Fracture Characterization Mapping for Regional Geologic Studies: The Hydrostructural Domain Approach, Ayer Quadrangle, Massachusetts Stephen B. Mabee And Joseph P. Kopera Office of the Massachusetts State Geologist Geosciences Department University of Massachusetts 611 North Pleasant Street Amherst, MA 01003 Abstract While traditional bedrock geologic maps contain valuable information, they commonly lack data on brittle fracture characteristics and distributions. The increased need for better understanding of groundwater flow behavior in bedrock aquifers has made this data critical. The concept of hydrostructural domains is used to redefine bedrock mapping units based on an assemblage of lithologic and fracture characteristics thought to be important controls on groundwater flow and recharge. These maps are constructed from detailed field observations and measurements of 2000-3000 fractures from 60-70 stations across a 7.5' quadrangle. Hydrostructural domains are displayed on the map as traditional lithologic units would be, with detailed descriptions and photos of the fracture systems contained in each hydrostructural “unit”. In the Ayer quadrangle, such domains closely correspond with bedrock lithology and ductile structural history. Steeply dipping metasedimentary rocks of the Merrimack Belt have pervasive, closely spaced, throughgoing fractures developed parallel to foliation, and therefore may provide excellent potential for vertical recharge and foliation-parallel flow. Where these rocks are intensely cut by a strong subhorizontal cleavage, a parallel fracture set dominates providing an opportunity for lateral flow. Massive granites generally have a well-developed, widely-spaced orthogonal network of fracture zones which may provide excellent local recharge. High-grade gneisses of the Nashoba formation have poorly developed fracture sets except near regional shear zones, where foliation parallel fractures and cross-joints may provide good vertical recharge and provide a strong northeast trending flow anisotropy. These maps are intended to provide regional-scale information to assist in site-specific groundwater investigations. We believe that such maps are an example of how new types of geologic maps can, and must, be developed to address changing societal needs. The next step is to determine if the qualitative descriptions provided by the hydrostructural domain can be translated into a quantitative measure of hydraulic properties. 169 to 186 Introduction The use of fractured-bedrock aquifers to meet private, public and commercial water supply needs is increasing in the New England region. Municipalities and water suppliers are finding it increasingly difficult to locate and develop water supplies in overburden aquifers because of contamination and a lack of suitable sites. As a result, water suppliers are going deeper into bedrock aquifers (Drew et al., 2001). Yet information on the factors that influence the availability and recharge characteristics of fractured bedrock aquifers in highly deformed crystalline metamorphic rocks is limited. The availability of water in fractured rock aquifers is particularly critical in eastern Massachusetts because growth and development along the coast, major transportation corridors, and in rural communities adjacent to large metropolitan areas is extensive. The I-495 corridor, a circumferential highway 20 miles west of Boston, has become the focus of recent growth. Professional office buildings, research and development parks associated with the computer industry, warehouses and light industry are springing up along this corridor, as are housing and condominium developments. Municipalities and water suppliers are simply unprepared for this onslaught of development and need help in understanding the complex dynamics of the ground water system. Since 2003, the Office of the Massachusetts State Geologist has been preparing a new suite of 1:24,000 scale mapping products referred to as fracture characterization maps (Mabee and Salamoff, 2004; Mabee, 2005; Kopera et al., 2006). These maps were created to fill a critical need for brittle fault and fracture information relevant to groundwater issues in bedrock environments that was not being met by traditional bedrock geologic mapping. This purpose of this paper is to describe briefly how these maps are constructed and utilized by the user community. We will provide an example of one of the fracture characterization maps from the 1:24,000-scale Ayer quadrangle in eastern Massachusetts. Study Area The Ayer quadrangle is located approximately 25 miles NW of Boston. Two major transportation routes traverse the quadrangle (Figure 1). These include Interstate 495 and Route 2. I-495 is the major transportation corridor connecting northern New England with the entire eastern seaboard megalopolis. Route 2 is the major east-west corridor that transits the northern half of Massachusetts connecting Boston with New York state. Both of these transportation routes and the communities adjoining them have been the focus of development and recent growth (Figure 1). The Ayer quadrangle encompasses parts of six communities, Groton, Ayer, Shirley, Harvard, Boxborough and Littleton (Figure 2). The population has grown by an average of 22% from 1980 to 2000 with Boxborough and Groton growing at 53% and 42%, respectively. An average of 283 acres per year are being converted from undeveloped to developed land in the six towns combined. Nearly 15% of the total land area in Groton has become developed in the 28 years extending from 1971 to 1999. Growth is expected to continue in the coming years. 2 Rt 2 Figure 1. Map showing the percentage of land area, by town, converted from undeveloped land (crops, pastures, forests, open space) to developed land (residential, commercial, industrial land uses, etc.) from 1977 to 1999 (from MassGIS). Note correlation between growth patterns and major state routes and interstates. The Ayer quadrangle is labeled Project 1. All of the communities rely primarily on groundwater to meet their potable water supply demands despite being within 30 miles of Boston. All water is supplied either through private domestic wells or limited municipal water supply systems. Ayer, Groton, Littleton and Shirley have municipal water supply systems comprised of gravel pack wells but most individuals not connected to public water rely on bedrock wells to meet demand. Harvard and Boxborough have no public water supply systems and must rely predominantly on bedrock wells. Geology In the broadest terms, Massachusetts is the amalgamation of rocks from three tectonic plates (Figure 3). These include rocks associated with the margin of Figure 2. Close-up view of the Ayer Quadrangle. Approximate location of Fort Devens shown in blue shaded area. 3 Laurentia, medial New England, which includes Ordovician-age intrusive and extrusive rocks associated with an island arc system and Proterozoic rocks of uncertain origin, and the Avalon plate. These plates were active during the early to middle Paleozoic (Robinson et al., 1993). Figure 3. Generalized map of Massachusetts showing the geographic distribution of lithotectonic packages and terranes into which the rocks of the State have been grouped (modified from Hatch, 1991). Regions discussed in this proposal are bold and all caps. Structural and metamorphic features in Massachusetts were produced during three orogenic events. These include the late Ordovician Taconian Orogeny (the docking of medial New England with Laurentia, affecting central and western Massachusetts), the Devonian Acadian orogeny (the docking of Avalon with amalgamated Laurentia and medial New England, affecting eastern and central Massachusetts predominantly) and the Pennsylvanian-Permian Alleghenian orogeny (metamorphism and reactivation of faults produced by the collision of Africa with North America). All structures were later modified and reactivated by extension in the Mesozoic during opening of the present day Atlantic Ocean (Robinson et al., 1993). Sandwiched between medial New England and Avalon is an enigmatic terrane referred to as the Nashoba terrane (Figure 3). Acceptance as a separate terrane did not take place until the early 1980’s following identification of the large terrane-bounding fault zones (Castle et al., 1976; Bell and Alvord, 1976). The Clinton Newbury fault zone delineates the western edge of the Nashoba terrane and separates it from the eastern portion of medial New England, an area known as the 4 Merrimack Belt (the Merrimack Belt lies within the Merrimack Synclinorium and forms the eastern half of medial New England) (Figure 3). Rocks of the Merrimack Belt are comprised of calcareous metasiltstones, phyllite, metasandstones and quartzites of Silurian and Ordovician age (Robinson and Goldsmith, 1991). The rocks immediately west of the Clinton-Newbury fault are metamorphosed to the lower greenschist facies and progressively rise in grade toward the northwest (Hepburn, 2004). In contrast, the rocks of the Nashoba terrane, east of the Clinton-Newbury fault, are multiply deformed and metamorphosed middle to upper amphibolite facies rocks (sillimanite and sillimanite-K feldspar zones) (Hepburn, 2004) consisting of largely metavolcanic materials to the east and metasedimentary rocks to the west (Goldsmith, 1991). The Ayer quadrangle is located at the junction between the Merrimack belt and the Nashoba terrane (Figure 4). The Clinton-Newbury fault passes through the southeast corner of the Figure 4. Generalized bedrock