Contaminant Residence Time Research Articles

Importance of Grid-Cell Area in the Estimation of Estuarine Residual Fluxes

Estimation of residual fluxes in estuarine cross sections is a very important procedure to establish, among others aspects, residence time of contaminants, circulation pattern, and sediment transport dynamics. However, the analytical procedure to obtain such values is not trivial and is presented in detail, demonstrating the importance of the grid-cell area as a weighting element in the calculation of spatial averages. The procedure is tested with four different grid designs and it is shown that grids with proportional columns and rows are the only ones that do not introduce statistical noise in the estimation of the residual fluxes. The four designs are also tested with data from a cross section of the Bahia Blanca Estuary (Argentina), results again show that the proportional columns and rows grid provide the best approach in calculating residual fluxes.

Passive water sampling for polynuclear aromatic hydrocarbons using lipidcontaining semipermeable membrane devices (SPMDs): Application to contaminant residence times

Lipid-containing semipermeable membrane devices are applied to the quantitative determination of the persistence of polynuclear aromatic hydrocarbons (PAH) in an irrigation water canal, a representative waterway. The uniform and reproducible sampling of the USGS design of lipid-containing SPMDs is exploited to measure PAH half-lives without requiring calculation of water concentrations. The trend in calculated PAH half-lives agrees with that expected if volatilization is a significant mechanism for loss from the canal with more volatile PAHs having shorter half-lives. The higher persistence of phenanthrene compared to anthracene is consistent with the greater stability of phenanthrene and suggests, with other physicochemical evidence, that other (reactive) mechanisms may be involved. The results of this study demonstrate the feasibility of the SPMD method for the measurement of residence times in aquatic and other compartments. Such information is essential for quantifying contaminant behavior in the environment and providing insights into contaminant fate.

Sensitivity analysis of solute transport in fractured porous media

We conducted a sensitivity analysis using a numerical model of fractured porous media to find out how porous media and fracture parameters affect solute transport. The 2‐dimensional, saturated fracture flow and transport code FRACTRAN was used to conduct the simulations. Seven parameters were considered: matrix hydraulic conductivity, matrix porosity, retardation of the matrix, gradient of the flow field, fracture retardation, fracture aperture, and fracture probability (which incorporates fracture spacing and fracture length). A Latin‐hypercube design was used to select a matrix of parameter values that minimized correlations among the design parameters. The results were summarized by examining least square fit of the relationship between the seven parameters and 50% breakthrough time and spread of the breakthrough curves, with nearly identical results. The matrix parameters, in particular hydraulic conductivity and porosity, had the greatest effect. As expected, fracture probability was nearly equivalent in importance. These results indicate that field characterization in fractured porous media should not only emphasize fracture location, which strongly influences directions of contaminant transport, but also matrix properties, which have a major influence on contaminant residence times and breakthrough concentrations.

Tracking a Pollutant in the Lower Fraser River: A Computer Simulation

Abstract A one-dimensional numerical model of the tidal portion of the Fraser estuary has been adapted to simulate the path and residence time of contaminants released in any of the lower reaches of the river. A brief discussion of the model’s original development and purpose is followed by an assessment of its accuracy in predicting currents as compared to water surface elevations. Since the flow computations in this type of model are vertically and laterally integrated, they had to be corrected to represent transport in mid-channel. These correction factors were established by several time series and spot measurements along the river, taking into account the local topography. The results were verified by tracking surface floats during three representative discharges.

Foliar Retention and Leachability of Submicron Plutonium and Americium Particles

AbstractSubmicron particles of Pu and Am were aerosolized, deposited onto foliage of bush bean (Phaseolus vulgaris L.), and their subsequent retention and behavior determined. Particles having mass median diameters of < 1 µm are not readily dislodged from leaf surfaces at wind speeds of −400 m min−1. Under conditions of simulated rainfall, weathering half‐times range from 164 to 1,000 days, and are dependent on both particle size and initial solubility. The residence time of contaminants on foliar surfaces prior to leaching influences subsequent foliar retention and leachate characteristics. Foliar retention of particles ranges from 20 to 92% and is dependent on particle size of Pu‐ and Am‐oxides, chemical form of Pu, and environmental conditions such as humidity, simulated rainfall, and acidity of the simulated rain.

Subsonic jet aircraft and stratospheric pollution

A significant number of thepresent long-range subsonic jet aircraft flights are in the stratosphere. Based on fuel consumption, an equivalent of 107 SST aircraft (Concorde) have been flying in the stratosphere each year from 1960 to 1970 (620 equivalent SST in 1990). Subsonic jets in 1990 will burn about 1.6 times as much fuel in the stratosphere as SST aircraft. Based on a Concorde emission index of 12.5 for NO, the amount of NOx emitted by subsonic jets in the stratosphere from 1960 to 1970 is equivalent to an average of 20 Concordes (444 Concordes in 1990). Available information concerning mixing and transport in the atmosphere indicates that both subsonic jets and the SST will fly in the same atmospheric environment in the upper troposphere and lower stratosphere insofar as dissipation of their exhaust pollutants is concerned. The tropopause is not an effective barrier to mixing of stratospheric and tropospheric air. Many processes contribute to mixing in the lower stratosphere where the residence time of contaminants is of the order of months instead of years.