Importance of Infiltration Rates for Fate and Transport of Benzene in High-Tiered Risk-Based Assessment Considering Korean Site-Specific Factors at Contaminated Sites

Widely used conservative approaches for risk-based assessments of the subsurface transport processes have been calculated using simple analytical equations or general default values. Higher-tier risk assessment of contaminated sites requires the numerical models or additional site-specific values for input parameters. Previous studies have focused on the development of sophisticated models fit into risk-based frameworks. Our study mainly aims to explore the applicability of site-specific parameters and to modify the risk-based fate and transport model according to the types of the site-specific parameters. To apply the modified fate and transport equation and the site-specific default infiltration range, this study assessed the source depletion, leachate concentrations, and exposure concentration of benzene, which is a representative organic contaminant. The numerical models consist of two continuous processes, the fate and transport of contaminants from (1) the soil to the groundwater table in the vadose zone and subsequently (2) from the groundwater table to exposure wells in the saturated zone. Spatially varied Korean domestic recharge data were successfully incorporated into site-specific infiltration parameters in the models. The numerical simulation results were expressed as transient time series of concentrations over time. Results presented the narrow range of predicted concentrations at the groundwater table when site-specific infiltration was applied, and the dilution–attenuation factors for the unsaturated zone (DAFunsat) were derived based on the prediction. When a contaminant travels to the longest path length of 10 m with a source depth of 1 m in the vadoze zone, the simulated DAFunsat ranged from 3 to 4. The highest DAFunsat simulation results are close to 1 when contaminants travel to a source depth of 5 m and the shortest path length of 1 m. In the saturated aquifer below the contaminated sites, the variation in exposure concentration with time at monitoring wells is detected differently depending on the depth of the saturated zone.

ECOLOGIA DE PEIXES DE RIACHOS DE CAVERNAS E OUTROS HABITAT SUBTERRÂNEOS

Brazil is rich in caves, with more than 20,000 officially registered. In addition to the caves, which develop in different types of rock, there are other subterranean habitats (hypogean) with bodies of water in the form of drainages (level base streams), outcrops of the water table (saturated zone) in flooded caves or in the form of pools and lakes within caves, in addition to upper aquifers formed by infiltration of water in the rock. In some cases, groundwater does not emerge in caves, but in alluviums close to rivers, representing a hyporeic zone. The Brazilian subterranean ichthyofauna is composed of fish restricted to caves and other subterranean habitats (generally categorized as troglobites / stygobites) or species that have well-established populations in these habitats, but which also occur in streams and bodies of water on the surface (categorized as troglophilics). Currently, there are more than 80 species of fish with troglobitic and troglophilic populations in Brazil. Some population studies show estimates of varying population sizes and densities, generally characterized by small populations; a tendency towards a sedentary lifestyle; low condition factor values and life cycle strategies tending to K within the r-K continuum. These characteristics are related to the unique abiotic conditions of these habitats, such as low, infrequent and often unpredictable supply of nutrients, which can represent an accentuated filter. In relation to conservation, subterranean fish are threatened and most of the species formally described are included in lists of threatened fauna in Brazil. Only four species have been included and evaluated globally (Stygichthys typhlops, Pimelodella kronei, Phreatobius cisternarum and Phreatobius sanguijuela).

Experimental Study on the Lateral Seepage Characteristics in the Tension Saturated Zone

To study the lateral seepage field in the tension saturated zone (TSZ), an experiment with no evaporation and precipitation infiltration was carried out in a self-made seepage tank filled up with fine sand. Based on the data and plots obtained, the lateral seepage field distribution features in the TSZ can be divided into three area for discussion: ascending area, descending area, and the nearly horizontal flow area. In the ascending and descending area, the total water potential gradient diminished from the recharge area to the discharge area and the seepage velocity was faster. In the nearly horizontal flow area, the total water potential gradient was lower and the seepage velocity was slower. The pressure potential gradually decreased horizontally from the recharge area to the discharge area, while in the vertical profile, it gradually decreased from the bottom to the top in the whole seepage area. In the absence of evaporation, the vertical water exchange among the saturated zone, TSZ, and unsaturated zone in nearly horizontal flow area is weak. Contrarily, in the ascending area and descending area, vertical water flows through both the phreatic surface and the upper interface of the TSZ. When there is lateral seepage in the TSZ, the thickness of the TSZ generally increases from the ascending area to the nearly horizontal area and then to the descending area. It should be pointed out that in the nearly horizontal area, the TSZ thickness is approximately equal to the height of the water column. Overall, the lateral seepage in the TSZ can be regarded as a stable siphon process, hence the siphon tube model can be further used to depict this lateral seepage.

Development of spatial permeability variations in English chalk aquifers

The Cretaceous Chalk in England forms dual-porosity aquifers, with low-permeability matrix and high-permeability network of fissures, which are predominantly stress-relief fractures that have been enlarged by dissolution. This enlargement is a function of the volume of water that has passed along a fracture (the flowrate effect) and its degree of chemical undersaturation. Feedback effects result in the development of a distinctive permeability structure, with four particular characteristics: i) troughs in the water table with high transmissivity and convergent groundwater flow; ii) substantial increases in transmissivities in a downgradient direction; iii) downgradient decreases in hydraulic gradient; and iv) discharge from the high-transmissivity zones to the surface commonly at substantial springs. This distinctive self-organised permeability structure occurs throughout unconfined chalk aquifers. Early enlargement of fissures at a depth of 50 - 100 m below the water table is slow, but is much more rapid close to the water table and in the uppermost bedrock due to non-linear dissolution kinetics. A modelled dissolution profile shows that more than 95% of dissolution takes place in the top 1 m of bedrock, and that enlargement of fissures in the saturated zone results from progressive dissolution occurring over a period of a million years or more.

Coupling unsaturated and saturated zone reactive transport : Development and benchmarking of the MTHP tool

<p>MTHP stands for <strong>M</strong>odflow <strong>T</strong>ransport <strong>H</strong>ydrus <strong>P</strong>hreeqC and aims to provide an effective coupling tool for simulating reactive transport in the unsaturated and saturated zones. It builds upon the existing codes HYDRUS-1D (Šimůnek et al., 2013) / HP1 (Jacques et al., 2018), MODFLOW (Harbaugh, 2005), MT3D-USGS (Bedekar et al., 2016) and PhreeqC (Parkhurst and Appelo, 2013).</p> <p>Two-way coupling between HYDRUS 1-D and MT3D-USGS has been implemented. HYDRUS-1D provides a mass flux of solute to the topmost saturated cell in MT3D-USGS. After one time step of solute transport has been solved in groundwater, the resulting solute concentration profile in the saturated zone is updated in HYDRUS. The code has been benchmarked against HYDRUS for a 1-D case but still requires to be adapted for 2 and 3-D cases when solute concentrations change in the unsaturated zone following lateral transport in groundwater.</p> <p>Coupling PhreeqC to HYDRUS 1-D was already implemented within HP1. Simulating geochemical reactions in the aquifer required coupling MT3D-USGS to PhreeqC. This has been implemented by adding a new module MCP (<strong>M</strong>ulti<strong>C</strong>omponent <strong>P</strong>ackage) to the MT3D-USGS code using a similar versatile approach as in HPx. MCP has been successfully benchmarked against examples from the similar PHT3D code (Prommer and Post, 2010).</p> <p>An application of this new module MCP for the simulation of redox plume development from a landfill, is presented. In this case study, reactive transport in the unsaturated zone is not included (i.e. only the MT3D-USGS – PhreeqC coupling is used), as the contamination source is suitably conceptualized to be at the water table surface. Kinetic degradation of dissolved organic carbon (DOC) in the presence of several electron acceptors is simulated. Observations of ion concentrations at different points in space and time are used to calibrate the MTHP simulations and investigate what is the acceptable level of process and parameter simplification.</p> <p>This research is part of the RESPONSE project, funded by the Belgian Science Policy within the framework of the BRAIN-be programme (contract BR/165/A2/RESPONSE).</p> <p><strong>References</strong></p> <p>Bedekar, V., Morway, E.D., Langevin, C.D., and Tonkin, M., 2016, <em>MT3D-USGS version 1: A U.S. Geological Survey release of MT3DMS updated with new and expanded transport capabilities for use with MODFLOW</em>: U.S. Geological Survey Techniques and Methods 6-A53, 69 p.</p> <p>Harbaugh, A.W., 2005, <em>MODFLOW-2005, The U.S. Geological Survey Modular Ground-Water Model — the Ground-Water Flow Process</em>, U.S. Geological Survey Techniques and Methods.</p> <p>Jacques, D., Šimůnek, J., Mallants, D., and van Genuchten, M. T., 2018, The HPx software for multicomponent reactive transport during variably-saturated flow: Recent developments and applications. <em>J. Hydrol. Hydromech</em>, 66(2), 211-226.</p> <p>Parkhurst, D.L. and Appelo, C.A.J., 2013, <em>Description of input and examples for PHREEQC version 3--A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations</em>: U.S. Geological Survey Techniques and Methods, book 6, chap. A43, 497 p.</p> <p>Prommer, H., and Post, V., 2010, <em>PHT3D: A Reactive Multicomponent Transport Model for Saturated Porous Media</em>, Version 2.10 User’s Manual.</p> <p>Šimůnek, J., Šejna, M., Saito, H., Sakai, M., and van Genuchten, M. Th., 2013, <em>The Hydrus-1D Software Package for Simulating the Movement of Water, Heat, and Multiple Solutes in Variably Saturated Media, Version 4.17</em>, HYDRUS Software Series 3, Department of Environmental Sciences, University of California Riverside, Riverside, California, USA, 342 p. </p>