Hydrology of the Crow Reservation
Recharge to the aquifers on the Crow reservation is mainly from infiltration of precipitation and streams. A smaller amount of recharge occurs by subsurface in-flow from outside the reservation, by infiltration from stock ponds and reservoirs where they have been constructed on outcrops, and from leakage across confining beds. Discharge from aquifers on the reservation is by evapotranspiration in outcrop areas and along the stream valleys, by springs and seeps, and by wells. Discharge also occurs by interformational leakage and by subsurface outflow from aquifers along boundaries of the outcrop area. The quality of water or the types and amounts of minerals dissolved in water depend on the chemicals and physical characteristics of the soil and rocks over or through which the water passes, the length of time the water is in contact with the soil and rocks, and other factors such as temperature and pressure (Crow Indian Tribe: Hydrology Resources Report).
Coalbed methane is natural gas trapped in underground coal deposits that also store thousands of gallons of water. Coalbed methane extraction requires that the coal seams be drained of water, emptying aquifers that may take hundreds of years to refill (Wyoming Coalbed Methane Leases Ruled Illegal (more info) ). In the Powder River Basin, the average coalbed methane well discharges 15,000 to 20,000 gallons of salty water per day, impacting surface soils, vegetation and aquatic animals. The BLM predicts that exploiting some 80,000 coalbed methane wells in Wyoming and Montana will discharge at least four trillion gallons of water over the next 15 years (Wyoming Coalbed Methane Leases Ruled Illegal (more info) ). Each coal deposit is different, so the amount of water that must be pumped to the surface varies. The amount of water also varies over the life of the well, with more water being removed from the coal seam initially, and then declining over time. For more information about the quantity and quality of water removed from coal seams in the Powder River Basin, refer to this United States Geological Survey report (more info) .
The composition of the water removed during coalbed methane production varies, even within coal deposits. For example, a USGS monitoring project in the Powder River Basin shows an increase in dissolved sodium (Na+) and bicarbonate (HCO3), along with an increase in total dissolved solids. In general water produced from coal seams can be saline (contains a high concentration of dissolved salts), or sodic (high in sodium concentration relative to concentrations of calcium and magnesium), or both.
- To learn more about the saline and sodic chemistry of coalbed methane co-produced water in Montana's Powder River Basin, click here (more info) .
- To learn more about why salt water is a constant threat to irrigated agriculture, click here (more info) .
- To learn more about the USGS's water composition and monitoring efforts underway in the Powder River Basin click here (more info) .
Local aquifers may or may not be affected by water extraction, depending on the local geology, but there is usually some drawdown measured. Various agencies now monitor water in the affected areas to learn more about this process. For more information about potential drawdown in the aquifer of Wyoming's Powder River Basin, refer to this US Bureau of Land Management report (more info) .
There are many possibilities of what is done with the water generated during coalbed methane production, depending on water composition. The water can be released on the surface and used to irrigate crops or water livestock if it is clean enough. There is some disagreement about what is clean enough, however. Water extracted from coal seams can negatively affect soil, crops and native plants. For example, using saline water for irrigation can eventually inhibit germination and plant growth. Excess sodium can change the physical properties of soil, leading to poor drainage and crusting. This in turn can affect both plant growth rates and crop yield. Other alternatives, such as re-injection, can be costly. Water rights relating to coalbed methane production can be contentious in the arid west, and to a degree remain unresolved.
- Learn more about coalbed methane co-produced water's affect on different types of soil (more info) .
- See how coalbed methane water can affect native plants (more info) .
- View an extensive set of possibilities for coalbed methane water disposal (more info) .
- Learn about the complicated water rights (more info) in the arid western states.
To further investigate the hydrology of the Crow Reservation, follow the links below.
Hydrology of the Crow Reservation
Resources containing information about the hydrology on the Crow Reservation.