Measurement of soil gas (methane, oxygen, hydrogen sulfide and carbon monoxide) concentrations in coal outcrops of the Fruitland Formation along the Basin rim is intertwined with bradenhead testing and water well monitoring. Local residents perceived an apparent increase in observed hydrogen sulfide odors in the Carbon Junction vicinity where the Animas River is crossed by US Highway #550-160 at Colorado State Highway #3 (known by locals as the High Bridge). These comments augmented the general concern that other exposures of the Fruitland coal might be venting gas, as was observed in the Los Pinos River Valley. Therefore, an early reconnaissance survey was accomplished by the BLM-SJRA in May, 1995 (BLM, 1995) between Moving Mountain (southeast of Durango, Colorado) on the southwest and the Florida River drainage to the east. Evidence of methane seepage from the outcrop was confirmed in several locations, primarily in topographically low-lying areas such as valleys defined by river and stream systems. Conversely, methane seepage was conspicuously absent at topographically higher elevations.
While no pre-CBM-development soil vapor
baseline data exist, the recognition of vegetation mortality in recent years
indicates degradation from prior soil gas conditions. High methane (and commensurately low oxygen) concentrations
similar to those observed in the Pine River Ranches vicinity have been observed
within the Southern Ute Indian Reservation along the Fruitland coal
outcrop. In the spring of 1995, a
Southern Ute Indian Tribal geologist noticed extensive soil gas venting from
Fruitland basal coal seams at an historic seep location on the western Basin
rim approximately 7 miles north of the New Mexico –Colorado State border. While the Valencia Canyon Gap seep had been
known to exist for many years, the venting intensity had notably increased. Gas streams flowing from quarter-inch
diameter soil vents were consistently transporting sand grains from land
surface to a height of several inches into the air. Pinon and juniper trees and sagebrush vegetation in the local
area were showing signs of stress.
Figure
5: Dead trees aligned over basal coal
The
stressed/dying trees were aligned congruent with the strike of Fruitland coal
seam outcrops (Figure 5). Field analyses of the venting gases in these
zones of vegetation mortality showed methane concentrations in excess of the
lower explosive limit of methane. Some
samples contained in excess of 200-ppm hydrogen sulfide. Soil oxygen levels were depleted to 0.1%
oxygen or less. Soon after this
discovery of toxic and flammable gasses, the access road was expediently closed
by Southern Ute Indian Tribal Council action due to public health and safety
concerns.
Figure 6: Dead sagebrush over Fruitland basal coal at
Soda Springs. (PC sandstone
outcrop at left)
Less
than year later, a half-mile long by fifty to seventy-five foot wide swath of
previously healthy pinon and juniper trees, sagebrush and saltbrush stood dead
as a stark testimony to recent environmental changes. This scenario is repeated in each of five major coal seams in the
Valencia Canyon Gap area. . The closest gas well with high water production is
0.5 miles to the east. This area
appeared to respond quickly to the nearby water extraction. Vegetation at other
sites more distant from high water producing wells appeared to respond more
slowly. Similar illustrations of
recently altered soil conditions detrimental to vegetative life were soon
noticed along the Fruitland coal seams in other areas (Appendix B: Map and Cross Sections 11). All together, locations along the Fruitland outcrop north of the
New Mexico State line account for more than eight miles of stressed/dead
vegetation. (See Figure 6 below.)
Each lineation
of dead vegetation corresponds to a coal seam of the Fruitland Formation;
however, to date the majority of coal outcrops are not accompanied by stressed
vegetation. Soil gas testing within
those areas manifesting strips of dead vegetation reveals severe oxygen
depletion, accompanied by high methane concentration. (Isolated locations were observed where dead vegetation is not
coincident with depleted soil oxygen, but probably relates to disease or other
factors). Methane concentrations in
the coalbeds along the outcrop have been documented at 1,000,000 parts per
million (100 percent methane). Hydrogen
sulfide concentrations in the soil exceed 2,000 parts per million at some soil
vapor monitoring sites.
With the
apparent exacerbation of gas seepage at Valencia Canyon Gap and several other
sites within the Southern Ute Indian Reservation, the BLM and the Southern Ute
Indian Tribe launched a surficial reconnaissance survey. Geologists walked the Fruitland outcrop
along the western flank of the San Juan Basin within the Southern Ute Indian
Reservation, making note of abandoned mine sites and strings of dead and
stressed vegetation or other evidence possibly linked to increased methane gas
seepage such as visible/audible gas vents. Following the initial
reconnaissance, the BLM focused efforts on further defining the extent and
concentration of soil gas constituents.
Approximately eight miles of the Fruitland outcrop was surveyed in
specific areas where preceding reconnaissance recorded conditions suggestive of
possible methane gas presence.
Locations exhibiting stressed/dying vegetation and discernable coal
outcrops were targeted for sampling. In
the absence of either of these physical indicators, samples were drawn every
several hundred yards along the outcrop. With the intent of monitoring the
ongoing concentration of methane, hydrogen sulfide, and oxygen in the soils, semi-permanent
soil vapor monitoring stations were established at the extremes of stressed
zones, within affected areas, and in transects established perpendicular to the
strike of the Fruitland outcrop. See Appendix B: Maps and Cross-Sections 12 for
the current design of soil vapor monitoring stations.
Additional
monitoring stations supplemented the initial soil tube arrays in areas such as
topographic lows (stream/erosion/fault valleys) in which no evidence of seepage
was detected, but at sites recognized as having the greatest latent potentials
for seepage. Lastly, monitoring stations were established at documented USGS
survey locations in horizons stratigraphically above the coalbeds in the upper
Fruitland and Kirtland formations, some near exploratory coalbed core holes or
gas wells. While most monitoring sites
were established by Fall 1995, forty-six were installed as late as Summer and
Fall 1998. The latter sites were installed
in response to proposed infill drilling activity along the northwestern flank
of the San Juan Basin. In all, 184
monitoring locations have been established along the north and western flank of
the Basin within the exterior boundaries of the Southern Ute Indian
Reservation. More sites are pending
installation to specifically monitor the outcrop influence of proposed
mitigation wells. Enervest (one of the
gas operators in this area) installed a 10-foot by 85-foot soil vapor collector
over the basal coal seam at the site of the Valencia Canyon Gap seep to monitor
cumulative gas flux and composition emanating from the outcrop. This collector (Figure 7 below) has been instrumental in collecting data,
especially during the time period subsequent to the slant-well mitigation
measure of Winter 1996.
Figure
7: Valencia Gap Soil Gas Collector
(These slant wells,
drilled into coal exposures in an effort to capture free gas in the near
subsurface before migrating to the outcrop, are further discussed under the
“Mitigation to Date” section.)
Concurrent
with the activity within the Southern Ute Indian Reservation, the discovery and
documentation of methane and hydrogen sulfide seeps along the Fruitland outcrop
north of the Southern Ute Indian Reservation and possible implications were
discussed in a public forum of GORT. A
consortium of efforts largely funded by the COGCC and industry (with BLM
participation) enabled an outcrop study extending from the Southern Ute Indian
Reservation northern boundary line on the southwest to the Archuleta County
boundary to the north and east. A soil
vapor reconnaissance was performed by Direct Geochemical (Stonebrooke, 1996) supplementing the
initial BLM reconnaissance of Spring 1995.
A fracture, cleat and coalbed mapping study was accomplished by the USGS
(Condon and others, 1997). L.T.
Environmental, Inc (L.T. Environmental, 1998) installed semi-permanent soil gas
measuring probes and soil gas flux chambers in 1997. An earlier study (BLM, 1994) detected methane at high
concentrations in domestic well water along the South Fork of Texas Creek (northwest
of Bayfield, Colorado). The subsequent
soil vapor testing in 1996 showed methane-saturated soils. Patches of dead vegetation confirmed the
lack of soil oxygen available to plant roots.
One hundred and sixty-one stations were ultimately installed north of
the Southern Ute Indian Reservation to allow periodic soil gas concentration
measurements. Six flux (soil gas flow)
measurement chambers and one weather station were positioned over soils
exhibiting micro-seepage of methane. These were equipped with solar panels and
data loggers to provide continuous data collection.
To date 346 soil vapor monitoring sites have been established in
the Fruitland outcrop along 45-50 miles of the northern and western San Juan
Basin rim. These sites were installed
through cooperation of the BLM, COGCC, Southern Ute Indian Tribe, La Plata
County and the gas industry. Monitoring
is primarily accomplished by the BLM, San Juan Field Office (SJFO) at intervals
ranging from monthly to quarterly. L.T.
Environmental is currently commissioned to collect data from the flux chamber
sites and the weather station under the auspices of the COGCC and
industry.