Sources and transport of phosphorus in the Western Lake Michigan Drainages

Abstract

In 1991, the U.S. Geological Survey (USGS) began full implementation of the National Water-Quality Assessment (NAWQA) Program. The goals of the NAWQA program are to (1) provide a nationally consistent description of water-quality conditions for a large part of the Nation's water resources, (2) define long-term trends (or lack of trends) in water quality, and (3) identify, describe, and explain, as possible, the major factors that affect the observed water-quality conditions and trends (Hirsch and others, 1988). To fulfill the goals of the NAWQA program, the USGS plans to examine 60 areas (study units) across the United States on a rotational cycle. The first 20 of these study units began intensive investigations in 1991. One of these study units is the Western Lake Michigan Drainages (WMIC) (fig. 1). The WMIC study unit contains many individual drainage basins that empty into Green Bay or directly into Lake Michigan. These basins can be grouped into four main subunits based on land use: the Northern-Forested subunit (88 percent forest and 8 percent agriculture), the Oconto-Peshtigo subunit (71 percent forest and 25 percent agriculture), the Fox-WolfPensaukee subunit (35 percent forest and 53 percent agriculture), and the Direct Lake Michigan subunit (13 percent forest and 73 percent agriculture) (fig. 1 and table 1). Within these subunits, extensive water-quality data have been collected at eight sites. Six sites have large drainage basins: Escanaba (1) and Ford (2) River Basins in Upper Michigan, Menominee River Basin (3) in Wisconsin and Upper Michigan (primarily forest), and Fox River Basin (5) (mixed agriculture and forest), and Manitowoc (7) and Milwaukee (8) River Basins (primarily agriculture) in Wisconsin. Two sites in Wisconsin have small drainage basins with one primary land use: Popple River Basin (4, forest) and White Creek Basin (6, agriculture). These eight are referred to as rivers and their watersheds are referred to as basins. These reference are identified in figures 1, 2, and 3 and described in table 1 . In this report, the inputs of total phosphorus into the reference basins, subunits, and entire WMIC study unit are quantified and used in conjunction with the export rates from the reference to estimate the phosphorus transported from the four subunits and the entire WMIC study unit, as well as to determine the importance of point sources to the overall phosphorus export. Phosphorus is essential for algal and macrophyte growth in aquatic environments, but in sufficiently high concentrations, it can lead to excess algae and macrophytes. Phosphorus transport from runoff has been identified as a primary water-quality concern throughout the WMIC study unit, especially downstream from large agricultural areas of the study unit, such as Lake Winnebago and Green Bay. With the belief that point sources of phosphorus in the study unit are under control, much of the current effort in improving water quality focuses on quantifying and reducing nonpointsource loading. Nonpoint-source loading of phosphorus into is primarily a function of land-use practices and the types of surficial deposits (Clesceri and others, 1986; U.S. Environmental Protection Agency, 1980); therefore, the inputs and exports of phosphorus described in this report are discussed with respect to these characteristics. Forested land covers more than 50 percent of the study unit (fig. 1) and dominates the northern part, where deciduous forests grade to evergreen NorthernForested Subunit