Investigating Groundwater in Montana Informs State Water Policy Decisions

The Ground Water Investigation Program at the Montana Bureau of Mines and Geology seeks to provide answers to communities about the status of local groundwater.

This article was updated on March 1st, 2016.

This 3D model created by the Montana Bureau of Mines and Geology Ground Water Investigation Program details the complex network of more than 2,000 miles of irrigation ditches in the Gallatin Valley, as well as how the groundwater flows in the area. Much of the groundwater in the valley is replenished by the unlined ditches, which are largely credited for maintaining groundwater levels in the area despite rapid urban and suburban development and irrigation practice changes. Groundwater in the valley flows north and west toward Nixon Gulch. Image Credit: Montana Bureau of Mines and Geology - Ground Water Investigation Program

This 3D model created by the Montana Bureau of Mines and Geology Ground Water Investigation Program details the complex network of more than 2,000 miles of irrigation ditches in the Gallatin Valley, as well as how the groundwater flows in the area. Much of the groundwater in the valley is replenished by the unlined ditches, which are largely credited for maintaining groundwater levels in the area despite rapid urban and suburban development and irrigation practice changes. Groundwater in the valley flows north and west toward Nixon Gulch. Image Credit: Montana Bureau of Mines and Geology – Ground Water Investigation Program

As in many western states, officials in Montana have long been concerned with water. Montana’s landmass is the size of Japan, and the hydrogeology of the state is as complex and varied as the landscape, which ranges from mountainous and forested in the west to short-grass prairie and badlands in the east.

The Ground Water Investigation Program (GWIP) at the Montana Bureau of Mines and Geology in Butte, Montana, researches pressing groundwater issues selected by the Montana Ground Water Assessment Steering Committee, which was legislatively authorized by the Montana State Legislature. GWIP examines such questions as groundwater/surface-water interactions and stream depletion, changing aquifer recharge by improved efficiency in irrigation methods, hydrologic effects of land-use changes, and water-quality impacts from septic tank density.

“The purpose of GWIP is to provide a mechanism to answer focused water resource issues throughout the state,” said Ginette Abdo, GWIP program manager. “The data and the interpretations that we make through our program are used to make better-informed water management decisions, whether that’s on a local scale or on a statewide legislative scale.”

GWIP was formed in early 2009 after the Legislature’s Water Policy Interim Committee recognized that the competition for water resources would only intensify, and that the lack of understanding about how groundwater systems in the state function necessitated hydrologic investigations to provide data for policy decisions as the state’s population grows.

Since its inception, 50 projects have been nominated to GWIP, and 10 detailed, peer-reviewed reports have been published with four more in review. GWIP data are archived online to allow public access and give regulators, senior and new water-rights holders, and other stakeholders a better understanding of groundwater systems.

Essentially, the data GWIP staff collect and interpret serve to provide scientific verification or refutation of anecdotal evidence of changes to groundwater systems.

“You go into these areas and you hear these stories about, ‘Well, 30 years ago I had no problem with my well.’ You hear a lot of anecdotal type stories,” Abdo said. “What we provide is the data and the science to help support or not support what we’re seeing.”

In addition to producing reports about specific areas, GWIP staff provide educational lectures to the Water Policy Interim Committee and others around the state.

“Data can be used by landowners to make decisions about what they have control over,” Abdo said. “Data could be used to make more regional decisions, such as ‘Should we line this ditch?’ On a statewide scale we’ve provided information to answer the question of stream depletion.”

GWIP staff use a number of methods to analyze the data they collect from test wells, including creating detailed maps in ArcGIS and by using spectral imagery from Landsat. These maps are as simple as showing the network of test wells, and as complex as showing which way groundwater flows in a particular area by generating simulations of the geology including layers and faults. These maps also can be used to calculate nitrate volumes in an area.

By combining the GIS modeling with Groundwater Modeling System data, GWIP staff are able to build 3D geologic systems that allow visualization by using data points to illustrate subsurface geology.

Irrigated acreage in the Four Corners area near Bozeman, Montana, decreased between 1953 and 2010 as subdivision and rural residential/commercial development increased. Image Credit: Montana Bureau of Mines and Geology Ground Water Investigation Program

Irrigated acreage in the Four Corners area near Bozeman, Montana, decreased between 1953 and 2010 as subdivision and rural residential/commercial development increased. Image Credit: Montana Bureau of Mines and Geology Ground – Water Investigation Program

In the Gallatin Valley, where Bozeman is located, one nearly completed GWIP project has examined how heavy urban and suburban development may or may not be depleting the groundwater system.

“Gallatin County is the fastest-growing county in the state, and the development pressure because of the current laws has been very heavy on groundwater development without municipal systems,” said Mary Sutherland, a GWIP research hydrogeologist, speaking on the process by which many people drill exempt wells.

In Montana, exempt wells are defined as those that use less than 35 gallons per minute and a maximum of 10 acre-feet per year of water. These are typically the type of well a rural homeowner installs to supply water to their house and yard. They are not subject to the state water righting process. Sutherland says some farmers and ranchers who have historic water rights dating back to the early 1900s are concerned that they’re losing their water to later established wells. The focus of the GWIP Four Corners project, in part, is to determine if such concerns are valid.

Sutherland uses groundwater and surface water data across the area during a particular time frame to create a smooth simulated surface to determine how groundwater moves.

“It’s a historically agricultural valley. Because it’s Bozeman and there’s been a lot of pressure on the city, there’s been a lot of urban sprawl,” Sutherland said. “How does that change the water system? There’s a theory of the many straws. Think of the milk shake as the groundwater. If you put 50 straws in, does it deplete significantly faster? We’re talking about a huge milk shake. Are these exempt wells that are not part of the permitting process, are they drying up the aquifer? That was why we were brought in.”

In addition to groundwater and surface water monitoring, GWIP staff measure irrigation, wells that pump water out of the system, and evapotranspiration levels to create a holistic view of groundwater situations throughout the state.

According to Sutherland, evapotranspiration is an important part of the water cycle, especially in a place like Montana where the climate can be hot, dry, and windy; abundant water during the spring melt becomes scarce in August. During times of scarcity, an accurate water budget is critical to manage water resources such as minimum in-stream flows for fish and irrigation requirements for crops.

GWIP staff use several methods, including METRIC (Mapping Evapotranspiration at high Resolution and with Internalized Calibration), a mapping method developed at the University of Idaho, to use energy balance data from Landsat remote sensing to compare to field measurements of evapotranspiration. This allows for highly accurate evapotranspiration rates and volumes in a study area.

Using spectral analysis (Landsat 5 imagery), project investigators were able to determine lawn/garden irrigation to calculate evapotranspiration in high-density subdivisions (left) and low-density subdivisions (right) in the Four Corners area. Image Credit: Montana Bureau of Mines and Geology Ground Water Investigation Program

Using spectral analysis (Landsat imagery), project investigators were able to determine lawn/garden irrigation to calculate evapotranspiration in high-density subdivisions (left) and low-density subdivisions (right) in the Four Corners area. Image Credit: Montana Bureau of Mines and Geology – Ground Water Investigation Program

In the case of the Gallatin Valley, Sutherland and her co-worker Dean Snyder have been able to use this remote sensing method to determine how lawn irrigation in densely and thinly populated subdivisions contribute to localized groundwater draw-down. In a nutshell, more densely populated subdivisions with smaller lawns impact groundwater less than subdivisions with expansive lawns.

Sutherland and Snyder’s work has demonstrated that despite the rapid development in the Gallatin Valley, the groundwater in the area is not being depleted at current population levels. Thanks to recharge from more than 2,000 miles of unlined irrigation ditches that twist and curve spiderweb-like through the valley, the groundwater levels have remained at historic levels despite changes in irrigation practices that input less water into the system.

Perhaps one of the greatest impacts of GWIP is that the models staff create are able to estimate the impact on groundwater in the future as population grows. According to 2010 census data, the population of Gallatin County was 89,513, making it the fourth-most populous county in Montana. The population has increased a whopping 32 percent since the 2000 census, when the county’s population was 67,831.

“The utility of the model is predictive situations,” Sutherland said. “If you have a valid way of representing change, the model will tell you how the system will react. We can apply the changes in the numeric model; this is what 30,000 more people would look like, tell me how the system will react.”

Ultimately, GWIP staff are confident in their science and hope the open-access data they produce will be used by decision-makers around the state.

The land-use change scenario modeling increasing development in the Four Corners area at a rate of growth similar to what the area was experiencing in 2008. Image Credit: Montana Bureau of Mines and Geology Ground Water Investigation Program

The land-use change scenario modeling increasing development in the Four Corners area at a rate of growth similar to what the area was experiencing in 2008. Image Credit: Montana Bureau of Mines and Geology – Ground Water Investigation Program

Tammy Swinney, district manager at the Gallatin Local Water Quality District, has worked with GWIP staff during the past few years and says the research the organization produces in invaluable to her organization.

“We’re a really small staff and have a limited budget. We can’t do the investigative groundwater research the Bureau can do,” she said. “Eventually it will be very helpful to decision-makers in the valley. The Gallatin is really experiencing some rapid growth. We’re seeing this landscape conversion from agricultural use to suburban and urban use. We have a myriad of irrigation ditches and we’ve intensely modified the landscape for water distribution for agriculture. We’re getting a clearer understanding of how the aquifers respond to conversion.”

Noting that forces outside human control, like climate change, will affect the groundwater in the Gallatin Valley, Swinney noted: “Our water supply is snowpack-driven. We need to get a handle on what is available out there, how much do we have, and how long we can support it.”

Affirmations like this help the Ground Water Investigation Program staff know that getting data into the hands of community leaders will help those people make good decisions for the future of their communities.

“I feel like we’re a tremendous resource for the state of Montana,” Abdo said. “We can help people in management roles; we keep gathering more information to help them make better decisions.”

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