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Input-Output Budgets of
Major Ions, Trace Elements,
and Mercury


4. Conclusions

An evaluation of the results of sampling efforts PPRP conducted as part of this Western Maryland Atmospheric Deposition Study indicate the following conclusions about wet deposition of major ions, trace metals, and mercury and to the Chesapeake Bay watershed. The results for major ions indicate that, HCWS:

  • Receives some of the highest atmospheric inputs of H+, NH4+, NO3- and SO42- in the eastern United States;
  • Is a net sink for H+, NH4+, NO3- and K+;
  • Exports 2 to 5 times more Na+, Ca2+, Mg2+ and SO42- than the throughfall input to the watershed; and
  • Is exhibiting signs of N saturation.
For example, NO3- export from HCWS is similar to that expected for forested watersheds that are at stage 2 along the N saturation continuum described by Aber et al. 1989. Despite the continuous export of NO3-from HCWS, annual NO3- export from HCWS accounted for only 9% of the total anion export of 1.8 keq ha-1 yr-1 in 1996-97. In contrast, however, SO42- export accounted for 78% of the total anion export from HCWS in 1996-97. Additionally, SO42- was the dominant anion associated with the net base cation export from HCWS. Net base cation export from HCWS, particularly Ca2+ and Mg2+, is of great concern because continued net watershed losses of these cations could lead to nutrient deficiencies in the vegetation.

The atmosphere is also an important source of trace elements to ecosystems of western Maryland. In comparison with other studies, we observed a strong regional gradient in wet deposition of many trace elements. For example, wet deposition of Al, Cr, Cu, Fe, Mn, Ni and Pb were highest in western Maryland, intermediate in central Maryland (Bear Branch), and lowest near the east coast (CBADS and Lewes, DE). This pattern reflects both the closer proximity to regional sources and higher precipitation amounts to the Allegheny Plateau of western Maryland.

The forest canopy at HCWS had little effect on the deposition of Al, As, and Pb (throughfall = wet deposition). In contrast, however, the forest canopy affected the deposition of Mn, Fe, Cu, Zn, Se, Cr, Cd, and Ni. The canopy was a net source of Mn, Fe, Cu, Zn, and Se; throughfall deposition for Mn was 60 times greater than wet deposition, and throughfall deposition of Fe, Cu, Zn, and Se were 30–50% greater than wet deposition rates. Higher throughfall Mn deposition involved both wash of dry deposition material and canopy leaching, but the enhanced deposition of the other trace metals are due primarily to wash off of dry deposited materials. Three trace metals (Cr, Cd, and Ni) had throughfall deposition rates that were 2 to 4 times lower than the wet deposition rates which suggests that the canopy was a net sink for these elements.

Our input – output budgets suggest that:

  • Atmospheric inputs of Al equaled the stream water outputs,
  • 50 to 90% of the atmospheric inputs of Pb, As, and Se were retained in HCWS,
  • About 25% of the atmospheric inputs of Fe, Cu, and Cr were retained in HCWS, and
  • HCWS was a net source of Zn, Ni and Cd.
The high export of Fe, Cu, Cr, Zn, Ni and Cd deserves further study because these trace elements are important micronutrients for aquatic organisms.

The wet deposition flux of Hg to western Maryland is similar to that of other rural sites within Maryland (e.g., CBL). The variability between sites, and between years, is largely a function of differences in rainfall amounts. Higher fluxes are seen in years of higher rainfall. The concentration of total Hg in rain did not correlate strongly with any other major ions or trace elements suggesting that the sources of Hg are complex, and no particular source profile predominantes. More detailed analysis will be required to understand the sources of Hg deposited at Piney Dam.

Methylmercury in precipitation is a small fraction of the total, being less than 0.5%. The throughfall flux at HCWS was about 25-35% higher than the wet-only flux. This relative increase in throughfall is less than has been found in other watershed studies. There are a number of potential reasons for this, which are being investigated in our continuing studies. The yield of Hg from the stream is low (20% or less) and is comparable to that found in other forested watersheds. Such a low yield suggests there is a buildup of Hg in soils, which likely mirrors the increased input of Hg to the atmosphere as a result of man’s activities.

The conclusions section is available on-line in pdf format (you will need the Adobe Acrobat reader to view it.)
The entire file (617 KB) is available on-line in zipped format (you will need WinZip or other extraction software to view it. Once unzipped, you will need the Adobe Acrobat reader to view it)


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This page was updated on March 30, 2001.