On Evaluating Characteristics of the Solute Transport in the Arid Vadose Zone

The transport of bromide (Br) under matric heads of 0, ?2, ?5, and ?10 cm using undisturbed soil columns was investigated for understanding the solute transport in arid soils. Undisturbed soil cores were collected at ground surface, directly below where tension infiltrometer measurements were made in the Amargosa Desert, Nevada, United States. Laboratory experiments were conducted by introducing water containing Br tracer into a soil column maintained at steady‐state conditions. The observed data of breakthrough curves (BTC) were well fitted to an one‐region model, except for the cores at saturation, and a core at the matric head of ?5 cm, from which the observed data were better fitted to a two‐region model. Fitted pore water velocities with the one‐region model ranged from 1.2 to 56.6 cm/h, and fitted dispersion coefficients (D) ranged from 2.2 to 100 cm²/h. Results for the core analyzed with the two‐region model indicated that D ranged from 27.6 to 70.9 cm²/h at saturation, and 25.7 cm²/h at the matric head of ?5 cm; fraction of mobile water (β) ranged from 0.18 to 0.65, and mass transfer coefficient (ω) ranged from 0.006 to 0.03. In summary, the water fluxes and Br dispersion coefficients at investigated matric heads were very high due to the coarseness of the soils and possibly due to preferential flow pathways. These high water fluxes and Br dispersion coefficients would lead to a higher risk of deeper leaching accumulating nitrate nitrogen to the groundwater, and have significant effects on the desert ecosystem.     

Percolation and Particle Transport in the Unsaturated Zone of a Karst Aquifer

Recharge and contamination of karst aquifers often occur via the unsaturated zone, but the functioning of this zone has not yet been fully understood. Therefore, irrigation and tracer experiments, along with monitoring of rainfall events, were used to examine water percolation and the transport of solutes, particles, and fecal bacteria between the land surface and a water outlet into a shallow cave. Monitored parameters included discharge, electrical conductivity, temperature, organic carbon, turbidity, particle-size distribution (PSD), fecal indicator bacteria, chloride, bromide, and uranine. Percolation following rainfall or irrigation can be subdivided into a lag phase (no response at the outlet), a piston-flow phase (release of epikarst storage water by pressure transfer), and a mixed-flow phase (increasing contribution of freshly infiltrated water), starting between 20 min and a few hours after the start of recharge event. Concerning particle and bacteria transport, results demonstrate that (1) a first turbidity signal occurs during increasing discharge due to remobilization of particles from fractures (pulse-through turbidity); (2) a second turbidity signal is caused by direct particle transfer from the soil (flow-through turbidity), often accompanied by high levels of fecal indicator bacteria, up to 17,000 Escherichia coli /100 mL; and (3) PSD allows differentiation between the two types of turbidity. A relative increase of fine particles (0.9 to 1.5 μm) coincides with microbial contamination. These findings help quantify water storage and percolation in the epikarst and better understand contaminant transport and attenuation. The use of PSD as "early-warning parameter" for microbial contamination in karst water is confirmed.     

Analytical Solutions for Flow Fields near Drain-and-Gate Reactive Barriers

Permeable reactive barriers (PRBs) are a popular technology for passive contaminant remediation in aquifers through installation of reactive materials in the pathway of a plume. Of fundamental importance are the degree of remediation inside the reactor (residence time) and the portion of groundwater intercepted by a PRB (capture width). Based on a two-dimensional conformal mapping approach (previously used in related work), the latter is studied in the present work for drain-and-gate (DG) PRBs, which may possess a collector and a distributor drain (“full” configuration) or a collector drain only (“simple” configuration). Inherent assumptions are a homogeneous unbounded aquifer with a uniform far field, in which highly permeable drains establish constant head boundaries. Solutions for aquifer flow fields in terms of the complex potential are derived, illustrated, and analyzed for doubly symmetric DG configurations and arbitrary reactor hydraulic resistance as well as ambient groundwater flow direction. A series of practitioner-friendly charts for capture width is given to assist in PRB design and optimization without requiring complex mathematics. DG PRBs are identified as more susceptible to flow divergence around the reactor than configurations using impermeable side structures (e.g., funnel-and-gate), and deployment of impermeable walls on drains is seen to mitigate this problem under certain circumstances.     

Methods and Results of Numerical Modeling of Reactive Solute Transport in River Flow

The equation of advection–diffusion transport of a reactive solute in a stream is solved by different methods. Procedures for evaluating the parameters of a four-node finite-difference approximation used to numerically integrate this equation are proposed. Numerical experiments based on the developed numerical procedures, software, and models of solute transport allowed, in particular, the evaluation of the sensitivity of pollutant concentration estimates to variations in the parameters of interaction processes and hydrodynamic dispersion for a lowland river.     

Simulation of Vapor Transport Through the Unsaturated Zone — Interpretation of Soil-Gas Surveys

Soil-gas surveys are becoming more widely accepted as a tool for the preliminary determination of the extent of soil and ground water contamination by volatile organic compounds (VOCs). The interpretation of the results of published soil-gas surveys has been necessarily limited and qualitative due to a lack of adequate models. There has been considerable effort in the recent past to characterize the transport and fate of pesticides in soil. However, the behavior of pesticides generally differ substantially from those of VOCs.
This paper presents a computer model developed to simulate the diffusive transport of VOC vapor through unsaturated soils using a two-dimensional, finite-difference, solution to Fick's second law of diffusion. An effective diffusion coefficient that incorporates the effects of tortuosity, moisture content, and soil organic carbon content is computed. Although the model has not been validated due to the unavailability of adequate field or laboratory data, nevertheless, sensitivity analyses demonstrate the importance of soil moisture and, secondarily, organic matter content in controlling the migration of VOC vapor through the unsaturated zone. The interpretation of soil-gas surveys can be complicated by unknown spatial heterogeneities in soil moisture and organic carbon content, temporal variations in moisture content, extent of contaminant migration as a non-aqueous phase liquid and by the unknown extent of VOC liquid and contaminated ground water.     

Analytical Solutions for Steady-State Gas Flow in Layered Soils with Field Applications

Soil vapor extraction (SVE) is widely used to remove volatile organic compounds from the vadose zone. Design of SVE systems rely largely upon vacuum responses and limited vapor concentration data measured during short-term soil gas extraction tests performed in single extraction wells. Interpretation of such vacuum data is often simply a rule of thumb as most field sites have layering complexity negating applicability of existing analytical models. This paper provides the derivation of an analytical model for steady, axisymmetric gas flow in heterogeneous (layered) soils from a single well. A general, variable flow boundary condition along the well screen represents actual conditions more closely than a uniform flow or uniform well pressure condition. Each soil layer is assumed homogeneous with anisotropic gas permeability. The solution is derived using the generalized integral transform technique and includes expressions for vacuum, velocities, and streamlines. The model is applied to the interpretation of multiple well tests at a field site and uses linear superposition to extend the flow model to multi-well extraction. The demonstration site included an array of vacuum monitoring data collected during nine individual well flow tests. A method of normalizing the vacuum data is illustrated that allowed the full data set to be employed in a single calibration effort. The test site also included a surface cap with an apparent vertical permeability two to three orders of magnitude smaller than the sands of the vadose zone. This large permeability contrast posed no difficulties in evaluating the solution.     

Analytical Solutions for Gas Flow Due to Gas Injection and Extraction from Horizontal Wells

Analytical solutions are developed for modeling the transient and steady-state gas pressure and the steady-state streamfunction fields resulting from gas injection and extraction from a pair of parallel horizontal wells. These solutions apply to cases in which the ground surface is open to the atmosphere, and in which the porous media is anisotropic but homogeneous. By neglecting end effects due to the finite length of the wells, the three-dimensional gas flow field is approximated as a two-dimensional cross section perpendicular to the wells. These solutions may be used to develop estimates of the horizontal well system behavior and to analyze horizontal well gas pump tests, and are useful for numerical model verification.     

Unsaturated Zone Flow Processes and Aquifer Response Time in the Chalk Aquifer,Brighton, South East England

The Chalk aquifer is one of the main sources of water in South East England. The unsaturated zone in the aquifer plays an important role controlling the time and magnitude of recharge and is major pathway for contaminant transport to the water table. A range of previous work has addressed flow processes in the Chalk unsaturated zone, but physical understanding is still incomplete. Here we present the results of a study on flow mechanism in the Chalk unsaturated zone using a combination of statistical analysis and novel laboratory methods. The study was undertaken at three sites (North Heath Barn [NHB], Pyecombe East [PE], and Preston Park [PP]) on the Chalk of the Brighton block, South East England. Daily and hourly time series data of groundwater level and rainfall were correlated. The results show that a slower groundwater level response to rainfall occurs during dry seasons (summer and autumn) when the amount of effective rainfall is less than 4 mm/day, with a thicker and drier unsaturated zone. A faster response occurs during wet seasons (winter and spring) when the daily effective rainfall exceeds 4 mm/day with a thinner and wetter unsaturated zone. Periods of very rapid response (within 15 h) were observed during wet seasons at NHB and PE sites, with unsaturated hydraulic conductivity (K_u) inferred to reach 839 mm/day. A slower response was observed at an urbanized site (PP) as a result of reduction in direct recharge due to reduced infiltration, due to presences of impermeable infrastructure covering the area around PP borehole. Laboratory measurements of K_u of the Chalk matrix using a geotechnical centrifuge show variation from 4.27 to 0.07 mm/day, according to the level of saturation. Thus, the rapid responses cannot be linked to matrix flow only but indicate the contribution of fracture and karstic flow processes in conducting water.