Redox and carbon cycling in mine lakes of the Upper Harz Mountains (Germany)

The Harz Mountains, situated in the north of Germany, are about 120 km long and about 40 km wide. Their highest moun­tain is Mount Brocken (1,141 m a.s.l.). The mountain range is known for ancient silver and base metal mining. Today the Harz Mountains are an impor­tant drinking water supply region for north­ern Germany.

About 70 lakes are situated around the town of Clausthal-Zellerfeld in the Western Harz Mountains. These lakes were constructed to save a continuous water supply to the ore mines about 200 – 500 years ago. The water depths range from about 3 to 15 m, the storage volume from about 10,000 to 600,000 m³. The lakes are an important part of the UNESCO World Heritage Site "Oberharzer Wasserregal". Most of the lakes are oligotrophic with pH values of about 7 and SEC values below 200 µS/cm (Bozau et al., 2015). Some of the lakes are used for the drinking water supply of nearby communities and are still important for the protection of floods.

From 2023 – 2025, the water column of selected mine lakes was investigated. Samples of the water column were analysed for major ions, trace metals and stable isotopes. In summer, the formation of a deep anoxic layer (hypolimnion) was observed in some lakes. The intensity of anoxic conditions depends on the summer temperatures, precipitation rates and wind conditions. There is a typical chemical stratification of the water column for every single lake. Shallow lakes showed stronger redox reactions than deeper lakes. Colder weather periods with high precipitation rates during the summer time can minimise the extent of the hypolimnion. SEC, bicarbonate, Fe and Mn are enriched in the anoxic layer leading to problems in the traditional treatment of drinking water. Nitrate and sulphate are depleted due the chemical reactions under anoxic conditions. The ratio Mn/Fe proved to be a very sensitive indicator for the formation of the hypolimnion. The δ¹⁸O and δ²H values in the water column of mine lakes also reflect the seasonal stratification. Due to evaporation effects at the water surface the highest δ¹⁸O and δ²H changes are found in mine lakes during summer time. The δ¹³C values in the the water column range between -24 … -13 ‰. The lowest δ¹³C values are found in the anoxic hypolimnion during summer time. Due to warmer and longer spring, summer and autumn seasons the formation of hypolimnia increased in the last years and the treatment of drinking water was adapted.

 

Bozau E, Licha T, Stärk HJ, Strauch G, Voss I, Wiegand B, 2015. Hydrogeochemische Studien im Harzer Einzugsgebiet der Innerste. Clausthaler Geowissenschaften 10, 35-46.