Glacial Geology and Aquifer Characteristics of the Big River Area, Central Rhode Island By Janet Radway Stone and David C. Dickerman INTRODUCTION part in Washington County; it encompasses parts of the Coventry Center, Hope Valley, Crompton, and Slocum The Rhode Island Water Resources Board 7.5-minute quadrangles. (RIWRB), which is responsible for developing the This investigation includes a characterization of State’s major water resources, is concerned that contin- the geologic framework and an evaluation of the ued growth in demand for water in Rhode Island will hydraulic properties of the stream-aquifer system. The severely stress water supplies. In the early 1960s, the Big River area stream-aquifer system is the surficial State proposed construction of a surface-water reser- aquifer and the network of rivers, brooks, lakes, and voir in the Big River Basin in central Rhode Island to ponds that overlie and are in hydraulic connection with meet the growing water demands, but, to date (2001) the aquifer. The surficial materials units of the Big the U.S. Environmental Protection Agency has not River area and the accompanying bedrock-surface given approval for construction of this reservoir. As an contours (pl. 1) were delineated on the basis of data alternative, the RIWRB would like to identify sites in collected during field mapping, seismic-refraction the Big River area stream-aquifer system where high- surveys, ground-penetrating radar surveys, logs from yield supply wells can be constructed to meet future all available wells and test holes, and results of water demands. A cooperative study was initiated previous investigations. during the summer of 1995 by the U.S. Geological Some streams in the study area lose surface Survey (USGS) and the RIWRB to evaluate ground water naturally to the underlying sand and gravel aqui- water as a source of water supply in the Big River area. fer, whereas others lose water to the aquifer by induced This report presents the initial results of a multi- infiltration of streamflow caused by ground-water year investigation of the three-dimensional characteris- withdrawals. Most large ground-water withdrawals are tics of the surficial aquifer system based on geologic from public-supply wells in the sand and gravel aquifer and geophysical field studies, and provides information in the Mishnock River Basin. Minimal ground-water needed to protect, develop, and manage the Big River withdrawals take place in the Big River Basin, because area stream-aquifer system. The Big River area most of the land is designated as open space and pro- includes the entire Big River drainage basin and part tected from development by State law. The primary of the Mishnock River drainage basin (fig. 1) and use of surface water in the study area is for recreation. covers approximately 35 mi2, including parts of the Water from the Flat River Reservoir, the largest towns of Coventry, Exeter, and West Greenwich. Most surface-water body in the study area, is used only for of the Big River area is in Kent County, with a small recreational purposes. Introduction 1 o o 71o45' 71o37'30" 71 30' 71 22'30" 42o00' Woonsocket Cumberland Burrillville North MASSACHUSETTS Smithfield Central Smithfield Lincoln Falls Glocester 41o52'30" North Pawtucket RHODE ISLAND Providence Providence Johnston Foster Scituate East CONNECTICUT Providence 71o15' Cranston 41o45' Barrington West Warren Warwick Warwick Coventry Big River Bristol 71o07'30" Study Area East Greenwich o 41 37'30" West Greenwich North Kingstown Portsmouth Tiverton Exeter Jamestown Little Hopkinton Middletown Compton 41o30' Richmond South Kingstown Newport Charlestown RHODE ISLAND SOUND 41o22'30" Narragansett Westerly 0 5 10 MILES 0 5 10 KILOMETERS BLOCK ISLAND SOUND ATLANTIC OCEAN 41o15' Block Island New Shoreham Figure 1. Location of study area and major streams in the Big River area, Rhode Island. 2 Glacial Geology and Aquifer Characteristics of the Big River Area, Central Rhode Island Acknowledgments conducted a water-resources investigation for the South Branch Pawtuxet River Basin, which includes the Big The authors express appreciation to Timothy River Basin. Brown, General Manager, Kent County Water Authority for providing well logs, aquifer-test data, and pumpage records for water-supply wells in the Topography and Mishnock River Basin. Special acknowledgment is Drainage made to Patrick Craft, formerly with the USGS, who analyzed aquifer-test data and calculated hydraulic Landforms in the Big River study area consist properties at six test sites. The authors also wish to of a series of north- to northwest-trending hills and thank Lance Ostiguy, USGS, who provided map digiti- valleys. Most hills that form the drainage basin divide zation and GIS (geographic information systems) are higher than 400 ft in altitude; the highest point is support, and Mark Bonito, USGS, for digital drafting. at 600 ft on the top of Raccoon Hill. Valley bottoms generally are below 300 ft, and contain a variety of glacial landforms such as flat-topped ice-contact Previous Studies (kame) deltas, sinuous esker ridges, glacial lake- bottom plains, and postglacial wetlands and flood- Surficial geologic maps have been published plains. The lowest altitude in the area is 240 ft along for the Hope Valley quadrangle (Feininger, 1962), the Mishnock River where the river exits the drainage Crompton quadrangle (Smith, 1956), and Slocum basin in the northeastern part of the area. The Big River quadrangle (Power, 1957). These maps are based on a Basin drains to the north and includes seven streams morphogenic classification of deposits, and generally ultimately tributary to the east-flowing Flat River and show an accurate distribution of glacial till, glacial South Branch Pawtuxet River: Raccoon Brook is a meltwater deposits, and postglacial deposits, but do north-flowing tributary to the south- and southeast- not describe the thickness and textural distribution of flowing Nooseneck River; Congdon River is a north- the surficial materials. The surficial geology of the flowing stream that joins the Nooseneck River to form Coventry Center quadrangle was not previously the north-flowing Big River; Carr River and other mapped. The bedrock geology was mapped for the north- and northwest-flowing tributaries join the Big Coventry Center quadrangle (Moore, 1963), Hope River in the central part of the study area. Bear Brook Valley quadrangle (Moore, 1958), Crompton quadran- flows north and east to join the Big River, and the gle (Quinn, 1963), and Slocum quadrangle (Power, Mishnock River originates in a large wetland area, and 1959). Many surficial geologic contacts and locations flows northerly to the South Branch Pawtuxet River. of nearly all bedrock outcrops shown on the surficial The northern end of the Big River Valley now is materials map (pl. 1) were taken from these earlier flooded by the Flat River Reservoir. quadrangle studies. The bedrock geology of the area has been updated on the new Bedrock Geologic Map of Rhode Island by Hermes and others (1994). Ground- Bedrock water maps have been published for the Crompton quadrangle (Allen and others, 1959), Hope Valley Bedrock controls the large-scale aspect of the quadrangle (Bierschenk and Hahn, 1959), Slocum landscape in the study area. The basin is underlain by quadrangle (Hahn, 1959), and Coventry Center quad- granitic rocks of the Scituate Igneous Suite (Hermes rangle (Mason and Hahn, 1960). Also, Gonthier (1966) and others, 1994), formerly called Scituate granite Introduction 3 gneiss (Quinn, 1971). The granite has undergone GLACIAL GEOLOGY Paleozoic metamorphism and has a gneissic structure and texture. Although locally massive, in most places Glacial deposits overlie bedrock in the Big River the rock is moderately foliated and compositionally area (plate) and range from a few feet to more than layered. In the western part of the study area, the strike 200 ft in thickness. Most of these materials are deposits of foliation and layering generally is north-northeast of the last two continental ice sheets that covered New with low (25–35˚) northwest dips; in the central part, England during the middle and late Pleistocene. Most the strike of foliation is north to north-northwest with deposits were laid down during the advance and retreat steep (greater than 70˚) to vertical dip. In the eastern- of the last (late Wisconsinan) ice sheet, which reached most part of the area, foliation trends are northeast to its terminus on Long Island, N.Y., about 21,000 years east-northeast with moderate (less than 40˚) northwest ago, and was retreating northward through the Big to north-northwest dips. Locally, Mesozoic-aged River area by about 17,000 years ago (Stone and Borns, diabase dikes intrude the granitic rock. Ubiquitous 1986; Boothroyd and others, 1998). The glacial depos- fractures and joints cut the bedrock; cross-cutting its are divided into two broad categories—glacial till fractures include near-horizontal unroofing joints and and glacial meltwater deposits. Till was deposited high-angle to vertical fractures generated by tectonic directly by glacier ice and is characterized as a non- stresses. The orientation of dominant sets of high-angle sorted matrix of sand, silt, and clay with variable fractures coupled with the orientation of foliation and amounts of pebbles, cobbles, and large boulders. Gla- layering in the bedrock control the erosional resistance cial meltwater deposits were laid down by meltwater in of the rock, hence the trend and shape of bedrock hills lakes and streams in front of the retreating ice margin and valleys. The bedrock valleys, which today are par- during deglaciation. These materials, also referred to as tially filled with glacial meltwater sediments, were glacial stratified deposits, consist of well-sorted to developed from deeply weathered zones in the bed- poorly sorted layers of gravel, sand, silt, and clay. Post- rock, modified by multiple episodes of continental glacial sediments, primarily floodplain alluvium and glaciation as glacial ice preferentially plucked the swamp deposits, are thin surface units and make up a fractured bedrock along zones of structural weakness.