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General hydrology

Types of water-quality data

Water sample data

Continuous water-quality data

Water-quality standards

Summary of water-quality data

Specific conductance

Sodium adsorption ratio

Nutrients

Suspended sediment

Trace elements

References

Summary of Water-Quality Data
for Sites in the Tongue River Surface-Water-Quality Monitoring Network, March-October 2005 Irrigation Season

This data summary provides a brief overview of general patterns observed in the water-quality data collected by the USGS during March-October 2005 for the 12 sites in the Tongue River surface-water-quality monitoring network in Montana and Wyoming (fig. 1). In 2005, a twelfth site was added to the monitoring network to gain better spatial resolution regarding the water-quality characteristics in the lower part of the Tongue River watershed. The additional site is located on the mainstem just upstream from the T&Y Diversion Dam near Miles City and the confluence of Pumpkin Creek. Details of the data-collection activities during 2005 are provided on the Project Plan page of this Web site. 

Figure 1. Monitoring sites in the Tongue River surface-water-quality monitoring program, 2005.

General Hydrology

General hydrologic conditions are summarized because the volume of runoff is an important factor affecting water quality. Streamflow data for specific sites can be viewed and retrieved by accessing the Home page of this Web site and then clicking on the site symbol on the study area map.

Streamflow in the Tongue River mainstem was below normal during March-April 2005 and was above normal for much of May, when a combination snowmelt and rainfall event occurred. Peak flows in May were noticeably higher than the long-term average for Tongue River at State Line (fig. 2). After the May runoff event, flows through October 2005 were sustained at levels similar to the long-term average. The 2005 flows are in contrast to the 2004 flows, which were below normal for the entire 8-month irrigation season. In 2004, substantially less runoff occurred and notable peak flows were absent.

Figure 2. Streamflow during March-October 2005 and long-term average streamflow for the period of record (1960-2005), Tongue River at State Line, Mont. The streamflow data for October 2005 are provisional. Streamflow during March-October 2004 also is plotted for comparison.

Similar streamflow conditions occurred in tributaries of the Tongue River during 2005, as illustrated in figure 3 for Prairie Dog Creek. Peak flows in tributaries during spring 2005 generally were larger than the long-term average peak flows, whereas near-normal conditions persisted during the summer months. In comparison, flows during 2004 were below normal for almost the entire 8-month period. 

Figure 3. Streamflow during March-October 2005 and long-term average streamflow for the period of record (1971-80, 2000-05), Prairie Dog Creek near Acme, Wyo. The data for October 2005 are provisional. Streamflow during March-October 2004 also is plotted for comparison.

The flows in 2005 may have affected various water-quality conditions by providing greater dilution of dissolved constituents, such as salts (major ions), and greater transport of suspended sediment and constituents adsorbed to sediment than in 2004. The large volume of runoff not only flushed the landscape and stream channels, but also replaced some of the water stored in Tongue River Reservoir with water having lower concentrations of dissolved constituents. Concentrations of dissolved salts in the reservoir presumably had increased during previous low-flow years owing to longer retention times and the effects of evaporative concentration.

Types of Water-Quality Data

The water-quality data summarized here includes field and laboratory analytical data for periodically-collected water samples and continuously-recorded data obtained from onsite monitors (fig. 4). Data are presented for March-October 2005, which coincides with the irrigation season.  

Figure 4.   Tongue River at Monarch, Wyo., July 2004. The onsite monitor that measures specific conductance is deployed in the vertical pipe attached to the bridge pier. The monitor is connected by cable to an electronic data logger in the gage house located to the right of the field-laboratory truck. Data are transmitted to a satellite from the antenna mounted above the gage house and displayed in real-time on the Tongue River network Web site.

Water-sample data

Water samples were collected periodically throughout the year at 12 sites in the Tongue River watershed. During the March-October 2005 irrigation season, samples were collected 15 times at mainstem sites and 10 times at tributary sites, except at Hanging Woman and Pumpkin Creeks, which were sampled only eight times due to extended dry periods. Samples were measured in the field for specific conductance (SC), pH, dissolved oxygen, and water temperature, and were analyzed in the laboratory for major ions, nutrients, suspended sediment, and trace elements. The sampling frequency for major ions remained the same as in 2004 (15 samples), whereas the sampling frequency for nutrients and trace elements was reduced from 12 times per year in 2004 to 6 times per year in 2005; about 4 nutrient and trace-element samples were collected during the irrigation season at most sites. Analytical results for samples can be viewed and retrieved on the Water-Quality Data page of this Web site.

Samples typically were collected at all of the network sites during the same week. This systematic approach was used to help ensure the comparability of data among sites by representing similar hydrologic, seasonal, and land-use conditions.

Continuous water-quality data

Continuous water-quality data include direct measurements of SC by onsite monitors and estimates of sodium adsorption ratio (SAR). The estimated SAR values are based on regression relations developed between SC and laboratory-measured SAR in samples. Continuous records of SC were collected from mid-March through October 2005 at most sites, except during periods of no flow or equipment malfunction. As described below, several levels of data resolution are available from the continuous water-quality data.

Real-time: SC was measured every 15 minutes by the onsite monitors and transmitted via satellite telemetry every 1-4 hours. Estimated SAR was calculated from each value of SC using regression equations. Graphs of these SC and estimated SAR values were displayed in near real-time on the Tongue River network Website. The SC and estimated SAR data displayed in real-time were “provisional” because these data had not yet been adjusted to account for sensor cleaning, calibration corrections, and other site conditions that might have affected the accuracy of the data. The data were approved as “final” after they were reviewed and necessary adjustments were made. The data presented in this summary and stored in the USGS National Water Information System (NWIS) represent the final values, which may differ slightly from the provisional data that were presented in real-time during 2005.

Daily means: Daily mean values were calculated from all values of SC and estimated SAR for each day. These daily means can be viewed and retrieved by clicking on the site symbol on the study area map located on the Tongue River network Web site, and then selecting “Time-Series: Daily data” from the drop-down menu of available data for the site.

Monthly means: Monthly mean values of SC and estimated SAR were calculated from the daily mean values for each month. For months that had multiple days of missing record, a general criterion was used that required at least nine daily mean values in a month to calculate a monthly mean. This criterion was set low due to the difficulty in obtaining daily record at several sites.

Water-Quality Standards

Specific Conductance and Sodium Adsorption Ratio.  The water-quality data obtained during 2005 are compared to selected State and Federal water-quality standards and guidelines in order to provide a perspective for data analysis. Water-quality standards for SC and SAR have been established for the Montana portion of the Tongue River watershed by the Montana Department of Environmental Quality (2003). Standards exist for individual water samples and for monthly means (tables 1 and 2). The standards vary depending on whether the measurement was made during the irrigation season or non-irrigation season and, for SC, whether the site is on the Tongue River mainstem or a tributary. The Wyoming Department of Environmental Quality has not established numeric standards for SC and SAR in the Wyoming portion of the Tongue River watershed. Consequently, SC and SAR values measured at all 12 network sites are compared to the State of Montana standards for consistency.

Table 1. State of Montana numeric standards for specific (or electrical) conductance (Montana Department of Environmental Quality, 2003) in microsiemens per centimeter at 25 degrees Celsius (µS/cm).  

Table 2. State of Montana numeric standards for sodium adsorption ratio (Montana Department of Environmental Quality, 2003).  

Nutrients. The U.S. Environmental Protection Agency (USEPA) recommends a guideline concentration of 1.50 milligrams per liter (mg/L) for total nitrogen, as N, and 0.075 mg/L for total phosphorus, as P, in the area encompassing the Tongue River watershed of Montana and Wyoming to prevent eutrophication (U.S. Environmental Protection Agency, 2001). Eutrophication is the accelerated growth of algae and aquatic plants that can lead to accumulation and decomposition of organic material. These guidelines represent target values that may be protective of beneficial stream uses.

Trace Elements. Water-quality standards for trace elements have been established for the protection of aquatic life by the States of Montana (Montana Department of Environmental Quality, 2006) and Wyoming (Wyoming Department of Environmental Quality, 2001). Some trace elements have standards that vary with water hardness and are calculated for each sample using hardness at the time of sampling and equations presented in the respective references for Montana and Wyoming.

Summary of Water-Quality Data

Specific Conductance

SC is a physical property of water that provides an easily measured and qualitative estimate of the total amount of dissolved ions, or salinity. SC is an important water-quality parameter in the Tongue River watershed because it provides a measure of the potential for irrigation water to induce physiological stress in plants and restrict plant growth. SC data are available for water-quality samples that were collected 8-15 times at each site during March-October 2005 (irrigation season). Continuous SC data are available from the onsite monitor at each site during the irrigation season, except during periods of no flow or equipment malfunction.

Water-sample data

SC values for samples collected from the Tongue River mainstem in 2005 (fig. 5) increased gradually in a downstream direction. In the mainstem, SC in samples at each site had relatively little change throughout 2005, ranging from about 200 to 430 µS/cm at Tongue River at Monarch, from about 320 to 800 µS/cm at Tongue River at Birney Day School Bridge, and from about 340 to 1,050 µS/cm at Tongue River at Miles City. The irrigation-season SC sample standard of 1,500 µS/cm for the Tongue River mainstem (Montana Department of Environmental Quality, 2003) was not exceeded in any sample collected from the mainstem sites (fig. 5).

SC values for samples collected from tributaries in 2005 (fig. 5) were more variable, with three of the five tributaries having substantially elevated SC compared to the mainstem. Median SC values in Goose and Pumpkin Creeks were similar to those in the mainstem, although Pumpkin Creek had a much larger range of SC values compared to Goose Creek and a notable maximum value (2,460 µS/cm). The highest SC measured in the tributaries was 3,180 µS/cm at Otter Creek at Ashland. The irrigation-season SC sample standard of 500 µS/cm for tributaries (Montana Department of Environmental Quality, 2003) was commonly exceeded in all of the tributaries sampled (fig.5).

Figure 5.   Specific-conductance (SC) values measured in samples collected at Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Sites are plotted in a downstream direction from left to right. The State of Montana irrigation-season SC sample standards for the Tongue River mainstem and tributaries (Montana Department of Environmental Quality, 2003) are included on the graph for reference. Boxplots are used to show the statistical distribution of the data.

Daily Mean Specific-Conductance Data

Daily mean SC values were calculated from SC values measured with the continuous onsite monitors. In the mainstem, daily mean SC in 2005 (fig. 6) fluctuated gradually and over a moderate range at most sites. At all the mainstem sites, the SC decreased during the May-June runoff season. After streamflow decreased in July, the SC gradually increased through the remainder of the season. In comparison, SC in the mainstem during 2004 did not have a large decrease during May-June because of the lack of substantial runoff. Instead, the SC fluctuated to a minor extent and generally held steady throughout the irrigation season.

Daily mean SC values generally increased in the downstream direction with the exception of the Tongue River at State Line. At this site, the SC was higher than at sites downstream from the Tongue River Reservoir during much of the summer. Daily mean SC values ranged from about 150 to 450 µS/cm at Tongue River at Monarch, from about 290 to 750 µS/cm at Tongue River at Birney Day School Bridge, and from about 400 to 1,300 µS/cm at Tongue River at Miles City. Daily mean SC at all mainstem sites during March-October 2005 was less than the State of Montana irrigation-season SC sample standard of 1,500 µS/cm.

Figure 6. Daily mean specific conductance (SC) for Tongue River mainstem sites, Montana and Wyoming, during March-October 2005. Daily mean values are not shown for days of no flow or equipment problems. The State of Montana irrigation-season SC sample standard for the Tongue River mainstem (Montana Department of Environmental Quality, 2003) is included on the graph for reference.

Daily mean SC values also were calculated for the five tributary sites in 2005 (fig. 7). Goose Creek near Acme typically had the lowest SC and the least fluctuation in daily means; values ranged from about 100 to 800 µS/cm. Daily mean SC at Hanging Woman, and Otter Creeks always exceeded 1,000 µS/cm. The highest SC in 2005 was recorded in Otter Creek at Ashland, with a maximum daily mean of about 3,200 µS/cm. The SC for Otter Creek in 2004 was similar in magnitude, but fluctuated less than in 2005. SC data are limited for Hanging Woman and Pumpkin Creeks in 2005 due to no flow for extended periods. SC data for Otter Creek in September are missing because of an equipment malfunction.

Daily mean SC during March-October 2005 at each tributary site commonly exceeded the State of Montana SC irrigation-season sample standard of 500 µS/cm, with the exception of Goose Creek on most days during early May to mid-July and Pumpkin Creek on several days during May, June, and October. In 2004, SC at Goose Creek exceeded the Montana irrigation-season sample standard of 500 µS/cm for most of the period.

Figure 7.   Daily mean specific conductance (SC) for Tongue River tributary sites, Montana and Wyoming, during March-October 2005. Daily mean values are not shown for days of equipment problems or no flow. The State of Montana irrigation-season SC sample standard for the Tongue River tributaries (Montana Department of Environmental Quality, 2003) is included on the graph for reference.

Monthly Mean Specific-Conductance Data

Monthly mean SC values were calculated from daily mean SC values. In the mainstem, monthly mean SC in 2005 (fig. 8) generally increased in the downstream direction during March-May. Monthly mean SC decreased substantially in June at all mainstem sites due to dilution by runoff from rain and snowmelt. As flow decreased in July, the monthly mean SC gradually increased or held steady throughout the remainder of the irrigation season. Mean monthly SC at Tongue River at State Line was nearly as high or higher than at the mainstem sites downstream from the Tongue River Reservoir during July-October.

At most sites in 2005, SC was more variable from month to month (fig. 8) than in 2004 possibly due to the diluting effect of peak flows, which were negligible in 2004. In 2005, the monthly mean SC at Tongue River at Miles City exceeded the irrigation-season monthly mean standard of 1,000 µS/cm (Montana Department of Environmental Quality, 2003) in March (1,010 µS/cm) and was near the standard in April (997 µS/cm). In 2004, mean monthly SC at Tongue River at Miles City exceeded the irrigation-season standard in May, July, August, and September.

Monthly mean SC varied substantially among tributary sites in 2005 (fig. 8). Data are missing for some of the months at tributary sites due to no flow. Monthly means commonly were higher than 2,000 µS/cm at Hanging Woman and Otter Creeks. Monthly means of SC at all tributary sites (fig. 8) exceeded the irrigation-season SC monthly mean standard of 500 µS/cm in every month (for which sufficient data existed to calculate a monthly mean) except at Goose Creek during May and June. In 2004, monthly mean SC at all tributary sites exceeded the irrigation-season SC monthly mean standard during the entire period.

Figure 8. Monthly mean specific conductance (SC) for Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Monthly means of SC are not shown for months in which fewer than nine daily mean values were available. The State of Montana irrigation-season SC monthly mean standards for the mainstem and tributaries (Montana Department of Environmental Quality, 2003) are included on the graph for reference.

Sodium Adsorption Ratio

 Sodium adsorption ratio (SAR) is a chemical property of water that provides a relative measure of the sodium content of water. It is calculated from the concentrations of calcium, magnesium, and sodium measured in samples. SAR is used to indicate the suitability of irrigation water for sustained soil and crop health. SAR laboratory values are available for all periodic water-quality samples, which were collected 8-15 times at each site during the irrigation season. Continuous data for estimated SAR are available for sites that had a suitable regression relation between SC and SAR. These relations are presented on the Estimating SAR page of this Web site.

Water-Sample Data

SAR values in samples collected from the Tongue River mainstem in 2005 (fig. 9) generally increased gradually in the downstream direction with a noticeable increase in the variability of SAR in the reach from Tongue River below Brandenberg Bridge to Miles City. SAR values generally were less than 1.5 at the upper mainstem sites ( Tongue River at Monarch to Birney Day School Bridge). SAR values for the lower mainstem sites (Tongue River below Brandenberg Bridge to Miles City) ranged from about 0.5-4.0. The irrigation-season SAR sample standard of 4.5 (Montana Department of Environmental Quality, 2003) was not exceeded in any sample from the mainstem. The SAR values and downstream patterns for mainstem sites in 2005 generally were similar to those in 2004.

AR values varied widely in samples collected from the tributaries in 2005 (fig. 9) and typically were slightly higher to much higher than values in the nearby mainstem sites. The lowest SAR values in the tributaries were measured in Goose Creek (equal to or less than 1). The highest SAR values were measured in Pumpkin Creek (range of about 5-15). The irrigation-season SAR sample standard of 4.5 was exceeded in all samples from Otter and Pumpkin Creeks, and in about one-half of the samples from Hanging Woman Creek. The irrigation-season SAR sample standard of 4.5 was not exceeded in any sample from Goose or Prairie Dog Creeks. In 2005, SAR values for samples collected from the tributaries were similar to values in 2004.

Figure 9. Laboratory sodium adsorption ratio (SAR) values for samples collected at Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Sites are plotted in a downstream direction from left to right. The Montana irrigation-season SAR sample standard for the Tongue River mainstem and tributaries (Montana Department of Environmental Quality, 2003) is included on the graph for reference. Boxplots are used to show the statistical distribution of the data.

Daily Mean Estimated Sodium Adsorption Ratio Data

Daily mean values of estimated SAR in 2005 are available for all seven mainstem sites (fig. 10). Estimated SAR generally increased in the downstream direction along the mainstem. Values ranged from about 0.2-0.4 at Tongue River at Monarch to about 0.7-3.0 at Tongue River at Miles City. One exception to the pattern of downstream increase is the Tongue River at State Line, where estimated SAR values exceeded the estimated SAR values at several downstream sites during July-September. This pattern is similar to that observed for SC.

Daily mean values of estimated SAR varied gradually at most mainstem sites, except for relatively rapid decreases during runoff in May and June. After June, the SAR gradually increased throughout the remainder of the season as flows decreased. Although a regression relation between SC and SAR has proven to be an acceptable method of estimating SAR at most sites, short-lived inflows to the mainstem from tributaries having substantially different SAR characteristics can cause considerable estimation errors. For example, occasional high inflows from Pumpkin Creek, with substantially higher SAR values than the mainstem, caused large discrepancies between measured and estimated SAR values in June at Tongue River at Miles City (fig. 10).

None of the daily mean values of estimated SAR for the seven mainstem sites exceeded the irrigation-season SAR sample standard of 4.5. Tongue River at State Line and above T&Y Diversion Dam have a period of missing record in May-June because high water washed out the equipment.

The downstream patterns and general range of values in 2005 were similar to those in 2004; however, a substantial decrease in SAR in May-June did not occur in 2004 due to the absence of large runoff events. Also, daily mean values of estimated SAR values at Tongue River at Miles City varied less in 2005 than in 2004.

Figure 10. Daily mean estimated sodium adsorption ratio (SAR) for Tongue River mainstem sites, Montana and Wyoming, during March-October 2005. Daily mean values are not shown for days of missing specific-conductance record. The State of Montana irrigation-season SAR sample standard for the Tongue River mainstem (Montana Department of Environmental Quality, 2003) is included on the graph for reference.

Daily means of estimated SAR in 2005 are available for four of the five tributary sites (fig. 11); estimated SAR is not available for Pumpkin Creek because the regression relation between SC and SAR is poorly defined. Daily means of estimated SAR varied widely among the tributaries. Estimated SAR at Goose Creek (range of 0.23-0.78) was similar to that at nearby mainstem sites, whereas estimated SAR at Prairie Dog Creek (range of 0.59-2.2) was slightly to moderately higher than that at Tongue River at State Line (range of 0.35-1.1). Estimated SAR was substantially higher in Hanging Woman and Otter Creeks than in the mainstem. Daily means of estimated SAR ranged from 4.0 to 5.2 at Hanging Woman Creek and from 4.8 to 6.8 at Otter Creek during the periods of available record.

Daily mean estimated SAR exceeded the SAR individual sample standard of 4.5 at Otter Creek throughout the period of available record (fig.11). Daily mean estimated SAR in Hanging Woman Creek exceeded the standard for about one-half of the period of available record. In 2005, the daily means of estimated SAR for the four tributary sites had a pattern similar to 2004, although SAR daily means in Otter Creek in 2004 were much less variable than in 2005.

Figure 11. Daily mean estimated sodium adsorption ratio (SAR) for Tongue River tributary sites, Montana and Wyoming, during March-October 2005. Daily mean values are not shown for days of missing specific-conductance record. The State of Montana irrigation-season SAR individual sample standard for the Tongue River tributaries (Montana Department of Environmental Quality, 2003) is included on the graph for reference.

Monthly Mean Estimated Sodium Adsorption Ratio Data

Monthly mean SAR values were calculated from daily mean SAR values. At the seven mainstem sites, monthly mean estimated SAR in 2005 (fig. 12) increased in a downstream direction during March-May. Monthly mean estimated SAR at mainstem sites decreased slightly in May and substantially in June, presumably due to the diluting effects of increased runoff. As flows decreased, SAR values gradually increased or held steady at mainstem sites throughout the remainder of the irrigation-season. Along the mainstem, none of the monthly means for estimated SAR (fig.12) exceeded the SAR irrigation-season monthly mean standard of 3.0 (Montana Department of Environmental Quality, 2003). Monthly means of estimated SAR at the mainstem sites in 2005 generally were slightly lower than in 2004, presumably due to a substantially larger spring runoff in 2005.

Monthly mean estimated SAR in 2005 varied widely among the four tributaries (fig. 12). The variation in monthly means of estimated SAR was minimal at Goose and Prairie Dog Creeks, with the exception of a moderately large increase in July at Prairie Dog Creek. At Goose and Prairie Dog Creeks, monthly mean estimated SAR decreased during May-June similar to the decreases in the mainstem; these decreases occurred on a smaller scale at Hanging Woman Creek and did not occur at Otter Creek. Month-to-month variation of estimated SAR generally was small at Hanging Woman Creek and moderate at Otter Creek.

Monthly means of estimated SAR at Goose and Prairie Dog Creeks did not exceed the State of Montana SAR irrigation-season monthly mean standard of 3.0, whereas all available monthly means exceeded the standard at both Hanging Woman and Otter Creeks. During August-October, there was no flow at Hanging Woman Creek; therefore, monthly means are not available. During September, equipment malfunctioned at Otter Creek; thus, monthly means were not calculated because fewer than nine days of record were available.

In 2005, the general pattern of monthly mean estimated SAR at the tributary sites was similar to that in 2004. Monthly means of estimated SAR at Goose and Prairie Dog Creeks did not exceed the State of Montana SAR irrigation-season monthly mean standard of 3.0 in 2004, whereas, all monthly means exceeded the standard at both Hanging Woman and Otter Creeks.

Figure 12. Monthly mean estimated sodium adsorption ratio (SAR) for Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Values are not shown for months for which fewer than nine daily mean values were available. The State of Montana irrigation-season SAR monthly mean standard (Montana Department of Environmental Quality, 2003) is included on the graph for reference.

Nutrients

Nutrients are nitrogen and phosphorus compounds that stimulate plant growth. Excessive nutrient enrichment may lead to accelerated growth of algae and other aquatic plants that may accumulate to nuisance levels and impair sensitive biota through either degradation of streambed habitat or depletion of dissolved oxygen as organic matter decomposes. A complete list of nutrient compounds analyzed is provided on the Project Plan page of this Web site. Data for total nitrogen and total phosphorus, which are measures of all the combined dissolved and suspended species of nitrogen and phosphorus, respectively, are summarized for samples collected during March-October 2005 to provide a general indication of spatial variations in nutrient concentrations and to compare values to recommended guideline concentrations.

Nutrient samples were collected 2-10 times at each site during the irrigation season. The sampling frequency for nutrients was reduced in 2005 to 4 samples during the irrigation season, with the exception of 10 samples at Tongue River above T&Y Diversion Dam. Only two samples at Hanging Woman Creek and three samples at Pumpkin Creek were collected due to extended periods of no flow. Because of the small number of samples at most sites, the description of nutrient concentrations is somewhat limited in the range of conditions characterized.

Total nitrogen

Total nitrogen concentrations in samples collected during March-October 2005 (fig. 13) varied to a moderate degree at most mainstem and tributary sites, with the exception of Tongue River at Miles City and Pumpkin Creek, which had a wider range and considerably higher concentrations relative to the other sites. The variability in median concentrations was relatively minor among all mainstem sites, except Tongue River at Miles City, where the median (1.58 mg/L) was about 1.5 times higher than medians at the upstream sites. The larger median concentration could be attributed to the high total nitrogen concentrations in water from Pumpkin Creek.

Generally, median total nitrogen concentrations in the tributaries were slightly higher than those in the mainstem and were less than 1 mg/L, with the exception of Pumpkin Creek (3.86 mg/L). Pumpkin Creek also had the highest total nitrogen concentration (4.79 mg/L) of all the network sites. The eutrophication guideline concentration of 1.50 mg/L was exceeded in one or more samples from Tongue River at State Line, Pumpkin Creek, and Tongue River at Miles City (fig.13).

In 2005, total nitrogen concentrations at each site were more variable than in 2004, with the exception of Pumpkin Creek. The eutrophication guideline concentration of 1.50 mg/L was exceeded only at Pumpkin Creek in 2004.

Figure 13. Total nitrogen concentrations analyzed in samples collected at Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Sites are plotted in a downstream direction from left to right. A recommended guideline concentration to prevent eutrophication (U.S. Environmental Protection Agency, 2001) is plotted on the graph for reference. Boxplots are used to show the statistical distribution of the data. For sites with three or fewer samples, sample concentrations are plotted directly.

Total phosphorous

Total phosphorus concentrations in samples collected in 2005 varied considerably at most sites. One notable exception occurred in samples collected at Tongue River at Tongue River Dam, which also had distinctly lower concentrations than the upstream site Tongue River at State Line (fig. 14). The decrease in total phosphorus concentrations immediately below the Tongue River Reservoir was similar to that observed for total nitrogen. Concentrations at mainstem sites increased in variabililty downstream from Tongue River at Tongue River Dam, although median concentrations did not change appreciably until Tongue River at Miles City. Median concentrations at mainstem sites were less than 0.05 mg/L, except for Tongue River at Miles City (about 0.6 mg/L). Median concentrations in the tributaries were higher than at nearby mainstem sites; all were less than 0.2 mg/L, except for Pumpkin Creek (1.18 mg/L). Most samples from Pumpkin Creek and Tongue River at Miles City had higher total phosphorus concentrations relative to other sites. The maximum concentration (2.74 mg/L) in the network was measured at Tongue River at Miles City. Only two samples at Hanging Woman Creek and three samples at Pumpkin Creek were collected, due to extended periods of no flow.

The USEPA eutrophication guideline of 0.075 mg/L was exceeded in one or more samples from all sites except for Tongue River at Tongue River Dam and Hanging Woman Creek. Almost all samples from Goose, Otter, and Pumpkin Creeks and Tongue River at Miles City exceeded the guideline.

Total phosphorus concentrations in 2005 had similar patterns as in 2004, although there were several noteworthy differences. Tongue River at State Line and Tongue River below Brandenberg Bridge had larger ranges of total phosphorus concentrations in 2005 and concentrations exceeded the eutrophication guideline in one or more samples. In contrast, no concentrations from samples collected at these two sites exceeded the guideline in 2004. Concentrations at Tongue River at Miles City also were higher in 2005 compared to concentrations in 2004; the median concentration in 2005 was more than ten times higher than the median concentration in 2004. Some of the higher concentrations in 2005 could be due to substantial runoff and associated increases in the transport of suspended sediment and adsorbed nutrients.

Figure 14. Total phosphorus concentrations analyzed in samples collected at Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Sites are plotted in a downstream direction from left to right. A recommended guideline concentration for the prevention of eutrophication (U.S. Environmental Protection Agency, 2001) is plotted on the graph for reference. Boxplots are used to show the statistical distribution of the data. (Note: total phosphorus concentrations are plotted on a logarithmic scale.) For sites with three or fewer samples, sample concentrations are plotted directly.

Suspended Sediment

Suspended sediment is the particulate material suspended in water. The sediment particles in suspension are derived from erosion of soil or stream channel materials, which are transported through the watershed at rates that vary with streamflow and sediment supply. Although standards do not exist for suspended sediment, the concentrations measured at network sites can indicate relative differences in the amount of overland or channel erosion from upstream sources. Some chemical constituents tend to adsorb to sediment particles; consequently, their total-recoverable concentrations commonly vary with the concentration of suspended sediment. Suspended-sediment samples were collected 8-15 times in 2005 at each site.

Suspended-sediment concentrations in samples collected in 2005 at the seven mainstem sites (fig. 15) generally were less than 100 mg/L, although nearly one-half of the samples from Tongue River at Miles City were higher than 100 mg/L. A notable decrease in concentration occurred at Tongue River at Tongue River Dam, which presumably resulted from sediment deposition in Tongue River Reservoir. Suspended-sediment concentrations in the mainstem increased gradually downstream from the dam. Concentrations in the tributaries generally were similar to or higher than concentrations at nearby mainstem sites. The highest suspended-sediment concentrations commonly were measured in Pumpkin Creek, which had a median concentration of about 7,890 mg/L and a maximum concentration of 20,100 mg/L. The relatively high concentrations in Pumpkin Creek may partly be due to the predominance of samples collected during runoff conditions although sediment concentrations throughout the range of discharge at this site were higher than at most other sites.

The general pattern of suspended-sediment concentrations among mainstem and tributary sites was similar for both 2005 and 2004. Although maximum concentrations in 2005 were somewhat higher at most sites, median concentrations were not markedly different between the two years.

Figure 15. Suspended-sediment concentrations in samples collected at Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Sites are plotted in a downstream direction from left to right. Boxplots are used to show the statistical distribution of the data. (Note: suspended-sediment concentrations are plotted on a logarithmic scale.)

Trace Elements

Trace-element samples were collected 2-10 times in 2005 at each site during the irrigation season. The sampling frequency for trace elements was reduced in 2005 to 4 samples during the irrigation season, with the exception of 10 samples at Tongue River above T&Y Diversion Dam. Only two samples at Hanging Woman Creek and three samples at Pumpkin Creek were collected due to extended periods of no flow. A complete list of trace elements analyzed is provided on the Project Plan page of this web site. The summary of trace-element data for March-October 2005 is intended to describe general patterns in the concentration data. Because of the small number of samples collected at most sites, the description of trace-element concentrations is somewhat limited in the range of conditions characterized.

One set of samples collected during summer 2005 was analyzed for mercury using a low-level method. This method provides results of concentrations in the nanogram per liter (ng/L) range. As more samples are collected and analyzed by this low-level method, the magnitude and spatial variations of mercury in the basin can be better characterized.

Of the 17 trace elements analyzed, concentrations of most were commonly less than State of Montana standards for aquatic life or human health; many concentrations were less than minimum reporting levels. Several elements (aluminum, arsenic, barium, beryllium, cadmium, chromium, mercury, nickel, selenium, and zinc) typically were much lower than standards at most sites.

Total-recoverable iron concentrations occasionally exceeded the State of Montana chronic aquatic-life standard at most mainstem and tributary sites. These concentrations typically were exceeded during runoff conditions when suspended-sediment concentrations increased. Total-recoverable copper and lead infrequently exceeded State of Montana aquatic-life chronic standards during runoff conditions at several mainstem sites upstream from Pumpkin Creek. Trace-element concentrations exceeded State of Montana aquatic-life standards more frequently at Pumpkin Creek than at any other site, with either chronic or acute standards being exceeded in one or more samples for arsenic, beryllium, cadmium, chromium, copper, iron, lead, nickel, and zinc. The elevated trace-element concentrations in Pumpkin Creek likely contributed to increased concentrations in the mainstem at Tongue River at Miles City, where trace-element concentrations exceeded standards more frequently than at mainstem sites upstream from Pumpkin Creek.

Several spatial patterns are evident from the concentration data for a few selected trace elements. These general atterns are shown for total-recoverable concentrations of copper, lead, and zinc (fig. 16) and dissolved concentrations of arsenic, copper, and selenium (fig. 17). The most notable overall pattern is that trace-element concentrations in each tributary generally were higher than concentrations in the mainstem upstream from the confluence with the tributary.

Total-recoverable copper, lead, and zinc concentrations at most mainstem sites tended to increase gradually in the downstream direction (fig. 16). However, concentrations generally decreased at Tongue River at Tongue River Dam, which could be a result of deposition of suspended sediment and adsorbed trace elements in the reservoir. The general downstream increase in total-recoverable concentrations in the mainstem most likely results from inflow of tributaries with higher total-recoverable trace-element concentrations. The pattern of high total-recoverable concentrations at Pumpkin Creek also is consistent with the high suspended-sediment concentrations measured at this site (fig. 15).

Along the mainstem, no consistent spatial pattern is discernable in dissolved arsenic, copper, and selenium concentrations (fig. 17). The decrease in total-recoverable concentrations of copper, lead, and zinc noted for Tongue River at Tongue River Dam (fig. 16) was not evident for dissolved arsenic concentrations. Dissolved concentrations of arsenic, copper, and selenium in the tributaries generally were higher than nearby mainstem sites.

The spatial pattern of the selected trace-element concentrations among sites in 2005 was generally similar to that of 2004 (Fig. 1, Fig. 2). Although maximum concentrations for most trace elements in 2005 were somewhat higher than in 2004 at most sites, median concentrations at most were not markedly different between the two years. One exception was Tongue River at Miles City, where median concentrations of the total-recoverable copper, lead, and zinc were notably higher in 2005 than in 2004.

Figure 16. Total-recoverable concentrations of selected trace elements for samples collected at Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Sites are plotted in the downstream direction from left to right. Boxplots are used to show the statistical distribution of the data. (Note: trace-element concentrations are plotted on a logarithmic scale.) For sites with three or fewer samples, sample concentrations are plotted directly. Concentrations reported as less than the laboratory reporting level are plotted as one-half the laboratory reporting level.

Figure 17. Dissolved concentrations of selected trace elements for samples collected at Tongue River mainstem and tributary sites, Montana and Wyoming, during March-October 2005. Sites are plotted in the downstream direction from left to right. Boxplots are used to show the statistical distribution of the data. For sites with three or fewer samples, sample concentrations are plotted directly. Concentrations reported as less than the laboratory reporting level are plotted as one-half the laboratory reporting level.

References

U.S. Environmental Protection Agency, 2001, Ambient water quality criteria recommendations – rivers and streams in nutrient ecoregion 4: EPA 822-B-01-013, 30 p.

Montana Department of Environmental Quality, 2003, Numeric standards for electrical conductivity (EC) and sodium absorption ratio (SAR), Rule 17.30.670, Administrative rules of Montana, chapter 30, sub-chapter 6, p. 17-2757 to 17-2758; accessible online at http://deq.state.mt.us/dir/legal/Chapters/CH30-06.pdf.

Montana Department of Environmental Quality, 2006, Montana numeric water quality standards: Circular DEQ-7, 40 p., Administrative Rules of Montana 17.30.619; accessible online at http://www.deq.state.mt.us/wqinfo/Standards/CompiledDEQ-7.pdf.

Wyoming Department of Environmental Quality, 2001, Water quality rules and regulations, Chapter 1, Wyoming surface water quality standards, 25 p., appendices A-G; accessible online at http://deq.state.wy.us/wqd/WQDrules/Chapter_01.pdf.

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