Forecasting Versus Predicting Solute Transport in Solution Conduits for Estimating Drinking-Water Risks

Vnos polutantov v kras ima lahko hitre in uničujoče posledice za pitno vodo. Ker je nevarnost izlitja vedno navzoča, je prav, da upravniki vodnih virov predvidijo scenarije onesnaženja še preden se to v resnici zgodi. Različne scenarije vnosa polutantov v kras lahko predvidimo z uporabo že znanih podatkov in meritev v vodonosniku. Napoved širjenja polutantov je najbolj zanesljiva, če temelji na rezultatih številnih sledenj med različnimi vhodnimi in izhodnimi točkami. Na ta način lahko primerne tranportne parametre določimo z analizo krivulj koncentracij. S kombiniranjem različnih krivulj in uporabo ustrezne aproksimacijske metode, lahko razultate združimo v eno samo krivuljo koncentracij, ki jo uporabimo za predvidenje širjenja polutantov. Širjenje polutantov lahko napovemo tudi z uporabo osnovnih tranportnih parametrov, ki veljajo za kraške prevodnike in ustreznih transportnih enačb. Rezultat so krivulje koncentracij, ki jih tudi uporabimo za napoved posledic možnega vnosa polutantov v kras. Obe metodi smo preizkusili na namišljenem vnosu polutantov v kraški kanal in pri obeh dobili sprejemljive rezultate. Metoda, ki temelji na uporabi transportnih enačb, se je izkazala za boljšo pri oceni potovalnega časa in koncentracije, ki smo ji izpostavljeni. Poleg tega je pri tej metodi delež povrnjenega sledila večji.Contaminant releases in karstic terranes can cause rapid and devastating affects on drinking-water supplies. Because future contaminant releases are likely it is necessary that local water managers develop release scenarios so as to be prepared prior to an actual contaminant release occurring. Release scenarios may be forecasted using appropriate historical data or they may be predicted using selected measured parameters. Forecasting contaminant releases to drinking-water supplies in karstic terranes is best accomplished by conducting numerous tracer tests from each potential source location to each exposure point so that acceptable solute-transport parameters for each solution conduit may be estimated from analyses of the breakthrough curves. Compositing the numerous breakthrough curves and fitting a quintic spline allows development of a single representative breakthrough curve that may then be used to forecast the effects of a release. Predicting contaminant releases is accomplished by combining basic measured field parameters for selected solution conduits in functional relationships for application in solute-transport models. The resulting breakthrough curve and solute-transport parameters can be used to predict the effects of a release. The forecasting and prediction methodologies were tested using a hypothetical release into a solution conduit developed in a karstic aquifer. Both methods were shown to produce reasonably acceptable results. The prediction methodology produced better time-of-travel results and better mass recovery and exposure concentration results than did the forecasting methodology

Dynamics and efficiency of a self-propelled, diffusiophoretic swimmer

Active diffusiophoresis - swimming through interaction with a self-generated, neutral, solute gradient - is a paradigm for autonomous motion at the micrometer scale. We study this propulsion mechanism within a linear response theory. Firstly, we consider several aspects relating to the dynamics of the swimming particle. We extend established analytical formulae to describe small swimmers, which interact with their environment on a finite lengthscale. Solute convection is also taken into account. Modeling of the chemical reaction reveals a coupling between the angular distribution of reactivity on the swimmer and the concentration field. This effect, which we term "reaction induced concentration distortion", strongly influences the particle speed. Building on these insights, we employ irreversible, linear thermodynamics to formulate an energy balance. This approach highlights the importance of solute convection for a consistent treatment of the energetics. The efficiency of swimming is calculated numerically and approximated analytically. Finally, we define an efficiency of transport for swimmers which are moving in random directions. It is shown that this efficiency scales as the inverse of the macroscopic distance over which transport is to occur.Comment: 16 pages, 11 figure

Solute trapping and diffusionless solidification in a binary system

Numerous experimental data on the rapid solidification of binary systems exhibit the formation of metastable solid phases with the initial (nominal) chemical composition. This fact is explained by complete solute trapping leading to diffusionless (chemically partitionless) solidification at a finite growth velocity of crystals. Special attention is paid to developing a model of rapid solidification which describes a transition from chemically partitioned to diffusionless growth of crystals. Analytical treatments lead to the condition for complete solute trapping which directly follows from the analysis of the solute diffusion around the solid-liquid interface and atomic attachment and detachment at the interface. The resulting equations for the flux balance at the interface take into account two kinetic parameters: diffusion speed $V_{DI}$ on the interface and diffusion speed $V_D$ in bulk phases. The model describes experimental data on nonequilibrium solute partitioning in solidification of Si-As alloys [M.J. Aziz et al., J. Cryst. Growth {\bf 148}, 172 (1995); Acta Mater. {\bf 48}, 4797 (2000)] for the whole range of solidification velocity investigated.Comment: Regular article in Physical Review E, Vol. 76 (2007

Impacts of Preferential Flow on Tc-99and Np-237 Vadose Transport in Soils at the Savannah River Site

Since the 1950s, the United States has produced approximately 90,000 metric tons of spent nuclear fuel (SNF) (Office of Nuclear Energy, 2022); however, no long-term storage solutions are available. Technecium-99 and neptunium-237, two fission products found in SNF, readily form highly mobile species in oxidizing conditions (Hu, 2008; Bondietti, 1979) and have respective half-lives of 2.13 x 105 and 2.14 x106 years (Hu, 2010). Considering these characteristics, 99Tc and 237Np are two risk-driving isotopes found in SNF storage. The process of macropore-facilitated preferential flow, transport through cracks within a soil matrix, has been recognized to increase radionuclide mobility (Bundt, 2000). The aim of this study is to clarify the influence of macropore structures (orientation and geometry) on the transport of 99Tc and 237Np in unsaturated sand and sandy clay loam (SCL) soils. Unsaturated column flow experiments were completed with Br (tracer), 99Tc, and 237Np on homogeneous and macroporous soil columns. CT scans of soil columns were acquired to generate 2D and 3D imaging of macropore networks. Retardation factors for Tc99 breakthrough were 1.30 in homogeneous SCL and ranged from 1.28 to 1.41 in macroporous SCL, indicating that macropore-facilitated bypass flow is limited in unsaturated conditions. However, 99Tc exhibited earlier initial arrival (IBV) and longer attenuation in macroporous SCL soils. Increasing dispersivity estimates, determined by fitting the two-region nonequilibrium convection dispersion equation (MIM), well describe the anomalous breakthrough trends observed in macroporous SCL soils. From these findings, we conclude that macropore structures oriented transverse to unsaturated flow act as capillary barriers (resulting in longer radionuclide attenuation). If vertically oriented, macropore structures act as capillary boundaries that isolate matrix flow to a fraction of the pore network (increasing the average porewater velocity and IBV)

Thermodynamic entropy production fluctuation in a two dimensional shear flow model

We investigate fluctuations in the momentum flux across a surface perpendicular to the velocity gradient in a stationary shear flow maintained by either thermostated deterministic or by stochastic boundary conditions. In the deterministic system the Gallavotti-Cohen (GC)relation for the probability of large deviations, which holds for the phase space volume contraction giving the Gibbs ensemble entropy production, never seems to hold for the flux which gives the hydrodynamic entropy production. In the stochastic case the GC relation is found to hold for the total flux, as predicted by extensions of the GC theorem but not for the flux across part of the surface. The latter appear to satisfy a modified GC relation. Similar results are obtained for the heat flux in a steady state produced by stochastic boundaries at different temperatures.Comment: 9 postscript figure

Oxytetracycline in Soil and Conditions that Select for Antibiotic Resistant Bacteria

The increased occurrence of antibiotic resistant bacteria (ARBs) in the environment is posing significant and increasing pressure on human health care in the U.S. and globally. The misuse, overuse and partial metabolism of antibiotics in humans and the animalproducing industry over the years has been accompanied by unintentional environmental antibiotic contamination. With the increased incidence of ARBs, attention has been paid to the environmental fate of antibiotics, including Oxytetracycline (OTC). OTC is one of the most commonly administered antibiotics to livestock and has been categorized by the World Health Organization (WHO) as “critically important ”because it is used as an alternative treatment of serious infections in humans and to treat diseases caused by bacteria that may be transmitted to humans from non-human sources. Even though OTC has a low potential for mobility due to its high sorption partition coefficient (Kd range between 115 to 269,097 L/Kg), OTC has been detected in surface and groundwater. OTC is strongly retained by soil component (alumninosilicates, organic matter, metal oxides) through multiple mechanisms, yet is still bioavailable to microorganisms suggesting a potential scenario for toxicity and/or emergence of antibiotic resistance. This work presents an inventory of reported OTC concentrations distributed in aqueous and solid media and an evaluation of the fate of OTC in the environment. OTC concentrations were compared to threshold limits that delineate selective windows of resistance to assess the selective potential for resistant bacteria. A model to predict OTC partition coefficients based on soil properties was developed considering the importance of sorption on the fate, mobility and bioavailability of OTC, the variability of reported sorption and the complex interaction of OTC with soil components. The fate and transport of OTC in soil was addressed by using a Two-site, One-rate Non-equilibrium model. Simulation results were compared to antibiotic resistant selection regions to evaluate which scenarios resulted in the potential for antibiotic resistant harboring. The model predicted soil-bound OTC concentration levels that were within the antibiotic resistance selection ranges. Therefore, surface application of slurry and diluted slurry with manure-associated OTC concentrations on the order of 101 to 102 g=Kg could potentially select for antibiotic resistant bacteria (ARB), particularly in cases where no incorporation of manure is considered. Predicted concentrations resulted in potential selection of ARBs throughout the entire simulation period (120 days) for slurry application without incorporation and up to 50 days for slurry application with homogeneous incorporation of manure to a depth of 10 cm. For the cases of diluted slurry application without incorporation, predicted concentrations resulted in potential selection of ARBs throughout the entire simulation period (120 days) and up to 30 days for diluted slurry application with homogeneous incorporation of manure to a depth of 10 cm. These results suggest that OTC concentrations in swine manure together with current waste management practices of land application of manure as fertilizer present the potential for selection of antibiotic resistance, in particular when no incorporation of manure is practiced. The incorporation of manure into soil effectively reduces OTC concentrations in soil and also in the aqueous phase to levels below antibiotic resistance selectivity. Incorporation of manure into soil and other manure management practices to reduce manureassociate OTC, such as stockpiling and composting, can be effective in minimizing the potential selection for antibiotic resistance. Additional research is needed to assess the microbial activity of soil-bound OTC to compare OTC concentration levels in soil with antibiotic selectivity ranges