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Nitrogen and Phosphorus in Streams in Part of the Upper Mississippi River Basin, Minnesota and Wisconsin, 1984-93

By Kroening, S.E.

Stream-water nitrogen- and phosphorus-concentration data from part of the Upper Mississippi River Basin were compiled and analyzed as part of a retrospective analysis of existing data for the Upper Mississippi River Basin study unit of the U.S. Geological Survey's National Water-Quality Assessment Program. This data analysis focused on data collected from water years 1984 through 1993 in a study area that encompassed 19,500 square miles in the eastern part of the study unit. The study area included the Mississippi River Basin from Royalton, Minnesota, to the outlet of Lake Pepin; the Minnesota River Basin from Jordan, Minnesota, to the confluence with the Mississippi River; and the entire drainage basins of the St. Croix, Cannon, and Vermillion Rivers. Sources of data were ambient stream-monitoring programs maintained by the Metropolitan Council Environmental Services, Minnesota Pollution Control Agency, U.S. Geological Survey, and the Wisconsin Department of Natural Resources.

Total nitrite plus nitrate nitrogen, total organic plus ammonia nitrogen, total nitrogen, and total phosphorus concentrations were significantly larger in the Minnesota, Straight, and Cannon Rivers, which drain areas with a large percentage of agricultural land, than in the St. Croix, Namekagon, and Kettle Rivers, which drain areas with a larger percentage of forested land. In the Mississippi River, nitrogen and phosphorus concentrations varied in the downstream direction. Mean concentrations of nitrate and total nitrogen were significantly larger downstream of the confluence with the Minnesota River than upstream. Mean total phosphorus concentrations were significantly larger downstream of wastewater discharges from Newport, Minnesota, to Lock and Dam 3 than mean concentrations upstream of the confluence with the Minnesota River.

INTRODUCTION

In 1991, the U.S. Geological Survey (USGS) began full implementation of the National Water-Quality Assessment (NAWQA) Program. Long-term goals of the NAWQA Program are to describe the status of and trends in the quality of the Nation's water resources, and to identify the major natural and anthropogenic factors that affect the quality of these resources. To meet these goals, nationally-consistent data useful to policy makers, scientists and managers are being collected and analyzed. Because assessment of the water quality in the entire Nation is impractical, major activities of the NAWQA Program take place within a set of hydrologic systems called study units. Study units comprise diverse hydrologic systems of river basins, aquifer systems, or both, and are assessed on a decadal cycle.

One of the first components of each NAWQA study-unit investigation is an analysis of existing water-quality data (retrospective analysis). Results of the retrospective analyses are used to provide a historical perspective of the water quality, assess the strengths and weakness of the existing information, and set initial priorities for water-quality sampling in the study unit.

The Upper Mississippi River Basin study unit (fig. 1) encompasses an area of about 47,000 mi2 (square miles), and includes the drainage of the Mississippi River from its source to the outlet of Lake Pepin, and the entire Minnesota, St. Croix, Cannon, and Vermillion River Basins. Water quality in the study unit is of concern due to reliance on good quality water by major municipalities, power plants, and other water users in the basin.

Within the study unit, most analyses of existing data were focused on a smaller study area, encompassing 19,500 mi2 in the eastern portion of the study unit (fig. 1) for this cycle of the NAWQA Program, which lasts from 1994 through 1999. This more detailed assessment was done to examine the effects of the Twin Cities metropolitan area (TCMA) on the water quality in the study unit. The smaller study area includes the part of the Mississippi River Basin from Royalton, Minnesota, to the outlet of Lake Pepin; the Minnesota River Basin from Jordan, Minnesota, to the confluence with the Mississippi River; and the entire drainage basins of the St. Croix, Cannon, and Vermillion Rivers.

Within the study area, the Mississippi River derives most of its discharge from its drainage upstream of the TCMA and its two major tributaries, the Minnesota and St. Croix Rivers (fig. 1). These tributaries respectively contribute, on average, 22 and 26 percent of the mean annual flow of the Mississippi River at Prescott, Wisconsin (18,600 cubic feet per second) [1]. Runoff in the study area varies spatially and temporally, but most of the annual runoff occurs in spring and early summer from snowmelt and from rainfall on nearly saturated soils [1].

Agriculture is the dominant land cover in the western and southern parts of the study area [1]. Corn, soybeans, and hay are the principal crops grown [1]. In contrast, mixed deciduous and coniferous forests, and wetlands are the primary land covers in the northeastern part of the study area [1]. The northern and central parts of the study area are transitional between forest and agriculture [1]. Cropland in these parts of the study area is limited due to rocky, marginally-fertile soils; hay and oats are the principal crops grown, primarily to feed livestock, mainly dairy cows [1].

Urban land use and land cover (including suburban land) is concentrated primarily in the TCMA (fig. 1). Data obtained from the U.S. Environmental Protection Agency's (USEPA) Permit Compliance System [2, 3] and the Metropolitan Council Environmental Services (MCES) [4] show that approximately 200 municipal wastewater treatment plants are permitted to discharge into streams in the study area. The largest single discharge to the river system within the study area is the Metropolitan Wastewater Treatment Plant (MWTP) [2, 4] which is located on the Mississippi River downstream from St. Paul, Minnesota (fig. 1). The MWTP treats municipal and industrial wastewater from the TCMA. Data obtained from the MCES [4] shows that the MWTP in 1993 had an average annual discharge rate of approximately 230 million gallons per day (MGD), approximately two percent of the mean annual discharge of the Mississippi River at Prescott, Wisconsin. Other wastewater treatment plants in the TCMA include the Empire Wastewater Treatment Plant (EWTP), located on the Vermillion River near Empire, Minnesota, and the Blue Lake (BLWTP) and Seneca (SWTP) plants, located on the Minnesota River between Jordan, Minnesota and the confluence with the Mississippi River. Data obtained from the MCES [4] show that the EWTP, BLWTP, and SWTP in 1993 had average annual discharge rates of approximately 8 MGD, 24 MGD, and 24 MGD, respectively.

SOURCES OF STREAM-WATER DATA AND METHODS OF DATA REVIEW

The data analyzed in this project were obtained from the Conventional River-Pollutant Monitoring Network of the MCES, the Ambient Stream-Water Monitoring Network of the Minnesota Pollution Control Agency (MPCA), the National Stream Quality Accounting Network of the USGS, and the Ambient Stream-Water-Quality Monitoring Network of the Wisconsin Department of Natural Resources (WDNR). Data were obtained for the following 10 streams in the study area: the Mississippi, Minnesota, St. Croix, Namekagon, Kettle, Rum, Sauk, Vermillion, Straight, and Cannon Rivers (fig. 1). The Namekagon River and the headwaters of the St. Croix River drain forested areas in the northeastern part of the study area. In contrast, the Minnesota, Vermillion, Straight, and Cannon Rivers drain areas in the southern and western parts of the study area with primarily agricultural land cover. The Sauk, Rum, and Kettle Rivers and the lower part of the St. Croix River drain the northern and central parts of the study area where the land cover is a mixture of forest and cropland.

The nitrogen and phosphorus compounds for which analyses were done during this project were total nitrite plus nitrate nitrogen, total ammonia nitrogen, total organic plus ammonia nitrogen, total nitrogen, total phosphorus, and dissolved orthophosphate. At sites sampled by the MCES, total nitrite plus nitrate nitrogen concentrations were estimated by summing total nitrate and total nitrite concentrations. Total nitrogen concentrations were similarly estimated for sites sampled by the MCES, by summing total nitrate, total nitrite, and total organic plus ammonia nitrogen concentrations. Total nitrite plus nitrate nitrogen will be referred to as nitrate in the remainder of this paper because total nitrite nitrogen concentrations were small relative to total nitrate nitrogen concentrations and most nitrate nitrogen in streams generally is in the dissolved form. Dissolved orthophosphate data was only available for the Mississippi, Minnesota, St. Croix, Namekagon, and Vermillion Rivers. An analysis of data from water years 1984 through 1993 was chosen because this time period was common to the majority of the sampling sites included in this report. Data analyzed from each site had at least a five-year period of record during the analysis period. Only samples collected by the MPCA during January, April, July, and October of each year were used in the assessment to eliminate biases due to the MPCA's sampling frequency, which focused on the ice-free season from March through October.

Data collected from two different agencies at sampling sites located within one mile of each other were combined because signed-rank [5] and Wilcoxon rank-sum tests [5] indicated there was no significant difference at the 0.05 significance level either in the values reported, or in the average concentrations during the analysis period. Because of these results, data from such sampling sites were presented together graphically as truncated boxplots [5].

A one-way analysis of variance (ANOVA) [5] and Tukey multiple comparison [5] were used to test differences in mean constituent concentrations between sites at the 0.05 significance level. Censored values were handled by substituting one-half the reporting limit for each censored value [5] because the amount of censored data was small, less than 25 percent for all constituents except dissolved orthophosphate in the Mississippi River near Royalton, Minnesota.

PRESENCE AND DISTRIBUTION OF NITROGEN

Within the study area, nitrate concentrations generally were less than the Maximum Contaminant Level (MCL) of 10 mg/L (milligrams per liter) as nitrogen established by the USEPA for drinking water [6]. However, concentrations exceeding the MCL were measured in some samples from the Minnesota, Vermillion, and Straight Rivers, and the Mississippi River at Nininger, Minnesota and at Lock and Dam 2. Exceedance of the MCL comprised less than 10 percent of the samples at each of these sites.

Nitrate concentrations (fig. 2) were largest in the streams draining agricultural areas and smallest in streams draining more forested areas. Mean concentrations were significantly larger in the Minnesota, Straight, Cannon, and Vermillion Rivers (2 to 6 mg/L) than mean concentrations in the Sauk, Rum, Kettle, Namekagon, and St. Croix Rivers (0.1 to 0.5 mg/L). Relatively large concentrations measured in the Vermillion River near Empire, Minnesota (fig. 2) were downstream of wastewater discharges.

In the Mississippi and St. Croix Rivers, nitrate concentrations increased significantly downstream. In the Mississippi River, mean concentrations were significantly larger downstream of the confluence with the Minnesota River (1.8 to 2.5 mg/L) than upstream (0.2 to 0.9 mg/L). In the St. Croix River, mean nitrate concentrations were significantly larger at the three sites on the lower 20 miles of the river (0.2 to 0.5 mg/L) than the mean concentration about 100 miles upstream near Danbury, Wisconsin (0.1 mg/L).

Total nitrogen concentrations also were largest in the streams draining agricultural areas, smallest in streams draining forested areas, and increased downstream in the Mississippi and St. Croix Rivers. Mean concentrations in the Minnesota, Straight, Cannon, and Vermillion Rivers (3 to 7 mg/L) were significantly larger than those in the St. Croix, Namekagon, and Kettle Rivers (0.7 to 1.0 mg/L). In the Mississippi River, mean concentrations were significantly larger downstream of the Minnesota River (3 to 4 mg/L) than upstream (1 to 2 mg/L). In the St. Croix River, concentrations were significantly larger near the confluence with the Mississippi River at Prescott, Wisconsin(1 mg/L) than about 100 miles upstream near Danbury, Wisconsin (0.7 mg/L).

Total organic plus ammonia nitrogen concentrations generally were largest in the Minnesota, Straight, and Cannon Rivers and the Mississippi River within the TCMA, and smallest in the streams draining forested areas and in the Vermillion River near Farmington, Minnesota. Mean concentrations in the Minnesota, Straight, Cannon, and Sauk Rivers, and in the Mississippi River within the TCMA (1.2 to 1.5 mg/L) were significantly larger than concentrations in the St. Croix, Namekagon, and Kettle Rivers, and in the Vermillion River near Farmington, Minnesota (0.3 to 0.7 mg/L).

The largest total ammonia nitrogen concentrations were measured in the TCMA, in the Mississippi and Minnesota Rivers downstream of wastewater discharges. Mean concentrations in the Mississippi River at Newport, Minnesota and Grey Cloud Island, Minnesota, and in the Minnesota River near the mouth (0.33 to 0.65 mg/L) were significantly larger than mean concentrations at most other sites (0.05 to 0.19 mg/L) except the Straight River near Clinton Falls, Minnesota (0.27 mg/L) and the Mississippi River at Nininger, Minnesota (0.30 mg/L).

PRESENCE AND DISTRIBUTION OF PHOSPHORUS

Similar to the results for nitrate, total phosphorus concentrations (fig. 3) were smallest in the streams draining forested areas and largest in the streams draining agricultural areas. Mean concentrations at sites on the Minnesota, Straight, Cannon, and Sauk Rivers (0.10 to 0.37 mg/L) were significantly larger than mean concentrations at sites in the St. Croix, Namekagon, and Kettle Rivers (0.02 to 0.07 mg/L). The large total phosphorus concentrations in the Vermillion River near Empire, Minnesota (fig. 3) were downstream of wastewater discharges.

In the Mississippi River, total phosphorus concentrations varied in the downstream direction. Mean concentrations increased significantly from 0.04 mg/L near Royalton, Minnesota to 0.10 mg/L at Lock and Dam 1. Downstream of wastewater discharges, mean concentrations were significantly larger from Newport, Minnesota, to Lock and Dam 3 (0.20 to 0.26 mg/L) than the mean concentration at Lock and Dam 1. Near the outlet of Lake Pepin, the mean concentration was smaller (0.19 mg/L) but was not significantly different from the mean concentrations measured immediately upstream.

Dissolved orthophosphate concentrations were largest in the Mississippi, Minnesota, and Vermillion Rivers downstream of wastewater discharges. In the Mississippi River, mean concentrations measured within the TCMA from Newport, Minnesota to Lock and Dam 2 (0.14 to 0.15 mg/L) were significantly larger than mean concentrations at all sites upstream of the confluence with the Minnesota River (0.01 to 0.06 mg/L). In the Minnesota River, the mean dissolved orthophosphate concentration was significantly larger downstream of wastewater discharges (0.16 mg/L) than near Jordan, Minnesota (0.09 mg/L). In the Vermillion River, concentrations were significantly larger downstream of wastewater discharges near Empire, Minnesota (0.79 mg/L) than near Farmington, Minnesota (0.06 mg/L).

SUMMARY AND CONCLUSIONS

As part of a retrospective analysis of existing water-quality data in the Upper Mississippi River Basin study unit of the National Water-Quality Assessment Program, the U.S. Geological Survey compiled and analyzed existing stream-water nitrogen and phosphorus concentration data collected from water years 1984 through 1993. The sources of the data were ambient stream-water monitoring programs maintained by the Metropolitan Council Environmental Services, Minnesota Pollution Control Agency, U.S. Geological Survey, and the Wisconsin Department of Natural Resources. Water-quality analyses focused on a 19,500-square-mile area in the eastern portion of the study unit and includes the drainage of the Mississippi River from Royalton, Minnesota, to the outlet of Lake Pepin; the Minnesota River drainage from Jordan, Minnesota, to the confluence with the Mississippi River; and the entire drainage basins of the St. Croix, Cannon, and Vermillion Rivers.

Analyses of the water-quality data showed concentrations of total ammonia nitrogen and dissolved orthophosphate generally were largest in the Mississippi and Minnesota Rivers within the Twin Cities metropolitan area. Nitrate, total nitrogen, and total phosphorus concentrations were larger in the Minnesota, Cannon, and Straight Rivers, which drain areas with a larger proportion of agricultural land, than in the St. Croix, Kettle, and Namekagon Rivers, which drain areas with a substantial amount of forested land. Relatively large nitrate, total nitrogen, total phosphorus, and dissolved orthophosphate concentrations in the Vermillion River were downstream of wastewater discharge. In the Mississippi River, nitrate, total nitrogen, and total phosphorus concentrations varied in the downstream direction. Nitrate and total nitrogen concentrations were larger downstream of the confluence with the Minnesota River than upstream. Total phosphorus concentrations were largest downstream of wastewater discharges from Newport, Minnesota, to Lock and Dam 3. Similarly, in the St. Croix River, nitrate and total nitrogen concentrations increased downstream. Concentrations of these two constituents were significantly larger at Prescott, Wisconsin, than about 100 miles upstream near Danbury, Wisconsin.

ACKNOWLEDGEMENTS

The author expresses appreciation to Terrie O' Dea and Lisa Dyste of the Metropolitan Council Environmental Services, Theresa Flom of the Minnesota Pollution Control Agency, and Arnie Leder of the U.S. Environmental Protection Agency for their assistance in compiling the data, and for their guidance during the preparation of this report.

REFERENCES

[1] Stark, J.R.; W.J. Andrews; J.D. Fallon; A.L. Fong; P.E. Hanson; K.E. Lee; S.E. Kroening; and R.M. Goldstein; 1996, "Water-quality assessment of the Upper Mississippi River Basin study unit, Minnesota and Wisconsin--Environmental Setting and Study Design", U.S. Geological Survey Water-Resources Investigations Report 96-4098, U.S. Geological Survey, Mounds View, Minnesota, 1996, 62 p.

[2] Flom, T., Minnesota Pollution Control Agency, written communication, 1994.

[3] Leder, A., U.S. Environmental Protection Agency, Region 5, written communication, 1995.

[4] Dyste, L., Metropolitan Council Environmental Services, electronic communication, 1996.

[5] Helsel, D.R., and R.M. Hirsch, Statistical methods in water resources, Elsevier Science Publishers, New York, USA, 1992, 522 p.

[6] U.S. Environmental Protection Agency, 1994, National primary drinking water standards, U.S. Environmental Protection Agency, EPA 801-F-94-001A, 4 p.