The Lagrangian Stochastic Advective-Reactive Approach to Modeling Solute Transport in Hydrological Systems

Abstract: The Lagrangian stochastic advective-reactive modelingapproach has been used for analyzing transport of bothnonreactive and reactive solutes in different hydrologicalsystems (structured soil, groundwater, mining waste rockdeposits and surface waters including single stream and networkof streams) and at different spatio-temporal scales (rangingfrom laboratory column-scale to catchment-scale). Further, afirst step has been taken to extending the Lagrangianstochastic advective-reactive modeling methodology to integratethe soil-groundwater-stream transport through a catchment.The modeling approach enabled us to explicitly quantify theeffects of solute transport through subsurface preferentialflowpaths in both laboratory and field scales, throughexperimental results from tracer tests in structured soil andmining waste rock deposits. Rapid preferential transport andassociated diffusional mass transfer of solute between mobileand immobile water regions yield the characteristicnonequilibrium behavior of early initial breakthrough and longtailing in solute breakthrough curves. The occurrence ofpreferential flow may considerably increase the total leachedmass of sorptive water pollutants, such as phosphorus that wasspecifically investigated in this thesis.Temporal moment analysis of solute transport in a singlestream and a network of streams has been carried out,accounting for heterogeneity in hydraulic geometry variablesand physicochemical mass transfer parameters. The resultsindicate that mass transfer processes in the bed sediment arerelatively more important than processes in the storage zonesof the streams. The probability of any percentage of solutemass arriving at the outlet is sensitive to the diffusive masstransfer rate in the bed sediment. This mass transfer rate isan uncertain parameter, which implies uncertainty also inpredictions of the solute transport process.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 flowpaths withassociated diffusional mass transfer processes extend therelevant time scales for the catchment-scale process overseveral orders of magnitude. Furthermore, increasinggroundwater recharge from the unsaturated zone and decreasingratio of average groundwater to stream velocity impliessmoother solute breakthrough curves that approach theobservations of fractal stream chemistry by Kirchner et al.(Nature, Vol. 403, pp. 524-527, 2000).The overall results show that the various mass transferprocesses that take place in different hydrological systemsresult in reduction of peak mass flux and increase in meanarrival time, relative to the stream and groundwater advectionprocesses only, due to prolonged tails. Such tailing impliesconsiderably increased temporal spreading of solute mass,further extending the relevant time scales for thecatchment-scale transport process and leading to low levelcontaminant concentrations leaching from the catchment for avery long time. Better understanding and quantification of thecatchment-scale transport process are essential for correctpredictions of possible adverse effects on water quality andfor judging the performance of different measures for waterquality protection or remediation.Key words:solute transport, groundwater transport,unsaturated zone, stream hydrology, stream networks, stochasticprocesses, mass transfer kinetics, preferential flow, temporalmoments, catchment hydrology, Lagrangian transport model.