Numerical modeling of an abiotic hyporheic mixing-dependent reaction: Chemical evolution of mixing and reactant production zones.

Mixing-dependent reactions occur where groundwater and surface water mix in shallow sediments (hyporheic zone) and can attenuate contaminants along upwelling flowpaths, thus reducing transport to surface water. Here we used MODFLOW/SEAM3D to numerically simulate prior laboratory observations of a mixing-dependent reaction between sodium sulfite (Na2SO3) and dissolved oxygen (DO) to produce sodium sulfate (Na2SO4). This reaction is not common in nature but is used as a surrogate for mixing-dependent DO consuming reactions of environmental significance. We evaluated how location and thickness of mixing zones and reaction product production zones dynamically respond to variations in hydraulic and chemical boundary conditions and reaction kinetic rate. Sensitivity analysis showed that location and thickness of mixing zones and reactant production zones were most sensitive to changes in the balance of hydrologic inflow from groundwater and surface water (inflow ratio). Mixing zone thickness for reactive DO calibrated to experimental data was thinner than that for the "DO tracer" (identical source location and concentration as DO but conservative tracer), indicating that as DO is consumed its mixing zone narrows. The SO4 production zone was consistently thicker than the DO mixing zone. Small changes in mixing/production zone thicknesses were linked to large changes in mass consumed and produced, indicating the potential for simpler field metrics like thickness to act as surrogates for more challenging measurements such as contaminant flux or consumption in monitoring natural attenuation. This study improves understanding of the evolution of hyporheic mixing-dependent reaction zones that occur even under steady state hydraulics, emphasizing their complex controls.

Food contact of paper and plastic products containing SiO2, Cu-Phthalocyanine, Fe2O3, CaCO3: Ranking factors that control the similarity of form and rate of release.

The paper industry is an important sector annually consuming kilotons of nanoforms and non-nanoforms of fillers and pigments. Fillers accelerate the rate of drying (less energy needed) and product cost (increasing the load of low-cost fillers). The plastic industry is another use sector, where coloristic pigments can be in nanoform, and many food containers are made of plastic. Use of paper to wrap both wet and dry food is consumer practice, but not always intended by producers. Here we compare the release behavior of different nano-enabled products (NEPs) by changing a) nanoform (NF) characteristics, b) NF load, c) the nano-enabled product (NEP) matrix, and d) food simulants. The ranking of these factors enables an assessment of food contact by concepts of analogy, specifically via the similarities of the rate and form of release in food during contact. Three types of matrices were used: Paper, plastic ((Polylactic Acid (PLA), Polyamide (PA6), and Polyurethane (PU)), and a paint formulation. Two nanoforms each of SiO2, Fe2O3, Cu-Phthalocyanine were incorporated, additionally to the conventional form of CaCO3 that is always contained in paper to reduce cellulose consumption. Tests were guided by the European Regulation EC 1935/2004 and EU 10/2011. No evidence of particle release was observed: the qualitative similarity (the form of release) was high regarding the food contact of all NEPs with embedded NFs. Quantitative similarity of releases depended primarily on the NEP matrix, as this controls the penetration of the simulant fluid into the NEP. The solubility of the NF and impurities in the simulant fluid was the second decisive factor, as dissolution of the NF inside the NEP is the main mechanism of release. This led to complete removal of CaCO3 in acidic medium, whereas Fe and Si signals remained in the paper, consistent with the low release rates in an ionic form. In our set of 16 NEPs, only one NEP showed a dependence on the REACH NF descriptors (substance, size, shape, surface treatment, crystallinity, impurities), specifically attributed to differences in soluble impurities, whereas for all others the substance of the nanoform was sufficient to predict a similarity of food contact release, without influences of size, shape, surface treatment and crystallinity.

Hyporheic transverse mixing zones and dispersivity: Laboratory and numerical experiments of hydraulic controls.

Mixing of surface water and groundwater in shallow sediments is important to biogeochemical cycling and contaminant migration, and is often used to define the hyporheic zone. Yet knowledge of mixing processes in hyporheic zones is supported by surprisingly few rigorous lab or field observations, and differ from those in deeper groundwater by presence of enhanced head gradients, sediment heterogeneity, and temporal fluctuations. In a laboratory sediment (sand) tank we photographed a conservative dye to analyze transverse mixing zones between upwelling groundwater and bidirectional hyporheic exchange flows. We then conducted numerical modeling to investigate processes behind observed phenomena and estimate dispersivities. We found that transverse mixing zones were thin (i.e. mixing thickness measured in direction of steepest concentration gradient, δ, less than 5 cm), consistent with a small calibrated transverse dispersivity (~0.1 mm) and prior lab studies conducted at similar scales. In steady-state experiments and simulations, δ and estimated dispersion coefficients increased with the surface water head drop driving exchange flows. Given relatively constant deeper groundwater heads, increased Δh led to increased mixing zone length for both steady-state and transient conditions, indicating larger bedforms or weaker gaining conditions enhance subsurface mixing. However, Peclet number and flux-related dilution index simultaneously increased and decreased, respectively, indicating that enhancement of subsurface advection outpaced that of dispersion. In transient experiments and simulations, δ was greater than for steady-state, probably from temporary addition of longitudinal dispersion. During transient experiments, δ exhibited temporal noise, perhaps due to the mixing zone moving past varying patterns of sediment packing. Our results provide basic knowledge of mixing zone behavior in hyporheic zones with implications for hyporheic zone definitions, solute transport, mixing-dependent reaction, and water quality.