Neil R. Thomson

highway stormwater runoff quality: development of surrogate parameter relationships

The development of relationships for predicting the impact of highway stormwater runoff is described. The predictive relationships are regression-based equations reflecting variations in the magnitude of the constituents of interest. The objective is to identify a subset of constituents that can be used as surrogates for the remaining constituents as a means of decreasing the costs of collection and measurement of highway stormwater runoff quality data.The Minnesota highway stormwater quality database complied in the late 1970s and early 1980s is employed in identifying the set of surrogate parameters. The findings indicate that total suspended solids, total dissolved solids, total volatile solids, and total organic carbon are effective surrogate parameters for numerous metals, ionic species, and nutrients. The findings also indicated that the developed ionic species constituent relationships are portable, while the metal and nutrient constituent relationships were limited to urban sites with similar environmental conditions. The development of these relationships represents a step towards remediating and preventing the problem of nonpoint source highway pollution.

Laboratory-scale in situ chemical oxidation of a perchloroethylene pool using permanganate

In situ chemical oxidation (ISCO) is an emerging technology for the destruction of some chlorinated solvents present in subsurface environments. A laboratory investigation using a physical model was designed to assess the effectiveness of using permanganate as an oxidant to reduce the mass of a perchloroethylene (PCE) pool. The physical model was filled with silica sand overlying a silica flour base, simulating a two-dimensional saturated sand zone overlying a capillary barrier. PCE was introduced into the model so that it rested on top of the silica flour base, forming a dense nonaqueous phase liquid pool. The experimental methodology involved flushing the model with a permanganate solution for 146 days. During this period, measurements of chloride were used to assess the extent of pool oxidation. Before and after the oxidant flush, the quasi-steady state dissolution from the PCE pool was evaluated. Additionally, tracer studies were completed to assess changes in the flow field due to the oxidation process. At the termination of the experiment nine soil cores extracted from the model were used to detect the presence of MnO2 deposits and to quantify the mass of PCE remaining in the system. Excavation of the remaining material in the model revealed that the MnO2 distribution throughout the model was consistent with that observed in the cores. The oxidant flush was concluded before all of the pure phase PCE had been completely oxidized; however, approximately 45% of the PCE mass was removed, resulting in a fourfold decrease in the quasi-steady state aqueous phase mass loading of PCE from the pool. Measurements of chloride during the oxidant flush and of PCE in the soil cores suggested that the oxidation reaction occurred primarily at the upgradient edge of the PCE pool. MnO2 deposits within the model aquifer decreased the velocity of water directly above the pool, and the overall mass transfer from the remaining PCE pool. The results of this experimental study indicate that ISCO using permanganate is capable of removing substantial mass from a DNAPL pool; however, the performance of ISCO as a pool removal technology will be limited by the formation and precipitation of hydrous MnO2 that occurs during the oxidation process.

Relaxation of transient ionization in the lower ionosphere

This paper presents a set of basic expressions and parameters to describe the relaxation of ionization in the upper atmosphere and lower ionosphere. Both spatial (diffusion) and chemical loss are included, along with the varying electrical conductivity caused by changing electron density and temperature. In particular, reasonable values for the attachment rates and recombination coefficients at these high altitudes are discussed. Example calculations are given involving studies into red sprites. However, the modeling and parameters are not restricted to these events, and might well be useful in a number of other modeling problems.

Sunrise effects on VLF signals propagating over a long north-south path

We present a detailed study of the times of amplitude minima observed on the 12-Mm path from NAA (24 kHz, 1 MW, Cutler, Maine) to Faraday, Antarctica, during the period 1990–1995. (NAA is a naval transmitter call sign.) This study represents the first account of the effect of the sunrise terminator when it is parallel to a propagation path at some times of the year. Since the NAA-Faraday path is within 3° of the north-south meridian, parallel orientation happens close to the equinoxes, while the maximum angle of incidence occurs during the solstices. During the solstices the terminator takes a significant length of time to cross the entire propagation path, so modal conversion effects are observed over a range of hours. During the equinoxes, however, the leading edge of the night-day transition region crosses the whole propagation path within 20 min. The interpretation of the timing of minima is consistent with modal conversion taking place as the sunrise terminator crosses the NAA-Faraday transmission path at specific, consistent locations. The timing of minima is remarkably consistent from year to year. Long wave propagation modeling is used to show that the location of nightside minima at an altitude of 45–75 km in the subionospheric waveguide represents the location of the sunrise terminator on the great circle path when dayside minima occur.

Two‐phase flow in a variable aperture fracture

In this paper a dynamic two-dimensional two-phase flow model for a single variable aperture fracture is developed. The model is based on a finite volume implementation of the cubic law and the conservation of mass for each liquid. The two-phase fracture flow system is represented by incompressible parallel plate flow within two-dimensional subregions of constant aperture. The fluid phase distribution is represented by an explicit definition of the phase presence at each location within the domain. To achieve this definition, a phase distribution is assigned to each fracture subregion. Knowledge of the phase distribution allows calculation of interface capillary pressure based on the fracture aperture. One-dimensional analytic solutions for two-phase flow are developed and used to verify the models behavior in one dimension. The model is verified against the Sandia Waste-Isolation Flow and Transport III model for the case of two-dimensional single-phase flow. Two-dimensional two-phase flow verification is performed qualitatively because no suitable analytic or physical model is currently available. Two-dimensional flow phenomena are investigated for variable aperture fractures generated using geostatistical methods. Results from these simulations illustrate the flow processes of phase isolation, pinching off of nonwetting phase globules, nonwetting phase refusal at the edges of tight regions, and downslope migration of a fluid countercurrent to flow of a less dense fluid.

Persistence of persulfate in uncontaminated aquifer materials.

Batch and stop-flow column experiments were performed to estimate persulfate decomposition kinetic parameters in the presence of seven well-characterized aquifer materials. Push-pull tests were conducted in a sandy aquifer to represent persulfate decomposition under in situ conditions. The decomposition of persulfate followed a first-order rate law for all aquifer materials investigated. Reaction rate coefficients (k(obs)) increased by an order of magnitude when persulfate concentration was reduced from 20 g/L to 1 g/L, due to ionic strength effects. The column experiments yielded higher k(obs) than batch experiments due to the lower oxidant to solids mass ratio. The kinetic model developed from the batch test data was able to reproduce the observed persulfate temporal profiles from the push-pull tests. The estimated k(obs) indicate that unactivated persulfate is a persistent oxidant for the range of aquifer materials explored with half-lives ranging from 2 to 600 d.

Experimental determination of the kinetic rate law for the oxidation of perchloroethylene by potassium permanganate

Flushing soils contaminated with trichloroethylene (TCE) and perchloroethylene (PCE) with a permanganate (MnO4-) solution has been shown to reduce the solvent content of the soil. Experiments were performed to quantify the rate at which KMnO4 oxidizes aqueous solutions of PCE over a range of concentrations. In a series of homogeneous reactors, aqueous phase PCE concentrations were monitored over time in nine experimental trials with excess oxidant concentrations ranging from 5 to 30 g/l. Analysis of the data was performed to quantify the oxidation reaction order with respect to PCE and KMnO4 and the reaction rate constant. The reaction between PCE and KMnO4 was determined to be first-order with respect to both PCE and KMnO4 with an overall specific reaction rate coefficient of 2.45+/-0.65 M(-1) min(-1).

A numerical investigation into factors affecting gas and aqueous phase plumes in the subsurface

Abstract An investigation into the face and transport of volatile organic compounds (VOCs) in the subsurface requires the consideration of contaminant mass in both the aqueous and soil gas phases. As a result of water/gas phase partitioning, contaminated by partitioning from underlying ground water pollution. Conversely, soil gas can be contaminated by partitioning from underlying ground water VOC plumes. This soil gas and aqueous phase interaction has motivated the popularity of soil gas sampling technology as a method of characterizing ground water VOC contamination. A finite-element-based numerical model was developed to accurately simulate the interaction between the soil gas phase and the aqueous phase. This interaction is complicated since the saturation of the aqueous phase varies dramatically across the capillary fringe. The two-phase flow equations for gas and water are used to describe the flow regime, while the advective-dispersive transport of the VOC is considered in both phases. Dissolution and volatilization from a non-mobile non-aqueous phase liquid is included as a volatile organic contaminant source. A deforming mesh allows the model to accurately track the water table movement, and a Eularian-Lagrangian formulation is used to control some of the numerical difficulties associated with the numerical solution of the advection-dispersion equation. An investigation into diffusion of a VOC from below the water table demonstrated that both the frequency and the magnitude of water table fluctuations have a profound influence on the degree of soil gas contamination. Two-dimensional large-scale, long-term simulations were performed to estimate the aqueous and soil gas phase plumes resulting from an immobilized trichloroethylene residual located in the unsaturated zone. The simulation results indicate that these plumes are very sensitive to the vertical position of the contaminant source. In addition, it was determined that seasonal fluctuations in soil gas VOC concentrations are primarily controlled by temperature fluctuations, while ground water VOC concentration fluctuations are primarily a result of infiltration fluctuations.

Investigating seismoionospheric effects on a long subionospheric path

We examine the possibility of earthquake precursors influencing the subionospheric propagation of VLF transmissions. We consider the long (12 Mm) path from northeastern United States to Faraday, Antarctica (65°S, 64°W), during 1990–1995 and investigate the subionospheric amplitude variation of signals from the NAA communication transmitter (24.0 kHz, 1 MW) in Cutler, Maine, with particular emphasis on possible changes induced by seismic events occurring in South America. We have analyzed the changes in timing of modal minima generated by the passage of the sunrise terminator over the Andes, i.e., the “VLF terminator time” (TT) method. The anomalous variations in timing throughout the year are of a size and occurrence frequency similar to those previously reported, i.e., ±0.5–1 hour and 1–2 per month. However, we find that in these anomalous cases, the time of the sunrise modal minimum does not change significantly, but rather, the minimum becomes insufficiently deep to be detected, and the time of the next nearest minimum is logged. Our analysis indicates that the occurrence rate of successful earthquake predictions using the TT method cannot be distinguished from that of chance. Additionally, the level of false earthquake prediction using the TT method is high.

VLF line radiation observed by satellite

VLF line radiation received by the ISIS 1 and 2 satellites over New Zealand is found to fall into two distinct classes. The first of these consists of magnetospheric lines (MLs) which are characterized by a broadband appearance and by frequency drifts of a few tens of hertz per minute, similar to those reported elsewhere. Both their initial frequencies and their frequency spacings were, however, found to be essentially random rather than multiples of 50 or 60 Hz. The hourly variation in ML occurrence showed no correlation with electrical load in possible electrical mains systems sources. No clear decrease in occurrence on weekends was evident. This first satellite survey with significant numbers of MLs found no evidence of a relationship with power line harmonics. The second class of satellite-received VLF lines consists of “tram lines” (TLs) which are characterized by their very narrow bandwidth and zero frequency drift. TLs appear to lie close to harmonics of 50 or 60 Hz. An example of this class is presented.