University of Rochester

Researchers Examine Rochester's Water Supply for Impact of Global Warming

March 25, 2002

Three researchers from the University of Rochester have collected a sediment core from the bottom of Hemlock Lake, the principal source of the city of Rochester's water, in the hopes of shedding some light on how our water supply may fare if the greenhouse effect continues to warm the globe. The results are to be presented at the Geological Society of America meeting in Springfield, Mass., on March 25.

Bill Chaisson, assistant professor of earth and environmental sciences at the University of Rochester, led the team that recovered a 32-centimeter core from the lake bed. The core contained sediment that accumulated over the past 130 years, allowing the researchers to extract paleontological and geophysical data and compare it to the weather reports in the Democrat and Chronicle back to 1870. The correlation was strong enough to spur the team to look into a second phase of the project-examining the lake sediment changes over the last several thousand years.

"We found that during droughts in our area the relative abundance of certain species of diatoms, a type of golden algae, was higher," says Chaisson. "We think that during the 1920s and 30s, which included the great Dust Bowl droughts out in the Midwest, the water level in Hemlock Lake dropped, causing more sediment, and hence more nutrients, to be suspended in the remaining water." Using this pattern as a guide, the team is going to look further back into Hemlock Lake's history, about 7,000 years, to get a grasp on how it behaved long before Rochester was tapping its water. This may shed much needed light on what may happen to the lake should the climate change dramatically-due to more frequent and intense local droughts that may result from global warming.

Chaisson, with SUNY Brockport biology researcher Mary Arnold, set out on Hemlock and a number of other Finger Lakes in a small boat armed with a coring device borrowed from Monroe County, which normally uses it to check for different types of contaminants in the lakes. The device, a sort of lead-weighted pipe, is lowered over the edge in the deepest part of the lake where sediment tends to accumulate uninterrupted. The pipe, about three feet long and an inch and a half wide, plunges into the muddy bottom, keeping layers of sediment in order. A rubber stopper then slides over the top of the pipe and as the pipe is hauled back to the boat, the layers of sediment rise with the pipe, held in by the suction provided by the stopper.

Rory Cottrell, a postdoctoral research associate in earth and environmental sciences at the University, measured the magnetic properties of the sediments and determined that there had been a significant increase in magnetic grain size during the early decades of the 20th century. This correlated well with Arnold's data showing significant increases in diatom species associated with greater nutrient enrichment. Both findings suggest lower lake levels associated with drought. So far, the team has found that more than half of the variation in the diatom concentration can be explained by a correlation with drought intensity. They are now looking into variations in land use around the lake to factor in its possible impact. For example, there was more logging in the Hemlock drainage basin during the first half of the 20th and last part of the 19th centuries than there has been since the 1950s. Increased erosion of sediment into the lake can supply nutrients to the diatoms, favoring certain species over others.

"The primary objective is to determine the relationship between lake water quality and climate," explains Chaisson. "It would appear that the lakes become more productive when their levels are lowered by drought. The water has been filtered since 1992, but treatment costs may increase if water quality is lowered more often by plankton blooms."

The current global warming trend may have had some precedent within the last 10,000 years. There is evidence that it was quite warm and dry in western New York about 7,000 years ago. Chaisson and his team are now looking to determine what impact that sustained heat wave had on Hemlock and other lakes in the area.