Inver Grove Heights is a suburb of St. Paul, Minnesota, population 34,000. The city has 589 stormwater drainage basins, many of which are “internally drained” due to the city’s kettle moraine topography, and typically have a 30 to 40-foot deep bowl-shaped catchment.
Beginning in 2009, the city began inspecting its basins as part of routine maintenance required by its Municipal Separate Storm Sewer System (MS4) permit. Inspections revealed that many basins were significantly filled with sediment, reducing their holding capacity.
Permit conditions and awareness of the potential for the accumulated sediments to be contaminated with polycyclic aromatic hydrocarbons (PAHs) led city staff to test sediment in 12 basins. Three of these were found to be contaminated with PAHs, with two having high levels of PAHs. These findings led to a ban on coal tar-based pavement sealcoats in November 2011.
The city embarked on its first PAH clean-up effort by targeting the smaller of the 2 basins found to be highly contaminated. This was a small project, requiring removal and landfilling of only 50 yards of sediment at a cost of $60 per yard, for the modest amount of $3,000.
The second basin however was a larger pond with heavy sediment accumulation. City staff estimated that a full clean-out would yield 7,000 yards of sediment. At a cost of $65 per yard, this option would have cost $455,000 for disposal alone. To reduce this cost, they chose to dredge only the area around the inlet of 2,300 yards of sediment, restoring part of the basin’s capacity for about $150,000 in disposal cost.
In 2012, the City successfully pursued a Minnesota state grant of $76,000 to begin the project. The first step was sediment testing ($3,400) to detect the extent of PAH contamination. This was followed by additional testing ($15,000) to pinpoint the most contaminated sediment. A licensed landfill within city limits agreed to accept the contaminated sediment for use as daily cover in a lined cell, reducing disposal costs to $36 per yard. The close proximity of the landfill also reduced transport costs. While the project will not fully restore the basin’s storage capacity, the final project cost of $155,000 included:
- Evaluating and testing sediment;
- Engineering, contracting, and work plans;
- Site preparation (including temporary access roads and storage areas);
- Mobilization and overhead;
- Excavating, transport and disposal of sediment;
- Site restoration.
City staff estimate that over 140 basins will show some PAH contamination; with nearly 100 having high levels of contamination. The local landfill will not have enough space to hold these additional sediments, meaning that disposal costs will jump from $36 to $65 per ton (or higher) with additional transportation costs. Estimates for removing PAH contamination from all basins in the City range from $1.5 to $4.0 million.
Preventing the contamination of stormwater sediment with PAH would also prevent the City from incurring high sediment disposal costs. Because national and Minnesota research had traced over 50% of PAH sediment contamination to coal tar sealcoats, the 2009 Minnesota legislature passed a law encouraging cities to pass ordinances banning coal tar sealcoats in order to be eligible for state grants to help pay for pond cleanout. To date, twenty eight Minnesota cities have passed coal tar sealcoat bans.
Many Wisconsin communities will be facing the cost of sediment removal from stormwater detention ponds in future years. To avoid additional costs related to disposal of PAH contaminated sediment, municipalities should consider eliminating the source of PAHs (e.g. coal tar-based asphalt sealcoat) to their MS4s.