Groundwater and Streams in Rhode Island's Big River Area

The Big River study area and the Big River Management Area. USGS, modified from Granato and others (2003).

The problem

Rhode Island gets plenty of rain, but water isn't always available everywhere in dry periods — especially in summer, when water levels drop, droughts loom and demand peaks with lawn watering, irrigation and coastal tourist populations. The Rhode Island Water Resources Board (RIWRB) worries demand could outstrip public supply in central and southern Rhode Island, where groundwater is the main drinking water.

The Big River area

The Big River and Mishnock River basins, headwaters of the South Branch Pawtuxet River, are relatively undeveloped and a potential source of high-quality drinking water.

• The Big River drains about 30 square miles, the Mishnock about 4; the Big River flows into the Flat River Reservoir.
• Forests and wetlands include Atlantic white cedar swamps and pitch pine–scrub oak barrens; the upper Big River supports native brook trout.
• After the 1960s drought the state bought 13.4 square miles for a reservoir, never built because of environmental concerns and changing needs. This Big River Management Area (BRMA) is kept as open space and a future water supply.
• The only large withdrawals are Kent County Water Authority wells in the Mishnock basin; most homes use private wells and septic systems.

An Atlantic white cedar swamp and a brook trout stream in the Big River area. USGS.

Twenty years of studies

For nearly two decades the RIWRB and USGS have asked how much groundwater the BRMA could supply while protecting its rivers, ponds and wetlands.

1. Mapping the aquifers (data from July 1996 to September 1998). Observation wells, geophysics, aquifer tests and monitoring — 27 monitoring wells, 18 test wells, 15 streambed piezometers, 10 stream stations and 8 ponds — plus records from 80 boreholes, 375 groundwater sites and 31 wells. The best aquifers are glacial sand and gravel 50 to 100 feet thick in the Big, Carr and Mishnock valleys; uplands have thin till over bedrock.

2. Modeling pumping and streams. MODFLOW groundwater models drew water-table maps and water budgets.

Groundwater flows from till and bedrock uplands toward the river valleys. USGS, modified from Granato and others (2003).

• Before development, about 93% of groundwater left the area as streamflow — so wells would mainly capture water that would otherwise feed streams.
• Some groundwater crosses from the Carr River subbasin into the Mishnock basin, so pumping in one can affect the other.
• Of 14 pumping scenarios at 12 candidate well sites, none avoided reducing streamflow. Scenario 10 — 4, 2 and 1 million gallons a day from the Big, Mishnock and Carr basins — cut average August flows by 25%, 51% and 24%.

Groundwater inflows and outflows, Big River area. USGS, modified from Granato and others (2003).

3. Optimizing withdrawals. Simulation–optimization models tested 31 scenarios at 13 sites. The stricter the minimum-streamflow rule, the less groundwater is available. Because demand peaks in summer when streams are lowest, planning for that halved the sustainable average withdrawal. Managing wells together helps; without it, withdrawals would need to be about 1–2 million gallons a day lower.

As more water must be left in streams, less groundwater is available. USGS, modified from Granato and Barlow (2005).

4. Wetlands. Five wetlands were studied through the 2009 growing season. All were connected to the aquifer; water stood within a foot of the surface more than 80% of the time at most spots. Mosses and herbaceous plants, shallow-rooted, would change first if soils dried. A refined model found that fine lake sediments under the Flat River Reservoir limit how much water pumping could draw from it — so nearby wetlands would drop more. In a proposed five-well, 2.5 million gallon a day scenario, water levels in wetlands near Reynolds Pond would fall 4 to 6 feet; the cedar swamp under 1 foot; the others under 0.3 foot.

Sampling wetland vegetation in the Big River area. USGS.

What it means

The models have helped the RIWRB site test wells, plan pumping tests and weigh the trade-off between developing groundwater and protecting streams and wetlands — and they underpin statewide water policy.

Sources

Based on David S. Armstrong, John P. Masterson, Keith W. Robinson and Kathleen M. Crawley, "Scientific Information in Support of Water Resource Management of the Big River Area, Rhode Island," U.S. Geological Survey fact sheet, with the Rhode Island Water Resources Board; a work of the United States government in the public domain. The map, charts and photographs are taken from the fact sheet's PDF.