To demonstrate the value of long-term, water-quality monitoring, the Michigan Department of Environmental Quality (MDEQ), in cooperation with the U.S. Geological Survey (USGS), initiated a study to evaluate potential trends in water-quality constituents for selected National Stream Quality Accounting Network (NASQAN) stations in Michigan. The goal of this study is to assist the MDEQ in evaluating the effectiveness of water-pollution control efforts and the identification of water-quality concerns.
The study included a total of nine NASQAN stations in Michigan. Approximately 28 constituents were analyzed for trend tests. Station selection was based on data availability, land-use characteristics, and station priority for the MDEQ Water Chemistry Monitoring Project. Trend analyses were completed using the uncensored Seasonal Kendall Test in the computer program Estimate Trend (ESTREND), a software program for the detection of trends in water-quality data. The parameters chosen for the trend test had (1) at least a 5-year period of record (2) about 5 percent of the observations censored at a single reporting limit, and (3) 40 percent of the values within the beginning one-fifth and ending one-fifth of the selected period. In this study, a negative trend indicates a decrease in concentration of a particular constituent, which generally means an improvement in water quality; whereas a positive trend means an increase in concentration and possible degradation of water quality.
The results of the study show an overall improvement in water quality at the Clinton River at Mount Clemens, Manistee River at Manistee, and Pigeon River near Caseville. The detected trend for these stations show decreases in concentrations of various constituents such as nitrogen compounds, conductance, sulfate, fecal coliform bacteria, and fecal streptococci bacteria. The negative trend may indicate an overall improvement in agricultural practices, municipal and industrial wastewater-treatment processes, and effective regulations.
Phosphorus data for most of the study stations could not be analyzed because of the data limitations for trend tests. The only station with a significant negative trend in total phosphorus concentration is the Clinton River at Mount Clemens. However, scatter-plot analyses of phosphorus data indicate decreasing concentrations with time for most of the study stations.
Positive trends in concentration of nitrogen compounds were detected at the Kalamazoo River near Saugatuck and Muskegon River near Bridgeton. Positive trends in both fecal coliform and total fecal coliform were detected at the Tahquamenon River near Paradise. Various different point and nonpoint sources could produce such positive trends, but most commonly the increase in concentrations of nitrogen compounds and fecal coliform bacteria are associated with agricultural practices and sewage-plant discharges.
The constituent with the most numerous and geographically widespread significant trend is pH. The pH levels increased at six out of nine stations on all the major rivers in Michigan, with no negative trend at any station. The cause of pH increase is difficult to determine, as it could be related to a combination of anthropogenic activities and natural processes occurring simultaneously in the environment.
Trends in concentration of major ions, such as calcium, sodium, magnesium, sulfate, fluoride, chloride, and potassium, were detected at eight out of nine stations. A negative trend was detected only in sulfate and fluoride concentrations; a positive trend was detected only in calcium concentration. The major ions with the most widespread significant trends are sodium and chloride; three positive and two negative trends were detected for sodium, and three negative and two positive trends were detected for chloride. The negative trends in chloride concentrations outnumbered the positive trends. This result indicates a slight improvement in surface-water quality be
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USGS Numbered Series
Trends in surface-water quality at selected National Stream Quality Accounting Network (NASQAN) stations, in Michigan