ANC Values Research Articles

Episodic acidification during snowmelt of high elevation lakes in the Sierra Nevada mountains of California

Atmospheric loads to dilute lakes in the Sierra Nevada mountains of California are very low, and fall almost entirely as snow. When acidic anions preferentially elute from melting snow, these low loads may nontheless be enough to acidify low ANC lakes. Two of the ten lakes included in the Sierra Episodes Study are discussed here: High Lake, the only lake in the study to become acidic during snowmelt; and Treasure Lake, typical of the remainder of the lakes. All lakes exhibited increases in NO3− concentrations during early snowmelt; these were accompanied by increases in base cations, primarily Ca2+. In the first few days of snowmelt, NO3− concentrations at High Lake increased more rapidly than concentrations of base cations, resulting in ANC values below zero. Export of both NO3− and SO42− from the watersheds exceeded the inputs from the snowpack, suggesting that other sources (e.g., watershed minerals, stored inputs from the previous summer, transformations of other inputs) of these anions are important.

Dilute, neutral pH standard of known conductivity and acid neutralizing capacity

By using the equilibrium computer program MINTEQA2, a quality control standard suitable for assessing measurement errors of pH, acid neutralizing capacity (ANC, formerly titration alkalinity) and conductivity of dilute, neutral pH solutions [ionic strength ≈ 4.6 × 10–4 mol kg–1 of solvent (water)] has been developed and tested. A dilute phosphate standard is prepared as a 200-fold dilution (by mass) of 0.025 mol kg–1(of solvent) KH2PO4 and Na2HPO4 standard buffer solution. At 25 °C and in equilibrium with 340 vppm (parts per million by volume) CO2, the standard has a theoretical pH of 6.89, a calculated ANC of 125 × 10–6 equiv kg–1(124.6 × 10–6 equiv dm–3) and a calculated conductivity of 37.6 × 10–4 S m–1. Theoretical pH and calculated ANC and conductivity values were also determined at equilibrium with laboratory CO2 concentrations ranging between 0 and 680 vppm. The dilute phosphate standard was measured repeatedly over a 6 month period at three laboratories. Measurements agreed well with the theoretical values (mean pH = 6.87, 2s=±0.100, n= 362; mean ANC = 124.5 × 10–6 equiv dm–3, 2s=±8.78, n= 294; mean conductivity = 37.7 × 10–4 S m–1 at 25 °C, 2s=±5.22, n= 308). The long-term systematic and random errors associated with the dilute phosphate standard are reduced for pH (because of reduced sensitivity to changes in CO2), and comparable for ANC and conductivity as compared to a dilute hydrogen carbonate standard (7.0 × 10–5 mol kg–1 NaHCO3 and 1.75 × 10–4 mol kg–1 KCl). For the dilute phosphate standard, among-day variation within a laboratory is the greatest source of error for ANC and conductivity measurements. For the dilute hydrogen carbonate standard, among-laboratory variation is the greatest source of error in pH measurements. Within-day variation is the greatest source of error in ANC measurements for the dilute hydrogen carbonate standard, while among-day variation within a laboratory is the greatest source of error for conductivity measurements of both dilute standards. Additional application of total speciation programs to the solving of design problems for quality control and assurance samples appears promising and justified.