Abstract
The primary purpose of this report is to present information
relating to flow-adjusted temporal trends in major-ion
constituents and properties for 16 sampling sites in the Tongue
and Powder River watersheds based on data collected during
1980–2010. In association with this primary purpose, the
report presents background information on major-ion characteristics
(including specific conductance, calcium, magnesium,
potassium, sodium adsorption ratio, sodium, alkalinity, chloride,
fluoride, dissolved sulfate, and dissolved solids) of the
sampling sites and coal-bed methane (CBM) produced water
(groundwater pumped from coal seams) in the site watersheds,
trend analysis methods, streamflow conditions, and factors that
affect trend results.
The Tongue and Powder River watersheds overlie the
Powder River structural basin (PRB) in northeastern Wyoming
and southeastern Montana. Limited extraction of coal-bed
methane (CBM) from the PRB began in the early 1990’s, and
increased dramatically during the late 1990’s and early 2000’s.
CBM-extraction activities produce discharges of water with
high concentrations of dissolved solids (particularly sodium
and bicarbonate ions) relative to most stream water in the
Tongue and Powder River watersheds. Water-quality of CBM produced
water is of concern because of potential effects of
sodium on agricultural soils and potential effects of bicarbonate
on aquatic biota.
Two parametric trend-analysis methods were used in
this study: the time-series model (TSM) and ordinary least
squares regression (OLS) on time, streamflow, and season.
The TSM was used to analyze trends for 11 of the 16 study
sites. For five sites, data requirements of the TSM were not
met and OLS was used to analyze trends. Two primary 10-year
trend-analysis periods were selected. Trend-analysis period 1
(water years 1986–95; hereinafter referred to as period 1) was
selected to represent variability in major-ion concentrations
in the Tongue and Powder River watersheds before potential
effects of CBM-extraction activities. Trend analysis period 2
(water years 2001–10; hereinafter referred to as period 2) was
selected because it encompassed substantial CBM-extraction
activities and therefore might indicate potential effects of
CBM-extraction activities on water quality of receiving
streams in the Tongue and Powder River watersheds. For sites
that did not satisfy data requirements for the TSM, OLS was
used to analyze trends for period 2 (if complete data were
available) or a 6-year period (2005–10).
Flow-rate characteristics of CBM-produced water were
estimated to allow general comparisons with streamflow
characteristics of the sampling sites. The information on
flow-rate characteristics of CBM-produced water in relation to
streamflow does not account for effects of disposal, treatment,
or other remediation activities on the potential quantitative
effects of CBM-produced water on receiving streams. In many
places, CBM-produced water is discharged into impoundments
or channels in upper reaches of tributary watersheds
where water infiltrates and does not directly contribute to
streamflow. For Tongue River at State line (site 4) mean
annual pumping rate of CBM-produced water during water
years 2001–10 (hereinafter referred to as mean CBM pumping
rate) was 6 percent of the mean of annual median streamflows
during water years 2001–10 (hereinafter referred to as
2001–10 median streamflow). For main-stem Tongue River
sites 5, 7, and 10, mean CBM pumping rate was 8–12 percent
of 2001–10 median streamflow.
For main-stem Powder
River sites (sites 12, 13, and 16), mean CBM pumping rates
were 26, 28, and 34 percent of 2001–10 median streamflows,
respectively.
For main-stem Tongue River sites analyzed by using
the TSM and downstream from substantial CBM-extraction
activities [Tongue River at State line (site 4), Tongue River
at Tongue River Dam (site 5), Tongue River at Birney Day
School (site 7), and Tongue River at Miles City (site 10)],
generally small significant or nonsignificant decreases in most
constituents are indicated for period 1. For period 2 for these
sites, the TSM trend results do not allow confident conclusions concerning detection of effects of CBM-extraction activities
on stream water quality. Detection of significant trends in
major-ion constituents and properties for period 2 generally
was infrequent, and direction, magnitudes, and significance of
fitted trends were not strongly consistent with relative differences
in water quality between stream water and CBM-produced
water. The TSM indicated significant or generally large
magnitude increases in median values of sodium adsorption
ratio (SAR), sodium, and alkalinity for period 2 for sites 5 and
7, which might indicate potential effects of CBM-extraction
activities on stream water. However, other factors, including
operations of Tongue River Reservoir, irrigation activities,
contributions of saline groundwater, and operations of the
Decker coal mine, confound confident determination of causes
of detected significant trends for sites 5 and 7. For all mainstem
Tongue River sites, trends for period 2 generally are
within ranges of those for period 1 before substantial CBM-extraction
activities.
For main-stem Powder River sites analyzed by using
the TSM [Powder River at Sussex (site 11), Powder River at
Arvada (site 12), Powder River at Moorhead (site 13), and
Powder River near Locate (site 16)], significant or generally
large magnitude decreases in median values of SAR, sodium,
estimated alkalinity, chloride, fluoride, specific conductance,
and dissolved solids are indicated for period 1. Patterns in
trend results for period 1 for main-stem Powder River sites
are consistent with effects of Salt Creek oil-brine reinjection
that started in 1990. Trend results for all main-stem Powder
River sites downstream from substantial CBM-extraction
activities (sites 12, 13, and 16) indicate evidence of potential
effects of CBM-extraction activities on stream water quality,
although evidence is stronger for sites 12 and 13 than for site
16. Evidence in support of potential CBM effects includes
significant increases in median values of SAR, sodium, and
estimated alkalinity for period 2 for sites 12, 13, and 16 that
are consistent with relative differences between stream water
and CBM-produced water. Significant increases in median
values of these constituents for period 2 are not indicated for
Powder River at Sussex (site 11) upstream from substantial
CBM-extraction activities. In interpreting the trend results,
it is notable that the fitted trends evaluate changes in median
concentrations and also notable that changes in median concentrations
that might be attributed to CBM-extraction activities
probably are more strongly evident during low to median
streamflow conditions than during mean to high streamflow
conditions. This observation is relevant in assessing trend
results in relation to specific water-quality concerns, including
effects of water-quality changes on irrigators and effects on
stream biota and ecology.