Open-File Report 2006–1226
Open-File Report 2006–1226
By Paul M. Barlow and Lance J. Ostiguy
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A numerical-modeling study was done to better understand hydrologic-system responses to ground-water withdrawals in the Hunt-Annaquatucket-Pettaquamscutt (HAP) stream-aquifer system of Rhode Island. System responses were determined by use of steady-state and transient numerical ground-water-flow models. These models were initially developed in the late 1990s as part of a larger study of the stream-aquifer system. The models were modified to incorporate new data made available since the original study and to meet the objectives of this study. Changes made to the models did not result in substantial changes to simulated ground-water levels, hydrologic budgets, or streamflows compared to those calculated by the original steady-state and transient models.
Responses of the hydrologic system are described primarily by changes in simulated streamflows and ground-water levels throughout the basin and by changes to flow conditions in the aquifer in three wetland areas immediately east of the Lafayette State Fish Hatchery, which lies within the Annaquatucket River Basin in the town of North Kingstown. Ground water is withdrawn from the HAP aquifer at 14 large-capacity production wells, at an industrial well, and at 3 wells operated by the Rhode Island Department of Environmental Management at the fish hatchery. A fourth well has been proposed for the hatchery and an additional production well is under development by the town of North Kingstown.
The primary streams of interest in the study area are the Hunt, Annaquatucket, and Pettaquamscutt Rivers and Queens Fort Brook. Total model-calculated streamflow depletions in these rivers and brook resulting from withdrawals at the production, industrial, and fish-hatchery wells pumping at average annual 2003 rates are about 4.8 cubic feet per second (ft3/s) for the Hunt River, 3.3 ft3/s for the Annaquatucket River, 0.5 ft3/s for the Pettaquamscutt River, and 0.5 ft3/s for Queens Fort Brook. The actual amount of streamflow reduction in the Annaquatucket River caused by pumping actually is less, 1.1 ft3/s, because ground water that is pumped at the fish-hatchery wells (2.2 ft3/s) is returned to the Annaquatucket River after use at the hatchery.
One of the primary goals of the study was to evaluate the response of the hydrologic system to simulated withdrawals at the proposed well at the fish hatchery. Withdrawal rates at the proposed well would range from zero during April through September of each year to a maximum of 260 gallons per minute [about 0.4 million gallons per day (Mgal/d)] in March of each year. The average annual withdrawal rate at the fish hatchery resulting from the addition of the proposed well would increase by only 0.13 ft3/s, or about 5 percent of the 2003 withdrawal rate. The increased pumping rate at the hatchery would further reduce the average annual flow in Queens Fort Brook by less than 0.05 ft3/s and in the Annaquatucket River by about 0.1 ft3/s (which includes some model error).
A new production well in the Annaquatucket River Basin is under development by the town of North Kingstown. A simulated pumping rate of 1.0 Mgal/d (1.6 ft3/s) at this new well resulted in additional streamflow depletions, compared to those calculated for the 2003 withdrawal conditions, of 0.8 and 0.2 ft3/s in the Annaquatucket and Pettaquamscutt Rivers, respectively. The source of water for about 30 percent of the well's pumping rate, or about 0.5 ft3/s, is derived from ground-water inflow from the Chipuxet River Basin across a natural ground-water drainage divide that separates the Annaquatucket and Chipuxet River Basins; the remaining 0.1 ft3/s of simulated pumping consists of reduced evapotranspiration from the water table.
Model-calculated changes in water levels in the aquifer for the various withdrawal conditions simulated in this study indicate that ground-water-level declines caused by pumping are generally less than 5 feet (ft). However, ground-water-level declines of as much as 20 ft were calculated near the fish hatchery and of as much as 18 ft were calculated near the new production well in the Annaquatucket River Basin. The larger water-level declines in these two areas are attributed to relatively low values of the transmissivity of the aquifer in these two areas. Average annual ground-water-level declines in the aquifer resulting from the increased withdrawal in the hatchery area are not substantially greater than those for the 2003 average annual pumping conditions.
Model-calculated hydrologic budgets for the aquifer in the three wetland areas near the fish hatchery indicate that the total inflow rate to each area is reduced by a maximum of 0.3 ft3/s for the 2003 average annual pumping conditions; these reductions are a maximum of 13 percent of the total inflow rate to each area for non-pumping conditions. The rates of reductions in ground-water flows to the wetland areas are not substantially different for the proposed additional withdrawals at the hatchery.
The transient model also was used to evaluate the effects of a simulated drought condition on streamflow and ground-water levels. The simulated drought condition was based on hydrologic conditions during 1957, which was determined by the Rhode Island Department of Environmental Management to represent a 1-in-25 year drought. For the withdrawal conditions evaluated, the simulated drought caused model-calculated monthly streamflows in the Annaquatucket River to be reduced by 1.0 to 3.1 ft3/s compared to those streamflows calculated for long-term average simulation conditions (that is, non-drought conditions). Model-calculated ground-water levels at the proposed well site at the fish hatchery for the drought condition declined by 1.0 to 3.7 ft compared to those levels calculated for non-drought conditions.
Results of the study must be viewed within the limitations of the quality of the data that are available for the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system and representation of the system with simulation models. Uncertainties in the model simulations result from a variety of factors, including uncertainties in the exact values of the hydraulic properties of the aquifers, rates of recharge to the aquifer, and the approaches used to simulate the many features of the hydrologic system (streams, evapotranspiration, flow conditions at ground-water drainage divides, and so forth).
The precision of the numerical models used in this study to determine hydrologic-system responses to ground-water withdrawals could benefit from refinement of localized information on the wetlands and the geology of the area near the fish hatchery. Model results in the area of the hatchery also would be improved by the use of a finer model-grid discretization to better simulate wetland-aquifer interactions and the impacts of withdrawals on wetlands and streamflow. Use of a finer discretization could be done as part of a broader data-collection and analysis study of wetland and hydrologic conditions at or near the hatchery. As a consequence of the spatial-discretization limitations of the numerical models developed for the HAP stream-aquifer system, the ability of the numerical models to predict hydrologic responses in the vicinity of the hatchery wells is limited. Furthermore, ground-water-flow conditions near the ground-water drainage divide that separates the Annaquatucket and Chipuxet River Basins are largely unknown. The understanding of hydrologic conditions near this divide would be improved by collection of hydrogeologic data in that area.
Abstract
Introduction
Purpose and Scope
Description of the Study Area and Water-Supply Wells
Numerical Models to Simulate Hydrologic-System Responses in the Stream-Aquifer System
Overview of the Original Numerical Models
Modifications to the Original Numerical Models
Conversion of Models to MODFLOW-2000
Increase in the Active Area of the Models
Simulation of Flow at Ground-Water Drainage Divides in the Annaquatucket River Basin
Modification of Aquifer Hydraulic Properties near Lafayette State Fish Hatchery and Town of North Kingstown Well 11
Conversion to Constant Saturated Thickness and Transmissivity
Simulation of Hydrologic-System Responses in the Stream-Aquifer System
Simulation Results for Original Withdrawal Conditions
Simulation Results for Alternative Withdrawal and Recharge Conditions
Simulations for Long-Term Average Recharge Conditions
Simulations for a Drought
Limitations of Numerical Models
Summary and Conclusions
References Cited
Appendix 1: Withdrawals from Selected Production Wells, 1999-2004
1–3. Maps showing—
1. Location of the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system, Rhode Island
2. Drainage boundaries to the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
3. Altitude and configuration of the water table in the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system, October 7–9, 1996
4. Graph showing average monthly withdrawal rates in the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system, 2003
5–8. Maps showing—
5. Spatial extent of active area of simulation models of the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
6. Grid and boundary conditions of the active areas of the modified simulation models of the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system for (A) rows 3–125, and (B) rows 126–205
7. Stream sites discussed in text for the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
8. Model-calculated steady-state water table with 1996 withdrawal rates, Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
9–11. Graphs showing—
9. Model-calculated mid-monthly streamflow at the outflow locations of the Hunt and Annaquatucket Rivers
10. Model-calculated end-of-month streamflow in Queens Fort Brook at its outflow (site J)
11. Model-calculated end-of-month streamflow in the Annaquatucket River at its outflow (site P)
12–14. Maps showing—
12. Model-calculated steady-state water table for simulation set 3 (2003 withdrawal rates), Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
13. Model-calculated steady-state drawdowns for simulation set 3 (2003 withdrawal rates), Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
14. Model-grid locations of wetlands near Lafayette State Fish Hatchery
15. Graphs showing model-calculated end-of-month ground-water levels at (A) State Fish Hatchery well 3, (B) State Fish Hatchery well 4, and (C) wetland site A
16–21. Maps showing—
16. Model-calculated steady-state contributing areas to water-supply wells in (A) the Hunt River Basin and (B) the Annaquatucket and Pettaquamscutt River Basins, for simulation set 3 (2003 withdrawal rates)
17. Model-calculated steady-state drawdowns for simulation set 4 (hatchery wells pumping at half their 2003 rates), Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
18. Model-calculated steady-state drawdowns for simulation set 5 (with State Fish Hatchery well 4 pumping), Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
19. Model-calculated steady-state water table for simulation set 6 (with State Fish Hatchery well 4 and town of North Kingstown well 11 pumping), Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
20. Model-calculated steady-state drawdowns for simulation set 6 (with State Fish Hatchery well 4 and town of North Kingstown well 11 pumping), Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
21. Model-calculated steady-state contributing areas to water-supply wells in the Annaquatucket River Basin for simulation set 6 (with State Fish Hatchery well 4 and town of North Kingstown well 11 pumping)
22–24. Graphs showing—
22. Long-term (1941–2003) average and 1957 monthly recharge rates, Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system
23. Model-calculated end-of-month streamflow at the outflow sites of (A) Queens Fort Brook and (B) Annaquatucket River for simulated long-term (1941–2003) average and drought conditions
24. Model-calculated end-of-month ground-water levels at (A) State Fish Hatchery well 4 and (B) wetland site A for simulated long-term (1941–2003) average and drought conditions
Barlow, P.M., and Ostiguy, L.J., 2007, Simulation of hydrologic-system responses to ground-water withdrawals in the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system, Rhode Island: U.S. Geological Survey Open-File Report 2006–1226, 51 p.
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