In northern regions, spring snowmelt generally results in an episodic decli
ne of surface water pH. Natural organic acids may be an important factor in
this type of pH change. We studied the variations in the acid/base charact
er of aquatic organic acids during spring snowmelt in 1997 at Svartberget,
a stream rich in total organic carbon (TOC) that is located in the boreal z
one of northern Sweden. Snowmelt at Svartberget was accompanied by a drop i
n stream pH of up to 1.8 pH units, as well as the dilution of base cation a
nd strong acid anion concentrations. Aluminum and TOC increased or decrease
d during snowmelt, depending on the sampling site within the 50-ha catchmen
t. Although there were distinct differences in pH, TOC, and major inorganic
ions in the runoff from three subcatchments, there was very little variati
on in the acid/base character of TOC. Thus organic acids in the Svartberget
catchment share a common set of acid/base properties despite markedly diff
erent subcatchment drainage patterns, vegetation, and soils. The dissociati
on behavior of organic acids at Svartberget could be described with high pr
ecision (R-2 = 0.91, P < 0.001, and n = 115) by a triprotic acid analog mod
el (pK(al) = 2.5, pK(a2) = 4.0, and pK(a3) = 5.8), together with the measur
ed site density of organic acids (8.6 <plus/minus> 0.8 mu eq (mg TOC)(-1)),
A Gaussian pK(a) distribution (mu = 4.20 and sigma = 1.43) predicted organ
ic acid dissociation with similar precision (R-2 = 0.91, p < 0.001, and n =
94). Variations in site density among the tributary streams could largely
be explained by aluminum complexation. Sites with lower measured site densi
ties had greater concentrations of organically bound Ai. Thus Al binding re
duces the potential release or neutralization of H+ by organic acids.