Critical Loads of Acid Deposition for Wilderness Lakes in the Sierra Nevada (California) Estimated by the Steady-State Water Chemistry Model

Abstract

Major ion chemistry (2000–2009) from 208 lakes (342 sample dates and 600 samples) in class I and II wilderness areas of the Sierra Nevada was used in the Steady-State Water Chemistry (SSWC) model to estimate critical loads for acid deposition and investigate the current vulnerability of high elevation lakes to acid deposition. The majority of the lakes were dilute (mean specific conductance = 8.0 μS cm−1) and characterized by low acid neutralizing capacity (ANC; mean = 56.8 μeq L−1). Two variants of the SSWC model were employed: (1) one model used the F-factor and (2) the alternate model used empirical estimates of atmospheric deposition and mineral weathering rates. A comparison between the results from both model variants resulted in a nearly 1:1 slope and an R 2 value of 0.98, suggesting that the deposition and mineral weathering rates used were appropriate. Using an ANClimit of 10 μeq L−1, both models predicted a median critical load value of 149 eq ha−1 year−1 of H+ for granitic catchments. Median exceedances for the empirical approach and F-factor approach were −81 and −77 eq ha−1 year−1, respectively. Based on the F-factor and empirical models, 36 (17 %) and 34 (16 %) lakes exceeded their critical loads for acid deposition. Our analyses suggest that high elevation lakes in the Sierra Nevada have not fully recovered from the effects of acid deposition despite substantial improvement in air quality since the 1970s.