Assessing groundwater coupling and vertical exchange in a meromictic mining lake with an SF6-tracer experiment

Groundwater–lake interaction in conjunction with surface mixing and vertical exchange were quantitatively examined for the monimolimnion in a small stratified mining lake. Although the lake is shallow (∼4.7 m), it shows a strong meromixis. The permanent chemocline, which is connected with stable density stratification, separates the mixolimnion from the anoxic monimolimnion at a variable depth of only 0.6–1.9 m. We conducted a tracer experiment, injecting ∼3 μmol of anthropogenic gas SF6 into the monimolimnion. With monthly measured vertical concentration profiles extended over 2 years we detected and quantified the groundwater coupling. An exchange time close to once per year was estimated for the monimolimnion by balancing the tracer. From CTD casts and thermistor chain data we extracted additional information about mixing. For the water and tracer balances, we included the erosion of the monimolimnion surface by strong daily convective mixing in the mixolimnion during the warm period. SF6 background measurements before spiking indicate that diffusive transport through the chemocline density step must be very low, i.e., close to the molecular level. The observed seasonal variability of the chemocline depth can be considered as the result of a virtually steady inflow of groundwater, which supports the growth of the monimolimnion, and strong seasonal mixing in the mixolimnion, which acts against the stratification and erodes the monimolimnion. Over longer time scales, the chemocline is to be found at the level where erosive processes balance the recharging of the monimolimnion from groundwater.