CONTAMINANTS IN THE UPPER MISSISSIPPI RIVER:  HISTORIC TRENDS, PRESENT 
STATUS, AND EMERGING CONCERNS

James G. Wiener and Mark B. Sandheinrich

University of Wisconsin–La Crosse, River Studies Center, 1725 State Street, 
La Crosse, WI 54601

The Upper Mississippi River receives contaminants from agricultural, 
industrial, municipal, and residential sources, including potentially toxic 
metals, pesticides, synthetic industrial compounds, and many other chemicals.  
Contamination of the river with mercury and lead rapidly followed settlement 
of the basin by Caucasian immigrants in the mid 1800s.  Concentrations of 
contaminants from industrial sources and sewage treatments plants are 
generally greatest in reaches within and downstream of metropolitan urban 
areas.  Lake Pepin, a natural lake extending 75-110 km downstream from the 
Twin Cities metropolitan area (Minneapolis-St. Paul, MN), traps particles and 
associated contaminants, decreasing the transport of potentially harmful 
pollutants from the metropolitan area, the Minnesota River basin, and other 
upstream sources into the reach of River downstream from the lake.

Certain contaminants–including methylmercury, PCB, DDT and its degradation 
products–readily bioaccumulate and can biomagnify to harmful concentrations 
in organisms in upper trophic levels of aquatic food webs.  Contamination of 
the aquatic food web with PCB was the probable cause of a sudden decline in 
populations of mink (Mustela vison) on the Upper Mississippi River National 
Wildlife and Fish Refuge during 1959-1965.  Institutional responses to 
bioaccumulative contaminants have included reduction of point-source 
discharges and issuance of fish-consumption advice.  The federal Clean Water 
Act, passed in 1972, has effectively reduced discharges of certain industrial 
pollutants from point sources to the river, thereby decreasing the exposure 
of fish and wildlife to compounds, such as PCB and methylmercury.  The 
decline in PCB contamination of riverine fish after the ban on their 
production and related efforts to reduce the release of PCB into the 
environment was followed by the gradual recovery of mink populations in the 
Refuge.  Similarly, the abundance of fish-eating birds, such as bald eagles 
(Haliaeetus leucocephalus) and double-crested cormorants (Phalacrocorax 
auritus), along the river corridor has increased since the banning of DDT and 
other eggshell-thinning insecticides, paralleling national population trends 
for these species.

Some sources of contaminants, particularly nonpoint sources, are not subject 
to the regulatory provisions of the Clean Water Act.  Much of the basin is 
intensively cultivated for corn and soybean production, and tributary streams 
receive agricultural chemicals and their degradation products via surface 
runoff and groundwater flow, acting as point sources of agricultural 
chemicals to the main-stem river.  The Upper Mississippi River basin upstream 
of its confluence with the Missouri River contributes much or most of the 
load of many pesticides present in the Mississippi River, even though the 
flow of the Upper Mississippi at that location is only about one-fifth of 
that from the entire Mississippi River basin.

Hundreds of recently synthesized chemicals from multiple sources enter the 
river, and the behavior and potential ecotoxicological effects of many of 
these compounds in aquatic systems are largely unknown.  Emerging 
contaminants (e.g., perfluorochemicals, pharmaceuticals, personal care 
products) and recently discovered mechanisms of adverse biological effects 
(e.g., endocrine disruption) pose substantial continuing challenges for 
scientists and environmental managers concerned with the ecological health of 
this complex ecosystem.

Keywords:  bioaccumulation, ecotoxicology, emerging contaminants, 
reproductive effects, spatial patterns, temporal trends