Environ. Sci. Technol. 1995, 29, 571-576
Comparison of Three Methods of Estimating Atmospheric Mercury KENNETH A. MORRISON, ELIZABETH S. KUHN, AND CARL J. WATRAS* Wisconsin Department of Natural Resources, UW-Trout Lake Station, 10810 CTY N, Boulder Junction, Wisconsin 54512
Three methods for estimating atmospheric Hg deposition were compared at a site in north central Wisconsin. Mean volume-weighted Hg concentrations varied 2-fold among methods: 8.6 ng/Lfor manual, event sampling (wet only); 12.3 ng/L for automated, wet-only collections; and 15.5 ng/L for triplicate bulk collections. Mean Hg loading estimates ranged from 0.24 to 0.44 p g m-2 week-’. Agreement among the three bulk collectors was good: 0.44 f 0.03 p g m-2 week-’ (mean f SD). Potassium dichromate preservative produced high blanks in the automated, wet-only sampler (2-55% of the measured Hg). Data from t w o of the collectors showed a strong negative correlation between Hg concentration and precipitation volume. This correlation became weaker when the data were lumped into multi-week intervals and disappeared altogether at a collection interval of 1 month. A negative power function fit the weekly data well, and simple modeling demonstrated that missing early storm stages and small rain events can strongly bias estimates of Hg concentration and loading.
Introduction Recent studies have generally implicated atmospheric transport and deposition as the major source of mercury (Hg) to remote lakes in the Northern Hemisphere (1-4). Mass balances generated for remote seepage lakes in northern Wisconsin, for example,demonstrated that direct Hg depositional fluxes were sufficient to account for the Hg in the water column and fish as well as the Hg accumulating within the sediments (5- 7). Several collection methodologies have been used in recent studies of atmospheric Hg deposition. Individual precipitation events have been sampled manually (6) or by modifying automated wet-only precipitation collectors (3, 8,9). Bulk Hg deposition has been measured by deploying bulk collectors (10) and by summing separate estimates of wet and dry deposition (6). In this paper, we compare three commonly used methods for measuring atmospheric Hg deposition: (a) weekly integrated bulk deposition collection; (b) weekly integrated automatic rain collection; and (c)manual, eventbased rain collection. All samplers were collocated at a site where there was a substantial previous record of Hg deposition (6,161and where additional meteorological data were available to support the Hg measurements. We also examine the relationship between Hg concentration in rain and rainfall amount and fit a simple mathematical model to these data. This model is used to demonstrate the importance of capturing both the early stages of large precipitation events and all small volume events. This research was part of a preliminary study by the Wisconsin Department of Natural Resources in developing a statewide network to monitor Hg deposition.
Methods Study Site. All deposition samplers were located at the NationalAtmospheric Deposition Program (NADP)station located in northern Wisconsin near the eastern shore of Trout Lake in Vilas County. The region is remote from heavy industrial activityand sparselypopulated (6.7 people/ km2).Tourism and forestry form the main economic base. The NADP site is a 40-m by 55-m grassy opening in a 30year-old Red Pine plantation. Vehicular traffic is excluded from the site. The samplers were arranged 3-5 m apart near the center of the clearing. Deposition Monitoring. Three types of atmospheric Hg deposition collectors were deployed at the site (Figure 1): (a) triplicate all-glass bulk deposition collectors, (b) an automated wet-only collector, and (c) an all-Teflon, manually-operated event collector. The three bulk collectorswere deployed in early June and sampled weekly throughout the study. The automatic wet-only collector was deployed in late June and also sampled weekly. The NADP site operator measured precipitation volume at the site, on the same weekly schedule, using a Belfort rain gauge and an Aerochem Metrics automatic sampler. Individual precipitation events were sampled manually during a 4-week period in late summer. * Corresponding author; FAX: 715-356-6866; E-mail address: WATRAC@dnrmai,dnr.wisc.gov.
0013-936)(195/0929-0571$09.00/0
@ 1995 American Chemical Society
VOL. 29, NO. 3. 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
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