Page Contents
Model Overview
Montana DEQ has developed a water quality model to evaluate salinity conditions in the Tongue River and to determine the amounts of salt being contributed to the river from various sources. DEQ is using a SWAT-Salt model for this project (Soil & Water Assessment Tool). SWAT was developed by the USDA's Agricultural Research Service (ARS), and is maintained jointly by ARS and the Texas A&M AgriLife Research to simulate runoff quality and quantity and predict the impacts, or effects, of land use changes and land management decisions. Recently, a salinity module was developed for use with SWAT to simulate salinity transport and investigate salinity best management practices in watersheds. This module is being used for the Tongue River watershed model. SWAT models are widely used throughout the world, and Montana DEQ has used SWAT for many other water quality projects.
What is Salinity and How is it Measured?
Salinity is a measure of how much dissolved salt is in the water and is easily measured as conductance – as electrical conductivity (EC) or as specific conductance (SC). Electrical conductivity is a measure of how well a substance conducts electricity, and dissolved solids (ions) conduct electricity through the water. The more cations (Na+, Ca2+, Mg2+, etc.) and anions (HCO3-, SO42-, Cl-, etc.) that are in the water, the higher the EC. Specific conductance is simply EC corrected to 25 degrees Celsius.
Montana DEQ uses both EC and sodium adsorption ratio (SAR) to evaluate salinity conditions. Over time, irrigation water high in EC will create soils that are high in salinity, which makes it harder for plants to uptake water and nutrients. Crop yields will decrease once EC rises above a plant’s specific threshold. Sodium adsorption ratio (SAR) is a measure of the suitability of water for irrigation, and is calculated using a ratio of sodium (Na) to calcium (Ca) and magnesium (Mg). A high SAR generally means high amounts of sodium in the water. Water with high levels of sodium can permanently damage agricultural soils; thus, a high SAR indicates water is less suitable for irrigation. Montana’s EC and SAR water quality standards are detailed on the Tongue River Water Quality Standards page.
Total dissolved solids (TDS) is a measure of the total amount of dissolved solids (by mass - usually in milligrams) per liter of Tongue River water. For the modeling portion of the project, TDS (in milligrams per liter) will be used as a surrogate for EC. TDS and EC have a strong positive correlation in the Tongue River data, and can be used to estimate one another with a fairly high degree of accuracy. Since TDS is mass-based, it is easier to model in a mass-balance hydrologic computer simulation such as SWAT.
How the Model is used to Evaluate Salinity
The SWAT-Salt model for the Tongue River is being used to help determine the existing and potential loads of electrical conductivity (EC) and sodium adsorption ratios (SAR) in the river under various flow and environmental conditions, and various land management scenarios. SWAT-Salt simulates the major cations in the water which can be used to correlate to conductance and total dissolved solids (TDS), and also calculate SAR. The model will simulate the stretch of the river from the Tongue River Reservoir dam to the river’s mouth at Miles City (Map 1), but takes the entire area of the watershed into account for determining the sources of salt to the river. The hydrology of the model is being calibrated with flow data from the Tongue River USGS gaging stations (Map 1), and EC, SAR, and TDS sample data collected from the gaging stations and from water samples collected from other locations along the river. Data is being used from the period of 2004 to 2013. The Tongue River Reservoir and the tributaries of Hanging Woman, Otter, and Pumpkin creeks all have USGS gaging stations with continuous flow data for the majority of the modeled timeframe (2004 to 2013), and are being treated as “point sources” within the model (Map 1).
Map 1: Modeled Area of the Tongue River Watershed
(Click on map to enlarge)
Where We Get Our Data
A complex watershed model such as SWAT requires many different types of data as input. Data used in the model includes climate data, soils data, land use data, topography, flow data, and water quality data. We use data collected by governmental agencies due to the stringent quality assurance (QA) procedures that they use.
For river flow and water quality data, data was obtained from either the U.S. EPA STORET database, or the USGS NWIS database. EPA's STORET database was decommissioned in June 2018 and replaced with the National Water Quality Portal which combines both EPA and USGS data, as well as data from States (including Montana), other federal agencies, and other data collection entities. These databases are available to the public and all of the data used for this project can be obtained from them. DEQ's Water Quality Portal Guidance can assist you in downloading data from the Portal, if needed. The model run period was from 1998 to 2013, but the model was analyzed from 2004 to 2013 (a ten year period), allowing the first few years of the run period to be used as an initial warm-up. All data was analyzed for only the ten year period from 2004 to 2013.
USGS gages 06307500 Tongue River at Tongue R Dam nr Decker MT, 06307600 Hanging Woman Creek near Birney MT, 06307740 Otter Creek at Ashland MT, and 06308400 Pumpkin Creek near Miles City MT were used to directly inform the model. These gages were used as inflows to the modeled system (Map 1 above).
USGS gages 06307616 Tongue R nr Birney MT, 06307830 Tongue R bl Brandenberg Bridge nr Ashland MT, 06307990 Tongue R ab T & Y Div Dam nr Miles City MT, and 06308500 Tongue River at Miles City MT (Map 1 above) were used to calibrate the model to existing conditions. These gages were not directly used in the model but rather certain parameters within the model were adjusted to make the modeling results more closely match data at these gages. In addition, data collected by DEQ and EPA in the watershed was also used where appropriate.
Additionally, point source data for the model was obtained for the various point sources in the Montana portion of the watershed. Groundwater quality data was obtained from sources mentioned above, the Montana Bureau of Mines & Geology, and some of the extensive groundwater monitoring performed on Otter Creek by private consultants.
Model Scenarios
DEQ is using the model to simulate "what if" scenarios to determine how possible changes in land management may affect salt loads in the river. Below is a bulleted summary of scenarios that were modeled.
Coal Bed Methane Production:
- Remove all CBM from the watershed
- Increase the number of CBM wells discharging to surface water in the watershed
- Lower permit discharge limits for CBM operations (stricter permit limits)
Coal Mines:
- Remove all coal mines from the watershed
- Lower permit discharge limits (stricter limits)
Irrigation:
- Remove all irrigation from the watershed
- Increase irrigation in the watershed
- Modify irrigation practices (e.g., T&Y Canal flow changes, increased sprinkler usage)
Tongue River Dam & Reservoir Operations
- Removal of the dam to represent natural flow conditions
- Seasonal reservoir flow release modifications
Combined Scenarios
- Increases in seasonal flow, no coal mine or CBM discharges
- Increases in seasonal flow, coal mine and CBM discharges at the EC/SAR standards
- Increases in seasonal flow, no CBM discharges, coal mine discharges at the EC/SAR standards
Model Results and Report
CBM and coal development had the largest impact to salinity levels, while agriculture and livestock impacts were minimal. The scenarios showed that high salinity levels in the Tongue River are predominantly caused by natural conditions, likely related to the geology and soils of the region. The water quality standard could not always be met even in a scenario that simulated pre-human natural conditions by removing all coal, CBM, agriculture, and livestock sources. A draft modeling report is available for review in the table below.
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Document
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Document Description
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Document Date
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Tongue River Salinity Model Report (final version)
Appendices
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The final Tongue River Modeling Report, detailing contributions of anthropogenic and natural sources of salinity to the Tongue River, is available. Contact Christy Meredith at Christy.Meredith@mt.gov or 406-444-0371 for more information. |
February 12, 2024 |
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Stakeholder Draft: Tongue River Salinity Modeling Report (5.65 MB)
Appendices (2.60 MB)
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Stakeholder Draft of Tongue River Modeling Report, detailing contributions of anthropogenic and natural sources of salinity to the Tongue River, is available for public review. Comments may be submitted until July 3, 2023 to Christy Meredith at Christy.Meredith@mt.gov, 406-444-0371
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May 2, 2023
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Tongue River Project Contacts
See the Tongue River Watershed Project Contacts page for a full description of each project team member’s role and for a list of additional contacts.
Page Released: July 6, 2016
Last Updated: May 24, 2023
