Ectogenic Meromixis of Lake Hallstättersee, Austria Induced by Waste Water Intrusions from Salt Mining
2011
Ficker, Harald | Gassner, Hubert | Achleitner, Daniela | Schabetsberger, Robert
Lake Hallstättersee is a holomictic alpine lake, which is influenced by salt mining since the middle Bronze Age. Beside the constant saline waste water load, two massive brine spills loaded the lake with additional 16,900Â tons sodium chloride (≈10,250Â tons Cl−) from 1977 to 1979 and 3,000Â tons salt (≈1,820Â tons Cl−) in 2005. The effect of waste water intrusions from salt mining on stratification of Lake Hallstättersee was analysed over a period of 40Â years. Water density, dissolved oxygen and total phosphorus (TP) concentrations were measured and an exponential model was fitted to describe the wash-out of chloride from Lake Hallstättersee after the brine spills. Furthermore, the time required returning to holomixis and steady chloride content after the second brine spill was estimated. During the whole sampling period the minimum and maximum volume-weighted annual mean chloride concentrations were 23.58Â mg/L in 1979 and 3.19Â mg/L in 1998. However, the mixing regime of Lake Hallstättersee, as well as the chloride concentrations, varied considerably and exhibited three holomictic and three meromictic periods between 1970 and 2009. Holomictic periods were observed when the yearly density gradient was below 0.06Â kg/m3, deepwater oxygen in spring above 4Â mg/L and consequently declining TP concentration in the deepest water layer below 60Â mg/m3, otherwise meromictic periods were observed. Our study showed that Hallstättersee was 13Â years ectogenic meromictic and 27Â years holomictic during the study period.
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