Development of a data base of community water-supply wells in New Jersey and a method of evaluate their sensitivity to contamination

Abstract

Well-construction and other well-attribute data for 2,598 community water-supply wells in New Jersey were compiled from existing data bases. The resulting data base is stored in a geographic information system and includes well-identification numbers, well-construction details and well characteristics, ratings of sensitivity to contamination, location data, and owner information. Information from this data base can be used by water managers to delineate wellhead-protection areas for water-supply wells. Ground-water flow models were used to simulate ground-water contributing areas and the travel times of ground water from the water table to the open interval of wells in typical aquifer settings. From this information, the sensitivity to contamination of wells in three types of aquifers glacial, Coastal Plain, and bedrock was evaluated. Hydrogeologic variables that were considered in this assessment include the presence or absence of confining units above the open interval of the well, the location of the well relative to the outcrop area of the aquifer penetrated by the well, and the depth to the top of the open interval. Wells with open intervals in glacial aquifers were considered to be sensitive to contamination from land surface because of (1) the absence of extensive confining units; (2) short groundwater travel times from the water table to the well; and (3) the typical construction characteristics of wells in glacial aquifers, which include shallow depth to the top of the open interval and shallow depth of the well. Wells screened in Coastal Plain aquifers were considered either (1) sensitive (wells in or less than 0.5 miles downdip from outcrop areas of confined aquifers and wells in unconfined aquifers, because the minimum travel time from land surface to the well likely is less than 12 years), or (2) insensitive (wells in confined aquifers greater than 0.5 miles downdip from the outcrop area, because the minimum travel time likely is greater than 12 years). Wells with open intervals in bedrock aquifers were considered sensitive to contamination because of (1) the geologic complexity of aquifer systems and absence of extensive confining units; (2) the relatively fast velocities of ground water in fractured zones within bedrock aquifers and the resulting short travel time of ground water from land surface to the wells; and (3) the typical construction characteristics of wells in bedrock aquifers, such as long open intervals and short casing lengths. All 245 wells in glacial and 1,002 wells in bedrock aquifers were considered sensitive to contamination because minimum travel times are most likely less than 12 years. In the Coastal Plain, 637 of 1,351 wells were considered sensitive to contamination because they are located in outcrop areas or less than 0.5 miles downdip from outcrop areas where minimum travel times are probably less than 12 years. INTRODUCTION The New Jersey Department of Environmental Protection (NJDEP) is responsible ftr ground-water protection and resource management within the State of New Jersey. The NJE HP developed a Wellhead Protection Program Plan as required by the 1986 Federal Safe Drinking Water Act Amendments (Section 1428). The purpose of the Wellhead Protection Program is to minimize the risk of water-supply-well contamination due to the discharge of ground-water contaminants at land surface. The Wellhead Protection Program was developed to enhance protection of three groups of potable-water-supply wells: public community supply wells, public noncommunity supply wells, and clusters of domestic supply wells. It provides this protection through the delineation of wellhead-protection areas and the implementation of regulations and other activities to minimize contamination from both point and nonpoint sources within these areas (New Jersey Department of Environmental Protection and Energy, 1991). Wellhead-protection areas, as defined by the NJDEP, are portions of the wells' groundwater contributing areas that are close to the wells. A wellhead-protection area is defined by two criteria: the average time of travel for ground water to reach a well from the water table, and the presence of hydrologic boundaries, such as faults, surface-water bodies, and confining units. Each wellhead-protection area is divided into three sequential tiers. Management controls are most stringent nearest the well, because contaminant sources near a well pose the greatest threat to ground-water quality. Tier 1 is intended to prevent sources of bacteria and viruses from discharging near the well; the travel time from its outer boundary to the well is 200 days. Tier 2 is intended to prevent discharges of hazardous materials that do not degrade rapidly in ground water in areas so close to the well that remediation is not possible; the time of travel from its outer boundary to the well is 5 years. Tier 3 is designed to allow remediation of contaminant discharges before the well is contaminated if the discharge can be identified and responded to rapidly; the maximum time of travel from Tier 3 is 12 years. The time-of-travel criterion results in wellheadprotection areas that are tailored to individual wells and are based on the well's pumping cap?°ity, the length of the well's open interval, the characteristics of the surrounding aquifer, and other variables specific to that well (New Jersey Department of Environmental Protection and Energy, 1991). To effectively manage and protect the ground-water resources of the State, an understanding of the sensitivity of aquifers and wells to contamination is necessary. The sensitivity of wells to contamination is related to hydrogeologic factors that determine the time of travel of water recharged from land surface to the open interval of the well. Hydrogeologic variables that may affect the time of travel are soil type, depth to water, depth to the top of the open interval, hydraulic properties of aquifers, position of the well within the flow system, and the presence of confining units above the open interval. The NJDEP Wellhead Protection Program establishes wellhead-protection areas on tl e basis of the assumptions that the recharge area of the well is in the immediate vicinity of the well, and that the protection area includes only those areas within a 12-year travel time of the well (New Jersey Department of Environmental Protection and Energy, 1991). Under this definition, community water-supply wells may be exempt from Wellhead Protection Program delineation regulations because the recharge area of the well is likely to be far from the well itself, and because ground-water travel time from the land surface to the well exceeds 12 years. The NJDEP, Bureau of Safe Drinking Water (BSDW), currently monitors the quality of water in about 2,600 public community supply wells in New Jersey (fig. 1). Well-construction, well-location, and other well-attribute data for these wells reside in various locations and formats, and are associated with varying degrees of accuracy and completeness. Therefore, the U.S. Geological Survey (USGS), in cooperation with the NJDEP, conducted an investigation during 1992-95 to (1) compile and organize a data base of well-construction and other well-attribute data for community water-supply wells within the State of New Jersey, and (2) develop methods to determine the sensitivity of these wells to contaminants discharged at land surface. This information can be used by water managers to appropriately delineate wellhead-protection areas for water-supply wells that are contamination-sensitive, and to exempt from wellhead-protection delineation regulations those wells that withdraw water from parts of aquifer systems that are insensitive to contamination. Purpose and Scope This report describes the sources of well-construction and other well-attribute data that were compiled in, and components of, a data base of 2,598 community water-supply wells in New Jersey. It also describes the method used to evaluate the sensitivity to contamination of wells in three types of aquifers: glacial, Coastal Plain, and bedrock. Also included are examples of areas contributing water to wells and times of travel of ground water for selected wells in these types of aquifers determined by using available ground-water flow models. In addition, the report also presents guidelines for determining the sensitivity to contamination of wells screened in confined aquifers. Description of the Study Area New Jersey is divided into four well-defined physiographic provinces that trend from northeast to southwest: the Valley and Ridge, Highlands, Piedmont, and Coastal Plain Provinces. For this evaluation, aquifers and aquifer systems in these provinces are classified into three types glacial, Coastal Plain, and bedrock-on the basis of similarities in hydrogeologic characteristics of the aquifers and typical well-construction characteristics. The stratigraphic and hydrogeologic characteristics of the geologic units in New Jersey are shown in table 1. Previous Investigations Previous investigations relevant to the current study are of four types: (1) evaluations of approaches and methods used to assess the sensitivity of aquifers and the vulnerability of ground water to contamination, (2) studies in which statistical and ground-water flow models were used to assess ground-water vulnerability and delineate ground-water contributing areas in New Jersey and hydrogeologically similar nearby areas, (3) studies in which ground-water flow models were developed to describe flow conditions in various parts of New Jersey, and (4) hydrogeologic investigations of various parts of New Jersey. Many approaches and methods have been used to assess the sensitivity of aquifers and the vulnerability of ground water to contamination. These methods range in complexity from simple evaluations of available map data to complex models of physical, chemical, and biological