Catchments as heterogeneous and multi-species reactors: An integral approach for identifying biogeochemical hot-spots at the catchment scale

Abstract

Summary From a biogeochemical perspective, catchments can be regarded as reactors that transform the input of various substances via precipitation or deposition as they pass through soils and aquifers towards draining streams. Understanding and modeling the variability of solute concentrations in catchment waters require the identification of the prevailing processes, determining their respective contribution to the observed transformation of substances, and the localization of “hot spots”, that is, the most reactive areas of catchments. For this study, we applied a non-linear variant of the Principle Component Analysis, the Isometric Feature Mapping (Isomap), to a data set composed of 1686 soil solution, groundwater and stream water samples and 16 variables (Al, Ca, Cl, Fe, K, Mg, Mn, Na, NH4, NO3, SO4, total S, Si, DOC, electric conductivity and pH values) from the Lehstenbach catchment in Germany. The aim was (i) to assess the contribution of the prevailing biogeochemical processes to the variability of solute concentrations in water samples taken from soils, in groundwater and in stream water in a catchment and (ii) to identify hot spots at the catchment scale with respect to 16 solutes along different flow paths. The first three dimensions of the Isomap analysis explained 48%, 30% and 11%, respectively, i.e. 89% of the variance in the data set. Scores of the first three dimensions could be ascribed to three predominating bundles of biogeochemical processes: (i) redox processes, (ii) acid-induced podzolization, and (iii) weathering processes. In general, the upper 1 m topsoil layer could be considered as hot spots along flow paths from upslope soils and in the wetland, although with varying extents for the different prevailing biogeochemical processes. Nearly 67% and 97% of the variance with respect to redox processes and acid induced podzolization could be traced back to hot spots, respectively, representing less than 2% of the total spatial volume of the catchment. In contrast, biogeochemical processes in deep groundwater had only minor effects on the biogeochemical turnover in catchment waters. Hot spots with respect to weathering processes along upslope and wetland flow paths could not be quantified due to effects of preferential sampling in soil solution samples. Predominating flow paths and biogeochemical processes crucial for the variability of stream water chemistry differed substantially for three streams but were consistent with presumed mixing ratios.