Vadose Zone Soil Research Articles

Migration of depleted uranium from a corroded penetrator in soil vadose zone in Bosnia and Herzegovina

Depleted uranium (DU) from corroded armor penetrators can migrate through the soil vadose zone and cause environmental problems, yet studies on such migration at former theatres of war are scarce. Here, we investigated vertical DU migration in a soil profile due to a penetrator (3–8 cm beneath the soil surface) corroded over 7 years in Bosnia and Herzegovina. The highest concentration of DU was ∼45,300 mg/kg at 6–10 cm, with the concentration decreasing markedly with increasing depth. The majority of the DU accumulated within the top 20 cm and the DU front reached ∼42 cm beneath the penetrator. In addition, particles with varying U concentrations (3–65 wt%) were observed at 0–15 cm, with U primarily co-located with O, Si, Al, maghemite, and hematite. Particularly, metaschoepite was identified at 6–10 cm. Finally, X-ray absorption spectroscopy analysis found U was hexavalent in the soil profile. These findings suggest that the downward migration of DU was likely present as a soluble form adsorbed on clay minerals and Fe oxides. Overall, we show that the rate of DU migration within the vadose zone is comparatively slow, although if the penetrator is left in the soil for decades, it could pose a serious long-term risk. Environmental ImplicationsOver 90 % of the depleted uranium (DU) penetrators fired in previous conflicts missed their armored targets and were left in the soil to corrode. The corroded penetrators can not only contaminate soil but also pose a risk to groundwater. The present study examined the migration of DU in a soil profile that included a DU penetrator that had been corroding for over 7 years. Studying the dynamics of DU migration is essential to develop effective remediation strategies to mitigate long-term environmental risks and safeguard ecosystems and human health from DU contamination.

Fiber-optic seismic sensing of vadose zone soil moisture dynamics

Vadose zone soil moisture is often considered a pivotal intermediary water reservoir between surface and groundwater in semi-arid regions. Understanding its dynamics in response to changes in meteorologic forcing patterns is essential to enhance the climate resiliency of our ecological and agricultural system. However, the inability to observe high-resolution vadose zone soil moisture dynamics over large spatiotemporal scales hinders quantitative characterization. Here, utilizing pre-existing fiber-optic cables as seismic sensors, we demonstrate a fiber-optic seismic sensing principle to robustly capture vadose zone soil moisture dynamics. Our observations in Ridgecrest, California reveal sub-seasonal precipitation replenishments and a prolonged drought in the vadose zone, consistent with a zero-dimensional hydrological model. Our results suggest a significant water loss of 0.25 m/year through evapotranspiration at our field side, validated by nearby eddy-covariance based measurements. Yet, detailed discrepancies between our observations and modeling highlight the necessity for complementary in-situ validations. Given the escalated regional drought risk under climate change, our findings underscore the promise of fiber-optic seismic sensing to facilitate water resource management in semi-arid regions.

Empirical relationship between vadose zone properties and diesel attenuation capacity: A complement for intrinsic vulnerability models

Evaluating and predicting the natural attenuation capacity (AC) of a vadose zone is essential for determining groundwater vulnerability to contamination from upper sources. However, it remains unclear how the physicochemical properties of vadose zone soils affect AC owing to their complexity and spatial heterogeneity. In this study, we developed a regression model for estimating the AC of a vadose zone against diesel using datasets from different soils with a wide range of physicochemical properties. Among the 17 properties, six (i.e., organic matter (OM), total phosphorous (TP), coefficient of uniformity, particle size (D30), van Genuchten’s n, saturation degree (SD)) were selected as primary regressors. The results indicate that biogeochemical factors, including OM and TP, have decisive effects on the AC. Finally, the regression model was expanded to a GIS-based spatial model and applied to Namyangju, Korea using the index-overlay method. The produced AC map showed a nonmonotonic decrease along the depth, and the areas closer to the water bodies generally represented low AC values, most likely due to the lower OM, TP, and higher SD. This study provides an empirical basis for future research initiatives for spatial and temporal AC dynamics, which complements conventional intrinsic groundwater vulnerability models such as DRASTIC.

Newly plant-derived carbon in the deeper vadose zone of a sandy agricultural soil does not stimulate denitrification

Analysis of stable carbon (C) isotopic signatures (δ13C) in various soil C pools provides useful information on soil C sources, transport, and availability. Understanding the extent of deeper soil (below 40 cm) sequestration and transport of plant derived C is of particular interest as this provides a source for microorganisms to drive biological denitrification (as indicated by denitrification enzyme activity, DEA) and hence mitigate the nitrate leaching to groundwater. Meanwhile, studies on deeper soil C sequestration are rare due to methodological constraints. This study was done in deeper vadose zone (0–160 cm) of a sandy agricultural soil in a humid and temperature zone after a C3-C4 vegetation change taking advantage of the marked isotopic differences between C3 and C4 plants. It took place in a site previously grown with C3 crops (beet, barley, grass), but where C4 crops (maize) were grown continuously for the last 20 years. The other site where C3 crops were continuously grown was used for comparison. Specifically, the δ13C signature in top and deeper soil layers was used to distinguish between old C3– and newly C4-plant derived C in four C pools, i.e., bulk soil C, hot- and cold-water extractable C, and respired CO2-C. The δ13C signature between C3 soils and C3-C4 shifted soils was similar for bulk soil C but significantly different for water extractable and respired C pools. Hence, we estimated that the contribution of newly derived C to bulk soil C was negligible, whereas the contributions to the other C pools amounted up to 28.4 % along the soil profile. This emphasizes the importance of simultaneously analysing δ13C signature in various soil C pools to accurately assess C vertical transport and distribution. The concentrations of cold-DOC and values of specific ultraviolet visible absorbance of the wavelengths 254 and 280 nm decreased from 50 to 130 cm soil depths, while they increased below these depths. However, this suggested rise in C chemical quality at the deepest soil depths did not cause an increase in soil respiration activity or DEA, which was attributed to the protective effects of iron and aluminium oxides on C decomposition. Upon the application of labile C and N substrates, the deepest soil layers displayed a significantly increased DEA, suggesting the presence of a relatively abundant population of active denitrifying organisms. Overall, this study documents the presence of plant-derived C in the deeper vadose zone. Meanwhile, this particular C pool might not be an important substrate to drive deep-soil denitrification due to constraints imposed by the protection by metal oxides.

Understanding the Effect of Seasonal Climate Variability on the Salinity in Unsaturated Agricultural Soil

Salinization/desalinization processes in the soil vadose zone are important to define agricultural irrigation and drainage schedules, especially in reclaimed crop areas. Numerical modeling of soil–climate interaction is a very helpful tool to understand soil salinity distribution and solute transport and therefore define efficient desalination solutions. A finite element analysis program Code_Bright was used to perform a coupled thermo-chemo-hydraulic analysis aiming at investigating the effect of climate actions on the distribution of soil salinity in depth, by modeling solute transport in the vadose zone under fresh/saline groundwater supply. The analysis separated first the effect of rain infiltration and evaporation, and then a real climate was considered as the boundary condition. A downward flow pattern induced by rainfall in the unsaturated zone resulted in a nonlinear salt leaching process. Significant differences in salt concentration between the surface and lower layer caused by rainfall resulted in a decrement in the leaching efficiency. Evaporation causes water to move upward and salt transport to the surface, thus enhancing the soil salinity above the evaporation front. The salinity above the groundwater table and below the evaporation front were less affected regardless of the salinity of the supplied groundwater. The model simulated the salt leaching process during the wet seasons and salt accumulation processes during the dry ones. The soil salinity and saturation at the soil surface have significantly responded to seasonal climate variability. A typical seasonal climate variability would result in a low salt leaching efficiency through years in the coastal reclamation area. These results would be helpful for the design of soil salinization management strategies, such as reducing salt accumulation by reducing evaporation or leaching the surface salt in the dry season, and increasing the drainage to promote leaching in the wet season.

Exploring recharge mechanisms of soil water in the thick unsaturated zone using water isotopes in the North China Plain

Exploring the groundwater recharge mechanisms in regions with thick unsaturated zones can enhance our understanding of intricate groundwater processes, especially in intensive agricultural regions such as the North China Plain (NCP). For example, to better answer the question of how deep soil water can be recharged, it is of utmost importance to illustrate the recharge mechanisms of groundwater under different land use types in the NCP. This study collected soil samples from four boreholes up to 18 m deep covering farmland and orchards with different stand ages and measured soil water content (SWC), stable and radioactive isotopes (δ18O, δ2H and 3H) of both soil water and groundwater. Results indicated that the movement of water through the deep vadose zone of soils was primarily piston flow based on a soil tritium profile. The infiltration rate was estimated to be 32.2 mm yr−1 based on the tritium peak method. The variations of soil water δ18O and line control excess (lc-excess) indicated that the orchards had lower evaporation effects and a higher precipitation offset than those in farmland soils. Furthermore, the deep soil water was replenished by intensive precipitation events (from July to September). The great soil water deficit and decreased deep drainage under pear orchards is usually closely related to huge water consumption by evapotranspiration (ET) via root uptake, which explains the increase of ET in pear orchards with increasing stand age. These findings have provided substantial insights into groundwater resource management in similar regions with limited water resources and recharge modeling for regions covered with thick unsaturated zones.

Vadose zone water stable isotope profiles for assessing groundwater recharge: Sensitivity to seasonal soil sampling

Groundwater recharge is widely recognized as being the most important parameter for the sustainable management of water resources. In semiarid environments, groundwater recharge can be quantified using the piston displacement method (PDM). From a single soil sampling campaign, the PDM relies on linking the deeper vadose zone soil pore water stable isotope composition (δ2HH2O and δ18OH2O) to the local meteoric water line (LMWL). However, the isotopic composition of precipitation changes seasonally, influencing the water isotope composition of the vadose zone over time. Thus, it is important to test whether the PDM is sensitive to seasonal soil sampling and whether the assessed recharge rate is independent of the time of sampling. This study investigates the effect of seasonal soil sampling on the distribution of vadose zone stable isotope composition to determine whether the sampling time influences the estimate of recharge rate from PDM. Soil samples were obtained along vertical profiles through the vadose zone in a semiarid region during the spring, summer, and autumn seasons. Specifically, the δ2HH2O and δ18OH2O of the soil pore water were determined along vertical profiles, and the PDM was applied to quantify the annual recharge. The δ2HH2O and δ18OH2O values range from −7.3 to −3.5 ‰ and from −54.4 to +7.41 ‰, respectively, and plot along a continuum with a slope less than the LMWL. Samples from deeper in the vadose zone profile had distinct ranges in isotopic composition between the three soil sampling campaigns, with isotopic composition of spring sampling dominated by lower values and those from autumn with higher values. Despite these differences, the resulting annual recharge rates from the different sampling campaigns are comparable (1.5 to 2 % of annual precipitation). Even though the pore water isotopic composition changed over time, the shift between the deeper vadose zone isotopic compositions and the LMWL remained relatively constant, leading to a similar recharge estimate over time. Therefore, the PDM-based recharge assessment in the tested semiarid environment is independent of the sampling time, which indicates that sampling for assessing groundwater recharge can be undertaken during any season.

Assessment of Metalaxyl migration through vadose zone of alluvial sandy soil using column experiment and HYDRUS numerical modeling

Contemporary research on pesticides/fungicides as potential sources of groundwater contamination, including their migration pathways, especially in the Western Bengal basin (WBB), is scarce. The present research intends to study the vulnerability of groundwater towards pollution from metalaxyl. Metalaxyl is a fungicide added anthropogenically to the sandy soil of WBB for the cultivation of crops like tomatoes, potatoes and mustard. The study explores the mechanics of metalaxyl adsorption in soil and its migration to the associated groundwater system. Chemical analyses show high concentrations of metalaxyl within groundwater (472.9 μg/L, maximum amount) from the study area (Nadia district of WBB). The groundwater ubiquity score of metalaxyl (4.6) depicts that it is very much prone to leach through the sandy soils of WBB to the underlying groundwater system. The results of column leaching experiments and their congruence to the findings of numerical modelling study using HYDRUS software confirm the fact. The adsorptive resilience of the studied soils towards metalaxyl is insignificant (soils of North Chandmari (S1) =0.1087 mg/g, Ghoragacha (S2) =0.21 mg/g, and Khaldarpara (S3) =1.771 mg/g). However, the presence of excess iron concentration may enhance the adsorptive capacity of the soil toward Metalaxyl, thereby limiting its migration toward the zone of saturation.

Influence of chain length on field-measured distributions of PFAS in soil and soil porewater

Soil and porewater concentrations measured for multiple PFAS were compiled from three field studies. The soil:porewater concentration ratios were shown to be functions of molar volume for all three data sets. Remarkable consistency was observed between the three sets of field-based measurements, indicating that PFAS distributions in the three soil systems exhibited similar magnitudes of overall retention. The relative contributions of solid-phase sorption and air-water interfacial adsorption to total retention were examined. The contribution of air-water interfacial adsorption was greater than that of solid-phase sorption for the longer-chain PFAS, whereas it was less than that of solid-phase sorption for the shorter-chain PFAS. These results show that the relative contributions of the two processes can vary as a function of the particular PFAS when the solid-phase sorption functionality deviates from that of air-water interfacial adsorption. This might occur for example when sorption is influenced by addition mechanisms beyond hydrophobic interaction, or when sorption and/or adsorption are nonlinear. Based on the results from all three data sets, soil concentrations are likely to be smaller than porewater concentrations for the shortest-chain PFAS. Conversely, soil concentrations will generally be significantly greater than porewater concentrations for longer-chain PFAS. The results from this study have implications for characterizing and evaluating PFAS distributions in vadose-zone soils.

Measuring Nitrate Leaching in the Vadose Zone of Loess Soils—Comparison of Batch Extraction and Centrifugation

The nitrate concentration in the subsoil moisture of the vadose zone is an important indicator for future groundwater quality, which is classically determined via centrifugation. Batch extraction is an inexpensive and easy alternative method, but whether these methods measure the same soil water, nitrogen species, and nitrate concentrations is unclear, in particular for loess soils. Two experiments were carried out to assess the differences in nitrate and other anion concentrations between centrifugated soil moisture (centrifugated at different speeds and times) and batch extractions (using double-distilled water and 0.01 M CaCl2). Batch extraction resulted in lower nitrate (−20%) and chloride (−15%) concentrations than centrifugation, mainly due to anion exclusion, where soil microporosity controls the contribution of diffusion, denitrification, and leaching processes. Vice versa, batch extraction overestimated the concentration of nutrients that occur as precipitates in or sorb the soil matrix, such as sulphate (+50%) and ammonium (+96%). Batch extractions can only be used as a proxy to determine actual nitrate concentrations of soil water. However, they are useful to monitor changes in nitrate leaching over time in response to (policy) measures taken. They can also be used as “early warning indicator” and to improve the reliability of spatial explicit monitoring networks.

Thermal Conduction Heating for a 125 ft Deep TCE Source—Multiple Lines of Evidence for Verification of Remedial Goals

AbstractThis paper describes thermal conduction heating of a trichloroethylene (TCE) source zone within vadose zone soils consisting of glacial till at depths of 50 to 125 ft below an active manufacturing facility. The performance objectives and metrics used to determine remediation success are presented. Access limitations inside the facility prevented traditional spatial soil confirmation sampling, which made performance assessment a unique challenge. Instead, the project team developed an approach utilizing multiple lines of evidence to document remedy completeness. These lines of evidence included achievement of target temperatures within the treatment volume, demonstration of asymptotic vapor concentrations extracted from the treatment volume, achieving modeled energy input goals, and verifying TCE concentrations in post‐treatment soil samples collected at the few accessible locations within the operational facility. The treatment volume was divided into three discrete treatment areas or heating groups, each constructed and heated independently in order to meet an aggressive project schedule. Additionally, each line of evidence was required to be met in each group. After approximately 283 days of heating, temperatures exceeded 90 °C at 95% of the temperature monitoring points, TCE levels had diminished to asymptotic levels in the recovered vapors, and mass removal rates from the treatment volume declined to minimal levels. Confirmatory soil sample results indicated that average TCE concentrations achieved were more than an order of magnitude below the remedial goal of 1 mg/kg.

Alfalfa in rotation with annual crops reduced nitrate leaching potential.

Rotation of perennial alfalfa (Medicago sativa L.) with annual crops has the potential to reduce nitrate-nitrogen (NO3 -N) in the vadose zone and increase soil organic carbon (SOC) sequestration. The objective of this study was to determine the long-term effects on SOC, NO3 -N, ammonium-N (NH4 -N), and soil water in the 7.2m depth with an alfalfa rotation compared with continuous corn (Zea mays L.). Soils from six pairs of alfalfa rotation versus continuous corn observation points were sampled to 7.2m depth in 0.3m increments. The uppermost 0.3m was divided into 0-0.15 and 0.15-0.30m. For the 0-7.2m depth, the alfalfa rotation compared with continuous corn had 26% less soil water (0.29vs. 0.39g cm-3 ) and 55% less NO3 -N (368vs. 824kg ha-1 ). The cropping system and NO3 -N concentration did not affect NH4 -N in the vadose zone. The alfalfa rotation compared with continuous corn had 47% higher SOC (105.96 Mg ha-1 vs. 72.12 Mg ha-1 ) and 23% higher total soil nitrogen (TSN) (11.99 Mg ha-1 vs. 9.73 Mg ha-1 ) in the 0-1.2m depth. The greater depletion of soil water and NO3 -N with alfalfa rotation was primarily below the rooting zone of corn, suggesting no negative implications for corn following alfalfa but greatly reduced potential of NO3 -N leaching to the aquifer with the alfalfa rotation. Alfalfa rotation compared with continuous corn is a means to greatly reduce the leaching of NO3 -N to the aquifer and improve the surface soil with the potential to increase SOC sequestration.

Vadose Zone Soil Flushing for Chromium Remediation: A Laboratory Investigation to Support Field‐scale Application

AbstractCr(VI) flushing from the vadose zone to the groundwater (with subsequent Cr(VI) removal in groundwater by pump‐and‐treat system) is a promising remedial technique that has recently been used at field scale. This laboratory study was conducted to provide the technical basis to design a field soil flushing strategy. The objectives were to (1) quantify the relationship between sediment Cr(VI) and Cr(III) mass and release rates and subsequent Cr(VI) leaching; (2) investigate different methodologies to maximize Cr(VI) leaching, and (3) investigate methods to minimize leaching of remaining residual Cr. Characterization of Cr‐contaminated sediments (Hanford Site, WA) exhibited Cr(VI) showed that leach rates that were correlated to different Cr surface phases. Sediments with low leachable Cr(VI) (<2 μg/g) leached Cr rapidly, so slow infiltration of water in a single pulse was sufficient to leach most Cr. In contrast, sediments with high Cr (2 to 200 μg/g) released some Cr(VI) quickly but 10 to 50% Cr(VI) slowly (tens to hundreds of hours). Efficient unsaturated leaching of these sediments required a different infiltration strategy that includes: multiple slow leach pulses with time between flushing cycles; the use of a surfactant to increase Cr leaching from low‐permeability zones, and the use of a reductant (Na‐dithionite or Ca‐polysulfide) in the final leach water was highly effective at decreasing residual Cr leaching. This study clearly demonstrated that the methodology of basing laboratory Cr flushing on parameters such as Cr release mass and rates could be used to improve the efficiency of soil flushing at field scale.

Transport and natural attenuation of benzene vapor from a point source in the vadose zone

The vadose zone is a very dynamic and active environment that directly affects natural attenuation and vapor intrusion of volatile organic compounds (VOCs). Therefore, it is important to understand the fate and transport of VOCs in the vadose zone. A column experiment combined with model study was conducted to investigate the influence of soil type, vadose zone thickness, and soil moisture content on benzene vapor transport and natural attenuation in the vadose zone. Vapor-phase biodegradation and volatilization to atmosphere for benzene are two main natural attenuation mechanism in the vadose zone. Our data showed that biodegradation in black soil is the main natural attenuation mechanism (82.8%) while volatilization is the main natural attenuation mechanism in quartz sand, floodplain soil, lateritic red earth and yellow earth (>71.9%). The R-UNSAT model-predicted soil gas concentration profile and flux were close with four soil column data except for yellow earth. Increasing the vadose zone thickness and soil moisture content significantly reduced the volatilization contribution while increased biodegradation contribution. The volatilization loss decreased from 89.3% to 45.8% when the vadose zone thickness increased from 30 cm to 150 cm. The volatilization loss decreased from 71.9% to 10.1% when the soil moisture content increased from 6.4% to 25.4%. Overall, this study provided valuable insights into clarifying the roles of soil type, moisture, and other environmental conditions in vadose zone natural attenuation mechanism and vapor concentration.

Effects of Underground Mining on Soil–Vegetation System: A Case Study of Different Subsidence Areas

The influence of coal mining subsidence on the surface ecological environment can be characterized as the evolution of a complex system. Examining the ecological damage caused by coal mining subsidence from the perspective of internal soil–vegetation system relationships is important for the ecological protection and restoration of mining areas. We investigated vegetation coverage, surface soil water and nutrient content, and vadose zone soil moisture in uniform and nonuniform areas of coal mining subsidence. Subsidence reduced vegetation coverage, surface soil moisture and nutrient content, and vadose zone soil moisture while increasing their spatial variability. These effects are more pronounced in areas with nonuniform subsidence. Subsidence also reduced the degree of soil–vegetation system correlation, which was also more pronounced in areas of nonuniform subsidence. Furthermore, a higher degree of soil–vegetation correlation was linked to decreased variability in soil moisture and nutrient content. Areas of nonuniform subsidence were characterized by greater preferential flow during rain infiltration, which contributed to the spatial variability of soil moisture and nutrient content and damage to vegetation growth. Our findings revealed that coal mining subsidence reduces both the quality of environmental factors and the degree of internal correlation between these factors, of which the preferential flow effect is an important underlying mechanism. These findings provide a theoretical basis for ecosystem management and the restoration of land damaged by coal mining subsidence.

Improved drought monitoring in teleconnection to the climatic escalations: A hydrological modeling based approach

Catchment-scale efficient agricultural drought monitoring and irrigation planning mostly depend on the accuracy of evapotranspiration (ET) estimates under different crop-growth phases. As indirect ET estimation under limited data availability scenario and complex paddy-field dynamics could not be sufficiently addressed by the conventional curve number (CN) based SWAT model, SWAT incorporates an add-in pothole module (SWAT-P) that conceptualizes the paddy-field hydrology with the empirical coefficient or equations resulting in poor ET estimates. To address these limitations, this study integrates an improved pothole methodology in SWAT (SWAT-EP) accounting the vadose-zone soil water dynamics under alternate ponding and drying conditions, ET variation under moisture abundant and stress conditions, and role of irrigation return flow generated in the paddy fields. The proposed approach was validated in the Kangsabati River integrated reservoir-catchment-command (12,014.70 km2) of the eastern India, and the results reveal that SWAT-EP outperformed the existing SWAT-P in reproducing the catchment-scale streamflow and ET flux with respect to the FAO-56 Penman-Monteith (PM) based ET estimates in all the four cropping seasons. The SWAT-EP derived Standardized Evapotranspiration Drought Index (SEDI) based drought severity and duration well-corroborate with the benchmark MOD16A2 derived drought estimates; whereas, the SWAT-P tends to overestimate the drought severity in the command area. The predictive uncertainty in drought monitoring was the lowest by SWAT-EP with relatively lower uncertainty was observed in the crop-growing locations of Kharagpur and Mohanpur. Moreover, the teleconnection between drought and climatic escalations corresponds to a better reproducibility of El Niño and La Niña phases by the SWAT-EP, while the SWAT-P performed un-satisfactorily across different spatiotemporal domains. This study endorses to adopt the proposed SWAT-EP model for river basin-scale drought monitoring and irrigation planning with prior validation in the diversified climatic and topographic conditions.

Simulated leaching of PFAS from land-applied municipal biosolids at agricultural sites

Biosolids are an important resource for agricultural practice but have recently received increased focus as a potential source of per- and polyfluoroalkyl substances (PFAS) in the environment. Few studies have investigated the transport of PFAS through the unsaturated zone under conditions relevant to biosolids application sites. Herein, the unsaturated flow and transport model HYDRUS is used to evaluate the leaching of per- and polyfluoroalkyl substances (PFAS) from land-applied biosolids used in agricultural practice to determine the impacts of PFAS leaching on underlying groundwater resources. This numerical case study was based on conditions and operations at two test sites in central Illinois where biosolids were applied at agronomic rates and where PFAS contents and desorption characteristics were previously characterized. Each site possessed different vadose zone soil textural heterogeneity. Simulations were performed under actual present-day meteorological conditions and extended 150 years beyond the initial biosolids application. These long-term simulations demonstrate how soil equilibrium sorption/desorption processes within the biosolids-amended surface soils effectively control the transport rate of individual PFAS to groundwater. Air-water interfacial (AWI) adsorption, which is sometimes considered to be a significant source of PFAS retention in vadose zone soils, was observed to have minimal impacts on PFAS leaching rates within the biosolids-amended surface soils at these sites. Additionally, the impact of AWI adsorption was found to be most significant for PFAS transport within the underlying vadose zone soils when these soils were more texturally homogeneous and considerably less significant within the texturally heterogeneous soils represented herein. The results of multiple long-term simulations were used to develop an empirical equation that relates predicted maximum PFAS pore-water concentrations reaching the saturated zone with changes in PFAS concentrations in the biosolids-amended soil for various biosolids re-application events. This approach is shown to be very useful in developing site-specific PFAS soil screening levels and/or maximum leachate levels for PFAS in support of establishing best management practices (BMPs) for land application of biosolids.

Characterization of relevant site-specific PFAS fate and transport processes at multiple AFFF sites

The relevance of multiple poly- and perfluoroalkyl substance (PFAS) fate and transport process across multiple sites was established through high-resolution characterization of the spatial distribution of PFAS within aqueous film forming foam (AFFF)-related source areas and the downgradient plumes. The maximum total PFAS concentrations in source area groundwater at the three study sites ranged from 6 to 51 mg/L but consistently decreased by several orders of magnitude with distance from the source area at all sites, indicating that non-destructive attenuation of PFAS occurred along each flow path. The relative distribution of different PFAS classes, including zwitterionic/cationic species, provided site-specific lines of evidence for retardation due to hydrophobic, air-water interfacial, and electrostatic partitioning processes, as well as impacts from biotransformation and matrix diffusion at multiple sites. The only site where one of these processes (air-water interfacial partitioning) was not supported by the data (Site 1) was attributable to disturbance of vadose zone soils as part of historic remedial efforts. In other cases, the magnitude that these processes influenced PFAS transport reflected site-specific conditions. This included apparent salting out of PFAS at Site 2 due to its elevated groundwater salinity, which has implications for plume migration in coastal areas. In addition, PFAS was present in lower-permeability soils at each site, suggesting that longer-term retention of PFAS is occurring in these zones. The finding that multiple processes were active at site-wide scales is consistent with expectations that these are naturally occurring reactions that should be relevant at most AFFF-impacted source zones.

Effects of Dissolved Organic Matter on the Adsorption of Norfloxacin by Sandy Soil in Vadose Zone: A Preliminary Field Study in Yellow River of Northern China

Effects of Dissolved Organic Matter on the Adsorption of Norfloxacin by Sandy Soil in Vadose Zone: A Preliminary Field Study in Yellow River of Northern China

Numerical Simulation Analysis of Nitrate Nitrogen Vertical Transport in Vadose Zone of Alluvial Soil Under Sewage Reusing Based on Hydrus-1d

Numerical Simulation Analysis of Nitrate Nitrogen Vertical Transport in Vadose Zone of Alluvial Soil Under Sewage Reusing Based on Hydrus-1d