Hydrologic and Borehole Geophysical Investigation of Bedrock Observation Wells at the University of Maine

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1 Hydrologic and Borehole Geophysical Investigation of Bedrock Observation Wells at the University of Maine Abstract Eric Rickert, Andrew Reeve, Frederick L. Paillet, University of Maine The University of Maine recently installed 5 clusters of monitoring wells on its campus to evaluate groundwater flow in a fractured bedrock aquifer and overlying glacial sediments. These well clusters are positioned in transects parallel to the Stillwater River and from the Stillwater River to the crest of a hill. These monitoring wells were installed to provide students with practical field experience and to give a physical context for hydrology classroom exercises. Present within each cluster of monitoring wells is an approximately 250- foot deep bedrock well, and up to 2 wells screened in the overburden. Hydrological Study A 17 to about 70 feet thick confining layer of glacial till overlie the fractured bedrock aquifer. Water levels within the bedrock and overburden wells have been monitored for the last year. A downward gradient was observed at the hillcrest, assumed to be the recharge area. A strong upward gradient was observed at the Stillwater River, the discharge area. A seasonally variable upward gradient was observed between the discharge and recharge area. A horizontal gradient of 0.01 was observed between the recharge area and the discharge area. Seasonal meter-scale variability in the water levels was measured in the bedrock wells. The hydrograph response of the water levels of the river and a bedrock well in the close vicinity demonstrates that the river is intimately connected to the nearby bedrock aquifer. Groundwater samples were collected from these wells and analyzed. Specific conductance in the bedrock wells ranged from 142 µs/cm in the recharge zone to 866 µs /cm in the discharge zone. Elevated chloride concentrations suggest that road salt has impacted the aquifer. Calcium concentration data indicates that sodium from the road salt is undergoing cation exchange and sorbing to mineral surfaces. Nitrogen concentrations measured in the ground water suggest that past agricultural activity has impacted the bedrock aquifer. Nitrogen concentrations tend to be lower in the shallow wells, suggesting that improved agricultural practices have reduced nitrogen loading to the bedrock aquifer. Geophysical Study Bedrock fractures within the boreholes were identified by a three-arm Mount Sopris 2CAA-1000 Caliper Probe. An anomalously large fracture, over-range of the instrument, was observed in one of the boreholes. Caliper logs indicate greater fracturing in the upper zone beneath the bedrock hill. A sub-horizontal fracture appears to occur across the site. Using a Mount Sopris HFP-2293 Heat Pulse Flow Meter it was demonstrated that two of the bedrock wells locations have only one dominant productive fracture, these correspond to the well with the largest yield and the smallest yield. The bedrock well in the close vicinity of the Stillwater River has three separate productive fractures and is the only borehole with measurable flow under ambient conditions. Additional water level monitoring, geochemical analysis and Heat Pulse Flow Meter logging is ongoing. 532

2 Figure 1 Site Location with Borehole Locations. 533

3 Introduction Description of Study The University of Maine is located in the town of Orono in central Maine, on the southwest corner of Marsh Island between the Stillwater River and the Penobscot River. The Penobscot River, located on the eastern edge of Marsh Island, is dammed upstream of the study area for power generation. Dams on the Stillwater River, east of Marsh Island are approximately two miles apart, bracketing the study area (figure 1.) The bedrock is a very low permeability metamorphic rock (Devonian Vassalboro Formation), overlain by low permeability glacial till and glaciomarine silt and clay (Presumpscot Formation). The bedrock aquifer has little intergranular pore space and groundwater flow is controlled by bedrock fractures. The bedrock boreholes were drilled using air-hammer water well-drilling rigs. The overburden wells were installed using a geotechnical boring drilling rig, using either hollow stem augers or drive and wash method with smooth casing. Statement of Purpose The purpose of this study was to characterize the bedrock aquifer using borehole geophysical equipment. Fracture locations and flow rates within the boreholes are used to evaluate fracture transmissivity and hydraulic heads associated with individual fractures. This information will be used to support educational activities in environmental classes at the University of Maine and provide basic information needed to conceptualize fractured bedrock aquifers in Maine. Methods Hydrological Study Telog data loggers with pressure transducers were placed in the boreholes at the recharge zone and at the discharge zone. Water levels were also taken using an electronic water-level indicator throughout spring, summer and fall (figure 2). All wells were surveyed using dual-frequency differential geographic positioning systems. Additionally, PPL Corp. the owner of the dams above and below the study site has supplied the water elevation data for the headwater and tailwater of the upper dam (figure 3). Data for the lower dam is not available, as it is not actively being used for power production. Water-chemistry samples have been taken at each of the well sites and at the river. Water samples were collected after the overburden wells had been purged. The bedrock wells were pumped until the ph, conductivity and temperature remained at a constant value and were not fully purged due to their large storage capacity. The samples were filtered (0.2 micron) during collection, cation samples were acidified, and samples were stored in coolers. Samples were analyzed for Mg, Ca, Na, K, SO4, Cl, Alkalinity, Total N, Specific Conductivity and ph (Table 1 and figure 4). Specific conductance and ph were measured in the field using electrical probes. Alkalinity was measured in the field by Gran titration (Stumm and Morgen, 1981) using a Hach digital titrator. Cations and anions were measured at the University of Maine s Environmental Chemistry Laboratory by inductively coupled plasma spectroscopy and ion chromatography, respectively. Vertical gradients between the bedrock borehole and the overburden wells were calculated for each well cluster. Vertical hydraulic gradients were calculated each time that water levels were collected by electronic water-level indicator using the depth to the most productive fracture in the borehole to the bottom of the overburden well screen. The value and date of the measurement for the maximum and minimum gradient are shown in table

4 Geophysical Study Bedrock fractures were identified and general condition of the borehole was examined by using a three-arm Mount Sopris 2CAA-1000 Caliper Probe. The 2CAA-1000 Caliper is lowered to the bottom of the borehole and measurements are made as the probe is raised in the borehole, measuring the diameter with three linked arms operating a single resistive sensor (figure 5). (Mount Sopris, 2001) Adiabatic flow conditions were measured at discrete intervals using a Mount Sopris HFP-2293 Heat Pulse Flowmeter. The water velocity in a borehole is calculated with this device by measuring the time a pulse of heated water takes to move a known distance within the water column. Flow conditions were also measured in the borehole while the well was pumped at up to 1.4 gal/min. Hydraulic head was measured in boreholes before and during pumping conditions. Borehole flow conditions were modeled as a continuous falling head test (Paillet, 1998; 2000). This model was used to simulate both adiabatic and pumped flow fields within the borehole. Fracture transmissivities and far field hydraulic heads were adjusted in the model to fit the field data. These modeled responses were plotted with the heat-pulse flowmeter field data along with the caliper logs (figures 6-11). Tabular Results Water Level Elevation (ft) Bryand (shallow) Bryand (deep) Stewart Lot (S) Stewart Lot (M) Stewart Lot (D) River Well River Elev. Farm (shallow) Farm (deep) Jun- Jul- Aug- Sep- Oct- Nov- Dec- Jan- 04 Feb- 04 Mar- 04 Apr- 04 May- 04 Jun- 04 Date Figure 2 Water-Level Data Collected by Electronic Water-Level Indicator 535

5 Water Elevation (ft) River Borehole and Tailwater Water Elevation Farm Well (ft) Jul- Aug- Sep- Oct- Nov- Dec- Jan-04 Mar-04 Apr-04 May-04 Date 142 River Borehole Tailwater Farm Well Figure 3 Hydraulic Head Data Recorded with Data Logger for the River Borehole and the Deep Farm Borehole and Visually observed Tailwater Elevations Cations Anions Mg Ca Na K SO4 Cl Alkalinity NO3 Total N S. Cond ph meq/l meq/l meq/l meq/l meq/l meq/l meq/l ueq/l meq/l ms/cm Su Bryand Shallow Bryand Bedrock Farm Shallow Farm Deep Farm Rd Shallow Farm Rd Med Farm Rd Bedrock River Bedrock Stillwater River Stewart Shallow Stewart Medium Stewart Bedrock Table 1 Water-Chemistry Summary 536

6 Exchange and Salt Contamination Mixing High Na+ low Cl- impact of previous marine inundation Vertical Hydraulic Gradients between Bedrock and Overburden Max Date Min Date Bryant /11/ /29/20 Stewart Parking Lot (S-D) /17/ /11/20 Stewart Parking Lot (M-D) /17/ /8/20 Farm Road (S-D) /30/ /29/20 Farm Road (M-D) /21/ /18/2004 Farm /29/ /17/20 Table 2 Maximum and Minimum Vertical Hydraulic Gradients 537

7 Caliper Logs Corrected for Elevation 50 Foot Interval Shallow Farm Borehole Deep Farm Borehole Farm Road River Borehole Stewart Parking Lot Bryand Figure 5 Caliper Logs of all 6 Boreholes T= ft 2 75 feet depth Flow (gal/min) Well Diameter (cm) T= 4. ft 2 92 feet depth -0.2 Depth (ft) 14 Adiabatic Pumped Modeled Pumped Modeled Adiabatic Caliper Figure 6 Deep Farm Borehole Caliper Log (cm), Flow Rates for Adiabatic and Pumped States, Modeled Flow Rates and Modeled Transmissivity. 538

8 T= ft 2 44 feet depth T= ft 2 60 feet depth 26 1 Flow (gal/min) T= ft 2 77 feet depth T= ft 2 90 feet depth Well Diameter (cm) Depth (ft) 14 Adiabatic Pumped Modeled Pumped Modeled Adiabatic Caliper Figure 7 Shallow Farm Borehole Caliper Log (cm), Flow Rates for Adiabatic and Pumped States, Modeled Flow Rates and Modeled Transmissivity T= ft 2 75 feet depth T= ft feet depth 18 Flow (gal/min) Well Diameter (cm) T= 4.18 ft feet depth Depth (ft) Adiabatic Pumped Modeled Pump Modeled Adiabatic Caliper Figure 8 Farm Road Borehole Caliper Log (cm), Flow Rates for Adiabatic and Pumped States, Modeled Flow Rates and Modeled Transmissivity. 539

9 T= feet Flow (gal/min) T= feet Well Diameter (cm) T= feet -0.5 Depth (ft) 13 Adiabatic Pumped Modeled Pumped Modeled Adiabatic Caliper Figure 9 River Borehole Caliper Log (cm), Flow Rates for Adiabatic and Pumped States, Modeled Flow Rates and Modeled Transmissivity T=3.55 ft 2 70 feet depth Flow Rates (gal/min) Well Diameter (cm) T= 1.45 ft 2 90 feet depth Depth (ft) Adiabatic Pumped Modeled Pump Modeled Adiabatic Caliper Figure 10 Stewart Parking Lot Borehole Caliper Log (cm), Flow Rates for Adiabatic and Pumped States, Modeled Flow Rates and Modeled Transmissivity. 540

10 T= 8.76 ft 2 25 feet depth 2 16 T= feet depth 1.5 Flow (gal/min) 1 T= 6.57 ft 2 55 feet depth Well Diameter (cm) T= 2.92 ft 2 95 feet depth T= 1.46 ft feet depth Depth (ft) T= 0.73 ft feet depth 14 Adiabatic Pumped Modeled Pump Modeled Adiabatic Caliper Figure 11 Bryand Borehole Caliper Log (cm), Flow Rates for Adiabatic and Pumped States, Modeled Flow Rates and Modeled Transmissivity Elevation (ft) Magnitude of Transmissivity (ft 2 /day) Stewart Farm Road Deep Farm Bryand Farm Shallow River Figure 12 Plot of Fractures Transmissivity vs Elevation Illustrating Log-Linear Relationship 541

11 Results and Discussion Hydrological Interpretation. The majority of the water-chemistry data (table 1) have a charge-balance error of less than 5 percent. The shallow well at the Bryand location has a 35 percent charge-balance error and an 11 percent charge-balance error for the Deep Farm Borehole and the sample from the Stillwater River. The water-chemistry data indicate that the cations are undergoing exchange as the water carries them from the recharge area (the Farm Bedrock Boreholes) to the discharge area (the River Bedrock Borehole). This agrees with the presumption that the flow direction will mimic the overlying topology. Stewart Parking Lot Borehole is anomalous with a ph of 9.2, high Na + concentration and low Cl - concentration. These parameters suggest inflow by trapped seawater in the overburden. The linear trend in the anion field of the Piper Plot (Piper, 1944) suggests that the anions are undergoing simple mixing with an elevated chloride source, likely due to contamination by de-icing salts (figure 4). Elevated nitrates and potassium levels found in several of the wells are indicative of the breakdown of fertilizers. The gaps in the data from the dam tailwater elevations and the gaps in the data loggers make detailed interpretation and extrapolation of individual storm events between the Stillwater River elevation and the River Bedrock Borehole difficult. However, the quick and flashy responses of the River bedrock borehole to changes in the river elevation make it apparent that the River borehole is intimately linked to the Stillwater River. Water levels for all of the wells on campus tend to have their lowest elevations in the fall and highest elevations in the winter months with ranges at the Farm Boreholes of over 3 meters and the minimum ranges were nearly a meter at the Bryand shallow well and at the River Borehole. The Farm Road Borehole had been artesian for much of the winter and spring, contributing to the evidence of the confined nature of the fractured bedrock aquifer. Vertical downward hydraulic gradients were observed at three of the four well clusters between the fractured bedrock aquifer and the glacial sediment overburden (table 2). The Farm Road Cluster of wells exhibits an upward vertical hydraulic conductivity. The Stewart Parking Lot cluster and the Bryant cluster mirror each other as one rises the other falls with relatively large gradients, maximums of 0.44 and 0.15 respectively. The Farm Road Cluster and the two boreholes at the Farm have much smaller vertical gradients than for the well clusters with considerably lower borehole transmissivities, with maximums of 0.09 and 0. respectively. Geophysical Interpretation. The Shallow Farm Borehole and the Deep Farm Borehole were drilled as close to the top of a local topographic divide as logistically possible. A downward borehole flow measured in both boreholes is near the lower detection limit of the heat pulse flowmeter and do not appear to correlate to any fracture flow found in the pumped state. This may be due to fractures that have transmissivities that are too small to be discerned during pumping conditions, but have sufficient differences in head values to drive the flow field in ambient conditions. This conflict may be due to the limitations of the method, as the determination of transmissivity of fractures by heat-pulse flowmeters is limited to two orders of magnitude (Paillet, 1998). In the deeper borehole the 75-foot fracture is dominant (111 ft 2 /day) while in the shallow borehole the 90-foot fracture is dominant (110.3 ft 2 /day), as shown in figures 6 and 7. The Farm Road Borehole has 3 major fractures contributing to the transmissivity of the borehole a fracture at 75 feet depth (62.4 ft 2 /day), a fracture at 105 feet depth (163.6 ft 2 /day) and a fracture at 120 feet depth (4.2 ft 2 /day). Some fractures that were clearly identified by the caliper log were found to be the fractures contributing to the transmissivity of the borehole, while others that were just as large, such as the fracture at 190 feet, were not shown to contribute to the borehole transmissivity (figure 8). The River Borehole was modeled as a simple three-fracture flow. This was required due to the extremely large response, ninety five percent of the transmissivity, at the near surface making any detailed observation of the individual flows below that fracture impractical. The River Borehole is the only borehole to demonstrate a strong flow in its ambient condition (figure 9). The strong upward adiabatic flow that increases both at the modeled fractures and between them is indicative of an aquifer discharge, which can be expected at a river. 542

12 The Stewart Parking Lot Borehole has a very low overall transmissivity and the majority, 70 percent, of the total transmissivity appears to be either bypassing the bottom of the casing or is a fracture at or near the bottom of casing (figure 10). A fracture at 90 feet has a transmissivity of only 1.45 ft 2 /day. The chemistry of the medium depth Stewart Parking Lot well is very similar to the deep borehole chemistry, suggesting that the water from the overburden is bypassing the casing through the upper zone with a transmissivity of 3.55 ft 2 /day. The Bryand Borehole has several fractures contributing to the transmissivity of the well. Six fractures are apparent in this borehole a fracture at 25 feet (8.76 ft 2 /day), a fracture at 40 feet (16.06 ft 2 /day), a fracture at 55 feet (6.57 ft 2 /day), a fracture at 95 feet (2.92 ft 2 /day), a fracture at 130 feet (1.46 ft 2 /day) and a fracture at 170 feet (0.73 ft 2 /day). As these fractures are not tremendously productive the contributions of the smaller fractures can more easily discerned than in most of the other wells (figure 11). The Stewart Parking Lot Borehole and the Bryand Borehole are both in the middle of the transect between the recharge zone and discharge zone and show the smallest of the transmissivities (total borehole transmissivities of 5 ft 2 /day and 36.5 ft 2 /day respectively). Whereas the Farm Road Borehole is also in the middle of that same transect but has a transmissivity (230 ft 2 /day) similar to the boreholes in the recharge zone (213 ft 2 /day for the deeper and 202 ft 2 /day for the shallower Farm Boreholes). Transmissivity of fractures within the study area show a sharp decrease with a decrease in elevation as shown on figure 12. While this does not correlate to an increase in transmissivity within each borehole with depth, the overall trend for the area is true. The northwestern portion of the study area has a much higher transmissivity while the southeastern portion of the study area has a much lower transmissivity. Conclusions Identifying the fracture locations with the caliper probe is an important first step in modeling the transmissive response of boreholes, but presence of fractures and their size at the wall of the borehole is not indicative of the magnitude of the transmissivity of the fracture. The largest fracture (<40 cm) in any of the boreholes studied was greater than the upper observation limit of the caliper probe but did not contribute to the transmissivity of the Shallow Farm Borehole. The most transmissive fracture (657.6 ft 2 /day) is at the River Borehole and is less than 16 cm in diameter. The Farm Boreholes are located within the recharge zone and the River Borehole is located in the discharge zone, as shown both by the water chemistry and the flow fields within the boreholes. The Stillwater River is gaining water from the fractured bedrock aquifer. The horizontal hydraulic gradient is or about 1 in 80 and the vertical hydraulic gradient is insignificant for all of the boreholes except for the River Borehole, but observable within the overburden as measured by water-levels within the well clusters. Vertical upward hydraulic gradients of 0. in the upper level and for the lower level were observed in the River Borehole. This characterization of the bedrock aquifer at the University of Maine will provide baseline data for educational and research activities. References Mount. Sopris Instrument Company, 2001, 2PCA-1000 PolyCaliper Probe and 2CAA-1000 Caliper Probe: PDF Format P/N Revision 3, p.3. Paillet, F.L., 1998, Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations: Water Resources Research, v. 34, no. 5, p Paillet, F.L., 2000, A field technique for estimating aquifer parameters using flow log data: Ground Water, 38, no. 4, p

13 Piper, A.M., A Graphical procedure in the geochemical interpretation of water analyses. Transactions of the American Geophysical Union, 25, p Stumm, W. and J.J. Morgan, Aquatic Chemistry, Wiley and Sons, New York. Biographical Sketches Eric Rickert Department of Earth Sciences 5790 Bryant Global Sciences Center University of Maine Orono, ME Tel: (207) Fax: (207) Eric_Rickert@umit.maine.edu Eric Rickert is presently studying the utilization of geophysics in hydrogeology while pursuing a MS in Earth Sciences from the University of Maine at Orono. Eric received a BS from Texas A & M University from the Department of Geophysics in Andrew Reeve, Ph.D Department of Earth Sciences 5790 Bryant Global Sciences Center University of Maine Orono, ME Tel: (207) asreeve@maine.edu Andrew Reeve is an Associate Professor at the University of Maine. His interests include wetland hydrology, groundwater modeling, and aquifer geochemistry Frederick L. Paillet Research Professor Department of Earth Sciences University of Maine Orono, ME Tel: Fax: fpaillet@maine.edu Fred Paillet joined the department in Maine after retiring from the U. S. Geological Survey in Before then he was chief of the Borehole Geophysics Research Project and conducted studies in all aspects of borehole geophysics applied to ground water. He has published numerous papers on the use of geophysical logs and borehole flowmeter data in the characterization of fractured bedrock aquifers. 544