The St. Croix River is an environmentally sensitive river that separates two major cities in Minnesota and Wisconsin. The St. Croix Crossing project has been decades in the making, partly because of the many historic, cultural and environmental features along the St. Croix National Scenic Riverway. There has been extensive community involvement on both sides of the river to determine whether a bridge should be built to accommodate traffic over the river, as well as to select the best location and alignment for the bridge.
This joint project between the Minnesota Department of Transportation (MnDOT) and the Wisconsin Department of Transportation ultimately will replace the 80-year-old Stillwater Lift Bridge with a new four-lane bridge to connect expressways on both sides of the river.
The current total project cost estimate ranges from $571 million to $676 million and includes construction, right-of-way, environmental protection and remediation, contingency, bonds and insurance, engineering and management, etc.
The St. Croix River is considered to be one of the most important and biologically diverse refuges in the U.S. for native mussels. Before contractors could even begin, the Minnesota Department of Natural Resources and MnDOT completed a removal and relocation of more than 4,000 native mussel species from the St. Croix Crossing construction worksite. The freshwater mussels—including specimens of the rare Higgins’ eye mussel—were relocated during a two-week period as part of preparation for the earliest bridgework. Without moving the mussels, the work could have threatened their habitat on the river shores.
The St. Croix River Crossing project is committed to ensuring the protection of the mussels and the natural environment of this area. The systems put into place to ensure that the river water is not contaminated during the construction process have been central to the construction process.
Treatment During Drilling
Edward Kraemer & Sons Inc. of Burnsville, Minn., is the general contractor for the first phase of this five-year, $100-million contract. Edward Kraemer & Sons, working together with WSB & Associates, chose Rain for Rent and HaloKlear because of the flexibility that their filtration system offers to treat an array of incoming construction water, including suspended solids and high pH. This team of contractors provided the expertise to manage the difficult environmental needs on this large, sensitive, high-profile construction project.
The drilling and setting of the bridge piers in Phase 1 started in March 2013. This initial stage must be complete before July 2014, with a contractor goal for completion in early 2014. The operation is 24/7 in this phase, as drilling into the river through gravel and limestone is done to set the piers. The contractors are drilling up to 200 ft deep, using a chemical-based lubricant for the drills that further complicates the mixture and treatment of the slurry. This slurry is pumped into hopper barges, which hold it temporarily. The barges then are taken to a land-based water treatment system (Stillwater Dock), where the water and sludge are separated, treated and analyzed, before clean water is returned to the river. Each barge holds up to 300,000 gal of water and sludge and 600 gal per minute of dirty water flows through the treatment system, which operates 24/7.
Controllers are set up on the system to monitor the pH and turbidity of the output after treatment. Regulations require the
pH to be between 6.0 and 8.0, and turbidity has to measure 25 ntu or lower. The controllers regulate the system, and if pH or turbidity levels rise above set tolerances, the alarm sounds and the water is rerouted immediately back through the system. This project cannot stop—it costs $198,000 per day in lost labor costs.
Obstacles on Day One
On the first day of operations, tests of the drilling mud revealed it contained polyacrylamides, which are used as a lubricant in the drilling industry. The drilling slurry sample changed from what was originally supplied to the contractor and WSB. The alum-based drilling sludge was changed from a positively charged to a negatively charged blend. This required a quick solution. Collaboration between Rain for Rent, WSB and HaloKlear facilitated a hybrid solution that still met all regulatory requirements. HaloKlear BHR-P50 was used to change the charge so that the LiquiFloc 2% product could effectively flocculate the contaminants from the water.
Returning Clean Water to the River
Once the new solution was provided and implemented, the treatment of the sludge continued as originally planned. At the water treatment system, the sediment-laden water enters the settling tanks on land through a pump with a suction screen to prevent clogging of the intake system. At the point of entry, a turbidity and pH reading is taken. The turbid water, upon intake, is exposed to HaloKlear LiquiFloc and LBP-2101 (BHR-P50) to begin the flocculation process. These were chosen because of their low toxicity—they are green, chitosan-based solutions derived from crab and shrimp shells. Static mixers are used between each application of the HaloKlear products to aid the flocculation. This injection amount varies according to the turbidity and pH readings taken at material intake.
The flocculated material flows through four 20,000-gal weir tanks that are 8 ft wide, 20 ft long and 8 ft deep. Upon completion of travel through the weir tanks, the water is pumped through a four-cylinder sand filtration system. Automated turbidity and pH reading equipment will signal whether the material is passing 25 ntu above background or ±1.0 above 7.0 pH, as the permit requires. If the material fails this threshold, the water will be returned to the holding barge and the process repeated. If the material meets these thresholds, it will be released back into the river. Additional polishing of all treated materials is accomplished with Rain for Rent sand filters. Due to the use of the HaloKlear products, there is no need to treat for pH. The onsite treatment also has a portable water quality monitoring system in place for real-time data capture. Finally, a residual test kit, provided by HaloKlear and administered by WSB, assures no polymers remain within the water stream being discharged into the St. Croix River.
Swimmable, Fishable Water
Presently, this project has treated approximately 9.5 million gal of clean, swimmable, fishable water to the pristine environment of the St. Croix River, and all requirements are being met. The drilling for the bridge pylons was completed as scheduled, before the first snowfall this winter, keeping the overall project on schedule. Collaboration on this project between Rain for Rent, WSB and HaloKlear was paramount to designing a system that worked in the environmentally sensitive area. The system’s successful performance will be a proven solution for future storm water treatment in the area. It was effective at meeting strict water quality requirements and able to treat multiple water conditions with excellent results.
“It’s with great pleasure to see that our HaloKlear technology is actively working to solve the most challenging environmental concerns on large construction projects,” said Tim Joyce, vice president and general manager of HaloSource. “Working with experts like Rain for Rent and WSB is key to ensuring the most cost-effective, environmental solutions are used in sensitive habitats like the St. Croix River.”
Drilling challenges overcome in environmentally sensitive St. Croix River