Colorado Water June 2005

• Year: 2005
• Title: Colorado Water
• Author: Water Center of Colorado State University
• Description: JUNE 2005 Issue
• Publications - Doc Type: Newsletter

- Surface And Groundwater Interactions In Coalbed Methane Waters
In The Powder River Basin, Wyoming
by
John D. Stednick
(Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University)
and
William E. Sanford
(Department of Geosciences, Colorado State University)
Coalbed methane (CBM) gas is formed in confined coal -
bed aquifers through biogenic processes and remains
trapped in coal fractures by overlying water pressure.
Pumping water from the coalbed aquifer decreases the
water pressure and allows methane gas to be released,
collected and subsequently distributed via pipelines. It
is estimated that a single CBM well in the Powder River
Basin (PRB) produces from 2 to 40 gallons of water
per minute, but varies with the aquifer pumped and the
density of wells. Approximately 0.6 trillion gallons of
product water eventually will be produced from CBM
extraction in Wyoming (DeBruin et al., 2000).
The variability of water chemistry produced by CBM
wells in the Powder River Basin is not clearly under-
stood. In general, the total dissolved solids in CBM dis-
charge waters increases
from the southeast to the
northwest in the Powder
River Basin as deeper
coal seams produce more
saline and alkaline waters
(Rice, 2000). CBM ex-
traction wells are placed
together in a manifold
system discharging to a
single point and releas-
ing often into ephemeral
stream channel systems,
constructed unlined
wellhead downstream in ephemeral stream channels
and to examine surface- groundwater interactions. We
collected CBM discharge water samples from discharge
points and analyzed for pH, major element concentra-
tions, and calculated the sodium adsorption ratio (SAR).
High SAR levels can increase the soil salinity, increase
the soil pH, decrease infiltration rates, and decrease soil
productivity. These parameters were analyzed in sur-
face and shallow groundwater samples collected down-
stream of wellhead discharges to determine changes in
CBM discharge water.
The coalbed methane discharge waters are generally
alkaline, high in sodium (Na) and bicarbonate (HCO3)
concentrations (Table 1). The chemical composition
of the discharge waters did not vary over time, similar
to other PRB studies (McBeth et al.,
2003). Mean pH increased from 7.4
to 8.8 in the stream from precipitation
of calcite, with a decrease in calcium
retention ponds and /or,
re- injected to the groundwater, or recently treated on site
to reduce salinity before discharge. A careful evaluation
of CBM chemistry to better determine potential uses of
CBM produced water and the ability to maintain state
water quality standards is needed.
The objective of this study was to examine CBM
discharge water chemistry as water moves from the
6
and increase in sulfate. The decreased
calcium increased the SAR. In gen-
eral, the water chemistry of the surface
water sampled at different points down-
stream did not vary, but differences in
groundwater chemistry were observed.
Discharge and surface waters are a Na-
HCO3 type water.
Groundwater chemistry as sampled at
2 and 4 foot depths, near the channel,
showed significantly higher concentrations of all salts.
Sodium concentrations in particular tended to increase
with soil depth, and are a Na -SO4 type water. As the
wellhead water is discharged into the channel, water
soluble salts are being dissolved and leached laterally
into the soil. Once the channel discharge decreases or
is stopped the soil reservoir of salts will migrate back
to the channel. This process has been confirmed when