Geochemistry of the River Rhine and the Upper Danube: Recent Trends and Lithological Influence on Baselines

Journal of Environmental Science for Sustainable Society, Vol. 1, 39-46, March 2007

GEOCHEMISTRY OF THE RIVER RHINE AND THE UPPER DANUBE: RECENT TRENDS AND LITHOLOGICAL INFLUENCE ON BASELINES

Jens HARTMANN 1, Nils JANSEN 2, Stephan KEMPE 3, Hans H. DÜRR 4

1 Postdoctoral Researcher, Institute of Applied Geoscience, Darmstadt University of Technology (Schnittspahnstrasse 9, D-64287 Darmstadt, Germany) E-mail: [email protected] 2 Ph.D.-Student, Institute of Applied Geoscience, Darmstadt University of Technology (Schnittspahnstrasse 9, D-64287 Darmstadt, Germany) E-mail: [email protected] 3 Professor, Institute of Applied Geoscience, Darmstadt University of Technology (Schnittspahnstrasse 9, D-64287 Darmstadt, Germany) E-mail: [email protected] 4 Postdoctoral Researcher, Faculty of Geosciences, Utrecht University (Heidelberglaan 2, 3508 TC Utrecht, The Netherlands) E-mail: [email protected]

Human pressure is now severe on most of the rivers worldwide. The long term fluxes of dissolved geogenic and biogenic matter are changing dramatically, causing notable changes in aquatic bioactivity. Typical patterns of anthropogenic pressure that influence eutrophication, salinization and chemical contamination are discussed. The heavily influenced rivers Rhine and upper Danube will be used as examples, also considering their geological settings. In the past decade sewage treatment reduced nitrate and orthophosphate loads in both basins. This influenced bioactivity in the rivers, causing less silica depletion due to diatom blooms in the Rhine. Therefore a notable increase in minima concentrations of dissolved silica can be observed. In the upper Danube, however, an increase in orthophosphate concentration since 2003 is noticeable; breaking the former decreasing trend, despite treatment efforts. The hydrochemistry of major ions in both basins is strongly influenced by the ratios of carbonate, siliciclastic sediment and igneous or metamorphic rock outcrops. In addition Mesozoic evaporites and salt mining were

responsible for extremely high levels of Cl, Na and SO4 in the Rhine, peaking in the 70s and 80s at concentrations of 350, 180 and 140 mg/l, respectively. Water basin management efforts cut former high levels to less than a half. Heavy metals and persistent organic pollutant concentrations are declining in the Rhine as well. A combination of climate change and anthropogenic water inputs resulted in an increase of water temperature of the Rhine by 3.5 °C during the past 50 years. In the upper Danube such a trend in water temperature can not be observed.

Key Words : Danube, Rhine, lithology, baseline, trend analysis, nutrients, silica, temperature, evaporite, salinization, water temperature

1. INTRODUCTION and aquatic biotope progressively used and transformed by humans. Human pressures influence Continental aquatic systems can be observed many basins in a way that the continental aquatic from two perspectives1): a) as a major link between system no longer can be considered as being atmosphere, biosphere, pedosphere, geosphere and controlled only by Earth system processes1)2)3). This oceans within the Earth system and b) as water source study reviews geochemical baselines and recent

39 Jens HARTMANN et al. trends for some major elements in the Rhine and station Jochenstein cover the period from 1982 till upper Danube. Due to recent progress in water basin 2005. However, only some parameters were management, anthropogenic pressures like measured during the entire period. In addition, data salinization and eutrophication are decreasing, from a detailed study on the Danube’s geochemical causing changes in annual baselines of element background levels for the years 1991/92 were used4). concentrations1). Examples of the influence of To evaluate the influence of lithology on the lithology on hydrochemical composition in the hydrochemical composition of the river Rhine and Danube are presented. In addition, a possible the upper Danube the distribution of 15 lithological influence of climatic change on water temperature is types was calculated6). For this a global lithological discussed. map was used, which was specifically conceived for analyzing the relationship between hydrochemical composition of rivers and lithology on large scales6). 2. HYDROLOGY In addition, the German Geological Map was used to significantly improve the resolution of lithological Detailed descriptions of the hydrological regime units for catchment areas located in Germany 7). of the Rhine and the upper Danube can be found Using this information, it was possible to identify elsewhere3)4)5). The Rhine basin has a size of 185,300 lithological units containing significant amounts of km2 and the main stem of the river is 1320 km long. evaporites. Those units are known to contribute to At station Bimmen/Lobith, close to the salinization. German-Dutch border, the Rhine has a tributary area of 159,500 km2 delivering a discharge of 2200 m3/s on long term average. The Alpine Rhine (upstream of 4. NUTRIENTS N, P AND SI Lake Constance) constitutes only 19 % of the catchment area, but delivers nearly half of the Dominating nitrogen (N)- and phosphorus discharge on average. The flow regime of the river (P)-species in both rivers are nitrate and Rhine is dominated by melt water and precipitation orthophosphate, representing a large proportion of runoff from the Alps in summer and by precipitation the total N- and P-load (Fig. 1 and 2). runoff from the mid-basin uplands in winter. Major Both basins are heavily influenced by agriculture. winter tributaries are Mosel, Main and Neckar. Below Lake Constance, the Rhine is additionally The Danube is the second largest river in Europe. influenced by large urban areas concentrating around Its upper course runs for 587 km through southern Stuttgart, the Area at the Rhine- Main- junction Germany. At station Jochenstein (close to Passau at (Frankfurt) and in North-Rhine-Westphalia the German-Austrian border) the catchment has a (Ruhrgebiet). During the 1970s and 80s the river size of 77,050 km2 with an average discharge of Rhine was one of the most heavily polluted rivers in 1,440 m3/s. The upper Danube flows through the world, carrying sometimes more than 800 mg/l of agricultural areas of the northern Alpine foreland. Its total dissolved matter. The upper Danube was not Hydrology is controlled to a large extent by alpine affected by such high pollution levels, because the runoff, provided mostly by Isar and Inn. Their catchment contains less urban areas and lacks salt discharge peaks during summer season4). Other mining industry. important tributaries are the rivers Iller, Regen and Since the 1970s efforts increased to reduce Lech. nutrient emissions by installing sewage treatment plants8). This resulted in a recovery since the 1990s with respect to orthophosphate (Fig. 1) and 3. DATA particulate P (not shown here) as both were emitted mainly from point sources. However, for the upper Data used in this study were provided by the Danube a recent increase in orthophosphate since German Federal Institute of Hydrology (BfG) and the 2003 can be observed (Fig. 2). Nevertheless Bavarian Administration for Environment. The concentrations are still below those of the Rhine Rhine is intensively monitored since the 1950s. River. Available data for station Bimmen/Lobith cover the Nitrate, which is also emitted by diffuse sources, period from 1954 to 2001. Monitoring is coordinated did not decrease as much as orthophosphate. Recent by the ICPR (International Commission for the annual averages (since 2000) are 2.7 mg/l nitrate-N Protection of the Rhine). Data for the Danube at (Rhine) and 2 mg/l nitrate-N (upper Danube).

40 GEOCHEMISTRY OF THE RIVER RHINE AND THE UPPER DANUBE

- - NO3 -N concentration, Bimmen/Lobith, Rhine NO3 -N concentration, Jochenstein, Danube 6 4.5

4.0 5 3.5

4 3.0

3 2.5 mg/l mg/l 2.0 2 1.5

1 1.0

0.5 0 1982 1984 1987 1990 1993 1995 1998 2001 2004 2006 1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 2004 3- PO4 -P concentration, Jochenstein, Danube 3- PO4 -P concentration, Bimmen/Lobith, Rhine 0.30 0.8

0.7 0.25

0.6 0.20 0.5 0.15 0.4 mg/l mg/l 0.10 0.3

0.2 0.05

0.1 0.00 1982 1984 1987 1990 1993 1995 1998 2001 2004 2006 0.0 1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 2004 Fig.2 Nitrate and orthophosphate concentration in the Si-concentration, Bimmen/Lobith, Rhine upper Danube at station Jochenstein, close to the border between Germany and Austria. 4

2) 3 dissolved silica concentrations . A similar effect can be observed for the Rhine 2 (Fig. 1). In the 80s nitrate and orthophosphate mg Si /l Si mg concentrations peaked. During this time dissolved 1 silica concentrations reached a minimum (Fig. 1). Since the beginning of the 90s an increasing trend 0 can be observed, mainly due to an increase in annual 1976 1979 1982 1985 1988 1991 1994 1997 2000 minima. Recent average concentrations of dissolved Fig.1 Nitrate, orthophosphate and dissolved silica silica in the Rhine at Bimmen/Lobith are around 2.5 concentrations in the river Rhine at monitoring station mg Si /l. Silica is mainly derived from weathering of Bimmen/Lobith. (For dissolved silica trends before and after silicates and an important nutrient for aquatic January 1990 are given. The annual average increase for ecosystems. The observed correlation of dissolved silica since 1990 is 0.07 mg Si /l a.) eutrophication and silica depletion due to bioactivity

pH, Bimmen/Lobith, Rhine Treatment efforts resulted in a noticeable decrease of N and P inputs that had an effect on the carbon 8.5 dynamics in the aquatic system. In the Neckar river-system (an important tributary of the Rhine), N 8.0 and P-concentration decreases are correlated with a

sharp decrease in the partial pressure of CO2 and an pH increase in pH, indicating changes in the 7.5 relationships between photosynthesis and respiration processes2)9). At the same time average dissolved 7.0 silica (DSi) concentrations in the main stem of the Neckar system increased significantly, in average by 1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 2004 0.13 mg Si /l per year between 1997 and 20029). This Fig.3 pH at station Bimmen/Lobith. pattern is present in 18 monitoring stations in the Neckar system, all showing significant increases in 41 Jens HARTMANN et al.

Fig.4 Lithological map of the Rhine and upper Danube catchments, defined by the stations Bimmen/Lobith and Jochenstein.

illustrates the need to take these processes into 5. LITHOLOGICAL INFLUENCE account when budgeting silica fluxes10). The recovery of the river Rhine is accompanied The most important factor controlling by a significant decrease in concentrations of heavy concentrations or specific fluxes of geogenic major metals and organic persistent pollutants8)11). Both ions is lithology6)12)13). Major ions are primarily kinds of contaminants declined in the past two derived from weathering processes and are in part decades to either below detection limits or below influenced by ecosystems through active ion legal limits. The pH recovered earlier, showing exchange14). lowest values in the 70s (Fig. 3). In the humid Central European climate, weathering rates of carbonates can be 10 times higher than silicate weathering rates. Those of evaporites

42 GEOCHEMISTRY OF THE RIVER RHINE AND THE UPPER DANUBE

Table 1 Aereal proportions of lithological units for the Rhine catchment above station Bimmen/Lobith and the upper Danube above station Jochenstein.

Upper Lithology Rhine Danube Organic Sediments 0.20% 2.06% Loess 2.78% 4.06% Dunes 0.87% 0.00% Alluvial Deposits 10.74% 7.11% Semi- & Unconsolidated Sediments 14.83% 21.79% Siliciclastic Sedimentary Rocks 33.96% 18.77% Mixed Sedimentary Consolidated Rocks 5.90% 1.25% Carbonate Rocks 19.08% 24.49% Acid Volcanic Rocks 0.38% - Basic & Intermediate Volcanic Rocks 2.25% 0.05% Fig.5 Lithological units containing significant amounts of Acid Plutonic Rocks 2.70% 3.99% evaporites. However, evaporites are not dominating the mineral Basic Plutonic Rocks 0.06% 0.12% composition in any lithological unit located at the Earth’s Metamorphic Rocks 2.03% 6.93% surface. Complex Lithology 3.75% 8.84% Water Bodies 0.47% 0.54% 15)16) can be 40 to 80 times higher . Even a small proportion of carbonates can influence river hydrochemistry significantly, leading to elevated more than 10 times higher than the suspended matter concentrations of Ca, Mg and bicarbonate6)12). load3). Since the 90s, emissions of the salt mines were Fig. 4 and Table 1 show the lithological reduced (Table 2). distribution and the proportions of distinguished However, compared to other rivers concentrations 17)18) lithological units for both catchments. Fig. 5 gives of Na, Cl and SO4 are still high . Fluctuations in additional information on geological units containing Ca concentrations are partially controlled by evaporites. The catchments of the Rhine and the dissolution of gypsum, indicated by the strong upper Danube are characterized by a high aerial correlation between Ca and SO4 (Fig. 7). proportion of carbonates17) explaining their high Table 2 compares tributaries of the Danube holding a alkalinity, pH and Ca- and Mg- concentrations (Table high proportion of igneous and metamorphic rocks 2). Early Mesozoic evaporite containing lithological with those holding high carbonate proportions. units (halite and gypsum) and late Paleaozoic Significant differences in maximum concentrations evaporites (halite, gypsum and sylvite) are another of dissolved silica, Ca and Mg are apparent. High important contributor to the total dissolved ions. proportions of carbonates are in accordance with They are responsible for high SO4 concentrations in high Ca and Mg concentrations. Silica maximum both basins and for high Cl and Na concentrations in concentrations are coincident with a large proportion the Rhine (Fig. 6 and 7). Salt mines as point sources of igneous/metamorphic rocks. are influencing salinity of the Rhine in addition to the The upper Danube at the German-Austrian border contribution of natural chemical weathering of contains an average concentration of dissolved silica surface rocks. However, evaporites do not outcrop at of 4 mg Si /l, which results in a specific flux of ~ 2.4 t the earth’s surface. Si /km2 a. The Rhine shows smaller dissolved silica Extraordinary high Cl and Na concentrations fluxes with ~ 1.1 t Si /km2 a. The world average is 1.6 (more than 300 and 150 mg/l, respectively) during t Si /km2 a, but can be more than 10 times higher in the 70s and 80s were consequences of mining hot spot regions like Japan, due to easily weatherable practices which included wastewater discharging volcanic rocks10). directly into the Rhine (Fig. 6). During this period the total annual dissolved matter load of the Rhine was

43 Jens HARTMANN et al.

Cl-concentration, Bimmen/Lobith, Rhine Ca-concentration, Bimmen/Lobith, Rhine 400 130 120 350 110 300 100 250 90 80 200 mg/l mg/l 70 150 60 100 50 40 50 30 20 1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 2004 1959 1964 1969 1974 1979 1984 1989 1994 1999 2004

Na-concentration, Bimmen/Lobith, Rhine SO4-concentration, Bimmen/Lobith, Rhine 200 180 180 160 160 140 140 120 120

100 100 mg/l mg/l 80 80 60 60 40 20 40

20 1960 1965 1971 1976 1982 1987 1993 1998 2004 1959 1964 1969 1974 1979 1984 1989 1994 1999 2004

K-concentration, Bimmen/Lobith, Rhine Ca-SO4 relationship during the 1970s, Bimmen/Lobith, Rhine 16 1.8 1970-1979 14 1998-2001 1.5 12

10 1.2 mmol/l 4

mg/l 8 0.9 SO 6 0.6 4

2 0.3 0.5 1.0 1.5 2.0 2.5 3.0 3.5 1960 1965 1971 1976 1982 1987 1993 1998 2004 Ca mmol/l Fig.6 Cl and Na concentrations in the Rhine at Bimmen/Lobith. Fig.7 Ca and SO4 concentrations at station Bimmen/Lobith,

Rhine. A significant correlation between SO4 and Ca 6. CLIMATE CHANGE concentrations is showing the influence of gypsum dissolution on the hydrochemistry of the river Rhine. In addition to direct anthropogenic pressures, climate change is influencing the aquatic system of both rivers3). In the 20th century global average upper Danube yield different results. The Rhine temperature rose around 0.6 °C and precipitation shows an increase in water temperature by ~ 3.5 °C or over the northern hemisphere increased3). 0.07 °C/a in the past five decades (Fig. 8), while the Consequences for the Rhine catchment are increases temperature of the Danube increases much less by in winter precipitation since around 1980, leading to 0.01 °C/a. Temperature data for the upper Danube, higher runoff. Shorter periods of snow cover are however, reach back only to 1982. observed in mountainous regions. This has In addition to climate change, direct consequences for average water temperatures. anthropogenic influences such as cooling water However, regressions for the Rhine River and the inputs from power plants or sewage treatment plants

44 GEOCHEMISTRY OF THE RIVER RHINE AND THE UPPER DANUBE

Table 2 Aereal proportions of lithology for selected tributaries of the upper Danube (including some downstream tributaries) are given in percentage (bold : dominant lithology). Geochemical base levels of major elements are based on three samples taken in the years 1991 and 19924). Tributaries downstream the German-Austrian border are marked with ‘A’. For comparison data from the stations Bimmen/Lobith (2000-2001) and Jochenstein (2000-2005) are given.

Igneous/ Alka- Carbo~ Other metamor- Si (mg/l) Ca (mg/l) Mg (mg/l) linity SO4 (mg/l) Cl (mg/l) Na (mg/l) K (mg/l) nates sediments phic rocks (mmol/l) Regen 94 4 2 1.5 – 4.3 15 – 17 3.3 – 3.6 0.6 – 1.6 20 – 47 3.8 – 22.2 4.7 – 16.4 1.5 – 2.8 Ilz 98 0 2 5.8 – 8 9 - 11 1.5 – 2.4 0.5 – 1.2 13 – 24 2.6 – 11.8 4.7 – 10.3 1.2 – 2.4 Kamp (A) 91 0 9 1.1 – 5.6 23 – 31 5.8 – 11.8 1.3 – 4.5 29 – 62 2.4 – 14.8 4.4 – 11.6 2.8 – 5.0 Krems (A) 96 0 4 2.8 – 5.5 54 – 57 13.2 – 16.9 2.4 – 4 27 – 38 3.7 – 19.6 6.1 – 9.8 2.6 – 3.1

Traun (A) 0 61 34 1.6 – 4.1 69 – 77 11.4 - 14.1 3.5 – 4.5 30 – 37 11.3 – 22.2 13.6 – 22 2.7 – 3.0 Enns (A) 19 63 28 2.2– 4.4 48 – 55 12.5 – 16.1 3.4 – 4.6 31 – 40 4.4 – 7.6 2.3 – 4.2 0.9 – 1.1 Ybbs (A) 0 69 31 1.5 – 3.9 68 – 78 16.3 – 25.8 3.9 – 7.8 38 – 47 6.1 – 7.2 3.0 – 11.4 1.3 – 3.5 Traisen (A) 0 66 34 0.8 – 2.5 69 – 79 19.6 – 26.1 4.8 – 5.5 52 - 56 1.3 – 5.5 6.5 – 12.8 1.8 – 2.3

Upper Data from reference 4), Passau, 2.4 – 5.2 34 - 59 8.5 – 15.8 4.6 – 6.6 26 - 32 1.7 – 12.3 5.6 – 10.1 1.9 – 2.3 Danube 1991-1992 Data station Jochenstein 56 13 3.1 26 16 10 2.3

(2000-2005) (43 -81) (10 – 21) (1.3 – 3.7) (18 – 37) (8 – 34) (6 – 16) (1.6 – 2.8) ~ 2.8 Rhine 2.5 75 11 54 88 48 4.4 (data < (Bimmen) (0.9 – 3.7) (63 – 89) (9 – 14) (37 – 68) (47 – 119) (26 – 64) (3.4 – 5.5.) 1990)

may have contributed to the increase in water temperature of the Rhine. This issue needs more discussion, as water temperature is also an important 7. CONCLUSIONS factor influencing aquatic bioactivity and hence dissolved silica fluxes. A comparison of geochemical base lines and the analysis of recent trends in concentrations of major Water temperature, Bimmen/Lobith, Rhine (increase: 0.07 °C/a) ions and nutrients for Rhine and upper Danube shows 30 that significant geochemical changes are occurring in 25 these rivers. Specifically nutrient levels and salt flux are decreasing since the 1980s8)11). Both rivers are 20 recovering from the anthropogenic syndromes such 15 as enhanced eutrophication, heavy metal °C contamination and salinization1). 10 For the river Rhine much longer and more 5 complete data sets are available, allowing long-term trend analysis. Data sets covering decades with high 0 1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 2004 sampling frequency are necessary to evaluate changes in dissolved matter fluxes due to changes in Water Temperature, Jochenstein, Danube (increase: 0.01 °C/a) land use, water basin management and climate 20 change. The observed changes will impact coastal bioactivity. Observed dynamics should be considered 15 if global or regional studies based on averages from older data sets are carried out. This is specifically

°C 10 important for global and regional nutrient budgets of coastal zones19). 5 ACKNOWLEDGMENT: Presented data were 0 provided by the German Federal Institute of 1981 1984 1987 1990 1993 1996 1999 2002 2005 Hydrology (BfG) and the Bavarian Administration Fig.8 Water temperature of the river Rhine at Bimmen/Lobith for Environment. and the upper Danube at Jochenstein.

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