Water Quality in Point Pelee Marsh

Point Pelee National Park, on the northwest shore of Lake Erie, contains one of the finest surviving wetlands in the Lower Great Lakes, providing habitat to a rich diversity of species. Over the past decade, however, concern has grown that the quality of water in parts of the marsh has deteriorated to such a point that its natural biodiversity is threatened by a variety of sources.

Elevated levels of nutrients, including phosphates and ammonia, were found in several open-water ponds in Point Pelee marsh, causing a prolific and unsightly growth of algae. The challenge facing researchers from Environment Canada's National Water Research Institute (NWRI) was to discover if the Park's septic systems were the source of the nutrients and find out how they were moving through the hydrogeological environment of the Park.

Very little was known about the mechanisms controlling groundwater flow at Point Pelee National Park. The marsh is separated from Lake Erie by barrier bars along its east and west sides. A barrier bar is a continuous offshore ridge of sand and gravel formed as a result of wave action and lake currents transporting and depositing the sediment. Because the highest concentrations of nutrients were found along the western bar—the main area of human activity—researchers suspected that they were leaching from septic systems to the groundwater, and flowing from there into the marsh.

Using field data and computer modelling techniques, NWRI researchers conducted hydrogeological and geochemical studies that are providing a wealth of information about the flow of groundwater in the Park and the movement and sources of nutrients. Their results show that the barrier bars are sufficiently wide that they do not allow any direct flow of lakewater between Lake Erie and the marsh. The groundwater flow regime between the lake and the marsh, however, is highly complex, with some areas of the barrier bar having a high potential to transport septic-system-derived contaminants to the marsh, and others having almost none.

The research team also discovered that, while the Park's septic systems contribute to excessive nutrient input at certain sites, they were not the major source at Sanctuary Pond—the area of the marsh with the highest nutrient concentration. Instead, the main contributor was the regeneration of nutrients from pond sediment—a natural process that takes place in all open-water ponds, but varies from one location to the next due to differing nutrient levels in the sediments. Work to better understand groundwater flow and its influence on nutrient cycling in the Park continues, and will assist in developing management strategies to improve water quality in affected areas.

NWRI researchers have also been grappling with a historical problem that poses a threat to Point Pelee's ecosystems. Between 1949 and 1970, DDT was used on a large scale to control pests in former orchards and mosquitoes in the Park. The pesticide was applied as a particulate spray over wide areas and also as "toss bombs" at specific sites or pools of water. By the late 1990s, it was expected that the DDT would have degraded. However, in 1998 it was detected in shallow soil at several locations.

Parks Canada asked NWRI scientists to confirm the reported high concentrations of DDT, identify the extent of the contaminated area, and determine the reason for the pesticide's persistence. They found that while DDT contamination is widespread in areas formerly occupied by apple orchards, it is restricted to the top few centimetres of the soil where the organic content is high. Through computer simulations, they verified that because of the highly adsorptive nature of DDT and its low solubility, it would be held in the organic-rich portion of the soil and not leach downward to the water table. Hence, the groundwater is not contaminated, and there is little potential for the DDT to be transported to the marsh via groundwater.

Research is now in progress to assess the impact of different soil micro-environments on DDT persistence and degradation pathways. Ultimately, this information will be used to develop technologies to remove and degrade the DDT without causing widespread destruction to the Parlés wildlife and vegetation.

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