Understanding fate and transport within fluvial systems requires accurate modeling of breakthrough curve (BTC) tails, which often display non-Fickian behaviors. However, it is unclear how anomalous processes relate to the physical and biological characteristics of the stream ecosystem. We use the Stochastic Mobile Immobile model (SMIM) to determine the impact of biofilm colonization among different substrate types on reach-scale transport velocity (V) and dispersion (D), rate of delivery to the subsurface (Λ), and retention within the subsurface (reflected by power law slope; β). During summer 2020 and 2021, we conducted a total of n=42 Rhodamine-WT releases in four experimental streams lined with contrasting substrata (sand, pea gravel, cobble, and a three-way mix) at the Notre Dame Linked Experimental Ecosystem Facility (ND-LEEF) in Indiana (USA). To explore the effect of biofilm colonization, we conducted releases under artificially shaded, early and late biofilm development, and senescent biofilm conditions. We found that replicated releases under constant conditions consistently reproduced stream BTCs and modeled transport parameters. Biofilm abundance, biofilm status (living versus dead), and substrate type produced significant variations in BTC shape and transport parameterizations. We found a non-linear relationship between algal biomass and V, where increases in biomass produced decreases in V at low biomass and increases in V at high biomass. Substrate type also predicted patterns in transport, with sand producing higher V, Λ, and β than larger substrata. These results suggest that substrate type acts as the primary driver and biofilm development the secondary control on transport in fluvial systems.

The benthic biolayer is a reaction hotspot that contributes disproportionately to whole-stream reactions, including aerobic respiration and contaminant transformation. Quantifying the relative contribution of the biolayer to whole-stream mass transformation remains challenging because it requires that hyporheic zone solute transport and reaction heterogeneity are explicitly captured within a single upscaled modeling framework. Here, we use field experiments and modeling to quantify mass transformation in the biolayer relative to other stream compartments. We co-injected and monitored several fluorescent tracers, including the reactive tracer resazurin, into a controlled experimental stream whose 8.5 cm hyporheic zone is isolated from groundwater. We characterized reactive transport in the water column and at multiple hyporheic zone depths by simultaneously fitting concentration time series measured at these locations to a new mobile-immobile model, using resazurin-to-resorufin transformation as an indicator of aerobic bioreactivity. Results show that the biolayer transformed 2× more resazurin to resorufin than all other stream compartments combined, and over half of all reactions occurred within 2 advective travel times through the reach. This hotspot and hot moment behavior is attributed to the biolayer’s propensity to rapidly acquire, transiently retain, and rapidly degrade stream-borne solutes. Model analysis shows that mass transformation is highest in the biolayer across a wide range of biolayer structural properties, including scenarios when the biolayer is less reactive than deeper regions of the hyporheic zone. Together, our results point to the biolayer as a common feature of streams and rivers that should be considered in network-scale models of river corridor biogeochemistry.

Ambient air pollution is an increasing threat to society, with rising numbers of adverse outcomes and exposure inequalities across the globe. Reducing uncertainty in health outcomes models and exposure disparity studies is therefore essential to develop policies effective in protecting the most affected places and populations. This study uses the concept of information entropy to study tradeoffs in mortality uncertainty reduction from increasing input data of air pollution versus health outcomes. We study a case scenario for short-term mortality from fine particulate matter (PM2.5) in North Carolina for 2001-2016, employing a case-crossover design with inputs from an individual-level mortality dataset and high-resolution gridded datasets of PM2.5 and weather covariates. We find a significant association between mortality and PM2.5, and the information tradeoffs indicate that in this case increasing information from mortality may reduce model uncertainty at a faster rate than increasing information from air pollution. We also find that Non-Hispanic Black (NHB) residents tend to live in relatively more polluted census tracts, and that the mean PM2.5 for NHB cases in the mortality model is significantly higher than that of Non-Hispanic White (NHW) cases. The distinct distribution of PM2.5 for NHB cases results in a relatively higher information value, and therefore faster uncertainty reduction, for new NHB cases introduced into the mortality model. This newfound influence of exposure disparities in the rate of uncertainty reduction highlights the importance of minority representation in environmental research as a quantitative advantage to produce more confident estimates of the true effects of environmental pollution.

The presence of solute concentration fluctuations at spatial scales much below the scale of resolution is a major challenge for modeling reactive transport in porous media. Overlooking small-scale fluctuations, which is the usual procedure, often results in strong disagreements between field observations and model predictions, including, but not limited to, the overestimation of e˙ective reaction rates. Existing innovative approaches that account for local reactant segregation do not provide a general mathematical formulation for the generation, transport and decay of these fluctuations and their impact on chemical reactions. We propose a Lagrangian formulation based on the random motion of fluid particles carrying solute concentrations whose departure from the local mean is relaxed through multi-rate interaction by exchange with the mean (MRIEM). We derive and analyze the macroscopic description of the local concentration covariance that emerges from the model, showing its potential to simulate the dynamics of mixing-limited processes. The action of hydrodynamic dispersion on coarse-scale concentration gradients is responsible for the production of local concentration covariance, whereas covariance destruction stems from the local mixing process represented by the MRIEM formulation. The temporal evolution of integrated mixing metrics in two simple scenarios shows the trends that characterize fully-resolved physical systems, such as a late-time power-law decay of the relative importance of incomplete mixing with respect to the total mixing. Experimental observations of mixing-limited reactive transport are successfully reproduced by the model.

The common-pool nature of groundwater resources creates incentives to overpump that contribute to their rapid global depletion. In transboundary aquifers, users are separated by a territorial border and might face substantially different economic and hydrogeologic conditions that can alternatively dampen or amplify incentives to overpump. We develop a theoretical model that couples principles of game theory and groundwater flow to capture the combined effect of well locations and user asymmetries on pumping incentives. We find that user asymmetries in either energy cost, groundwater profitability or aquifer response tend to dampen incentives to overpump. However, combinations of two or more asymmetry types can substantially amplify common-pool overdraft, particularly when the same user simultaneously faces comparatively higher costs (or aquifer response) and profitability. We use this theoretical insight to interpret the emergence of the Disi agreement between Saudi Arabia and Jordan in association with the Disi-Amman water pipeline. By using bounded non-dimensional parameters to encode user asymmetries and groundwater connectivity, the theory provides a tractable generalized framework to understand the premature depletion of shared aquifers.

In an effort to better represent aerosol transport in meso- and global-scale models, large eddy simulations (LES) from the NCAR Turbulence with Particles (NTLP) code are used to develop a Markov chain random walk model that predicts aerosol particle vertical profiles in a cloud-free marine atmospheric boundary layer (MABL). The evolution of vertical concentration profiles are simulated for a range of aerosol particle sizes and in a neutral and an unstable boundary layer. For the neutral boundary layer we find, based on the LES statistics, that there exist temporal correlation structures for particle positions, meaning that over short time intervals (T= 500 s, or T/Tneut= 0.25), particles near the bottom of the boundary are more likely to remain near the bottom of the boundary layer than being abruptly transported to the top, and vice versa. For the unstable boundary layer, a similar time interval of T= 500 s (T/Teddy= 0.39) exhibits weaker temporal correlation compared to the neutral case due to the strong non-local convective motions. In the limit of a large time interval, T= 2000 s (T/Teddy= 1.56), particles have been mixed throughout the MABL and virtually no correlation exists. We leverage this information to parameterize a Markov chain random walk model that accurately predicts the evolution of vertical concentration profiles for the range of particle size and stability tested in LES, even over short time intervals which exhibit substantial correlation. The new methodology has significant potential to be applied at the subgrid level for coarser-scale weather and climate models.

In this work we employ a reduced-order basis of conservative chemical components to model reactive transport using a Lagrangian (particle tracking) method. While this practice is well-understood in the Eulerian (grid-based) context, its adaptation to a Lagrangian context requires a novel reformulation of particle transport properties. Because the number of conservative-species particles need not change during simulation, spatial resolution stays constant in time, and there is no increase in computational expense due to increasing numbers of product particles. Additionally, this treatment simplifies the interaction between equilibrium and kinetic reactions and allows the use of species-dependent transport operators at the same time. We apply this method to model a suite of simple test problems that include equilibrium and kinetic reactions, and results exhibit excellent match with base-case Eulerian results. Finally, we apply the new method to model a 2D problem concerning the mobilisation of cadmium by a CO$_2$ leak, showing the potential applicability of the proposed methodology.

We propose an approach to upscale solute transport in spatially periodic porous media. Our methodology relies on pore scale information to predict large scale transport features, including explicit reconstruction of the solute plume, breakthrough curves at fixed distances, and spatial spreading transverse to the main flow direction. The proposed approach is grounded on the recently proposed trajectory-based Spatial Markov model (tSMM), which upscales transport based on information collected from advective-diffusive particle trajectories across one periodic element. In previous works, this model has been applied solely to one-dimensional transport in a single periodic pore geometry. In this work we extend the tSMM to the prediction of multi-dimensional solute plumes. This is obtained by analyzing the joint space-time probability distribution associated with discrete particles, as yielded by the tSMM. By comparing numerical results from fully resolved simulations and predictions obtained with the tSMM over a wide range of Péclet numbers, we demonstrate that the proposed approach is suitable for modeling transport of conservative and linearly decaying solute species in a realistic pore space and showcase the applicability of the model to predict steady state solute plumes. Additionally, we evaluate the model performance as a function of numerical parameters employed in the tSMM parameterization.