An Alternate Method for Creating a Statewide Isopleth Map of Total Mercury Wet Deposition with an Example for Indiana

Martin Risch, Kathleen Fowler, and Nancy Baker
U.S. Geological Survey, 5957 Lakeside Blvd., Indianapolis, IN 46278

Valuable information about mercury in precipitation is illustrated by the National Atmospheric Deposition Program (NADP) through isopleth maps of annual mercury concentrations and mercury wet deposition at sites in the Mercury Deposition Network (MDN) in North America. These maps show the locations of MDN sites and list either the annual precipitation-weighted average total mercury concentration or the annual mercury wet deposition. More importantly, these maps illustrate an interpretation of the spatial distribution of ranges of mercury concentrations or mercury wet deposition, primarily in eastern North America, by use of colored isopleths. The isopleths are prepared with software in a geographic information system (GIS) that uses an inverse-distance weighting algorithm to estimate mercury concentrations or mercury wet deposition values for cells in a grid overlying eastern North America.

Isopleth maps of mercury wet deposition for an individual state derived from the NADP North America maps can lack the desired level of detail because the spatial distribution of sites is too sparse, and because the isopleth ranges are too broad for state-level interpretations. An alternate method for preparing a statewide map of total mercury wet deposition was utilized in an example for Indiana. Because mercury wet deposition is computed as the product of mercury concentration and precipitation, a wet deposition isopleth map was made by using precipitation data from a dense array of 127 National Weather Service (NWS) cooperative observer sites in Indiana.

To prepare the mercury wet deposition maps with the alternate method, a GIS was used to create an isopleth map of precipitation-weighted average mercury concentrations from MDN sites in Indiana and surrounding states, with data from 2001–2006. The Indiana map isopleths had a 0.5 nanogram per liter (ng/L) concentration range interval, compared with a 2 ng/L range found in the NADP maps. Average annual precipitation (2001–2006) for the 127 NWS sites was overlain with the mercury concentration isopleths map in order to assign a mercury concentration value for each NWS site. The concentration value that was associated with each NWS site was multiplied by the precipitation value for that site to obtain a mercury wet deposition value. The GIS then was used to create an isopleth map of mercury wet deposition for Indiana, based on the values computed for the 127 NWS sites.

The poster presents the methods and results of the alternate method, compares it with results from the (traditional) NADP method, and discusses limitations and uses of the new maps.