New Remediation Technique Tops the LIST

Organic waste from sewage and pulp and paper mills, and chemicals from steel, petroleum and other industries have contaminated sediment in many of the world's harbours, lakes, rivers and canals. In addition to affecting water quality and aquatic life, some of these contaminants combine with others—and with compounds that are naturally present in saltwater—to create foul-smelling, corrosive and highly toxic aerosols.

In some places, such as Asia, the problem has reached such proportions that the health of people living near these waterways and the harvesting of fish and shellfish have been affected. In Canada, the problem is less severe, but it has still reduced water quality in some areas and caused the growth of tumours in some species of fish.

The most common way of dealing with sediment contamination is to scoop or vacuum up the sediment with a barge-mounted dredger and haul it away for long-term storage or for treatment. Unfortunately, trucking and disposal are expensive and pose the risk of an accident or leak. Storing contaminated material on site is handier, but reduces the area of usable land at the site. Dredging itself also has drawbacks—it has a significant impact on habitat, may not remove all of the sediment in question, and may not be feasible if the sediment is unstable or the water too deep. Dredging is also not sustainable over the long term—a problem if remediation must be repeated periodically to address added contamination.

In searching for a better way to deal with extensive sediment contamination in Hamilton Harbour and Sault Ste. Marie, Ontario, scientists with Environment Canada's National Water Research Institute (NWRI) in Burlington have created one of the first commercialized on-site remediation processes. The Limnofix In-Situ Sediment Treatment Technology (LIST) uses an underwater harrow towed behind a boat to till the contaminated sediment and inject it with a chemical oxidant, usually calcium nitrate. Since calcium nitrate is also a nutrient, it must be injected deep into the sediment to prevent it from escaping into the water column and boosting the growth of algae.

Any rich organic waste, such as untreated sewage, can convert sulphates, which are found in industrial waste and occur naturally in sea water, to create a toxic, odorous and corrosive hydrogen sulphide gas. The oxidant injected in the LIST process promotes the aerobic biodegradation of contaminants by providing oxygen to the bacteria in the sediment, and by oxidizing the sulphides that impede this natural process. Although oxidization takes place relatively quickly, bioremediation can take several months—depending on the type and severity of the contamination.

LIST has been demonstrated successfully in bench and pilot-scale studies in Canada, the United States, Europe and Asia by Golder Associates—LIST's commercial licensee—with support from NWRI. In 1998, a full-scale treatment was carried out near the old airport in Hong Kong, where sewage-contaminated sediment in the marine environment was causing serious odour problems and corroding nearby buildings and aircraft. Within weeks, the sediment had turned from black to brown, more than 95 per cent of the sulphides were remediated, and the odour had dissipated significantly.

A five-year pilot project initiated the same year in Salem, Massachusetts, is also showing positive results. The site, an inter-tidal mudflat contaminated with coal tar from a coal gasification plant, underwent two LIST treatments in 1998 and 1999 using an injection system towed behind a tractor at low tide. It is now 90-per-cent remediated.

Future uses of LIST are equally promising. Environment Canada is providing technical support for a full-scale remediation to take place on the Shing Mun River in Hong Kong. Through Golder Associates, LIST has been accepted by the United States Naval Facilities Engineering Service Center to clean up some American naval sites. In the meantime, scientists at NWRI continue to explore alternatives—such as capping contaminated sediment with a layer of material fortified with chemical additives—to create a suite of techniques that will be effective in a wide range of situations.

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