Tracer experiment with sulfur hexafluoride to quantify the vertical transport in a meromictic pit lake

Abstract:

This paper describes a tracer experiment with sulfurhexafluoride (SF₆) in the monimolimnion of the meromictic mining lake Merseburg-Ost 1b. In October 1998, 1.1 mmol (<simeq>160 mg) of the conservative gas SF₆ was released at the site of greatest depth 3.5 m above the sediment to observe its vertical spreading. An easy-to-use system to collect ∼0.5 L water samples using evacuated flasks was developed. The headspace technique, gas chromatographic separation and ECD-detection were used to determine SF₆. The main objective is the evaluation of mean vertical diffusion coefficients for SF₆ and heat from periodically measured SF₆ and CTD profiles. During the study period of 1¹/₂ years, the heat transport was estimated to be molecular in the stratified portion. A thermal flux from the sediments of 0.23 W/m² was found necessary to balance the heat. In the region of high stability (N ²∼10^-2 s^-2) diffusivities for SF₆ were ∼ 10^-8 m²/s, whereas in the lower part of the monimolimnion both tracers resulted in K∼ 7 · 10^-6 m²/s. We found K to be approximately proportional to N ^{-2.4 ± 0.2}.