The MDN is the only network providing a longterm record of total mercury (Hg) concentration and deposition in precipitation in the United States and Canada. All MDN sites follow standard operating procedures and have uniform precipitation chemistry collectors and gages. The automated Aerochem (ACM) collector has the same basic design as the NTN collector but is modified to preserve mercury. Modifications include a glass funnel, connecting tube, bottle for collecting samples, and an insulated enclosure to house this sampling train. The funnel and connecting tube reduce sample exposure to the open atmosphere and limit loss of dissolved mercury. As an additional sample preservation measure, the collection bottle is charged with 20 mL of a one percent hydrochloric acid solution.
Site operators collect samples every Tuesday morning. With each MDN sample, the entire sampling train is replaced with one that is cleaned by the Mercury Analytical Laboratory (HAL) at the Wisconsin State Laboratory of Hygiene, Madison, Wisconsin. Rigorous cleaning ensures that each sampling train component is essentially mercury-free. The HAL supplies the collection bottles already charged with the hydrochloric-acid preservative. By following those procedures and stringent sampling protocols, the MDN is able to report mercury concentrations below 1 part per trillion (<1 nanogram/liter).
All MDN samples are sent to the HAL, which analyzes all forms of mercury in a single measurement and reports this as a total mercury concentration. Currently, a subset of MDN sites are also analyzed for methyl mercury. The HAL reviews field and laboratory data for completeness and accuracy, and flags samples that were mishandled, compromised by precipitation collector failures, or grossly contaminated. The HAL delivers all data and information to the NADP Program Office for final checks and resolution of remaining discrepancies. Data then are made available on the NADP Web site
Information on joining the network is available here.
The Mercury Problem
For thousands of years, civilizations ancient and modern have found mercury to be useful in many ways. Worldwide industrial use of mercury in mining and manufacturing processes, as well as emission during power generation, has greatly increased concentrations of mercury in the environment. Unfortunately, appreciation of mercury ‘s adverse health effects on humans is only recent. Currently, U. S. health concerns do not center on exposure to elemental mercury, but rather on very low methyl-mercury contamination in fish.
Mercury persists in the environment
Mercury is in a class of chemicals called persistent bioaccumulative toxins. Mercury persists in the environment for long periods by cycling back and forth between the air and soil surface, all the while changing chemical forms. Atmospheric lifetimes of elemental mercury are estimated to be up to two years, and as methyl mercury in the soils for decades. Mercury is never removed from the environment; it is just moved to other locations and eventually buried under soils and sediments.
Mercury accumulates in the food chain
Mercury accumulates in biological tissue through complex reactions (bioaccumulation), many of which are still unknown. We do know that several bacteria incorporate environmental inorganic mercury into their bodies through chemical conversion to several organic mercury compounds, collectively called methyl mercury (Me-Hg). This Me-Hg form is more toxic and more difficult to remove from bacterial systems than inorganic mercury. Any higher-level organisms that consume these bacteria also consume the Me-Hg. This cycle repeats up the food chain, with each higher predator consuming more and more Me-Hg, ultimately arriving in fish. Estimates suggest that Me-Hg can accumulate more than a million-fold in the aquatic food chain.
Mercury reaches the human population
As humans consume fish, the Me-Hg in the fish is also consumed. By consuming Me-Hg faster than our bodies can remove it, we bioaccumulate Me-Hg also. By consuming less Me-Hg contaminated foods, concentrations in bodies will decrease. This idea has led to the fish-consumption warnings for mercury.
Mercury as a neurotoxin
Neurotoxicity is the most important health concern with mercury. Methyl mercury easily reaches the bloodstream and is distributed to all tissues; it can also cross the normally protective blood-brain barrier and enter the brain. It will also readily move through the placenta to developing fetuses, and so is of particular concern to pregnant women. Low-level exposure is linked to learning disabilities in children, along with interference in reproduction of fish-eating animals. As well, both methyl mercury and mercuric chloride are listed by EPA as possible human carcinogens.
How does mercury reach lakes, rivers and oceans?
Mercury reaches the surface water primarily through atmospheric deposition, both wet and dry. Wet deposition is pollution, washed out of the atmosphere by rain. Dry deposition is pollutants that are deposited to the ground, trees, etc. from the atmosphere. Wet deposition is deposited pollutants, but through washing out of the atmosphere by rain. Estimates suggest mercury wet deposition accounts for 50% to 90% of the mercury load to most inland water bodies and estuaries in the U.S. (REFERENCE). Elemental (Hg0), divalent (Hg+2), and particulate mercury (Hgp) are each important in both wet and dry deposition, most likely dominated by the Hgp and Hg2+ forms, since Hg0 is only slightly soluble in water and has very low deposition velocities. Wet deposition rates are variable, and tend to have summer maximums. Dry deposition rates to forest canopies may be higher than wet deposition rates in terrestrial ecosystems.
These variable pathways to water bodies, primarily depositing inorganic forms of mercury, lead to mercury in water bodies. And through conversion to Me-Hg, and bioaccumulation, we have Me-Hg magnifying up the food chain.
Scientific and regulatory communities need to know where mercury is being added to the environment — at what rates, in what concentrations, and by what routes. Which brings us to why the Mercury Deposition Network is sampling for mercury in the environment.
Maps / GIS
Annual gradient maps of precipitation-weighted mean concentrations and deposition are available for MDN. These maps are available in PDF and grid formats.
About the maps
In 2011, the NADP Technical Committee instituted changes in the way NADP annual concentration and deposition maps are produced. It was decided to modify the original map series from a discrete contour map style to a continuous color gradient map style while incorporating an external, highly resolved precipitation dataset. More information about the process used to create these maps is available below.
NADP annual concentration and deposition maps represent a modeled, spatial interpolation of quality-controlled point observation data from NTN and MDN sites. Uncertainty in the measurements and the spatial interpolation is an unavoidable part of the process. The intended purpose of these maps is to represent regional trends in concentration and deposition data. As such, we do not have measurement sites near local sources of air pollutants, such as urban areas and industrial sites. Uncertainty within these maps varies geographically and has not been quantified. Higher levels of uncertainty can be expected when looking at regions with large topographic variability, near urban and industrial areas, and in regions isolated from nearby NADP sites. Uncertainty also increases when viewing the maps at smaller scales and evaluating small-scale features.
Users of the maps are warned to use caution when making decisions based upon localized estimates within the map where measurements were not taken.
The concentration gradient maps are generated using the Inverse Distance Weighting interpolation method.
The deposition surfaces are not directly interpolated. Instead, they are generated as a product of two interpolated surfaces – the corresponding concentration surface, and a precipitation surface that combines NADP-measure precipitation values with modeled estimates
The annual composite precipitation surfaces are derived from an adapted version of a high resolution precipitation model developed by the PRISM Climate Group, and supplemented with NADP precipitation observations. PRISM stands for “Parameter-elevation Regression on Independent Slopes Model”. The PRISM modeled precipitation estimates incorporate point observation data, a reliable digital elevation model (DEM), and expert knowledge of complex climatic variables that result in high resolution, continuous, digital grid estimates of total annual precipitation.
NADP modified the original PRISM surfaces by adding in the additional NADP precipitation observations. The annual precipitation surfaces were created by using an inverse distance weighting (IDW) method to calculate a weighted value for every grid cell within a 30 km radius of each NADP precipitation site. The weighted values are calculated using a combination of the PRISM modeled precipitation data, and the NADP observed precipitation values. The weighting function was established so that as you approached the edge of the 30 km radius the values of the weighted grid cells approached that of PRISM. Outside of the 30 km radius the annual precipitation grid cells were populated using only PRISM data.
Compared to generating a precipitation surface using only NADP station data, this process should result in annual precipitation, and subsequently deposition maps with improvements in the estimation of interpolated values, especially in regions of highly complex terrain such as the Rocky Mountains.
Acceptable Use Policy
If you intend on reprinting any of these maps for use in a publication, please read our Use Conditions.
There are two collectors approved for MDN use, one is a modified wet-only deposition collector used by NADP for other observations and the other is commercially available. Both collectors utilize a sampling chimney, and are enclosed and insulated. A heater is used to maintain the winter temperature between 10 and 15 °C to prevent samples from freezing. Heat from the interior rises up the chimney to warm the collection funnel which melts ice and snow on contact with the funnel. A ventilation fan pulls filtered air through the sampler during warm weather to moderate the inside temperatures.
The mercury sampling train consists of an approximately 128-mm diameter, borosilicate glass funnel, an acid-cleaned, wide-bore (3-mm) capillary tube, and either a 1L or 2L PETG bottle. The capillary tube is connected to the funnel by a ground glass fitting. The opposite end of the capillary tube is expanded to form a sphere with a diameter slightly larger than the mouth of the sample bottle. A small hole in the sphere allows water to drain into the sample bottle. Sample bottles are pre-charged with 20.0 mL of dilute (0.12 M) hydrochloric acid as a preservative to prevent microbial action and provides sufficient chloride ion to complex mercury in solution and prevent conversion to and loss of HgO.
The field operators receive a pre-cleaned sampling train each week. Every Tuesday, the exposed sampling train is removed using clean techniques and returned to the MDN laboratory, Wisconsin State Laboratory of Hygiene in Madison, WI (Hg Analytical Laboratory, “HAL”), along with the sample bottle containing any collected precipitation. All operators wear plastic gloves when handling the sampling train and follow special procedures to avoid contaminating the sample. Any overflow from the bottle is collected and measured to determine the total precipitation amount, but it is not included with the sample sent to the lab.
Each site is also equipped with a weighing-bucket rain gauge that provides a continuous record of rainfall amounts. Rainfall is recorded to the nearest 0.01 in. The rain gauge also monitors the precipitation sampler, recording whether the sampler was properly open during wet periods and closed during dry periods.
Each Tuesday precipitation samples are collected through acid cleaned glass sampling trains into pre-acidified PETG square media bottles. Samples trains and bottles are cleaned, bagged, quality assured and shipped each week. Upon receipt to the laboratory, samples are evaluated, logged in, weighed to assess capture efficiency and tracked through analysis. Total Mercury analysis is performed in the Trace Element Clean Laboratory (TECL) on a Tekran 2600 following USPEA Method 1631 Revision E. Monthly compositing for Methyl Mercury following USEPA 1630 is an option.
Field results are verified weekly, lab results are reported monthly. Lab reports include weekly precipitation, Mercury concentrations and deposition for each site.
The current listing of laboratory SOPs are available. Copies are available upon request.