Acid/base character of organic acids in a boreal stream during snowmelt

In northern regions, spring snowmelt generally results in an episodic decline 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 character 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 zone of northern Sweden. Snowmelt at Svartberget was accompanied by a drop in stream pH of up to 1.8 pH units, as well as the dilution of base cation and strong acid anion concentrations. Aluminum and TOC increased or decreased during snowmelt, depending on the sampling site within the 50-ha catchment. Although there were distinct differences in pH, TOC, and major inorganic ions in the runoff from three subcatchments, there was very little variation in the acid/base character of TOC. Thus organic acids in the Svartberget catchment share a common set of acid/base properties despite markedly different subcatchment drainage patterns, vegetation, and soils. The dissociation behavior of organic acids at Svartberget could be described with high precision (R² = 0.91, p < 0.001, and n = 115) by a triprotic acid analog model (pK_a1 = 2.5, pK_a2 = 4.0, and PK_a3 = 5.8), together with the measured site density of organic acids (8.6 ± 0.8 μeq (mg TOC)^-1). A Gaussian pK_a distribution (μ = 4.20 and σ = 1.43) predicted organic acid dissociation with similar precision (R² = 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 densities had greater concentrations of organically bound A1. Thus A1 binding reduces the potential release or neutralization of H⁺ by organic acids.