Arsenic Occurrence, Sources, Mobilization, and Transport in

Chapter 13

Arsenic Occurrence, Sources, Mobilization, and Transport in Groundwater in the Newark Basin of New Jersey

Downloaded by CORNELL UNIV on June 6, 2017 | http://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch013

M. E. Serfes, S. E. Spayd, and G. C. Herman New Jersey Geological Survey, P.O. Box 427, Trenton, NJ 08625

Arsenic concentrations as high as 215 ug/L in ground water occur in bedrock aquifers of the Newark Basin in New Jersey. This basin is a Mesozoic aged half graben containing red, gray and black mudstone, shale and sandstone with basic igneous intrusions and flows. In 2000-01, random sampling of 94 domestic wells in a 200 square mile study area revealed that 15 percent have arsenic concentrations exceeding 10 ug/L. Those wells generally have low dissolved oxygen concentrations, < 3 mg/L, and pH values range from 7.5 to 8.2. Analyses of red, gray and black mudstone and shale yielded maximum As concentrations of 13, 50 and 240 ppm respectively. Pyrite (FeS ) in the black shale contains up to 40,000 ppm As. A spatial association between high As in ground water and black shale is observed. Hematite and clays in red mudstone are also potential sources of As. Mobilization of As via pyrite oxidation and desorption from hematite and clays are potential mechanisms. Transport of As is aided by competitive adsorption, an alkaline pH, and suboxic aqueous environment. 2

© 2005 American Chemical Society

O'Day et al.; Advances in Arsenic Research ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction On February 22, 2002 The United States Environmental Protection Agency (USEPA) lowered the arsenic drinking water standard for public water supplies from 50 micrograms per liter (ug/L) to 10 ug/l with a compliance date of January 2006 (1). Estimated risks of lung or bladder cancers in individuals with lifetime exposure to drinking water with more than 10 ug/L arsenic exceed 1 to 2 in 1000 (2). Human exposure to inorganic As in drinking water has been linked to internal and external cancers and non-cancer related health impacts (3, 4). Arsenic in ground-water supplies is both an international and national issue with countries on all continents being affected (5). The State of New Jersey has proposed to implement a stricter arsenic standard of 5 ug/L. A spatial analysis of arsenic concentrations in New Jersey ground water was conducted in 1998 by the New Jersey Geological Survey (NJGS); New Jersey Department of Environmental Protections (NJDEP) in anticipation of the new drinking water standard. Concentrations of arsenic from wells sampled in the cooperative NJDEP/United States Geological Survey (USGS) Ambient Ground Water Quality Network (AGWQN) and other studies in New Jersey were analyzed as a function of Physiographic Province (6, 7, 8, 9). The initial assessment showed that arsenic concentrations are generally less than the reporting limit of 1 μg/L in all provinces except in that part of the Piedmont Province that is underlain by bedrock of a Mesozoic aged tectonic extension produced half graben called the Newark Basin (NB). There, 25 out of 45 wells sampled had arsenic detections that ranged from 1 to 19 μg/L (Figure 1). A subsequent analysis of public supply well data from the NJDEP Bureau of Safe Drinking Water showed a similar pattern. Most notably, a public supply well in the western N B in New Jersey had a consistent arsenic concentration of ~ 45 ug/L since it's installation in 1995. In January 1999 a private well sampled as part of the A G W Q N in the western N B in New Jersey was found to have 57 ug/L arsenic. Together, these results prompted research studies that focused on assessing the occurrence, sources, mobilization, transport and treatment of arsenic in ground water in the western N B in New Jersey (10, 11, 12, 13). This paper mainly summarizes our findings concerning the occurrence and geologic sources of arsenic in a 200 square mile study area in the central N B , where the highest arsenic concentrations in ground water had been found (Figure 1). Also, hypotheses concerning the mobilization and transport of arsenic in this hydrogeologic setting are proposed and discussed.


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Background Arsenic Aqueous Chemistry Arsenic is a heavy metalloid element with atomic number 33 and has only one natural isotope with atomic mass 75. It has four valence states in the natural environment: -3, 0, +3 and +5. However, in ground water it generally forms inorganic oxyanions of arsenite As (III), [ H A s 0 ° below pH 9.2], and arsenate As (V), [H As0 " at pH < 6.9 and H A s 0 " at higher pH values] (5). Although not significant in most natural ground water, organically bound arsenic such as the methylated species monomethylarsonic acid ( M M A ) and dimethylarsinic acid ( D M A ) may be produced where arsenic and organic contaminants occur together. Arsenic is particularly mobile within the pH range 6.5 to 8.5 that is commonly found in ground water. It can be present under both reducing and oxidizing conditions. The species of arsenic occurring at a particular location are mainly controlled by the pH, Eh and possibly microbiological activity (15). 3

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Potential Arsenic Sources in the Newark Basin Potential sources of arsenic in ground water in the N B are arsenical pesticides and the decomposition of minerals containing arsenic. Arsenical pesticides were widely used in this country, including New Jersey, from the late 1800s until the mid- to late 1900s (16). The greatest use in New Jersey was in fruit orchards (17). Arsenical pesticides are not very water soluble and bind tightly to soil particles. Studies in North Dakota, South Dakota, Wisconsin and Minnesota all conclude that ground water is largely unaffected by past arsenical pesticide use (15). Therefore, arsenic from arsenical pesticide use in agricultural areas is generally not very mobile in soils and not considered a major source in ground water. The application of phosphate fertilizers can however release slugs of arsenic via competitive adsorption where they are applied (15). However, sulfide oxidation and iron oxide reduction are considered the most important mechanisms for As being released into groundwater (15).

Newark Basin Hydrogeology In New Jersey, the Piedmont Physiographic Province is mostly underlain by a Triassic and Jurassic aged (195-225 million year old) half graben, a tectonic depression produced in a tensional environment, called the Newark Basin (NB) (Figure 1).



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Figure 1. Rift basins of eastern North America and the Newark Basin (modifiedfrom 14) are shown in (a), (b) is a map of New Jersey showing the Newark Basin and area of detail, and(c)Js the area of detail showing the 200 square mile study area and location of wells sampled. Fifteen percent of wells sampled had greater than 10 ug/L arsenic. The Passaic and Lockatong Formations had highest occurrence and concentratiions of As.

The N B contains coarse to fine grained non-marine sedimentary rocks associated with igneous diabase intrusions and basalt flows. These geologic materials have been faulted and gently folded, with strata generally dipping 15 degrees to the northwest, perpendicular to major normal fault trends. Four lithic groups are, from youngest to oldest: 1. 2.

Basalt interlayered with sedimentary rock (early Jurassic), mudstone, siltstone, sandstone and conglomerate. The Passaic Formation (Triassic), mainly cyclical sequences of red mudstone, siltstone and sandstone with intermittent gray mudstone and black shale.


179 3.


4.

The Lockatong Formation (Triassic), black (organic rich), gray and red cyclical sequences of argillitic mudstone, siltstone and shale containing lenses of pyrite and calcite. The Stockton Formation (Triassic), mainly comprised of arkosic sandstone, siltstone and shale derived from fluvial sediments.

The Lockatong and Passaic Formations were mostly deposited in deep (black), transitional (gray) and shallow (red) lake environments. Detailed stratigraphie relationships, including the identification of specific members in the Newark Basin, are reported from the Newark Basing Coring Project (18). The unweathered Passaic and Lockatong Formations in the Newark Basin are generally characterized as anisotropic, leaky, multi-layered aquifer systems ( L M A S ) with bed-parallel water-bearing zones (WBZs) sandwiched between thicker nonconductive zones (19, 20). Flow between WBZs occurs as leakage via fractures that cross-stratigraphic layering or as cross flow in well boreholes. Hydraulic conductivity, and therefore preference for flow, is greatest parallel to bedding strike, somewhat less in the dip direction and least perpendicular to bedding (21, 22). Weathered bedrock near the surface is more hydraulically isotropic and possesses greater storage, but is considered less permeable than that in the deeper parts of the aquifer (19). Hydrogeologic investigations in shallow weathered sedimentary rock with dipping beds at the Oak Ridge National Laboratory (ORNL) facility yielded a comprehensive understanding of contaminant transport in that setting (23, 24). In the shallow weathered zone there, near surface-storm flow and deeper water-table flow are largely controlled by topography with permeable weathered WBZs acting as conduits between the two. Recharge to the deeper unweathered bedrock aquifer mainly occurs where deep WBZs intersect the shallow weathered zone. Although uncertainties still exist, the conceptual flow model used here for the Passaic and Lockatong Formations is based on the shallow weathered and deeper unweathered bedrock models described above. Work to better understand ground-water flow characteristics in the Newark Basin continue to be conducted by the NJGS and others.

Arsenic Investigation and Results Detailed Spatial Analysis of Arsenic Occurrence in the Central Newark Basin In 1999-2000, the NJGS used a Geographic Information System to conduct a detailed spatial analysis and investigation on the occurrence of arsenic in ground water as a function of bedrock lithology. The study was concentrated in



180 a 200 square mile area in the western N B in New Jersey where high concentrations of arsenic in well water were found (Figure 1). A grid-based approach using square mile cells, and random selection within blocks, was used to assure that the sampling was spatially distributed within the study area rather than being mainly clustered in high population areas i f a simple random approach was used (25). The goal was to collect a representative well-water sample and field parameter data: pH, dissolved oxygen, specific conductivity, and temperature from at least one domestic well per cell. Untreated samples were collected mainly from outside taps after water had flowed for at least 15 minutes and field parameters stabilized. A l l samples were analyzed using ICPM S at Environmental Health Laboratories, a New Jersey certified laboratory, in Indiana using E P A Method 200.8 - Determination of Trace Elements by Inductively Coupled Plasma - Mass Spectrometry. Three well records per cell for a total of 600 well records total were obtained by conducting a well record search using the New Jersey Department of Environmental Protection Division of Water Supply database. Generally the most recent well records in each cell were used to simplify the determination of the property owner. Letters requesting volunteers for this project were sent out to the well owners and the first to respond was the one sampled. Out of the 200 cells, volunteers were found in 94. Cluster sampling in two areas where at least one well had an arsenic concentration greater than 40 ug/L was also conducted.

Sample Results O f the 94 wells sampled, 15 percent have arsenic concentrations exceeding 10 ug/L. Well water from the Passaic and Lockatong Formations have the highest arsenic concentrations and frequency of occurrence (10, 11, 12). The Passaic Formation is the more aerially extensive of the two. Arsenic concentrations greater than 10 ug/L generally have low dissolved oxygen (DO) concentrations (DO < 3 mg/L) and pH values range from 7.5 to 8.2 (Figure 2). Arsenic concentrations greater than 40 ug/L are associated with suboxic (DO < 1.0 mg/L) ground water and a pH of about 8. Subsequent sampling also identified several areas in the N B where some wells have even higher concentrations than found in the original reconnaissance. Several wells in diabase near the contact with sedimentary rock in the southwestern N B in New Jersey have up to 95 ug/L arsenic. Approximately 10 miles east of those wells, a well in the Lockatong Formation has up to 150 ug/L arsenic. Also, wells drawing water from a sulfide mineralized zone in the Passaic Formation under a basalt flow in the north-central N B in New Jersey have arsenic concentrations that vary up to 215 ug/L.



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Φ m 20

5 10

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pH (standard units)

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Arsenic versus pH

Figure 2. Arsenic versus dissolved oxygen and pH in ground water in the western NB in New Jersey. This is a compilation of data from the 94 wells sampled there. Note that a DO < 3 mg/L and a pH between 7.5 and 8.2 are the optimal range for arsenic greater than 10 ug/L.

Dissolved Oxygen (mg/L)

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3 30

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Ch 40

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Arsenic versus Dissolved Oxygen


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182 Table I. Arsenic speciation in 9 private residential wells in the Passaic Formation. Concentrations in μg/L.


Species As total As+ 3 As+ 5 MMA DMA

Weill 22.3 0.3 22.0

Arsenic Occurrence, Sources, Mobilization, and Transport in

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