Successful restoration of Lake Gross-Glienicker (Berlin, Brandenburg) with combined iron treatment and hypolimnetic aeration
Klaus-Dieter Wolter & Wilhelm Ripl
Technische Universität Berlin
Department of Limnology
e-mail: cf. >Kontakt
Introduction
Lake Gross-Glienicker is situated in Federal Republic of Germany at the border between the federal states Berlin and Brandenburg. Until the sixties the lake had high water quality and extended submerged vegetation. Since the seventies untreated sewage from military barracks with 2,000 soldiers was discharged into the lake. Secchi depth decreased to below 30 cm, submerged vegetation disappeared. Mean total phosphorus concentration increased to 0,54 mg/l P (1989-1992) (cf. fig. 1). Oxygen in hypolimnion was depleted. Subsequent temporary smells of hydrogen sulphide and death of fish occurred.
After re-unification since 1990 sewage was diverted for treatment. Nevertheless the lake did not react to this diminishing of loadings. Therefore authorities for water management of Berlin ("Senatsverwaltung für Stadtentwicklung, Umweltschutz und Technologie") engaged the authors to develop measures for oligotrophication of the lake.
In this paper, restoration measures and their success 6 years after execution are documented.
Description of Lake Gross-Glienicker before restoration
The catchment area (20 km2) of Lake Gross-Glienicker is mainly used for farming, forestry, and settlements. The littoral is used for bathing, recreation, and gardens. Since 1990 the lake has no continuous in- and outflow. Morphometrical, hydrological and limnological data are listed in tab. 1. The lake is dimictic and during winter mostly ice-covered for several weeks.
The lake developed thermal stratification in April or May. Already from June oxygen in hypolimnion was depleted and hydrogen sulphide was formed. During total mixing in October oxygen saturation in the whole lake often decreased to less than 20 % with the consequence of death of fish. In summer a dense algae bloom established. During total mixing phosphorus concentration lay between 0.5 and 0.7 mg/l tot-P (fig. 1).
Before 1992 sediment surface (0-25 cm) consisted of sapropel with high water and loss of ignition content. The content of iron at the sediment surface was low (20 mg/g dm Fe, cf. tab. 1).
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before restoration, |
after restoration, |
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until 1992 |
since 1993 |
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location |
52°28´ N, 13°6´50´´ W |
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hydrology |
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catchment area |
~ 20 km2 |
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nominal hydraulic residence time |
~ 4 a |
~ 16 a |
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morphometry |
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lake area |
0.67 km2 |
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maximum length |
1.5 km |
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maximum width |
0.4 km |
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lake volume |
4.3 Mio m3 |
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maximum depth |
10.75 m |
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mean depth |
6.5 m |
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water |
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summer Secchi depth: 5 %-percentile |
0.86 m |
2.1 m |
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95 %-percentile |
2.6 m |
5.2 m |
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mean total phosphorus concentration |
0.54 mg/l P |
0.038 mg/l P |
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mean total nitrogen concentration |
2.1 mg/l N |
1.3 mg/l N |
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sediment (0-2.5 cm layer) |
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mean iron content |
20 mg/g dm Fe |
160 mg/g dm Fe |
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mean phosphorus content |
2.1 mg/g dm P |
4.0 mg/g dm P |
Tab. 1: Morphometrical, hydrological and limnological data of Lake Gross-Glienicker (Sources: unpublished depth contour map Senatsverwaltung für Stadtentwicklung und Umweltschutz 1992; own calculations).
Restoration measures
In 1992 a restoration concept for Lake Gross-Glienicker with the aim of improvement of water quality was developed by the authors. For this task experience from Sweden and restoration projects in Germany were used (e.g. Ripl 1978, Björk 1988). Until 1992 partial dredging, hypolimnetic aeration as well as pumping lake water through a mobile phosphate elimination plant were discussed as restoration measures. Because of the size of the lake, the low phosphorus binding capacity of the sediments, and economical reasons these measures were regarded as ineffective.
Because of sewage diversion in 1990 and relative long hydraulic residence time of Lake Gross-Glienicker (tab. 1) a sediment/water treatment for in-lake phosphorus precipitation and increasing iron concentration in sediments (Ripl 1978, Wolter 1994) was planned to improve water quality significantly. The treatment was carried out in autumn and winter 1992/1993 with 500 g Fe per square meter lake area. Concentration of iron at sediment surface increased to 160 mg/g dm Fe (tab. 1). To decrease the formation of sulphide in hypolimnion and in the iron enriched sediment, iron treatment was combined with hypolimnetic aeration, which is operated during summer stratification.
Fig. 1: Secchi depth, total nitrogen, nitrate and nitrite nitrogen, and total phosphorus 1989 to 1998 before and after iron treatment of Lake Gross-Glienicker. Volume weighted means for total volume, epilimnion, and hypolimnion.
The lake after restoration
After iron treatment total phosphorus concentration of lake water reached a minimum of 12 µg/l P (volume weighted mean) by co-precipitation with iron in mid March 1993. Since that time total phosphorus stabilised at a mean value of 38 µg/l P (fig. 1). Phytoplankton biomass decreased markedly, too. Secchi depth after treatment was practically always more than 2 m (tab. 1). Simultaneously mean nitrogen concentrations decreased by coupled nitrification-denitrification at the sediment-water boundary layer from 2.1 to less than 1.3 mg/l N (fig. 1). During the highly eutrophic phase a lot of sapropel with easily degradable organic substance accumulated in the sediments.
1998 for the first time after restoration aeration was not continuously operated during whole summer stratification. Despite increased oxygen depletion in deep water phosphate was not released from the sediments. After iron treatment the former vegetation free littoral zones are slowly colonised by Elodea canadensis, Zannichellia sp. and above all Myriophyllum sp.
Conclusions
Iron treatment combined with measures for oxidation of sapropel and excessive denitrification at the sediment surface seems to be one of the most effective restoration measures for eutrophicated lakes. For oxidation of sediments nitrate treatment (Ripl 1978, 1994) or hypolimnetic aeration can be utilised. The high effectiveness of iron treatment is based on the in-lake immobilisation of phosphorus by binding to iron and the fixation of sulphide in the sediment. High iron content in sediments provides a dynamic buffer, which can be released together with phosphorus, facilitating renewed phosphorus co-precipitation under aerobic conditions. By oxidation of easily degradable organic substance the energetic drive for re-mobilisation of phosphorus from sediments is removed.
By people restoration of Lake Gross-Glienicker with iron treatment is considered as most successful. Total phosphorus in lake water was reduced by 93 %. 1998 it was possible, to reduce aeration. A further improvement of lake functionality can be expected along with the development of submersed macrophytes.
Literature
Björk, S. (1988): Redevelopment of lake ecosystems - a case study approach. Ambio 17(2): 90-98
Ripl, W. (1978): Oxidation of lake sediments with nitrate - A restoration method for former recipients.- Institute of Limnology, University of Lund. ISSN 0348-0798. 151 pp.
Ripl, W. (1994): Sediment treatment. In: M. Eiseltová (ed.): Restoration of lake ecosystems - a holistic approach. International Waterfowl and Wetlands Research Bureau, Slimbridge. IWRB Publication 32: 75-81
Wolter, K.-D. (1994): Phosphorus precipitation. In: M. Eiseltová (ed.): Restoration of lake ecosystems - a holistic approach. International Waterfowl and Wetlands Research Bureau, Slimbridge. IWRB Publication 32: 63-68