Stormwater pond muck: What is it—and why should it matter to you?
August 22, 2018
August 22, 2018
What are we doing with excavated sediments? Here’s how municipalities can save thousands of dollars while conserving landfill space.
When you walk on the shores of a pond in an urban park, you likely aren’t thinking about the muck that accumulates on that pond’s floor. But there is an important story to be told about these water features—and the muck that lies beneath the surface.
Did you know that urban stormwater management (SWM) ponds are designed to provide flood protection and water quality improvement benefits? This is how they work: Rain and snow melt produce overland runoff (stormwater), which flows over urban surfaces, such as roads, driveways, rooftops, lawns, gardens, etc. Stormwater is channeled through underground storm sewers before it ultimately flows into a SWM pond. Ponds slowly release stormwater to provide flood protection for surrounding communities. This slow release also improves water quality before it is discharged to receiving aquatic habitats, such as creeks, rivers, and lakes.
As stormwater flows over urban surfaces, it picks up soil particles, which “muddies” and degrades water quality in the form of total suspended solids (TSS). Overland runoff also picks up contaminants, which typically originate from common sources such as lawns, gardens, roof shingles, asphalt pavements, and automobile emissions or leaks.
SWM ponds allow contaminated TSS particles to settle out of the water column and into pond basins. The improved water then flows out of the pond to a receiving system (i.e. natural creek, engineered channel, etc.). Over time, TSS continues to accumulate as a sediment layer in the pond basin. Routine removal of this sediment restores stormwater storage capacities, which allows the pond to continue protecting public safety and property from flooding dangers. Routine sediment excavations are essential for enhancing water quality treatment efficiencies, which helps protect natural fish habitats and drinking water resources.
Is landfill disposal the only option? Not always.
SWM ponds come in a variety of sizes, which accumulate different sediment volumes over time. General predictions of sediment accumulation rates and volumes can be made for fully developed catchment areas. For example, an average residential neighborhood with a pond measuring 2 hectares in area could produce approximately 1,000 cubic metres of sediment every 10 years. To put this into perspective, this sediment volume would fill approximately 100 dump trucks.
Excavated sediments from residential SWM ponds are typically sent to municipal landfill facilities, which could generate landfill tipping fees of approximately $100,000 for this volume of sediment. But it can be difficult to find landfills that are willing to take large sediment volumes that are being produced by thousands of SWM ponds across Canada, and especially in the province of Ontario. For example, the Waterloo Region Landfill Facility estimates that combined sediment volumes from local SWM pond sediments could exceed the landfill’s current capacity within 10 years. This presents a significant problem for SWM pond owners and landfill operators, which could lead to delayed sediment cleanouts and increased flooding risks.
Each province in Canada sets contaminant limits to determine if sediment requires regulated waste management—which may involve landfill disposal or safe reuse elsewhere. These limits are important for the protection of human and ecological health.
Risk evaluations can determine that SWM pond sediments with specific contaminant levels and sources may be safely reused in variety of ways. For example, sediments containing road asphalt particles may be reused as topsoil amendment materials for re-vegetating asphalt highway boulevards. In this example, the safe diversion of sediment from landfill disposal would save thousands of dollars for the municipality, while also conserving precious landfill space.
Yes, it is! Two pilot beneficial use pilot studies have recently been approved in Ontario.
Pilot study #1: The Ontario Ministry of Environment, Conservation and Parks (MECP) has recently permitted the City of Waterloo to use SWM pond sediment as a road boulevard topsoil amendment material. This represents a municipal cost saving of over $30,000 in landfill disposal fees for this one SWM pond.
Pilot study #2: The Canadian Food Inspection Agency has recently permitted the City of Guelph to use SWM pond sediment as a fertilizer product on Grand River Conservation Authority tree nursery soils. This represents a municipal cost savings of over $290,000 in landfill disposal fees for this one SWM pond.
Regulatory support and research on this topic continues to develop. The MECP is incorporating SWM pond sediment beneficial use requirements into the new Excess Soil Best Management Practices guide. The MECP is also funding a pilot study to evaluate beneficial use risks and opportunities associated with highway SWM pond sediments. Additional research efforts continue across Canada.
So, the next time you see a SWM pond, remember that these greenspace features require routine sediment removal to protect people and the environment from flooding and water quality impacts. Also remember that SWM pond sediment may be treated as a valuable resource rather than as a costly and environmentally unsustainable waste. There’s a lot going on with SWM pond muck—and hopefully we’ll see many more beneficial use examples soon.