Environment -- Lurking In Spring Snowmelt: The `Acid Pulse' Of Pollution

Could acid snow be the next acid rain? In parts of the West, scientists are studying if and how airborne pollutants are settling in mountain snowpacks, only to be flushed out as part of spring meltwaters.

In a bunker buried beneath the snow 9,600 feet up Mammoth Mountain in California's Sierra Nevada range, pipes are beginning to drip with meltwater. Here and elsewhere across North America, the spring sun is working to clear away 6.8 million square miles of winter snow cover.

According to geographer David A. Robinson of Rutgers University, who monitors such things by satellite, it's the fourth largest snow cover in the past 21 winters. That means spring will bring floods to many regions as well as much-needed replenishment for reservoirs and groundwater systems.

In high alpine basins like those in the Sierras and the Rocky Mountains, however, spring snowmelt also poses one of the greatest dangers of the year for mountain lakes and streams.

That's because snow has a unique ability to concentrate the airborne pollutants that fall with it or on it all winter, then flush out most accumulated impurities in the first batch of meltwater that drains from the pack. The phenomenon is called the "ionic pulse" - or, as humans pour more nitrates and sulfates from burning fossil fuels into the atmosphere, the "acid pulse."

Alpine lakes are especially vulnerable to concentrated doses of acid runoff, and for the past decade a growing group of scientists has been monitoring the acid pulse at research stations around the world.

So far, snow monitoring, combined with year-round water sampling at key lakes and streams in the high Sierras and Rockies, shows the waters have not been harmed by the acid pulse. But the danger signals are clear. In streams along the eastern slope of the Rockies in Colorado and southern Wyoming, for instance, snow researcher Mark Williams of the Institute of Arctic and Alpine Research (INSTAAR) in Boulder has found nitrate levels so high that they rival those of badly acidified lakes in the Adirondacks.

"These streams are not acid yet, but the pH has dropped slightly as the nitrogen concentration has increased steadily over the past couple of years," says Williams. Fortunately for the Rockies, the nitrate buildup isn't paired with the high sulfate concentrations that have combined to kill off zooplankton, insects and other tiny creatures in so many acidified Adirondack lakes.

Eastern mountain basins, including the Adirondacks and the Canadian Shield, are also more affected by acid rains than acid snow. Their snow melts several times during a typical winter and doesn't store pollutants for release in a single potentially deadly "pulse." But in the West, where the pack collects all winter and then melts, "the real culprit is spring snowmelt," says Williams.

Along the Colorado Front Range, Williams notes, the snow carries pollutants emitted from power plants in northwest Colorado as well as the infamous "brown cloud" generated by Denver, Fort Collins and Boulder to the east. The high Sierras are more fortunate, getting most of their snow from frontal storms off the Pacific that pick up little of the human-generated pollution from West Coast cities.

Nevertheless, during the mid-1980s a team led by ecologist John Melack of the University of California, Santa Barbara, reported two major instances when the pH of Emerald Lake in Sequoia National Park in the Sierras dropped into the acid range. Melack also set up plastic enclosures within the lake and tested the impact of lowered pH on invertebrates and the trout that eat them. He found these high Sierra ecosystems "extraordinarily sensitive to acidification."

"I think alpine areas are early warning systems," Williams notes. "These are the most sensitive ecosystems to acidification, so you would expect to see an effect on alpine systems before you would in lower basins."

In the Cascades, Central Washington University chemistry professor Clint Duncan and his students have found that the chemistry of the high lakes is still "dominated by geology rather than atmospheric input." Because the bedrock is volcanic or granitic and weathers very slowly, the Cascade lakes naturally contain very few dissolved solids, and so far rain and snow over the region do not appear to have increased that load, Duncan notes.

For the past two years, Duncan and his team have been monitoring rain and snow chemistry year-round for the National Park Service using samples collected at the Paradise Ranger Station at 5,000 feet on Mount Rainier.

"What we have found at Mount Rainier is that, although most of the annual precipitation falls as snow, the concentration of contaminants is much higher in the

rain," Duncan notes. Most of the contaminants are picked up as the air masses stall for a time and then move inland over the Puget Basin, he says.

Snow research is a labor-intensive enterprise. This time of year teams ski into remote mountain basins, dig pits through 18 to 20 feet of undisturbed snowpack and sample the chemistry and water content of each layer. At the University of California's Mammoth Mountain Snow Science Laboratory - an ocean cargo container buried partly underground near a ski slope and reachable in winter by climbing down a "Santa Claus tower" - an octopuslike array of pipes collects meltwater at nine different points and funnels it into the bunker. There it is monitored for dissolved impurities.

The phenomenon of acid pulse begins in the clouds, where raindrops pick up particles and gas molecules as they form. When raindrops freeze into ice crystals, the impurities get pushed to the surface. As these snowflakes fall, they pick up more pollutants.

On the ground, intricate and lacy snowflakes tend to melt slightly and recrystallize, clumping into coarse-grained snow. The process pushes all the impurities from multiple flakes to the outer surface of these new grains, leaving these pollutants even more concentrated and available to be carried away with the first surge of meltwater. In a joint study in the Sierras, Williams and Melack found that the first 20 percent of meltwater carries off about 80 percent of the dissolved pollutants stored in the seasonal snow pack.

Alpine areas are particularly vulnerable to acidification because they tend to have thin acid soils, exposed bedrock such as granite (rather than alkaline minerals like limestone), very little groundwater storage and sparse vegetation. There are few opportunities to buffer or neutralize acid runoff before it reaches lakes or streams.

Even in alpine basins, however, plants can usually soak up all the excess nitrogen that drains into the soil from melting snow and use it to fertilize extra growth, Williams notes. That makes the findings from the Rockies even more unusual.

"What's really unexpected is that during the summertime when vegetation is cranked up and nitrates always go down to zero because of prolific plant growth, we're seeing nitrates flowing out of the watershed (along the Colorado Front Range)," says Williams. "What we're seeing is nitrogen saturation - the basin has gotten to a point where it just can't take up any more nitrogen, so it's exporting it in stream waters."

His findings are based on data taken since 1984 at three sites: the U.S. Forest Service Glacier Lakes Ecosystems Experiment Site in southern Wyoming, Loch Vale in Rocky Mountain National Park, and Niwot Ridge west of Boulder.

A central goal of snow researchers is to combine all this field data into computer models that can predict the size, timing and chemistry of the acid pulse across a whole rugged basin or watershed.

Roger Bales of the University of Arizona says scientists are able to describe what the acid pulse will look like at a single point in a watershed. But it's not simple to translate that information into a picture of how all the snow in an alpine basin will melt and drain into a lake or a stream.

Snow melts - and releases its pollutants - unevenly depending on tree cover, depth of the snow, slope angle, amount of sun or shade, whether the slope faces north or south, and irregularities such as "ice lenses," which disrupt the percolation of water through the pack.

The pipes bringing meltwater into the Mammoth Mountain lab from nine different points provide Bales' team with a miniature model of this differential melting process. He plans to test his computer models with field data obtained from watersheds such as Emerald Lake in the Sierra, the Snowy Range in Wyoming, the Chilean Andes and eventually Tien Shan in China.

Bales says: "We hope our models will be useful for scenario testing: What happens if we double the precipitation? Or double the acid in the precipitation? What happens if we increase the solar radiation because there's more carbon dioxide in the atmosphere?"

In other words, what is likely to happen to our alpine water supplies or the chemistry of mountain lakes and streams if humans emit more air pollution or global warming causes more winter precipitation to fall as rain, rather than snow?

Indeed, snow fields cover 30 percent of the Earth's land surface each winter and may themselves play a major role in global climate change, acting like giant mirrors to reflect sunlight back into the atmosphere. For this reason, as well as the need to understand global water and pollutant cycling, snow researchers are experimenting with monitoring snow from satellites.

UC Santa Barbara geographer Jeff Dozier says it is theoretically possible to measure water content, thickness, density, wetness and other characteristics of snow by using remote sensors. The sensors aboard the current Landsat satellite, however, were designed to monitor duller features such as vegetation, and are effectively blinded by the bright glare of snow.

Dozier is lead scientist for the National Aeronautics and Space Administration's Earth Observing System program, which is scheduled to launch a successor to Landsat in 1998. That satellite will carry sensors capable of monitoring solar radiation reflected from snow - a feature needed for measuring contaminants in the pack. Because of budget cutbacks, however, it will not carry the type of synthetic aperture radar needed to measure water content or depth of the snowpack.

That means teams of snow researchers will still be skiing into remote mountain basins each spring to dig pits and read the record of human-generated pollution stored in the snowpack.