The air-water exchange of C-15-C-31 n-alkanes in a precipitation-dominatedseepage lake

The air-water exchange of semivolatile II-alkanes in Crystal Lake, a small precipitation-dominated seepage lake in northern Wisconsin, was investigate d with modeling and mass balance approaches, The results suggest that atmos pheric deposition contributes approximately 80% of the allochthonous input of il-alkanes to Crystal Lake. Atmospheric deposition accounts for about 50 % of the total annual input of n-alkanes to Crystal Lake, and an additional 30% is contributed by in situ production of planktonic n-alkanes (Sigma C- 15, C-17, C-19) Contributions to the particle dry flux of terrestrial n-alk anes (Sigma C-25. C-27, C-29, C-31) by pine pollen dispersal and by dry dep osition of particles containing leaf waxes are similar in magnitude and con stitute about 60% of the atmospheric input, with particle wet deposition be ing responsible for the remainder. Approximately 30% of the atmospheric inp ut of the il-alkanes occurs during a two-week episode of pine pollen disper sal in spring. Concentration gradients between gaseous, n-alkanes in the at mosphere and dissolved n-alkanes in the water column of Crystal Lake favor volatilization of n-alkanes from the lake surface; however, distributions o f dissolved n-alkanes are characteristic of bacteria, and therefore are con tained in organic matter and not available for air-water exchange. The esti mated net atmospheric input of terrestrial n-alkanes is about 20% less than the settling sediment flux, Additional allochthonous sources of the terres trial n-alkanes might include diffuse surface runoff or episodes of coarse- particle deposition. The discrepancies in the results from the modeling and mass balance approaches indicate that direct measurements of air-water exc hange rates and measurements of the seasonal variations of particle size di stributions in air and rain would greatly improve our ability to quantify a ir-water exchange rates of n-alkanes. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.