Development of the Chromium Public Health Goal

This site features a presentation about the development of a public health standard for chromium in drinking water in California. The presentation includes useful information about chromium availability, toxicity, and carcinogenic effects. Educational levels: Graduate or professional

Edge waves in the presence of strong longshore currents

A form of the linear, inviscid shallow water wave equation which includes alongshore uniform, but cross-shore variable, longshore currents and bathymetry is presented. This formulation provides a continuum between gravity waves (either leaky or edge waves) on a longshore current, and the recently discovered shear waves. In this paper we will concentrate on gravity wave solutions for which V(x)/c < 1, where V(x) is the longshore current, and c is the edge wave celerity. The effects of the current can be uniquely accounted for in terms of a modification to the true beach profile, h'(x) = h(x) [I - V(x)/c]¯², where h(x) is the true profile and h'(x) is the effective profile. This is particularly useful in conceptualizing the combined effects of longshore currents and variable bottom topography. We have solved numerically for the dispersion relationship and the cross-shore shapes of edge waves on a plane beach under a range of current conditions. Changes to the edge wave alongshore wavenumber, K, of over 50% are found for reasonable current profiles, showing that the departure from plane beach dispersion due to longshore currents can be of the same order as the effect of introducing nonplanar topography. These changes are not symmetric as they are for profile changes; IKI increases for edge waves opposing the current flow (a shallower effective profile), but decreases for those coincident with the flow (a deeper effective profile). The cross-shore structure of the edge waves is also strongly modified. As lkl increases (decreases), the nodal structure shifts landward (seaward) from the positions found on the test beach in the absence of a current. In addition, the predicted variances away from the nodes, particularly for the alongshore component of edge wave orbital velocity, may change dramatically from the no-current case. Many of the edge wave responses are related to the ratio V max/c, where V max is the maximum current, and to the dimensionless cross-shore scale of the current, lkl x(V max), where x(V max) is the cross-shore distance to V max. This is most easily understood in terms of the effective profile and the strong dependence of the edge waves on the details of the inner part of the beach profile. Inclusion of the longshore current also has implications regarding the role of edge waves in the generation of nearshore morphology. For example, in the absence of a current, two phase-locked edge waves of equal frequency and mode progressing in opposite directions are expected to produce a crescentic bar. However, in the presence of a current, the wavenumbers would differ, stretching the expected crescentic bar into a welded bar. A more interesting effect is the possibility that modifications to the edge waves due to the presence of a virtual bar in the effective profile could lead to the development of a real sand bar on the true profile. These modifications appear to be only weakly sensitive to frequency, in contrast to the relatively strong dependence of the traditional model of sand bar generation at infragravity wave nodes

Wave variance partitioning in the trough of a barred beach

The wave-induced velocity field in the nearshore is composed of contributions from incident wind waves (f > 0.05 Hz), surface infragravity waves (f σ²/gβ), where ƒ is the frequency, σ = 2πf, k is the radial alongshore wavenumber (2π/L, L being the alongshore wavelength), β is the beach slope, and g is the acceleration due to gravity. Using an alongshore array of current meters located in the trough of a nearshore bar (mean depth ≈ 1.5 m), we investigate the bulk statistical behaviors of these wave bands over a wide range of incident wave conditions. The behavior of each contributing wave type is parameterized in terms of commonly measured or easily predicted variables describing the beach profile, wind waves, and current field. Over the 10-day period, the mean contributions (to the total variance) of the incident, infragravity, and shear wave bands were 71.5%, 14.3% and 13.6% for the alongshore component of flow (mean rms oscillations of 44,20, and 19 cm s¯¹, respectively), and 81.9%, 10.9%, and 6.6% for the cross-shore component (mean rrns oscillations of 92, 32, and 25 cm s¯¹, respectively). However, the values varied considerably. The contribution to the alongshore (cross-shore) component of flow ranged from 44.8- 88.4% (58.5-95.8%) for the incident band, to 6.2-26.6% (2.5-32.4%) for the infragravity band, and 3.4- 33.1 % (0.6-14.3%) for the shear wave band. Incident wave oscillations were limited by depth-dependent saturation over the adjacent bar crest and varied only with the tide. The infragravity wave rms oscillations on this barred beach are best parameterized by the offshore wave height, consistent with previous studies on planar beaches. Comparison with data from four other beaches of widely differing geometries shows the shoreline infragravity amplitude to be a near-constant ratio of the offshore wave height. The magnitude of the ratio is found to be dependent on the Iribarren number, ξ₀ = β(H/L₀)¯1/2. Shear waves are, as previous observation and theory suggest (Oltman-Shay et al., 1989; Bowen and Holman, 1989), significantly correlated with a prediction of the seaward facing shear of the longshore current

Development of a Health-Protective Drinking Water Level for Perchlorate

We evaluated animal and human toxicity data for perchlorate and identified reduction of thyroidal iodide uptake as the critical end point in the development of a health-protective drinking water level [also known as the public health goal (PHG)] for the chemical. This work was performed under the drinking water program of the Office of Environmental Health Hazard Assessment of the California Environmental Protection Agency. For dose–response characterization, we applied benchmark-dose modeling to human data and determined a point of departure (the 95% lower confidence limit for 5% inhibition of iodide uptake) of 0.0037 mg/kg/day. A PHG of 6 ppb was calculated by using an uncertainty factor of 10, a relative source contribution of 60%, and exposure assumptions specific to pregnant women. The California Department of Health Services will use the PHG, together with other considerations such as economic impact and engineering feasibility, to develop a California maximum contaminant level for perchlorate. We consider the PHG to be adequately protective of sensitive subpopulations, including pregnant women, their fetuses, infants, and people with hypothyroidism

Considering Changes in Exposure and Sensitivity in an Early Life Cumulative Risk Assessment

A cumulative risk assessment is generally intended to address concurrent exposure by all exposure routes to a group of chemicals that share a common mechanism of toxicity. However, the contribution of different exposure routes will change over time. This is most critical when estimating risks to infants and children because their exposure sources change rapidly during the first few years of life because of dietary and behavioral changes. In addition, there may be changes in sensitivity to toxicants during this time period, associated with various developmental stages. Traditional risk assessments do not address this progression. Examples of how these factors might be incorporated into an early life risk assessment are provided for lead, dioxins and furans, and organophosphate pesticides. The same concepts may apply to other potentially susceptible subpopulations, such as the elderly. </jats:p