Characterization of the Structure of River-Bed Gravels Using Two-Dimensional Fractal Analysis

This paper is concerned with the application of fractal analysis to understand the structure of water-worked gravel-bed river surfaces. High resolution digital elevation models, acquired using digital photogrammetric methods, allowed the application of two-dimensional fractal methods. Previous gravel-bed river studies have been based upon sampled profiles and hence one-dimensional fractal characterisation. After basic testing that bed elevation increments are Gaussian, the paper uses two-dimensional variogram surfaces to derive directionally dependent estimates of fractal dimension. The results identify mixed fractal behavior with two characteristic fractal bands, one associated with the subgrain scale and one associated with the grain scale. The subgrain scale characteristics were isotropic and sensitive to decisions made during the data collection process. Thus, it was difficult to differentiate whether these characteristics were real facets of the surfaces studied. The second band was anisotropic and not sensitive to data collection issues. Fractal dimensions were greater in the downstream direction than in other directions suggesting that the effects of water working are to alter the level of surface organisation, by increasing surface irregularity and hence roughness. This is an important observation as it means that water-worked surfaces may have a distinct anisotropic signal, revealed when using a fractal type analysis.     

Reactivated strike–slip faults: examples from north Cornwall, UK

Several strike–slip faults at Crackington Haven, UK show evidence of right-lateral movement with tip cracks and dilatational jogs, which have been reactivated by left-lateral strike–slip movement. Evidence for reactivation includes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or negative (left-lateral) displacements at fault tips.

Evidence for the relative age of the two strike–slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks formed during left-lateral slip show no deformation; (2) some of the tip cracks associated with right-lateral movement show left-lateral reactivation; and (3) left-lateral displacement is commonly recorded at the tips of dominantly right-lateral faults.

The orientation of the tip cracks to the main fault is 30–70° clockwise for right-lateral slip, and 20–40° counter-clockwise for left-lateral slip. The structure formed by this process of strike–slip reactivation is termed a “tree structure” because it is similar to a tree with branches. The angular difference between these two groups of tip cracks could be interpreted as due to different stress distribution (e.g., transtensional/transpressional, near-field or far-field stress), different fracture modes or fractures utilizing pre-existing planes of weakness.

Most of the d–x profiles have similar patterns, which show low or negative displacement at the segment fault tips. Although the d–x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements show various shapes depending on the amount of displacement and the slip sequence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fault tips due to late-stage tip propagation. Whereas for a smaller slip followed by larger slip with opposite sense, the d–x profile would be flatter with no reverse displacement at the tips. Reactivation also decreases the ratio of d_max/L since for an original right-lateral fault, left lateral reactivation will reduce the net displacement (d_max) along a fault and increase the fault length (L).

Finally we compare Crackington Haven faults with these in the Atacama system of northern Chile. The Salar Grande Fault (SGF) formed as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven.     

A comparison of cytochrome P4501A (CYP1A) mRNA inducibility in four species of Atlantic coast anadromous fishes

Quantification of levels of cytochrome P4501A (CYP1A) gene expression in sentinel species of fishes has been proposed as a management tool to evaluate contamination of aquatic systems. Based on preliminary studies, we hypothesized that differences in CYP1A mRNA inducibility among individuals, populations, or species might lead to spurious conclusions when using this approach in environmental monitoring programs. To address this possibility, we quantitated and compared CYP1A mRNA induction levels in four species of common Atlantic Coast estuarine fish: smooth flounder, hogchoker, striped bass, and Atlantic tomcod, which were treated with model chemicals (beta naphthoflavone (β-NF), or benzo[a]pyrene at 10 ppm) known to induce CYP1A mRNA, or were exposed to contaminated environments. Species-specific CYP1A DNA probes were generated from PCR (polymerase chain reaction) amplification of genomic DNA using conserved oligonucleotide primers, and, along with cloned rainbow trout and Atlantic tomcod CYP1A cDNA probes were used to quantify CYP1A mRNA levels in northern blot analyses. Successful PCR amplification of CYP1A hybridizable DNA fragments was observed for all four species. Results from northern blot analyses showed large differences in CYP1A mRNA induction among species; only Atlantic tomcod exhibited significant induction of CYP1A mRNA for both chemically treated (97-fold) and environmentally exposed fish (34-fold). Significant, although lower, levels of induction were observed in β-NF treated (14-fold) smooth flounder, but not in environmentally exposed smooth flounder. Only low levels (not significant) of CYP1A gene induction were detected in hogchokers and striped bass. We conclude that CYP1A mRNA inducibility differed significantly among fish taxa perhaps due to differences in regulation of gene expression, suggesting that careful selection of sentinel species should be exercised prior to the use of CYP1A mRNA induction in environmental monitoring programs. However, the significance of differences in CYP1A mRNA inducibility in relation to higher level biological endpoints has yet to be determined.     

Case Histories of Shallow Time Domain Electromagnetics in Environmental Site Assessment

Time domain electromagnetic (TDEM) soundings were utilized in site assessment studies for the purpose of accomplishing two objectives: (1) measuring the migration of brines from oil and gas field evaporation pits, and (2) mapping the continuity of clay strata. Both case histories are representative of common objectives in site assessment. The data for both examples were analyzed by one-dimensional ridge regression inversions.
Both case histories illustrate that TDEM is an effective method for determining the lateral and vertical resistivity distribution (geoelectric section) of the subsurface in the depth range from about 5 m to 100 m, and it is known from other investigations that depths of investigations up to 2500 m can be achieved with TDEM.
Frequently, the lateral extent of contaminant plumes emanating from localized sources are of limited areal extent. The case history involving the use of TDEM to map a brine plume shows that a TDEM measurement has a relatively small zone of influence, so that meaningful information about the lateral and vertical extent of the plume can be obtained. Both case histories demonstrate the ability of TDEM to determine geoelectric sections below layers of low resistivity.     

The transformation and export of phosphorus from Wetlands

There is widespread acceptance of the phosphorus retention capability of wetlands even though research findings are often inconclusive and contradictory. The results of a one year phosphorus budget study indicate that internal wetland processes may transform sediment bound phosphorus to plant available orthophosphorus. While total phosphorus imports were nearly double the total phosphorus exports for the study wetland, orthophosphorus exports were 22 per cent greater than imports. This study supports the recent finding that wetlands have limited capability to retain orthophosphorus and indicates that wetlands may even increase the export of orthophosphorus. The generally accepted nutrient retention function of wetlands and their possible role in eutrophication is thus questionable.     

Comparison of heat and bromide as ground water tracers near streams

Heat and bromide were compared as tracers for examining stream/ground water exchanges along the middle reaches of the Santa Clara River, California, during a 10-hour surface water sodium bromide injection test. Three cross sections that comprise six shallow (<1 m) piezometers were installed at the upper, middle, and lower sections of a 17 km long study reach, to monitor temperatures and bromide concentrations in the shallow ground water beneath the stream. A heat and ground water transport simulation model and a closely related solute and ground water transport simulation model were matched up for comparison of simulated and observed temperatures and bromide concentrations in the streambed. Vertical, one-dimensional simulations of sediment temperature were fitted to observed temperature results, to yield apparent streambed hydraulic conductivities in each cross section. The temperature-based hydraulic conductivities were assigned to a solute and ground water transport model to predict sediment bromide concentrations, during the sodium bromide injection test. Vertical, one-dimensional simulations of bromide concentrations in the sediments yielded a good match to the observed bromide concentrations, without adjustment of any model parameters except solute dispersivities. This indicates that, for the spatial and temporal scales examined on the Santa Clara River, the use of heat and bromide as tracers provide comparable information with respect to apparent hydraulic conductivities and fluxes for sediments near streams. In other settings, caution should be used due to differences in the nature of conservative (bromide) versus nonconservative (heat) tracers, particularly when preferential flowpaths are present.     

Look out, aurorae about!

Jim Wild recounts the rationale and success of AuroraWatch, a service that alerts you when you might be able to see aurorae.