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Abstract "Heft 12"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des
 Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.

Schriftleitung: F. Hädrich

Heft 12

Peter Trüby

Elementumsatz in einer bewässerten Pararendzina der
südlichen Oberrheinebene unter besonderer Berücksichtigung der Schwermetalle

Freiburg im Breisgau 1983

ISSN 0344-2691


In conjunction with planned high-water protection measures, the large-scale inundation of former flood plain locations in the range of the southern upper Rhine valley is being discussed. The proposed areas are for the most part two-layer Pararendzinas with pine forests. The goal of this investigation was to assess the effects.
A field experiment was carried out with measurements of soil water budget and element turnover (Na, K, Mg, Ca, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, P, and Cl). The growth and nutritional condition of an experimental pine stand was also investigated. A further point of the study was the optimization of the methods used, as well as a critical evaluation of the experimental design and the obtained results.
The sites were irrigated with 60 mm of water per week over a 19 month period. This is the equivalent of about 5 times the average yearly precipitation. In order to assess the influence of water quality on the element turnover and the forest growth, two plots each were irrigated with groundwater and Rhine water. For the purposes of comparison 4 control plots were also set up.
To assess the initial conditions, soil and needle analyses were carried out before the beginning of irrigation. Among other things, the total elemental contents in coarse material ( 2 mm), fine earth, sand, silt and clay were determined. The fraction of the elements bound in carbonates was also measured for coarse material and fine earth by determining the HCl-soluble fraction. Because the soil is rich in carbonates, Ca is the dominating element. The levels of heavy metals tend toward values found in relatively unpolluted soils. The vertical distribution of P, Zn, Cd, and Pb is similar to that of organic matter. The distribution of carbonates over particle size fractions indicates a secondary enrichment of carbonates in the top soil layer.
For Ca , Mg, Mn, Cd, and Cu, the HC1-extractable fraction is more than 50 % of the total. For Al, Fe, Cr, and Ni it is less than
2 %.
The variability of the total element content and the HCl-soluble fraction was high. Depending on the element and horizon, the coefficient of variation ranged up to 74 1. An assessment of supply changes in the soil resulting from irrigation, through a control measurement involving a simple collection of 8 random samples at the end of the irrigation period, was therefore not possible for any of the elements.
To determine the element turnover, the concentrations and fluxes in incident precipitation, irrigation water, and soil water were monitored. Soil water was collected by means of Al203-cerami c tension lysimeters.
The element concentrations in incident precipitation tend toward the same general levels as those found in the southern Black Forest and in the Soiling (except for Cu and Zn).
The concentrations in the groundwater and Rhine water are very similar to one another. P is the only element found in comparatively higher levels in Rhine water. Heavy metal concentrations in Rhine water are slightly above those in groundwater; however, in no instance do they approach the drinking water standards, lying generally from one to several orders of magnitude below these limits. The dominating elements are Ca, Na, Mg, K, and Cl.
The concentrations in soil water are largely determined by those in the irrigation water. No significant differences in concentration were found between the groundwater and Rhine water. Because of the high permeability of the substrate, the concentration changes resulting from infiltration in the soil were seldom more than 1/3 of the input concentrations.
The concentrations in soil water are not controlled by solution equilibria with poorly soluble carbonates, hydroxides or hydroxy-carbonates. Thermodynamic equilibrium concentrations for the most part deviate strongly from the average measured concentrations, in extreme cases even by up to eight orders of magnitude.
Particularly in the large pores, the soil/water system lies well-removed from thermodynamic equilibrium conditions.
Through the irrigation, the input of Na, K, Mg, and Ca is increased 10 to 100 times over that in incident precipitation.
The flux of heavy metal deposition in incident precipitation hardly differs from that of the southern Black Forest. Deposition of Cu and In, however, is clearly lower than in the Soiling.
Despite intensive irrigation by Rhine water, the input of heavy metals with precipitation still represents a substantial component of the total input. For Cd it is even the major source.
A climatic water balance forms the foundation for the calculation of element fluxes in the soil water. Compared to the control plots, irrigation leads to a drastic increase in output amounts.
The element flux balance is clearly negative for Na, K, Mg, Ca, and Cu, indicating a mobilization of soil stores of these elements. The magnitude of the filter effect varied between 10 and 30 % on the irrigated plots. On the control plots it is much higher, up to 80 %.
The pine stands responded to the irrigation with a distinct growth increase. In no instance do the needle analyses show significant content differences between the irrigated and control plots.
These results clearly show that, with respect to the investigated elements, the planned flooding,of forested sites in the plain of the upper Rhine will not lead to any noteworthy impairment of groundwater quality. On the other hand with optimum watering, wood production can be markedly increased. Moreover, the function of the soil as a filter can make a moderate contribution toward improving Rhine water quality.
The error analysis shows that even to this day, the trace analysis of soil remains a problem. Analytical methods need to be refined for Ni and Cr, and especially for Cd. For the remaining elements, the spatial variability greatly exceeds the analytical error, and the obtained precision is sufficient.
The Al203-cerami c used in the collection of soil water is inappropriate for the assessment of elemental concentrations over time. A large share of the output is bound to the lysimeter plates through sorption processes. Accordingly, this must be taken into account in the element flux balance.

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