Dynamics Of Contaminant Transport Research Articles

Flow and contaminant transport dynamics in clay-amended barriers through flushing experiments and multi-porosity-based modeling

Clay-amended barriers are widely used to prevent hazardous leachate percolation from landfill to subsurface. The performance of these barriers is mostly evaluated through numerical simulations with limited experimental investigation through leachate flushing experiments. To bridge this gap, contaminant loading and its flushing experiments were carried out to assess the performance of clay-amended composite materials as landfill liners. River sand (Sa), loamy soil (Ns), and alternative waste materials like fly ash (Fa) and flushed silt (Si) were used to prepare the composites. Composites fulfilling the hydraulic conductivity (<10−7 cm/s) and compressive strength (200 kPa) criteria were selected for contaminant loading and its flushing experiments to understand the fate of fluoride ions. The experimentally determined hydraulic conductivity (Ks) values for all the composites were in the order of 10−8 cm/s. The experimental breakthrough curves exhibited skewed shape, long tailing, and dual peaks. Dual porosity and dual permeability with immobile water models were employed to simulate these curves, revealing that preferential flow pathways and random chemical sorption sites significantly affect solute transport in clay-amended barriers. Further, scanning electron microscopy and energy-dispersive X-ray spectroscopy were employed to trace the preferred path of fluoride ions through the barrier. The removal efficiency and temporal moments were used to determine the percentage mass retained, mean arrival time, and spreading within the barrier. The highest solute mass was retained by sand-clay barrier (SaB30) (91%), followed by loam-clay barrier (NsB30) (59%), fly ash-clay barrier (FaB30) (38%), and silt-clay barrier (SiB30) (4%) with the least mass. The lowest mean arrival time was calculated for NsB30 (269 h) and the highest for SaB30 (990 h), with FaB30 (384 h) and SiB30 (512 h) having values in between. This study concludes that validating the design hypothesis of clay-amended barriers through contaminant loading and its flushing studies leads to an effective and sustainable design.

Surrogate-Based Uncertainty Analysis for Groundwater Contaminant Transport in a Chromium Residue Site Located in Southern China

Numerical modeling is widely acknowledged as a highly precise method for understanding the dynamics of contaminant transport in groundwater. However, due to the intricate characteristics of environmental systems and the lack of accurate information, the results are susceptible to a significant degree of uncertainty. Numerical models must explicitly consider related uncertainties in parameters to facilitate robust decision-making. In a Chromium Residue Site located in southern China (the study area), this study employed Monte Carlo simulation to assess the impact of variability in key parameters uncertainty on the simulation outcomes. Variogram analysis of response surface (VARS), global sensitivity analysis, and an XGBoost (version 2.0.0)-based surrogate model was employed to overcome the substantial computational cost of Monte Carlo simulation. The results of numerical simulation indicate that the contaminant is spreading downstream towards the northern boundary of contaminated site near Lianshui River, threatening water quality. Furthermore, migration patterns are complex due to both downstream convection and upstream diffusion. Sensitivity analysis identified hydraulic conductivity, recharge rate, and porosity as the most influential model parameters, selected as key parameters. Moreover, uncertainty analysis indicated that the variability in key parameters has a minimal impact on the simulation outcomes at monitoring wells near the contaminant source. In contrast, at wells positioned a considerable distance from the contaminant source, the variability in key parameters significantly influences the simulation outcomes. The surrogate model markedly mitigated computational workload and calculation time, while demonstrating superior precision and effectively capture the non-linear correlations between input and output of the simulation model.

Mechanistic insights into contaminant transport dynamics in the saturated porous system in the presence of low permeability region using numerical simulations and temporal moment analysis.

The influence of low permeability porous media (LPPM) on contaminant transport dynamics in saturated porous systems was investigated using numerical simulations and temporal moments of contaminant concentrations. Two-dimensional flow and contaminant transport simulations were conducted, considering various parameters such as longitudinal dispersivity (ranging from 15 to 60m), the ratio of transverse to longitudinal dispersivity (ranging from 0.05 to 0.2), retardation factor (ranging from 1 to 4), and hydraulic gradient (ranging from 0.005 to 0.02) for both homogeneous and heterogeneous porous systems. The findings revealed significant differences in the transport behavior of conservative and highly reactive contaminants between the porous systems without and with the LPPM region. The center of mass of contaminant plume and peak concentration zone were observed inside the LPPM region for the heterogeneous porous system, especially during the source off-loading period. Furthermore, asymmetric distributions of the zeroth temporal moment (ZTM), mean residence time (MRT), and variance of the breakthrough curve (BTC) were observed along the longitudinal distance within the LPPM region for heterogeneous porous system, highlighting the impact of heterogeneity on contaminant plume evolution dynamics. The moment analysis results provided insights into the influence of LPPM region on time-averaged contaminant transport dynamics in adjacent porous systems. These findings can help risk managers understand the complex fate and transport dynamics in heterogeneous porous systems. Future studies could explore the modelling of multispecies contaminants in heterogeneous saturated porous systems subjected to fluctuating water table.

More Than a First Flush: Urban Creek Storm Hydrographs Demonstrate Broad Contaminant Pollutographs.

Stormwater runoff clearly impacts water quality and ecological health of urban receiving waters. Subsequent management efforts are often guided by conceptual models of contaminant "first flushes", defined by disproportionate concentrations or mass loads early in the storm hydrograph. However, studies examining the dynamics of contaminant transport and receiving water hydrology have primarily focused on "traditional" stormwater contaminants and point sources, with less evaluation of chemically complex nonpoint pollution sources. Accordingly, we conducted baseflow and storm sampling in Miller Creek, a representative small, urban watershed in the Puget Sound region (WA, USA). We comprehensively characterized organic contaminant profiles and dynamics via targeted quantification of 35 stormwater-derived chemicals, complementary nontarget HRMS analyses, and surrogate chemical metrics of ecological health. For quantified analytes, total daily baseflow loads were 0.8-3.4 g/day and storm event loads were ∼80-320 g/storm (∼48 h interval), with nine contaminants detected during storms at >500 ng/L. Notably, urban creek "pollutographs" were much broader than relatively sharp storm hydrographs and exhibited transport-limited (rather than mass-limited) source dynamics, with immediate water quality degradation during low-intensity precipitation and continued mobilization of contaminant mass across the entire hydrograph. Study outcomes support prioritization of source identification and focused stormwater management efforts to improve water quality and promote ecosystem function in small urban receiving waters.

Evaluation of hydrochemical variations in coastal alluvial aquifers of Prakasam district, A.P, India

The unconfined coastal alluvial aquifers, although occurring in limited extent along ocean tracts, cater to the growing water requirements of most of the seaside habitation. By virtue of the physical location, the chemistry of groundwater in marine environment undergoes rapid modifications and is also susceptible to contamination. Through this paper, an attempt is made to chronicle the temporal changes in water chemistry and extent of contamination in the sandy aquifers utilizing the closely monitored data of 13 sampling sessions spread over 4 years. The water is of weak alkaline nature and less mineralized, electrical conductivity (EC) being <1000 μS/cm in many samples. Major ion composition indicates that 60 to 80% of the samples are polluted with nitrates. Apart from nitrate, the water is also contaminated with respect to pH, EC, Ca2+, Mg2+, Cl−, and SO4 2− as about 5 to 20% of samples in different seasons have some of these parameters in more than the maximum permissible limits of Bureau of Indian Standards drinking water standards. Ionic abundance is in the order of Cl− > Na+ > Ca2+ > HCO3 − > Mg2+ > SO4 2− > NO3 −. Na+ and Cl− are almost in similar proportions implying affect of coastal climate on water quality. Even though spatial variations among constituent species are limited, the water can be classified into two distinct facies viz. Ca-Mg-Cl and Na-Cl-SO4 types. The stoichiometry analysis of long-term water chemistry data enunciate that multiple processes were involved in evolution of aquatic chemistry. Water-rock interaction with the aquifer matrix and carbonate dissolution was the most significant. It was further influenced by evaporation process and anthropogenic inputs which are authenticated by mean ionic ratios of Na+/Cl−, SO4 2−/Cl, Mg2+/Ca2+, and HCO3 −/Cl−. Nitrate contamination, which is more rapid in habitat areas, can be attributed to poor sewerage and domestic waste disposal mechanism. Fertilizer inputs and continuous irrigation and agriculture activity can be accounted for the nitrate enrichment in agriculture areas. With the contaminant transport dynamics being uniform in the aquifer, difference in the intensity of contamination can be attributed to land use variations. Point source pollution from domestic and animal waste landfills is causing enormous damage to the groundwater. The study substantiate that shallow phreatic coastal aquifers suffers more from human interference than coastal climate effect.

Model of Non-equilibrium Multiphase Contaminant Transport in Lake Water-Sediment System

The model of contaminant transport between the abiotic components of the lake water-sediment system is presented. The implementation of the model, which uses radioactive material as a contaminant, and the evaluation of parameter values are discussed in the work. A semi-analytical solution of the simulation of the dynamics of contaminant transport in the lake water and one-dimensional sediment solute transport including non-equilibrium processes is presented. The model includes a concept of dynamics of the contaminant sorption in lake water and sediment compartments, considering the specific porous structure of sediments, the contaminant material exchange between the liquid and solid phases of sediments. The key processes included in the model are sedimentation, resuspension, diffusive exchange of solute at the lake water-sediment interface and advection-diffusion in sediment solute. Modelling of the contaminant transport in both spheres (lake water and bottom sediments) is influenced by non-equilibrium and transformation reactions. Special attention was paid to the contamination balance between the two spheres in the interface area. The ranges of boundary and initial conditions were extended, and final results were obtained using an accurate and robust numerical inversion calculation based on the De Hoog algorithm. Therefore, the model can be used in experimental measurements interpreting the contaminant profile in lake sediments as well as a part associated with the comprehensive determination of the volumetric activity, in the estimation of irradiation doses due to radionuclides released into the lake water.

Computer modeling of surface interactions and contaminant transport in microstructures during the rinsing of patterned semiconductor wafers

Rinsing microstructures on a patterned semiconductor wafer is modeled. The simulation results are presented for two cases when the surfaces of a trench as the microstructure are made of a single material, or two different materials. The dynamics of contaminant removal from the microstructure surfaces and its dependence on the geometrical structure, physical characteristics of the surfaces, and contaminant diffusivity are presented. The results show that in the case of a trench with two different materials, the cleaning dynamics of the trench bed strongly depends on the stacking order of the materials. When the upper material has a smaller desorption rate coefficient, the dynamics of contaminant transport develops a smaller rate at some point in time that depends on the thickness of the layers.

The combined use of heat-pulse flowmeter logging and packer testing for transmissive fracture recognition

This paper presents an improved borehole prospecting methodology based on a combination of techniques in the hydrogeological characterization of fractured rock aquifers. The approach is demonstrated by on-site tests carried out in the Hoshe Experimental Forest site and the Tailuge National Park, Taiwan. Borehole televiewer logs are used to obtain fracture location and distribution along boreholes. The heat-pulse flow meter log is used to measure vertical velocity flow profiles which can be analyzed to estimate fracture transmissivity and to indicate hydraulic connectivity between fractures. Double-packer hydraulic tests are performed to determine the rock mass transmissivity. The computer program FLASH is used to analyze the data from the flowmeter logs. The FLASH program is confirmed as a useful tool which quantitatively predicts the fracture transmissivity in comparison to the hydraulic properties obtained from packer tests. The location of conductive fractures and their transmissivity is identified, after which the preferential flow paths through the fracture network are precisely delineated from a cross-borehole test. The results provide robust confirmation of the use of combined flowmeter and packer methods in the characterization of fractured-rock aquifers, particularly in reference to the investigation of groundwater resource and contaminant transport dynamics.

Laboratory and numerical investigation of transport processes occurring above and within a saltwater wedge

Salt wedges divide coastal groundwater flow regime into two distinct regions that include a freshwater region above the saltwater-freshwater interface and a saltwater region below the interface. Several recent studies have investigated saltwater transport in coastal aquifers and the associated flow and mixing processes. Most of these studies, however, have either focused on studying the movement of salt wedge itself or on studying contaminant transport processes occurring above the wedge. As per our knowledge, so far no one has completed laboratory experiments to study contaminant transport processes occurring within a saltwater wedge. In this study, we completed laboratory experiments to understand contaminant transport dynamics occurring within a saltwater wedge. We used a novel experimental approach that employed multiple neutral-density tracers to map and compare the mixing and transport processes occurring above and within a saltwater wedge. The experimental data were simulated using SEAWAT, and the model was used to further investigate the saltwater flow and transport dynamics within a wedge. The laboratory data show that the transport rates active within the wedge are almost two orders of magnitude slower than the transport rates active above the wedge for the small-scale experimental system which is characterized by very low level of mixing. The numerical results, however, postulate that for large-scale systems involving higher levels of mixing (or dispersion) the transport rate active within the wedge could be comparable or even higher than the rates active above the wedge. More field or laboratory studies completed under high dispersion conditions are needed to further test this hypothesis.

Integrated Dynamic Modeling of Contaminant Fate and Transport within a Soil–Plant System

Contaminants found in the soil and in the atmosphere frequently find their way into plants. Because plants are at the bottom of the food chain, analysis of this pathway contributes to health risk assessment studies. On the other hand, plants that exist at a contaminated site have a potential effect on contaminant transformation and migration within the soil or the atmosphere. In this study, a modeling framework was developed to integrate the plant pathway into soil contaminant transport models. A soil–plant system model was developed by coupling soil moisture distribution, contaminant transport, plant life cycle, and plant pathway models. The outcome unifies single‐medium continuous models with multimedia compartmental models in a flexible framework. The framework recognizes that plants are dynamic biologic systems that regulate their life cycle in interaction with existing conditions in the ambient environment, which significantly influence the dynamics of the overall complex system. The model developed was applied to a hypothetical contamination scenario where the effect of plants on contaminant migration within the system was investigated. Also, the outcome of the plant pathway as it responds to water flow and contaminant transport dynamics was analyzed. A mass balance analysis found that the processes of volatilization and root water uptake are very critical in determining the contaminant fate within the system. A sensitivity analysis showed that the contaminant concentration within the plant is significantly affected by the variation in the values of the retardation factor, transpiration stream concentration factor, and contaminant half‐life within the plant. The outcome of these applications reflects the effect of multiple levels of complexity associated with plant growth and root water uptake representations within the soil.

Computer programs for estimating substrate flux into steady‐state biofilms from pseudoanalytical solutions

AbstractFixed‐film processes employing microorganisms attached to an inert surface (biofilms) are widely used for biological treatment of municipal and industrial wastewater. For optimal design and analysis of these processes, mathematical models are necessary that describe the dynamics of contaminant transport within these biofilms and the associated contaminant utilization by the microorganisms. However, these governing equations that typically involve Fickian diffusion for contaminant transport and Monod kinetics for contaminant utilization are inherently nonlinear and have no closed form solutions except under special conditions. This can restrict their use in the classroom as cumbersome numerical techniques must be used for their solution. This problem is well documented in the literature and several authors have presented pseudoanalytical solutions that replace numerical solutions with algebraic equations. In the present study, we present pseudoanalytical solution‐based computer programs for estimating substrate flux and biofilm thickness for a steady‐state biofilm. Depending upon the intended end use, these programs can either partially or totally automate the solution process. In the partial automation mode, they can serve to enhance student understanding of important concepts related to steady‐state biofilms, while complete automation can help bring more challenging and realistic problems associated with steady‐state biofilms into the classroom. The programs have been tested on MATLAB version 5.0 and are available as freeware for educational purposes. © 2002 Wiley Periodicals, Inc. Comput Appl Eng Educ 10: 26–32, 2002; Published online in Wiley InterScience (www.interscience.wiley.com.); DOI 10.1002/cae.10017

Sewage contamination in the upper Mississippi River as measured by the fecal sterol, coprostanol

The molecular sewage indicator, coprostanol, was measured in bed sediments of the Mississippi River for the purpose of determining sewage contamination. Coprostanol is a non-ionic, non-polar, organic molecule that associates with sediments in surface waters, and concentrations of corpostanol in bed sediments provide an indication of long-term sewage loads. Because coprostanol concentrations are dependent on particle size and percent organic carbon, a ratio between coprostanol (sewage sources) and cholestanol + cholesterol (sewage and non-sewage sources) was used to remove the biases related to particle size and percent organic carbon. The dynamics of contaminant transport in the Upper Mississippi River are influenced by both hydrologic and geochemical parameters. A mass balance model incorporating environmental parameters such as river and tributary discharge, suspended sediment concentration, fraction of organic carbon, sedimentation rates, municipal discharges and coprostanol decay rates was developed that describes coprostanol concentrations and therefore, expected patterns of municipal sewage effects on the Upper Mississippi River. Comparison of the computed and the measured coprostanol concentrations provides insight into the complex hydrologic and geochemical processes of contaminant transport and the ability to link measured chemical concentrations with hydrologic characteristics of the Mississippi River.

Physical and geochemical characteristics of suspended solids, Wilton Creek, Ontario

To understand the nature of sediment-associated nutrient and contaminant transport dynamics in fluvial systems, a stormflow sampling program of suspended solids is reported for one water year in a representative rural diffuse source catchment of southeastern Ontario. Bulk samples of subsieve suspended solids were obtained using field-portable continuous-flow centrifuge apparatus. The physical and geochemical properties of suspended solids show no significant intersite differences over reaches of 1500–2000 m, yet display distinctive seasonal trends. Systematic seasonal changes in particle size, organic content, and Ca, P, Mn, Al, Ti, Fe, and K appear to reflect the changing role of partial area hydrology. Ca, P, and Mn are bioaccumulated by stream algae. Mineral signature is relatively constant over the year.