Multidimensional simulation of PFAS transport and leaching in the vadose zone: impact of surfactant-induced flow and soil heterogeneities

PFAS are emergent contaminants of which fate and transport in the environment remain poorly understood. As surfactants, adsorption at air-water interfaces and solid surfaces in soils complicates the retention and leaching of PFAS in the vadose zone. Recent modeling studies accounting for the PFAS-specific nonlinear adsorption processes predicted that the majority of long-chain PFAS remain in the shallow vadose zone decades after contamination ceases—in agreement with many field measurements. However, some field investigations show that long-chain PFAS have migrated to tens to a hundred meters below ground surface. These discrepancies may be attributed to model simplifications such as a one-dimensional (1D) representation of the homogeneous vadose zone. Another potentially critical process that has not been fully examined by the 1D models is how surfactant-induced flow (SIF) influences PFAS leaching in multidimensions. We develop a new three-dimensional model for PFAS transport in the subsurface to investigate the multidimensional effects of SIF and soil heterogeneities. Our simulations and analyses conclude that 1) SIF has a minimal impact on the long-term leaching of PFAS in the vadose zone, 2) preferential flow pathways generated by soil heterogeneities lead to early arrival and accelerated leaching of (especially long-chain) PFAS, 3) the acceleration of PFAS leaching in high water-content preferential pathways or perched water above capillary barriers is more prominent than conventional contaminants due to the destruction of air-water interfaces, and 4) soil heterogeneities are among primary sources of uncertainty for predicting PFAS leaching and retention in the vadose zone.

Microbial Hotspots in Lithic Microhabitats Inferred from DNA Fractionation and Metagenomics in the Atacama Desert

The existence of microbial activity hotspots in temperate regions of Earth is driven by soil heterogeneities, especially the temporal and spatial availability of nutrients. Here we investigate whether microbial activity hotspots also exist in lithic microhabitats in one of the most arid regions of the world, the Atacama Desert in Chile. While previous studies evaluated the total DNA fraction to elucidate the microbial communities, we here for the first time use a DNA separation approach on lithic microhabitats, together with metagenomics and other analysis methods (i.e., ATP, PLFA, and metabolite analysis) to specifically gain insights on the living and potentially active microbial community. Our results show that hypolith colonized rocks are microbial hotspots in the desert environment. In contrast, our data do not support such a conclusion for gypsum crust and salt rock environments, because only limited microbial activity could be observed. The hypolith community is dominated by phototrophs, mostly Cyanobacteria and Chloroflexi, at both study sites. The gypsum crusts are dominated by methylotrophs and heterotrophic phototrophs, mostly Chloroflexi, and the salt rocks (halite nodules) by phototrophic and halotolerant endoliths, mostly Cyanobacteria and Archaea. The major environmental constraints in the organic-poor arid and hyperarid Atacama Desert are water availability and UV irradiation, allowing phototrophs and other extremophiles to play a key role in desert ecology.

Scaling up microbial dynamics for soil carbon cycling models

<p>Soils are heterogeneous at all scales and so are the biogeochemical reactions driving the cycling of carbon (C) and nutrients in soils. While the microbial processes involved in these reactions occur at the pore scale, what we observe at the soil core or pedon scale depends on how micro-scale processes are integrated in space (and time). This integration step requires accounting for the inherent patchiness of soils, but models used to describe element cycling in soils typically assume that conditions are well-mixed and that kinetics laws developed for laboratory conditions hold. Similarly, the response functions used in models to capture the effects of environmental conditions on C and nutrient fluxes neglect the contribution of spatial heterogeneities, which might alter their shape. There is therefore a need to re-evaluate model structures to test whether they can account for micro-scale heterogeneities. Alternatively, one can ask why some models are clearly successful in capturing observations despite neglecting soil heterogeneities. In this contribution, we present examples of how soil heterogeneities – in particular the spatial placement of soil microorganisms and their substrate – may affect decomposition kinetics and microbial responses to soil drying. We show that the kinetics laws used in current models are different from the kinetics obtained by integrating microbial dynamics at the micro-scale, and that respiration responses to soil drying may vary depending on soil heterogeneity. These results thus highlight structural uncertainties in current models that we propose can be assessed using existing ‘scale-aware’ methods to derive macro-scale model formulations. Model advances will need to be supported by empirical evidence bridging the gap between pore and core (or larger) scales, but can also provide new theory-based hypotheses for novel experiments.</p>

Inverting a comprehensive crop model in parsimonious data context using Sentinel 2 images and yield map to infer soil water storage capacity.

<p>Soil Water storage Capacity (SWSC) is an important quantity in the field of hydrology and agronomy to represent the hydrological functioning of a territory and/or the dynamics of a crop. SWSC spatial variability is often strong resulting from heterogeneity in texture and structure as well as soil depth. In situ measurement of SWSC is expensive, destructive and cannot be considered over a large area. Therefore, the characterization of SWSC by non-destructive methods is a mean of addressing the mapping issue. In this study we took profit of the new capacities offered by the Sentinel 2 mission, which allows characterizing relevant features in vegetation dynamic linked to stresses. In addition, yield map offers an additional source of information. Both yield and vegetation development are sensitive to several factors as the water and nitrogen supply, crop installation or pest. To isolate the influence of water supply, and therefore access parameters involved in the SWSC, an option is to delineate the effect of such factors by inverting a crop model able to simulate the observation together with the representation of most of influencing factors. The STICS crop model implemented in this study is suitable to consider interactions between carbon, nitrogen and water cycles, plant development and farming practices. The issue is then to demonstrate that parsimony in field characterization can be overcome by using satellite and yield observations to implement and invert comprehensive model such as STICS. A sensitivity analysis (Lammoglia et al. 2019) indicates that once plant variety parameters are calibrated, the parameters linked to crop installation, as the sowing depth and the sowing density, the initial soil mineral nitrogen and the SWSC are the main quantities to consider in an inversion procedure. The GLUE Bayesian method was used to retrieve the different parameters. The procedure was tested on non-irrigated winter durum wheat in a Mediterranean context in south-eastern France. The approach was evaluated in farm context 20 on heterogeneous fields over three years (2016-2018). Evaluation was made either by comparing inverted SWSC to observations and/or assessing the crop model performances on subsequent years. Soil heterogeneities are well captured by the method, but some heterogeneities interpreted as soil heterogeneities might be artefacts. A multi-year analysis is then necessary to get the permanent features that are most likely linked to soil properties. Discussion on the adding value of combining both soil vegetation dynamic (FAPAR, LAI) and yield, on the inversion strategy (calibration steps, data being considering, initialisation) and on the cost function (to reduce the impact of uncertainties on crop parameters) was made. The study has shown that LAI/FAPAR and yield observations make the use of complex model in data parsimonious context possible. In particular, the study highlights the importance of having frequent image acquisition, as it allows to capture short feature as the senescence rate which appears as an important proxy of the availability of water in the soil.</p><p>Lammoglia, A. Chanzy & M. Guerif, “Characterizing soil hydraulic properties from Sentinel 2 and STICS crop model” doi:10.1109/MetroAgriFor.2019.8909266, pp 312-316</p>

Characterization of soil permeability in the former Lake Texcoco, Mexico

The geotechnical subsoil conditions of the former Lake Texcoco represent a complex sequence of highly compressible lacustrine clays interbedded with layers and seams of harder and more permeable materials. Although the mechanical properties of these deposits have been extensively studied in the past, the information about their hydraulic properties is scarce. Currently, a comprehensive characterization of the hydraulic conductivity of this site has become necessary because of the construction of the New Mexico International Airport (NAIM). The present study describes a systematic evaluation of the hydraulic conductivity in the former Lake Texcoco through three different in-situ methods (well permeameter, LEFRANC and piezocone dissipation test). The measurements, taken from 155 locations, show a high spatial variability, with ranges spanning more than two orders of magnitude. The results also reveal that the estimated permeabilities vary significantly among methods. These discrepancies reflect the scale dependency of the hydraulic conductivity in the area caused by soil heterogeneities. A comparison of the presented results with previous studies demonstrates that piezocone tests provide representative results for the clayey formations, while LEFRANC tests better estimate the hydraulic conductivity of the permeable strata. Besides, CPTu tests yield more consistent values of hydraulic conductivity, with smaller dispersion than well permeameter and LEFRANC tests.

Subsurface flow and large-scale lateral saturated soil hydraulic conductivity in a Mediterranean hillslope with contrasting land uses

The lateral saturated hydraulic conductivity,Ks,l, is the soil property that mostly governs subsurface flow in hillslopes. Determinations ofKs,lat the hillslope scale are expected to yield valuable information for interpreting and modeling hydrological processes since soil heterogeneities are functionally averaged in this case. However, these data are rare since the experiments are quite difficult and costly. In this investigation, that was carried out in Sardinia (Italy), large-scale determinations ofKs,lwere done in two adjacent hillslopes covered by a Mediterranean maquis and grass, respectively, with the following objectives: i) to evaluate the effect of land use change onKs,l, and ii) to compare estimates ofKs,lobtained under natural and artificial rainfall conditions. HigherKs,lvalues were obtained under the maquis than in the grassed soil since the soil macropore network was better connected in the maquis soil. The lateral conductivity increased sharply close to the soil surface. The sharp increase ofKs,lstarted at a larger depth for the maquis soil than the grassed one. TheKs,lvalues estimated during artificial rainfall experiments agreed with those obtained during the natural rainfall periods. For the grassed site, it was possible to detect a stabilization ofKs,lin the upper soil layer, suggesting that flow transport capacity of the soil pore system did not increase indefinitely. This study highlighted the importance of the experimental determination ofKs,lat the hillslope scale for subsurface modeling, and also as a benchmark for developing appropriate sampling methodologies based on near-point estimation ofKs,l.