Abstract
AbstractAn environmental concern with hydraulic fracturing for shale gas is the risk of groundwater and surface water contamination. Assessing this risk partly involves the identification and understanding of groundwater–surface water interactions because potentially contaminating fluids could move from one water body to the other along hydraulic pathways. In this study, we use water quality data from a prospective shale gas basin to determine: if surface water sampling could identify groundwater compartmentalisation by low‐permeability faults; and if surface waters interact with groundwater in underlying bedrock formations, thereby indicating hydraulic pathways. Variance analysis showed that bedrock geology was a significant factor influencing surface water quality, indicating regional‐scale groundwater–surface water interactions despite the presence of an overlying region‐wide layer of superficial deposits averaging 30–40 m thickness. We propose that surface waters interact with a weathered bedrock layer through the complex distribution of glaciofluvial sands and gravels. Principal component analysis showed that surface water compositions were constrained within groundwater end‐member compositions. Surface water quality data showed no relationship with groundwater compartmentalisation known to be caused by a major basin fault. Therefore, there was no chemical evidence to suggest that deeper groundwater in this particular area of the prospective basin was reaching the surface in response to compartmentalisation. Consequently, in this case compartmentalisation does not appear to increase the risk of fracking‐related contaminants reaching surface waters, although this may differ under different hydrogeological scenarios.
Highlights
We propose that surface waters appear to interact with a weathered bedrock layer via shallow circulation through glaciofluvial sands and gravels of the superficial deposits, there is no chemical evidence to suggest that deeper groundwater from the Mercia Mudstone Group, or below, was rising to the surface in response to the Woodsfold fault acting as a barrier
In prospective shale gas basins identifying groundwater compartmentalisation and groundwater– surface water interactions is important for understanding potential contaminant pathways
Using surface water quality data from a prospective basin we showed that bedrock geology was a significant factor influencing surface water quality across the prospective basin, implying regional-scale groundwater–surface water interactions despite the near-ubiquitous presence of superficial deposits with an average thickness of 30–40 m
Summary
The rapid expansion of hydraulic fracturing (fracking) to exploit unconventional shale gas reservoirs in the United States has led to a range of environmental concerns: induced seismicity (Davies, Foulger, Bindley, & Styles, 2013); water usage and contamination (Kondash, Lauer, & Vengosh, 2018; Vengosh, Jackson, Warner, Darrah, & Kondash, 2014; Vengosh, Warner, Jackson, & Darrah, 2013); fugitive methane (CH4) emissions (Boothroyd, Almond, Qassim, Worrall, & Davies, 2016; Boothroyd, Almond, Worrall, Davies, & Davies, 2018); human health effects (Currie, Greenstone, & Meckel, 2017); air quality and noise (Goodman et al, 2016); and surface footprint (Clancy, Worrall, Davies, & Gluyas, 2018). Potential contamination of surface waters and groundwater from spills or subsurface contaminant migration has been a common concern (Vidic, Brantley, Vandenbossche, Yoxtheimer, & Abad, 2013). As surface waters and groundwater can be hydraulically connected by pathways, contamination of either water body could result from surface activities, for example, spills and surface water discharge (Gross et al, 2013; Olmstead, Muehlenbachs, Shih, Chu, & Krupnick, 2013), or from the potential subsurface upward migration of formation fluids, stray gas or injected fluids (usually predominantly water but chemicals can be added to: reduce friction; help carry proppants; prevent biological growth and metal corrosion; and remove drilling mud damage) (Myers, 2012; Osborn, Vengosh, Warner, & Jackson, 2011; Warner et al, 2012). Palmer & Lewis, 1998; Worrall & Kolpin, 2004) In both cases an essential part of understanding the vulnerability of surface waters or groundwater is identifying groundwater–surface water interactions, which are indicative of potential pathways contaminants may follow
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