Status and Trends of Atmospheric Deposition

What is the amount and extent of atmospheric deposition in Maryland? The answer to this question is critical to determining whether deposition poses a problem to natural resources in the State. How is the amount and extent of deposition changing? This question is critical to determining if measures to control deposition, especially the Clean Air Act Amendments of 1990, are having the desired effect.

Acidic deposition has been the target of substantial research for many years culminating in the establishment of a nationwide precipitation chemistry monitoring program at approximately 200 sites under the direction of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). The results of this monitoring consistently show that Maryland and the Chesapeake Bay watershed are within the region of most acidic precipitation in the United States. Maryland and the Bay watershed also receive some of the highest concentrations of sulfate and nitrate in wet deposition in the Nation.

The Maryland Critical Loads Study subsequently confirmed that the geology in these regions of the state allowed resources to be at risk from continued acidic deposition even with the reduction in sulfate promised by the Clean Air Act Amendments of 1990.

The 1987 Maryland Synoptic Stream Chemistry Survey found 33% of streams statewide to be sensitive to acidification (defined as streams with ANC less than 200 Feq/l). The highest proportions of acid-sensitive streams were in the Coastal Plain (74%) and Appalachian Plateau (53%).

Trends in precipitation chemistry for the Chesapeake Bay region have been tracked using data from 24 NADP/NTN and Pennsylvania Atmospheric Deposition Monitoring Network (PADMN) sites. Analysis has demonstrated that concurrent with 1995 emissions reductions, sulfate and hydrogen ion concentrations in precipitation in Maryland and much of the Mid-Atlantic region were lower (note: pH is inversely related to the hydrogen ion concentration) in 1995 than previous years (1983-1994); nitrate concentrations were not as widely affected. Similar reductions have also been reported for the Northeastern U.S.

Three graphs above : Trends in precipitation chemistry at White Rock, Maryland Based on these results it appears that implementation of the 1990 CAAA may have resulted in lower sulfate concentrations in precipitation in Maryland and the mid-Atlantic region. Concurrent with these sulfate reductions have been similar (nearly one for one) reductions in hydrogen ion concentrations. In contrast, nitrate concentrations, as well as chloride and the base cations, do not appear to have been affected. The 1995 declines in sulfate and hydrogen ion concentrations were not attributable to changes in precipitation rates. Overall, these results lend support to the conclusion that the CAAA of 1990 has reduced acidic deposition in Maryland and the Mid-Atlantic region.

Graph above: Recent changes in annual sulfate ion concentrations in precipitation, measured at 24 NADP/NTN and PADMN stations. The percent change represents the difference between 1995 observed ion concentrations and the seasonalized trend modeled from 1983-94 results.

Nutrients: Nitrogen and ammonia deposition to the region contribute significantly to Chesapeake Bay nutrient loadings. Although emissions of NO_x have begun to decline, nitrate concentrations in precipitation did not exhibit a dramatic decrease, as was noted for sulfate. Concentrations show little if any decline in 1995. Controls of point source emissions alone will not likely be sufficient to reduce nitrate deposition, given that vehicular and other area sources make up a large proportion of total emissions of nitrogen oxides. Ammonium, an additional contributor to nutrient loadings, showed a slight increase in precipitation concentrations over the region. Additional contributions of organic nitrogen add to total loadings.

(Source: Chesapeake Bay Program Indicator Workshop 1995)

Toxic contaminants: Atmospheric deposition provides various amounts of trace elements and organic contaminants, in some cases comparable to riverine loads from point and nonpoint sources. Although the extent of our knowledge of trace element and organic contaminant deposition is limited, further steps are being taken to fully address these inputs to Maryland's aquatic systems. Lead has exhibited declines in both emissions and deposition. Information on other trace elements and organic contaminants is at this point insufficient to describe trends or to fully characterize loadings.

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This page was updated on 4/7/04