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Arsenic, Boron, and Fluoride Concentrations in Ground Water in and Near Diabase Intrusions, Newark Basin, Southeastern Pennsylvania

U.S. Geological Survey Scientific Investigations Report 2006-5261

By Lisa A. Senior and Ronald A. Sloto

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During an investigation in 2000 by the U.S. Environmental Protection Agency (USEPA) of possible contaminant releases from an industrial facility on Congo Road near Gilbertsville in Berks and Montgomery Counties, southeastern Pennsylvania, concentrations of arsenic and fluoride above USEPA drinking-water standards of 10 g/L and 4 mg/L, respectively, and of boron above the USEPA health advisory level of 600 g/L were measured in ground water in an area along the northwestern edge of the Newark Basin. In 2003, the USEPA requested technical assistance from the U.S. Geological Survey (USGS) to help identify sources of arsenic, boron, and fluoride in the ground water in the Congo Road area, which included possible anthropogenic releases and naturally occurring mineralization in the local bedrock aquifer, and to identify other areas in the Newark Basin of southeastern Pennsylvania with similarly elevated concentrations of these constituents. The USGS reviewed available data and collected additional ground-water samples in the Congo Road area and four similar hydrogeologic settings.

The Newark Basin is the largest of the 13 major exposed Mesozoic rift basins that stretch from Nova Scotia to South Carolina. Rocks in the Newark Basin include Triassic through Jurassic-age sedimentary sequences of sandstones and shales that were intruded by diabase. Mineral deposits of hydrothermal origin are associated with alteration zones bordering intrusions of diabase and also occur as strata-bound replacement deposits of copper and zinc in sedimentary rocks.

The USGS review of data available in 2003 showed that water from about 10 percent of wells throughout the Newark Basin of southeastern Pennsylvania had concentrations of arsenic greater than the USEPA maximum contaminant level (MCL) of 10 g/L; the highest reported arsenic concentration was at about 70 g/L. Few data on boron were available, and the highest reported boron concentration in well-water samples was 60 g/L in contrast to concentrations over 5,000 g/L in the Congo Road area. Although concentrations of fluoride up to 4 mg/L were reported for a few well-water samples collected throughout the Newark Basin, about 90 percent of the samples had concentrations of 0.5 mg/L or less.

The USGS sampled 58 wells primarily in 5 areas in the Newark Basin, southeastern Pennsylvania, from February 2004 through April 2005 to identify other possible areas of elevated arsenic, boron, and fluoride and to characterize the geochemical environment associated with elevated concentrations of these constituents. Sampled wells included 12 monitor wells at an industrial facility near Congo Road, 45 private-supply wells in Berks, Montgomery, and Bucks Counties, and 1 private-supply well near Dillsburg, York County, an area where elevated fluoride in ground water had been reported in the adjacent Gettysburg Basin. Wells were sampled in transects from the diabase through the adjacent hornfels and into the unaltered shales of the Brunswick Group. Field measurements were made of pH, temperature, dissolved oxygen concentration, and specific conductance. Samples were analyzed in the laboratory for major ions, nutrients, total organic carbon, dissolved and total concentrations of selected trace elements, and boron isotopic composition.

Generally, the ground water from the 46 private-supply wells had relatively neutral to alkaline pH (ranging from 6.1 to 9.1) and moderate concentrations of dissolved oxygen. Most water samples were of the calcium-bicarbonate type. Concentrations of arsenic up to 60 g/L, boron up to 3,950 g/L, and fluoride up to 0.70 mg/L were measured. Drinking-water standards or health advisories (for constituents that do not have standards established) were exceeded most frequently (about 20 percent of samples) for arsenic and boron and less frequently (6 percent or less of samples) for total iron, manganese, sulfate, nitrate, lead, molybdenum, and strontium. In water from 12 monitor wells at the industrial facility on Congo Road, concentrations of arsenic up to 61 g/L, boron up to 5,240 g/L, and fluoride up to 6.51 mg/L were measured, and drinking-water standards or health advisories were exceeded most frequently (more than 30 percent of samples) for manganese, boron, strontium, and arsenic and less frequently (8 to 25 percent of samples) for chloride, sulfate, fluoride, ammonia, iron, and selenium.

Statistical comparisons of constituent concentrations in five sampling areas (including the Congo Road area) and three main lithologies using the nonparametric Kruskal-Wallis test found few statistically significant differences for water from private-supply wells in the five sampling areas, but those differences included arsenic and boron concentrations. Arsenic concentrations were lowest in the Jacksonwald area of Berks County, and boron concentrations were lowest in the Quakertown area of Bucks County. Many differences in water quality were found in the comparison of private-supply and monitor-well samples; water from the monitor wells had higher concentrations of numerous constituents, including major cations, chloride, fluoride, ammonia, total organic carbon, barium, cobalt, iron, lithium, manganese, nickel, selenium, strontium, and uranium.

Statistically significant differences were not found in arsenic and boron concentrations when water from the private-supply wells and monitor wells were compared, suggesting that elevated arsenic and boron concentrations are not restricted to the industrial site and can occur naturally in ground water elsewhere in the Newark Basin. Statistically significant differences in concentrations of some major ions and trace elements and for boron isotopic compositions were found when water from wells in the three main lithologies--diabase, hornfels, and unaltered shales--was compared, although significant differences were not found in concentrations of arsenic, boron, or fluoride. Concentrations of most constituents tended to be lowest in water from wells in diabase and highest in water from wells in the shales and generally intermediate in water from wells in the hornfels, but water from wells in the diabase and hornfels tended to have higher concentrations of silica and vanadium than water from wells in the unaltered shales. Water from wells in the diabase tended to have water most enriched in the heavier isotope of boron, 11B. The boron isotope compositions of most water samples from private-supply wells in all the sampled areas indicate natural mineral sources, such as datolite, for boron.

Relations between chemical constituents were explored using the nonparametric Spearman rho correlation test. In water samples from 46 private-supply wells, arsenic correlated most strongly and positively with pH, boron, and molybdenum. Arsenic also correlated positively with selenium, uranium, nickel, lithium, fluoride, and strontium, and negatively with total organic carbon, copper, and dissolved oxygen. Arsenic concentrations may be controlled partly by pH affecting adsorption of the anion arsenate. All samples with pH of 8 or higher had arsenic concentrations greater than the MCL of 10 g/L, whereas no sample with pH of 7 or lower had arsenic concentrations that exceeded the MCL. The correlation of arsenic with many of the other trace elements suggests similar geochemical controls and (or) distribution in the aquifer materials.

For wells completed in and near diabase, the percentage of wells with ground-water concentrations of arsenic above the MCL of 10 g/L appears to be greater than for the Newark Basin as a whole (about 20 percent compared to 10 percent), suggesting some arsenic enrichment in the rocks and (or) a favorable geochemical environment to mobilize arsenic. Also, the amount of boron in and near diabase intrusions probably is higher than in the Newark Basin as a whole. Boron may have been introduced or remobilized during the intrusion of diabase in the basin, especially where connate brines or residual brines associated with evaporites may have been present. Only a few elevated fluoride concentrations, other than those related to anthropogenic sources at the industrial facility near Congo Road, were measured in samples from wells in and near diabase, indicating limited local natural fluoride enrichment. Possible natural sources of arsenic, boron, and fluoride in the rocks include minerals deposits associated with the diabase intrusions and minerals in unaltered shales.

Table of Contents

     Purpose and Scope
     Study Area
     Previous Investigations
Hydrogeologic Setting
          Geologic Structure
          Geologic Units
               Precambrian and Paleozoic Rocks
               Mesozoic Rocks
          Effects of Thermal Metamorphism
          Mineral Deposits in the Newark Basin
               Magnetite Skarn and Skarn/Replacement Deposits
               Hornfels Copper Deposits
               Diabase-Hosted Vein and Late-Stage Igneous Segregation Deposits
               Sediment-Hosted and Stratabound Replacement Deposits
     Ground-Water System
Arsenic, Boron, and Fluoride in Ground Water
     Geochemical Controls on Arsenic, Boron, and Fluoride in Ground Water
     Review of Available Data
     Methods of Data Collection and Analysis
          Ground-Water Sample Collection
          Sample Analysis
     Summary of Ground-Water Composition
          Major Ions and Characteristics
          Trace Elements
          Boron Isotopes
     Relation to Hydrogeologic Setting
          Congo Road Area
          Jacksonwald Area
          Kibblehouse Quarry Area
          New Hope Area
          Quakertown ARea
     Comparison of Sampling Areas and Lithologies
     Relation to Trace Elements and Other Chemical Constituents
     Hydrogeochemical Controls on the Distribution of Arsenic, Boron, and Fluoride in Ground Water
Summary and Conclusions
References Cited
Appendix 1--Mines and Prospects in the Newark Basin

This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.

View the full report in PDF 11.7MB

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