Pavel Krám1, Jakub Hruška1, Kevin Bishop2
1Czech Geological Survey, Dept. of Geochemistry, Klárov 3, 118 21 Prague 1, Czech Republic;
kram@cgu.cz
2Swedish University of Agricultural Sciences, Dept. of Environ. Assessment, POB 7050, Uppsala, SE-750 07 Sweden
ABSTRACT
Long-term streamwater patterns in two catchments underlain by different bedrock (leucogranite at Lysina vs.
serpentinite at Pluhův Bor) were studied by intensive field monitoring in 1989-2002 and computer modeling which simulated the period 1851-2030. A marked decrease in the concentration of acidifying compounds including sulfate was evident in the monitored precipitation inputs. Streamwater sulfate declined at mean rates of 28 µeq L-1 yr-1 (Lysina) and 50 µeq L-1 yr-1 (Pluhův Bor) during the monitoring period. At the chronically acidic Lysina catchment, pH increased at a mean rate of 0.02 pH units yr-1, however pH did not change appreciably at Pluhův Bor. Rates of the decrease in sulfate concentration in the streamwater at Lysina and Pluhův Bor were faster than the declines in any other reported European catchment. Pre-industrial streamwater pH of 5.5 was simulated by the MAGIC model at Lysina. According to the model, the pH dropped below 3.9 in the 1980s and should increase to 4.3 by 2030. Soil base saturation declined dramatically from about 25% to only 6% at Lysina. This severely acidified site will not return to good environmental conditions in the upcoming decades. According to MAGIC, pH was at a similar level (between 6.9 and 7.2) during the whole simulation period as well as the soil base saturation (between 84 and 95%) at the acid resistant Pluhův Bor catchment.
Therefore it is obvious that the Pluhův Bor was able to mitigate long-term acidification. The two catchments served as valuable end-members of central European spruce regions of middle altitude.
Keywords acidification recovery, Czech Republic, leucogranite, long-term monitoring, MAGIC model, serpentinite, small catchments, water chemistry
INTRODUCTION
Severe impacts of air pollution on forested and aquatic environments were documented in the Czech Republic (Moldan and Schnoor, 1992). There is an interest in long-term monitoring of small forested catchments to document changes in biogeochemical patterns (Likens et al., 1996). There is also considerable interest in computer simulations of these patterns (Tiktak and van Grinsven, 1995), especially the predicted recovery from acidification (Evans et al., 2001).
The main objective of this study was to investigate long-term streamwater patterns in two catchments with different geology. These catchment serve as end-members of sensitivity to acidic atmospheric deposition. Another objective was to simulate expected recovery after a decade of massive reduction of sulfur emissions in the “Black Triangle” of central Europe. The MAGIC model was also tested in this application in very acidic as well as alkaline environments.
SITE DESCRIPTION AND METHODS
The catchments are situated in the Slavkov Forest (Slavkovský les), in western Bohemia, Czech Republic (Fig 1). The 27.3 ha Lysina catchment is underlain by base cation-poor leucogranite and the 22.0 ha Pluhův Bor catchment is underlain by base cation-rich (predominantly Mg) serpentinite. They are both forested
have an average age of 50 years at Lysina. At Pluhův Bor, the dominant spruce stands are about 100 years old. The elevation at Pluhův Bor is slightly lower than at Lysina (Fig 1). Mean annual air temperature is 5oC at Lysina and 6oC at Pluhův Bor.
Surface water flow from the catchments was monitored continuously since 1989 at Lysina and 1991 at Pluhův Bor using V-notch weirs and water level recorders. Runoff samples were collected usually weekly.
Bulk precipitation from the open areas was collected since 1990, and throughfall (precipitation below tree canopy) since 1991. Soils were sampled mainly in 1993, tree tissue samples were collected in 1994.
Procedures for chemical analyses are described in detail in Krám et al. (1997).
MAGIC (Model of Acidification of Groundwater in Catchments) was designed to reconstruct the historical development of acidification and to predict future drainage water and soil chemistry (Cosby et al., 1985).
It uses a lumped representation of geochemical processes in the catchment. Water fluxes, atmospheric deposition, net vegetation uptake, weathering rates, and properties of organic acids are required as external inputs to the model. MAGIC simultaneously solves equilibrium reactions between the soil exchanger and soil solution, usually in annual time steps. The simulation outputs are fluxes and volume-weighted concentrations in soil water and surface water as well as basic soil chemistry. Version 5.01 of MAGIC (Cosby, 1991) was used in this application.
Fig 1: Maps of the Lysina and Pluhův Bor catchments in the western Czech Republic. Topographic contours are shown every 5 meters. Elevation range of Lysina is 829-949 m a.s.l., the elevation of Pluhův Bor is 690-804 m a.s.l. Only the major stream channels are shown.
RESULTS AND DISCUSSION
Pluhův Bor is drier than the Lysina catchment. Mean annual precipitation in open areas was about 950 mm at Lysina compared to approximately 850 mm at Pluhův Bor. Less precipitation at Pluhův Bor reflects its lower altitude compared to Lysina and partially also a rainfall shadow east of the major peaks of the Slavkov Forest. Mean annual throughfall was approximately 820 mm at Lysina, and 670 mm at Pluhův Bor.
The lower throughfall flux was influenced by slightly higher temperatures at Pluhův Bor. Annual runoff averages are 430 mm at Lysina and 270 mm at Pluhův Bor. Calculated evapotranspiration was 390 mm at Lysina and 400 mm at Pluhův Bor, given the reported throughfall and runoff values and the assumption of zero storage.
Lysina exhibited incomplete neutralization of mineral acids and it showed low streamwater pH (Fig 2) and higher concentrations of Al. Pluhův Bor exhibited efficient neutralization of acids by weathering of Mg-rich silicates. It showed near neutral streamwater pH (Krám et al., 1997, 2000). Measured Gran alkalinity (by titration) was always negative at Lysina, even during baseflow conditions (Fig 3). However the alkalinity was very high at Pluhův Bor, except during snowmelt periods or large storms.
Marked decreases in the concentration of major acidifying compounds (SO4, F, H), dust representing solutes (Al, Si) and trace metals (As, Pb, Be) was evident in the bulk precipitation and throughfall during the 1990s.
The strong decline in atmospheric deposition of sulfur in the study area in the 1990s (Hruška et al., 2002) resulted in changes of streamwater chemistry in the study catchments. Time series of streamwater concentrations showed a significant decrease of SO4 at Lysina and Pluhův Bor over the periods 1989-2001 and 1991-2001, respectively. SO4 declined at mean rates of 28 µeq L-1 yr-1 (Lysina) and 50 µeq L-1 yr-1 (Pluhův Bor). Annual volume-weighted mean SO4 declined from 568 µeq L-1 in 1990 to 232 µeq L-1 in 2000 at Lysina (Fig 4), while pH increased at a mean rate of 0.02 pH units yr-1. Pluhův Bor experienced an extremely large decline of SO4 from 1035 µeq L-1 in 1992 to 332 µeq L-1 in 2000. Despite the large decrease in SO4 concentrations, pH did not change appreciably at Pluhův Bor. A decrease in concentrations of Mg (13 µeq L-1 yr-1) and Ca (2 µeq L-1 yr-1) were observed in this catchment. At Lysina the decline of SO4
was accompanied by a decrease of base cations at a mean rate of 16 µeq L-1 yr-1 (11 µeq L-1 yr-1 by Ca, 3 µeq L-1 yr-1 by Mg). Charge balance acid neutralization capacity (ANC) increased by 15 µeq L-1 yr-1 at Lysina and by 35 µeq L-1 yr-1 at Pluhův Bor. Rates of the decrease in the concentration of sulfate and the increase of ANC in the streamwater at Lysina and Pluhův Bor were faster than the changes in any other monitored European catchments (Evans et al., 2001; Fölster and Wilander, 2002).
The MAGIC model (Cosby, 1991) was applied to estimate streamwater and soil chemistry between 1851-2030 at Lysina (Hruška et al., 2002) and at Pluhův Bor. Estimated atmospheric deposition was used for 1851-1989, and the measured deposition was used in 1990-2000. The average of the measured deposition from the last two years (1999-2000) was applied for the future deposition in 2001-2030. According to the MAGIC simulation, sulfate concentrations in streamwater peaked in the middle 1980s (Fig 4), several years after the peak of sulfur deposition in 1979 (due to soil adsorption of part of the incoming sulfur).
Similarly the decrease of concentrations of sulfate will be slowed down partially by desorption of previously stored sulfur in the soil. Both catchments should be close to a new equilibrium with respect to sulfur in 2030.
Streamwater pH at Lysina will increase to 4.3 (Fig 5), and soil base saturation will increase to 6.2% by 2030 (from a minimum of 5.7% estimated for 2002). Pre-industrial pH was estimated to be 5.5 and soil base saturation 24.7%. The loss of base cations was caused predominantly by atmospheric acidity, but intensive forestry was responsible for approximately one third of the net base cation loss via accumulation in harvested biomass at Lysina (Hruška et al., 2002). Severely damaged sites, under continued pressure from forestry, will not suddenly (if ever) return to a good environmental condition when the acid deposition input is only partially reduced. According to MAGIC, the Pluhův Bor streamwater pH will stabilise by 2030 (pH 7.1) at a level similar to the simulated pre-industrial value (pH 7.2) (Fig 5). Similarly, soil base saturation, simulated as 95% in 1851, declined to 84% in 1993 and was predicted to remain at 84% in 2030.