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


1. Introduction
1a. Background

Atmospheric deposition may be one of the most important non-point sources of nitrogen (N) and trace metals (e.g., Pb, Zn, Cd, As, Se and Hg) to Chesapeake Bay (Tyler 1988; Baker et al. 1997; Mason et al. 1997a, b; Castro and Morgan 1999). For example, past studies suggest that the atmosphere contributes between 25 to 80% of the total N loadings to the Chesapeake Bay (Fisher and Oppenheimer 1991; Hinga et al. 1991, Tyler 1998, Boynton et al. 1995; Jaworski et al. 1997; Castro et al. 1999). Studies of atmospheric deposition of trace elements (e.g., Al, Cr, Zn and Hg) suggest that about 50% of the total loading to the mainstem of the Chesapeake Bay is from direct deposition to surface waters (Baker et al. 1997; Scudlark et al. 1994; Mason et al. 1999).

Estimates of atmospheric N and trace metal loadings to Chesapeake Bay, however, are highly uncertain primarily because of our lack of understanding of the retention and transport of atmospheric inputs by the watershed (Valigura et al. 1994 Castro et al. 1997) to the Bay. Understanding the retention and export of N from forests in the Chesapeake Bay watershed is critical for several reasons:

  • Forests are the dominant land use in the watershed, accounting for 58% of the total land area (Linker et al. 1996);
  • Nitrogen export from forests has not been quantified with great accuracy (Castro et al. 1997), is subject to significant spatial and temporal variability (Morgan et al. 1994; Eshleman et al. 1998; Dow and DeWalle 1997; Wington et al. 1996; Gardner et al. 1996), and may be increasing as forests reach maturity (Peterjohn et al. 1996); and
  • Studies of trace element export from Chesapeake Bay forests have been limited to a single study in north-central Maryland that showed that atmospheric inputs of many trace elements (e.g., Al, Zn, Ni, Mn and Cd) are mostly transmitted through the watershed, while others (e.g., Cu, Cr, Se, As, Fe and Pb) are largely retained, presumably as organically-bound phases (Church et al. 1998).

1b. Objectives

The primary objective of this study was to increase our understanding of the retention and movement of major ions and trace elements in forested watersheds. Our specific objectives were to:

  • Measure precipitation concentrations and estimate wet deposition rates of major ions (H+, K+, Na+,Ca+2, Mg+2, NO3-, SO4-2, and Cl-), trace metals (Al, As, Cd, Cu, Cr, Fe, Mn, Ni, Pb, Se, and Zn), and mercury (total and methymercury) to a forested watershed on the Allegheny Plateau of western Maryland;
  • Measure throughfall concentrations and estimate throughfall deposition rates of major ions, trace metals in the same forested watershed;
  • Measure stream water concentrations and estimate export rates of major ions, trace metals in the same watershed that we measured the throughfall inputs; and
  • Determine, by mass balance, watershed scale input-output budgets of major ions, trace metals and mercury.

The introduction 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.