REGIONAL SIMULATION of GROUND-WATER FLOW in a FRACTURED BEDROCK AQUIFER, NEW HAMPSHIRE

National Ground Water Association
March 13-15, 2002, Denver, Colo.

Thomas J. Mack, U.S. Geological Survey
James R. Degnan, U.S. Geological Survey
Richard Bridge Moore, U.S. Geological Survey

Introduction

Ground water is being withdrawn from bedrock aquifers in New Hampshire's Seacoast region in increasing amounts in response to accelerated population growth and accompanying water demands. Ground-water resource development can result in the diversion of fresh ground water that would otherwise discharge to estuaries and the ocean, which can lead to salt-water intrusion and change the balance of ground-water and surface-water interactions. Methods are needed however, for water-resource managers to evaluate the effects of development on coastal New Hampshire's regional hydrologic budget.

This study evaluates a regional bedrock aquifer system that extends from New Hampshire's rapidly growing seacoast to upland areas 25 miles inland (figure 1). Specific objectives of the study were to: 1) characterize the regional ground-water flow system and estimate fresh ground-water discharge to Great Bay, New Hampshire; and 2) examine the use of ground-water-flow simulation to assess fresh ground-water resources in the region. The area simulated (study area) includes the New Hampshire watersheds draining to Great Bay and the Atlantic Ocean. The model described below is sufficiently detailed for approximating ground-water flow in the region of Great Bay but is not intended to provide accurate flow simulations at all model boundaries.

Model Construction

A finite-difference ground-water-flow model, using MODFLOW-2000 (Harbaugh and others, 2000), was constructed using a preprocessor (Winston, 2000) capable of handling geographic information system (GIS) data layers with fine (100-foot) resolution at a regional (tens of miles) scale. The study area covers approximately 700 mi², ranging from the coast to upland areas with an elevation over 1,400 ft (figure 2).

The study area was discretized into various horizontal cell sizes, ranging from 200 by 200 ft at Great Bay, to 1,250 by 1,250 ft at the boundaries of the model. The study area was discretized vertically into 3 layers to simulate unconsolidated surficial materials, in layer 1, and bedrock in layers 2 and 3. Model layers are oriented parallel to the land-surface with layer 1 convertible (water table), and layers 2 and 3 non-convertible. In an assessment of ground-water flow in a crystalline bedrock watershed in central New Hampshire, Harte and Winter (1995) indicated that topography and heterogeneity within the bedrock system are major controls on ground-water flow. In the present simulation, detailed topography was incorporated by using digital-elevation model (DEM) data, with a 100-foot horizontal grid resolution, to create model layer and boundary elevations (river stages). The outer model boundary, coincident with regional watershed divides in most places, and the bottom of layer 3 are simulated as no-flow boundaries. Relatively large watersheds surrounding Great Bay, such as the Lamprey River watershed, were simulated to ensure that the hypothesized model boundaries would not adversely influence surface or ground-water flows at Great Bay and other coastal areas. Simulation of the upper Cocheco and Salmon Falls watersheds were truncated, at a location sufficiently removed from the coastal drainages, to reduce model size and complexity.

Surficial materials simulated in model layer 1 include stratified glacial sediments and glacial till. These sediments have hydraulic conductivities that span at least two orders of magnitude. In the study area, stratified-drift aquifers are discontinuous (covering less than 15 percent of the study area), are generally less than 40 ft thick, and overlie fairly continuous till deposits that are generally less than 20 ft thick. The hydraulic conductivity of stratified-drift sediments is generally orders of magnitude greater (10 to 100+ ft/d) than till (<1 to 1+ ft/d). Because of the large hydraulic conductivity contrasts within and between upper (1) and lower (bedrock) model layers, precise representation of layer 1 hydraulic conductivity and thickness is not warranted for a regional flow simulation. Therefore model layer 1 uses generalized hydraulic conductivity values based on published transmissivity coverages (Mack and Lawlor, 1992; Moore, 1990; Stekl and Flanagan, 1992). For model layer 1, in areas of stratified-drift sediments, hydraulic conductivities of 10, 50, 100, and 200 ft/d, were assigned to areas of transmissivity of 1 to 1000; 1,001 to 2,000; 2,001 to 4,000; and greater than 4,000 ft 2 /d. All other areas, not covered by large water bodies, were assumed to be cover by till or other fine-grained sediments, and were assigned a hydraulic conductivity of 1 ft/d. Model layer 1 was simulated with a uniform 20 ft thickness to represent an average glacial sediment thickness in the study area. Additionally, a relatively thin upper layer allows for realistic numerical representation of streams in model layer 1.

Heterogeneity in model layers 2 and 3, the bedrock aquifer, is obtained from a statewide digital coverage of bedrock well-yield probabilities (Moore and others, in press). Additional bedrock heterogeneity was included by incorporating fracture-correlated lineament information, recently collected at Great Bay (Degnan and Clark, in press), into the bedrock well-yield probability model using the methods of Moore and others (in press). In a similar ground-water flow simulation in a crystalline bedrock aquifer, Daniel and others (1997) found that transmissivity, storage, and well yield vary similarly and used one as an index for the other. In this study, bedrock well-yield probabilities were used as an index of heterogeneity in bedrock hydraulic conductivity. The bedrock well-yield probabilities were treated as a multiplier array in a parameter estimation process, using MODFLOW-2000 (Hill and others, 2000), to estimate bedrock hydraulic conductivities. Model layers 2 and 3 are each 400 ft thick. Thus, the thickness of model layer 2 corresponds to a typical bedrock well depth in the study area.

All streams and rivers in the study area were simulated, using the MODFLOW-2000 river package, with water-surface elevations obtain from the DEM. River bed hydraulic conductivity and thickness are not known. Regionally rivers are assumed to be fairly well connected to the aquifer and a high river-bottom hydraulic conductance was used. Coastal water bodies (all tidal water bodies including Great Bay and the Atlantic Ocean), were simulated as a constant-head boundary (with a head of 0 ft) in layer 1. These water bodies were simulated with layer 1 thicknesses increased to the estuary or ocean depth. A high hydraulic conductivity (10,000 ft/d) were used for coastal water bodies to create flow-through cells in the model.

Calibration

A steady-state model was used to simulate long-term average flow conditions. The model was developed to assess the effects of the regional ground-water-flow system on Great Bay and does not account for various water uses, such as ground-water withdrawals or inter-basin water transfers. Estimates of selected model parameters (recharge and horizontal and vertical hydraulic conductivities of layers 1 though 3) were evaluated through a parameter-estimation process. In this study, model parameters were adjusted regionally, and not at individual cells, or groups of cells, to fit the simulation to observed data. A close model fit can be obtained by adjusting model parameters in selected areas or cells of the model, however a better understanding of the entire flow system is obtained by avoiding selectively adjusting individual cell input values. Simulations were calibrated to water levels measured at 309 bedrock wells, located throughout the study area and measured at various times, and 226 “synoptic” bedrock water levels, collected in the vicinity of Great Bay (Robert Roseen, University of New Hampshire, written communication 2001), during one week in June 2000. The synoptic water levels, measured near Great Bay, were given a greater weight in the parameter estimation process because they had at least an order of magnitude better elevation accuracy (about 0.1 ft) than the historically measured water levels. The historical water-level data were used to provide a minimal level of calibration information elsewhere in the study area. Simulated watershed discharges were compared to long-term annual average daily streamflow discharge at 5 subwatersheds in the study area, ranging from 5 to 183 mi 2 in size. Through this process, a regional recharge rate of 16 in/yr was selected. Hydraulic conductivities derived through the parameter estimation process were within the range of values presented for surficial aquifers in the study area (Mack and Lawlor, 1992; Moore, 1990; Stekl and Flanagan, 1992) and values identified for bedrock in another regional ground-water-flow simulation in New Hampshire (Tiedeman and others, 1997).

Ground-Water Flow Simulation

Model fit was assessed by examining simulated and observed heads and discharges. Heads in the Great Bay area (synoptic water levels) were simulated with an average residual (simulated minus observed) of 9.5 ft. The synoptic water levels were from domestic wells, measured after a short period of inactivity, and may be low due to prior well use. Simulated heads in the remainder of the study area (historical water levels) had an average residual of –6.6 ft. Overall, the agreement between simulated and observed water levels is very good (figure 3) and the residual for all water-level data is near zero. Simulated average daily discharges at 5 subwatersheds with contrasting sizes (figure 4) were well represented by the model. The ratios of simulated to observed flow for the watersheds were: Lamprey River (183 mi2) 104 percent, Dudley Brook (5 mi2) 75 percent, Winnicut River (estimated discharge 14 mi2) 96 percent, Oyster River (12 mi2) 79 percent, and the Mohawk Brook (9 mi2) 97 percent. Considering that model parameters were adjusted regionally, not adjusted by individual watershed, the overall model fit is very good and indicates that geohydrologic processes are well represented.

Simulated fresh ground-water discharge to Great Bay was approximately 2.13x10 6 ft 3 /d or 16 million gallons per day (mgd). The simulated inflow is comparable to the fresh ground-water discharge estimates of Brannaka and others (2002) who estimate a discharge to Great Bay at the shoreline (primarily the tidal zone), of approximately 5.5 mgd. Model results suggested that half of the freshwater discharge to Great Bay occurs through leakage from the underlying bedrock aquifer and about two thirds of the fresh ground-water discharge originates from the west side of the bay. The total simulated fresh ground-water discharge to all tidal water bodies in the study area is estimated to be approximately 50.9 mgd, or about one tenth the total simulated model flux (500 mgd).

Summary and Conclusions

The ground-water-flow simulation presented here contributes to the understanding of the geohydrology of the study area . Although the model provided a good simulation of regional flows and water levels in the Great Bay area, it is calibrated with limited and generalized information. Sustainability of ground-water withdrawals and potential reductions in fresh ground-water discharges to the coastal zone is a concern for water-resource managers. With additional and more detailed data, for example additional head and streamflow data, this modeling approach should be useful for providing water-resource managers with a means to quantify ground-water resources in the region and assess the effects of current and future ground-water withdrawals on the resource.

Brannaka, L.K, Ballestero, T.P., Mack, T.J., and Roseen, R, 2002, Inflow loadings from ground water to the Great Bay Estuary: CICEET Progress Report for the period 2/01/01 through 7/31/01, University of New Hampshire, Durham, New Hampshire.

Coakley, M.F., Keirstead, Chandlee, Brown, R.O., and Kiah, R.G., 2001, Water resources data, New Hampshire and Vermont, Water year 2000: U. S. Geological Survey Water-Data Report NH-VT-00-1, 192 p.

Daniel, C.C., III, Smith, D.G., and Eimers, J.I., 1997, Hydrogeology and simulation of ground-water flow in the thick regolith-fractured crystalline rock aquifer system of Indian Creek basin, North Carolina: U.S. Geological Survey Water-Supply Paper 2341-C, 137 p.

Degnan, J.R. and Clark, S.F., in press, Fracture Correlated Lineaments at Great Bay, Southeastern New Hampshire: U.S. Geological Survey Open File Report 02-13, 1 plate, 14 p.

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000, MODFLOW-2000, The U.S. Geological Survey modular ground-water-flow model--user guide to modularization concepts and the ground-water flow process: U.S. Geological Survey Open-File Report 00-92, 121 p.

Harte, P.T. and Winter, T.C., 1995, Simulation of flow in crystalline rock and recharge from overlying glacial deposits in a hypothetical New England setting: Ground Water, vol. 33, n. 6, p 953-964

Hill, M.C., 1998, Methods and guidelines for effective model calibration: U.S. Geological Survey Water Resources Investigation Report 98-4005, 90 p.

Hill, M.C., Banta, E.R., Harbaugh, A.W., and Anderman, E.R., 2000, MODFLOW-2000, The U.S. Geological Survey modular ground-water model—User guide to the observation, sensitivity, and parameter-estimation process and three post-processing programs: U.S. Geological Survey Open-File Report 00-184, p. 209

Mack, T.J., and Lawlor, Sean, 1992, Geohydrology and water quality of stratified-drift aquifers in the Bellamy, Cocheco, and Salmon Falls River Basins, Southeastern, New Hampshire: U.S. Geological Survey, Water-Resources Investigation Report 90-4161, 65 p., 6 pls.

Moore, R.B., 1990, Geohydrology and water quality of stratified-drift aquifers in the Exeter, Lamprey, and Oyster River Basins, Southeastern, New Hampshire: U.S. Geological Survey, Water-Resources Investigation Report 88-4128, 61 p., 8 pls.

Moore, R.B., Schwarz, G.E., Clark, S.F., Walsh, G.J., and Degnan, J.R., in press, Factors related to well yield in the fractured-bedrock aquifer of New Hampshire: U.S. Geological Survey, Professional Paper 1660, 2 plates.

Roseen, R., Brannaka, L.K., Ballestero, T.P., 2001, Assessing estuarine groundwater nutrient loading by thermal imagery and field techniques verified by piezometric mapping: A methodology evaluation: in Geological Society of America Meeting, November 5-8, 2001 Session no. 17, Boston, Mass.

Stekl, P.J., and Flanagan, S.M., 1992, Geohydrology and water quality of stratified-drift aquifers in the Lower Merrimack and Coastal River Basins, Southeastern, New Hampshire: U.S. Geological Survey, Water-Resources Investigation Report 91-4025, 75 p., 7 pls.

Tiedeman, C.R., Goode, D.J., Hseih, P.A., 1997, Numerical Simulation of ground water flow through glacial deposits and crystalline bedrock in the Mirror Lake Area, Grafton County, New Hampshire: U. S. Geological Survey Professional Paper 1572, 50 p.

Tiedman, C.R., Goode, D.J., and Hsieh, P.A., 1998, Characterizing a ground water basin in a New England mountain and valley terrain: Ground Water, v. 36, no. 4, p. 611-620.

Winston, R.B., 2000, Graphical User Interface for MODFLOW, Version 4: U.S. Geological Survey Open-File Report 00-315, 27 p.

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