Small scale heterogeneity of soil chemical properties: Effects on spring water chemistry

Abstract

Previous soil and spring water analyses in small catchments revealed low pH values in the spring water during high discharge events. This paper analyses the potential which small scale heterogeneity of soil acidity may have to explain decreasing spring water pH as a result of high discharge. Soil aggregates were collected from a C-horizon of a Spodosol in the Fichtelgebirge. Exchangeable cations and soil solution were examined on both samples from the surfaces and the cores of aggregates which were obtained by a mechanical separation procedure. The Reuss-Johnson soil chemical equilibrium model was used to predict soil and spring water pH values as a function of acidic input and soil air CO2 concentration in equilibrium with both aggregate fractions. Ranges of acidic input from 160-570 μeq L-1 and soil air CO2 concentrations from 0.1 to 3 Vol. % were considered. The model predicted spring water pH values from 5.0 to 5.3 for the acidic aggregate surface samples (base saturation = 12.5%) and from 6.8 to 7.2 for the aggregate core samples (base saturation = 32.1%). The results suggest that small scale acidity gradients may expand the range of predictable spring water pH values. However, very low pH values (<5) still need additional explanation. © WILEY-VCH Verlag GmbH, 2000.