Scholarly article on topic 'Assessing dissolved methane patterns in central New York groundwater'

Assessing dissolved methane patterns in central New York groundwater Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Lauren E. McPhillips, Anne Elise Creamer, Brian G. Rahm, M. Todd Walter

Abstract Study region Groundwater in Chenango County (central New York State, USA), which is underlain by Devonian sedimentary bedrock. This region has conventional natural gas wells and is under consideration for future shale gas development using high-volume hydraulic fracturing. Study focus The study examines current patterns of dissolved methane in groundwater, based on 113 samples from homeowner wells in the spring of 2012. Samples were analyzed for methane and other water quality parameters, and each well characterized by its landscape position and geology. Statistical testing and regression modeling was used to identify the primary environmental drivers of observed methane patterns. New hydrological insights for this region There was no significant difference between methane concentrations in valleys versus upslope locations, in water wells less than or greater than 1km from a conventional gas well, and across different geohydrologic units. Methane concentrations were significantly higher in groundwater dominated by sodium chloride or sodium bicarbonate compared with groundwater dominated by calcium bicarbonate, indicating bedrock interactions and lengthy residence times as controls. A multivariate regression model of dissolved methane using only three variables (sodium, hardness, and barium) explained 77% of methane variability, further emphasizing the dominance of geochemistry and hydrogeology as controls on baseline methane patterns.

Academic research paper on topic "Assessing dissolved methane patterns in central New York groundwater"

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Journal of Hydrology: Regional Studies

journal homepage www.elsevier.com/locate/ejrh

Assessing dissolved methane patterns in central New York groundwater

Lauren E. McPhillipsa*, Anne Elise Creamera1, Brian G. Rahmb, M. Todd Waltera

a Department of Biological and Environmental Engineering, Cornell University, 111 Wing Dr, Ithaca, NY, USA

b New York State Water Resources Institute, 1103 Bradfield Hall, Ithaca, NY, USA

ABSTRACT

Study region: Groundwater in Chenango County (central New York State, USA), which is underlain by Devonian sedimentary bedrock. This region has conventional natural gas wells and is under consideration for future shale gas development using high-volume hydraulic fracturing. Study focus: The study examines current patterns of dissolved methane in groundwater, based on 113 samples from homeowner wells in the springof 2012. Samples were analyzed for methane and other water quality parameters, and each well characterized by its landscape position and geology. Statistical testing and regression modeling was used to identify the primary environmental drivers of observed methane patterns. New hydrological insights for this region: There was no significant difference between methane concentrations in valleys versus upslope locations, in water wells less than or greater than 1 km from a conventional gas well, and across different geohydrologic units. Methane concentrations were significantly higher in groundwater dominated by sodium chloride or sodium bicarbonate compared with groundwater dominated by calcium bicarbonate, indicating bedrock interactions and lengthy residence times as controls. A multivariate regression model of dissolved methane using only three variables (sodium, hardness, and barium) explained 77% of methane variability, further emphasizing the dominance of geochemistry and hydrogeology as controls on baseline methane patterns.

© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

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ARTICLE INFO

Article history:

Received 17 March 2014

Received in revised form 28 May 2014

Accepted 4 June 2014

Keywords: Dissolved methane Groundwater quality Natural gas drilling Geochemistry New York groundwater

* Corresponding author. Tel.: +1 607 269 7732. E-mail addresses: lem36@cornell.edu (L.E. McPhillips), ac864@ufl.edu (A.E. Creamer), bgr4@cornell.edu (B.G. Rahm), mtw5@cornell.edu (M.T. Walter). 1 Present address: Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL, USA.

http://dx.doi.org/10.1016/j.ejrh.2014.06.002

2214-5818/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction

1.1. Summary ofMarcellus Shale gas issue in New York

Natural gas development is not an entirely new issue in New York State, with the first United States natural gas well installed in 1821 in Fredonia, NY (Kappel and Nystrom, 2012). Currently there are several thousand active natural gas wells, primarily located in the western and central regions of the state (NYSDEC, 2010). However, portions of the state that are underlain by the Marcellus Shale are being considered for extensive natural gas development. The Marcellus Shale underlies several states, including Pennsylvania, Ohio, and West Virginia, and contains approximately 141 trillion cubic feet of gas - enough to sustain current national energy needs for several years (USEIA, 2012). However, the extremely low permeability of this formation requires the use of unconventional technologies, horizontal drilling and high-volume hydraulic fracturing, to extract economically viable gas yields (Soeder and Kappel, 2009). While these methods are being utilized in many states, New York currently (as of May 2014) has a moratorium on the use of high-volume hydraulic fracturing as the New York State Department of Environmental Conservation (NYSDEC) develops regulations to be included in a supplement to the current Generic Environmental Impact Statement that governs oil and gas exploration (NYSDEC, 2011).

1.2. Concern of possible groundwater contamination

Potential environmental impacts being assessed by NYSDEC include the risk of contamination of groundwater resources due to shale gas development and hydraulic fracturing (NYSDEC, 2011). One concern is that high-pressure injection of large volumes of fracturing fluids could lead to contamination of aquifers. There is additional concern that methane could seep through or along improperly cemented gas well casings and into groundwater (Vidic et al., 2013). In other states currently allowing the use of these technologies, there have been reported instances of groundwater contamination. In Pennsylvania, between 2008 and 2011, there were two major cases of stray gas migration into groundwater, each affecting more than 15 drinking-water wells, though neither of these cases was specifically linked to hydraulic fracturing; rather the problem was deemed to be faulty casing of gas wells (Considine et al., 2012). A recent study in Pennsylvania found increased amounts of dissolved methane in groundwater within a kilometer of hydraulically fractured gas wells, however, no evidence of chemical contamination of groundwater due to drilling fluids was found (Osborn et al., 2011). Several replies to the paper by Osborn et al. (2011) contested the conclusion that methane contamination was due to hydraulic fracturing, noting there were a lack of baseline data and that much of the sampling occurred in the Dimock region of Pennsylvania, which was known to have methane migration issues from faulty gas well casings (Davies, 2011; Saba and Orzechowski, 2011; Schon, 2011). A follow-up study that included a more extensive dataset distributed across several counties in northeastern Pennsylvania similarly found increased methane concentrations with proximity to shale gas wells (Jackson et al., 2013). Two other studies in Pennsylvania found no evidence of increased methane in drinking-water wells as a result of natural gas drilling (Boyer et al., 2012; Molofsky et al., 2013), though one noted a few instances of water quality changes during pre-drilling and post-drilling (Boyer et al., 2012). In 2011, the U.S. Environmental Protection Agency found evidence of hydraulic fracturing chemicals in drinking-water wells in Pavillion, Wyoming, though the geology and hydrology of this site is considerably different than the Marcellus Shale region in the eastern part of the U.S. (USEPA, 2011). In another region of shale gas development in the U.S. - the Fayetteville Shale region of Arkansas - geochemical investigations did not find evidence that methane or major ion chemistry in shallow groundwater had been influenced in any way by shale gas drilling activities (Kresse et al., 2012; Warner et al., 2013).

1.3. Necessity of an understanding of baseline conditions

As New York considers lifting its moratorium on high-volume hydraulic fracturing, it is important to be able to accurately assess any potential cases of groundwater contamination due to these drilling technologies. Thus it is essential that there is an understanding of the existing baseline conditions

with regards to groundwater quality in New York (Riha and Rahm, 2010). Such a baseline would ideally include assessment of total suspended solids and a broad range of solutes, particularly chemicals known to be included in most fracturing fluid additives, as well as dissolved methane. Other parameters such as dissolved oxygen and volatile organic compounds could be informative baseline metrics as well, but these are not addressed in this paper.

With regard to methane monitoring, it is particularly useful to measure its isotopic composition (§13C-CH4 and/or §2H-CH4); this can provide information on the source reservoir of methane, and whether it was created biologically or thermogenically (Schoell, 1980; Laughrey and Baldassare, 1998; Revesz et al., 1980). Often, biologically produced methane is present in shallower geologic formations and unconsolidated deposits and thermogenic methane more in deeper, thermally mature formations. There can be wide variation of isotopic signatures among various methane-bearing formations (Baldassare et al., 2014). A survey of gas wells across western and central New York found that gas from wells tapping Upper and Middle Devonian formations had an average §13C-CH4 = -44.7 ±3.9%o (n = 8) while wells finished in Lower Devonian or Silurian formations produced gas with a considerably different signature, averaging §13C-CH4 = -36.3 ±3.0% (n = 9) (Jenden et al., 1993). Isotopic signatures of dissolved methane, particularly in shallow aquifers, can represent mixing of gases from multiple source reservoirs (Osborn and McIntosh, 2010; Baldassare et al., 2014).

1.4. Review of other studies examining dissolved methane patterns

There has been some work in some areas of New York and nearby states to characterize dissolved methane patterns in aquifers. One U.S. Geological Survey (USGS) study found that 9% of wells sampled in New York had methane concentrations above the recommended level of 10 mg L-1 (Kappel and Nystrom, 2012). Many of these wells were finished in Devonian-aged black shale or in confined glacial sand and gravel aquifers overlying the shale. Black shales are rich in organic carbon, typically leading to thermogenic methane production as the sediments are buried (NYSDEC, 2011). In this case, the black shale was presumed to be the source of the methane in the sampled water (Kappel and Nystrom, 2012). A recent USGS investigation focused specifically on isolating geologic and topographic controls on groundwater methane in south-central New York. Sampling locations in valleys had a higher proportion of methane concentrations in excess of 0.1 mgL-1 compared to upland wells and had predominantly thermogenic isotope signatures. Confined valley aquifers had the highest methane concentrations. The authors concluded that the likely source of the valley methane was underlying saline groundwater (Heisig and Scott, 2013). A USGS study in West Virginia found that groundwater methane levels over 10 mg L-1 were also linked to geology and topography; water wells in valleys and in regions dominated by low-sulfur coal deposits tended to have higher methane levels (Mathes and White, 2006).

In neighboring Pennsylvania, investigations of dissolved methane patterns yielded mixed results. Studies by one group found higher groundwater methane concentrations and very thermogenic isotope signatures in close proximity to existing gas wells (Osborn et al., 2011; Jackson et al., 2013) but no correlation to other factors such as topographic position or tectonic deformation (Jackson et al., 2013). Another group found no relationship between dissolved methane in groundwater and proximity to gas wells, but did find topographic and geochemical relationships where methane concentrations were higher in valleys as well as in groundwater dominated by sodium chloride or sodium bicarbonate (Molofsky et al., 2013). In northeastern Pennsylvania, a multivariate regression of methane patterns using landscape and hydrogeologic factors found gas well proximity, groundwater residence time, and well depth relative to certain geologic strata to be most dominant, though only 28% of variation in methane was explained with the regression (Pelepko, 2013). A fourth study found no correlation between groundwater methane and proximity to gas wells, but did not examine other landscape characteristics that might be driving observed values (Boyer et al., 2012).

1.5. Objectives of this study

The objectives of this study were to obtain groundwater quality data from domestic wells in central New York in order to (1) investigate baseline distributions of dissolved methane and other water

quality parameters, including major cations and anions, and (2) to analyze dissolved methane patterns using a variety of statistical techniques in order to understand environmental drivers of the observed patterns.

2. Methods

2.1. Study area

The chosen study area was Chenango County, which is a 2315 km2 (894mi2) region (US Census, 2012) located in the glaciated Appalachian Plateau portion of central New York State (McPherson, 1993). The county is dominated by agricultural and forested land (Crandall, 1985). Surficial geology is characterized by unconsolidated glacial till that mantles the bedrock uplands except on hilltops, north-facing hillslopes, and truncated spur hillsides where the till is absent and bedrock crops out at the land surface; with major valleys containing thicker sediments comprised of alluvium and glacialfluvial outwash and glaciolacustrine fine sand, silt, and clay (Cadwell, 1991; Hetcher et al., 2003; Hetcher-Aguila and Miller, 2005). Bedrock in the county is dominated by Upper and Middle Devonian shale with sandstone, siltstone, limestone and black shale also present in some formations (Fig. 1). Underlying stratigraphy is shown in Fig. 1b.

As of April 2012, there were 93 natural gas wells in the county, with 33 of these wells considered active. Drilling density, considering all existing wells, varies across the county, from 0 in several townships to 0.48 wells km-2 in Smyrna Township (Fig. 2). These wells primarily produce from the Oriskany and Herkimer Sandstones and Oneida Conglomerate (NYSDEC, 2012). However, advances in drilling technologies have resulted in interest by natural gas companies to produce natural gas from organic-rich shales. In south-central New York, two organic-rich shale formations that have been targeted are the Marcellus Shale and Utica Shale, with the Marcellus Shale becoming less desirable toward the northern portion of Chenango County where the formation is less than 1500 feet deep (Selleck, 2010a). Since unconventional drilling is significantly different than conventional drilling, New York has been in the process of developing supplemental regulations (Supplemental Generic Environmental Impact Statement, SGEIS) which are pending the approval of the NYSDEC as of May 2014 (NYSDEC, 2013).

Most county residents obtain their drinking water from groundwater, with residents in the major river valleys generally tapping the glaciofluvial sand and gravel aquifers, in which, some aquifers are confined. Residents in the uplands primarily tap into bedrock aquifers (McPherson, 1993).

Fig. 1. Primary bedrock type (a) and generalized stratigraphy (b) for Chenango County, NY. Bedrock geology data was obtained from Fisher et al. (1970) and stratigraphy information was obtained from RCG (2013), Selleck (2010b), and USGS (2013).

75°50'0"W 75°40'0"W 75°30,0"W 75°20'0"W

Fig. 2. Location ofthe 113 sample groundwater wells in Chenango County, NY with active and inactive gas wells (NYSDEC, 2012) also noted. Well locations are overlain on a Digital Elevation Model (DEM) (obtained from USGS) to show general topography. Town and City of Norwich boundaries are also denoted.

2.2. Field sample collection

In late 2011, Cornell Cooperative Extension collaborators placed newspaper ads in Chenango County newspapers to recruit residents who would allow us to obtain samples from their water wells in exchange for receipt of a free water quality report. Interested county residents who responded to the ad were accepted into the study; only drilled wells as opposed to dug wells or springs were included in this analysis. The 113 wells included in this analysis were distributed across the county (Fig. 2). Water samples were obtained from each of these homeowner wells between March and June 2012. The samples were taken from the closest accessible location to the well, which was often a spigot just past the water pressure tank in the basement. Water collection also occurred prior to the treatment system, if there was one. Water was initially run to purge the pipes and pressure tank of stagnant water, for at least five minutes. A one liter pre-cleaned amber glass bottle was filled with water to be used for sediment and solute analysis. A second water sample was then taken for dissolved gas analysis per standard methods of the USGS Reston Dissolved Gas Laboratory (Busenberg et al., 1998). For this method, flexible Masterflex Tygon tubing was attached to the spigot using a hose connector and water was run into a large bucket. The tubing was then inserted to the bottom of a 125 mL glass

serum bottle and the bottle filled with water. With the water still running, the bottle was lowered into the bucket and then the tube was removed. After making sure no bubbles were adhering to the inside of the bottle, a butyl rubber stopper was inserted in the bottle neck. A syringe needle was then inserted into the stopper that allowed the stopper to fully seal the bottle without having any remaining headspace. After sealing each bottle, the needle was removed, the bottle was removed from the full bucket, and the labeled sample bottles were stored in a cooler.

2.3. Sample processing

Upon return to the Cornell Soil and Water Lab, a subsample of water for anion and cation analysis was removed from the amber collection bottle after ensuring it was well-mixed. The subsample was filtered to 0.45 |im and all samples were stored at 4 °C until analysis. Analysis of total cations/metals was performed using a Jarrell Ash ICP-AES (Inductively Coupled Plasmography with Atomic Emission Spectrometer) for Ba, Ca, Cu, Fe, K, Mg, Na and ICP-MS (Inductively Coupled Plasmography with Mass Spectrometer) for As, Cd, Cr, Pb, Mn, Hg, and Se. Hardness was calculated as CaCO3 equivalent based on calcium and magnesium concentrations. Analysis of anions (NO3-, NO2-, SO42-, Cl-, HCO3-/CO32-) was performed on a Dionex ICS-2000 Ion Chromatograph with IonPac AS-18 analytical column, 25 |L sample loop, and 21 mM KOH eluent. Due to the high pH of the mobile phase, carbonate species were analyzed as CO32-. Since the speciation cannot be resolved with this method, results are represented as 'HCO3- + CO32-'. Bromide data were not available due to interference from the end of the carbonate peak, which occurred with this chromatographic method. This issue was unable to be resolved at the time of analysis. Carbonate data were considered usable based on consistently good calibration curves (R2 > 0.98) using peak height rather than peak area to deal with the interference with the bromide peak.

The unfiltered remainder from the amber collection bottle was analyzed within seven days for specific conductance and total suspended solids (TSS). Specific conductance was measured using a Fisher Scientific bench-top meter. TSS was determined by filtering 450 mL of sample through standard 934-AH glass fiber filters and determining the difference of oven-dry mass before and after filtration.

Water samples for dissolved gas extraction were stored at 4 °C until analysis, which occurred within two days of original sampling. The initial step was to remove a subsample of water to allow for sampling of headspace gas according to the phase equilibration technique (Davidson and Firestone, 1988; Kampbell and Vandegrift, 1998). In order to be able to remove water from the full glass sampling bottle without contacting ambient air, a Tedlar bag filled with high purity helium was attached to tubing and a 21 gauge syringe needle, and the needle was inserted in the bottle stopper. A syringe was then inserted in the stopper and 20 mL of water sample was removed. The 20 mL water sample was injected into a pre-evacuated 125 mL serum bottle capped with a rubber septum. The headspace in this bottle was filled with high purity helium to equalize the internal pressure. The bottles were kept at 4 °C for 24 h, at which point they were removed and shaken vigorously for ten seconds to ensure gas equilibration. A gas sample was then removed from the headspace via syringe and injected into a pre-evacuated 12 mL Labco Exetainer. Gas samples were then sent to the UC Davis Stable Isotope Laboratory for analysis of methane concentration and §13C-CH4 using a Thermo Scientific GasBench-PreCon trace gas system interfaced to a Delta V Plus IRMS (Isotope Ratio Mass Spectrometer). The original concentration of dissolved gas in the water samples was then calculated using partition coefficients based on the temperature of sample incubation (Lomond and Tong, 2011).

2.4. Data analysis

ArcGIS 10 (ESRI, Inc.) geographic information system software was used to spatially analyze the data. Water sampling locations were classified according to their distance to the closest existing natural gas well, as well as their topographic position (valley vs. upslope). The samples were also classified by the geohydrologic units in which the water well was finished (bedrock formations vs. unconsol-idated sand and gravel). Locations of existing natural gas wells in Chenango County were obtained from the NYSDEC (NYSDEC, 2012), and a threshold of 1000 m was used to group water wells into 'close' or 'far' from a gas well (Osborn et al., 2011). Topographic position was determined using two methods. Following Molofsky et al. (2013), one method determined location in a valley according

to distance to the nearest stream. Locations within 305 m (1000 feet) of a stream were considered to be valleys, where streams were defined using the USGS National Hydrography Dataset (NHD). A second approach focused on the geohydrologic setting and used surficial geology maps (Cadwell, 1991) and georeferenced USGS maps of valley-fill aquifers in Chenango County (McPherson, 1993) to classify 'valley' wells as those located in mapped valley-fill aquifers. These approaches were similar to the methodology used by a recent USGS study in south-central New York; however, their valley delineation factored in additional parameters including stream slope and elevation change between streams and adjacent uplands (Heisig and Scott, 2013). Well finishing geology in this study was determined as a specific bedrock formation or unconsolidated sand and gravel fill by using information on well depth (as reported by the homeowner) along with depth to bedrock estimated from USGS survey maps (McPherson, 1993) and bedrock geology maps (Fisher et al., 1970). Finishing geology was only determined for locations where well depth was reported by the homeowner.

R (The R Project for Statistical Computing) was used for statistical analysis of the data. For statistical analysis of all analytes, values below the method detection limit were treated as being equal to their analyzed values (Gilliom et al., 1984). The Mann-Whitney non-parametric test was used to analyze the dissolved gas data, as grouped according to proximity to gas wells and topographic position (valleys vs. upland). A non-parametric test was chosen due to the skewed distribution of the methane dataset and since log transformation of the data was not sufficient to normalize the distribution. For any analysis of §13C-CH4 data, values were excluded for samples where the methane concentration was below the method detection limit of 0.01 mgL-1. The Kruskal-Wallis non-parametric test combined with a pairwise comparison ('kruskalmc' in R package 'pgirmess') was used where there were more than two groupings for methane data. It was used to evaluate differences between methane according to the geohydrologic units that the drinking-water wells tapped as well as across groundwater geochemical categories, as classified using major cation and anion data for the water samples (Deutsch, 1997). In order to classify the geochemical water type, a Piper diagram of major groundwater cations and anions that were detected in the samples was generated using Rockworks software (Rockware, Inc.). Multi-variate regression was used to determine what landscape setting or chemical parameters could best explain observed methane patterns. The factors initially included in the regression were chosen using a Pearson correlation analysis to assess what variables were most closely correlated with methane concentrations. Prior to regression analysis, methane and all other chemical analytes that were considered as explanatory variables were natural-log-transformed, due to their skewed distributions; the only variables considered in the regression that were not transformed were distance to streams and distance to active or existing gas wells.

3. Results and discussion

3.1. Baseline distribution of methane and dissolved solids

The tested groundwater samples from Chenango County met most federal drinking-water standards, with a few exceptions (Table 1). Among the measured constituents, manganese concentrations exceeded the USEPA SMCL (U.S. Environmental Protection Agency Secondary Maximum Contaminant Level) of 50 |igL-1 in 31 samples, chloride concentration exceeded the SMCL of 250 mgL-1 in one sample, and barium concentration exceeded the USEPA MCL (Maximum Contaminant Level) of 2 mg L-1 in one sample. 42 sampled wells yielded water that is considered 'hard' (>120 mg CaCO3 L-1) but this is a nuisance and not a health risk. For dissolved gas, there were no methane concentrations that exceeded the 10 mg L-1 'watch' limit set by the Office of Surface Mining (Eltschlager et al., 2001) and 63 out of 113 total samples (56%) had methane concentrations less than 0.01 mg L-1 (the method detection limit). These results are comparable to the recent USGS study in south-central NY (primarily extending southwest of Chenango County), in