Scientific Investigations Report 2006–5271
Scientific Investigations Report 2006–5271
By John P. Masterson, Jason R. Sorenson, Janet R. Stone, S. Bradley Moran¹, and Andrea Hougham²
¹University of Rhode Island Graduate School of Oceanography ²University of South Carolina-Columbia
The body of the report is available in PDF Format (8,215 KB)
The Salt Pond region of southern Rhode Island extends from Westerly to Narragansett Bay and forms the natural boundary between the Atlantic Ocean and the shallow, highly permeable freshwater aquifer of the South Coastal Basin. Large inputs of fresh ground water coupled with the low flushing rates to the open ocean make the salt ponds particularly susceptible to eutrophication and bacterial contamination. Ground-water discharge to the salt ponds is an important though poorly quantified source of contaminants, such as dissolved nutrients.
A ground-water-flow model was developed and used to delineate the watersheds to the salt ponds, including the areas that contribute ground water directly to the ponds and the areas that contribute ground water to streams that flow into ponds. The model also was used to calculate ground-water fluxes to these coastal areas for long-term average conditions. As part of the modeling analysis, adjustments were made to model input parameters to assess potential uncertainties in model-calculated watershed delineations and in ground-water discharge to the salt ponds.
The results of the simulations indicate that flow to the salt ponds is affected primarily by the ease with which water is transmitted through a glacial moraine deposit near the regional ground-water divide, and by the specified recharge rate used in the model simulations. The distribution of the total freshwater flow between direct ground-water discharge and ground-water-derived surface-water (streamflow) discharge to the salt ponds is affected primarily by simulated stream characteristics, including the streambed-aquifer connection and the stream stage. The simulated position of the ground-water divide and, therefore, the model-calculated watershed delineations for the salt ponds, were affected only by changes in the transmissivity of the glacial moraine.
Selected changes in other simulated hydraulic parameters had substantial effects on total freshwater discharge and the distribution of direct ground-water discharge and ground-water-derived surface-water (streamflow) discharge to the salt ponds, but still provided a reasonable match to the hydrologic data available for model calibration. To reduce the uncertainty in predictions of watershed areas and ground-water discharge to the salt ponds, additional hydrogeologic data would be required to constrain the model input parameters that have the greatest effect on the simulation results.
Abstract
Introduction
Hydrogeologic Framework
Geologic Setting
Hydrologic Setting
Freshwater Flow Beneath Salt Ponds
Development of Ground-Water-Flow Model
Model Discretization and Boundaries
Spatial Discretization
Hydrologic Boundaries
Hydrologic Stresses
Recharge
Pumping
Hydraulic Properties
Model Calibration
Water-Level Data
Streamflow Data
Geochemical Tracers
Simulation of Ground-Water Flow to Salt Ponds
Delineation of Ground-Water Recharge Areas
Calculation of Ground-Water Fluxes to Salt Ponds
Simulation of Changes in Selected Hydraulic Parameters
Effects on Total Flow to Coast
Recharge
Hydraulic Conductivity
Effects on the Distribution of Freshwater Discharge to Salt Ponds
Effects on Ground-Water Discharge within Salt Ponds
Simulation of High- and Low-Flow Discharge to Salt Ponds
Summary and Conclusions
Acknowledgments
References Cited
Appendix 1 Use of Continuous Resistivity-Profiling Techniques to Characterize the Freshwater Extent Beneath the Coastal Waters of the Salt Pond Region of Southern Rhode Island
1-3. Maps showing:
1. Production wells, water-supply districts, long-term streamflow-gaging stations, and long-term observation well in the Salt Pond region of southern Rhode Island
2. Watershed boundaries of the salt ponds in the South Coastal Basin aquifer
3. Surficial geology and altitude of bedrock surface. Sections A-A’ and B-B’ are shown in figure 4
4. Geologic cross sections (A and B) showing deposits of southern Rhode Island
5. Generalized cross section of the Salt Pond region
6. Diagram of ground-water flow in the South Coastal Basin aquifer
7.Map showing extent and distribution of boundary conditions for ground-water-flow model of South Coastal Basin aquifer
8. Section showing model representation of geologic section B-B’
9. Graph showing long-term water levels for U.S. Geological Survey observation well Charlestown 18 (CHW-18) from 1954 through 1961
10–11. Maps showing:
10. Observation wells, ponds, and streamflow-gaging stations from which data were used for model calibration, South Coastal Basin aquifer
11. Delineation of ground-water recharge areas to production wells, streams, and salt ponds in the South Coastal Basin aquifer
12. Diagrams showing possible ground-water flow conditions in the South Coastal Basin aquifer where the ground-water divide is A, coincident with the surface-water divide; B, north of the surface-water divide; and C, south of the surface-water divide
13. Map showing changes in the model-calculated recharge area to Winnapaug Pond in response to changes in the simulated hydraulic-conductivity values representing the Charlestown moraine in the ground-water-flow model of the South Coastal Basin aquifer
14. Graphs showing A, changes in model-calculated freshwater flow to Ninigret Pond in response to changes in simulated hydraulic parameters from low- to high-flow values; and B, changes in model-calculated freshwater flow to Green Hill Pond in response to changes in simulated hydraulic parameters from low- to high-flow values
1. Average pumping rates for production wells in the South Coastal Basin aquifer, southern Rhode Island, 1995–99
2. Lithology and hydraulic conductivity of lithologic units used in the ground-water-flow model of the South Coastal Basin aquifer
3. Measured water levels for selected observation wells in the modeled area (1954-61), and the model-calculated water-level altitudes for simulated current (1995–1999) pumping and recharge conditions, South Coastal Basin aquifer
4. Water levels for selected ponds in the modeled area determined by plane-table surveys between 1953 and 1957 and reported on U.S. Geological Survey 1:24,000 topographic maps, and the model-calculated water levels for simulated current (1995–1999) pumping and recharge conditions, South Coastal Basin aquifer
5. Measured streamflows for selected locations on streams in the modeled area, and the model-calculated streamflows for simulated current (1995–1999) pumping and recharge conditions, South Coastal Basin aquifer
6. Estimated ground-water discharge to selected salt ponds in the modeled area, and the model-calculated ground-water discharge to these ponds for simulated current (1995–1999) pumping and recharge conditions, South Coastal Basin aquifer
7.Model-calculated hydrologic budget for the South Coastal Basin aquifer under current (1995–1999) pumping and recharge conditions
8. Changes in model-calculated freshwater fluxes to selected salt ponds in response to changes in simulated hydraulic parameters, South Coastal Basin aquifer
Masterson, J.P., Sorenson, J.R., Stone, J.R., Moran, S.B., Hougham, A., 2007, Hydrogeology and simulated ground-water flow in the Salt Pond region of southern Rhode Island: U.S. Geological Survey, Scientific Investigations Report 2006–5271, 56 p.
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