Abstract
The time trend of α- and γ-hexachlorocyclohexane (HCH) isomers in Lake Superior water was followed from 1986 to 2016, the longest record for any persistent organic pollutant (POP) in Great Lakes water. Dissipation of α-HCH and γ-HCHs was first order, with halving times (t1/2) of 5.7 and 8.5 y, respectively. Loss rates were not significantly different starting a decade later (1996–2016). Concentrations of β-HCH were followed from 1996–2016 and dissipated more slowly (t1/2 = 16 y). In 1986, the lake contained an estimated 98.8 tonnes of α-HCH and 13.2 tonnes of γ-HCH; by 2016, only 2.7% and 7.9% of 1986 quantities remained. Halving times of both isomers in water were longer than those reported in air, and for γ-HCH, they were longer in water than those reported in lake trout. Microbial degradation was evident by enantioselective depletion of (+)α-HCH, which increased from 1996 to 2011. Volatilization was the main removal process for both isomers, followed by degradation (hydrolytic and microbial) and outflow through the St. Mary’s River. Sedimentation was minor. Major uncertainties in quantifying removal processes were in the two-film model for predicting volatilization and in microbial degradation rates. The study highlights the value of long-term monitoring of chemicals in water to interpreting removal processes and trends in biota.
Highlights
The five Laurentian Great Lakes bordering Canada and the United States have been recipients of persistent and toxic chemicals for many decades
Inputs came through direct discharge and runoff from the watersheds, but were often dominated by atmospheric deposition, especially for the larger lakes.[1−3] The Great Lakes have responded rapidly to declines in atmospheric concentrations of persistent organic pollutants (POPs), such as polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs),[4−6] and burdens in fish have decreased.[7−9] Over time, the lakes have shifted from being net recipients to “secondary sources” of some POPs due to revolatilization from lake water.[4,6]
PCBs were lost from Lake Superior (LS), mainly by volatilization, with a halving time (t1/2) of 3.5 y between 1978 and 1992.11,12 The OCP toxaphene increased in the water column of LS from 1950 to the mid-1970s and incurred net loss by volatilization through the 1990s
Summary
The five Laurentian Great Lakes bordering Canada and the United States have been recipients of persistent and toxic chemicals for many decades. Inputs came through direct discharge and runoff from the watersheds, but were often dominated by atmospheric deposition, especially for the larger lakes.[1−3] The Great Lakes have responded rapidly to declines in atmospheric concentrations of persistent organic pollutants (POPs), such as polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs),[4−6] and burdens in fish have decreased.[7−9] Over time, the lakes have shifted from being net recipients to “secondary sources” of some POPs due to revolatilization from lake water.[4,6] Since the early 1990s, The Canada-U.S Integrated Atmospheric Deposition Network (IADN) and the Canadian Great Lakes Basin Monitoring and Surveillance Network (GLB) have monitored temporal trends of POPs and other chemicals of emerging concern in Great Lakes air and precipitation. Periodic reports have documented temporal trends in air concentrations, and atmospheric deposition/volatilization flows of toxic chemicals to and from the lakes. PCBs were lost from LS, mainly by volatilization, with a halving time (t1/2) of 3.5 y between 1978 and 1992.11,12 The OCP toxaphene increased in the water column of LS from 1950 to the mid-1970s and incurred net loss by volatilization through the 1990s.3,13
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