Wisconsin’s Fox River, which flows 37 miles from Lake Winnebago to Lake Michigan’s Green Bay, is one of North America’s best walleye fisheries. Some years, nearly 200,000 walleye are pulled from the river by anglers, and the sport generates about $6 million annually for the local economy. But even though they’re said to be delicious, Fox River walleye are almost never kept and eaten. “Most people won’t eat fish from the Fox River,” explains Fran Barbeau, owner of Fathead Fran’s Bait Shop in Green Bay. “It’s ingrained into their head.”
Fox River is contaminated with polychlorinated biphenyls (PCBs), persistent organic pollutants similar to dioxin that are linked to immune and circulatory problems, liver damage, increased cancer risk, and developmental problems in infants and children. PCBs were introduced to the Fox in the 1950s by paper mills making carbonless copy paper. The Department of Natural Resources (DNR) estimates that between 1954 and 1971 (when PCBs were banned) about 700,000 pounds of PCBs were dumped into the river and settled to the bottom, where they tend to bind with sediment. Now, 620 pounds of PCBs leach into the bay each year, contaminating fish and other aquatic life.
Some Wisconsin residents are apprehensive about dredging and think of it as “underwater rototilling” that stirs up muck … and PCBs. But in fact, even though more than 3.8 million cubic yards of sediment will eventually be sucked up from 500 acres of river bottom by crews working around the clock, dredging on this project is extremely precise. “People say that we do ‘surgical’ dredging,” explains James R. Dunkley, the survey systems manager for J.F. Brennan Co. Inc., the marine contractor in charge of dredging and capping operations on the Fox River project. Brennan, based in LaCrosse, Wis., has assigned three dredges to this project; one will keep sediment moving through a 12-inch pipeline, and two smaller dredges will keep an 8-inch line busy. The 8-inch dredge is prized for its ability to work in water as shallow as 2 feet, which limits the amount of work that has to be done, less efficiently, with cranes or other equipment. Each dredge is equipped with two Trimble R5 GNSS receivers, one to measure elevation and horizontal position, and one to compute heading.
The custom-built aluminum survey vessels are also equipped with Trimble R5 GNSS receivers, a decision that was made after the receivers were first used on a similar dredging project near Toledo, Ohio. “In Toledo, we were having serious problems with coverage,” Dunkley explains. “Tree canopy, tall buildings, high river banks--we had outages all the time. Switching to Trimble R5 GNSS receivers capable of tracking more satellites solved that immediately. We haven’t had one problem since switching to the new receivers; they’ve been phenomenal.”
Constant access to satellites, and position, is extremely important for a round-the-clock operation like the Fox River project. “Without accurate and constant survey information coming in, we wouldn’t be able to work as precisely,” Olander explains.
To keep up with all these demands, surveyors use hydrographic survey techniques to produce topographic maps of original and dredged river bottom, and compare the two to verify quantities and position. Control is based on a network of state plane coordinates provided by the DNR and verified and extended by Brennan surveyors. Accurate position provided by GNSS receivers is combined with data from depth-sounding equipment by computers running hydrographic software. After processing to calibrate information with satellite data and daily measurements of the speed of sound through water (see sidebar), the data is used to create maps.
The maps are also used to plan future work, as well as update river bottom maps that reside on the dredge’s onboard computers. Displays are color keyed to show topographic information and to compare existing and plan surfaces. “We can directly access the dredge computer’s hard drives from anywhere with an Internet connection, most importantly, for example, from our onsite office,” Olander explains, “which is state-of-the-art for this type of work.” In fact, dredge computers are updated wirelessly in real time--as soon as the information is available from surveyors, dredge operators see it on their screens. Since the dredges also have RTK capacity, operators always know where they are in relation to areas of concern. “It’s like they have a real-time working model of what’s going on out there,” Olander says, “and there’s even a correctly scaled image of the dredge right on the screen, which helps them be more efficient.”
“For example,” he continues, “if they’re cutting on a slope or contoured bottom, they can follow a contour and be more precise. We pride ourselves on the accuracy and efficiency of our dredging. Right now, with the GNSS RTK and this software, I’m confident we’re within six inches at all times, maybe within three or four inches. For this kind of work, that’s remarkable.”
Capping is essentially dredging in reverse, as the dredges are used to lay clean soil down over contaminated areas, sealing in PCBs and preventing leaching. Surveyors still have to measure daily to be sure that the correct depth of clean soil is being layered down.
The Fox River cleanup is happening at a time when the river is improving in other aspects, as well. Federal clean water laws have improved overall water quality tremendously, with dramatic results according to the DNR. In 1972, just three species of fish were living in the Fox River; now there are more than 40. Walleyes and spotted muskies--which can be more than 4 feet long and are highly prized by Midwest anglers--have made an amazing recovery, and there are currently 10 walleye tournaments held each year on the Fox.
All of this has also been good for the local economy. The project employs more than 100 locals, and contracts worth hundreds of millions are going to more than 20 local, state and regional firms.
Best of all, dangerous PCBs are being removed and stabilized so that they don’t find their way into animals and humans. This means healthier children, less cancer and liver damage in everyone, and a far healthier environment. So, even though the original contamination was a bad thing, the response is positive. Things are getting better.
Surveyors usually find their job satisfaction in things like accuracy and efficiency, in the way their work resolves boundary disputes or helps large infrastructure projects get built on time and on budget. The surveyors working on the Fox River remediation can feel these satisfactions--after all, they’re working on one of the world’s largest environmental projects. But they also experience an additional satisfaction: By keeping this remediation precisely on track, they’re making Green Bay and the Fox River safer and cleaner, and they’re protecting the health of many generations to come.
How Fast is Sound?The speed of sound is a constant, right, like the speed of light?
Well, no. The usual figure given, 768 mph (Mach 1), is actually the speed of a sound wave propagating in certain conditions: dry air at 68ºF. But sound travels about 3.3 times faster in liquids and about 15 times faster in solids. Moreover, the speed is affected by factors like temperature, pressure, salinity, and turbidity (suspended sediment).
Brennan survey crews use fathometers to precisely measure water depth. Since fathometer measurements depend on the timing of echoes (like the ‘pings’ of submarine sonar), surveyors must know how fast sound is traveling through water while the fathometer is being used. That’s every time, so crews measure the speed of sound before and after every survey. “It can change a lot depending on conditions like rain events and temperature,” Dunkley says. “It also changes with the seasons--the peak is at midsummer.”
To measure, crews use a special sensor that is cast over the side of a boat. It measures the speed of sound at every foot in the water column to account for varying conditions in different strata of the water, and this information is used to correct fathometer readings. “By addressing factors like the speed of sound, we propagate less error,” Dunkley says. “And that’s part of the reason we can measure so accurately.”