Preliminary Evaluation of Methylmercury Concentrations for Wet Deposition Samples Collected in North America, 2003-2013

Gregory Alan Wetherbee, USGS CO, United States, Branch of Quality Systems, Denver, CO, United States, Mark F. Rhodes, University of Illinois, Prairie Research Institute, Illinois State Water Survey, Champaign, IL, United States, David Allen Gay, National Atmospheric Deposition Program, Madison, United States, Robert C. Brunette, Eurofins Frontier Global Sciences, Bothell, WA, United States, Eric M. Prestbo, Tekran Instruments Corporation, Toronto, ON, Canada and Martin R. Risch, U.S. Geological Survey, Indiana Water Science Center, Indianapolis, IN, United States

Contact First Author: Gregory Alan Wetherbee; wetherbe@usgs.gov

Abstract ID#: 35766

 

English Abstract:
The National Atmospheric Deposition Program /Mercury Deposition Network (NADP/MDN) recently released data for methylmercury (MeHg) concentrations in 5,375 non-filtered samples of wet-only deposition. Samples were collected at 74 locations in the USA and Canada during 2003-2013.

Concentrations are censored at a mass-based RL=0.050 ng in samples collected before April 18, 2007 and at RL=0.0025 ng on or after that date, relevant for samples with a minimum volume of 45 mL. Mean MeHg concentrations for samples pre- and post-April 18, 2007 are statistically different per the Peto-Prentice test. The mean MeHg concentration is 71 percent lower for samples collected after April 18, 2007 due to an increased percentage of low-concentration samples above the RL. Approximately 66% of the MeHg determinations are less than the RLs.

 Assigned quality rating codes classify 92.8 percent of the samples as “valid”. Associated notes codes qualify the data based on various sample characteristics, including visible debris type (bird, insect/animal, plant, ash/soot, and cloudiness). Consideration of these codes is crucial to proper use of the data. Approximately 63% of valid samples with MeHg concentrations above the RL contained visible debris, indicating a highly significant relation (p<.001) between debris presence and quantifiable MeHg as measured by a chi-squared contingency analysis. The mean MeHg concentration in samples noted as containing debris is 1.7 times the mean for samples with no debris per the maximum likelihood estimator comparison test (p<0.001) .

A mean MeHg concentration of 0.122 + 0.450 ng/L and median of 0.050 ng/L is calculated by Regression on Ordered Statistics (ROS) for the complete dataset. Exclusion of both samples considered invalid and samples with visible debris shifted the mean concentration to 0.078 + 0.254 ng/L and median of 0.019 ng/L per ROS. Concentrations in the range 0.1 – 1.0 ng/L follow a log-normal distribution on a normal quantile plot, with higher concentrations departing from this distribution at 1 ng/L. Interestingly, 67 percent of MeHg concentrations greater than 1 ng/L were not coded as having observable debris.

Preliminary analysis indicates network-wide temporal variation in concentrations along with regional (spatial) patterns.

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