Lake sediments during the transient eutrophication period: Reactive-transport model and identifiability study

Abstract

The modeling of diagenetic processes in the sediment of deep eutrophic lakes requires a dynamic approach covering seasonal changes to decadal trends. Due to the large base of scientific knowledge, the description of environmental systems is often based on complex simulation models that contain parameterizations of a large number of processes. The parameters of such models are usually not identifiable from a data set available for a specific system. In such situations, identifiability analysis techniques are useful in order to find identifiable subsets of parameters and to gain insight into the degree of non-identifiability. In this study a dynamic diagenetic model was developed to gain insight into the history of sedimentation and diagenetic processes in the sediments of Lake Zug, Switzerland, over the past 400 years. This reconstruction was constrained by measured data of 11 pore water concentration profiles, 6 particulate mass fractions in the sediment, sediment porosity and sediment core dating. According to the model, methanogenesis dominated the mineralization in the anoxic zone below the sediment–water interface with a fraction of 77% of organic matter degraded via this pathway. The contributions of Mn, Fe, and sulfate reduction to the organic matter degradation were less important (6%, 2%, and 15% respectively). The detailed analysis of an aerobic degradation at the sediment–water interface requires high-resolution measurements of dissolved substances. The model reproduced the main features of the seasonal variation of dissolved ammonia depth profiles quite well but failed to reproduce a sharp phosphate peak that was observed during summer, which indicates either spatial heterogeneity or short-term dynamics in the phosphorus release. Based on the time dependence of sedimentation fluxes, the model described changes in the accumulation of FeS in sediments from 1940 to 1994 as a result of increasing eutrophication, which opens the perspective for paleolimnological reconstructions based on quantitative diagenetic models. Based on identifiability analysis techniques this study shows that for a diagenetic model of the sediment of Lake Zug, an exceptionally large number of model parameters (25) can be identified from the available data. The result, based on numerical identifiability criteria, is verified by an automatic parameter estimation procedure. Furthermore, it is discussed if the identified parameter values are reasonable, or if the model parameters or model deficiencies that are fitted lead to a significant bias in their estimates.