Barbara Coffin, Larkin McPhee: Documentary was carefully researched

Barbara Coffin, Larkin McPhee: Documentary was carefully researched

We appreciated David Mulla’s commentary ("’Troubled Waters’ tells only part of the water-quality story," Oct. 10) noting that our documentary tells a "powerful story about unintended pollution." However, Mulla was wrong in stating that the narrative is "marred with inaccuracies."

Mulla said it is inaccurate to claim that Midwestern corn and soybean fields are the main culprit contributing to the Gulf of Mexico hypoxia (the dead zone). His statement is baffling, since he cites the same research we used — a U.S. Geological Survey study stating that "corn and soybean cultivation is the largest contributor of nitrogen" (52 percent) to the Gulf of Mexico. Yes, there are other factors that affect the size of the dead zone from year to year, but there is no question that nitrogen runoff from agricultural lands is the primary driver.

Mulla also downplayed the damage caused by agricultural runoff to Minnesota’s lakes, by pointing out that many of the state’s lakes are impaired by mercury and not by excess nutrients and associated algal blooms. But the fact that we also have a problem with atmospheric contaminants such as mercury does not diminish the harm caused by farm runoff. Indeed, more than half of the 440 state lakes listed as impaired because of excess phosphorus are in the agricultural regions of southern Minnesota. Another 25 percent of these impaired lakes are in the greater Twin Cities area, demonstrating that urban runoff can be equally harmful to water quality. 

Another criticism was leveled at our statement about drinking-water contamination by nitrates. Municipalities throughout the Upper Midwest spend millions of dollars to remove nitrates from drinking water. In the film, we showcase the world’s largest nitrate removal plant, constructed to protect the drinking water of Des Moines, Iowa. Closer to home, the city of Hastings recently upgraded its water treatment plant for nitrate removal. Studies by USGS scientists have identified a large swath of the corn and soybean belt as a high-risk area for groundwater nitrate contamination.

The film briefly mentions the possibility that wetland drainage may have contributed to increased flood frequency, which Mulla dismissed as a "popular myth," saying that "catastrophic floods … were caused by extreme climatic events, not agricultural drainage." We don’t dispute that fact, nor does the film, but the effect of wetland drainage on less extreme events cannot be so easily dismissed. The U.S. Fish and Wildlife Service has documented wetland losses in excess of 85 percent in the intensively farmed counties of southern Minnesota. And many scientists contend that this loss of storage capacity has likely altered river flows.

Finally, Mulla said that the film incorrectly attributes the accelerated infilling of Lake Pepin to agricultural practices and speculates that increased rainfall is to blame for the sediment increase. However, a recent synthesis on sediment sources in the Minnesota River basin by a team of scientific experts (including Mulla) concludes that "most of the sediment entering Lake Pepin comes from the Minnesota River Basin and that the rate of sediment supply has increased by approximately an order of magnitude over the [last] 150 years. Widespread agricultural development over that time clearly plays a dominant role in this history, although changes in climate may have also contributed."

Mulla stated that "Minnesotans need to have a focused discussion on ways to improve water quality …" and that he "appreciates the filmmaker’s effort to start this important conversation." We accept this recognition but dispute the critique.

The film’s production was carefully vetted by a team of scientific experts. We stand by its accuracy, its presentation of a complex environmental problem and its message of hope.

Barbara Coffin is executive producer and Larkin McPhee is director, producer and writer of "Troubled Waters: A Mississippi River Story."

http://www.startribune.com/opinion/commentary/104821434.html?page=1&c=y