Elemental mass balances, and episodic and ten-year changes in the chemistry of surface water, Acadia National Park, Maine : final report

Heath, Richard; Kahl, Jeffrey; Norton, Stephen; Brutsaert, Willem

Water quality data collected in 1988-90 at Acadia National Park were compared to data from the same sites collected in 1982-84. 'The data show that ANC has increased by a mean of 5.5 ueqL-1. This small change in ANC was not statistically significant (p=0.05) compared to 1982-84. In contrast to the regional trend of decreasing SO4, mean sulfate concentrations were 2.2 ueqL-1 higher, also not significant at the 0.05 level. We did not re-sample stream stations studied in the early 1980s. These results for Acadia National Park in the context of other data in Maine are discussed in Kahl et al. (1993).' 'Major episodic acidifications have been observed on numerous occasions in first-order brooks and second-order streams at Acadia National Park. Contributing factors include natural acids from soil solutions, and H2SO4 and HNO3 from acid precipitation. However, in some acidic events, short-term declines of up to 2 pH units and 130 ueqL-1 ANC were caused by HCL from soil solutions (Heath et al., 1992), rather tan by H2SO4 or HNO3 from precipitation. The major evidence is: 1) SO4 concentrations were constant or decreased during the pH depression, 2) Cl concentrations were greatest at the time of lowest pH, and 3) Na:Cl ratios decreased from values much greater than those in precipitation (a result of chemical weathering), to values equal to or less than those in precipitation.' 'Other mechanisms of episodic acidification include dilution by increased flow, increases in NO3 concentrations from precipitation or the large soil N pool, and increased export of organic acidity from soils (Kahl et al., 1992), but these were often insufficient to cause the observed increases in acidity. Increases in SO4 were even more rarely associated with episodic acidification. Many of the episodic data are interpreted to represent surface water acidifications due primarily to an ion-exchange 'salt effect' of Na+ for H+ in soil solution, and secondarily to dilution, neither of which are directly related to acidic deposition.' 'The salt effect phenomenon was observed principally in first-order, upland sites with thin soils. Higher order sites at lower elevation are too well buffered to readily acidify, although the salt effect was evident in some of these streams. High concentrations of salt relative to normal levels are necessary for the effect to be apparent. The prerequisite condition for a major episodic salt-effect acidification is acidic soils. Percolation of solutions with high salt concentrations through these soils causes the salt-effect. The salty solutions can be from either an especially salt-laden wet precipitation event, or as a result of accumulation of marine salts from dry deposition onto soils or snow, followed by wet precipitation and/or snow melt. Acidic precipitation is a contributing but non-essential factor in these episodic acidifications. It is not possible with the data available to estimate the contribution of acidity in precipitation to salt effect acidifications.' 'Estimates of watershed mass balances indicate that the Upper Hadlock Pond watershed is exporting products of mineral weathering (base cations, Si, and Al) in amounts similar to other watersheds in eastern Maine. However, more N is retained in the Acadia catchments than at two comparable catchments 60 km north of Acadia. Leaching of nitrogen from terrestrial systems does not presently appear to be nearly as high as in some other regions of the northeastern U.S. (Aber et al., 1989; Kahl et al., 1993).' 'Based on the mass balance calculations, dry deposition of Cl and SO4 occurs in amounts equivalent to wet deposition. Substantial amounts of base cations and NO3 are also deposited in dry form, but calculation of the fractions is not possible with available data.' (Abstract continued in Notes)