VIRTUS, a model of virus transport in unsaturated soils.

As a result of the recently proposed mandatory groundwater disinfection requirements to inactivate viruses in potable water supplies, there has been increasing interest in virus fate and transport in the subsurface. Several models have been developed to predict the fate of viruses in groundwater, but few include transport in the unsaturated zone and all require a constant virus inactivation rate. These are serious limitations in the models, as it has been well documented that considerable virus removal occurs in the unsaturated zone and that the inactivation rate of viruses is dependent on environmental conditions. The purpose of this research was to develop a predictive model of virus fate and transport in unsaturated soils that allows the virus inactivation rate to vary on the basis of changes in soil temperature. The model was developed on the basis of the law of mass conservation of a contaminant in porous media and couples the flows of water, viruses, and heat through the soil. Model predictions were compared with measured data of virus transport in laboratory column studies and, with the exception of one point, were within the 95% confidence limits of the measured concentrations. The model should be a useful tool for anyone wishing to estimate the number of viruses entering groundwater after traveling through the soil from a contamination source. In addition, model simulations were performed to identify parameters that have a large effect on the results. This information can be used to help design experiments so that important variables are measured accurately.     

Modeling Fate of RDX at Demolition Area 2 of the Massachusetts Military Reservation

The movement of explosive RDX residue from soil to groundwater at Demolition Area 2 of the Massachusetts Military Reservation was studied with mathematical models to assess its utility and limitations and to determine requisite model improvements and data needs. The Munitions Residue Characterization and Fate model, which is based on the MEPAS source term model for soil modified for solid phase residue with dissolution, the MEPAS vadose zone model, and the MEPAS aquifer model were used in this study. All three models were applied within ARAMS? to facilitate model-to-model connectivity for computing movement of RDX from soil to vadose zone, and from vadose zone to groundwater. Model parameters and a hypothetical RDX residue loading rate were adjusted to match model results to measured RDX concentrations in surface soil and in groundwater. Through an iterative process, a loading rate of 1 kg/yr for RDX residues applied for 10 years starting in 1978 was found to fit measured conditions 20 years later. Model results were sensitive to the C4-RDX dissolution flux rate and the aqueous RDX degradation rate. Results indicate that dissolution of C4 and degradation of RDX in soil and groundwater could be quite slow, and such processes warrant further study. Mechanistic models such as those presented will be useful for estimating fate of constituent residue in soil and transport to receiving waters for evaluating range residue carrying capacity and compliance issues.     

Soil inactivation of DNA viruses in septic seepage

AIMS: To generate field-relevant inactivation data for incorporation into models to predict the likelihood of viral contamination of surface waters by septic seepage. METHODS AND RESULTS: Inactivation rates were determined for PRD1 bacteriophage and Adenovirus 2 in two catchment soils under a range of temperature, moisture and biotic status regimes. Inactivation rates presented for both viruses were significantly different at different temperatures and in different soil types (alpha = 0.05). Soil moisture generally did not significantly affect virus inactivation rate. Biotic status significantly affected inactivation rates of PRD1 in the loam soil but not the clay-loam soil. Adenovirus 2 was inactivated more rapidly in the loam soil than PRD1 bacteriophage. CONCLUSIONS: Virus inactivation rates incorporated into models should be appropriate for the climate/catchment in question with particular regard to soil type and temperature. Given that PRD1 is similar in size to adenoviruses, yet more conservative with regard to inactivation in soil, it may be a useful surrogate in studies of Adenovirus fate and transport. SIGNIFICANCE AND IMPACT OF THE STUDY: A better understanding of the factors that govern virus fate and transport in catchments would facilitate the design of barrier measures to prevent viral contamination of surface waters by septic seepage.     

Effect of Soil Fabric on Transport of a LNAPL through Unsaturated Fine-Grained Soils: A Centrifugal Model Study

Centrifugal model tests were performed to study the impact of the fabric of a fine-grained soil on transport of a light non-aqueous phase liquid (LNAPL). An image processing technique was developed to extract contaminant transport and fate data from the centrifugal model. Two unconsolidated sites with different moisture contents and a saturated site consolidated due to self-weight were simulated using the centrifuge. The LNAPL migrated in the vertical direction as a narrow plume and formed a free product pool above the saturated zone in unsaturated and unconsolidated soils. However, the LNAPL migrated in the horizontal direction before moving in the vertical direction as a broad plume in the consolidated site. The test results showed that the final width of the plume in the unsaturated zone of the consolidated site was nearly two times as large as that for the unconsolidated sites. In addition, the rate of leak from the underground storage tanks (USTs) on consolidated soils was substantially higher when compared with those on the unconsolidated state. The comparison of LNAPL saturation profiles at the centerline of the centrifugal models during leakage showed that, depending on the soil fabric at a given time and depth, the LNAPL phase would be different; i.e., mobile or immobile (residual) in the same soil type. The test results provided additional insight into contribution of soil fabric on transport and fate of contaminants. The soil fabric controls the geological and hydro-geological properties of fine-grained soils and hence the contamination plume.     

Virus movement in soil during saturated and unsaturated flow

Virus movement in soil during saturated and unsaturated flow was compared by adding poliovirus to sewage water and applying the water at different rates to a 250-cm-long soil column equipped with ceramic samplers at different depths. Movement of viruses during unsaturated flow of sewage through soil columns was much less than during saturated flow. Viruses did not move below the 40-cm level when sewage water was applied at less than the maximum infiltration rate; virus penetration in columns flooded with sewage was at least 160 cm. Therefore, virus movement in soils irrigated with sewage should be less than in flooded groundwater recharge basins or in saturated soil columns. Management of land treatment systems to provide unsaturated flow through the soil should minimize the depth of virus penetration. Differences in virus movement during saturated and unsaturated flow must be considered in the development of any model used to simulate virus movement in soils.     

Virus movement in soil during saturated and unsaturated flow.

Virus movement in soil during saturated and unsaturated flow was compared by adding poliovirus to sewage water and applying the water at different rates to a 250-cm-long soil column equipped with ceramic samplers at different depths. Movement of viruses during unsaturated flow of sewage through soil columns was much less than during saturated flow. Viruses did not move below the 40-cm level when sewage water was applied at less than the maximum infiltration rate; virus penetration in columns flooded with sewage was at least 160 cm. Therefore, virus movement in soils irrigated with sewage should be less than in flooded groundwater recharge basins or in saturated soil columns. Management of land treatment systems to provide unsaturated flow through the soil should minimize the depth of virus penetration. Differences in virus movement during saturated and unsaturated flow must be considered in the development of any model used to simulate virus movement in soils.     

Modeling jet fuel (JP-8) fate and transport in soils with plants

Vegetation is often used to clean up soils and groundwater contaminated with organic contaminants. Plant-induced upward water movement may draw organic contaminants spilled near the watertable to the more aerated near-surface soil. The objective of this study was to develop and verify a 1-D model of fate and transport of JP-8, a kerosene-based jet fuel, in soil. The modeling approach considered the advective and dispersive transport of jet fuels dissolved in groundwater, which may undergo simple first-order decay or linear adsorption. The governing partial differential advection dispersion equation was solved in one dimension. Data from an experiment of fate and transport of JP-8 with plant-induced upward water movement were used to verify the model. Simulated results with different scenarios described the experimental results well for different depths above the contaminated zone in both vegetated and unvegetated columns. Advection was the dominant mechanism near the contaminated zone and advection with retardation and decay was used to fit the data away from the contaminated zone. Results indicated that the soil water movement impacted the transport and concentration of JP-8 in the soil columns. This model can be used to simulate the fate of JP-8 associated with phytoremediation and evapotranspiration.     

农田生态系统中除草剂阿特拉津的环境行为及其模型研究进展

阿特拉津是世界范围内广泛使用的除草剂,在曾经使用阿特拉津的国家的地下水、河流和湖泊中已经检测到阿特拉津的残留,长期暴露在该有机污染物的环境中,势必对动物的生殖与繁衍及人体的健康产生不良影响。介绍了阿特拉津国内外的研究情况,着重评述了阿特拉津的吸附机制与影响因素、化学降解、生物降解、生态毒理学评价以及模型对其环境行为的描述,目的旨在帮助理解田间阿特拉津迁移机制及其如何污染地下水。农田生态系统中阿特拉津的迁移规律决定其环境行为,它是进行环境和健康风险评价的基础,目前应用较多的迁移模型是经典的对流-扩散方程,该模型已广泛运用于水-土系统中化学物质的迁移,且室内模拟的实验结果能较好解释田间现象,因此,开展阿特拉津在稳定流场饱和/非饱和室内土柱中的混合置换实验及批量平衡吸附实验,可获得反映阿特拉津迁移的穿透曲线特性及阿特拉津吸附性能的分配系数,结合数学模型拟合阿特拉津在土壤中非平衡迁移曲线,用拟合参数预测不同深度土壤中阿特拉津浓度变化和累积淋溶量动态规律,应该是今后研究工作的重点。     

A Study of Vadose Zone Transport Model VLEACH

Application of vadose zone transport models has been hampered by lack of model validation. Difficulties to validate vadose zone models using field data not only come from model assumptions that are uncertain to the subsurface transport processes but also from the uncertainties associated with soil contaminants’ release time and quantity, soil sampling, sample transport, and analytical procedures. This article first conducts a test of a popularly used vadose zone transport VLEACH by comparing model results with a set of laboratory soil column infiltration and volatilization study data. The comparison shows a close agreement between the VLEACH model results and the laboratory data. Second, the sorption coefficient K_d calculated in VLEACH is compared with field data. The comparison indicates that VLEACH may overestimate the mass leached from soil to groundwater. The article also discusses the selection of the model simulation timestep, the vertical dimension increment, the Courant criterion, and the lower boundary condition using the sensitivity analysis method based on a case study of soil remediation for trichloroethylene. The procedures presented in this paper are important to practical model application and modification. This level of work should be routinely conducted for any new or modified version of vadose zone models.     

The development of chemical‐specific,risk‐based soil cleanup guidelines results in timely and cost‐effective remediation

Millions of dollars of limited state cleanup funds are spent each year in New Hampshire to identify, sample, excavate, and treat thousands of tons of contaminated soil. Cost analyses of numerous sites indicated that soil remediation costs alone reach upwards of $300,000.00 per site. The New Hampshire Department of Environmental Services “Interim Policy for Management of Soils Contaminated from Spills/Releases of Virgin Petroleum Products”; (DES, 1989, 1991) set conservative remediation goals based on total petroleum hydrocarbons in 1989 using the Leaching Potential Analysis method (California Luft Manual, 1989). A current review of available literature and several case histories indicated that chemical‐specific soil cleanup levels may be more appropriate for establishing remedial goals. New chemical‐specific soil cleanup guidelines using a risk‐based approach have been developed. These new guidelines are conservatively based using two principal considerations: (1) an assumed soil exposure scenario that estimated the human health risks associated with potential long‐term exposure to site soils via ingestion, inhalation and dermal contact and (2) the estimated fate and transport of chemicals of concern in the soil unsaturated zone. The first consideration assumed a total cancer risk that did not exceed 1 × 10^‐6. The second consideration employed the use of the SEasonal SOIL Compartment (SESOIL) model which simultaneously models water transport, sediment transport, and pollutant fate (US EPA, 1981). Several state soil standards from Oregon, Wisconsin, Massachusetts, and other states were extensively reviewed in order to develop a level of confidence that use of the SESOIL model was appropriate. A series of “sensitivity”; analyses was also performed in order to evaluate the response of the model to changes in various input parameters unique to New Hampshire's hydrogeologic conditions. Generic soil cleanup guidelines were developed for 24 petroleum‐based volatile and semivolatile chemicals of concern to be applied statewide. Site‐specific soil cleanup guidelines will be allowed if it can be demonstrated that insertion of site‐specific data into the model will not adversely affect groundwater quality. As a result of the above processes, timely and much more cost‐effective remediation will be achieved while still maintaining a high degree of protection of the groundwater quality and human health.     

Inactivation of MS-2 phage and poliovirus in groundwater

Since temperature affects the inactivation rate of viruses in natural water systems, the aim of this study was to determine if a temperature shift could influence the structural integrity of model viruses. When crude lysates of MS-2 phage were seeded into groundwater microcosms and incubated at 27 degrees C, complete virus inactivation took place in eight days. The temperature was then shifted to 4 degrees C. Three days after the temperature shift, a two-log increase in virus titer (reactivation) occurred. However, when purified MS-2 lysates were added to groundwater microcosms, no reactivation was obtained. No reactivation of poliovirus took place when similar microcosm experiments were done. The sedimentation coefficients of MS-2 shifted from 80S to 58S, 48S, 37S, 32S, and 18S as inactivation proceeded in groundwater and distilled water controls. Similarly, the sedimentation coefficients of polioviruses changed from 156S to 142S, 135S, 117S, 105S, 95S, and 80 S as inactivation took place. There was no correlation between % virus inactivation and % decrease in virions with intact sedimentation coefficients, as reported earlier for poliovirus inactivated by chlorine. The results presented support our hypothesis that virus inactivation proceeds gradually, involving the rearrangement and (or) loss of capsomere components that may eventually lead to the ejection of nucleic acids. The intermediate particles generated as inactivation proceeds may be in a reversibly inactivated state, and may revert back to a fully infectious state when chemical components stabilize the virus particle.     

Enterovirus inactivation in soil.

The inactivation of radioactively labeled poliovirus type 1 and coxsackievirus B 1 in soils saturated with surface water, groundwater, and septic tank liquor was directly proportional to temperature. Virus persistence was also related to soil type and the liquid amendment in which viruses were suspended. At 37 degrees C, no infectivity was recovered from saturated soil after 12 days; at 4 degrees C, viruses persisted for at least 180 days. No infectivity was recovered from dried soil regardless of temperature, soil type, or liquid amendment. Additional experiments showed that evaporation of soil water was largely responsible for the decreased recovery of infectivity from drying soil. Increased rates of virus inactivation at low soil moisture levels were also demonstrated.     

Enterovirus inactivation in soil.

The inactivation of radioactively labeled poliovirus type 1 and coxsackievirus B 1 in soils saturated with surface water, groundwater, and septic tank liquor was directly proportional to temperature. Virus persistence was also related to soil type and the liquid amendment in which viruses were suspended. At 37 degrees C, no infectivity was recovered from saturated soil after 12 days; at 4 degrees C, viruses persisted for at least 180 days. No infectivity was recovered from dried soil regardless of temperature, soil type, or liquid amendment. Additional experiments showed that evaporation of soil water was largely responsible for the decreased recovery of infectivity from drying soil. Increased rates of virus inactivation at low soil moisture levels were also demonstrated.     

Biodegradation, sorption, and transport of 2,4-dichlorophenoxyacetic acid in saturated and unsaturated soils.

The fate of an organic contaminant in soil depends on many factors, including sorption, biodegradation, and transport. The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was used as a model compound to illustrate the impact of these interacting factors on the fate of an organic contaminant. Batch and column experiments performed with a sandy loam soil mixture under saturated and unsaturated conditions were used to determine the effects of sorption and biodegradation on the fate and transport of 2,4-D. Sorption of 2,4-D was found to have a slight but significant effect on transport of 2,4-D under saturated conditions (retardation factor, 1.8) and unsaturated conditions (retardation factor, 3.4). Biodegradation of 2,4-D was extensive under both batch and column conditions and was found to have a significant impact on 2,4-D transport in column experiments. In batch experiments, complete mineralization of 2,4-D (100 mg kg-1) occurred over a 4-day period following a 3-day lag phase under both saturated and unsaturated conditions. The biodegradation rate parameters calculated for batch experiments were found to be significantly different from those estimated for column experiments.     

Environmental inactivation of Cryptosporidium oocysts in catchment soils

AIMS: To generate field-relevant inactivation rates for Cryptosporidium oocysts in soil that may serve as parameter values in models to predict the terrestrial fate and transport of oocysts in catchments. METHODS AND RESULTS: The inactivation of Cryptosporidium oocysts in closed soil microcosms over time was monitored using fluorescence in situ hybridization (FISH) as an estimate of oocyst 'viability'. Inactivation rates for Cryptosporidium in two soils were determined under a range of temperature, moisture and biotic status regimes. Temperature and soil type emerged as significantly influential factors (P < 0.05) for Cryptosporidium inactivation. In particular, temperatures as high as 35 degrees C may result in enhanced inactivation. CONCLUSIONS: When modelling the fate of Cryptosporidium oocysts in catchment soils, the use of inactivation rates that are appropriate for the specific catchment climate and soil types is essential. FISH was considered cost-effective and appropriate for determining oocyst inactivation rates in soil. SIGNIFICANCE AND IMPACT OF THE STUDY: Previous models for predicting the fate of pathogens in catchments have either made nonvalidated assumptions regarding inactivation of Cryptosporidium in the terrestrial environment or have not considered it at all. Field-relevant inactivation data are presented, with significant implications for the management of catchments in warm temperate and tropical environments.     

Virus and Bacteria Removal from Wastewater by Rapid Infiltration Through Soil

A rapid infiltration land wastewater application site, composed of unconsolidated silty sand and gravel, which has been in continuous operation for over 30 years was examined for the accumulation and/or migration of a tracer virus (coliphage f2), indigenous enteroviruses, and enteric indicator bacteria in the soils and underlying groundwater. Tracer f2 penetrated into groundwater together with the front of percolating primary effluent and was not observed to concentrate on the upper soil layers. The tracer virus concentration in a 60-foot (about 18.3-m)-deep observation well directly beneath the wastewater application area began to increase within 48 h after application to the soil. The tracer level in this well stabilized after 72 h at a level of approximately 47% of the average applied concentration. Indigenous enteroviruses and tracer f2 were sporadically detected in the groundwater at horizontal distances of 600 feet (about 183 m) from the application zone. Laboratory soil adsorption studies confirmed the poor virus adsorption observed at the site. This was especially true on surface soils when contained in wastewater. Enteric indicator bacteria were readily concentrated on the soil surface by filtration on the soil surface mat. However, during tracer f2 virus tests, comparison studies with fecal Streptococcus revealed that bacteria capable of penetrating the surface were able to migrate into the groundwater. They were detected at the same locations as tracer and enteric viruses.     

Transport and survival of bacterial and viral tracers through submerged-flow constructed wetland and sand-filter system

Untreated or improperly treated wastewater has often been cited as the primary contamination source of groundwater. The use of decentralized wastewater treatment systems has applicability around the world since it obviates the need for extensive infrastructure development and expenditures. The use of a submerged flow constructed wetland (CW) and a sand filter to remove bacterial and viral pathogens from wastewater streams was evaluated in this study Salmonella sp. and a bacteriophages tracer were used in conjunction with the conservative bromide tracer to understand the fate and transport of these organisms in these treatment systems. Viral breakthrough numbers in the sand filter and CW were similar with a Spearman Rank correlation of 0.8 (P<0.05). In the CW, the virus exhibited almost a 3-log reduction, while in the sand filter, the viruses exhibited a 2-log reduction. The bacterial tracers, however, did not exhibit similar reductions. Low numbers of bacteria and viruses were still detectable in the effluent streams suggesting that disinfection of the effluent is critical. The survival of the tracer bacteria and viruses was as expected dependent on the biotic and abiotic conditions existing within the wastewater. The results suggest that the microbial removal characteristics of decentralized wastewater treatment systems can vary and depend on factors such as adsorption, desorption and inactivation which in turn depend on the design specifics such as filter media characteristics and local climatic conditions.     

Calculation of soil cleanup criteria for volatile organic compounds as controlled by the soil‐to‐groundwater pathway: Comparison of four unsaturated soil zone leaching models

Four computer models that predict leaching of chemicals in the unsaturated soil zone were used to calculate example soil cleanup criteria for volatile organic compounds, using a hypothetical environmental scenario. The criteria were calculated so that allowable groundwater concentrations for the chemicals were not exceeded. The models used were the Pesticide Root Zone Model (PRZM) and the Seasonal Soil Compartment Model (SESOIL) from the U.S. Environmental Protection Agency, the Sanitary Landfill Model (SLM1) from Oregon State University, and the Integrated Moisture and Aqueous Contaminant Transport model (IMPACT) under development for the State of New Jersey. The hypothetical scenario assumed a water table depth of 10 ft, a contaminated zone from 0 to 4 ft, and sandy loam soil properties. Transport times to groundwater were similar for all four models. The calculated soil criteria for many chemicals using the four models agreed to within an order of magnitude. In a few instances, SLM1 and PRZM predicted much lower cleanup criteria than the other two models because volatilization losses were not modeled. Calculated criteria were often quite low when degradation was assumed to be zero. When estimated degradation rates were employed, criteria were sometimes considerably higher.     

Derivation of a Liquid-to-Soild Ratio for ASTM Method D3987-85 for Soils Containing Chromite Ore Processing Residue Using Selected Unsaturated Zone Models

While most health-based cleanup levels for hexavalent chromium [Cr(VI)] in soil are established to protect the general population against cancer, the New Jersey Department of Environmental Protection (NJDEP) has established a Cr(VI) soil cleanup criterion to protect Cr(VI)-sensitive individuals against allergic contact dermatitis (ACD). For this criterion, exposure is assumed to occur when a Cr(VI)-sensitized individual contacts a puddle in which rainfall has dissolved Cr(VI) from the associated surface soils. To simulate the movement of Cr(VI) from surface soil into puddle water after rainfall events, this soil cleanup criterion requires that the Cr(VI) concentration extracted from soil using ASTM Method D3987-85 at a 2:1 liquid-to-solid ratio (LSR) be less than 25?mg/L. However, this LSR is based on several simplifying assumptions that may not be appropriate for many soil types. In this article, three unsaturated zone transport models—Chemflo, SWIM, and HYDROGEOCHEM—are evaluated for developing LSRs for sites with soil containing chromite ore processing residue (COPR). Based on this evaluation, SWIM was found to be the most appropriate model for simulating Cr(VI) transport into puddles because of its ability to simulate rainfall and evaporation rate changes during and following precipitation events. A more refined evaluation using only SWIM was performed to determine the effects of various factors on COPR-specific LSRs, including saturated hydraulic conductivity, daily rainfall rate, daily evaporation rate, and groundwater depth. The refined evaluation demonstrated that the LSR ranges from 1.4 to 30, with a mean of 7.4 and a median of 7.0. Thus, an LSR of 7:1 is the most appropriate to represent the puddle scenario at COPR sites.     

Uptake of saline groundwater by plants: An analytical model for semi-arid and arid areas

An analytical model, based on unsaturated zone water and solute balances, was developed to describe the uptake of saline groundwater by plants in dry regions. It was assumed that: i. initially, the profile had low water and salt contents to some depth; ii. both water and solutes move upwards from the water table by piston flow due only to plant water extraction; iii. the uptake of water concentrates solutes in the soil solution until some threshold salinity is reached, above which plants can no longer extract water due to osmotic effects; iv. uptake of the groundwater does not affect the water table level; and v. uptake of groundwater is only limited by transmission of groundwater through the soil. Model predictions were compared with measurements of groundwater uptake made over 15 months at five sites in aEucalyptus forest in a semi-arid area, using independently measured model parameters. Depth and salinity of groundwater, and soil type varied greatly between sites. Predicted groundwater uptake rates were close to measured values, generally being within ∼ 0.1 mm day^-1. Sensitivity analysis showed that groundwater depth and salinity were the main controls on uptake of groundwater, while soil properties appeared to have a lesser effect. The model showed that uptake of groundwater would result in complete salinisation of the soil profile within 4 to 30 yr at the sites studied, unless salts were leached from the soil by rainfall or flood waters. However, a relatively small amount of annual leaching may be sufficient to allow groundwater uptake to continue. Thus groundwaters, even when saline, may be important sources of water to plants in arid and semi-arid areas.