November 27, 2009

DAVID GREEN

Dust and run-off entering a waterway is a major symptom of construction pollution.

FEATURE | Demolition & environmental engineering

Demand for environmental monitoring on the rise

Construction still sends ‘ridiculous’ amounts of dust as sediment to waterways and ponds

PETER KENTER

correspondent

Moving dirt from Point A to Point B has been a staple of construction projects since humans first began to build. Concerns over erosion and whether soil enters sensitive watercourses are a far more recent phenomenon.

Construction projects are increasingly seeking the assistance of environmental monitors to help prevent and mitigate erosion and other environmental problems, says David Green, an aquatic biologist and an expert in environmental monitoring for construction with Ontario-based Natural Resource Solutions Inc.

To counter the issues of run off, in 2006 the Greater Golden Horseshoe Area Conservation Authorities developed The Erosion and Sediment Control Guideline for Urban Construction.

“We were seeing a lot of construction projects going in a bad way,” says Green. “But the document wasn’t a cookbook. It was designed to support everyone in doing a better job. Handling sediment control on a construction site is a very challenging endeavour that requires a lot of creativity. But, despite efforts to control the problem on construction sites, we’re still losing ridiculous amounts of sediment to the water system, both through the movement of water and wind.”

Green notes sediment that enters a watercourse can have more than a temporary environmental impact. Excess sediment continues to move downstream year after year.

“The sediment can stay in the system for hundreds of years,” he says.

“In some cases you could have huge slugs of sediment from a construction site that can deposit mid-channel and create additional erosion on either bank of a watercourse as water is forced to flow around it.”

Traditionally, engineers have handled the job of environmental monitoring, with specialists called in as needed.

Green says that more construction project owners are calling in an environmental monitor earlier in the game more so as the potential for liability increases.

Since 2006, the Ontario Ministry of Transportation has pre-qualified environmental monitors who have expertise in both environmental disciplines and construction contracts.

Green, who is pre-qualified, says understanding construction contracts is often a key factor in determining erosion and sediment control issues.

“I can look at contract documents, then look at the site and tell you the sediment control issues they’ll be up against,” says Green. “In most cases there’s no deliberate effort to overlook the problem. It’s a contractor’s job to bring in a competitive bid, then show the environmental authorities that a project is constructible, but in many cases what might appear reasonable doesn’t work in practice. If a contractor is dewatering a stream area as part of a construction effort, and they want to use the standard approach of employing sandbags as instream plugs to achieve that end, it may appear reasonable to an environmental agency. But if they screen the project through a monitor, it may be noticed that those sandbags are going to be placed on very thick organic material. Recognizing that a good seal on a soft bottom system is extremely difficult, those sand bags may keep sinking and sinking as they compress the underlying layer. The only effective way to control the water may be to drive sheet pile to depth to block the channel.”

Green says that environmental monitors are most effective when called into a project early on, rather than later.

“Something as simple as phasing construction to limit soil exposure can go a long way to help reduce sediment problems,” he says.

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