MONTANA BUREAU OF MINES AND GEOLOGY INFORMATION PAMPHLET 11 DISCUSSION GROUNDWATER QUALITY IN THE SHIELDS RIVER BASIN OF SOUTH-CENTRAL MONTANA: The two primary sources of methane in groundwater are biogenic and thermocatalytic. ASSESSMENT OF OIL AND GAS DRILLING IMPACTS AND BASELINE CONDITIONS Biogenic methane, common in shallow ground- Daniel D. Blythe, Montana Bureau of Mines and Geology water, is formed from bacterial reduction of organic material (Clark and Fritz, 1997). Ther- OVERVIEW mocatalytic methane is the major component in 2 natural gas extracted from sedimentary basins; The Shields River Basin covers nearly 855 mi (547,048 acres) of mostly open range and some irrigated fi elds it forms from the breakdown of higher mass hy- adjacent to the Shields River and its tributaries. The basin is mostly in northern Park County, but extends into drocarbons at elevated temperatures. There are parts of Gallatin and Meagher Counties. The basin is bounded by the Bridger Range to the west, the Crazy Moun- advanced analytical techniques to determine the tains to the east, and low hills on the north (fi g. 1). Clyde Park and Wilsall have populations of less than 300 (U.S. source of methane in groundwater (Clark and Census Bureau). Farming and ranching are the main industries. Fritz, 1997). Concentrations of methane were Bedrock in the Shields River Basin consists of Mississippian through Tertiary sedimentary rocks with some too low to use these techniques in this study. Tertiary intrusive rocks in the Crazy Mountains. Quaternary alluvium occurs in the drainage bottoms, and terrace Half the wells with low-level methane concen- gravels are present at levels above the streams. The terraces are most notable between the Shields River and the trations were within 2 mi of the recent oil and Crazy Mountains. gas wells, and the others were over 10 mi away There are records of approximately 1,300 domestic, stock, and public water supply wells in the basin (GWIC, (fi g. 6). Fractures in the subsurface can signifi - 2016); most wells serve domestic purposes (fi g. 1). Typically, bedrock aquifers supply water wells completed with- cantly increase groundwater velocities; how- in 300 ft of the land surface. Figure 6. Organic sampling results. No BTEX, DRO, GRO, or ethane was detected. Low-level methane concentrations were detected in 6 wells and ethylene in 3 wells. ever, an unusual set of fractures would have Since 2000, increased oil and natural gas production from organic-rich shale formations in North Dakota and Concentrations were well below the recommended threshold value of 10 mg/L to exist to transport methane across formation Montana has occurred, largely due to advances in directional drilling and hydraulic fracturing. These advances (Eltschlager and others, 2001). Three of the wells with detectable methane were boundaries and nearly 10 mi from the recent have opened up areas for exploration that had seen little to no recent oil and gas exploration (King, 2012). Within resampled in 2014; neither methane nor ethylene was detected. drilling. the Shields River Basin, exploration companies have identifi ed the Cody and Mowry shales as potential shale-gas Groundwater fl ow paths and velocities likely vary widely between and within formations across the basin. McMa- hon and others (2014) concluded that in areas with slower velocities, subsurface contamination may go undetected if the monitoring period is short or the monitoring wells are far from the source. Therefore, long-term monitoring is needed from monitoring points close to energy development to increase the likelihood of early detection of contami- nation. CONCLUSIONS The MBMG collected samples from surface water, springs, and water wells to assess the groundwater resource utilized by residents of the Shields River Basin. Analytical results show that the water in the basin is of good quality and suitable for public water supplies, domestic or stock use, and irrigation. Generally, concentrations were below drinking water health standards, TDS concentrations < 500 mg/L, and there was an absence of organic constit- uents. Methane and ethane occurred at low concentrations in samples from a few wells in 2013, but were below detection in follow-up samples in 2014. The results show that groundwater in the sampled areas has not been impacted by oil and gas drilling. REFERENCES

Blythe, D.D., 2015, Shallow groundwater quality and geochemistry in the Shields River Basin, south-central Montana: Butte, Montana Tech of The University of Montana, M.S. thesis, 72 p. Clark, I., and Fritz, P., 1997, Environmental isotopes in hydrogeology: New York, Lewis Publishers, 328 p. Eltschlager, K.K., Hawkins, J.W., Ehler, W.C., and Baldassare, F., 2001, Technical measures for the investigation and mitigation of fugitive methane hazards in areas of coal mining: Pittsburgh, Pa.: Offi ce of Surface Mining Reclamation and Enforcement, http://www.osmre.gov/ resources/library/ghm/methane.pdf [Accessed 7/7/13]. King, G.E., 2012, Hydraulic fracturing 101—What every representative, environmentalist, regulator, reporter, investor, university researcher, neighbor and engineer should know about estimating frac risk and improving frac performance in unconventional gas and oil wells: Soci- ety of Petroleum Engineers Hydraulic Fracturing Conference, Woodlands, Tex., February 6–8, 2012, 80 p. McMahon, P.B., Caldwell, R.R., Galloway, J.M., Valder, J.F., and Hunt, A.G., 2014, Quality and age of shallow groundwater in the Williston basin, Montana and North Dakota: Groundwater. doi: 10.1111/gwat.12296 Montana Board of Oil and Gas, 2015, Department of Natural Resources and Conservation, Board of Oil and Gas Conservation, http://bogc. Figure 1. The Shields River Basin is located north of Livingston in south-central Montana. About 1,300 well records dnrc.mt.gov/ [Accessed 5/24/2016]. show that 83 percent of wells serve domestic purposes and 8 percent provide stockwater. The remaining 9 percent serve irrigation, fi re protection, and commercial uses. There are 24 public water supply wells; Clyde Park and Wilsall Montana Bureau of Mines and Geology, 2014, 2016 Ground Water Information Center, http://mbmggwic.mtech.edu/ [Accessed 5/24/2016]. both use groundwater for their town supplies.

For more information, contact: Dan Blythe, 406-496-4379, [email protected] targets, although in the southern part of the basin their bases are nearly RESULTS 10,000 ft below land surface. North and west of Wilsall the formations are about 3,100 to 5,800 ft below land surface (fi gs. 2, 3). Between 2007 and Concentrations of arsenic, fl uoride, and nitrate 2009, seven oil and gas exploratory wells were drilled (fi gs. 3, 4; MBOG, were below drinking water health standards. The 2016). The drilling raised concerns about potential degradation of ground- selenium concentration of 87 micrograms per water quality. Since 2009, all seven exploratory wells have been plugged liter (μg/L) in one sample exceeded the health and abandoned (fi g. 4). standard of 50 μg/L. Total dissolved solids (TDS) concentrations of 803 and 557 milligrams per liter WATER-QUALITY SAMPLING (mg/L) in two samples exceeded the aesthetic quality standard of 500 mg/L (fi g. 5). In cooperation with the Shields Valley Watershed Group and the Park County Conservation District, the Montana Bureau of Mines and Geology’s Results for organic constituents in samples (MBMG) Ground Water Assessment Program sampled wells, springs, and from 24 wells, 2 springs, and 3 surface-water surface water in the basin to assess impacts from the recent drilling and to locations showed no detectable concentrations establish current baseline water quality. The MBMG selected sample loca- of BTEX, DRO, GRO, or ethane. Low-level meth- tions based on an aquifer susceptibility analysis, the current distribution of ane concentrations were detected in 6 wells, and water wells, depths to oil and gas target formations, and the hydrogeologic ethylene was detected in 3 wells. The methane setting (Blythe, 2015). For comparative purposes, locations were selected concentrations ranged from 0.0139 to 0.184 mg/L, near and distant from the 2007–2009 oil and gas drilling (fi g. 3). and ethylene ranged from 0.0075 to 0.0138 mg/L (fi g. 6). All results were below the recommended In 2013, 33 domestic wells, 2 springs, and 3 surface-water sites across threshold value of 10 mg/L for potentially explo- the basin were sampled. Samples were analyzed for major ions, trace Figure 4. Oil and gas exploration companies drilled seven oil and gas wells in sive environments (Eltschlager and others, 2001). 18 2 the basin between 2007 and 2009. By 2013, the companies had plugged and metals, water isotopes ( O and H), and tritium. Additionally, a subset of There is no human health standard for methane. samples was analyzed for benzene, toluene, ethylbenzene, xylene (BTEX), abandoned six of the seven wells. One well (pictured here) was “shut-in.” By 2016, the owner had also plugged and abandoned this well (MBOG, 2016). The MBMG resampled three of the wells with de- methane, ethane, ethylene, diesel range organics (DRO), and gasoline tectable methane in 2014; methane and ethylene range organics (GRO). were below detection in these samples. STUDY LIMITATIONS The absence of methane and eth- Figure 2. A geologic stratigraphic ylene in the 2014 resampling does not column for the Shields River Basin shows that water wells are com- negate the low-level results from 2013. pleted within about 300 ft of land Error in sample collection or laboratory surface. Oil and gas exploration analysis may have occurred. Readers targets, the Cody and Mowry shale should consider the analytical results explored between 2007 and 2009, are as much as 12,000 ft below land for organic constituents in the context surface. of groundwater fl ow paths, groundwa- ter velocities, and the ability of widely distributed wells to characterize aquifer systems at the basin-wide scale. In a similar study from the Williston Basin (McMahon and others, 2014), ground- water velocities in aquifer systems at an intermediate depth were slow; they concluded that any contaminants from energy development would still be less than 0.5 km (0.3 mi) from their source. Groundwater fl ow paths and velocities in the basin are not known. Wells installed for domestic uses may not be as well suited to detect Figure 5. Water quality in the basin is of good quality, with most samples having total dissolved solids concentrations less than 500 mg/L. The aesthetic water-quality standard contamination as monitoring wells (500 mg/L) was exceeded in two groundwater samples. Figure 3. The MBMG selected sample sites in 2013 based on aquifer susceptibility, existing well completed in specifi c water-bearing distribution, depth to oil and gas target formations, and hydrogeologic setting. Star symbols show the locations of oil and gas wells drilled in 2007–2009. Labels show the depths below land surface zones. Well logs within the MBMG of the formations explored for oil and gas. Ground Water Information Center (GWIC) contain driller-reported lithology data that varies in quality and detail. Comprehensive lithology data were unavailable for the sampled domestic wells used in this study.