by Jason Ziemer
Low Impact Development (LID) is a set of principles and practices that incorporates environmental considerations into the design, construction and function of a wide variety of projects. Preserving and improving long–term, nonpoint source water quality in stormwater runoff has been a common focus of many LID projects. This is typically accomplished by preserving natural waterways, reducing impervious surfaces and using natural drain features such as bioswales and wetlands.
Until now, little attention has focused on the contribution to overall stormwater pollution that comes from runoff during the construction phase. Stormwater impacts are often only minimally addressed during the construction phase of projects because many people underestimate the overall contribution of fine sediments and nutrients to receiving waters. Although the duration of construction stormwater impacts are relatively short (a few months to a few years), the impact to the surrounding habitat can last for decades.
Why focus on Construction–Phase Stormwater Impacts?
Construction stormwater runoff is considered to be one of the three most contributing sources of stormwater pollution. It can have tremendous impact to downstream aquatic resources, such as fish, because it can smoother spawned eggs, it lowers visibility which decreases feeding capability, and can even raise water temperature due to the absorption of UV radiation. Studies have shown that the chronic level for cold water species of salmonids is at 10 NTU. The pollutant concentrations can be ten times that in post–construction stormwater and can amount to dump–truck loads of sediment.
Sustainable water treatment solutions
The Highpoint project in Seattle, WA, is located in the drainage basin of Longfellow Creek (one of Seattle's few remaining native salmon spawning habitats). It has incorporated several LID techniques to minimize the impacts of runoff from the project.
The 120–acre project has employed an emerging technology that has demonstrated great success at reducing pollutant loads. Chitosan enhanced sand filtration (CESF) is a stormwater treatment system that utilizes a bio–polymer made from recycled crab shells to coagulate the fine particles suspended in the water. The particles and associated pollutants are then removed from stormwater by pressurized sand filters.
Without CESF, a project of this magnitude could contribute as much as 335 tons of sediment to the Puget Sound basin (an ESA–protected marine water and central focus of the Puget Sound Partnership action agenda). This is a greater impact than a decade of post–construction sediment contribution.
To date, the CESF system has treated tens of millions of gallons and discharged the water below 10 NTUs. The technology has the ability to reduce turbidity levels that conventional methods are unable to achieve and resembles water quality found in natural environments. CESF technology is a BMP that minimizes the impact of the equivalent of 10 years of post–construction runoff in a single wet season and is a suitable design consideration for any LID project. With the ability to operate using bio–diesel and rechargeable batteries, the system further demonstrates LID principles.
About the Author:
Jason Ziemer is an environmental scientist whose work has focused on municipal, construction and industrial stormwater management. He holds two US patents on chitosan–based stormwater treatment technologies.
