this group of outcrops (Fig. 5). The vector mean of all measured northeast dipping fractures in this set is 338o, 60o. Thus there appears to be a close match between the orientation of north-northwest trending fractures observed at Station C and the orientation of the main water-bearing fracture zone intercepted by wells BTW-2, -3, and -5 (332o, 69o). The correlation is illustrated in Figure 4 by the bold vertical line on the right-hand side of the histogram that has been included to denote the trend of the water-bearing fracture zone (152o/332o): the frequency peak for the corresponding fracture set occurs only four degrees away at 148o. The rose diagram for Station C (Fig. 6) dramatically illustrates the same azimuthal correlation: the trend of the water-bearing zone is indicated by the black arrows on the perimeter of the plot.

There is also a strong correlation between the calculated orientation of the water-producing fracture zone and faults observed in outcrops located about 4,400 feet south of the test-drilling site. The vector mean orientation of eight slickensided surfaces measured at Station C is 329o, 63o. In addition to attributes indicative of brittle failure, these faults are characterized by evidence of ductile deformation and shearing (in the form of a stretch lineation oriented parallel to slickenlines and overall grain size reduction) and abundant iron oxide encrustation. Slickenlines observed on north-northwest trending fault planes generally pitch at steep angles (72 to 87o) to the north indicating dip-slip movement.

Given the character of drill cuttings from pertinent borehole intervals and the fact that the orientation of the water-producing fracture zone in BTW-2, BTW-3, and BTW-5 is similar to those of faults observed in nearby outcrops, it is reasonable to conclude that the water-bearing fracture zone intersected by the wells is a fault and/or ductile shear zone. Pumping test results and the distance of separation between BTW-5 and BTW-3 demonstrate that this zone is continuous and hydraulically transmissive over a distance of at least 160 feet whereas geophysical data suggest that the fracture zone extends for a minimum of at least 210 feet along strike.

Bedrock Aquifer Response to Pumping

A long-term pumping test was performed at the site to determine the sustainable yield of the aquifer, assess potential impacts on the surficial aquifer and nearby stream and wetlands, and to evaluate any trends in measured MTBE concentrations during an extended period of withdrawals. Bedrock well BTW-2 was pumped at an average rate of approximately 69 gpm for 123 days.

Fracture fabric analysis and pumping test data confirm that BTW-2, BTW-3, and BTW-5 are hydraulically connected and likely tap the same north-northwest trending water-bearing fracture. It was also surmised that BTW-6 might be indirectly connected to the main water-bearing fracture zone in BTW-2, BTW-3, and BTW-5 by the north-northeast or east-northeast trending fracture sets that correspond with frequency peaks at 25 and 78o, respectively, in Figure 4. As displayed in Figure 7, the long-term testing suggests a hydraulic connection between these wells as the drawdown at BTW-6 falls on the same distance-drawdown line as BTW-2, BTW-3, and BTW-5. Wells BTW-1 and BTW-7, however, have a distinctly different distance-drawdown relationship as compared to the other bedrock wells (See Figure 7). Although limited hydraulic connection is evident between wells BTW-1, BTW-7, and BTW-2, the connection is much less pronounced than observed in the other bedrock wells. A plan view of the drawdown after 90 days of pumping is displayed in Figure 8. Note from Figure 8, the correlation between the drawdown contours and the rose diagram (Fig. 6) displaying the orientation of fractures in the vicinity of the site.

In addition to monitoring bedrock wells during the pumping test, overburden wells, streamflow, and riparian wetland wells were monitored. No discernible impacts to overburden wells, streamflow or conditions in riparian wetlands were attributed to pumping during the testing performed for this project, which occurred during the 2002 drought.

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Based on these observations, it was concluded that the majority of water withdrawn from BTW-2 was derived from groundwater in the bedrock aquifer flowing through the main north-northwest trending fracture zone.

Groundwater Quality Response to Pumping

The quality of groundwater was assessed in order to identify any need for treatment (e.g., pH control, softening) prior to use and to assess the potential for contamination. The first groundwater samples were collected following the execution of several step-rate tests and short-term constant-rate pumping tests on the various wells. These samples indicated the presence of low levels (less than 5 µg/l) of methyl tertiary-butyl ether (MTBE) in several of the bedrock wells. In response to this observation, a survey of potential MTBE sources in the area was developed, additional sampling was performed, and an on-going assessment of MTBE fate and transport in the bedrock system was begun. Perhaps not surprisingly, the survey of potential MTBE sources indicated approximately a dozen within ½ mile of the site, with three properties in which MTBE had been measured in excess of 1 mg/l in groundwater. Given the uncertainty regarding the potential source, the fracture fabric analysis was critical in the successful conceptualization of MTBE fate and the development of a monitoring and management strategy.

As indicated in Figure 9, groundwater extraction strongly affects water levels in the fractured bedrock aquifer. The observed depth to water increased sharply early in the extended pumping test and remained relatively steady through the later portions. Following the completion of the test, the water level quickly rebounded. Similarly, during a short period of groundwater use in April 2003, water level responded rapidly. As described above, groundwater extraction resulted in a sharp head gradient from both BTW-3 and BTW-5 to BTW-2. In the absence of groundwater extraction, the inferred head gradient is downward from BTW-5 to BTW-2 to BTW-3 along the trend of the major fracture zone. Thus, in the absence of pumping, the major source of water to BTW-2 is from the north-northwest along the fracture (i.e., through BTW-5). Under pumping, water is derived from both directions along the fracture.

The spatial and temporal pattern of observed MTBE concentration during groundwater withdrawals strongly suggests that the contaminant comes to BTW-2 from the direction of BTW-3. With one exception, under pumping or soon after pumping, the relative concentration of MTBE in the wells along the north-northwest trending fracture zone is as follows: BTW-3 > BTW-2 > BTW-5. The one exception (indicated in Figure 9 with a highlighted ?), is likely attributable to a laboratory performance issue. In absence of pumping, the concentration of MTBE in all three wells declines and eventually is non-detectable. It is also noteworthy, that the concentration of MTBE in BTW-2 (and likely BTW-3) increases relatively steadily as pumping continues. On the other hand, the MTBE concentration in BTW-5 is largely unaffected by pumping.

The candidate MTBE sources are arrayed in two locations. One cluster (several potential sources and one documented to have high MTBE in groundwater) exists approximately one mile to the north-northwest, essentially astride the inferred trend of the major water-bearing fracture zone. The other cluster (two sites with confirmed MTBE in excess of 1 ppm) lies about 1/2 mile west-southwest of BTW-2, well off of the trace of the major water-producing fracture zone. The second cluster of sites occurs near several mapped outcrops and in close proximity to the western limit of the stratified (alluvial) overburden materials where they lap up onto the bedrock highlands along the west side of the valley (see Figure 2). No significant development of any kind is located to the south-southeast along the trace of the major water-bearing fracture zone.

The weight of evidence suggests that the source area to the west-southwest is the most likely to have affected the bedrock aquifer at BTW-2. MTBE is apparently not transported from the north (i.e., through BTW-5) and the potential sources that lie in that direction. MTBE does appear to be derived from the south (i.e., BTW-3).

The east-northeast trending fracture set at Station C correlates with the frequency peak at 78o in Figure 4. Almost one quarter of all fractures measured at Station C belong to this fracture set. The east-northeast trending fractures measured at Station C have strikes that range between 62 and 104o and average about 81o. Field observations indicate that members of this fracture set are typically developed as sub- to non-persistent meso- scale joints of limited areal extent. It is therefore unlikely that a single through-going fracture would

515 hydraulically connect a distant contaminant source with the high-capacity bedrock wells on the east side of the railroad tracks. Rather it is more plausible that a series of interconnected east-northeast trending fractures might create a less efficient and more tortuous hydraulic connection between the source and the wellfield.

It is particularly noteworthy that the azimuth of a line connecting BTW-3 and one of the suspected sources of MTBE, a former gas station with documented subsurface releases located about 2,400 feet west-southwest of the wellfield, is 74o. The line of sight between the potential source and receptor is shown on the fracture trend histogram for Station C (Fig. 4) as a bold vertical line centered at 74o. The histogram illustrates that the inferred flow direction from the suspected source to the wellfield under pumping conditions is generally coincident with the orientation of the east-northeast trending fracture set.

A plausible interpretation is that under pumping conditions, groundwater flow in the bedrock aquifer is induced along the east-northeast trending fracture set, from the potential source area toward BTW-3. At BTW-3, the east-northeast trending flow path is intersected by the through-going, north-northwest trending fracture zone, and the contaminated groundwater then flows from BTW-3 along it to BTW-2. In the absence of a pumping stress, the natural head gradient is restored and the MTBE is largely purged from the main fracture zone and/or transport of MTBE along the secondary east-northeast trending fracture set is significantly reduced.

DISCUSSION

The techniques described in this paper are those that are sometimes applied during water supply development of bedrock aquifers. The advantages such techniques bring to a water supply development program are very clear: the chances of encountering a major fracture set with exploratory boreholes were greatly improved. In fact, three of four wells sited using these techniques were able to intercept a major water yielding fracture and did so at a depth that allowed for adequate available drawdown within each borehole. In contrast, two wildcat wells sited based on convenience failed to intersect major water-bearing fractures. Given the high cost of bore holes as well as the importance of penetrating large, hydraulically conductive fractures to well yield, any improvement in the probability of well success is critical.

The development of a conceptual hydrogeological model of the site and the interpretation of geophysical surveys and pumping test data were facilitated by fracture fabric analysis. Four important phenomena were explainable based on the results of the fracture fabric analysis and the other methods of investigation employed:

• The first observation is the isolation of the overburden aquifer and the adjacent river from the hydraulic influence of bedrock wells. The nature of the fracture system (and the overlying basal till) strongly suggests that the bedrock aquifer is recharged at locations removed from the site including areas of extensive bedrock outcrops located west, northwest, east, and southeast of the site. • Small fractures located within the rock to the east of the production wells have poor hydraulic connection to the production wells and have little or no detectable MTBE. This is consistent with the frequency distribution of fracture orientations defined during the initial mapping effort. • One of the wildcat wells, BTW-6, had good hydraulic connection to the production wells as well as the source(s) of MTBE. These circumstances are consistent with evidence of fractures defined by the fracture trace and fracture fabric analyses. • While there are several candidate sources of MTBE to the bedrock aquifer, the fracture analysis as well as the response of water level and groundwater quality to extended pumping strongly suggests that one of them is the most likely source.

In summary, the application of the fracture fabric analysis as well as more routine (though rigorous) aquifer pumping tests and investigation of water quality allowed for the successful development of a supplemental water supply. Given the constraints at the site, success had to include a relatively high yielding well, the absence of adverse hydraulic influence to other water resources, and manageable contaminant threats.

We believe that application of fracture fabric analysis can be fruitful not only in the development of bedrock aquifers but in the investigation of contamination of bedrock aquifers. The advantages to such relatively low

516 cost, non-invasive techniques to the conceptualization of bedrock flow systems are clear. When supplemented with properly targeted well installations, aquifer pumping tests, and correlation with contaminant distribution patterns, a strong working knowledge of the hydraulics and constituent transport characteristics of the bedrock aquifer can emerge.

REFERENCES

Gale, J.E., 1982, Assessing the permeability characteristics of fractured rock: Geological Society of America Special Paper 189, p. 163-181

Makurat, A., 1985, The effect of shear displacement on the permeability of natural rough joints: in, Hydrogeology of Rocks of Lower Permeability, International Association of Hydrogeologists Memoirs, v. 17, parts 1 and 2, p. 99-106.

Volckmann, R.P., 1977, Bedrock geologic map of the Holliston and Medfield Quadrangles, Middlesex, Norfolk, and Worcester Counties, Massachusetts: U.S. Geological Survey, Miscellaneous Investigations Map I-1053, 1/48,000-scale.

Biographical Sketches

Daniel W. Folan 2 Technology Park Dr. Westford, MA 01886; 978/589-3054; fax: 978/589-3100; [email protected]. Dr. Folan has more than 15 years experience in the environmental and water resources industries. He specializes in hydrogeological and contaminant geochemistry. Dr. Folan holds a Ph.D. in Geochemistry from Colorado School of Mines. He is a PG in IN, ME, and NH and is a LSP in the Commonwealth of Massachusetts.

Dennis S. Albaugh 401 Gilford Avenue, Suite 220, Gilford, NH 03249-7536; phone: 603/524-8866; fax: 603/524-9777; [email protected]. Mr. Albaugh is ENSR’s Director of Hydrogeology and Water Supply. He has 26 years of experience in environmental geologic consulting. As a structural geologist, his areas of expertise include: fracture fabric analysis, air photo interpretation, and remote sensing. He holds an A.B. in Earth Science from Dartmouth College and an M.S. in Geophysics and Geotectonics from Cornell University.

A. Curtis Weeden, Jr. 2 Technology Park Dr. Westford, MA 01886; 978/589-3066; fax: 978/589-3100; [email protected]. Mr. Weeden is a Project Hydrogeologist who has over 6 years of experience in hydrogeological and water resources related field, including groundwater modeling, hydrogeological characterization, impact analysis, environmental permitting and well field design. He holds an M.S. degree in Hydrology from the University of Arizona.

Mark Gerath 2 Technology Park Dr. Westford, MA 01886; 978/589-3189; fax: 978/589-3100; [email protected]. Mr. Gerath is a Senior Program Manager for Water Resources. He specializes in water supply and wastewater discharge permitting and has project experience across the United States. His experience includes both surface water and groundwater resources as well as contamination assessment. He holds M.S. degrees from Cornell University and MIT.

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Figure 1: Conceptual block diagram showing general site geology and likely source area

Figure 2: Site Map

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Figure 3: Fracture trend histogram for outcrops near wellfield and project site. Frequency of occurrence is shown as a function of fracture-strike azimuth. Frequency peaks are labeled in degrees measured clockwise from true north.

Figure 4: Fracture trend histogram for Station C located about 4,400 feet south-southwest of wellfield. Frequency of occurrence is shown as a function of fracture-strike azimuth. Frequency peaks are labeled in degrees measured clockwise from true north. Bold vertical line on right corresponds to calculated strike of major water-bearing fracture zone in wells BTW-2, BTW-3, and BTW-5. Bold vertical line near center of histogram denotes direction from suspected contaminant source to wellfield.

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Figure 5: Contoured stereonet plot of poles to fracture planes measured at Station C (n = 63). Schmidt net, lower hemisphere projection. Contoured intervals, from light to dark, are: 3 to 6%, 6 to 9%, 9 to 12%, 12 to 15%, and > 15% per 1% area. The highest concentration of poles corresponds to the north-northwest trending fracture set which dips predominantly to the northeast.

Figure 6: Rose diagram for fractures measured at Station C (the same data set as displayed in the corresponding fracture trend histogram, Fig. 4). The prominence of the north-northwest trending fracture set is readily apparent. The trend of the water-bearing fracture zone intercepted by BTW-2, BTW-3, and BTW-5 is indicated by the arrows outside the perimeter of the plot.

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BTW-2 pumping 68.35 gpm over 90 days BTW-7 10

20 Bedrock Wells NOT within Fracture Network 30

40 BTW-1

BTW-6 50

BTW-3 Bedrock Wells within Fracture Drawdown (feet) 60 BTW-5 Network

70

80

90

BTW-2

100 1 10 100 1000 10000 Distance from BTW-2 (feet)

Figure 7: Observed aquifer drawdown with distance from pumping well illustrating the importance of contact with main water-producing fracture set.

Figure 8: Observed aquifer drawdown after 90 days of pumping BTW-2 at approximately 69 gpm.

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45 0

40 Short-Term Groundwater Use 20 35 ll L) 30 40 We g n ug/ i p m on ( i u

t 25 a P r 60

20 (ft) in h t p e

15 Long-Term Pumping Test 80 r D te MTBE Concent MTBE in "Down-Fracture" Well Wa

10 MTBE in Pumping Well 100 5 MTBE in"Up-Fracture" Well

0 ? 120 3 3 02 03 02 03 - - - - l-0 y-0 ep-02 ug-03 ep-03 an-03 un-03 an-04 Ju Oct-02 Apr-03 Oct-03 J J J Mar-03 Mar-04 Feb-03 Feb-04 Nov Nov S Dec A S Dec Ma Date

Figure 9: Time series of bedrock aquifer water level and MTBE concentration in three wells located along the major water-bearing fracture zone. MTBE concentrations in BTW-2 and BTW-3 increase under extended aquifer pumping but that observed in BTW-5 is much less sensitive. This has important implications for the conceptualization of the source(s) of MTBE as well as delineation of aquifer flow paths.

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