James W. Weaver

A screening model for nonaqueous phase liquid transport in the vadose zone using Green‐Ampt and kinematic wave theory

In this paper, a screening model for flow of a nonaqueous phase liquid (NAPL) and associated chemical transport in the vadose zone is developed. The model is based on kinematic approximation of the governing equations for both the NAPL and a partitionable chemical constituent. The resulting governing equation is a first-order, quasi-linear hyperbolic equation to which the generalized method of characteristics can be applied. This approach generally neglects the contribution to the NAPL flux from capillary pressure gradients. During infiltration under ponded conditions, or when the NAPL flux exceeds the maximum effective conductivity of the soil, the effect of capillary suction is included in the model through the usage of the Green-Ampt model. All of the resulting model equations are in the form of ordinary differential equations which are solved numerically by a variable time step Runge-Kutta technique. Results from a simple column experiment were used to evaluate the vadose zone flow model assumptions. Independently measured parameters allow simulation without calibration of the model results. The match of the model to the data suggests that the model captures the qualitative behavior of the experimental system and is capable of an acceptable degree of quantitative agreement.

DEVELOPMENT AND VERIFICATION OF A SCREENING MODEL FOR SURFACE SPREADING OF PETROLEUM

Overflows and leakage from aboveground storage tanks and pipelines carrying crude oil and petroleum products occur frequently. The spilled hydrocarbons pose environmental threats by contaminating the surrounding soil and the underlying ground water. Predicting the fate and transport of these chemicals is required for environmental risk assessment and for remedial measure design. The present paper discusses the formulation and application of the Oil Surface Flow Screening Model (OILSFSM) for predicting the surface flow of oil by taking into account infiltration and evaporation. Surface flow is simulated using a semi-analytical model based on the lubrication theory approximation of viscous flow. Infiltration is simulated using a version of the Green and Ampt infiltration model, which is modified to account for oil properties. Evaporation of volatile compounds is simulated using a compositional model that accounts for the changes in the fraction of each compound in the spilled oil. The coupling between surface flow, infiltration and evaporation is achieved by incorporating the infiltration and evaporation fluxes into the global continuity equation of the spilled oil. The model was verified against numerical models for infiltration and analytical models for surface flow. The verification study demonstrates the applicability of the model.

Quantifying Bank Storage of Variably Saturated Aquifers

Numerical simulations were conducted to quantify bank storage in a variably saturated, homogenous, and anisotropic aquifer abutting a stream during rising stream stage. Seepage faces and bank slopes ranging from 1/3 to 100/3 were simulated. The initial conditions were assumed steady-state flow with water draining toward the stream. Then, the stream level rose at a constant rate to the specified elevation of the water table given by the landward boundary condition and stayed there until the system reached a new steady state. This represents a highly simplified version of a real world hydrograph. For the specific examples considered, the following conclusions can be made. The volume of surface water entering the bank increased with the rate of stream level rise, became negligible when the rate of rise was slow, and approached a positive constant when the rate was large. Also, the volume decreased with the dimensionless parameter M (the product of the anisotropy ratio and the square of the domains aspect ratio). When M was large (>10), bank storage was small because most pore space was initially saturated with ground water due to the presence of a significant seepage face. When M was small, the seepage face became insignificant and capillarity began to play a role. The weaker the capillary effect, the easier for surface water to enter the bank. The effect of the capillary forces on the volume of surface water entering the bank was significant and could not be neglected.

A screening model for simulating DNAPL flow and transport in porous media: theoretical development

In the last two decades there has been an increased awareness of the contamination of groundwater due to the presence of denser-than-water nonaqueous phase liquids (DNAPLs). Numerous theoretical, experimental and numerical investigations have been conducted to study the various processes that impact aquifer contamination. These studies have provided us with greater insight into the individual processes and the complex nature of the problem. In spite of this progress, there still exists a need within the environmental community for a simple tool that will allow us to analyze a DNAPL contamination scenario from free-product release to transport of soluble constituents to downgradient receptor wells. Such a model may be useful in source term characterization for DNAPL releases to groundwater. The objective of this manuscript is to present the conceptual model and formulate the equations and modules which are utilized in this screening model. Three hypothetical releases are simulated and the results discussed to demonstrate the application and usefulness of this model. Due to its simplicity and ease of use, this screening model will be useful to industry, regulatory agencies and educators for estimating the impact of a DNAPL release on an aquifer.

Derivation Of Spreading Parameters Of Oil At Sea For Oil Spill Modeling

ABSTRACT Oil spill models are commonly used to simulate the large-scale (tens to hundreds of kilometers) transport of oil spills in the oceans. The values of the spreading parameters of these models are obtained empirically by fitting to observed slicks, thus they do not account explicitly for the effects of waves. In addition, there is little success in using these values to predict the spread at smaller scales (tens of meters to a few kilometers). This works attempts to better understand the physics of oil movement in the ocean by focusing on the small-scale mechanisms. The investigation also leads to evaluation of small-scale spreading parameters. The Random Walk Method is used in a Monte Carlo simulation framework to track the transport of oil due to the effects of waves, buoyancy, and turbulent diffusion. The small-scale spreading parameters are then calculated using the Method of Moments. Our results indicated that the approach for using a spreading coefficient becomes after a time equal to about 30...

Estimating the Rate of Natural Bioattenuation of Ground Water Contaminants by a Mass Conservation Approach

Recent field and experimental research has shown that certain classes of subsurface contaminants can biodegrade at many sites (e.g., Borden. etal. 95, Lovely. etal. 94). A number of factors influence the rate of biodegradation, which helps determine the ultimate extent of contamination. Estimation of degradation rate constants is necessary for determining the impact of contaminants on environmental receptors. The purpose of this note is to show one method for estimating rate constants from field data that relies upon application of a standard solution of a linear ordinary differential equation (ODE).

THE MOVEMENT OF OIL AT SEA DUE TO IRREGULAR WAVES

ABSTRACT Our previous work investigated the transport of oil under regular waves at sea. This work considered irregular waves represented by a JONSWAP spectrum, which is a more realistic situation. Particle tracking was used in a Monte Carlo framework to evaluate the combined effects of wave kinematics and turbulent diffusion on the transport of oil droplets at sea. The centroids, variance and spreading coefficients of oil spills with various wave parameters were found in this study. Turbulent diffusion was assumed to be velocity-dependent, and an empirical formulation adopted from subsurface hydrology was adopted. Five hundred neutrally-buoyant oil “particles” were placed at the water surface and tracked for 1 hour. The vertical movement of the plume appeared to be comparable to the significant wave height (about one meter herein), and to decrease with depth. The increase in wind fetch caused an increase in transport and spreading of the plume. The results found in this study can be used by spill respond...

Exposure Assessment Modeling for Hydrocarbon Spills into the Subsurface: Sensitivity to Soil Properties

Hydrocarbons which enter the subsurface through spills or leaks may create serious, long-lived ground-water contamination problems. Simplified models of the separate phase flow of the hydrocarbon and its dissolution into ground water may be appropriate for gaining insight into the significant phenomena, emergency response, or generic simulation for regulatory development. The paper outlines the components of a set of screening models for this problem and focuses on parameter sensitivity. Tabulated values of soil properties are used to model releases in typical soil materials. The availability of standard deviations of parameter values allows assessment of model response with regard to typical parameter variability. This knowledge has important consequences for emergency response applications which tend to rely on tabulated data instead of site specific data. Ultimate interest usually lies with dissolved aqueous concentrations, so the parameter sensitivity is assessed through concentrations predicted for down-gradient wells, as well as other significant aspects of the model results.