Survey: Methane in water near Silt
A review of more than a half-century worth of water-quality data in western Colorado’s Piceance Basin found that most groundwater detections of methane were in an area south of Silt, where a possible link between such detections and natural gas drilling has long been debated.
The assessment by the U.S. Geological Survey also found that salinity and selenium concentrations, a major concern in the Lower Gunnison River basin, are generally trending downward there.
The information comes in two new USGS reports intended to provide a baseline look at groundwater and surface-water quality in the Piceance Basin to address concerns about possible changes as large-scale energy development and population growth occurs. The project was the result of a voluntary effort between energy companies and local, state and federal agencies.
The USGS compiled and evaluated data from 1,545 wells and compared them to U.S. Environmental Protection Agency drinking water standards. It also compared 347 surface-water sites to EPA and state standards in a separate report. The samples were taken between 1946 and 2009.
The study area covers about 9,500 square miles from north of Rangely to south of Delta, and from Glenwood Springs to the Utah border.
There are no drinking-water standards for methane, but it can reach explosive levels in confined spaces. Methane concentrations were available for 874 wells, primarily in Garfield County, and it was detected in 24 percent of those wells, the USGS found. It said methane detections greater than 1 milligram per liter were found in 75 samples. Most methane detections and high-methane concentrations were near the Mamm Creek-Divide Creek area south of Silt, it said.
The USGS noted that methane can be biogenic, resulting from microbial degradation of organic matter, or thermogenic, a process that involves heat and pressure and generally occurs far below freshwater aquifers. It said thermogenic methane’s presence in water wells can result naturally, such as through migration of gas along naturally occurring fracture zones from deeper formations, or from oil and gas operations.
An ongoing study paid for by Garfield County has been looking at the methane issue south of Silt. Geoffrey Thyne, a geologist who has consulted for the county, previously has suggested a link between methane presence and drilling, but industry representatives have challenged some of his findings.
The USGS noted that some studies have used methane isotopic compositions to determine its origin. It said 37 samples involving high-methane levels had isotopic methane data, which indicated the methane in the Garfield wells was from both biogenic and thermogenic sources.
An encouraging finding by the agency involved benzene, toluene, ethylbenzene and xylene, volatile organic compounds that occur in some natural gas reservoirs. It found benzene in just 11 wells out of 808 for which benzene concentrations were available. Five of those exceeded the drinking standard and were in Garfield and Rio Blanco counties. Toluene was found in 38 wells out of 808 for which data were available, but in no cases was it above the allowable standard. There were few detections of ethylbenzene or xylene, and in no case did they exceed the standard.
The USGS found that dissolved solids concentrations commonly exceeded nonmandatory EPA standards in basin wells. These consist of substances such as sodium, magnesium, potassium and nitrogen.
The USGS also reported “significant downward trends in selenium” on the Gunnison River at Delta and near Grand Junction.
“High selenium concentrations correlate with high salinity concentrations; thus, when salinity control efforts are conducted in selenium-rich areas in the Lower Gunnison River Basin, both salinity and selenium have the potential to decrease,” it said.
It also found that about 30 percent of phosphorus samples for surface water basinwide exceeded the EPA’s recommended standard.