By Aldo V. Vecchia
Water-Resources Investigations Report 00-4019
Prepared in cooperation with North Dakota Department of Health
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The Souris River Basin is a 24,600-square-mile basin located in southeast Saskatchewan, north-central North Dakota, and southwest Manitoba. The Souris River Bilateral Water Quality Monitoring Group, formed in 1989 by the governments of Canada and the United States, is responsible for documenting trends in water quality in the Souris River and making recommendations for monitoring future water-quality conditions. This report presents results of a study conducted for the Bilateral Water Quality Monitoring Group by the U.S. Geological Survey, in cooperation with the North Dakota Department of Health, to analyze historic trends in water quality in the Souris River and to determine efficient sampling designs for monitoring future trends. U.S. Geological Survey and Environment Canada water-quality data collected during 1977-96 from four sites near the boundary crossings between Canada and the United States were included in the trend analysis.
A parametric time-series model was developed for detecting trends in historic constituent concentration data. The model can be applied to constituents that have at least 90 percent of observations above detection limits of the analyses, which, for the Souris River, includes most major ions and nutrients and many trace elements. The model can detect complex nonmonotonic trends in concentration in the presence of complex interannual and seasonal variability in daily discharge. A key feature of the model is its ability to handle highly irregular sampling intervals. For example, the intervals between concentration measurements may be be as short as 10 days to as long as several months, and the number of samples in any given year can range from zero to 36.
Results from the trend analysis for the Souris River indicated numerous trends in constituent concentration. The most significant trends at the two sites located near the upstream boundary crossing between Saskatchewan and North Dakota consisted of increases in concentrations of most major ions, dissolved boron, and dissolved arsenic during 1987-91 and decreases in concentrations of the same constituents during 1992-96. Significant trends at the two sites located near the downstream boundary crossing between North Dakota and Manitoba included increases in dissolved sodium, dissolved chloride, and total phosphorus during 1977-86, decreases in dissolved oxygen and dissolved boron and increases in total phosphorus and dissolved iron during 1987-91, and a decrease in total phosphorus during 1992-96.
The time-series model also was used to determine the sensitivity of various sampling designs for monitoring future water-quality trends in the Souris River. It was determined that at least two samples per year are required in each of three seasons--March through June, July through October, and November through February--to obtain reasonable sensitivity for detecting trends in each season. In addition, substantial improvements occurred in sensitivity for detecting trends by adding a third sample for major ions and trace elements in March through June, adding a third sample for nutrients in July through October, and adding a third sample for nutrients, trace elements, and dissolved oxygen in November through February.
Abstract
Introduction
Data used for water-quality trend analysis
Time-series model framework
Trend analysis results
Significant trends for Sherwood sites
Significant trends for Westhope sites
Sampling design considerations
Efficient sampling designs for Sherwood sites
Efficient sampling designs for Westhope sites
Summary
References
Appendix A: Statistical methods used to conduct water-quality trend analysis and sampling design
Appendix B: Water-quality data and time-series model output
Figures
1. Map showing location of Souris River Basin, Saskatchewan, North Dakota, and Manitoba, and locations of sites used in study
2. Graph showing recorded daily discharges and fitted low-frequency component, Souris River near Sherwood, North Dakota, 1977-96
3. Graph showing recorded dissolved sulfate concentrations and fitted low-frequency component showing discharge-related variability, Souris River south of Glen Ewen, Saskatchewan, and Souris River near Sherwood, North Dakota, 1977-96
4. Graph showing flow-adjusted dissolved sulfate concentrations and fitted trend, Souris River south of Glen Ewen, Saskatchewan, and Souris River near Sherwood, North Dakota, 1977-96
5. Graphs showing computed seasonal standard deviation for the high-frequency component of dissolved sulfate concentration, cross-correlation at a 0-day lag with the high-frequency component of discharge, and autocorrelation at a 30-day lag, Souris River south of Glen Ewen, Saskatchewan, and Souris River near Sherwood, North Dakota
6. Graph showing residuals from periodic autoregressive moving-average (PARMA) model fitted to high-frequency component of dissolved sulfate concentrations, Souris River south of Glen Ewen, Saskatchewan, and Souris River near Sherwood, North Dakota, 1977-96
7. Graphs showing sampling designs for hypothetical flow-adjusted concentrations and autocorrelation function used to generate data
8-20. Graphs showing trends in flow-adjusted concentrations, Souris River south of Glen Ewen, Saskatchewan, and Souris River near Sherwood, North Dakota, 1977-96:
8. Dissolved oxygen
9. Dissolved calcium
10. Dissolved sodium
11. Dissolved sulfate
12. Dissolved chloride
13. Dissolved solids
14. Total ammonia
15. Total nitrogen
16. Total phosphorus
17. Dissolved and total arsenic
18. Dissolved and total boron
19. Dissolved iron
20. Dissolved manganese
21-33. Graphs showing trends in flow-adjusted concentrations, Souris River near Westhope, North Dakota, and Souris River near Coulter, Manitoba, 1977-96:
21. Dissolved oxygen
22. Dissolved calcium
23. Dissolved sodium
24. Dissolved sulfate
25. Dissolved chloride
26. Dissolved solids
27. Total ammonia
28. Total nitrogen
29. Total phosphorus
30. Dissolved and total arsenic
31. Dissolved and total boron
32. Dissolved iron
33. Dissolved manganese
34. Graphs showing characteristic trends in dissolved oxygen, major ions, nutrients, and trace elements for different sampling designs, Souris River south of Glen Ewen, Saskatchewan, and Souris River near Sherwood, North Dakota
35. Graphs showing characteristic trends in dissolved oxygen, major ions, nutrients, and trace elements for different sampling designs, Souris River near Westhope, North Dakota, and Souris River near Coulter, Manitoba
Tables
1. Sites used for trend analysis
2. Summary of water-quality data for selected constituents, Souris River south of Glen Ewen, Saskatchewan, and Souris River near Sherwood, North Dakota
3. Summary of water-quality data for selected constituents, Souris River near Westhope, North Dakota, and Souris River near Coulter, Manitoba
4. Sampling schedules considered for trend analysis
5. Efficient sampling designs, Souris River south of Glen Ewen, Saskatchewan, and Souris River near Sherwood, North Dakota
6. Efficient sampling designs, Souris River near Westhope, North Dakota, and Souris River near Coulter, Manitoba
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