State environmental officials from Minnesota, Michigan and Pennsylvania are challenging a decades-long practice that allows freighters to rinse coal, iron ore, limestone and other residues into Lake Superior and the rest of the Great Lakes.

"Minnesota solid waste rules prohibit the disposal of solid waste into waters of the state of Minnesota, including Lake Superior," wrote Paul Eger, assistant commissioner of the Minnesota Pollution Control Agency, in a letter [PDF] to the U.S. Coast Guard.

Federal law and an international treaty also forbid ships from discharging materials into Lake Superior, but Congress allowed a 15-year exception for dumping of "dry cargo residues" in 1993, a policy that the Coast Guard wants to make permanent.

Shippers routinely wash out their vessels to prevent cross-contamination of cargo. Ship owners and the Coast Guard say the waste sinks quickly and does not harm water quality. But Eger said coal has washed up on the Minnesota shore.

"Coal has been observed and collected by the Minnesota Pollution Control Agency on the shoreline beach of Minnesota Point, Duluth," he wrote. "This coal did not dissolve or dissipate in the waters of the lake, but instead floated and accumulated along the beach in Duluth."

Eger told me that the coal spill was in 2007. "We think a reasonable approach is to sweep the spill dry during loading and unloading. Most of the spills occur then," he said.

Coast Guard wants documentation but not a ban
The interim policy expires Sept. 8. The Coast Guard is proposing a rule that would ask shippers to document what they wash into the lake, but it would not prohibit the practice. The Coast Guard estimated that 2 million pounds of residue were hosed into the Great Lakes in 2001, the last year for which it provided data.

The Lakes Carriers Association, a U.S.-flag vessel trade group, supports making the 1993 exception permanent. Association president James H.I. Weakly said in a letter to the Coast Guard that the total amount of material washed into the lakes is "minute." He noted that 165 million tons of cargo is shipped on the lakes each year.

A consortium of environmental groups, including Alliance for the Great Lakes, Great Lakes United, Lake Ontario Waterkeeper and the National Wildlife Federation, noted that 2 million pounds of waste dumped every year since the 1930s add up.

"Given the sheer volume of cargo that has been dumped over this time frame and the likelihood that such sweepings would be concentrated in ports and shipping lanes, it is presumptuous to state that bottomlands in these areas suffer only 'minor, indirect adverse effects' without conclusive evidence to demonstrate such a statement," the groups wrote.

Recreational boaters are banned from dumping waste into the Great Lakes and subject to hefty fines, a fact noted by those seeking an end to the practice by shippers.

The dry cargo residue issue has not received the attention that ballast water dumping and its problems with invasive species has, but the hearings and public comments on the residue have been active in July as the deadline for finalizing the rule approaches. "We want to be on record in this opportunity for the rule to be tightened up," Eger said.