Solute transport in the integrated soil-groundwater system

Abstract: A Lagrangian stochastic travel time approach has been usedfor investigating dominant process impacts on (non-reactive andreactive) solute transport in the integrated soil-groundwatersystem. A first step has also been taken to extending thisLagrangian stochastic travel time approach to integrate thesoil-groundwater-stream transport through a catchment.The chosen modeling approach enabled us to explicitlyaccount for several important hydrological-physical mechanismsthat may affect subsurface solute transport and spreading. Inaddition to random flow heterogeneity, we incorporated theeffects of the source distribution (in particular, thelongitudinal source extent that implies considerablevariability in solute pathway lengths to a control plane),non-uniformity in the mean groundwater flow due to groundwaterrecharge along the mean flow direction, and the possibleexistence of preferential flow and less mobile (or immobile)water. Various biogeochemical mass transfer and transformationprocesses (for instance, natural attenuation processes such asbiological degradation, radioactive decay andsorption/desorption) may also be accounted for in theconsidered modeling approach.Theoretical stochastic analyses of groundwater transporthave commonly not considered longitudinal source extents andnon-uniformity in the mean flow due to groundwater recharge. Weshow the importance of both these factors for relevantassessment of expected solute breakthrough at some downstreamcontrol plane. In particular in the near-field of sources thathave large extent, such as for basin-wide contaminationproblems, these factors have a pronounced effect on contaminanttransport and spreading, also increasing the total mass ofdegradable contaminant that may leach to a downstream controlplane.The relative importance of rapid preferential transport andassociated diffusive mass transfer of solute between mobileandimmobile water regions increases with increasing content ofimmobile water and decreasing rate of diffusive mass transfer.Further, the occurrence of preferential flow may alsoconsiderably increase the total leached mass of reactive waterpollutants, such as herbicides that was specificallyinvestigated in this thesis.The modeling of coupled solute transport through anintegrated soil-groundwater-stream system shows thatsignificant solute transport by groundwater to streams, and thepossible existence of subsurface preferential flow paths withassociated diffusive mass transfer processes, extend therelevant time scales for the catchment-scale process overseveral orders of magnitude. Better understanding andquantification of the subsurface catchment-scale transportprocess are essential for correct predictions of possibleadverse effects on water quality and for judging theperformance of different measures for water quality protectionor remediation.In general, application of the Lagrangian stochastic traveltime approach to field-specific experiments yielded robustmodel parameter values, in terms of mobile and immobile watercontents, rates of diffusive mass transfer between mobile andimmobile water zones, and herbicide reaction parameter values(retardation factors, first-order rates of mass transferbetween mobile and adsorbed phase, and first-order degradationrates in the dissolved and adsorbed phase). The results arereproducible and consistent, with other comparable experimentaldata reported in the literature.Keywords: solute transport, groundwater transport,integrated soil-groundwater system, unsaturated zone,stochastic processes, preferential flow, catchment hydrology,Lagrangian transport model, dominant process impacts.