The concentration intervals of conductivity follow the percentiles of the measuring network operated by the Senate within the “upper aquifer” GWL 2. According to Schleyer & Kerndorff (1992), concentrations above 840 µS/cm are said to be subject to „anthropogenic influences“, whereas – according to Kunkel et al. (2003) – those up to 1,000 µS/cm are said to be of „natural“ origin.
In the context of the regional assessment of the data stock from hydrogeological explorations (LUA 1996; the same data stock that was used in the surrounding area within the page segment) concentration ranges up to 500 µS/cm were considered to be “background values“ in Brandenburg. However, the current assessment of the groundwater condition in Brandenburg (LUA 2002) indicates that conductivities determined at the five measuring sites in the immediate vicinity of Berlin have increased in the mean time to 1,000 µS/cm and more. Values ranging from 600 to 1,200 µS/cm are regarded as “typical“ of Berlin (Fugro & Hydor 2002).
This tendency is also shown very clearly by the calculated area surveys. Concentrations in the range of 500 µS/cm and lower are seen in the urban area only in the mainly wooded suburban areas (Tegel and Spandau forests and south of the Müggelsee lake, but not in the Grunewald forest). In the surrounding area these ranges occur more frequently. Primarily in the northeastern part of the city this seems to be plausible even today. The value measured at the Zepernick measuring site in 2000 was still below 1,000 µS/cm.
Within the city, a tendency of groundwater conductivities increasing from the suburbs towards the city centre is noticed. Whereas in areas of confined groundwater (under glacial marl in the Barnim or Teltow regions) no significantly lower conductivities are seen compared to areas without confined groundwater (leaching fields do not show a clear influence either), areas with clearly increased conductivities (above 1,500 µS/cm) are concentrated in the densely built-up areas in the districts of Mitte (area around Nordbahnhof – a former train station), Prenzlauer Berg (Landsberger Allee / Storkower Straße), Kreuzberg (Gleisdreieck / Yorckbruecken), Schoeneberg and Wilmersdorf (Volkspark). These areas comprise land suspected of being polluted. The Wilhelmsruh industrial estate along the S-Bahn track with numerous polluted measuring sites and dumps can be mentioned as another area where increased contents were measured also for many other parameters.
Areas whose conductivity limiting values are exceeded are very rare and occur only in a few isolated core areas of the above-mentioned areas.
The area around Hahneberg in Spandau occupies a special position, because it is the largest area with values above the limit. Concentrations of almost all other parameters are also very high and therefore the area must be regarded as being clearly impaired. Hahneberg came into being as a rubble dump and has been identified as an area suspected of being polluted by hazardous substances. A number of groundwater measuring sites of the landfill programme have been set up in its vicinity, which all show clearly increased concentrations. The calculation results in areas whose parameters are linked with those in the vicinity of the northeastern industrial estate near Brunsbuetteler Damm.
However, conductivities ranging from 1,000 to 1,500 µS/cm (yellow areas) are relatively widely diffused within the city (including areas with open development) and can be called current „background noise“.
From the geochemical point of view, chloride is extremely mobile. Hence its action in groundwater resembles that of an ideal tracer, that is to say that in most cases it is not retained by permeable rock. Sources of increased chloride concentrations in groundwater may be wet salts, which are used by the city environmental services (BSR) during the winter season to improve road safety. This practice has been clearly reduced in Berlin in the last several years and is mainly limited to streets with rain water sewers. Markedly increased chloride contents of the groundwater, which are not caused by geogenic rising hypolimnetic water, may be regarded as indicators of isolated cases of wastewater discharge or pollution resulting from landfills. Specialized literature (Schleyer & Kerndorff 1992) refers to concentrations of more than 80 mg/l in the groundwater of North Germany as being “influenced by anthropogenic factors”. Members of the regional working-group on water (LAWA) describe concentrations up to 66 mg/l as „natural“.
Values up to 50 mg/l are described as background values in Brandenburg, whereas in Berlin values ranging from 14 to 95 mg/l are considered „typical” (Fugro & Hydor 2002) of the upper aquifer (only GWL 2). Brose & Brühl (1993) mention 9 to 69 mg/l as characteristic values for forest locations in Berlin. Due to glacial erosion of Rupelian, hydraulic contact with the lower saline water level became possible in several areas of Berlin. In principle, the rising of saline groundwater can be intensified by anthropogenic influences (relieving pressure by pumping water from layers above Rupelian). For specific locations in the Berlin area, this problem is of special importance owing to the intensive utilization of groundwater (drinking-water supply, individual water-supply installations, lowering of the groundwater level due to building work).
Within the city boundaries, the possibility of higher concentrations in the aquifers close to the surface due to geogenic salinification is limited; examples are an area in the northern part of the Neukoelln district or the main aquifer (GWL 2) in the area of Schmoeckwitzer Werder. Unfortunately, this could not be confirmed by data, since there are no measuring sites in these areas. In several catchment areas of Berliner Wasserbetriebe (Friedrichshagen waterworks, Beelitzhof), the impact of local saline water on wells is known. In problematic areas, these influences are countered by adapting pumping strategies. For the time being, an area-wide problem is not assumed to exist. The catchment areas of extraction galleries are continuously monitored by Berliner Wasserbetriebe and the problem of rising saline water due to possible climate change (relief of pressure owing to a more limited formation of new groundwater) will be monitored on an area-wide basis by including deeper measuring sites in Berlin’s basic measuring network.
Concentrations below 50 mg/l are almost exclusively found in the wooded suburbs. Also in Brandenburg, concentrations below 50 mg/l are found mainly in areas in the immediate vicinity of Berlin. But there are also large areas to the south of the city, where concentrations above 50 mg/l are found. They seem to be linked to the leaching fields that were mostly operated until 1990.
Within the city, areas with increased chloride contents are interrelated with those having increased conductivities. However, clearly increased contents of more than 100 mg/l are limited to small areas. Such areas exist only in Spandau (Hahneberg), Mitte (Nordbahnhof, a former railway station) and Prenzlauer Berg. Contents above the threshold value of 250 mg/l were identified only in one area (480 mg/l were measured at a measuring site to the northwest of Hahneberg). In the light of these findings, a significantly increased chloride pollution of the Berlin groundwater due to diffuse harmful substances cannot be assumed.
Sulphate is a rock constituent that is readily dissolved in water and relatively quickly leached out. Current anthropogenic sulphate inputs into the ground and groundwater are very high and of diverse origin. In Brandenburg, concentrations up to about 100 mg/l are regarded as background contents; Schleyer & Kerndorff (1992) presented similar findings (100 to 150 mg/l). Kunkel et al. (2003) mention contents of up to 200 mg/l.
Under forests, in contrast, contents corresponding in magnitude to the threshold value derived from TrinkWV (240 mg/l) were regarded as typical in the recent past. Kabelitz (1990) mentions concentrations of up to 1,200 mg/l in Berlin aquifers that date back to upper Pleistocene. Renger et al. (1989) give sulphate concentrations from 5 to 42 mg/l for precipitation measured in a stand of pine trees in the Grunewald forest. However, the reason for the markedly increased sulphate concentration in the Berlin groundwater are the numerous dumps of building and other types of rubble around the city that resulted from World War II (SenStadtUm 1986): the impact of domestic wastewater is also mentioned, but plays a secondary role (Wurl 1995). A characteristic feature of the mainly gypsum-like deposits is that they are scattered across the entire urban area in a more or less diffuse pattern. That is why the Berlin building rubble dumps must be regarded as forming the interface between diffuse and spot sources of inputs. Numerous very small (natural and artificial) cavities were also filled with large quantities of rubble.
We want to illustrate the impact of these dumps by way of an example: Siebert (1956) concluded that the huge quantities of debris and rubble that were dumped around Teufelsberg in the Grunewald forest in the mid-1950s did not affect the groundwater condition yet. In those days, a groundwater measuring site situated directly in the western runoff of Teufelsberg showed a sulphate content of around 50 mg/l. But in the context of the Hydrogeological Structural Model for the Tiefwerder waterworks (GCI & AKS 1998) it was pointed out that the sulphate concentration measured at the same point had increased to more than 400 mg/l in the meantime.
With the exception of small wooded areas, sulphate concentrations below 100 mg/l are no longer measured in the northwestern and southeastern parts of the city. In areas of Brandenburg that are extending to the south and west of the city, most sulphate values are above 100 mg/l. Values are above 180 mg/l in the entire inner-city area (the mean sulphate value being 114 mg/l). Spatial relationships to building rubble dumps exist in many areas (Wilhelmsruh, Spandau, Teufelsberg). The highest sulphate concentration (above 360 mg/l) is measured in densely built-up inner-city areas. An area of about 30 km2 extending along both banks of the lower reaches of the Panke River toward the east up to the Friedrichshain district is affected. In some places, sulphate concentrations of more than 800 mg/l are measured (for instance at Eberswalder Straße south of the Jahn sports field where a mean value of 872 mg/l was determined). The calculation of the retention time of percolating water showed periods of less than 50 years for that area on the southern edge of Barnim, so that the scenario that is based on the materials input caused by dumping after World War II seems to be plausible.
The extremely high sulphate content measured in the southwestern part of the Grunewald forest along the eastern bank of the Havel River is another special case. Here again, values up to 900 mg/l (872 mg/l at Havelchaussee and 922 mg/l in the Havel hills) were measured. At least for the latter measuring site, debris and building rubble dumps can hardly be considered as a cause. According to Fugro & Hydor (2002), the cause of such high contents is the groundwater depletion as a consequence of the water intake by wells operated by the Beelitzhof waterworks. This intake could have resulted in the aeration of formerly saturated areas and thus in the oxidation of sulphidic sulphur dispersed in the sediment. This explanation was also given by Sommer von Jarmersted (1992) for the extremely high sulphate contents measured at these sites. Furthermore, the strata-log sheet of one of these measuring sites includes sapropel strata from the Eemian warm period directly 19-25 m above the filter, which could be another source of sulphidic sulphur.
To check this hypothesis, we include the following figures showing the hydrograph curves of measuring sites 1172 and 1276. The elevation of measuring site 1172 is about 40 m, that of measuring site 1276 about 49 m. That is to say that in both cases the groundwater level curves, as referred to the elevation, range from 10 m to 20 m. At overall amplitudes of 5 m, the marked fluctuation of the groundwater surface (which is free in this case) is clearly shown. These amplitudes cannot be explained by natural causes. A clear decline of groundwater levels is seen at both measuring sites up to the late seventies, which can only be explained by an increase in the water intake by waterworks. Whether or not the extremely high sulphate content is indeed caused by the oxidation of pyrite can only be determined when a hydrogeochemical balance of the constituents of the groundwater, which has very high hardness but is not acidic, is drawn up. Other causes need to be investigated.