Controversy surrounding groundwater contamination from shale gas development started to gain a significant media presence in 2009, coinciding with the early boom in the Marcellus Shale region. The 2010 documentary Gasland presented shocking images of homeowners lighting their tap water on fire, while industry groups maintained that there is no proof that hydraulic fracturing has ever caused water contamination. More recent EPA investigations regarding potential links between hydraulic fracturing and groundwater contamination in Dimock, PA and Pavillion, WY have only added fuel to the fire.

In the face of such polarized debate, how can readers distill fact from fiction? To lay out a useful framework for assessing the risk of groundwater contamination from hydraulic fracturing and the shale gas extraction process at large, this post will:

• Explain groundwater flow and the processes that lead to contamination • Characterize the pathways-related directly, indirectly, or not at all to hydraulic fracturing-through which groundwater contamination could occur • Describe the technical tools used to understand groundwater contamination as well as the challenges that remain

How does groundwater flow and how are contaminants transported? It is a common misperception that groundwater aquifers are open bodies of water like underground rivers or lakes. In fact, groundwater aquifers are layers of sediment (such as sand, gravel or clay) or rock that transmit water through pore spaces or rock fractures. This water flow is highly variable, but generally very slow. Faster moving aquifers transmit water a few feet per day; water in slower aquifers might move only a couple of inches per year. All rock has natural pores and fractures, but low permeability rock may take hundreds or even thousands of years to transport a substantial volume of water any noticeable distance.

If contaminants enter a groundwater aquifer, they are generally pulled along with the natural groundwater flow, and they also spread out and disperse through the aquifer over time. Some chemicals adhere to soil or rock particles; others biodegrade over time. Because of these natural processes-dispersion, retardation, and degradation-chemical releases into aquifers may never reach nearby water wells at detectable or harmful levels. However, large releases into fast moving aquifers could pose a significant risk of contamination.

Contamination pathways In order for hydraulic fracturing fluid or methane from a fractured shale formation to enter a groundwater aquifer and potentially contaminate a water well, there must be a pathway capable of transporting a large volume of fluid over a reasonably short timeframe. Alternatively, some of the chemicals and methane observed in water wells near hydraulic fracturing sites may come from other industrial or natural sources through other pathways.

Several potential contamination pathways have been widely discussed in relationship to hydraulic fracturing. Assessing the relative risk of contamination from each is outside the scope of this post, but stay tuned for upcoming IHS CERA research on this topic.

Potential pathways to transport hydraulic fracturing fluid and formation methane:

• Subsurface migration: There is little potential for hydraulic fracturing fluid or methane to travel from the shale formation upwards through several thousand feet of low permeability rock to an aquifer as the result of high pressure from hydraulic fracturing. However, faults or fractures-either naturally occurring or induced by hydraulic fracturing- in the overburden could increase the potential for migration.

• Abandoned oil and gas wells: If a shale well intersected or came close to an abandoned oil or gas well from a conventional reservoir, the abandoned well could provide a pathway for fluid migration. This has been discussed significantly in Pennsylvania, where there are thousands of undocumented abandoned wells.

• Surface spills: Millions of gallons of fracturing fluid, flowback, and produced water are managed on the surface using pits, tanks, trucks, and pipes. Spilled fluid could seep into the ground and eventually infiltrate groundwater aquifers.

• Well casing failure: Poorly constructed or cemented wells could lead to fluid leakages or more catastrophic blowouts. These releases could occur close to the surface, potentially at depths comparable to groundwater aquifer depths.

Potential alternatives leading to water well contamination near shale gas wells:

• Biogenic methane: Methane, produced by microbial activity, can occur naturally in small pockets in the ground near the surface. It is possible for methane in these pockets to migrate over time and enter aquifers.

• Background water quality: Some contaminants contained in fracturing fluid and found in nearby water wells may have been present in the groundwater aquifer before shale development, either occurring naturally or as the result of previous industrial contamination.

• Poor water well construction: Faulty construction of water wells can lead to chemical or bacterial contamination that could be mistaken for contamination related to shale gas development.

Which pathways can cause contamination?

Given the variety of potential pathways for contamination, data collection and analysis are required to understand which pathways have caused or may cause groundwater contamination. Technical tools available include:

• Screening models: Using the basic principles of groundwater flow and contaminant transport described above, engineers can make rough, order-of-magnitude assessments of whether a release of a certain size and concentration could feasibly contaminate a water well some distance away.

• Calibrated site models: Using data gathered by sampling wells in the area, engineers can approximate the groundwater flow and direction, which is useful in identifying the location of the contamination source. Such modeling is dependent on having a large amount of high quality data, which is not regularly collected.

• Chemical analysis: Analysis of the chemical composition of water samples can help determine whether methane is biogenic or thermogenic and whether contaminants are traceable to fracturing fluid. This is particularly useful if baseline monitoring is performed before drilling activity begins, so that any changes can be monitored.

Challenges remain, however. Significant regional differences in geology are likely to make some pathways more relevant in some areas than others. Moreover, each site has different groundwater flow patterns, but a lack of site-specific data makes them difficult to discern. Even if some case studies are able to properly identify the source of contamination, they will likely not yield conclusions that could be generalized to other areas.

Looking forward The risk of groundwater contamination from shale gas development is a complex issue characterized by uncertain science, diverse stakeholders, polarizing media attention, and state-by-state regulation. More research is needed to fully understand the risk of contamination from potential pathways. Moreover, increased fact-based discussion among stakeholders can simultaneously help industry develop best management practices to maintain their social license to operate, help regulators focus on the most important issues, and help communities protect themselves from harm while realizing the economic benefits of natural gas development.

This article was published by S&P Global Commodity Insights and not by S&P Global Ratings, which is a separately managed division of S&P Global.