by Peter Janiesch and Jutta Rach

In order to minimise the ecological consequences of the lowering of groundwater levels during open-cast lignite mining, extensive water infiltration systems were installed during the excavation of Garzweiler I and the planning for Garzweiler II. They are intended to protect the wet forest areas in the neighbouring areas of the huge extraction sites. The Department of Biology at the University of Oldenburg has investigated whether these systems actually work. The results are positive.

In the north of the Rhenish mining district, in the catchment area of the Garzweiler open-cast mine, there is a contiguous wetland area of international importance, the "Schwalm-Nette Nature Park". Over the centuries, a diverse landscape with mill streams, ponds and wetland forests in the lowlands has been created here, the likes of which can hardly be found anywhere else in Central Europe. Wetland forests, including alder swamp forests, were still a typical feature of the landscape in Central Europe a hundred years ago. Today, they have almost completely disappeared due to drainage in connection with the development of industrial agriculture. They are among the most endangered plant communities in Central Europe. Furthermore, this type of landscape represents a cultural and historical heritage that should not be destroyed.

Once the Garzweiler I open-cast mine has been worked out, the Garzweiler II open-cast mine is to be put into operation immediately afterwards from 2006. At the same time as the authorisation procedures are still ongoing, an emotional and ideological debate about the possible ecological consequences has been heating up for years. But what are the actual dangers threatening the natural environment and how can they possibly be avoided?

More than ten years ago, the lowering of groundwater due to open-cast mining led to shifts in the ecological balance in near-natural ecosystems and destroyed groundwater-dependent ecosystems.

In the meantime, an extensive system for the infiltration of treated sump water has been installed to prevent or minimise groundwater lowering. The infiltration volumes in the area affected by the Garzweiler I open-cast mine were increased from 13 to 47 million cubic metres of water between 1992 and 1998.

The approved lignite plan of 1995 stipulates that the wet forest areas are to be protected in their "species-rich diversity and characterised by groundwater-dependent biocoenoses". In order to achieve this goal, the infiltration measures for Garzweiler II must be continued and increased to approx. 90 million cubic metres of feed water per year. On the basis of extensive investigations and expert opinions on the effectiveness of these measures, the 1995 basic environmental impact assessment (EIA) came to the conclusion that the Garzweiler II mining project is acceptable overall in terms of its impact on the environment.

Independently of this finding, an additional investigation has been underway since 1996 at the Buscher Bruch and the Ellinghovener Bruch on a branch of the Schwalm. The aim of these investigations is to gain additional detailed knowledge about the possible effects in practice. The questions are:

• Can already influenced wetlands be converted back into near-natural ecosystems by raising the groundwater level or can the ecological conditions be changed in such a way that near-natural conditions can develop in the foreseeable future?
• Can intact communities actually be maintained in their current ecological state by supporting the groundwater or does the introduction of sump water lead to a shift in the ecological balance?

In a joint project with the Universities of Düsseldorf, Bonn and our working group in Oldenburg, a comprehensive ecological investigation programme was compiled in cooperation with the State Institute for Ecology in NRW to answer these questions.

Years can pass before the plant cover in wetlands reacts to changing environmental conditions. Using ecosystem-based nutrient-ecological investigations in the soil, we want to try to make early statements about the condition of a site which, with its numerous abiotic and biotic factors, has a direct influence on plant occurrence.

The main factor in a wetland is the groundwater, which has a decisive influence on plant growth through both its quantity and its chemistry. If the groundwater level drops, not only the direct influence of the water on the flora and fauna can be detected, but also the indirect effect on the nutrient turnover in the soil through increased oxygen levels. The fen peat is slowly destroyed by the decomposition of the organic substance (process of mineralisation).

During the decomposition of organic matter, higher-molecular nitrogen compounds are broken down to the ammonium level and converted into nitrate using oxygen. Long-term studies by our working group have shown that wetland forest communities exhibit characteristic annual balances of nitrogen mineralisation depending on their degree of trophic and peat decomposition. In swamp forests disturbed by drainage, nitrogen mineralisation rates can be measured to be 20 times higher than in the corresponding near-natural plant communities. The mineralisation rates determined thus provide characteristic values for wetland forest communities that can be used to determine the ecological status of an area affected by drainage.

The lowering of the groundwater level caused by open-cast mining over the last 15 years is clearly visible in the wet forests in the Ellinghoven area. There, the higher sites, which are more strongly influenced by the drainage, show a clear peat subsidence and a changed floristic composition with worm fern and raspberry. Only the wetter sites in the immediate vicinity of the stream resemble near-natural areas of uninfluenced swamp forests.

The determination of mineral nitrogen production in 1994 showed that the values measured in Ellinghoven were too high for swamp forests at almost all sites. Above all, the nitrate content was far too high, which not only promotes the growth of nitrate-loving plants (nitrophytes), but also leads to acidification of the soil. Only in the area characterised by marsh sedges, about 20 m away from the stream, did the nitrogen supply change abruptly. High amounts of ammonium were produced, but these were seven times higher than the mineralised amounts of nitrogen in similar plant communities in near-natural streams.

In 1993, water began to infiltrate the Ellinghoven area via seepage slits and wells in order to gradually raise the groundwater level. In 1995, the water front reached the quarry area and the first positive effects could be detected in the soil even before changes in the vegetation became visible. Ammonium production at the site near the stream not only decreased significantly, it also showed an even distribution over the vegetation period in 1995. The degree of nitrification also decreased at almost all sites.

In order to fulfil the requirements laid down in the 1995 lignite plan, a start was made at an early stage to protect unaffected wetland forests by recharging the groundwater. In the Buscher Bruch wetland forest area, the groundwater level would already have dropped by several decimetres without additional injection of sump water. With its wetland communities, the Buscher Bruch is a near-natural wetland. In addition to iris and alder swamp forests, it is also home to nutrient-poor birch swamp forests and the now rare bogbean bushes.

As part of the nutrient-ecological investigations in the Buscher Bruch, it was possible to show with our method in the three investigation years 1994, 1995 and 1997 that the strongly groundwater-dependent plant communities within the area have very low annual nitrogen balances, as is typical for natural plant communities. During the four-year study period, the unchanged stability of even the most sensitive plant communities was observed despite the artificial support of the groundwater. The four-year observation of the nutrient-ecological situation in the Buscher Bruch therefore shows that artificial groundwater recharge is a suitable means of effectively protecting wet forests.

One question that remains unanswered, however, is what will happen when the actual sump water reaches the area? It is possible that the increased carbonate content of the sump water will change the soil biological conditions and thus have an influence on the species composition of the plant communities in the Buscher Bruch. However, as azonal communities, quarry forests are able to grow on nutrient-rich, clayey soils, such as those found in the Schwalm-Nette region, as well as on base-poor sands, such as those found in the north-west German lowlands. Nutrient-poor peat moss sub-communities occur in both landscapes, even in the middle of intensively farmed areas if the groundwater regime is optimal. However, if the groundwater level changes, the protective function is lost and the floristic composition will change over the course of a few years. Maintaining the natural dynamics of groundwater levels is therefore of central importance. If this can be technically realised, ecosystems can be effectively protected even in open-cast mines of this size.

Ecosystem analyses of nitrogen supply in wet forests are a suitable tool for predicting the ecological status even before changes in the vegetation become apparent. Rewetting after lowering the groundwater level is technically possible, but problematic. The regeneration process can take several decades. If the groundwater level is lowered for too long, permanent damage can also occur.

However, as the Buscher Bruch example shows, subsidence can be effectively prevented by infiltration measures. The limited change in groundwater quality that occurs over time can be buffered by the plant communities while maintaining the natural groundwater regime.

Technical-ecological progress combined with extensive research makes even such major interventions as open-cast lignite mining calculable. The experience gained here can also be used to preserve or regenerate threatened wetland communities elsewhere. Conservation must always take precedence over intervention. Another point should not be forgotten: Recultivation after the end of open-cast mining also opens up the possibility of creating ecologically valuable landscapes and ecosystems that have been destroyed over the last 100 years by the development of industrial agriculture.

The authors

Prof. Dr Peter Janiesch (57), a biologist specialising in physiological ecology and Director of the Botanical Garden, was appointed to the University of Oldenburg in 1982. He studied at the University of Münster, where he also gained his doctorate and habilitated. His research focus includes the mineral balance of plants and the anaerobic metabolism of marsh plants.

Diplom biologist Jutta Rach, a graduate of the University of Oldenburg and a research assistant in the project described above since 1993, is currently writing her dissertation on "Geochemical investigations into the rewettability of peat".