Desorption of p, p′-DDE from sediment during resuspension events on the Palos Verdes shelf, California: a modeling approach

Abstract

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.