Groundwater in the Williston Basin: How Much Is There, and What Changes It?

U.S. Geological Survey Fact Sheet 2018–3046, September 2018. By Joanna N. Thamke, Andrew J. Long and Kyle W. Davis.

The USGS Water Availability and Use Science Program (formerly the Groundwater Resources Program) is assessing groundwater across the United States — the status of the nation's principal aquifer systems, how changes in land use, water use and climate may affect them, and how much water there will be for people and ecosystems. The Williston Basin study aims to track how the basin's water budget changes over space and time, by understanding how water enters, moves through and leaves the groundwater system, and to build tools such as computer models to forecast the effects of natural and human stresses.

Water and energy in the Williston Basin

The basin is shallow and wide, underlying about 135,000 square miles of Montana, North Dakota and South Dakota and Manitoba and Saskatchewan.

• Three aquifer systems — the glacial, lower Tertiary and Upper Cretaceous — are up to 3,000 feet thick and sit on shale that walls them off from saline aquifers below.
• The climate is semiarid: about 6 inches of precipitation a year in the dry southwest, more than 30 inches in the east. About 10 percent of it recharges the groundwater.
• Surface water is heavily allocated and not always reliable where streams run intermittently, and groundwater levels have dropped markedly in places — pushing users to other sources and prompting conservation recommendations.

Oil and gas. The basin has been an important oil and gas region since the 1950s. Since the mid-2000s, horizontal drilling and hydraulic fracturing have opened formations such as the Bakken and Three Forks — methods that need large volumes of fresh water, mostly from shallow aquifers or surface water. Water used per well for fracturing rose about sixfold from 2005 to 2014, to an average of about 2.4 million gallons a well in 2014. Luckily, one of the wettest periods, 2007–14, coincided with the boom.

The basin's three uppermost aquifer systems and their seven hydrogeologic units — the most accessible groundwater, lying on up to 3,000 feet of shale. USGS, modified from Long and others (2018).

Building the model

Groundwater availability is more than how much can be pumped: it depends on water quantity and quality, climate, human use, aquifer properties, and changes in storage and in how easily water can be drawn. The USGS first built a 3D framework of the geology and hydrology — aquifer thickness, depth to water — and a conceptual model of flow directions and water amounts, then a computer model of groundwater flow through the three systems to test how human activity and climate could change storage and water levels.

What the study found

• Near the surface, the water table follows the rolling land, and water flows from uplands toward streams.
• In the deeper aquifers, water flows in a regional pattern from southwest to northeast, little affected by the surface.
• Recharge comes mainly from precipitation and seepage from streams and reservoirs; irrigation and inflow from outside the basin add little.
• The deepest system has the smallest water budget — less than a third of either shallower system — yet supplies about 70 percent of the basin's well withdrawals.
• Most groundwater discharges to streams and reservoirs. Pumping is less than 5 percent of the budget, but withdrawals rose from 1960 to 2005 and have lowered water levels locally.

Shallow groundwater flows toward rivers and streams; deeper groundwater flows toward the northeast of the basin. USGS, modified from Long and others (2018).

Three scenarios

1. Flowing wells. Some wells in the Fox Hills and Hell Creek aquifers flow without a pump, under the aquifer's natural pressure. Simulating their continuous flow from 1960 to 2035:

• Water levels fall most in 1960–2005, but the area of drawdown keeps growing afterward.
• The biggest declines — more than 100 feet — are near the Yellowstone, Missouri and Little Missouri Rivers, where most flowing wells are. They level off after 2010 as falling pressure slows the wells, and nearly a quarter of the simulated wells stop flowing by 2035.
• Modelled flows are much larger than measured ones, possibly because poorly sealed or corroded wells leak water into overlying aquifers instead of discharging it at the surface.

As total flow from the simulated flowing wells (black) rose, groundwater levels (blue) fell. Background photo of the Hell Creek Formation by Kevin Dennehy, USGS.

2. A 10-year drought across the basin. Water levels fall most — by as much as 230 feet — in the normally wet east; the dry west is less sensitive to drought.

3. A 10-year drought plus more pumping for energy, 2006–15. Reduced recharge from the drought affected the water budget more than the extra energy pumping. Wells for energy development have a small regional effect on water levels, but can have a substantial local one.

Sources

• Thamke, J.N., Long, A.J., and Davis, K.W., 2018, Williston Basin groundwater availability, United States and Canada: U.S. Geological Survey Fact Sheet 2018–3046. https://doi.org/10.3133/fs20183046 · https://pubs.usgs.gov/publication/fs20183046
• Long, A.J., Thamke, J.N., Davis, K.W., and Bartos, T.T., 2018, Groundwater availability of the Williston Basin, United States and Canada: U.S. Geological Survey Professional Paper 1841. https://doi.org/10.3133/pp1841
• The figures and the scenario results come from the fact sheet's PDF; the web version's text is out of order. Its photographs from a private studio are not reproduced here.
• Rewritten in hubnx's own words.