When Can Inverted Water Tables Occur Beneath Streams?

Decline in regional water tables (RWT) can cause losing streams to disconnect from underlying aquifers. When this occurs, an inverted water table (IWT) will develop beneath the stream, and an unsaturated zone will be present between the IWT and the RWT. The IWT marks the base of the saturated zone beneath the stream. Although a few prior studies have suggested the likelihood of an IWT without a clogging layer, most of them have assumed that a low‐permeability streambed is required to reduce infiltration from surface water to groundwater, and that the IWT only occurs at the bottom of the low‐permeability layer. In this study, we use numerical simulations to show that the development of an IWT beneath an unclogged stream is theoretically possible under steady‐state conditions. For a stream width of 1 m above a homogeneous and isotropic sand aquifer with a 47 m deep RWT (measured in an observation point 20 m away from the center of the stream), an IWT will occur provided that the stream depth is less than a critical value of 4.1 m. This critical stream depth is the maximum water depth in the stream to maintain the occurrence of an IWT. The critical stream depth decreases with stream width. For a stream width of 6 m, the critical stream depth is only 1 mm. Thus while theoretically possible, an IWT is unlikely to occur at steady state without a clogging layer, unless a stream is very narrow or shallow and the RWT is very deep.     

Case studies of earthquake-induced effects on concrete channels

Case studies are presented for two concrete lined channels shaken by strong ground motions during the 1994 Northridge earthquake, the high speed channel (HSC) and bypass channel (BC), on the Los Angeles Department of Water and Power’s Van Norman Complex. Performances of these two channels as they pass through different subsurface materials identify several important seismic aspects and the critical role of geotechnical earthquake engineering in assessing channel behaviors. Preliminary evaluations using deta...     

Inhomogeneous distribution of polycyclic aromatic hydrocarbons in different size and density fractions of contaminated sediment from Auckland Harbour, New Zealand: an opportunity for mitigation

Polycyclic aromatic hydrocarbons (PAHs) in sediment from Auckland Harbour (New Zealand) are not distributed evenly throughout bulk sediment, but highly concentrated in coarser, low-density fractions. Concentrations of 24 PAHs, measured in sediment that was separated into six size fractions that were furthermore separated into two density sub-fractions by flotation in sodium-polytungstate solution (rho = 2.15 g cm(-3)), varied between 4-103 microg g(-1)dw among grain size fractions and 2-998 microg g(-1)dw for density sub-fractions. Highest PAH concentrations were measured in the low density, 125-250 microm fraction. All sediment fractions had a similar relative PAH composition, dominated by >3-ring PAHs, suggesting a common pyrogenic origin. Low density material had 10-200 times higher PAH concentrations and 10-100 times higher organic carbon (OC) content, yet differences in OC content only partially accounted for variations in PAH concentration. Low density particles contributed more than 75% of the Sigma PAH, while comprising only 3% of bulk sediment dry weight. This may have significant utility for contaminant mitigation efforts in Auckland Harbour.     

HISTORICAL HAZARDS: THE GEOGRAPHY OF RELICT INDUSTRIAL WASTES*

Current inventories of hazardous waste sites seldom include older, unreported sites, thereby creating a serious shortcoming in attempts to assess risks posed by one type of technological hazard. Based upon a review of urban morphogenesis, hazardous material use in industry, and past waste management practices, this paper presents a model of historical hazards accumulations. Reviews of past industrial waste disposal practices in three Illinois urban settings illustrate the model's effectiveness.     

The potential impact of shell fragment distributions on high-frequency seafloor backscatter

There has been much recent activity on developing and testing high-frequency seafloor volume scattering models and on the related high-resolution characterization of seafloor volumes. This paper will address another plausible volume scattering mechanism that has not yet received much attention-distributions of broken shell fragments in sediment (commonly referred to as shell hash). The shell fragments are modeled as nonaggregating spherical scatterers, and the spatial distribution of the shell pieces is determined using the Percus-Yevick packing factor. Computer simulations of the multiple scattering and a single scattering model are used to study the power backscattered by shell hash sediments as a function of the volume of scatterers and the frequency of the incident wave (10-100 kHz). Parameter values for simulations are obtained by stereological analysis of x-ray computed tomography scans of sediment cores.