Using Fractran Fracture Flow Modeling in Tandem with Modflow to Assist in the Development of Wellfield Protection Zones for Municipal Wells in Bedrock T.K. Wiezel 1, G.G. Violette 1 and S.T. Hamilton 2 1 Neill and Gunter, Fredericton, NB 2 New Brunswick Department of the Environment and Local Government, Fredericton, NB Abstract The New Brunswick Department of the Environment and Local Government has initiated a long term Wellfield Protection Program which helps to ensure source water protection of municipal groundwater supplies. The province has been encouraging state-of-the-art groundwater protection strategies through the use of numerical models. Of the 58 municipalities in New Brunswick relying on municipal groundwater wells, nearly 60 percent draw their water supply from fractured bedrock. Because of the low storage in bedrock for a given flow, fracture flow tends to be faster and extends to greater distances than flow in porous media. Fracture flow modeling has not been widely used in the consulting industry due to the overall difficulties in setting up the fracture flow conceptual and mathematical models and the limited budgets associated with these studies. Often porous media flow models such as MODFLOW are used, with the assumption that bedrock fractures can be considered as an equivalent porous medium. For the Village of Aroostook in western New Brunswick, which draws its drinking water supply from two bedrock wells on the valley wall of the Saint John River, Wellfield Protection Zones were developed by combining the numerical results from MODFLOW with results from FRACTRAN, a two-dimensional numerical model that simulates groundwater flow through discretely-fractured porous media, such as bedrock. The results obtained by using both models were combined to produce Wellfield Protection Zones for the Village. The MODFLOW simulations assume that flow through bedrock fractures can be considered as an equivalent porous medium, and represent the current state of wellfield modeling. The FRACTRAN simulations incorporate a two-dimensional randomly-generated fracture network within the bedrock, and provide a closer conceptual interpretation of groundwater flow and particle travel to the wells by adding flow through bedrock fractures to the overall groundwater flow system. In general, the addition of a fracture network by use of the FRACTRAN model increased the distance of particle travel for each simulation. Several randomly-generated fracture networks were investigated with similar results. It was found that Wellfield Protection Zones based on the MODFLOW simulations alone were generally controlled by the model boundary conditions, whereas Protection Zones based on the FRACTRAN simulations were wider and more locally controlled by the fracture network, particularly near the wells, and extended 50 to 80 percent farther downgradient of the wells. With the addition of fracture flow modeling, the area of the Wellfield Protection Zones was increased by a factor of eight for Protection Zone A and a factor of five for Protection Zones B and C. 566
1. INTRODUCTION The New Brunswick Department of Environment and Local Government (NBDELG) has initiated a long term Wellfield Protection Program which provides the regulatory framework to ensure the protection of municipal groundwater supplies for continued safe use of groundwater resources in the Province of New Brunswick. Over 50 municipalities rely on municipal wells for the supply of water for a variety of uses. The protection of water quality and water quantity is essential to protect the health and safety of the public and to avoid well replacement and/or the process of identifying new water sources. During October 2000 the Wellfield Protected Area Designation Order (WfPADO) under the New Brunswick Clean Water Act was enacted in the Province of New Brunswick. This regulation is being used to protect groundwater recharge areas, also know as wellfield protected areas, that are sources of water for municipalities in New Brunswick. The WfPADO places restrictions and in some cases prohibits land use and chemical storage within wellfield protected areas in order to help protect the source of municipal drinking water supplies. The Village of Aroostook water supply is extracted via two production wells from groundwater stored in fractured bedrock in western New Brunswick. The Village is located in the Upper Saint John River valley at the confluence of the Saint John and Aroostook Rivers, near the New Brunswick-Maine border. This study focuses on the development of Wellfield Protection Zones using numerical modeling techniques for the Village of Aroostook wellfield. The primary objective is to delineate the wellfield into three distinct protection zones. Because of the low storage in bedrock for a given flow, fracture flow tends to be faster and extends to greater distances than flow in porous media. Fracture flow modeling has not been widely used in the consulting industry due to the overall difficulties in setting up the fracture flow conceptual and mathematical models and the limited budgets associated with these studies. This study incorporates the use of fracture flow software in the development of protection zones using both MODFLOW and FRACTRAN. 2. DESCRIPTION OF THE WELLFIELD The study area in Figure 1 includes the wellfield with the location of each well identified and the recharge area for the production wells. The definition of the study area is based on the arrangement of the physical features that form the area such as topography, hydrology, geology and hydrogeology. The production wells PW1 and PW2 are constructed within bedrock, on the valley wall of the Saint John River. Pertinent well information is presented in Table 1. Bedrock in this region has been described as moderately fractured with calcite veins dipping at 45 degrees (Milburn et al. 1994). Craig (1995) and Aqua Terra (1998) suggests that the beds (depositional layers) strike to the northeast and dip at a steep vertical angle. This bedrock hydrogeological setting is more difficult to model using Visual MODFLOW, which is primarily suited for porous media modeling. Often porous media flow models such as MODFLOW are used, with the assumption that bedrock fractures can be considered as an equivalent porous medium. In this study, the modeling results from Visual MODFLOW have been inspected and adapted to represent fracture flow, using the FRACTRAN fracture flow model. 3. PROTECTION AREA DEVELOPMENT GUIDELINES The production wells draw from an underground source of water which is vulnerable to contamination from land use activities. Recharge to the wellfield occurs primarily from precipitation and snowmelt with a small contribution from streams. The majority of the recharge to the wells is considered to be from storage in fractures within the bedrock aquifer. The threat to the aquifer arises from the potential release of chemicals at or near ground surface which could infiltrate the subsurface and reach groundwater flowing to the production wells. Mixed land use activities at the ground surface could lead to the release of contaminants to the subsurface environment. Once released, liquid 567
Figure 1 Study Area Table 1 Production Well Details for the Village of Aroostook Parameter Production Well PW1 PW2 Top of ground elevation (m) 90 121 Total well depth (m) 83.8 106.7 Total casing depth (m) min. 7 min. 7 Intake screen none none Depth to static water level (m) 14.1 7.9 Depth to pumping water level (m) 46.0 12.0 Pump location below ground (m) ± 60 ± 60 Average pumping rate (igpm) 70 90 Average daily extraction (m 3 /d) 57 153 contaminants can travel long distances through porous or fractured media depending on subsurface conditions and the type of contaminant. Simulations of groundwater flow have been carried out to demonstrate the possible pathways which groundwater and contaminants may follow while the production wells are in operation. The experience gained from NBDELG and consultants during prior development of the Wellfield Protection Studies in the Province of New Brunswick has lead to the establishment of a generic terms of reference. The information presented in the terms of reference formed the basis for the development of protection zones for the wellfield. 568
4. NUMERICAL MODELING Through the use of mathematics and computer modeling techniques, groundwater flow can be reproduced for known conditions and simulated for hypothetical conditions. The wellfield protection area was delineated into three protection zones based on groundwater travel time: Zone A: Bacteria and Virus Contaminant Zone 100 days (sand and gravel) or 250 days (bedrock) Zone B: Petroleum Contaminant Zone 100 or 250 days to 5 years Zone C: Persistent Contaminant Zone 5 years to 25 years The range in travel times provides an allowance for the attenuation capabilities of the media in which the contaminants travel. In this study, the primary pathway for contaminants is through fractures in the bedrock and the travel times used to develop the protection zones are those within the fractured bedrock aquifer. 4.1 Conceptual Model Based on the interpreted local boundary conditions in the study area, the model domain is rectangular with dimensions of 2.2 km by 2.5 km. Boundary conditions are the Saint John River to the east, a drainage divide between the Saint John and Aroostook Rivers to the north and local/regional drainage divides to the south and west. The model domain is rotated by 10 degrees clockwise from north to better conform to the interpreted boundaries. Figure 2 shows the MODFLOW finite-difference grid and boundaries (with rotation), and Figure 3 shows the FRACTRAN randomly-generated fracture network (no rotation). Figure 2 MODFLOW Finite-Difference Grid Figure 3 FRACTRAN Fracture Network For this study, flow through bedrock fractures is identified as the groundwater flow system. Flow through fractures is typically faster and more erratic than flow through a porous medium. Varying fracture apertures, fracture orientations and discontinuities may contribute to several discrete preferential pathways for groundwater travel. This can lead to a stepped effect of groundwater flow, where the overall flow system is not as smooth and continuous as with a porous medium flow is being driven by a pressure gradient but can be limited to specific pathways in the fractures and is dependent on fracture interconnectivity (shown in Figure 3). This tends to result in protection zones that are wider and greater in aerial extent. To best represent the fractured bedrock groundwater flow system, numerical groundwater flow models were constructed using the computer codes MODFLOW and FRACTRAN. MODFLOW (McDonald and Harbaugh 1988) has gained wide acceptance as a robust model for groundwater flow through porous media. FRACTRAN (Sudicky and McLaren 1998) is described as a highly-efficient numerical model for simulation of groundwater flow through discretely-fractured porous media such as bedrock. Results from the two numerical models were 569
used to produce the most probable groundwater flow pattern through the fractured bedrock aquifer to the production wells. The result of the combined modeling is a more reliable delineation of wellfield protection zones when compared to those of only one type of model. 4.2 Input Parameters The aquifer properties in the models were based on hydraulic testing at the two pumping wells and on previous work in surrounding areas with similar bedrock geology. For the numerical models, hydraulic conductivity is 1.0 m/day, total aquifer porosity is 0.16, storativity is 0.002, and specific yield/effective porosity is 0.14. Randomly-generated fractures in FRACTRAN were given an aperture (fracture opening) of 2 mm. Of the two pumping wells, PW2 supplies approximately 80 percent of the required potable water, and PW1 supplies the remaining 20 percent. PW2 operates for approximately 8 hours/day at 6.8 L/s (90 igpm) while PW1 operates for approximately 3 hours/day at 5.3 L/s (70 igpm). The pumping rates used in the models were for 24 hours of continuous pumping from each well at the operating pumping rate. This assumption results in conservative final protection zones, and is typical of these studies in New Brunswick. The rates used are 589 m 3 /day for PW2 (6.8 L/s x 24 hours of pumping) and 458 m 3 /day for PW1 (5.3 L/s x 24 hours of pumping). Precipitation in the area owing to rain and snowmelt is approximately 1100 mm/year (NRCC 1995). It is estimated that 30 percent (360 mm/year) enters the ground as recharge to groundwater, as entered in the model. 4.3 Numerical Models MODFLOW simulations were generated using a telescoping finite-difference grid to obtain a general sense of groundwater flow to the wells. Preliminary simulations using MODFLOW yielded protection zones that extended mainly west and were long and thin, owing to the relatively steep local gradient. These results reflected the protection zones if the aquifer were a porous sand and gravel deposit, and as such are not entirely representative of the natural groundwater fracture flow system near the production wells. In order to more accurately represent flow conditions to the production wells, consideration was also given to fractures in the bedrock aquifer which likely contribute to the bulk of groundwater flow. The MODFLOW simulations were modified by constructing a model in FRACTRAN using the same input and boundary conditions, with the addition of a randomly-generated discrete fracture network to better consider particle travel times in fractured bedrock. The model grid and high conductivity patterns were constructed to trend in a northeast-southwest direction, as observed in the area from outcrops and previous drilling. For the most part, the addition of a fracture network increased the distance of particle travel for each given time interval, particularly in the north-south direction (width), and correspondingly increased the size of the wellfield protection zones. This is to be expected, as groundwater flow through fractured media such as bedrock is limited to finite, thin pathways (fractures) and thus would migrate toward the well from greater distances. 4.4 Calibration and Sensitivity Analysis Data from a six-hour pumping test performed at each well in February 2003 were used in the calibration. Model calibration of groundwater flow conditions was also referenced to the available reported conditions of water levels in the production wells and water surfaces in nearby streams and ponds. The calibrated model was used to simulate the flow of groundwater particles to the production wells and the results of those simulations were used to develop protection zones for the wellfield. Model sensitivity to random fracture networks was examined by generating several unique random fracture networks, and it was found that there was very little change in the overall groundwater flow system and resulting protection zones among the varying fracture networks. Table 2 presents the resulting dimensions of the three protection zones for the sensitivity analysis. It can be seen in Table 2 that, for each zone, the fracture aperture width has a direct influence on the dimensions of the protection zones the length and width of each zone generally decreases with increasing fracture aperture. In general, the randomly-generated fracture network, represented by the random seed in Table 2, did not show a significant trend in affecting the dimensions of the protection zones. 570
Table 2 Sensitivity Analysis for Fracture Network Protection Zone Zone A Zone B Zone C Fracture Aperture Width (mm) 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 Approximate Dimensions Random Seed for of Protection Zone Numerical Model Length (m) Width (m) -10 400 190-30 320 200-100 310 160 Average 343 183-10 200 110-30 320 160-100 200 160 Average 240 143-10 160 90-30 190 140-100 200 160 Average 183 130-10 480 200-30 480 280-100 440 200 Average 467 227-10 400 210-30 640 320-100 320 280 Average 453 270-10 320 160-30 530 340-100 350 320 Average 400 273-10 1080 350-30 1080 400-100 1000 400 Average 1053 383-10 1040 280-30 700 360-100 760 280 Average 833 306-10 800 260-30 840 400-100 680 360 Average 773 340 571
4.5 Groundwater Flow Simulations For all groundwater flow simulations, the figures are presented with output from both the MODFLOW and FRACTRAN models. The simulated static groundwater flow conditions, without wells pumping, are shown in Figure 4. Groundwater flows from the higher elevated regions of the study area to the lower lying regions. In general, the direction of groundwater flow is generally east toward the Saint John River. The interpretation of static groundwater flow shown in Figure 4 indicates that groundwater discharges to the Saint John River. a. MODFLOW b. FRACTRAN Figure 4 Simulated Static Groundwater Flow Conditions The simulated groundwater flow conditions during pumping are shown in Figure 5. In this simulation, the two production wells are pumping at the rates presented in Table 1. The particle tracking for this simulation indicates that the wells intercept groundwater that would naturally flow from west to east toward the Saint John River. In addition, Figure 5 shows that the effects from pumping are local and mostly upgradient. a. MODFLOW b. FRACTRAN Figure 5 Simulated Pumping Groundwater Flow Conditions The travel time method is a common approach in the delineation of protection zones for wellfield protection. Using the numerical groundwater models, particle tracking of hypothetical groundwater particles delineated the zones within the wellfield protected area. Figure 6, Figure 7 and Figure 8 show the travel times simulated for the Aroostook wellfield for the selected time periods of 250 days, 5 years and 25 years, respectively. 572
a. MODFLOW b. FRACTRAN Figure 6 Simulated 250 Day Particle Travel Time a. MODFLOW b. FRACTRAN Figure 7 Simulated 5 Year Particle Travel Time a. MODFLOW b. FRACTRAN Figure 8 Simulated 25 Year Particle Travel Time 573
5. WELLFIELD PROTECTION ZONES The travel times at each well generated by the MODFLOW and FRACTRAN simulations were used to define the wellfield protected areas. Figure 9 shows the three resulting zones for the wellfield with a total extraction of 1,047 cubic metres per day from the two wells. A different pumping scenario would produce different wellfield protection areas. In order to analyze the results of the two models, the particle tracking plots shown in Figures 6, 7 and 8 were used. This raw data was used to ensure accurate and uncompromised results. Figure 9 Resulting Wellfield Protection Zones Zone A is delineated using a groundwater travel time of 250 days. Typically, this time would allow contaminants such as bacteria and viruses to be attenuated naturally. Zone A is roughly circular in shape for both wells, and extends for a radius of approximately 75 metres for PW1 and 125 metres for PW2. The area of Zone A is 15,000 m 2 (1.5 hectares) for PW1 and 42, 000 m 2 (4.2 hectares) for PW2, for a total protection area of 5.7 hectares. Zone B is delineated using a groundwater travel time of 5 years. Within this time and distance contaminants such as hydrocarbons may attenuate naturally or can be remediated in sufficient time to protect the production well. Zone B extends upgradient and mainly west for PW1 and southwest for PW2, to a maximum distance of approximately 300 metres from the wells. The area of Zone B, encompassing the area of Zone A, is 57,000 m 2 (5.7 hectares) for PW1 and 161,000 m 2 (16.1 hectares) for PW2, for a total protection area of 21.8 hectares. Zone C is delineated using a groundwater travel time of 25 years. Within this time and distance persistent contaminants can typically be controlled to minimize or prevent any impact to the production wells. Zone C also extends upgradient and mainly west for PW1 and southwest for PW2, to a maximum distance of approximately 1200 metres from the wells. The area of Zone C, encompassing the area of Zones A and B, is 348,000 m 2 (34.8 hectares) for PW1 and 631,000 m 2 (63.1 hectares) for PW2, for a total protection area of 97.9 hectares. 574
In general, the addition of a bedrock fracture network by use of the FRACTRAN model increased the distance of particle travel for each simulation in length and width. Tables 3 and 4 present dimensions of the wellfield protection zones generated using both MODFLOW and FRACTRAN. The right hand column offers the increase factor for each dimension when the fracture network was added to the flow system. Table 3 Comparison of Downgradient Distances between MODFLOW and FRACTRAN Zone Distance of Protection Zone Dowgradient of Well (m) MODFLOW FRACTRAN PW1 PW2 Average PW1 PW2 Average Percent Increase A 30 50 40 30 50 40 -- B 35 60 48 50 90 70 50 % C 40 65 53 60 130 95 80 % Table 4 Comparison of Wellfield Protection Areas between MODFLOW and FRACTRAN Zone Area of Wellfield Protection Zone (hectares) MODFLOW FRACTRAN PW1 PW2 Total PW1 PW2 Total Increase Factor A 0.1 0.6 0.7 1.5 4.2 5.7 7.9 B 0.8 3.5 4.3 5.7 16.1 21.8 5.0 C 4.00 15.5 19.5 34.8 63.1 97.9 5.0 Wellfield protection zones based on the MODFLOW simulations alone were generally controlled by the model boundary conditions, whereas protection zones based on the FRACTRAN simulations were wider and more locally controlled by the fracture network, particularly near the wells. The addition of fractures to the flow system using the FRACTRAN model extended the protection zones by 50 to 80 percent farther downgradient of the wells, as shown in Table 3. In addition, with fracture flow modeling, the area of the wellfield protection zones was increased by a factor of eight for Protection Zone A and a factor of five for Protection Zones B and C, as shown in Table 4. 6. CONCLUSIONS The following conclusions have been developed based on the findings of this study. 1. Hydrogeological conditions in the fractured bedrock are complex, and as such numerical modeling was carried out using two models: MODFLOW and FRACTRAN. 2. Wellfield protection zones were generated around each well using results from both numerical models, giving protection zones that were wider and greater in extent than with one model alone. 3. The addition of bedrock fractures in the numerical flow system increased the downgradient distance of the zones by 50 to 80 percent from the production wells. 4. The addition of bedrock fractures in the numerical flow system also increased the area of the wellfield protection zones by a factor of eight for Zone A and a factor of five for Zones B and C. 575
LIST OF REFERENCES Aqua Terra Investigations Inc., 2000, Village of Plaster Rock Wellfield Protection Study, Plaster Rock, NB: Unpublished report submitted to the New Brunswick Department of the Environment and Local Government. Aqua Terra Investigations Inc., 1998, L utilization du puits TW97-3, Village de Saint-André. Projet ATI-116: Unpublished report submitted to the Village of Saint-André, New Brunswick. Craig Hydrogeologic Inc., 1995, A Municipal Water Supply Wellfield Protection Study for the Village of Saint-André, NB Draft Report: Unpublished report submitted to the New Brunswick Department of the Environment and Local Government. Gallagher, R., 1997, Evaluation of Groundwater Quality at Two Hydrogeologically Distinct Agricultural Watersheds in New Brunswick: Masters of Science in Engineering Thesis, University of New Brunswick. Map NR-1, 2000, Bedrock Map of New Brunswick: NB Department of Natural Resources and Energy, Minerals and Energy Division. McDonald, M. and Harbaugh, A., 1988, A modular three-dimensional finite-difference ground-water flow model: U.S. Geological Survey. Techniques of Water-Resources Investigation, Book 6, Modeling Techniques, Chapter A1. Milburn, P., MacQuarrie, K., and Gallagher, R., 1994, Deep Groundwater Quality as Affected by Agriculture at Two Sites in New Brunswick: Canadian Society of Agricultural Engineering, Regina, Saskatchewan. National Research Council Canada, 1995, National Building Code of Canada: Issued by the Canadian Commission on Building and Fire Codes. 11 th Edition. Neill and Gunter Limited, 2004, Wellfield Protection Study Final Report, Village of Aroostook, NB: Unpublished report submitted to the New Brunswick Department of the Environment and Local Government, September 19, 2004. Neill and Gunter Limited, 2003, Groundwater Exploration Report, Saint-André, NB: Unpublished report submitted to the New Brunswick Department of the Environment and Local Government, May 14, 2003. Rampton, V.N., 1984, Map 1594A Surficial Geology of New Brunswick: Geological Survey of Canada, Department of Energy, Mines, and Resources. Sudicky, E. and McLaren, R., 1998, FRACTRAN User s Guide: An efficient simulator for two-dimensional, saturated groundwater flow and solute transport in porous or discretely-fractured porous formations: Waterloo Centre for Groundwater Research, University of Waterloo. Biographical Sketches T. K. Wiezel, E.I.T. Kent has been employed at Neill and Gunter for two years. He works with the Environmental Sector of the company s Fredericton, NB office, and specializes in numerical modeling of groundwater flow systems, thermal energy storage, and pumping test analysis. Kent has a Bachelor of Science in Engineering degree from the University of New Brunswick and a Master of Applied Science degree from Dalhousie University. P.O. Box 713, Fredericton, NB, Canada, E3B 5B4 kwiezel@ngl.ca phone: (506) 452-7000 fax: (506) 452-0112 G.G. Violette, P.Eng. Mr. Violette is the Manager of the Environmental Sector in Neill and Gunter s Fredericton, NB office. His areas of expertise include 24 years of experience in the area of groundwater and contaminant transport modeling, water supply exploration and development, and contaminated sites management. Mr. Violette has a Bachelor of Science in Engineering degree and a Master of Science in Engineering degree, both from the University of New Brunswick. P.O. Box 713, Fredericton, NB, Canada, E3B 5B4 gviolette@ngl.ca phone: (506) 452-7000 fax: (506) 452-0112 S.T. Hamilton, P.Eng. Shawn is the Manager of the Wellfield Protection Program at the New Brunswick Department of the Environment and Local Government. He works with over 50 municipalities in the province to complete wellfield protection plans, which are developed to protect the source of municipal groundwater supplies. Shawn has a degree in Geological Engineering from the University of New Brunswick. P.O. Box 6000, Fredericton, NB, Canada, E3B 5H1 shawn.hamilton@gnb.ca phone: (506) 444-2671 fax: (506) 457-7823 576