RISK ASSESSMENT METHODOLOGY FOR KARST AQUIFERS: (1) ESTIMATING KARST CONDUIT-FLOW PARAMETERS

Quantitative ground-water tracing of conduit-dominated karst aquifers allows for reliable and practical interpretation of karst ground-water flow. Insights into the hydraulic geometry of the karst aquifer may be acquired that otherwise could not be obtained by such conventional methods as potentiometric-surface mapping and aquifer testing. Contamination of karst aquifers requires that a comprehensive tracer budget be performed so that karst conduit hydraulic-flow and geometric parameters be obtained. Acquisition of these parameters is necessary for estimating contaminant fate-and-transport. A FORTRAN computer program for estimating total tracer recovery from tracer-breakthrough curves is proposed as a standard method. Estimated hydraulic-flow parameters include mean residence time, mean flow velocity, longitudinal dispersivity, Peclet number, Reynolds number, and Froude number. Estimated geometric parameters include karst conduit sinuous distance, conduit volume, cross-sectional area, diameter, and hydraulic depth. These parameters may be used to (1) develop structural models of the aquifer, (2) improve aquifer resource management, (3) improve ground-water monitoring systems design, (4) improve aquifer remediation, and (5) assess contaminant fate-and-transport. A companion paper demonstrates the use of these hydraulic-flow and geometric parameters in a surface-water model for estimating contaminant fate-and-transport in a karst conduit. Two ground-water tracing studies demonstrate the utility of this program for reliable estimation of necessary karst conduit hydraulic-flow and geometric parameters.     

Terrain Discontinuity Effects in the Regional Flow of a Complex Karstified Aquifer

Karst aquifers are characterized by spatial heterogeneity due to the presence of highly permeable channels and conduits in low-permeable fractured rocks (matrix block). Recent studies have reported a close relationship between surface and subsurface water in karstic regions due to the water flow through a complicated network of paths formed by fracture intersections. Subsurface flow in karstified aquifers ranges between conduit flow, in large passages with relatively high flow velocities, and diffuse flow, in the matrix block where Darcy’s law is still valid. In this paper, we present the simulation of a complex karstified aquifer system in Crete, Greece, where the presence of main faults drastically affects the regional flow. A discrete fracture approach in conjunction with an equivalent porous medium approach was adopted to simulate the mixed flow in the area of interest. The simulation results have shown that the length and the orientation of the dominant faults, primarily during the rainy season, affect the flow field.     

Migration of pollutants in groundwater. VI. Flushing of DNAPL droplets/ganglia

Models for describing the flushing of DNAPL from contaminated aquifers are developed, and the dependence of the calculated cleanup times on the model parameters is explored. Diffusion transport from isolated DNAPL droplets, from low-permeability porous spherical domains containing distributed DNAPL droplets, and from low-permeability porous planar lamellae containing distributed DNAPL is analyzed, and the resulting expressions then coupled with the equations for advective transport of dissolved VOC by means of natural uniform flow and a system of injection and recovery wells generating a two-dimensional flow field. The models are readily run on currently available microcomputers. The results of computations with the models are consistent with the severe tailing and slow rates of remediation which are generally observed when DNAPLs are removed by flushing.     

Ambient Air Quality Monitoring Network Design for Assessing Human Health Impacts from Exposures to Airborne Contaminants

Existing methods of establishing ambientair quality monitoring networks typically evaluateonly parameters related to ambient concentrations ofthe contaminant(s) of interest such as emissionsource characteristics, atmospheric transport anddispersion, secondary reactions, depositioncharacteristics, and local topography. However,adverse health risks from exposures to airbornecontaminants are a function of the contaminant andthe anatomic and physiologic characteristics of theexposed population. Thus, ambient air qualitymonitoring networks designed for the protection ofpublic health or for epidemiological studiesevaluating adverse health impacts from exposures toambient air contaminants should account for bothcontaminant characteristics and human healthparameters. A methodology has been establishedwhich optimizes ambient air quality monitoringnetworks for assessments of adverse human healthimpacts from exposures to airborne contaminants byincorporating human health risk assessmenttechniques. The use of risk assessment techniquesas the basis for designing ambient air qualitymonitoring networks will help to target limitedfinancial and human resources to evaluate humanhealth risks from exposures to airbornecontaminants.     

Evaluating pollution potential of leachate from landfill site, from the Pune metropolitan city and its impact on shallow basaltic aquifers

Leachate produced by municipal solid waste dumping site near the metropolitan city of Pune, India was examined for its pollution potential and impact on surrounding shallow basaltic aquifers. Twenty-eight physico-chemical parameters during post- and pre-monsoon seasons (Nov 2006 and May 2007) were determined to assess the seasonal variation in the leachate pollution index (LPI) as well as in the groundwater quality. The leachate demonstrated higher LPI value during pre-monsoon, comparable to those at other metropolises outside India. Potentially toxic leachates derived from the dumping site have largely influenced the adjoining basaltic aquifers through two different modes of transport. Despite high contents of heavy metals (Al, Cd, Cr, Cu, Co, Fe, Mn, Ni, Pb and Zn) in the leachate, the aquifers in the close proximity of landfill site are least polluted by metallic contaminants possibly due to redox controls. Various geoenvironmental features governing the dispersal of leachate contaminants in the basaltic aquifers under semi-arid climatic regime have been identified and discussed. Although a few remedial measures have been suggested to mitigate the impact of leachate percolation and dispersion, the present study demands for a proper solid waste management in metropolitan cities.     

Sensitive Parameters In Predicting Exposure Contaminants Concentration In A Risk Assessment Process

A sensitivity analysis (SA) was conducted on the analytical models considered in the risk-based corrective-action (RBCA) methodology of risk analysis, as developed by the American Society for Testing of Materials (ASTM), to predict a contaminant‘s concentration in the affected medium at the point of human exposure. These models are of interest because evaluations regarding the best approach to contaminated site remediation are shifting toward increased use of risk-based decision, and the ASTM RBCA methodology represents the most effective and internationally widely used standardized guide for risk assessment process. This paper identifies key physical and chemical parameters that need additional precision and accuracy consideration in order to reduce uncertainty in models prediction, thereby saving time, money and engineering effort in the data collection process. SA was performed applying a variance-based method to organic contaminants migration models with reference to soil-to-groundwater leaching ingestion exposure scenario. Results indicate that model output strongly depends on the organic-carbon partition coefficient, organic-carbon content, net infiltration, Darcy velocity, source-receptor distance, and first-order decay constant.     

Assessment of phenol infiltration resilience in soil media by HYDRUS-1D transport model for a waste discharge site

The movement of contaminants through soil imparts a variety of geo-environmental problem inclusive of lithospheric pollution. Near-surface aquifers are often vulnerable to contamination from surface source if overlying soil possesses poor resilience or contaminant attenuation capacity. The prediction of contaminant transport through soil is urged to protect groundwater from sources of pollutants. Using field simulation through column experiments and mathematical modeling like HYDRUS-1D, assessment of soil resilience and movement of contaminants through the subsurface to reach aquifers can be predicted. An outfall site of effluents of a coke oven plant comprising of alarming concentration of phenol (4–12.2 mg/L) have been considered for studying groundwater condition and quality, in situ soil characterization, and effluent characterization. Hydrogeological feature suggests the presence of near-surface aquifers at the effluent discharge site. Analysis of groundwater of nearby locality reveals the phenol concentration (0.11–0.75 mg/L) exceeded the prescribed limit of WHO specification (0.002 mg/L). The in situ soil, used in column experiment, possess higher saturated hydraulic conductivity (K _S ?=?5.25?×?10^?4 cm/s). The soil containing 47 % silt, 11 % clay, and 1.54 % organic carbon content was found to be a poor absorber of phenol (24 mg/kg). The linear phenol adsorption isotherm model showed the best fit (R ²?=?0.977, RMSE?=?1.057) to the test results. Column experiments revealed that the phenol removal percent and the length of the mass transfer zone increased with increasing bed heights. The overall phenol adsorption efficiency was found to be 42–49 %. Breakthrough curves (BTCs) predicted by HYDRUS-1D model appears to be close fitting with the BTCs derived from the column experiments. The phenol BTC predicted by the HYDRUS-1D model for 1.2 m depth subsurface soil, i.e., up to the depth of groundwater in the study area, showed that the exhaustion point was reached within 12 days of elapsed time. This clearly demonstrated poor attenuation capacity of the soil to retard migration of phenol to the groundwater from the surface outfall site. Suitable liner, based on these data, may be designed to inhibit subsurface transport of phenol and thereby to protect precious groundwater from contamination.     

Parameter values to model the soil ingestion pathway

Soil ingestion is an important exposure pathway for contaminants that are not otherwise very mobile in the environment. Health of both humans and animals can be affected. This paper summarizes the literature and recommends models and probabilistic parameter values for risk assessment applications. Models of the pathway require estimates of the amounts of soil ingested, the concentration of contaminants relative to the original soil, and the bioavailability in the gut of the contaminants ingested with soil. Using a lead-contaminated sandbox as an example, the modelling recommendations suggest that a child typically may consume 50 mg d^–1 of the sandbox soil, the soil ingested will have a tenfold higher lead concentration than the original soil, and the lead will be as bioavailable as if ingested as inorganic lead in water.The Canadian Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Migration of pollutants in groundwater. V. Modelling the removal of DNAPL droplets by flushing

Mathematical models are developed for the flushing of droplets of dense nonaqueous phase liquids (DNAPLs) distributed in aquifers. The kinetics of the diffusion of dissolved volatile organic compound (VOC) from the droplets into the moving liquid is included in the models. Models are developed for the flushing of DNAPL droplets in lab columns, in aquifers with a one-dimensional radial flow field and in quiescent aquifers in which a single well screened at the bottom is used to bring about the flushing. Some representative results are given.     

Colloid-Associated Groundwater Contaminant Transport in Homogeneous Saturated Porous Media: Mathematical and Numerical Modeling

During the past two decades, significant efforts have been made to study contaminant transport in the presence of colloids. Several researchers reported that colloidal particles could enhance the migration of contaminants in groundwater by reducing retardation factor. When the colloidal particles are present in the aquifer, the subsurface system can be considered as a three-phase system with two solid phases and an aqueous phase. The interaction between contaminants, colloids, and solid matrix should be considered in assessing the fate and transport of the contaminant in the groundwater flow system. In this study, a one-dimensional numerical model is developed by employing a fully implicit finite difference method. This model is based on mass balance equations and mass partition mechanisms between the carriers and solid matrix, as well as between the carriers and contaminants in a saturated homogeneous porous medium. This phenomenon is presented by two approaches: equilibrium approach and fully kinetic first-order approach. The formulation of the model can be simplified by employing equilibrium partitioning of particles. However, contaminant transport can be predicted more accurately in realistic situations by kinetic modeling. To test the sensitivity of the model, the effect of the various chemical and physical coefficients on the migration of contaminant was investigated. The results of numerical modeling matched favorably with experimental data reported in the literature.     

Cleanup of fractured rock aquifers: Implications of matrix diffusion

As contamination moves through a fractured rock aquifer, it tends to diffuse from the flowing fracture water into the rock's essentially stagnant pore water. This process tends both to retard a contamination plume's advance through a fractured rock aquifer and to substantially increase the difficulty of purging contamination from the aquifer. A mathematical model has been developed to evaluate the potential impact of this phenomenon upon water quality restoration in fractured rock aquifers. The numerical modeling reveals that cleanup of fractured rock aquifers will, in many cases, require many decades, even centuries, to achieve, particularly where substantial improvements in water quality are sought. The parameters which most strongly govern the degree to which matrix diffusion prolongs the aquifer restoration process are the rock's matrix porosity, fracture spacing, and matrix diffusivity, the chemical identity of the contaminant(s), and the length of time the aquifer has been contaminated.Since sedimentary rocks tend to have both relatively high matrix porosities and matrix diffusivities, it can be particularly difficult to purge contamination from sedimentary rock aquifers. Crystalline rocks, in contrast, typically have lower matrix porosities and matrix diffusivities, and therefore undergo more rapid cleanup. However, even in crystalline rocks, attainment of very high degrees of water quality improvement may be problematic. Numerical modeling also indicates that conventional groundwater pump and treat programs are not likely to be very effective in speeding up aquifer restoration if the rate limiting step in the process is diffusion of contaminants from the rock matrix.     

基于健康风险的三氯杀螨醇生产设备表面污染物筛选值推算的初步研究

运用确定性和概率性人体健康风险评估的方法,推算一般工业暴露情景和拆卸清理暴露情景下三氯杀螨醇生产设备表面污染物的筛选值.结果表明:p,p'-滴滴涕、p,p'-滴滴滴和p,p'-滴滴依基于确定性风险评估的设备表面筛选值在一般工业暴露情景下分别为0.224 mg/m2、0.214 mg/m2和0.151 mg/m2,在拆卸...     

A novel high-frequency groundwater quality monitoring system

High-frequency, long-term monitoring of water quality has revolutionized the study of surface waters in recent years. However, application of these techniques to groundwater has been limited by the ability to remotely pump and analyze groundwater. This paper describes a novel autonomous groundwater quality monitoring system which samples multiple wells to evaluate temporal changes and identify trends in groundwater chemistry. The system, deployed near Fresno, California, USA, collects and transmits high-frequency data, including water temperature, specific conductance, pH, dissolved oxygen, and nitrate, from supply and monitoring wells, in real-time. The system consists of a water quality sonde and optical nitrate sensor, manifold, submersible three-phase pump, variable frequency drive, data collection platform, solar panels, and rechargeable battery bank. The manifold directs water from three wells to a single set of sensors, thereby reducing setup and operation costs associated with multi-sensor networks. Sampling multiple wells at high frequency for several years provided a means of monitoring the vertical distribution and transport of solutes in the aquifer. Initial results show short period variability of nitrate, specific conductivity, and dissolved oxygen in the shallow aquifer, while the deeper portion of the aquifer remains unchanged—observations that may be missed with traditional discrete sampling approaches. In this aquifer system, nitrate and specific conductance are increasing in the shallow aquifer, while invariant changes in deep groundwater chemistry likely reflect relatively slow groundwater flow. In contrast, systems with high groundwater velocity, such as karst aquifers, have been shown to exhibit higher-frequency groundwater chemistry changes. The stability of the deeper aquifer over the monitoring period was leveraged to develop estimates of measurement system uncertainty, which were typically lower than the manufacturer’s stated specifications, enabling the identification of subtle variability in water chemistry that may have otherwise been missed.     

Temporal moments analysis of preferential solute transport in soils

Temporal moments analysis of solute breakthrough curves is used to investigate the preferential leaching of chloride, nitrate and phosphate through an Australian soil. Recent studies have shown that current models and methods do not adequately describe the leaching of nutrients through soil, often underestimating the risk of groundwater contamination by surface-applied chemicals, and overestimating the concentration of resident solutes. This inaccuracy results primarily from ignoring soil structure and non-equilibrium between soil constituents, water and solutes. Therefore simple models are required to accurately characterise solute transport in natural and agricultural soils under non-equilibrium conditions. A multiple sample percolation system, consisting of 25 individual collection wells was constructed to study the effects of localised soil heterogeneities on the transport of nutrients (NO₃ ^–, Cl^–, PO₄ ^{3 –}) in the vadose zone of an agricultural soil predominantly dominated by clay. Using data collected from the multiple sample percolation experiments, this paper compares and contrasts the performance of temporal moments analysis with two mathematical models for predicting solute transport, the advective-dispersion model with a reaction term (ADR) and a two-region preferential flow model (TRM) suitable for modelling preferential transport. The values for solute transport parameters predicted by temporal moments analysis were in excellent agreement with experimental data and results from ADR and TRM. It is concluded that temporal moments analysis when applied with other physical models such as the ADR and TRM, provide an excellent means of obtaining values for important solute transport parameters and gaining insight of preferential flow. These results have significant ramifications for modelling solute transport and predicting nutrient loadings.     

Empirical evaluation of spatial and non-spatial European-scale multimedia fate models: results and implications for chemical risk assessment

Multimedia environmental fate models are commonly-applied tools for assessing the fate and distribution of contaminants in the environment. Owing to the large number of chemicals in use and the paucity of monitoring data, such models are often adopted as part of decision-support systems for chemical risk assessment. The purpose of this study was to evaluate the performance of three multimedia environmental fate models (spatially- and non-spatially-explicit) at a European scale. The assessment was conducted for four polycyclic aromatic hydrocarbons (PAHs) and hexachlorobenzene (HCB) and compared predicted and median observed concentrations using monitoring data collected for air, water, sediments and soils. Model performance in the air compartment was reasonable for all models included in the evaluation exercise as predicted concentrations were typically within a factor of 3 of the median observed concentrations. Furthermore, there was good correspondence between predictions and observations in regions that had elevated median observed concentrations for both spatially-explicit models. On the other hand, all three models consistently underestimated median observed concentrations in sediment and soil by 1-3 orders of magnitude. Although regions with elevated median observed concentrations in these environmental media were broadly identified by the spatially-explicit models, the magnitude of the discrepancy between predicted and median observed concentrations is of concern in the context of chemical risk assessment. These results were discussed in terms of factors influencing model performance such as the steady-state assumption, inaccuracies in emission estimates and the representativeness of monitoring data.     

Highlighting the implications of selenium dispersion from disposal of Kahota Industrial Triangle area, Islamabad, Pakistan using three-dimension solute transport model

This study highlights the implications of selenium (Se) dispersion in groundwater flow regimes of Kahota Industrial Triangle area located adjacent to the Soan River, Islamabad. Initially, a regional groundwater 3-D flow model has been developed, calibrated to the known observed heads of 24 water wells, verified, and confirmed that convergence has actually arrived to satisfy the steady state condition. Later, the transient simulation was carried out adding in the known recharge, storage factor, porosity, and observed drawdown matched with the simulated drawdown that appears to fall in close agreement with a difference of 0.25 m. As such the steady state groundwater model has facilitated to understand the mechanism of groundwater flow regimes in reference to the implications of selenium dispersion from disposal of Kahota Industrial Triangle area. Thirty-five water samples were collected mainly from the industrial water wells for the evaluation of heavy metals. Selenium being the major contributor of pollution has been short listed to monitor its dispersion using a solute transport model modular three-dimensional transport model (MT3D). Chemical parameters related to selenium characteristics including horizontal and vertical transverse dispersivity/longitudinal dispersivity, effective molecular diffusion coefficient and bulk density of the porous medium of aquifers have been used in MT3D contaminant transport model. MT3D is run for 30 years in steady state condition. As usual first run did not produce the exact field conditions. Therefore, the contaminant transport model is calibrated against the 32 values of observed selenium concentrations in boreholes by minor adjustments in the chemical parameter values. The final calibration has been achieved with residual value of 3.88 × 10^???5 Kg/m³. Seven hypothetical observation wells are used to monitor the selenium concentrations over a long-term period of time.     

Modelling the dispersion of treated wastewater in a shallow coastal wind-driven environment,Geographe Bay,Western Australia: implications for environmental management

Numerical models are useful for predicting the transport and fate of contaminants in dynamic marine environments, and are increasingly a practical solution to environmental impact assessments. In this study, a three-dimensional hydrodynamic model and field data were used to validate a far-field dispersion model that, in turn, was used to determine the fate of treated wastewater (TWW) discharged to the ocean via a submarine ocean outfall under hypothetical TWW flows. The models were validated with respect to bottom and surface water current speed and direction, and in situ measurements of total nitrogen and faecal coliforms. Variations in surface and bottom currents were accurately predicted by the model as were nutrient and coliform concentrations. Results indicated that the ocean circulation was predominately wind driven, evidenced by relatively small oscillations in the current speeds along the time-scale of the tide, and that dilution mixing zones were orientated in a predominantly north-eastern direction from the outfall and parallel to the coastline. Outputs of the model were used to determine the ‘footprint’ of the TWW plume under a differing discharge scenario and, particularly, whether the resultant changes in TWW contaminants, total nitrogen and faecal coliforms would meet local environmental quality objectives (EQO) for ecosystem integrity, shellfish harvesting and primary recreation. Modelling provided a practical solution for predicting the dilution of contaminants under a hypothetical discharge scenario and a means for determining the aerial extent of exclusion zones, where the EQOs for shellfish harvesting and primary recreation may not always be met. Results of this study add to the understanding of regional discharge conditions and provide a practical case study for managing impacts to marine environments under a differing TWW discharge scenario, in comparison to an existing scenario.     

An analytical model for the assessment of pesticide exposure levels in soils and groundwater

The movement and degradation of pesticide residues in soils and groundwater are complex processes affected by soil physical, (bio)chemical, and hydrogeological properties, climatic conditions, and agricultural practices. This work presents a physically-based analytical model suitable for long-term predictions of pesticide concentrations in groundwater. The primary interest is to investigate the impact of soil environment, related physical and (bio)chemical processes, especially, volatilization, crop uptake, and agricultural practices on long-term vulnerability of groundwater to contamination by pesticides. The soil is separated into root and intermediate vadose zones, each with uniform properties. Transport in each soil zone is modeled on the basis of complete mixing, by spatial averaging the related point multiphase-transport partial differential equation (i.e., linear-reservoir models). Transport in the aquifer, however, is modeled by a two-dimensional advection-dispersion transport equation, considering adsorption and first-order decay rate. Vaporization in the soil is accounted for by assuming liquid-vapor phase partitioning using Henry's law, and vapor flux (volatilization) from the soil surface is modeled by diffusion through an air boundary layer. Sorption of liquid-phase solutes by crops is described by a linear relationship which is valid for first-order (passive) crop uptake. The model is applied to five pesticides (atrazine, bromacil, chlordane, heptachlor, and lindane), and the potential for pesticide contamination of groundwater is investigated for sandy and clayey soils. Simulation results show that groundwater contamination can be substantially reduced for clayey soil environments, where bio(chemical) degradation and volatilization are most efficient as natural loss pathways for the pesticides. Also, uptake by cross can be a significant mechanism for attenuating exposure levels in ground-water especially in a sandy soil environment, and for relatively persisting pesticides. Further, simulations indicate that changing agricultural practices can have a profound effect on vulnerability of groundwater to mobile and relatively persisting pesticides.     

Humic Acid Transport in Water-Saturated Porous Media

The presence of humic acid (HA) in water poses environmental problems because it enhances the transport of several contaminants. A series of column experiments was conducted toward studying HA transport in different porous media under various pH, ionic strength, and flow rate conditions. The results showed that decreasing pH and increasing ionic strength increased adsorption and therefore delayed the transport of HA in porous media. However, increasing flow rate accelerated the transport of HA in porous media. The effects of pH, ionic strength, and flow rate varied with the solid matrix and were more evident in sands of smaller average particle diameter than in those with larger ones. These factors must be considered when predicting the environmental fate of HA. The results also suggested that HA adsorption was an important process controlling HA transport and should be considered in studies of HA behavior in porous media.     

The distribution and extent of heavy metal accumulation in song sparrows along Arizona’s upper Santa Cruz River

Heavy metals are persistent environmental contaminants, and transport of metals into the environment poses a threat to ecosystems, as plants and wildlife are susceptible to long-term exposure, bioaccumulation, and potential toxicity. We investigated the distribution and cascading extent of heavy metal accumulation in southwestern song sparrows (Melospiza melodia fallax), a resident riparian bird species that occurs along the US/Mexico border in Arizona’s upper Santa Cruz River watershed. This study had three goals: (1) quantify the degree of heavy metal accumulation in sparrows and determine the distributional patterns among study sites, (2) compare concentrations of metals found in this study to those found in studies performed prior to a 2009 international wastewater facility upgrade, and (3) assess the condition of song sparrows among sites with differing potential levels of exposure. We examined five study sites along with a reference site that reflect different potential sources of contamination. Body mass residuals and leukocyte counts were used to assess sparrow condition. Birds at our study sites typically had higher metal concentrations than birds at the reference site. Copper, mercury, nickel, and selenium in song sparrows did exceed background levels, although most metals were below background concentrations determined from previous studies. Song sparrows generally showed lower heavy metal concentrations compared to studies conducted prior to the 2009 wastewater facility upgrade. We found no cascading effects as a result of metal exposure.