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Lake Sediments May Not Faithfully Record Decline of Atmospheric Pollutant Deposition Handong Yang* Environmental Change Research Centre, University College London, Pearson Building, Gower Street, London, WC1E 6BT, U.K. atmospheric Hg deposition to the lake surface, i.e. all the Hg from atmospheric direct deposition has sunk into the lake bottom. The ratio of “b” to “a” estimates the fraction of the atmospheric Hg flux which has been transported to the lake from the catchment.2 Similar to Hg, other pollutants were also buried in the lake sediments via contributions from direct deposition and catchment inputs. Once contemporary b/a ratios for a pollutant in the sediments of a lake are relatively constant through time, it indicates that changes of the contemporary pollutant fluxes in the sediments are in the same proportion as those in the atmospheric pollutant deposition fluxes. Hence, the pollutant record of the lake sediments reveals the history of the atmospheric pollutant deposition. When pollutants deposited into the terrestrial catchment surface, parts make their way to the lake, and parts absorbed to the surface soils or plants, then get stored and build up pollutant concentrations on the catchment surfaces.3 In fact, the catchment inputs of pollutant to a lake are not the only way in which a fraction of the deposited pollutant to the catchment directly makes its way to the lake, but a combination result involving physical and chemical processes. For example, when ake sediments have been extensively used as archives for rainwater passes through or runs off over the soils, pollutants revealing pollution history for decades. Remote lakes only dissolved from catchment soils, or washed out with the eroded receive pollutants from atmospheric deposition and in the soils, then into the lake. If atmospheric pollutant deposition absence of long-term monitoring, they have been used for fluxes are stable, pollutant inputs from a lake catchment to the studying atmospheric deposition. Since the introduction of the lake can be balanced and keep stable, while other conditions are 210 Pb dating method in the early 1970s, it has been possible to constant. If atmospheric pollutant deposition increases, catchprovide accurate chronologies for sediments to establish time ment surface soils have more chances to catch and concentrate based pollution history for the last c. 150 years, and as sediment the pollutants. Catchment erosion is a major cause of lake 210 Pb calculations can also provide sediment accumulation sediments. With an increase in pollutant concentration in the rates, contemporary pollutant deposition fluxes to the sedicatchment surface soils, erosion can bring more pollutants into ments can be evaluated.1 the lake, and more pollutants can also be dissolved into the Sediments deposited into a lake basin are derived from two rainwater and delivered to the lake. During the periods when sources: atmospheric direct deposition and terrestrial catchatmospheric pollutant deposition fluxes are stable or in ment inputs. Pollutants that enter the lake are associated with increase, contemporary b/a ratios can be relatively constant, these two sources. and therefore, the sediment pollutant record reveals more or By using sediment mercury (Hg) records in remote lakes in less similar proportional changes to the deposition, as show in North America to link with atmospheric deposition and Figure 1 before 1970. If a lake has gone through a high catchment input, Swain et al.2 concluded that sediment Hg pollutant deposition period, the pollutant concentration fluxes have good linear relationships with the ratio of terrestrial increases in the catchment surface. Consequently, the release catchment area to lake area, which can be written as follows: of this pollutant from the catchment also increases, and this release is independent of the later decline in pollutant F = a + b × A C /AL deposition. When pollutant deposition declines, especially in a considerable decline, catchment pollutant inputs derived from Where F is Hg flux in lake sediments, AC/AL is the ratio of erosion or dissolving from the soils into the lake are likely to be terrestrial catchment area (AC) to lake area (AL). The intercept unchanged in the same proportion as in the deposition. Thus, of the regression in this relationship indicates that the Hg accumulation rate in the sediments for a lake with no terrestrial catchment is “a”. If losses of Hg from the lake by evasion or Received: September 14, 2015 outflow are negligible, “a” should predict the rate of Published: October 16, 2015
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© 2015 American Chemical Society
12607
DOI: 10.1021/acs.est.5b04386 Environ. Sci. Technol. 2015, 49, 12607−12608
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Environmental Science & Technology
atmospheric pollution, impact from a polluted catchment can not be ignored.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Appleby, P. G. Three decades of dating recent sediments by fallout radionuclides: a review. Holocene 2008, 18, 83−93. (2) Swain, E. B.; Engstrom, D. R.; Brigham, M. E.; Henning, T. A.; Brezonik, P. L. Increasing rates of atmospheric mercury deposition in Midcontinental North America. Science 1992, 257, 784−787. (3) Yang, H.; Rose, N. L.; Battarbee, R. W.; Boyle, J. F. Mercury and lead budgets for Lochnagar, a Scottish mountain lake and its catchment. Environ. Sci. Technol. 2002, 36, 1383−1388. (4) Burt, T. P.; Howden, N. J. K. North Atlantic Oscillation amplifies orographic precipitation and river flow in upland Britain. Water Resour. Res. 2013, 49, 3504−3515. (5) Yang, H.; Smyntek, P. Use of the mercury record in Red Tarn sediments to reveal air pollution and the implications of catchment erosion. Environ. Sci. processes Impacts 2014, 16, 2554−2563.
Figure 1. Conceptual diagrams showing pollutant fluxes to lake sediments when pollutant deposition was in increase before 1970, followed by (A) a slower decline in catchment inputs compared with direct deposition, or (B) an increase in catchment inputs derived from an increase in catchment erosion, while atmospheric direct deposition has declined.
the b/a ratios are likely to increase, and therefore, lake sediments do not record the decline in atmospheric pollutant deposition faithfully. Due to concerns regarding environmental pollution, metal emission and production of persistent organic pollutants (POPs) have been reduced considerably in the European countries in recent years. For example, anthropogenic Hg and Pb emissions in the UK have declined sharply by over 90% since the 1970s. The decline trends have been shown in many lake sediment records across the UK for the same period. However, it is rare to see the reduction in the sediments at a similar proportion as metal emissions or POP productions in the UK. Since a huge amount of pollutants have been deposited and stored in the catchment surface soils of many lake sites, the release of the stored pollutants in the soils has become the dominate source for the lakes, and the lake sediments do not show decline, or decline levels are not as large as expected (Figure 1A).3 Furthermore, climate change has been linked to an increase of extreme weather events in recent years. With increases in extreme weather events, catchment soils become more fragile and moved more easily into the lake,4 enhancing the release of the stored pollutants in the catchment into the lake sediments (Figure 1B).5 The fact that lake sediments do not faithfully record decline of atmospheric pollutant deposition indicates that if a lake has terrestrial catchment, recovery from a pollution in the lake is likely to be delayed following reduction in the atmospheric deposition. Responsible governments are willing to reduce pollutant emission to the environment. Reductions in atmospheric depositions of different pollutants have been documented in many regions, and the reduction can be a common phenomenon worldwide. When we use lake sediments to reveal atmospheric pollution, especially a reduction in 12608
DOI: 10.1021/acs.est.5b04386 Environ. Sci. Technol. 2015, 49, 12607−12608
