New perspectives on the effects of natural gas extraction on groundwater quality

October 8th, 2013

Dr. Zacariah L. Hildenbrand, Dr. Brian E. Fontenot, Doug D. Carlton Jr. & Prof. Kevin A. Schug

Over the past decade natural gas has emerged as a versatile source of energy and has been described as a transition fuel. It has facilitated the shift from coal to renewable energy resources while helping reduce CO 2 emissions and curtail production of industrial chemicals by the power sector.1 Advancements in unconventional drilling techniques, such as hydraulic fracturing and shale acidisation, have made the extraction of natural gas from previously inaccessible deep shale formations both practical and profitable.

Hydraulic fracturing involves a highly pressurised injection of water, proppants, and chemical additives to expand fissures or fractures in the shale formation to release the trapped gases. Acidisation uses large quantities of hydrochloric acid under low pressure to dissolve sediments and solids, which serves to increase the permeability of the shale formation.

Despite the effectiveness of these technologies in liberating sequestered natural gas, they are not without environmental risk and are a cause for concern in today’s society. Anxieties over environmental stewardship, in conjunction with the prospect of using natural gas as a catalyst in achieving energy independence, have provided the impetus for a number of investigations designed to characterise the relationship between unconventional gas extraction and groundwater quality.

At the forefront of the unconventional natural gas extraction discussion are concerns over the potential migration of methane gas, the leaching of harmful chemical compounds, and the mishandling of produced waste. Each of these can have potentially negative effects on the surrounding groundwater. In areas where deep shale formations co-exist with shallow aquifers, methanethe main component of natural gascan leach into private water wells from both natural and anthropogenic processes.1

Methane occurs naturally in two forms, biogenic methane and thermogenic methane. Biogenic methane is produced at shallow depths as a byproduct of bacterial metabolism. Thermogenic methane, the primary target of unconventional natural gas extraction, is formed by geological processes at depths exceeding 1,000 m as a function of high temperature and pressure transforming decomposing organic material into methane gas.

Isotopic2 and hydrocarbon ratio3 analyses have been used to determine the source of methane found in private water wells in the Marcellus shale of Pennsylvania. The majority of methane detected was characteristic of deep, thermogenic methane that could only have been liberated through unconventional drilling activities. Methane was detected in approximately 80% of the collected samples2 with concentrations reaching their highest levels in close proximity to natural gas wells. The root cause of methane contamination events could be attributed to the opening of fractures by unconventional drilling activities that allowed thermogenic methane to migrate into water wells from abandoned historical gas wells.4

In the case of Pennsylvania, approximately 350,000 legacy oil and gas wells have been drilled and the exact locations of ~100,000 of these are unknown (Pennsylvania Department of Environmental Protection). Additionally, instances of methane and chemical contamination can result from gas well casing failures, a phenomenon that occurs in approximately 3% of new gas well operations.1 Changes induced by hydraulic fracturing can also facilitate advective transport of fracturing fluid and flowback into groundwater aquifers depending on the hydraulic conductivity and the presence of water-filled voids in the geological formation.5

In addition to the aforementioned pathways where groundwater can be directly affected by unconventional drilling activity, our research team at The University of Texas at Arlington has examined the effects of unconventional natural gas extraction on groundwater quality and found evidence for indirect mechanisms that could potentially lead to groundwater contamination.6

In a recent peer-reviewed study published in Environmental Science and Technology, our team sampled 100 private water wells to assess the potential effects of natural gas extraction on water quality in the Barnett Shale formation of north Texas (see Figure 1). Our analyses revealed levels of heavy metals above the United States Environmental Protection Agency’s Maximum Contaminant Limit for Drinking Water (MCL) in private water well samples collected near natural gas extraction sites. Most notably, 29 of the 91 samples collected within 5 km of an active natural gas extraction site had arsenic concentrations above the MCL of 10 parts per billion (ppb), with a maximum concentration of 161 ppb.

The maximum concentration, discovered in one well, was nearly 18 times greater than both the maximum concentration sampled from private water well samples located more than 14 km from any active gas wells and the maximum concentration sampled from historical data collected in the Barnett shale prior to the expansion of unconventional extraction activities. We also found selenium and strontium at elevated concentrations, with selenium detected exclusively within 2 km of natural gas wells.

One plausible explanation for the observed results involves large withdrawals of groundwater used in hydraulic fracturing operations that could cause localised declines in the water table. Such decreases can be associated with higher arsenic content in waters drawn from shallow water wells.7 Another scenario to explain elevated heavy metals could be the mechanical vibrations produced from unconventional drilling activity. In this scenario, vibrations from nearby intense drilling activity could mechanically disturb a poorly maintained private water well that has accumulated rust, sulfate, and/or carbonate scale. Once the rust and scale in the water well are disturbed, arsenic, selenium, and strontium that were previously bound in oxide complexes could be mechanically liberated and released into the well water.6

Our understanding of the relationship between groundwater quality and unconventional natural gas extraction is evolving. Future studies should focus not only on direct mechanisms of contamination from unconventional drilling activities such as fluid and methane leaks, but also on indirect mechanisms that could potentially lead to groundwater contamination. Additionally, a greater emphasis needs to be placed on the collection of baseline measurements prior to unconventional drilling activities.

In highly productive regions like the Bakken shale of North Dakota or the Barnett, Eagle Ford, and Wolfcamp shales of Texas, drilling operators are not required to perform baseline testing, making the characterisation of potential industrial contamination events extremely difficult. States like California have proposed legislation where groundwater monitoring efforts would be required before and after any well stimulation, as well as proposals for procedures to safely recycle or dispose of produced and flowback wastewaters. Whether these proposed laws are established and become more widely accepted remains to be determined. Regardless, the collection of baseline measurements prior to any natural gas extraction is the most direct way to quantify the environmental effects of unconventional drilling activity, and will greatly enhance our understanding of the relationship between shale gas extraction and groundwater quality.

References:

1. Vidic, R. D., Brantley, S. L., Vandenbossche, J. M., Yoxtheimer, D. & Abad, J. D. Impact of shale gas development on regional water quality. Science 340, 1235009, doi:10.1126/science.1235009 (2013).

2. Osborn, S. G., Vengosh, A., Warner, N. R. & Jackson, R. B. Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing. Proceedings of the National Academy of Sciences of the United States of America 108, 8172-8176, doi:10.1073/pnas.1100682108 (2011).

3. Jackson, R. B. et al. Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction. Proceedings of the National Academy of Sciences of the United States of America 110, 11250-11255, doi:10.1073/pnas.1221635110 (2013).

4. Jackson, R. E. et al. Groundwater protection and unconventional gas extraction: the critical need for field-based hydrogeological research. Ground water 51, 488-510, doi:10.1111/gwat.12074 (2013).

5. Saiers, J. E. & Barth, E. Potential contaminant pathways from hydraulically fractured shale aquifers. Ground water 50, 826-828; discussion 828-830, doi:10.1111/j.1745-6584.2012.00990.x (2012).

6. Fontenot, B. E. et al. An evaluation of water quality in private drinking water wells near natural gas extraction sites in the barnett shale formation. Environmental science & technology 47, 10032-10040, doi:10.1021/es4011724 (2013).

7. Reedy, R. C., Scanlon, B. R., Nicot, J. P. & Tachovsky, J. A. Unsaturated zone arsenic distribution and implications for groundwater contamination. Environmental science & technology 41, 6914-6919 (2007).

Dr. Zacariah L. Hildenbrand received his Bachelors of Science and Ph.D. from the University of Texas at El Paso. He is currently a visiting scholar at The University of Texas at Arlington where he is a lead scientist and project manager for several research studies analyzing groundwater quality in the Barnett and Cline Shale formations. Dr. Brian E. Fontenot graduated with a Ph.D. in Quantitative Biology from The University of Texas at Arlington in 2009. His past research focused on genetics, ecology, and hybridization in animals, but he currently uses his background in statistical analysis and experimental design as part of a team of researchers at The University of Texas at Arlington studying water quality in areas of natural gas extraction in the Barnett Shale formation of Texas. Doug D. Carlton Jr. is a Ph.D. student in the chemistry and biochemistry department at UT Arlington. Kevin Schug is Associate Professor in the Department of Chemistry and Biochemistry at the University of Texas at Arlington (UTA). Kevin received his B.S. degree in Chemistry in 1998 from the College of William and Mary, and later his Ph.D. degree in Chemistry from Virginia Tech in 2002 under the supervision of Prof. Em. Harold M. McNair.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.