- freely available
Environments 2015, 2(2), 200-220; https://doi.org/10.3390/environments2020200
2.1. Study Area and Data Preparation
2.2. Landscape Analyses
|1) How many watersheds have oil and gas (O/G) development?|
|2) How many watersheds have streams within 30 m or 60 m of O/G development?|
|3) How many watersheds have impaired streams within 30 m or 60 m of O/G development?|
|4) How many watersheds contain wildland trout streams within 30 m or 60 m of O/G?|
|5) How many watersheds contain environmental justice areas and O/G development?|
|6) How many watersheds contain both drinking water intakes (DWI) and O/G?|
|7) How many watersheds have DWI within 1, 5 and 10 km downstream of O/G|
|8) What are the population densities within 3 km of unconventional O/G sites?|
|9) What is the amount (area) of forest interior loss due to O/G development?|
|10) How has O/G development changed forest structure?|
- Questions (Table 1) related to occurrence and proximity were addressed using GIS intersection and buffering routines. Streams located within 30 m or 60 m of a gas exploration site (e.g., Table 1, Question 2) were estimated by expanding (buffering) the sites by those distances and then intersecting the expanded sites with the streams. The outcome (result) of these operations was used to estimate the number of watersheds with gas exploration sites within 30 m or 60 m of a stream, a wildland trout stream and an impaired stream. The 30-m threshold distance was based on the Governor’s Marcellus Shale Advisory Commission report recommendation that sites should not be within 30 m of a stream , and we added 60 m to evaluate the effect of including a more conservative threshold. Simple GIS intersections (without buffering) were used to estimate the number of watersheds with gas exploration sites, the number of watersheds with environmental justice areas and gas exploration sites, the number of watersheds with surface drinking water intakes and gas exploration sites and the number of exceptional value watersheds with gas exploration sites. We downloaded both impaired streams and impaired water bodies, but only used impaired streams in the analysis, because there were no impaired water bodies within 60 m of unconventional or conventional gas exploration.
|Unconventional Oil and Gas|
|Near-by permits||Number of sites||Average area (km2)||Total area (km2)|
|Conventional Oil and Gas|
- GIS network functions were used to estimate the number of watersheds with gas exploration sites upstream of a surface drinking water intake. Streams were used as the network; surface drinking water intakes defined the starting points; and the gas exploration sites defined the stopping points. The stopping points were defined by computing the Euclidean distance between all gas exploration sites within a 350-m buffer of a stream. The results from this analysis depend on the values assigned to the model parameters. We used stream distances of 1 km, 5 km and 10 km between drinking water intakes and gas exploration sites. These distances represent the length over which biotic and abiotic in-stream processes can remove or dilute pollutants [43,44]. For nitrogen, a possible component of unconventional oil and gas development wastewater , it is well established that in-stream dilution or removal is inversely related to stream size, such that it tends to persist in very large streams . The Monongahela and Allegheny Rivers are within the study region and are important sources of drinking water. We chose the 10-km distance based on the presence of large rivers in the study region. Further, many of the numerous possible constituents in fracking wastewater [7,45] may be novel  and, therefore, may persist downstream regardless of stream size. Our selection of a 350-m distance between gas exploration sites and streams is a conservative interpretation of the results reported by Boyer et al. , who compared pre- and post-fracking groundwater well samples and found elevated concentrations of bromide, an indicator of the presence of fracking fluids, after initiation of fracking. Boyer et al.  suggested a minimum distance of 915 m (3000 ft) between fracking sites and groundwater wells based on their results. Bromide, can react with the disinfection products used in drinking water treatment plants to create byproducts (i.e., disinfection byproducts) that can present health risks . The GIS network functions were applied also to the conventional gas exploration sites using the same distance parameters. Our rationale was that wastewater from conventional oil and gas development, including coal-bed methane, presents many of the same water quality issues as unconventional gas development [23,24,46]. For example, benzene, a carcinogenic compound , is a constituent of wastewater from conventional oil and gas development [24,46]. In addition to the drinking water intakes, there are also centralized drinking water (CWT) facilities in Pennsylvania that are potentially a major source of bromide and other contaminants [23,45,48].
- Population estimates within 3 km of a gas exploration site were determined using dasymetric analyses and GIS buffering. Dasymetric methods distribute population estimates to individual pixels based on land cover . We downloaded dasymetric maps based on NLCD 2006 census data from the U.S. Environmental Protection Agency’s (EPA’s) EnviroAtlas website . Dasymetric population estimation within a specified distance of conventional oil and gas development was not undertaken, because it does not include horizontal drilling.
- The forest fragmentation effects of conventional and unconventional gas extraction were not based on patch and edge measurement, because such measurements are poorly suited for the detection of forest fragmentation change. We used change in forest interior as our indicator of forest fragmentation. The four forest NLCD classes (deciduous forest, evergreen forest, mixed forest and woody wetlands) were used to define the forest class for the analyses . Forest interior was estimated using moving windows [51,52,53]. Moving windows is a well-established image processing technique where a geometric shape (typically a square) is passed over a raster map one pixel at a time; a mathematical operation is performed using the pixels within the geometric shape, and the result of the mathematical operation is assigned to the center pixel in the geometric shape. We measured forest interior by counting the number of forest pixels inside moving windows that had side lengths of 50 m (5 pixels), 110 m (11 pixels) and 150 m (150 pixels) and assigning the result to the center pixel of the window. We also used a less conservative threshold of 90% to define interior. The 110-m side length scale was chosen for consistency with the study by Harper , and the 50-m and 150-m side length scales were included because forest interior is a scale-dependent characteristic. Forest interior change was based on a comparison of the 10 m × 10 m NLCD with and without the embedded gas exploration maps.
- The change in the amount of forest interior was supported by a structural analysis of forests based on mathematical morphology [55,56]. Mathematical morphology (Section 6 in the Supplemental Information) classifies a feature (e.g., forest) into structural classes, such as core (interior), edge, bridge (corridor), perforated (non-forest “hole” in interior forest) and patch (isolated). The main input parameters for mathematical morphology are connectivity and edge width. We used eight neighbor connectivity and a 100-m (10 pixel) edge width . Interior was defined as 100% forest within the moving window.
|Indicator: Watersheds with…||Conventional Oil and Gas||Unconventional Oil and Gas||Both||Total|
|Oil and Gas (O/G) development||366||310||192||484|
|Streams within 30 m of O/G development||192||94||51||235|
|Streams within 60 m of O/G development||236||124||67||293|
|Impaired streams within 30 m of O/G development||16||2||0||18|
|Impaired streams within 60 m of O/G development||32||4||2||34|
|Wildland trout streams (= 240)|
|Wildland trout streams within 30 m of O/G development||1||0||0||1|
|Wildland trout streams within 60 m of O/G development||3||1||1||3|
|Study area exceptional value watersheds (EVAL) (= 63)|
|EVAL and O/G Development||10||9||4||15|
|Environmental justice (EJ) areas (= 125)|
|Environmental justice (EJ) areas and O/G development||38||14||14||38|
|Drinking water intakes (DWI) (= 187)|
|DWI and O/G development||73||64||46||91|
|DWI within 1 km downstream of O/G development||9||0||0||9|
|DWI within 5 km downstream of O/G development||28||8||6||30|
|DWI within 10 km downstream of O/G development||36||18||14||40|
|Population Range||No. of Sites|
|1 to 99||711|
|100 to 499||365|
|500 to 999||69|
|1000 to 9999||69|
|Pf = 100||Window||Size|
|5 × 5||11 × 11||15 × 15|
|Pf = 90|
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