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

39

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: hartmann@geo.tu-darmstadt.de 2 Ph.D.-Student, Institute of Applied Geoscience, Darmstadt University of Technology

(Schnittspahnstrasse 9, D-64287 Darmstadt, Germany) E-mail: jansen@geo.tu-darmstadt.de

3 Professor, Institute of Applied Geoscience, Darmstadt University of Technology (Schnittspahnstrasse 9, D-64287 Darmstadt, Germany)

E-mail: kempe@geo.tu-darmstadt.de 4 Postdoctoral Researcher, Faculty of Geosciences, Utrecht University

(Heidelberglaan 2, 3508 TC Utrecht, The Netherlands) E-mail: h.durr@geo.uu.nl

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

Continental aquatic systems can be observed from two perspectives1): a) as a major link between atmosphere, biosphere, pedosphere, geosphere and oceans within the Earth system and b) as water source

and aquatic biotope progressively used and transformed by humans. Human pressures influence many basins in a way that the continental aquatic system no longer can be considered as being controlled only by Earth system processes1)2)3). This study reviews geochemical baselines and recent

Jens HARTMANN et al.

40

trends for some major elements in the Rhine and upper Danube. Due to recent progress in water basin management, anthropogenic pressures like salinization and eutrophication are decreasing, causing changes in annual baselines of element concentrations1). Examples of the influence of lithology on hydrochemical composition in the Danube are presented. In addition, a possible influence of climatic change on water temperature is discussed.

2. HYDROLOGY Detailed descriptions of the hydrological regime

of the Rhine and the upper Danube can be found elsewhere3)4)5). The Rhine basin has a size of 185,300 km2 and the main stem of the river is 1320 km long. At station Bimmen/Lobith, close to the 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 Lake Constance) constitutes only 19 % of the catchment area, but delivers nearly half of the discharge on average. The flow regime of the river Rhine is dominated by melt water and precipitation runoff from the Alps in summer and by precipitation runoff from the mid-basin uplands in winter. Major winter tributaries are Mosel, Main and Neckar.

The Danube is the second largest river in Europe. Its upper course runs for 587 km through southern Germany. At station Jochenstein (close to Passau at the German-Austrian border) the catchment has a size of 77,050 km2 with an average discharge of 1,440 m3/s. The upper Danube flows through agricultural areas of the northern Alpine foreland. Its Hydrology is controlled to a large extent by alpine runoff, provided mostly by Isar and Inn. Their discharge peaks during summer season4). Other important tributaries are the rivers Iller, Regen and Lech. 3. DATA

Data used in this study were provided by the

German Federal Institute of Hydrology (BfG) and the Bavarian Administration for Environment. The Rhine is intensively monitored since the 1950s. Available data for station Bimmen/Lobith cover the period from 1954 to 2001. Monitoring is coordinated by the ICPR (International Commission for the Protection of the Rhine). Data for the Danube at

station Jochenstein cover the period from 1982 till 2005. However, only some parameters were measured during the entire period. In addition, data from a detailed study on the Danube’s geochemical background levels for the years 1991/92 were used4).

To evaluate the influence of lithology on the hydrochemical composition of the river Rhine and the upper Danube the distribution of 15 lithological types was calculated6). For this a global lithological map was used, which was specifically conceived for analyzing the relationship between hydrochemical composition of rivers and lithology on large scales6). In addition, the German Geological Map was used to significantly improve the resolution of lithological units for catchment areas located in Germany 7). Using this information, it was possible to identify lithological units containing significant amounts of evaporites. Those units are known to contribute to salinization.

4. NUTRIENTS N, P AND SI Dominating nitrogen (N)- and phosphorus

(P)-species in both rivers are nitrate and orthophosphate, representing a large proportion of the total N- and P-load (Fig. 1 and 2).

Both basins are heavily influenced by agriculture. Below Lake Constance, the Rhine is additionally influenced by large urban areas concentrating around Stuttgart, the Area at the Rhine- Main- junction (Frankfurt) and in North-Rhine-Westphalia (Ruhrgebiet). During the 1970s and 80s the river Rhine was one of the most heavily polluted rivers in the world, carrying sometimes more than 800 mg/l of total dissolved matter. The upper Danube was not affected by such high pollution levels, because the catchment contains less urban areas and lacks salt mining industry.

Since the 1970s efforts increased to reduce nutrient emissions by installing sewage treatment plants8). This resulted in a recovery since the 1990s with respect to orthophosphate (Fig. 1) and particulate P (not shown here) as both were emitted mainly from point sources. However, for the upper Danube a recent increase in orthophosphate since 2003 can be observed (Fig. 2). Nevertheless concentrations are still below those of the Rhine River.

Nitrate, which is also emitted by diffuse sources, did not decrease as much as orthophosphate. Recent annual averages (since 2000) are 2.7 mg/l nitrate-N (Rhine) and 2 mg/l nitrate-N (upper Danube).

GEOCHEMISTRY OF THE RIVER RHINE AND THE UPPER DANUBE

41

Treatment efforts resulted in a noticeable decrease of N and P inputs that had an effect on the carbon dynamics in the aquatic system. In the Neckar river-system (an important tributary of the Rhine), N and P-concentration decreases are correlated with a sharp decrease in the partial pressure of CO2 and an increase in pH, indicating changes in the relationships between photosynthesis and respiration processes2)9). At the same time average dissolved silica (DSi) concentrations in the main stem of the Neckar system increased significantly, in average by 0.13 mg Si /l per year between 1997 and 20029). This pattern is present in 18 monitoring stations in the Neckar system, all showing significant increases in

dissolved silica concentrations2). A similar effect can be observed for the Rhine

(Fig. 1). In the 80s nitrate and orthophosphate concentrations peaked. During this time dissolved silica concentrations reached a minimum (Fig. 1). Since the beginning of the 90s an increasing trend can be observed, mainly due to an increase in annual minima. Recent average concentrations of dissolved silica in the Rhine at Bimmen/Lobith are around 2.5 mg Si /l. Silica is mainly derived from weathering of silicates and an important nutrient for aquatic ecosystems. The observed correlation of eutrophication and silica depletion due to bioactivity

NO3- -N concentration, Jochenstein, Danube

1982 1984 1987 1990 1993 1995 1998 2001 2004 20060.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

mg/

l

PO43--P concentration, Jochenstein, Danube

1982 1984 1987 1990 1993 1995 1998 2001 2004 20060.00

0.05

0.10

0.15

0.20

0.25

0.30

mg/

l

Fig.2 Nitrate and orthophosphate concentration in the upper Danube at station Jochenstein, close to the border

between Germany and Austria.

NO3--N concentration, Bimmen/Lobith, Rhine

1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 20040

1

2

3

4

5

6m

g/l

PO4

3--P concentration, Bimmen/Lobith, Rhine

1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 20040.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

mg/

l

Si-concentration, Bimmen/Lobith, Rhine

1976 1979 1982 1985 1988 1991 1994 1997 20000

1

2

3

4

mg

Si /

l

Fig.1 Nitrate, orthophosphate and dissolved silica

concentrations in the river Rhine at monitoring station Bimmen/Lobith. (For dissolved silica trends before and after

January 1990 are given. The annual average increase for dissolved silica since 1990 is 0.07 mg Si /l a.)

pH, Bimmen/Lobith, Rhine

1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 2004

7.0

7.5

8.0

8.5

pH

Fig.3 pH at station Bimmen/Lobith.

Jens HARTMANN et al.

42

illustrates the need to take these processes into account when budgeting silica fluxes10).

The recovery of the river Rhine is accompanied by a significant decrease in concentrations of heavy metals and organic persistent pollutants8)11). Both kinds of contaminants declined in the past two decades to either below detection limits or below legal limits. The pH recovered earlier, showing lowest values in the 70s (Fig. 3).

5. LITHOLOGICAL INFLUENCE The most important factor controlling

concentrations or specific fluxes of geogenic major ions is lithology6)12)13). Major ions are primarily derived from weathering processes and are in part influenced by ecosystems through active ion exchange14).

In the humid Central European climate, weathering rates of carbonates can be 10 times higher than silicate weathering rates. Those of evaporites

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

GEOCHEMISTRY OF THE RIVER RHINE AND THE UPPER DANUBE

43

can be 40 to 80 times higher15)16). Even a small proportion of carbonates can influence river hydrochemistry significantly, leading to elevated concentrations of Ca, Mg and bicarbonate6)12).

Fig. 4 and Table 1 show the lithological distribution and the proportions of distinguished lithological units for both catchments. Fig. 5 gives additional information on geological units containing evaporites. The catchments of the Rhine and the upper Danube are characterized by a high aerial proportion of carbonates17) explaining their high alkalinity, pH and Ca- and Mg- concentrations (Table 2). Early Mesozoic evaporite containing lithological units (halite and gypsum) and late Paleaozoic evaporites (halite, gypsum and sylvite) are another important contributor to the total dissolved ions. They are responsible for high SO4 concentrations in both basins and for high Cl and Na concentrations in the Rhine (Fig. 6 and 7). Salt mines as point sources are influencing salinity of the Rhine in addition to the contribution of natural chemical weathering of surface rocks. However, evaporites do not outcrop at the earth’s surface.

Extraordinary high Cl and Na concentrations (more than 300 and 150 mg/l, respectively) during the 70s and 80s were consequences of mining practices which included wastewater discharging directly into the Rhine (Fig. 6). During this period the total annual dissolved matter load of the Rhine was

more than 10 times higher than the suspended matter load3). Since the 90s, emissions of the salt mines were reduced (Table 2).

However, compared to other rivers concentrations of Na, Cl and SO4 are still high17)18). Fluctuations in Ca concentrations are partially controlled by dissolution of gypsum, indicated by the strong correlation between Ca and SO4 (Fig. 7). Table 2 compares tributaries of the Danube holding a high proportion of igneous and metamorphic rocks with those holding high carbonate proportions. Significant differences in maximum concentrations of dissolved silica, Ca and Mg are apparent. High proportions of carbonates are in accordance with high Ca and Mg concentrations. Silica maximum concentrations are coincident with a large proportion of igneous/metamorphic rocks.

The upper Danube at the German-Austrian border contains an average concentration of dissolved silica of 4 mg Si /l, which results in a specific flux of ~ 2.4 t Si /km2 a. The Rhine shows smaller dissolved silica fluxes with ~ 1.1 t Si /km2 a. The world average is 1.6 t Si /km2 a, but can be more than 10 times higher in hot spot regions like Japan, due to easily weatherable volcanic rocks10).

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

Lithology Rhine Upper

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%

Acid Plutonic Rocks 2.70% 3.99%

Basic Plutonic Rocks 0.06% 0.12%

Metamorphic Rocks 2.03% 6.93%

Complex Lithology 3.75% 8.84%

Water Bodies 0.47% 0.54%

Fig.5 Lithological units containing significant amounts of evaporites. However, evaporites are not dominating the mineral

composition in any lithological unit located at the Earth’s surface.

Jens HARTMANN et al.

44

6. CLIMATE CHANGE In addition to direct anthropogenic pressures,

climate change is influencing the aquatic system of both rivers3). In the 20th century global average temperature rose around 0.6 °C and precipitation over the northern hemisphere increased3). Consequences for the Rhine catchment are increases in winter precipitation since around 1980, leading to higher runoff. Shorter periods of snow cover are observed in mountainous regions. This has consequences for average water temperatures. However, regressions for the Rhine River and the

upper Danube yield different results. The Rhine shows an increase in water temperature by ~ 3.5 °C or 0.07 °C/a in the past five decades (Fig. 8), while the temperature of the Danube increases much less by 0.01 °C/a. Temperature data for the upper Danube, however, reach back only to 1982.

In addition to climate change, direct anthropogenic influences such as cooling water inputs from power plants or sewage treatment plants

Ca-concentration, Bimmen/Lobith, Rhine

1959 1964 1969 1974 1979 1984 1989 1994 1999 200420

30

4050

6070

80

90

100

110120

130

mg/

l

SO4-concentration, Bimmen/Lobith, Rhine

1959 1964 1969 1974 1979 1984 1989 1994 1999 200420

40

60

80

100

120

140

160

180

mg/

l

Ca-SO4 relationship during the 1970s, Bimmen/Lobith, Rhine

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Ca mmol/l

0.3

0.6

0.9

1.2

1.5

1.8

SO

4 m

mol

/l

1970-1979 1998-2001

Fig.7 Ca and SO4 concentrations at station Bimmen/Lobith,

Rhine. A significant correlation between SO4 and Ca concentrations is showing the influence of gypsum dissolution

on the hydrochemistry of the river Rhine.

Cl-concentration, Bimmen/Lobith, Rhine

1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 2004

50

100

150

200

250

300

350

400m

g/l

Na-concentration, Bimmen/Lobith, Rhine

1960 1965 1971 1976 1982 1987 1993 1998 2004

20

40

60

80

100

120

140

160

180

200

mg/

l

K-concentration, Bimmen/Lobith, Rhine

1960 1965 1971 1976 1982 1987 1993 1998 2004

2

4

6

8

10

12

14

16

mg/

l

Fig.6 Cl and Na concentrations in the Rhine at

Bimmen/Lobith.

GEOCHEMISTRY OF THE RIVER RHINE AND THE UPPER DANUBE

45

may have contributed to the increase in water temperature of the Rhine. This issue needs more discussion, as water temperature is also an important factor influencing aquatic bioactivity and hence dissolved silica fluxes.

7. CONCLUSIONS A comparison of geochemical base lines and the

analysis of recent trends in concentrations of major ions and nutrients for Rhine and upper Danube shows that significant geochemical changes are occurring in these rivers. Specifically nutrient levels and salt flux are decreasing since the 1980s8)11). Both rivers are recovering from the anthropogenic syndromes such as enhanced eutrophication, heavy metal contamination and salinization1).

For the river Rhine much longer and more complete data sets are available, allowing long-term trend analysis. Data sets covering decades with high sampling frequency are necessary to evaluate changes in dissolved matter fluxes due to changes in land use, water basin management and climate change. The observed changes will impact coastal bioactivity. Observed dynamics should be considered if global or regional studies based on averages from older data sets are carried out. This is specifically important for global and regional nutrient budgets of coastal zones19). ACKNOWLEDGMENT: Presented data were provided by the German Federal Institute of Hydrology (BfG) and the Bavarian Administration for Environment.

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/ metamor- phic rocks

Carbo~nates

Other sediments Si (mg/l) Ca (mg/l) Mg (mg/l)

Alka- linity

(mmol/l)SO4 (mg/l) Cl (mg/l) Na (mg/l) K (mg/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 Danube

Data from reference 4), Passau, 1991-1992 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

Data station Jochenstein (2000-2005) 56

(43 -81)13

(10 – 21)3.1

(1.3 – 3.7)26

(18 – 37)16

(8 – 34) 10

(6 – 16) 2.3

(1.6 – 2.8)

Rhine (Bimmen) 2.5

(0.9 – 3.7)75

(63 – 89)11

(9 – 14)

~ 2.8 (data < 1990)

54 (37 – 68)

88 (47 – 119)

48 (26 – 64)

4.4 (3.4 – 5.5.)

Water temperature, Bimmen/Lobith, Rhine (increase: 0.07 °C/a)

1949 1954 1960 1965 1971 1976 1982 1987 1993 1998 20040

5

10

15

20

25

30

°C

Water Temperature, Jochenstein, Danube (increase: 0.01 °C/a)

1981 1984 1987 1990 1993 1996 1999 2002 2005

0

5

10

15

20

°C

Fig.8 Water temperature of the river Rhine at Bimmen/Lobith

and the upper Danube at Jochenstein.

Jens HARTMANN et al.

46

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(Received February 7, 2007)