<p>The interplay between hydraulic and chemical processes in streams and adjacent storage zones, e.g. streambed sediments, is of crucial importance for the large-scale transport of released contaminants. This thesis presents a methodology for tracer experiments and the development of mechanistic transport models. </p><p>We conducted four field tracer experiments with the reactive tracer chromium, ⁵¹Cr(III) and/or the conservative tracers potassium iodide (KI) and tritium (³H₂O), along 11 km of the Lanna Stream in Skara County, Sweden, and along a 30 km reach of the Säva Stream in Uppland County, Sweden. The field monitoring included sampling of tracer in the surface water as well as in the streambed sediments. A simultaneous injection of tritium and chromium facilitated an independent evaluation of the hydraulic transport into and out of the sub-surface storage zones. The difference in transport behaviour between the two tracers, were attributed to chemical reactivity.</p><p>In terms of idealised transport models we found that the reactivity of chromium could be characterised by simple chemical concepts. The local equilibrium assumption (LEA), the irreversible kinetics assumption (IKA) and the reversible kinetics assumption (RKA) were applied in the experimental evaluations. An independent evaluation of the streambed transport revealed that the impact of reaction kinetics was substantial. Model calculations and results from a chemical extraction procedure indicated that the chemical reactions affecting the chromium transport were to some extent irreversible.</p><p>This thesis presents a number of exact analytical solutions to the governing partial differential equations. The main theoretical contribution is the incorporation of variable coefficients for stream discharge and sediment porosity, which were measured in field.</p>

The interplay between hydraulic and chemical processes in streams and adjacent storage zones, e.g. streambed sediments, is of crucial importance for the large-scale transport of released contaminants. This thesis presents a methodology for tracer experiments and the development of mechanistic transport models. We conducted four field tracer experiments with the reactive tracer chromium, 51Cr(III) and/or the conservative tracers potassium iodide (KI) and tritium (3H2O), along 11 km of the Lanna Stream in Skara County, Sweden, and along a 30 km reach of the Säva Stream in Uppland County, Sweden. The field monitoring included sampling of tracer in the surface water as well as in the streambed sediments. A simultaneous injection of tritium and chromium facilitated an independent evaluation of the hydraulic transport into and out of the sub-surface storage zones. The difference in transport behaviour between the two tracers, were attributed to chemical reactivity. In terms of idealised transport models we found that the reactivity of chromium could be characterised by simple chemical concepts. The local equilibrium assumption (LEA), the irreversible kinetics assumption (IKA) and the reversible kinetics assumption (RKA) were applied in the experimental evaluations. An independent evaluation of the streambed transport revealed that the impact of reaction kinetics was substantial. Model calculations and results from a chemical extraction procedure indicated that the chemical reactions affecting the chromium transport were to some extent irreversible. This thesis presents a number of exact analytical solutions to the governing partial differential equations. The main theoretical contribution is the incorporation of variable coefficients for stream discharge and sediment porosity, which were measured in field.

It is important to understand the processes that influence the critical zone (CZ) evolution to ensure its sustainability. This thesis reports on laboratory and field experiments designed to measure the behavior of biogenic and lithogenic chemical species and their interaction in the CZ from column to pedon to catchment scales. We postulated that interactions between organic matter and rock-derived metals drive coupled processes of carbon stabilization and chemical weathering and denudation in the Jemez River Basin Critical Zone Observatory (JRB-CZO). First, we observed that secondary mineral coatings (Al and Fe (oxy)hydroxides) on primary silicate surfaces play a major role in sequestering aromatic and "humified" dissolved organic matter (DOM) into sorbate form, significantly retarding their subsurface transport. Further, reinfusion to OM-reacted-porous-media of a different DOM source resulted in exchange reactions consistent with a zonal model of OM adsorption at mineral surfaces. This dissertation also aimed to examine the influence of water and DOM fluxes on the CZ weathering processes. Rare earth elements (REE) were selected because of their coherent trends in reactivity toward organic ligands common to soils. Specifically, trends in REE fractionation were explored for their utility to inform on biogeochemical weathering processes in forested terrain in the JRB-CZO. Mineral weathering mechanisms are expected to differentially influence REE release, fractionation, and transport and the relative importance of such processes should be reflected in REE signatures of bulk soil, pore and surface waters. Our studies showed: (1) REE depletion trends with depth in bulk soils are correlated with topographically-induced variation in water and dissolved organic carbon (DOC) flux (reflected in negative correlations between total water and C fluxes) and solid phase REE concentrations measured at the same depths; (2) REE and DOC concentrations in stream waters were strongly correlated during snowmelt periods of high discharge, consistent with REE complexation and mobilization in association with organic ligands during shallow subsurface flow; (3) preferential sequestration of Eu occurs during formation of secondary Mn(IV)-oxides, explaining patterns of Eu enrichment in bulk soils; and (4) the incremental increase in positive Ce-anomalies with depth in bulk soils are apparently controlled by adsorption/co-precipitation with secondary Fe-(oxy)hydroxide minerals.