Contaminated Marine Sediments Assessment and Remediation (1989) / Chapter Skim
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Computer Simulation of DDT Distribution in Palos Verdes Shelf Sediments
Computer Simulation of DDT Distribution in Palos Verdes Shelf Sediments
Pages 178-198
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From page 178... ...
Arnold University of Arizona and Alex Steele Los Angeles County Sanitation Districts ABSTRACT Prior to impos ition of effective source control measures in 1970, large quantities of DDT were discharged to the Los Angeles County municipal sewer system and subsequently to the Pacific Ocean. among sediments of of the DDT lies 10 Much of this material accumulated the Palos Verdes shelf.
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From page 179... ...
Effluent, including some 150 metric tons of suspended solids per day, is discharged via a system of ocean outfalls to waters of the Palos Verdes shelf. During the last 15 years, JWPCP effluent quality has shown marked improvements, in terms of solids emissions and concentrations of specific contaminants including total DDT (Figure 2)
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From page 180... ...
Hendricks (1978, 1982, 1984, 1988) modeled the fate of chemical tracers in Palos Verdes sediments by combining a solids deposition model designed to yield the discharge- and current-dependent pattern of particulate fluxes to the shelf with a sediment resuspension model that produced time-dependent profiles of select contaminants in the Whites Point sediments.
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From page 181... ...
1 1 1 0 15 30 45 60 75 90 CORE DEPTH cm Figure ad FIGURE 3 Calculated and observed concentrations of volatile solids or DOT in Palos Verdes sediments among monitoring stations along the 61-m depth contour. Volatile solids concentrations were calculated using the basic sedimentation model without considering surface mixing or diffusion.
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From page 182... ...
_~ DATA CORE DEPTH cm Figure ah resuspension and redeposition) and the ratio of effluentrelated to natural particulate material among local sediments, 2.
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From page 183... ...
Residual uncertainties associated with mechanistic, state-of-the-art sediment models preclude their use for prediction of Palos Verdes surface sediment quality characteristics as a function of the projected treatment level at JWPCP. Here we apply an empirical approach to sediment quality modeling on the Palos Verdes shelf in order to avoid scientific uncertainties that have frustrated more mechanistic treatments.
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From page 184... ...
These were compared to profile data collected at several grid locations across the shelf to determine goodness-of-fit. By varying input parameter values, it was possible to select the most appropriate local background sedimentation rate and fraction of effluent solids deposited within specific grid boundaries.
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From page 185... ...
Initial estimates of a molecular diffusion coefficient were developed from the literature of mass transport through porous media and later fitted using 1985 DDT profile data from Figure 1 monitoring stations. Sediment Quality Projection Parameter estimates from the mixing model were used to project sediment quality characteristics as a function of the anticipated JWPCP suspended solids MER.
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From page 186... ...
A statistical measure of goodness-of-fit has not yet been incorporated into the model structures, so model parameters were visually determined. Order-one model calculations and sediment measurements of volatile solids and DDT concentrations were within reasonable agreement at all 61-m monitoring stations along the Palos Verdes shelf.
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From page 187... ...
level. Doubling the solids mass emission rate is predicted to lower the year-2005, surface sediment concentration of DOT by a factor of four relative to the secondary treatment alternative.
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From page 188... ...
. Sediment volatile solids concentrations respond to variation in both background sedimentation rate and relative size of the outfall-related solids contribution.
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From page 189... ...
In all cases, the solid "data" line represents 1985 LACED monitoring data; the broken "model" line represents results of the sedimentation (no mixing) model calculated using the parametric values summarized below; and the broken "mix/dif" line represents results of the sedimentation/mixing model corresponding to the parameter estimates given.
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From page 190... ...
(cm) 5A 250 0.5 - SB 750 0.5 5C 500 0.3 - 5D 500 0.7 5E 500 0.5 10- 5F 500 0.S 10-7 5G 500 0.5 10 8 4 cm 5H S00 0.5 10-8 8 cm NOTES: The percent effluent solids deposited in the grid sector containing station 6C was uncorrected for degradation of volatile solids.
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From page 191... ...
is capable of reproducing sediment profiles of volatile solids and DDT concentrations, and (2) exhibits a satisfactory degree of sensitivity relative to selection of parameter estimates, it provides an incomplete mechanical framework within which to identify the primary determinants of DDT distribution.
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From page 192... ...
3. Natural sedimentation rapidly decreases with distance beyond the break of the Palos Verdes shelf.
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From page 193... ...
In light of Marine Board instructions to consider engineering remedies for contaminated sediments, it is worthwhile considering the magnitude of problems imposed by contaminants in place on the Palos Verdes shelf and the likelihood of finding engineered solutions to these problems. On the bases of specific chemical and biological indicators of sediment quality, Palos Verdes sediments are among the most impacted in the nation.
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From page 194... ...
Engineered remediation strategies of a nature applicable in shallower waters in which contamination is more contained are inappropriate in this setting. SUMMARY AND CONCLUS IONS The primary physical determinants of the distribution of refractile, nondiffusing chemicals among sediments of the Palos Verdes shelf 61-m depth contour include sedimentation of natural and outfall-related particles and bioturbation of surface sediments.
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From page 195... ...
Southern California Coastal Water Research Project Biennial Report 1983-1984' W Bascom, ed.
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From page 196... ...
The fraction of effluent solids reaching specific grid sectors or "solids fraction" was initially estimated from the modeling efforts of LACED personnel using Hendricks model for sediment deposition on the Palos Verdes shelf. Values were subsequently fitted to reproduce volatile solids profiles in sediment cores.
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From page 197... ...
sediment flux: Sflux Ssr + Snat Note: Snag was originally estimatesd from the depth of elevated DOT concentrations at the TIC monitoring site. Values were subsequently fitted to reproduce sediment volatile solids profiles.
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From page 198... ...
Mass emission rate (metric tons/yr) fraction of volatile solids that are biologically degraded fraction of total solids that are volatile volatile solids flux of sewage origin to sediment (mg/cm2-yr)
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