Shortly after the onset of CBM production in La Plata
County in the late 1980’s, a local citizens group voiced concern about an
alleged increase in natural gas contamination of domestic water wells. U.S. Representative Ben Nighthorse Campbell
initiated the formation of a committee to address the concerns. As an outgrowth of the Campbell Committee,
the U.S. Geological Survey began a study in July 1990. This study focused on documenting the
occurrence of natural gas in near-surface ground water and in soils adjacent to
gas wells in the Animas River Valley in the San Juan Basin between Durango,
Colorado, and Aztec, New Mexico. From analysis of water chemistry in samples
collected from near-surface aquifers at domestic water wells, the study sought
to identify and map the occurrence, determine potential sources, and suggest
possible pathways through which natural gas might migrate to near-surface
aquifers. Included was the
investigation of the relationship between methane concentrations and mapped
geologic fractures.
This study (Chafin, 1994) showed measurable concentrations
of methane (greater than the detection level of 0.005 mg/L) in 34 percent of
the samples tested, with bedrock wells exhibiting higher concentrations than
alluvial wells. Hydrogen sulfide was often found associated with elevated concentrations of entrained methane. On the
basis of a thermogenic isotopic signature (Appendix C: Chart 10) and molecular
composition of the gas isolated from the water of some domestic wells showing
similarity to gas collected from producing horizons, the latter were depicted
as probable sources of the methane. (The isotopic character of carbon atom
distribution in water-entrained methane can be confounded by the fact that
methylotrophic bacteria often oxidize methane.
Following methane oxidation, the stable isotopic ratio of the carbon
atom population tends to indicate a false maturity and may result in misleading
assumptions. The deuterium isotope and
chemical composition of the entrained gas can be utilized to minimize confusion
over this issue.) Shuey (1990) reviewed
gas composition data of samples drawn from domestic water wells and seeps
between Bondad, Colorado, and a few miles south of Aztec, New Mexico. He concluded that approximately half of the
samples contained gas similar in character to that produced from Fruitland
Formation coalbeds. Beckstrom and Boyer
(1991) determined that the gas isolated from three conventional gas well
surface casings (bradenhead gas) was chemically and isotopically consistent
with Fruitland coalgas and hypothesized that gas migration had occurred upward
from the Fruitland Formation along uncemented well-bore annuli of conventional
gas wells. Proposed methane migration
pathways to water wells having a thermogenic gas signature include deficiencies
in well casing integrity, a lack of adequate annular isolation through the
Fruitland coal horizons, cathodic protection wells, seismic test holes, bedrock
water wells, and natural joints and fractures.
Conversely, methane isolated from water wells having
carbon isotopic signatures reflecting biogenic sources was attributed to
microbial action in near-surface regimes such as sewage lagoons, septic fields,
swampy areas, or within the groundwater aquifer itself. While the accumulation of methane in these
domestic water wells may represent environmental hazards, the implicated
sources are not under the auspices of oil and gas regulatory agencies. Therefore, investigations into biogenic
methane sources have been excluded.
With the rapidly increasing CBM development in the
years 1989 to 1991, La Plata County residents expressed concern that
anticipated increases in drilling activity and production from the Fruitland
Formation coals might adversely affect their water wells. In response, the San Juan Basin Oil and Gas
Coordinating Committee (with
representation from state, local, and Federal agencies, the Southern Ute Indian
Tribe, gas industry operators, citizen groups and private citizens) was formed
in 1989 to study the effects of oil and gas development, with an emphasis on
groundwater quality issues. The need
for a baseline of water quality was recognized, and in February 1991, the
Colorado Oil and Gas Conservation Commission (COGCC) established a Groundwater
Task Force to initiate a study to provide baseline data in La Plata and Archuleta
Counties (Velez, 1993). This
Groundwater Task Force was comprised of the COGCC, BLM, San Juan Citizens
Alliance, Southern Ute Indian Tribe, La Plata County, San Juan Health
Department, Colorado Division of Water Resources – Water Quality Control Division
and State Engineers Office, Office of Senator Ben Nighthorse Campbell, Colorado
Department of Local Affairs, and private citizens. The State of Colorado,
Department of Local Affairs Energy Impact Assistance Fund and gas industry
contributions to the COGCC Environmental Response Fund provided the needed
monies. A total of 324 wells were
sampled in 1991, with analyses being completed by several laboratories. Headspace methane concentrations were
reported in parts-per-million (ppm) in contrast to the USGS study, which
reported in milligrams of methane per liter of water. The latter has become the accepted standard for reporting methane
entrainment in groundwater. Unfortunately,
quality control split-samples for methane concentration differed substantially
between laboratories. Credibility of
the study suffered. Nevertheless,
analyses showed 81 wells to have methane above the detection limit of 7 ppm in
the headspace. The study also
identified specific water wells devoid of measurable methane contamination,
establishing a baseline at these locations.
Twenty-eight water wells having in excess of 1000-ppm methane in the
headspace were isolated and samples from sixteen of these were submitted to the
USGS laboratory in Denver for stable carbon isotope determination. Using a breakpoint of -55 per mil (0/00 ),
ten samples appeared to be of biogenic origin while six indicated thermogenic
origins with potential relationship to gas producing horizons.
A
Colorado Western Slope groundwater quality monitoring study (Schenderlein, 1993) evolved out of an
agreement between the Colorado Department of Health and the Office of the
Colorado State Engineer, Department of Natural Resources, Division of Water
Resources. A grant from the
Environmental Protection Agency funded this 1992 study to determine the extent
of groundwater contamination attributable to non-point source activities on the
Western Slope Area of Colorado. During
this study twenty water wells and springs were sampled in the San Juan Basin in
the summer of 1992; nineteen wells were re-sampled in the fall of 1992. Seventy percent of the first round of
samples and eighty-five percent of the second round of samples exhibited
quantifiable methane concentrations above the lower detection limit of 0.005 milligrams
methane per liter of water. Twenty-five
percent of the first round of samples and sixteen percent of the second round
of samples divulged concentrations exceeding one milligram methane per liter of
water, with two revealing concentrations of methane greater than ten milligrams
methane per liter of water. No
isotopic determinations were reported.