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

 

3. Results and Discussion
3a-1. Precipitation

From June 1, 1996 to May 31, 1997, we measured a total of 122 cm of precipitation at Piney Dam. In contrast, the Piney Dam weather station measured a total of 131 cm of precipitation, which was the 4th highest annual total recorded over their 10-year precipitation record. The 9 cm between site differences in total precipitation is related to differences in measurement techniques (Belfort vs. wedge gauge) and local wind patterns.

HCWS received 144 cm of precipitation, 22 cm more than Piney Dam. Higher precipitation at HCWS is related to orographic differences across Garrett County, Maryland. Long-term records show a regional gradient in precipitation, with the highest precipitation near Oakland and lower precipitation near Piney Dam (Stone and Matthews 1974; University of Maryland Office of Climatology 1998).

3a-2. Major Ions

Wet deposition of most major ions, except Na+ and K+, were 2 to 5 times higher in the growing season than in the dormant season. Nitrate, SO4-2, and H+ exhibited the most striking seasonal differences in wet deposition rates. This seasonal pattern reflects seasonal variations in the major ion sources, differences in precipitation scavenging by snow (dominant form of precipitation in dormant season), or meteorological conditions. If local anthropogenic sources were the dominant source of these ions, we would then expect higher deposition in the dormant rather than the growing season because more coal and wood are burned locally during the dormant season (winter and fall). Some of the enhanced major ion inputs could also be related to increased growing season precipitation, which was 1.1 times greater than precipitation during the dormant season.

The ionic composition of wet deposition at Piney Dam was dominated by H+ and SO4-2 ions. Annual volume-weighted mean concentration of H+ was 66 ueq L-1, which accounted for 70% of the total cation equivalents. Ammonium, the next most abundant cation, had an annual volume-weighted concentration of 16 ueq L-1, and accounted for 17% of the total cation equivalents. The remainder of the total cation equivalents (13%) was accounted for by the sum of the base cations (K+, Na+, Ca+2 and Mg+2). Annual average volume-weighted concentration of SO4-2 was 51 ueq L-1 and accounted for 58% of the total anion equivalents. Nitrate, the second most abundant anion, had an annual volume-weighted concentration of 29 ueq L-1 and accounted for 32% of the total anion equivalents. Chloride accounted for the remainder (10%) of the total anion equivalents.

With the exception of Na+, major ion concentrations in wet deposition at Piney Dam were at the high end of the range of the long-term (10-year) average annual volume-weighted wet deposition concentrations reported for other monitoring stations in our region. For example, wet deposition at Piney Dam had the highest concentrations of H+, K+and NH4+; second highest concentrations of Ca+2, NO3-, and Cl-; and third highest concentrations of SO4-2. Variations in precipitation concentrations among these sites, however, ranged from 0.3 ueq L-1 for K+ up to 15 ueq L-1 for H+. This lack of large regional variation in major ion concentrations in wet deposition suggests that a reasonable estimate of wet deposition of major ions to HCWS can be made using concentration data from Piney Dam and wet deposition amounts measured at HCWS. This extrapolation method, which accounts for spatial variations in wet deposition amounts and assumes that the precipitation concentrations are similar among sites, has also been used by other researchers (e.g., Dow and DeWalle 1997) to estimate wet deposition of major ions for sites without site specific precipitation chemistry data.

Wet deposition of H+, K+, NH4+, NO3- and SO4-2 to HCWS were at the high end of the range in annually averaged volume-weighted wet deposition rates reported for many other sites in the eastern United States. Relatively high wet deposition to HCWS reflects both the slightly higher (0.3 to 15 ueq L-1) precipitation concentrations and higher rainfall amounts. Hydrogen deposition to HCWS was almost identical to the long-term annual average H+ deposition at the Fernow Experimental Forest in Tucker County, West Virginia, but was 87 to 448 eq ha-1 yr-1 greater than H+ deposition at the other selected sites. Our K+ deposition rate was 4 to 16 eq ha-1 yr-1 higher than the long-term average K+ deposition for the selected sites. High K+ deposition to HCWS may reflect local differences in soil composition. Our high NH4+ deposition is most likely related to our daily collection protocol, which minimizes the amount of NH4+ that is converted to NO3- by microbial processes in the collection bottle. Wet deposition of SO4-2 to HCWS was 36 to 294 eq ha-1 yr-1 higher than SO4-2 deposition at 10 of the sites, but was 36 and 202 eq ha-1 yr-1 less than SO4-2 wet deposition to two nearby sites in WV and PA, respectively. Collectively, these results suggest that HCWS receives some of the highest wet deposition rates of H+, SO4-2, NO3- and NH4+ in the eastern United States.


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This page was updated on March 30, 2001.