Dartmouth scientists report mechanism that may explain seasonal spikes in water contamination

Posted 05/29/02

The first melt of spring brings hope for warmer days, but may also bring the greatest concentration of toxic chemicals to streams, lakes, and hibernating life. At a meeting of the American Geophysical Union this week, a group of researchers, led by Dartmouth earth sciences professor Xiahong Feng and graduate student Valisa Nez, report a mechanism that may explain these seasonal spikes in water contamination. This study shows that two natural processes, ice formation and water flow, contribute to raise contamination levels of the first melting snow, or meltwater, of the season.

To explore the relative impact of these processes in the absence of human intervention, the scientists needed a large, isolated snowbank. They found the perfect snow in the infamous Donner Pass region of California (once the subject of a documentary on cannibalism), home to the Central Sierra Snow Laboratory. Samples of snow at different depths in the bank were taken in April, 2001, as well as samples of freshly fallen snow and snow meltwater throughout the winter and spring of 2000-2001.

In their research, investigators found the highest concentrations of nitrates and sulfates - two components of acid rain - in the first meltwater of spring. Acid rain is formed when nitrates and sulfates in the atmosphere react with hydrogen in air or water to form nitric and sulfuric acid. When large amounts of these acids enter lakes and streams all but the most resilient fish may be killed.

In cold weather, atmospheric pollutants catch a ride on snowflakes, settling on the ground with newly fallen snow. As the snow collects, the pollutants go on the move. Pushed outward by developing ice structures, the chemicals accumulate near the outer surfaces of snow crystals. This makes them more likely to dissolve into the first meltwater as temperatures warm.

How the melted snow flows through a snow bank can be just as important in determining meltwater contamination levels. When water flows uniformly through a blanket of snow, it dissolves a representative sample of pollutants. Meltwater can also flow in discrete narrow paths, to avoid ice crystals for instance, dissolving only the pollutants in those pathways. Here's where things get trickier. The fact that meltwater interacts with less of the snow does not mean that it dissolves a smaller proportion of contaminants. The area along the surfaces of ice crystals tends to have a higher concentration of impurities, so the season's first meltwater may dissolve more contaminants from its narrow path than it would from a uniform blanket of snow.

The researchers illustrated these conclusions by showing that nitrates and sulfates were concentrated together after the snow went through its process of packing and ice formation. If this were the only process involved, then meltwater nitrate and sulfate concentrations should reflect the packed snow concentrations. Instead, the researchers found the chemicals concentrated further in the meltwater samples. This led to the conclusion that meltwater flow patterns initially raise the level of contaminants. These levels drop off as pollutants are drained from the snow.

While nitrates and sulfates are not the only contaminants found in snowpacks and meltwater, the researchers believe these two chemicals fairly represent the entire population. To confirm this assumption, further studies tracing other chemical contaminants are planned for the team. The research team includes Dartmouth's Valisa Nez, Xiahong Feng, Carl Renshaw, and Stefan Stürup; University of California at Berkeley's James Kirchner; and Central Sierra Snow Laboratory's Randall Osterhuber.

By Molly Hammell
Science Writing Intern
Center for Environmental Health Sciences at Dartmouth