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Desorption rates for sediment-associated contaminants during resuspension events are investigated for a site on the Palos Verdes shelf, California. This study focuses on p,p′-DDE, the most abundant DDT compound present in an effluent-affected near-surface sediment deposit on the Palos Verdes shelf. Frequency, magnitude, and duration of resuspension events are estimated from bottom boundary layer measurements and a 17-yr record of surface wave conditions. Desorption of p,p′-DDE from sediment in suspension is calculated using a radial diffusion model for porous aggregates. Suspended sediment concentrations and size distributions during resuspension events are found using a shelf sediment-transport model. Calculations are made for a 60-m site where measurements of near-bed flow and sediment properties, including p,p′-DDE concentration and percent organic carbon by size fraction, are available. The results indicate that the time scales of p,p′-DDE desorption from the finest sediment fractions (<20 μm) are typically much shorter than the duration of resuspension events, which average 1.6 days at the study site, whereas desorption time scales are almost always much longer than event duration for the coarser size fractions (>63 μm); the median bed grain size is about 45 μm. Our calculations suggest that 25–50% of the mass of p,p′-DDE in the surface active layer of the bed is lost during a resuspension event; mean active layer depth is 0.2 cm and mean p,p′-DDE concentration in the uppermost 2 cm of the bed at the study site is 11 μg−1. Total annual loss depends on the number of events per year. Wave data indicate an average of 10 resuspension events per year at a depth of 60-m on the Palos Verdes shelf from 1982 to 1999. This leads to an estimated average loss of p,p′-DDE by desorption during wave-driven resuspension of 3–7 μg cm−2 yr−1 for the 60-m site. The range in average loss rates is due to the large range in published values of the partition coefficient for DDE. [Copyright &y& Elsevier]

We combine direct observations, longer-term wave data, and model calculations to characterize resuspension and transport of fine-grained, effluent-affected sediment on the Palos Verdes shelf. Near-bed waves, currents, and suspended sediment concentrations were monitored during the winter of 1992–93 with a bottom tripod and current-meter mooring at a 63-m-deep site. Wave conditions that winter were moderate (∼2 year recurrence interval), and mean current was alongshelf to the northwest; currents were not significantly correlated with wave conditions. Seven wave events during the winter (December–March) produced near-bed wave orbital velocities at the study site in excess of 14 cm s−1, the observed threshold for significant resuspension. Three of these events occurred during the bottom tripod deployment and are characterized by the highest persistent suspended sediment concentrations in the tripod record. Suspended sediment flux was alongshelf to the northwest for 5 of the 6 wave events for which current data were available; one event occurred during low southeast currents. Measured suspended sediment concentration and grain size generally agree with values that were calculated using a shelf sediment transport model with no adjustment of parameters from values determined for two muddy sites on the northern California shelf. We extend our seasonal observations to a period of almost 2 decades by applying the observed thresholds for wave-driven resuspension to near-bed wave conditions calculated from NDBC Buoy 46025 surface wave data. An average of 10 resuspension events per year, with an average duration of 1.6 days, were identified at a water depth of 60 m; the number of events dropped to 3 per year at 90 m, beyond the shelf break. For the majority of these events, calculated net suspended sediment flux is toward the northwest (alongshelf) at an average rate of 140 kg m−1 h−1; about a third of the events have net southeastward flux at an average rate of 30 kg m−1 h−1. The calculated thickness of the resuspended surface layer of the bed was less than 1 cm for all events at 60 m. [Copyright &y& Elsevier]

Abstract: Sediment transport and the potential for erosion or deposition have been investigated on the Palos Verdes (PV) and San Pedro shelves in southern California to help assess the fate of an effluent-affected deposit contaminated with DDT and PCBs. Bottom boundary layer measurements at two 60-m sites in spring 2004 were used to set model parameters and evaluate a one-dimensional (vertical) model of local, steady-state resuspension, and suspended-sediment transport. The model demonstrated skill (Brier scores up to 0.75) reproducing the magnitudes of bottom shear stress, current speeds, and suspended-sediment concentrations measured during an April transport event, but the model tended to underpredict observed rotation in the bottom-boundary layer, possibly because the model did not account for the effects of temperature–salinity stratification. The model was run with wave input estimated from a nearby buoy and current input from four to six years of measurements at thirteen sites on the 35- and 65-m isobaths on the PV and San Pedro shelves. Sediment characteristics and erodibility were based on gentle wet-sieve analysis and erosion-chamber measurements. Modeled flow and sediment transport were mostly alongshelf toward the northwest on the PV shelf with a significant offshore component. The 95th percentile of bottom shear stresses ranged from 0.09 to 0.16Pa at the 65-m sites, and the lowest values were in the middle of the PV shelf, near the Whites Point sewage outfalls where the effluent-affected layer is thickest. Long-term mean transport rates varied from 0.9 to 4.8metrictonsm−1 yr−1 along the 65-m isobaths on the PV shelf, and were much higher at the 35-m sites. Gradients in modeled alongshore transport rates suggest that, in the absence of a supply of sediment from the outfalls or PV coast, erosion at rates of ∼0.2mmyr−1 might occur in the region southeast of the outfalls. These rates are small compared to some estimates of background natural sedimentation rates (∼5mmyr−1), but do not preclude higher localized rates near abrupt transitions in sediment characteristics. However, low particle settling velocities and strong currents result in transport length-scales that are long relative to the narrow width of the PV shelf, which combined with the significant offshore component in transport, means that transport of resuspended sediment towards deep water is as likely as transport along the axis of the effluent-affected deposit. [Copyright &y& Elsevier]

Long-term (60-yr) predictions of vertical profiles of p,p′-DDE concentrations in contaminated bottom sediments on the Palos Verdes shelf were calculated for three locations along the 60-m isobath using a numerical solution of the one-dimensional advection–diffusion equation. The calculations incorporated the following processes: sediment deposition (or erosion), depth-dependent solid-phase biodiffusive mixing, in situ diagenetic transformation, and loss of p,p′-DDE across the sediment-water interface by two mechanisms (resuspension of sediments by wave action and subsequent loss of p,p′-DDE to the water column by desorption, and desorption from sediments to porewater and subsequent molecular diffusion to the water column). A combination of field measurements, laboratory analyses, and calculations with supporting models was used to set parameters for the model. The model explains significant features observed in measurements made every 2 years from 1981 to 1997 by the County Sanitation Districts of Los Angeles (LACSD). Analyses of available data suggest that two sites northwest of the Whites Point sewage outfalls will remain depositional, even as particulate supply from the sewage-treatment plant and nearby Portuguese Bend Landslide decreases. At these sites, model predictions for 1991–2050 indicate that most of the existing inventory of p,p′-DDE will remain buried and that surface concentrations will gradually decrease. Analyses of data southeast of the outfalls suggest that erosion is likely to occur somewhere on the southeast edge of the existing effluent-affected deposit, and model predictions for such a site showed that erosion and biodiffusion will reintroduce the p,p′-DDE to the upper layer of sediments, with subsequent increases in surface concentrations and loss to the overlying water column. [Copyright &y& Elsevier]

The Palos Verdes peninsula is a short, very narrow (<3 km) shelf in southern California that is bracketed by two large embayments. In May 1992, arrays of up to 4 moorings and 2 benthic tripods were deployed in a yearlong study of the circulation processes over this shelf and the adjacent slope. Wind stress, coastal sea level, atmospheric pressure and wave records were obtained from offshore sites and from coastal stations surrounding Palos Verdes. Bottom stress calculated for the mid-shelf sites using a boundary-layer model and data from the above instruments indicated the bottom drag coefficient over this shelf is about 0.003. Currents flow toward the northwest along the shelf and upper slope. Speeds are generally around 20–30 cm/s. There was no obvious seasonal structure in the flow. The first EOF for subtidal alongshelf current accounted for nearly 70% of the variance at sites on the shelf and upper slope. The dominant fluctuations had periods between 5 and 20 days, periods longer than seen in the regional wind stress field. Coastal sea level and the alongshore gradient in sea level had a similar concentration of energy in the 5–20 day frequency band. About 30% of the alongshelf flow was coherent with the alongshelf pressure gradient; currents flowed down the pressure gradient with minimal phase lag. Winds accounted for only 15–20% of the variance in subtidal currents, but the measured effect of wind stress was large. A 1 dyne/cm2 wind stress was associated with a 20–30 cm/s alongshore current.Both the regional wind stress and the alongshelf pressure gradients had spatial scales much larger than found on this small shelf. Subtidal flows forced by these regional fields were set up in the adjacent, much broader basins. The currents amplified as they moved onto the narrow shelf between the basins. Hence, local wind-driven currents had anomalously large amplitudes. The momentum equations for alongshelf wind or pressure gradients did not balance because some of the measured terms were associated with regional fields, others with local process. Our observations suggest that it is more difficult to determine which measured fields reflect the local processes in regions with rapidly changing topography. [Copyright &y& Elsevier]