Annual Elemental Input/Output Estimates for Two Forested Watersheds in Eastern Tennessee

Abstract

AbstractAtmospheric inputs of SO2−4, NO−3, PO3−4, NH+4, Ca2+, Mg2+, Na+, and H+ via wet and dry deposition were compared to stream water outputs at two study sites, Camp Branch and Cross Creek, during the period 1978 to 1983. The Cross Creek site is located 19 km from a major coal‐fired point source and has been subjected to approximately 30 yr of elevated atmospheric input while Camp Branch is located in an area removed from the possible local influences of a major point source. Hydrologic comparison of mean streamflow output to incoming precipitation indicates a greater loss of water (64%) to streamflow at Cross Creek than Camp Branch (56%). No site differences (p = 0.05) could be detected in mean annual wet input for any element. The dryfall comparison indicated only a higher level of Mg2+ input at Cross Creek. The relative contributions of anions and cations in both wetfall and dryfall was similar at both sites with SO2−4 > NO−3 > PO3−4 for the anions and H+ > Ca2+ > NH+4 > Na+ > Mg2+ > K+ for the cations. Export of anions in streamflow had the same relative relationship as observed for wetfall and dryfall. The SO2−4 ion accounted for 86% of the measured anion export at Camp Branch and 91% at Cross Creek. Although SO2−4 export varies between sites as a function of flow, concentration values remain relatively constant from year to year. Cation export was dominated by Mg2+ at Camp Branch, followed by Ca2+ > Na+ > K+ > NH+4 > H+. At Cross Creek, the order of cation export was Mg2+ > Ca2+ > Na+ > K+ > H+ > NH+4. Comparing base cation inputs to outputs, there is a mean efflux of 203 molc ha−1 yr−1 at Cross Creek. Although SO2−4 retention rates are 78% of input, it is still the dominant measured companion anion, although there is circumstantial evidence for a substantial HCO−3 contribution. The H+ ion input is also almost completely retained within the system. Base cation loss at Cross Creek was 31% higher than that at Camp Branch. However, it is difficult to demonstrate any significant impact of the elevated historical S input because nutrient export was highly correlated with streamflow and streamflow volume was 29% higher at Cross Creek.