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Small portable Airmetric sampler
As environmental scientists sift through data from a Division of Air Quality study of wind-blown dust particles from the exposed Great Salt Lake shoreline, officials already are contemplating how to initiate additional studies, if and when the lake level drops again.
“There is a lot of chemistry in the lake that we haven’t quite got a handle on,” said Dianne Nielson, executive director of the Utah Department of Environmental Quality (DEQ). “My objective in conducting this study was focused on fine particle size and what was in the sediment from all sources – both natural and manmade.”
Others hoped the study would shed some light on mercury exposure, but results demonstrated no significant risks of airborne mercury from the dust. Even so, environmental officials are cautious about drawing any conclusive evidence about mercury risks.
“Even though the tests didn’t find significant amounts of mercury or selenium in the dust particles doesn’t mean they are not there,” said Bruce Allen, an environmental scientist with the Air Monitoring Center who conducted the two-year study in 2004 and in 2005, noting the quartz fiber filters used to collect the samples don’t completely capture gaseous mercury. “We just don’t have the equipment to be able to capture those mercury compounds,” Allen added. “Our portable samplers are good for dust collection but not volatile metals.”
“Mercury is really a difficult thing to monitor,” added Nielson.
Even so, the study does provide some insight into the types of toxins that have been deposited in the lake that can be whipped up during windstorms. “Because of the terminal nature of the Great Salt Lake it serves as a sink basin for all kinds of activities,” Allen said. “We did find significant levels of sulfur, chlorine, potassium, iron and strontium, which are more likely related to the types of rocks and sediment in the basin than any manmade activities.”
Exposed Lake Bed Offered Opportunity for Study
Now that the lake levels have risen there is no need to continue to study, although officials don’t rule out the possibility in the future. “What the study tells me is the next time we see the lake level go down we need to be prepared for a more rigorous sampling program,” said Nielson. “We want to understand any impact on people’s health of fine particulates that are picked up from the lake beds.”
In 2004, the drought took its toll on the Great Salt Lake, leaving about 70,000 acres of exposed shoreline, the largest high-and-dry expanse since 1963. “After six consecutive years of drought the Great Salt Lake was at an all-time low, which amounted to about a 14-foot drop in lake levels,” observed Bob Dalley, manager of the Air Monitoring Center. “We had a unique opportunity to do some air monitoring tests of windblown dust of lake sediment,” added Rick Sprott, director of the Division of Air Quality. “We had a sense that the dry lake bed does contribute to PM 2.5, which is the fine particulate matter in the air that causes health problems for asthmatics, children and the elderly.
It all came about during negotiations on how to clean up the groundwater contaminated by a century of Kennecott and Utah Copper Company mining operations in Bingham Canyon. One proposal under consideration is to discharge the reverse osmosis contaminants which contain selenium into the Great Salt Lake. But before environmental officials agree to do that, Nielson directed that a study be undertaken to determine a selenium standard for the lake. In doing so, she also saw an opportunity to find out what substances have historically flowed into the lake. “It really was a dust problem,” she said. “But I also was concerned about the chemistry and the risk. The lake is unusual because of the trace elements and metals.”
Dalley devised an air monitoring plan that included placing small portable Airmetric samplers at three selected sites around the Great Salt Lake. Allen collected 24-hour air samples, specifically monitoring for total suspended particulate matter and a smaller fraction that are 10 microns or smaller, otherwise known as PM10. Later, he sent the samples that contained the highest concentrations for chemical analysis to a lab in Tigard, Oregon. “The study’s primary objective was to assess the metal content of the wind-blown dust off the Great Salt Lake and its secondary objective was to evaluate the respirable and fugitive dust exposures stemming from the lake’s shoreline,” said Allen.
How the Sampling was done
Allen chose three locations to set up his portable battery-powered air samplers: one about one mile north of Saltair, another on the north end of the causeway near the park entrance to Antelope Island and the other at the Farmington Bay Bird Refuge. “We picked the locations that were accessible,” Allen said.
There were some bumps along the way trying to find the right sites. “In 2004, we used a site near the Central Davis Sewer District that was about three miles from the Great Salt Lake. Initially, we took 4-wheelers out and got stuck in the mud trying to find a suitable sampling site. So we ended up moving the sampling sites to a more accessible point near the treatment plant. In 2005, that site was moved to Farmington Bay Bird Refuge,” Allen said. “And, were I to do it again, I would move the Antelope Island site about one mile to the west where winds frequently stirred up immense clouds of dust from the exposed shoreline.”
Allen secured the portable samplers on telephone poles or tripods about 10 feet above the ground to avoid tampering. “I would set up the samplers a day in advance where the winds were predicted to exceed 20 miles per hour and no rain was in the forecast,” he said.
Portable samplers were calibrated at the outset of each year’s study and flow audits were performed at the end of each season. A margin of error of plus or minus 3 percent in flow rates was measured. For each study period, filters with the highest mass loadings were sent to Chester LabNet in Oregon for x-ray fluorescence elemental analysis.
In 2005, two samples from the Saltair location contained readings higher than the National Ambient Air Quality Standard (NAAQS) for PM10 set at 155 micrograms per cubic meter of air sampled (155 ug/m3). Lab results for mercury and selenium levels were compared to the reported laboratory “uncertainty term,” which represents measurement noise. Allen concluded there were no significant levels of the metals found in the air samples, because in order to be determined “significant” the results had to be at least twice the uncertainty term and none of them were.
“Respirable dust concentrations occasionally exceeded current NAAQS during high-wind conditions,” Allen said. “Those were found at the Saltair location, which was the site nearest the exposed lakebed. Much lower levels of particulate were measured at the other sites, which were at least a mile to three miles away from the lakebed.”