Emerging organic contaminants in groundwater: A review of sources, fate and occurrence

応用水文 = Applied hydrology

At the watershed scale, soil moisture is the major control for rainfall–runoff response, especially where saturation excess runoff processes dominate. From the ecological point of view, the pools of soil moisture are fundamental ecosystem resources providing the transpirable water for plants. In drylands particularly, soil moisture is one of the major controls on the structure, function, and diversity in ecosystems. In terms of the global hydrological cycle, the overall quantity of soil moisture is small, ∼0.05%; however, its importance to the global energy balance and the distribution of precipitation far outweighs its physical amount. In soils it governs microbial activity that affects important biogeochemical processes such as nitrification and CO2 production via respiration. During the past 20 years, technology has advanced considerably, with the development of different electrical sensors for determining soil moisture at a point. However, modeling of watersheds requires areal averages. As a result, point measurements and modeling grid cell data requirements are generally incommensurate. We review advances in sensor technology, particularly emerging geophysical methods and distributed sensors, aimed at bridging this gap. We consider some of the data analysis methods for upscaling from a point to give an areal average. Finally, we conclude by offering a vision for future research, listing many of the current scientific and technical challenges.

/pdf/soil-moisture-measurement-for-ecological-and-hydrological-1jak2uhu52.pdf

Soil Moisture Measurement for Ecological and Hydrological Watershed-Scale Observatories: A Review

[1] We explore and review the value of soil moisture measurements in vadose zone hydrology with a focus on the field and catchment scales This review is motivated by the increasing ability to measure soil moisture with unprecedented spatial and temporal resolution across scales We highlight and review the state of the art in using soil moisture measurements for (1) estimation of soil hydraulic properties, (2) quantification of water and energy fluxes, and (3) retrieval of spatial and temporal dynamics of soil moisture profiles We argue for the urgent need to have access to field monitoring sites and databases that include detailed information about variability of hydrological fluxes and parameters, including their upscaled values In addition, improved data assimilation methods are needed that fully exploit the information contained in soil moisture data The development of novel upscaling methods for predicting effective moisture fluxes and disaggregation schemes toward integrating large-scale soil moisture measurements in hydrological models will increase the value of soil moisture measurements Finally, we recognize a need to develop strategies that combine hydrogeophysical measurement techniques with remote sensing methods

/pdf/on-the-value-of-soil-moisture-measurements-in-vadose-zone-21mx8q831k.pdf

On the value of soil moisture measurements in vadose zone hydrology: a review

/pdf/concepts-of-hydrological-connectivity-research-approaches-20rhnezwvz.pdf

Concepts of hydrological connectivity: Research approaches, pathways and future agendas

Surface contaminant plumes emanating from waste disposal facilities are often denser than the ambient groundwater. Under certain conditions these dense plumes may become unstable, contaminating larger regions of an aquifer. The behavior of contaminant plumes with different densities was examined in three flow containers packed with homogeneous porous media simulating unconfined aquifers. Glass beads and a medium sand were used as the porous media. A horizontal ambient groundwater stream was established in each flow container using deionized water. The contaminant plumes consisted of NaI or NaBr solutions introduced into each flow container from a line source located on top of the porous medium and extending over the total width of the container. Optical tracers were added to the salt solutions to allow flow visualization. Results show that, for a given porous medium, dense plumes were either stable or unstable depending on the magnitude of the horizontal flow velocity, the contaminant leakage rate, and the density difference between the contaminant solution and the ambient groundwater. When a dense plume becomes unstable, lobe-shaped gravitational instabilities develop which are unsteady and three dimensional. Our experimental results suggest that the behavior of dense contaminant plumes overlying a less dense groundwater stream in a homogeneous porous medium depends on the magnitude of certain nondimensional parameters. It appears that gravitational instabilities begin to develop when the values of these nondimensional parameters exceed certain critical values.

Behavior of dense aqueous phase leachate plumes in homogeneous porous media

https://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.files/fileID/13926

Surfactant enhanced recovery of tetrachloroethylene from a porous medium containing low permeability lenses. 1. Experimental studies.

A review is presented of original multidimensional, intermediate-scale experiments involving nonaqueous phase liquid (NAPL) flow behavior, imaging, and detection and quantification with solute tracers. In a companion paper (Oostrom et al., 2006a), experiments related to aqueous dissolution and enhanced remediation were discussed. The experiments investigating flow behavior include infiltration and redistribution experiments with both light and dense NAPLs in homogeneous and heterogeneous porous medium systems. The techniques used for NAPL saturation mapping for intermediate-scale experiments include photon-attenuation methods such as gamma and X-ray techniques, and photographic methods such as the light reflection, light transmission, and multispectral image analysis techniques. Solute tracer methods used for detection and quantification of NAPL in the subsurface are primarily limited to variations of techniques comparing the behavior of conservative and partitioning tracers. Besides a discussion of the experimental efforts, recommendations for future research at this laboratory scale are provided.

A Review of Multidimensional, Multifluid, Intermediate-Scale Experiments: Flow Behavior, Saturation Imaging, and Tracer Detection and Quantification

For many transport problems involving one-dimensional flow a significant amount of solute might move in the direction transverse to the flow. In this study the advection-dispersion equation was solved analytically to evaluate transient solute transport in a two-dimensional semi-infinite isotropic porous medium (half plane) with a step change in concentration along the inlet during one-dimensional flow. The solution was obtained with Laplace and Fourier transforms and verified with various numerical and analytical solutions. The same technique was used to obtain analytical solutions for some other concentration distributions at the inlet under conditions of either two- or three-dimensional dispersion.

Analytical solutions of the one‐dimensional advection equation and two‐ or three‐dimensional dispersion equation

Simultaneous movement of oil, water, and air in a sandy porous medium was investigated experimentally under transient flow conditions and results were compared to numerical simulations employing a finite element multiphase flow code. The liquid hydrocarbon was Soltrol 170, a low-density branched alkane mixture. Liquid saturations were measured using a collinear dual-energy gamma radiation apparatus and liquid pressures were measured using hydrophilic (untreated) and hydrophobic (treated) ceramic tensiometers connected to pressure transducers. The experimental regime was selected to impose monotonically draining water and total liquid saturation paths to avoid hysteretic effects. Measured saturations and pressures are compared to values obtained from numerical simulations of the experiment using a finite element solution of the governing multiphase flow equations assuming negligible gas pressure gradients. Functional relationships between permeabilities k, saturations S, and capillary pressures P employed in the numerical model were estimated by two calibration methods which require different degrees of experimental effort. Measured transient water saturation versus oil-water capillary head data agreed well with predictions from static air-water S-P relations and interfacial tension data. Transient total liquid saturation versus air-oil capillary head data deviated more severely from the scaled air-water S-P data, possibly reflecting noncompliance with the assumption of negligible gas pressure gradients. Reasonably good agreement was observed between measured and numerically simulated water and oil saturations and pressures in space and time. Sensitivity of the numerical results to calibration method was not great.

/pdf/measurement-and-simulation-of-one-dimensional-transient-37k5gx60w9.pdf

Measurement and simulation of one-dimensional transient three phase flow for monotonic liquid drainage

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