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T. A. Harris: An Investigation of Arsenic and Sulfate Levels in Groundwater to Determine Potential Contamination from the Trenton Hydrocarbon Reserves

Groundwater provides the public drinking water supply for 60% of the state of Indiana (1); 90% of Indiana's rural population depends on the availability of groundwater (2). The necessity of maintaining an uncontaminated supply of groundwater has led the state of Indiana to adopt laws designed to prevent the polluting of these aquifers. However, careless human activities upon and below the surface have already contaminated groundwater in several areas, and the possibility of contamination exists in other locations as well. One location where such contamination may already have occurred is the region of the Trenton gas and oil field.

At the turn of the century, one of the nation's first oil rushes exploited the priceless oil and gas resources of the Trenton Limestone (also called the Trenton Group), which lies beneath northeastern Indiana (Fig. 1) and northwestern Ohio. As the accessibility of the Trenton Field's oil and gas diminished, many of the wells were abandoned. Often no precautions were taken to seal these wells to prevent the movement of material from one rock layer to another. Each of these unsealed wells represents a possible non-point source of contamination for groundwater supplies. Modern attempts to locate and seal these wells have been hampered by the lack of records from earlier times.

An examination of groundwater aquifer quality data, collected both inside and outside the Trenton Field, may help to determine whether contamination is originating with non-point sources derived from the Trenton. To begin, I will briefly review the geologic history of the Trenton Group, and discuss the oil and gas industry in that region. To determine whether contamination from abandoned Trenton wells may be occurring, I have chosen two probable resulting contaminants as indicators: arsenic and sulfur (sulfide and sulfate) minerals. Both minerals are found with deposits of fossil fuels such as petroleum, and are naturally occurring forms in the Trenton Group. Finally, I will examine the groundwater resources of the Trenton area today, with an overview of the aquifer locations and their water quality.

History of the Trenton Field

Geologic Formation

The formation of the Trenton Field area has been studied in detail by Keith of the Indiana Geological Survey (3–6). Two factors control the presence of the oil and natural gas in the Trenton Field of Indiana, Ohio, and southern Michigan: the structure of the topography of the area when the rocks were deposited and lithophied, and the diagenesis or formation of the rocks themselves (6).

Three major structural features underlie the surface of Indiana (Fig. 2).

  1. The Michigan Basin in northwestern Indiana is a region where the bedrock is depressed or sunken.
  2. The Cincinnati arch, an area of uplift, follows the eastern border of the state—as the Kankakee arch, it extends across northern Indiana into northwestern Illinois; as the Findlay arch, it stretches into northwestern Ohio.
  3. The Illinois Basin lies in the southwestern part of Indiana.

The two structures that determined the location and the origin of the Trenton Field are the Michigan Basin and the Cincinnati arch.

The Michigan Basin and the Cincinnati arch have shifted slightly since the Ordovician (6). In their studies of the Michigan Basin, Taylor and Sibley (9) note that the present center of deposition is slightly northwest of the center's location during the Ordovician; and Fara and Keith (6) note that the top of the Kankakee arch has migrated southward since the Ordovician. This shifting of large-scale structures has affected the formation of the reservoirs of the Trenton Field, and the formation of the rocks associated with the Trenton and the Black River groups. The hydrocarbon reserves present in the Black River Limestone may be interconnected with those of the Trenton Group (6).1 The deposition, diagenesis, occurrence, and weathering of the strata found in these locations were also affected by the shifting.

Strata of the Trenton

Only three groups of strata influence the location of the Trenton hydrocarbons: the Black River Group, deposited during the Middle Ordovician, the Trenton Group, deposited during the Middle to Late Ordovician, and the Maquoketa Group, deposited after the Trenton during the Late Ordovician. Both the Trenton and Black River groups began as limestones.

The Black River Group. The Black River Group consists of finely crystalline to lithographic limestones; some of the upper member has undergone dolomitization. According to Fara and Keith (6), the dolostones of the Black River Group formed with the regional dolostone of the Trenton Group. In northwestern Indiana, the dolostone grades into the limestone, resulting in poor porosity (8).

The Trenton Group. The Trenton Group consists of skeletal sand facies—in this case, skeletal fragments of organisms cemented together by lime mud or calcite cement. Both non-ferroan and ferroan calcite cements are present; non-ferroan calcite cement was deposited initially, followed by ferroan calcite cement. Iron loss as a result of the iron being incorporated into iron sulfides (for example, pyrites) led to a gradual increase in the non-ferroan calcite cement. The skeletal sand facies are divided by tempestites (6). At some undetermined time, the limestone underwent dolomitization.

Within both the present limestones and the dolomites are several areas described as hardgrounds. The lower hardgrounds consist of a mineralized rind containing glauconite, pyrite, and collophane. These lower hardgrounds are generally about 5 mm thick. The hardground at the uppermost contact surface consists of a 10-mm collophane zone (6). Collophanes are phosphatic sediments, typically P2O5 sediments which range from cryptocrystalline to amorphous under x-ray. Pyrite framboids and anhedra quartz silt are commonly distributed throughout the phosphate zone (6). This zone may also be slightly enriched in other elements due to the reducing conditions and amount of organic matter necessary for the formation of this type of rock (Nelson Shaffer, personal communication).

The Maquoketa Group. Overlying the Trenton Field are the shales of the Maquoketa Group. The composition of the Maquoketa influenced that of the Trenton: the dewatering of the Maquoketa Shale led to the first dolomitization of the Trenton Limestone (4, 6, 9). In addition, the Maquoketa Group comprises the basement rock dividing the upper glacial till from the Trenton Limestone for much of the Trenton Field.

The upper part of the Maquoketa consists of gray to blue-gray shales. In some places these shales may contain limestone or turn a darker color. Darker colors indicate a more reducing environment and also indicate the possible presence of organic matter. Both of these conditions are necessary for the formation of the sulfide minerals commonly found in small groups or scattered anhydras throughout the lower shales of the Maquoketa (6, 11). Brown color also indicates the presence of iron oxides (10).

The lower portion of the Maquoketa are all shale and the lowest 30 to 100 feet are dark brown to black (8). Petrographic studies of equivalent rock groups in Ohio show the presence of illite and chlorite minerals. The chlorite contains equal portions of iron and magnesium (11). The illite is high in iron, possibly indicating that magnesium was released as other minerals weathered to form the illite. Iron and magnesium were necessary for the formation of the ferroan dolomitic cap rock.

Dolomitization. After formation of the limestone, three (possibly four) steps of dolomitization occurred (6). Most studies agree that the first dolomitization to occur was the formation of the ferroan dolostone cap rock in the uppermost part of the Trenton Limestone. This dolomitization probably followed the dewatering of the overlying Maquoketa Shale during compaction of the sediments (6, 9). The dolostone was formed by the substitution of magnesium for the calcium in the rock. In the cap rock, porosity was not increased, and the rock remained medium crystalline and nonporous. except for sporadic 1 to 4 mm mesovugs and 4 to 32 mm sulfide-lined megavugs (6).

The regional dolostone, as the dolostone underlying the ferroan cap dolostone is referred to in Keith (5), was formed after the cap rock by late-burial dolomitization. In this second episode, warmed fluids may have migrated updip into the Cincinnati arch to the top of the anticline where they were contained in the limestones by the already formed nonporous dolostone (Fig. 3). These fluids then dissolved the calcite cement; the limestone recrystallized to form a coarsely crystalline, non-ferroan dolostone, parts of which are porous (6).2

The Fara and Keith (6) model for the formation of the regional dolomite is based on the similarity of the geometry of the regional dolomite to the model of a classic entrapment of fluids by an anticlinal structure. Paleogeographic maps by Droste and Shaver (12) show that the area of nonsubsidence was mostly north of the present Kankakee arch. Droste and Shaver's data correspond with the data compiled by Taylor and Sibley (9) concerning the location of the subsidence of the Michigan Basin.


1. The emphasis here will remain with the Trenton and Black River groups, although hydrocarbons have been noted in other formations. For example, Keith (3) mentions a study by Patton and Dawson (7) that describes oil production in southern Jay County from the Knox Dolomites. The Knox Dolomites, now referred to as the Knox Supergroup, are located in the strata that lies below that of the Trenton and Black River limestones; the Knox Supergroup was deposited during the Late Cambrian to Early Ordovician. Although Jay County lies within the Trenton Field, these deposits will not be considered in this study.

2. The porosity in dolomitization can be caused by the differently shaped crystals of dolomite replacement compared to those of the original calcite rock (10). Solution by fluids may also have increased the porosity (6).


Figure 1. The Trenton Field as it is found in Indiana. Deposits to the north and east are also considered a part of the Trenton. (G. L. Carpenter, T. A. Dawson, and Stanley J. Keller, Petroleum Industry in Indiana, Geological Survey Bulletin no. 42-N, Bloomington, Ind.: Indiana Department of Natural Resources, 1975. Used with permission of the Indiana Department of Natural Resources.)

Figure 2. Major structural features that influenced the development of the Trenton oil and gas field as it is found in Indiana. Oil deposits migrated from the area of the Michigan Basin up into the Kankakee and Cincinnati arches. (G. L. Carpenter, T. A. Dawson, and Stanley J. Keller, Petroleum Industry in Indiana, Geological Survey Bulletin no. 42-N, Bloomington, Ind.: Indiana Department of Natural Resources, 1975. Used with permission of the Indiana Department of Natural Resources.)

Figure 3. A model for dolomitization episodes of the Trenton Group. (A) Geometry during dolomitization. Dewatering of the overlying Maquoketa Shale into the upper Trenton causes dolomitization of a ferroan dolostone cap rock. The cap rock trapped hydrothermal solutions, leading to the formation of the regional dolostone. (B) Present geometry. The continuing movement of the Kankakee arch southward and continued subsidence of the Michigan Basin caused the shifting of the regional dolostone of the Trenton to its present position. (Modified with permission from Daniel R. Fara and Brian D. Keith, "Depositional Facies and Diagenic History of the Trenton Limestone in Northern Indiana," in The Trenton Group Upper Ordovician Series of Eastern North America: Deposition, Diagenesis, and Petroleum, edited by Brian D. Keith, AAPG Studies in Geology no. 29, Tulsa, Okla.: American Association of Petroleum Geologists, 1988.)

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