Fireworks shower perchlorate into water bodies
Perchlorate levels in a lake in Oklahoma rose significantly after fireworks displays but eventually returned to normal.
Fireworks spiraling into the night sky bring cheer and warmth to people watching from below. They also deliver significant levels of perchlorate into nearby water bodies, according to new research published today on ES&T's Research ASAP website (DOI: es0700698).
Scientists have long suspected that fireworks contribute to perchlorate contamination, but few studies have "directly examined" that connection, says study leader Richard Wilkin, an environmental geochemist at the U.S. EPA's National Risk Management Research Laboratory. This paper takes the research a step further and definitively links fireworks displays to surface water contamination. But the perchlorate levels don't last: Wilkin and his team also found that the contaminant's concentrations fell to background levels after 1–2 months, possibly because of microbial degradation.
Perchlorate in the environment is a human-health concern as well as a risk to wildlife. Levels as low as 100 micrograms per liter (µg/L) are known to cause thyroid problems in fish. Perchlorate has been shown to come from many different sources. Natural sources include lightning discharges and fertilizers mined from Chilean nitrate deposits, and anthropogenic sources include ammonium perchlorate, a major component of rocket fuel.
Because of the dearth of data on the contribution of fireworks to surface-water bodies, Wilkin and his team sampled water from Wintersmith Lake in Oklahoma, before fireworks and for several days after events during July of 2004, 2005, 2006, and November 2005.
Background levels of perchlorate in the lake averaged 0.043 µg/L. Within 14 hours of the fireworks displays, these levels rose 24–1028 times. The authors attribute the wide range in perchlorate concentrations immediately after the fireworks to several factors, including overall quantity of the fireworks used, efficiency with which the perchlorate was oxidized during the burning of the fireworks, and wind direction and velocity. The levels of perchlorate usually peaked at 1 day following the display and then gradually petered out after 20–80 days.
Given that the perchlorate disappearance rate was similar to previously observed microbial degradation rates and was temperature dependent—reaching a minimum at 12.4 °C—the team suspected microbial involvement.
They tested this hypothesis with microcosm experiments in the lab. The microbial fauna from the lake successfully degraded the perchlorate, whereas sterilized lake waters showed no change in perchlorate levels. The rate of degradation was dependent on the nitrate concentrations—the microbes preferred to use nitrate for food before attacking the perchlorate.
"The real impact of it is in helping to fill data gaps in our understanding of potential sources of perchlorate to the environment," says environmental toxicologist, Todd Anderson of Texas Tech University.
The findings also illustrate that "natural systems have the ability to naturally degrade perchlorate over a short time interval," says Wilkin. However, Purnendu (Sandy) Dasgupta, a perchlorate expert at University of Texas Arlington, says that because diffusion is also temperature dependent, disappearance of perchlorate could also be due to gradual diffusion of the contaminant into lake waters. There are several trace heavy metals that also increase with fireworks that can be considered as tracers, he says. "If perchlorate disappeared but this tracer concentration remained stable, one could follow the argument with greater faith that it just did not disappear by dilution into deeper waters."
The study "helps to emphasize the fact that perchlorate exposure is not always from a well-documented contamination issue," says Andrew Jackson, an environmental engineer and perchlorate expert at Texas Tech University. Many transient sources, such as bleach, used for water treatment among other things, or certain herbicides "can act as exposure sources," he says.
However, the study "keeps the whole exposure question open," says Jackson. "Nobody is really addressing what the potential exposure is from all these different sources."
