Alder Sweep To Measure Cargo Deposits
By Peter Passi
Duluth News Tribune
Published October 13, 2006
Using sophisticated electronics, the U.S. Coast Guard cutter Alder is mapping portions of Lake Superior 300 to 500 feet below the surface.
The sonar images collected by researchers aboard the Alder will be used to help assess the effects of cargo sweeping on the Great Lakes. Cargo sweeping is the practice of cleaning — usually spraying out — a ship’s holds to prevent cross-contamination of materials.
The overboard cargo adds up. Each year, about 2 million pounds of residual cargo winds up in the Great Lakes, according to federal estimates.
A U.S. Department of Transportation study found that American-flagged carriers deposited about 548 tons of cargo in the Great Lakes as a result of sweeping during the 2004-05 season.
To learn about the accumulated effects of cargo sweeping, the Alder was equipped with a downrigger on Sept. 19. Suspended below this downrigger was a 5-foot-long torpedo-shaped “tow-fish” that used sonar to map the contours of the lake bottom. Researchers tried to maintain a cushion of about 100 feet between the “flying” probe and the lakebed at all times.
In addition to mapping Lake Superior’s bottom, the sonar also provides information about the density of material found there. The stronger the return signal from the bottom, the denser the material found there generally is, said Jamie Maughan, project manager for CH2MHill, a Denver-based engineering and environmental consulting firm retained to collect data for the study.
Maughan said researchers probably will devote special attention to areas with the strongest sonic rebounds, as these may be surfaces littered with materials such as taconite pellets, limestone or coal. Together, the three cargoes account for all but an estimated 1 percent of the cargo washed into the lakes through sweeping.
The Alder, based in Duluth, has proven a valuable tool for researchers because of its sophisticated global-positioning equipment and its ability to accurately hold station even in rough water. A map of the lake’s bottom emerged strip by strip, as the Alder made multiple passes over study areas.
“We’re the perfect platform,” said Lt.j.g. Kenny Pepper, an officer aboard the Alder. “For us, it’s pretty much like mowing a lawn.”
The Alder focused on two mile-wide sections of the lake’s bottom:
r The first area, about 19 nautical miles northeast of Duluth along the established trackline for lakers outbound from the Twin Ports, measured 20 nautical miles.
r The second study area, about 25 nautical miles east of Silver Bay along the established trackline for vessels outbound from that port, stretched 17 nautical miles.
Coast Guard Lt. Cmdr. Mary Sohlberg said similar sonar research will explore parts of Lake Michigan and Lake Erie this year.
In April, researchers will return to collect 2-foot-deep soil samples from the test sites.
These soils will be tested for toxicity, forms of aquatic life and nutrients that could fuel algae growth. The samples will be compared with soil from less-disturbed areas.
Sohlberg said scientists plan to take a snapshot not only of the current lake bottom but to model what it will look like in the future, if cargo sweeping practices remain unchanged. She hopes to have a draft environmental impact statement completed by the fall of next year. The document will be subject to scientific peer review.
Environmentalists concerned about the effects of sweeping have called for greater regulation study of the issue.
The concept of a study has been embraced by many industrial interests, too.
“We’re cooperating fully,” said Glen Nekvasil, vice president of corporate communications for the Lake Carriers Association, a Cleveland-based trade organization representing the operators of American-flagged lakers. “We believe this study is important because future decisions need to be based on good, hard scientific data.”
Of course, Nekvasil added that carriers are optimistic the study will verify that existing policies governing sweeping are “environmentally sound.”
Vessels are authorized to sweep their holds only in certain portions of the Great Lakes, mostly in deep water far from shore.