Contaminant Plume Research Articles

Computational Fluid Dynamics Simulations to Assess Spatial Variability and Optimal Ventilation Scenarios for Biological Laboratory Exposures

Introduction: A significant amount of uncertainty exists regarding potential human exposure to laboratory biomaterials and organisms in Biosafety Level 2 (BSL-2) research laboratories. Computational fluid dynamics (CFD) modeling is proposed as a way to better understand potential impacts of different combinations of biomaterials, laboratory manipulations, and exposure routes on risks to laboratory workers. Methods: In this study, we use CFD models to simulate airborne concentrations of contaminants in an actual BSL-2 laboratory under different configurations. Results: Results show that ventilation configuration, sampling location, and contaminant source location can significantly impact airborne concentrations and exposures. Depending on the source location and airflow patterns, the transient and time-integrated concentrations varied by several orders of magnitude. Contaminant plumes from sources located near a return vent (or exhaust like a fume hood or ventilated biosafety cabinet) are likely to be more contained than sources that are further from the exhaust. Having a direct flow between the source and the exhaust (through-flow condition) may reduce potential exposures to individuals outside the air flow path. Conclusion: Designing a BSL-2 room with ventilation and airflow patterns that maximize through-flow conditions to the return/exhaust vents and minimize dispersion and mixing throughout the room is, therefore, recommended. CFD simulations can also be used to assist in characterizing the impacts of supply and return vent locations, room layout, and source locations on spatial and temporal contaminant concentrations. In addition, proper placement of particle sensors can also be informed by CFD simulations to provide additional characterization and monitoring of potential exposures in BSL-2 facilities.

Simulation-Optimization Approach for Siting Injection Wells in Urban Area with Complex Hydrogeology.

Managed aquifer recharge has become a standard water resources management practice to promote the development of locally sustainable water supplies and combat water scarcity. However, installation of injection wells for replenishment purposes in urban areas with complex hydrogeology faces many challenges, such as limited land availability, potential impacts on municipal production wells and known subsurface contamination plumes, and complex spatially variable hydraulic connections between aquifer units. To assess the feasibility and cost-effectiveness of injecting advanced treated water (ATW) into a complex urban aquifer system, a Simulation-Optimization (SO) model was developed to automate a systematic search for the most cost-effective locations to install new wells for injecting various quantities of ATW, if feasible. The generalized workflow presented here uses an existing MODFLOW groundwater model-along with advanced optimization routines that are publicly available-to flexibly accommodate a multiobjective function, complex constraints, and specific project requirements. The model successfully placed wells for injection of 1 to 4 MGD of ATW in aquifers underlying the study area. The injection well placement was primarily constrained by avoiding excessive impact on environmental sites with underlying groundwater plumes. The largest costs were for well installation and piping to the wells from the existing ATW pipes. This workflow is readily adaptable to other sites with different complexities, decision variables, or constraints.

Uptake of Per- and Polyfluoroalkyl Substances by Fish, Mussel, and Passive Samplers in Mobile-Laboratory Exposures Using Groundwater from a Contamination Plume at a Historical Fire Training Area, Cape Cod, Massachusetts.

Aqueous film-forming foams historically were used during fire training activities on Joint Base Cape Cod, Massachusetts, and created an extensive per- and polyfluoroalkyl substances (PFAS) groundwater contamination plume. The potential for PFAS bioconcentration from exposure to the contaminated groundwater, which discharges to surface water bodies, was assessed with mobile-laboratory experiments using groundwater from the contamination plume and a nearby reference location. The on-site continuous-flow 21-day exposures used male and female fathead minnows, freshwater mussels, polar organic chemical integrative samplers (POCIS), and polyethylene tube samplers (PETS) to evaluate biotic and abiotic uptake. The composition of the PFAS-contaminated groundwater was complex and 9 PFAS were detected in the reference groundwater and 17 PFAS were detected in the contaminated groundwater. The summed PFAS concentrations ranged from 120 to 140 ng L-1 in reference groundwater and 6100 to 15,000 ng L-1 in contaminated groundwater. Biotic concentration factors (CFb) for individual PFAS were species, sex, source, and compound-specific and ranged from 2.9 to 1000 L kg-1 in whole-body male fish exposed to contaminated groundwater for 21 days. The fish and mussel CFb generally increased with increasing fluorocarbon chain length and were greater for sulfonates than for carboxylates. The exception was perfluorohexane sulfonate, which deviated from the linear trend and had a 10-fold difference in CFb between sites, possibly because of biotransformation of precursors such as perfluorohexane sulfonamide. Uptake for most PFAS in male fish was linear over time, whereas female fish had bilinear uptake indicated by an initial increase in tissue concentrations followed by a decrease. Uptake of PFAS was less for mussels (maximum CFb = 200) than for fish, and mussel uptake of most PFAS also was bilinear. Although abiotic concentration factors were greater than CFb, and values for POCIS were greater than for PETS, passive samplers were useful for assessing PFAS that potentially bioconcentrate in fish but are present at concentrations below method quantitation limits in water. Passive samplers also accumulate short-chain PFAS that are not bioconcentrated.

Climate‐influenced hydrobiogeochemistry and groundwater remedy design: A review

AbstractThe process of designing a remedy for contaminated groundwater historically has not commonly included climate‐future, hydrologic, and biogeochemical aquifer characteristics. From experience, the remedy design process also has not consistently nor directly integrated or projected future hydrologic and biogeochemical effects of the human‐induced or developed environment—aka the anthropogenic influence—on potential remedy performance. The apparent practice of (1) not regularly assessing anthro‐influenced hydrological (termed here as anthrohydrology) or biogeochemical characteristics (collectively hydrobiogeochemistry) of a site and (2) rarely accounting for future climatic shifts as design factors in remedy design may be due, in part, to the general practice‐level view that groundwater remediation systems (whether in situ or ex situ) have seldom been anticipated to last more than a few years (or one or two decades at the most). Second, methods to reliably and quantitatively estimate site‐specific, climate‐future shifts in groundwater conditions using global and/or regional climate models and the resultant impacts on contaminant plume characteristics have not been readily available. The authors here suggest that while the concept of remedy design resilience and durability, within an envelope of climate change and anthropogenic influence, has been discussed in some technical circles as a component of “sustainable remediation,” we have found that direct application of these technical concepts in quantifiable terms remains rare. By incorporating the potential influence of future hydrobiogeochemical scenarios into remedy design, however, the design process could account for reasonable climate‐induced influence on the groundwater system for a given site. These scenarios could then be applied within the remedy selection process to assess performance durability under potentially changing hydrologic, biological, and chemical conditions.

Interactions of waste disposal site leachate with the Merida karst aquifer, Mexico

Hydrogeochemical tools were applied to evaluate the effect caused by the presence of municipal waste leachate on the balance of a karstic aquifer. The system evaluated correspond to the aquifer underlying the area of the old municipal waste disposal site (MWDS) in the city of Merida, Mexico. A contaminant plume identified with chloride ion concentrations was detected. Groundwater samples were collected from boreholes located along a flow line, starting at the downstream boundary of the dump site. The parameters involved in the carbonate system were analyzed, obtaining a decline in calcium (Ca2+) and magnesium (Mg2+) concentrations for high pCO2 values. This behavior is explained by the presence of high bicarbonate (HCO3–) content, as a result of the waste decomposition reactions. The bicarbonate ions behaved as a buffer preventing pH reduction. In addition to this, the calcium and magnesium species indicate that the formation of complexes affected the mineral solubility where leachate has the greatest influence. The transport of minerals, in moles, between boreholes located along the groundwater flow line, was determined through inverse modeling. It is concluded that the leachate affects the carbonate balance, as determined by the transfer of minerals along the flow line; it was observed that mixed waters with a higher percentage of leachate tend to dissolve carbonate minerals. This is the analogous to findings reported in other leachate contaminated aquifers.

Monitoring of in-situ chemical oxidation for remediation of diesel-contaminated soil with electrical resistivity tomography

In-situ chemical oxidation (ISCO) with persulfate, an electrically conductive oxidant, provides a powerful signal for noninvasive geophysical techniques to characterize the remediation process of hydrocarbon contaminants. In this study, remediation with ISCO is conducted in laboratory sandboxes to evaluate the ability of electrical resistivity tomography (ERT) for monitoring the base-activated persulfate remediation process of diesel-contaminated soil. It was found that the resistivity of contaminated sand significantly decreased from 846 Ω·m to below 10 Ω·m after persulfate injection, and all measured chemical parameters showed a noticeable increase. Natural degradation and contamination plume migration were not evident in a reference sandbox without treatment. The area with a resistivity ratio < 0.95 based on imaging before and after injection indicated downward migration of the oxidation plume due to density-driven flow. A comparison between remediation and reference sandboxes showed that the observed resistivity decrease can be due to both contaminant degradation as well as the oxidation plume itself in the contaminated source zone. In contrast, the resistivity decrease in the area with low contamination concentration is attributed to the oxidation plume alone. The derived relationships between resistivity and contaminant indicators further emphasize that the contribution of contaminant consumption to resistivity change in the source area is 25.6%, while it is <16% in the low or non-contaminated area. Although this study showed that resistivity is not solely affected by the chemical transformation of diesel components, it can be combined with sampling data to allow an assessment of the effectiveness of ISCO treatment and to identify target areas for subsequent treatment.

Modelling Plume Development with Annual Pulses of Contaminants Released from an Airport Runway to a Layered Aquifer, Evaluation of an In Situ Monitoring System

In cold climates, the use of de-icing chemicals in the winter can lead to groundwater contamination, especially when used in large quantities, such as at airports. Oslo Airport, Gardermoen, is situated on Norway’s largest rain-fed aquifer. Potassium formate is used to remove ice from runways and propylene glycol from airplanes; the organic parts are degradable. Most of the wells to monitor the spread of de-icing chemicals in the underlying aquifer have well screens near the groundwater level, while the runways and the source of de-icing chemicals are near the groundwater divides, where vertical flow is expected. The objective of this study is to demonstrate the importance of layers and time-varying recharge on the spreading of contaminant plumes in an aquifer near a groundwater divide. This is done with numerical modelling. The model results show increased vertical transport of the added tracer in the presence of horizontal layers, both continuous and discontinuous, in the aquifer. With certain distributions of hydraulic conductivity, Ks, we demonstrate that deeper monitoring wells are required. With the scenarios modelled here, time-varying recharge has a weaker effect on plume distribution. Measured concentrations of potassium and total organic carbon show the cyclic effect of seasonally varying recharge of contaminants, and an asymptotic accumulation of concentration over time, that is consistent with the model runs. In conclusion, groundwater monitoring systems near a groundwater divide should include multi-level samplers to ensure control of the vertical plume movement.

English

The Osogbo central solid waste dumpsite in Osun State, Nigeria falls within the Ilesha Schist Belt, southwestern basement complex of Nigeria. Thirty-three (33) Vertical Electric Sounding (VES), and 2-D Electric Resistivity Tomography (ERT) data were acquired across various locations in the study area using ABEM Terrameter SAS 300, respectively. Schlumberger configuration with a maximum spread length of 100 m, for the VES was used, while four traverses of ERT were conducted, using Dipole-Dipole Array with average traverse length of 112.50 m. A smoothness-constrained least-squares inversion was then applied to the ERT data using a 2-D inversion algorithm [that is, DIPRO for Windows, version 4.01. Generally, the results defined the contaminated leachate plumes as electrically conductive, with anomalous resistivity values. Both the VES and the 2-D tomography identified zones of low resistivity values, between 24 and 67 ?m, from 10 to 20 m deep, as leachate contaminant plumes. Some of the weathered and fractured basement rocks indicated that the subsurface soil and groundwater have been contaminated by the leachates, which appears to have migrated outside landfill site. Zones within the sub-surface, with resistivity values relatively high (&gt; 300 &Omega;m) and depth above 30 m might be pollutant free. Solid wastes management calls for the adoption of the best practicable environmental management approach through the governments&rsquo; enforcement of circular economic policies, using extended producer responsibility (EPR) model and construction of engineered landfills to replace unsanitary landfills. The conduct of environmental impact assessment (EIA), prior to commencement of operations and phytoremediation of closed sites is also crucial. Key words: Tomography, leachate, pollution, contaminant plume.

Solution of the backward problem for the space-time fractional diffusion equation related to the release history of a groundwater contaminant

Abstract Finding the history of a groundwater contaminant plume from final measurements is an ill-posed problem and, consequently, its solution is extremely sensitive to errors in the input data. In this paper, we study this problem mathematically. So, firstly, existence and uniqueness theorems of a quasi-solution in an appropriate class of admissible initial data are given. Secondly, in order to overcome the ill-posedness of the problem and also approximate the quasi-solution, two approaches (computational and iterative algorithms) are provided. In the computational algorithm, the finite element method and TSVD regularization are applied. This method is tested by two numerical examples. The results reveal the efficiency and applicability of the proposed method. Also, in order to construct the iterative methods, an explicit formula for the gradient of the cost functional J is given. This result helps us to construct two iterative methods, i.e., the conjugate gradient algorithm and Landweber iteration algorithm. We prove the Lipschitz continuity of the gradient of the cost functional, monotonicity and convergence of the iterative methods. At the end of the paper, a numerical example is given to show the validation of the iterative algorithms.

Contaminant Back Diffusion from Low-Conductivity Matrices: Case Studies of Remedial Strategies.

Recalcitrant groundwater contamination is a common problem at hazardous waste sites worldwide. Groundwater contamination persists despite decades of remediation efforts at many sites because contaminants sorbed or dissolved within low-conductivity zones can back diffuse into high-conductivity zones, and therefore act as a continuing source of contamination to flowing groundwater. A review of the available literature on remediation of plume persistence due to back diffusion was conducted, and four sites were selected as case studies. Remediation at the sites included pump and treat, enhanced bioremediation, and thermal treatment. Our review highlights that a relatively small number of sites have been studied in sufficient detail to fully evaluate remediation of back diffusion; however, three general conclusions can be made based on the review. First, it is difficult to assess the significance of back diffusion without sufficient data to distinguish between multiple factors contributing to contaminant rebound and plume persistence. Second, high-resolution vertical samples are decidedly valuable for back diffusion assessment but are generally lacking in post-treatment assessments. Third, complete contaminant mass removal from back diffusion sources may not always be possible. Partial contaminant mass removal may nonetheless have potential benefits, similar to partial mass removal from primary DNAPL source zones.

Experiments and Simulations on Plume Spreading by Engineered Injection and Extraction in Refractive Index Matched Porous Media

AbstractEngineered injection and extraction (EIE) is an in situ groundwater remediation strategy that imposes an engineered spatially and temporally varying velocity field to spread an amendment into the contaminant plume to promote mixing and contaminant degradation through reaction. Here we present a coordinated suite of laboratory experiments and numerical simulations that apply EIE using two pumping sequences: A folding sequence that manipulates plume geometry by stretching and folding, and an oscillating sequence that imposes velocity perpendicular to the plume interface in accordance with previously‐reported active spreading theory. Laboratory experiments investigated spreading and mixing of a non‐reactive tracer using refractive index matched porous media and laser‐induced fluorescence, with remarkable reproducibility between replicates. Numerical simulations matched the experimental results and simulated reactive transport by post‐processing sequentially injected non‐reactive tracer plumes representing the contaminant and the amendment assuming rapid reaction (i.e., high Damköhler number). This study provides the first experimental verification of EIE using Darcy‐scale imaging, demonstrates a novel experimental method to mimic reactive transport at the high‐Damköhler number limit, and confirms active spreading theory.

Hydrodynamic Decontamination of Groundwater and Soils Using ZVI

Polluted aquifers can be decontaminated using either ZVI (zero valent iron) permeable reactive barriers (PRB) or injected ZVI. The placement of ZVI within the aquifer may take several decades to remediate the contaminant plume. Remediation is further complicated by ZVI acting as an adsorbent to remove some pollutants, while for other pollutants, it acts as a remediation catalyst. This study investigates an alternative aquifer decontamination approach to PRB construction or n-Fe0 injection. The alternative approach reconstructs the potentiometric surface of the aquifer containing the contaminant. This reconstruction confines the contaminant plume to a stationary, doughnut shaped hydrodynamic mound. Contaminated water from the mound is abstracted, decontaminated, and then reinjected, until all the water confined within the mound is decontaminated. At this point, the decontaminated mound is allowed to dissipate into the surrounding aquifer. This approach is evaluated for potential use in treating the following: (i) immiscible liquid plumes; (ii) miscible contaminant and ionic solute plumes; (iii) naturally contaminated aquifers and soils; and (iv) contaminated or salinized soils. The results indicate that this approach, when compared with the PRB or injection approach, may accelerate the decontamination, while reducing the overall amount of ZVI required.

Predicting future well performance for environmental remediation design using deep learning

In this study, we developed a deep learning (DL) framework with a multi-channel three-dimensional convolutional neural network (MC3D-CNN) to predict well performance and thereby assist future environmental remediation design. Such prediction of extraction well performance at designated locations is critical for configuring pump-and-treat (P&T) well network design and operation, setting reasonable target closure dates for overall remedying, and estimating remedy costs. The framework is developed with operational and monitoring data routinely collected during P&T remedy operations, including well extraction and injection rates as well as in situ contaminant concentrations. Traditionally, the collected data were rarely used for purposes other than assessing past well performance and the accuracy of the conceptual site model. However, recent advances in data-driven computational approaches enable better use of the large datasets to inform future well performance, enhance site characterization, and improve remediation planning. In this study, we established a DL framework to integrate transient three-dimensional contaminant plumes and multiple aquifer properties (e.g., hydraulic conductivity and hydrostratigraphic maps) to identify characteristic patterns controlling and representing extraction well mass recovery, aiming at providing future mass recovery estimates for existing wells and candidate wells at any proposed locations. We evaluated our framework by using a realistic synthetic dataset generated from a well-calibrated flow and transport model used in the 200 West Area of the U.S. Department of Energy’s Hanford Site in southeastern Washington state. The multi-channel feature in our framework allows integration of various types and temporal densities of training datasets for DL model development. Overall, we found that the trained DL model achieved an accuracy of over 90% in ranking extraction well performance in validation datasets, and over 80% in predicting high-performance-ranking well locations. This data-informed approach provides a flexible tool to support adaptive site management, streamline decision-making, and potentially reduce remediation time and costs. Our DL framework can be used as a filtering tool to improve the current P&T network optimization design by reducing the number of candidate well locations.

Segmented trenches for recovering contaminant plumes from lined waste impoundments

This study examined the capability of segmented trenches with various placements to remediate groundwater contaminated by small holes in lined waste impoundments such as landfills. A groundwater flow and mass transport model simulated a small contaminant source above an unconfined alluvial aquifer. After the leachate plume evolved for 1000 days, the source was shut off and clean-up trials started. Decoupled trench configurations, with one segment at the downgradient edge of a plume, and the other at high concentrations within the plume, expedited clean-up while limiting downgradient travel. At some sites, relatively efficient schemes documented here may be effective clean-up alternatives for contemporary landfills.

Experimental investigations on spreading and displacement of fluid plumes around an injection well in a contaminated aquifer

ｄe concentration of the injected fluid decays smoothly around the location of the corresponding sharp interface, which is beyond the theoretical range. This concentration spread should be considered in redefining outer mixing boundaries around injection well between the injected and ambient contaminated solutions. In this study, a radial transport experiment was performed (naphthol green B was first injected as a contaminant plume followed by potassium permanganate as the remediation agent) to mimic the interface mixing in more realistic conditions. The radial advection–dispersion model (e.g., dominated by interfacial driving, R (C/C0 = 0.5), combined with dispersion mixing) was used to obtain the position of the “boundary”, rf, at the mixing fringe of the injected fluid. The results show that the injected oxidant front ran after the contaminated plume at a migration velocity of more than 1.29 times, increasing the mechanical dispersion difference between potassium permanganate and naphthol green B. The increase in the injection rate and ambient groundwater flow velocity improved the mixing of oxidant and contaminated plume (the area difference between the contaminated plume and the oxidant decreased) but also accelerated the contaminated plume coverage. Changing the injection mode (such as linear injection) was an effective method, which enhanced the mixing of the oxidant-contamination plume and weakly expanded the contaminated plume (The swept area, 254 cm2, was the smallest among the three injection modes). The relationship between injection and mixing needs to be further strengthened.

Evaluation of the contamination of the subsurface and groundwater by monoaromatic hydrocarbons in an eastern Amazonian town in northern Brazil

The present study evaluated the contamination by monoaromatic hydrocarbons (BTEX: benzene, toluene, ethylbenzene, and xylene) of the subsurface of gas stations and adjacent residences in the Amazonian town of Bragança, in northern Brazil. We combined the ground penetrating radar (GPR) geophysical tool, to detect possible plumes of contamination by BTEX, with gas chromatography (GC) and mass spectrometry (MS), which were used to diagnose the presence of BTEX in water samples obtained from boreholes located within the study area. The GPR identified low-amplitude reflections in the vadose zone of two gas stations, whose attenuation near the surface was due to the low permittivity recorded in the radargrams, which may have been caused by the reduced permittivity of the hydrocarbons (gas vapor) in comparison with water. The GC–MS identified benzene, toluene, ethylbenzene and/or xylene in 13 water samples (19.7% of the samples collected). Only three of the samples contained two contaminants, invariably toluene and xylene, whereas all the others contained only a single BTEX. Toluene and xylene were the BTEX compounds found most frequently in the samples. However, none of the samples collected in the study exceeded the threshold established by Brazilian legislation for BTEXs in drinking water. The integrated application of GPR and CG-MS produced satisfactory results for the diagnosis of the contamination of the subsurface of the gas stations surveyed in the present study.

Magnetic signatures of a creosote oil contaminated site: case study in São Paulo, Brazil

Soils and groundwater contamination modifies the physical–chemical conditions of the environment, altering natural biogeochemical processes of the ground. As a result, several mineral transformations occur, in which iron plays a decisive role. The presence of iron enables the study of magnetic properties, improving the understanding of the geophysical signatures of highly dynamic environments (e.g., biogeochemical hotspots and contamination plumes). In this work, we seek to identify creosote biodegradation related to the precipitation of magnetic minerals on sediments at a contaminated site in São Paulo, Brazil. Several rock magnetism analyses were carried out to provide the magnetic mineralogy of the samples in terms of their composition, size, and abundance. We conducted high-temperature thermomagnetic curves, frequency-dependent magnetic susceptibility, anesthetic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) data, superparamagnetic concentration and dipole moment (SPCDM), and scanning electron microscopy (SEM) analyses. The magnetic signatures of the contaminated samples suggest an increase of superparamagnetic grains in the water table fluctuation zone if compared to the magnetic signatures of the uncontaminated samples. Thermomagnetic curves of contaminated samples showed a lower heterogeneity of the magnetic mineral phases than the uncontaminated ones. This work contributes to the advancement of the understanding of how natural biogeochemical processes are impacted by human actions, such as soil contamination, and even by climate change, which should affect soil redox conditions in periods of drought and flooding.

Integration of hydrochemical and induced polarization analysis for leachate localization in a municipal landfill

Landfills have been identified as a significant concern to the surrounding surface and groundwater ecosystem because of the discharge of leachate. To tackle the uncertain localization of the contamination plume due to low sampling densities, a combination of hydrochemical analysis and induced polarization survey (IP) is employed to characterize the leachate in a municipal landfill. The polarization effect in the contaminated area is significantly higher than expected for landfill sites, but relatively low chargeability zones (<100 mV/V) indicating the distribution of leachate are observed inside high conductivity (>600 mS/m) areas. With reliable geophysical results confirmed by similar formation factors from both field and laboratory data, the abnormal high polarization effect is influenced by installed steel sheet piles next to the survey cable. In addition, we successfully identify linear relationship between the geophysical responses and dominant inorganic conservative compounds (Cl- and Na+) from the leachate plume. The gentle variations of borehole chemical parameters show that the plume is not affected by a continuous contamination source any more, indicating that the steel sheet pile effectively cut off the contamination from the leachate tanks. In conclusion, the integration of IP and hydrochemical data is an excellent way to locate contaminated zones and monitor the behaviors of leachate plume in the landfill.

A case study on the migration of nuclides in the coupled vadose zone-groundwater system at a proposed spent fuel reprocessing site in Gansu, China

Nuclear energy represents a relatively novel clean energy source, and potential nuclear contamination must be considered while developing the nuclear industry. In the context of a hypothetical nuclear accident, a nuclide migration model was established based on regional geological and hydrological data to investigate the migration characteristics of the radionuclides 3H, 99Tc(VII), 137Cs, and 79Se(IV) in a proposed spent fuel reprocessing site (SFRS) in Gansu, China. The temporal and spatial evolution of these four nuclides as well as their effects on the environment and local population were discussed in detail. The model of nuclide migration in the vadose zone was coupled to a large-scale regional nuclide migration model of groundwater in the form of boundary conditions. The results under the least favorable conditions showed that nuclides with high adsorption rates (137Cs and 79Se(IV)) remained in the vadose zone and decayed out completely. In contrast, nuclides with low adsorption rates (3H and 99Tc(VII)) penetrated the vadose zone and polluted the groundwater. The vadose zone delayed the arrival of 3H and 99Tc(VII) to the phreatic surface and reduced their concentrations. It was predicted that the concentration of 3H in the groundwater would fall below the threshold after 40 years, and the distance from the leakage area to the leading edge of the contamination plume would be 374 m. 99Tc(VII), which has a slightly higher adsorption rate and a lower decay rate than 3H, spread 833 m after 3000 years when its concentration was below the threshold. 3H and 99Tc(VII) did not migrate to downstream farms to threaten the safety of residential drinking water. This research presents a safety assessment of a nuclear area in Gansu, China, and provides a significant reference for the future construction of nuclear facilities and nuclear contamination control.

Waterborne virus transport and the associated risks in a large lake

Waterborne enteric viruses in lakes, especially at recreational water sites, may have a negative impact on human health. However, their fate and transport in lakes are poorly understood. In this study, we propose a coupled water quality and quantitative microbial risk assessment (QMRA) model to study the transport, fate and infection risk of four common waterborne viruses (adenovirus, enterovirus, norovirus and rotavirus), using Lake Geneva as a study site. The measured virus load in raw sewage entering the lake was used as the source term in the water quality simulations for a hypothetical scenario of discharging raw wastewater at the lake surface. After discharge into the lake, virus inactivation was modeled as a function of water temperature and solar irradiance that varied both spatially and temporally during transport throughout the lake. Finally, the probability of infection, while swimming at a popular beach, was quantified and compared among the four viruses. Norovirus was found to be the most abundant virus that causes an infection probability that is at least 10 times greater than the other viruses studied. Furthermore, environmental inactivation was found to be an essential determinant in the infection risks posed by viruses to recreational water users. We determined that infection risks by enterovirus and rotavirus could be up to 1000 times lower when virus inactivation by environmental stressors was accounted for compared with the scenarios considering hydrodynamic transport only. Finally, the model highlighted the role of the wind field in conveying the contamination plume and hence in determining infection probability. Our simulations revealed that for beaches located west of the sewage discharge, the infection probability under eastward wind was 43% lower than that under westward wind conditions. This study highlights the potential of combining water quality simulation and virus-specific risk assessment for a safe water resources usage and management.