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Environmental Sustainability Division

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Geostatistical Mapping of Effluent-Affected Sediments off the Coast of Southern California

Objectives: The objective of this project was to use geostatistical methods to map the thickness and contaminant concentrations of polluted sediments on a portion of the coastal shelf of Southern California, to provide estimates of the uncertainty attached to those maps and to determine the locations of additional samples for costly chemical analyses.

Approach: Variogram analysis of geological and geophysical data was used to provide quantitative models for the spatial distribution of the contaminated sediments. Ordinary kriging was used to provide a map of the sediment thickness. An advanced form of indicator geostatistics was also used to calibrate the relationship between the thickness of the sediments and the level of contamination. Sequential indicator simulations of the thickness and concentrations were then used in a Monte Carlo approach to provide probabilistic maps of the region and the uncertainty associated with those estimates throughout the mapped area.

Results: Maps were produced for the client that provided estimates of the thickness and concentration of contaminated sediments for each grid block in the map area. In addition, maps were produced showing a 90 percent confidence interval for the thickness and concentration levels in each grid block. The probabilistic maps were used to evaluate a number of sample locations to determine which would be most suitable for additional geochemical analyses, allowing the client to identify the locations that provided the most valuable information at the lowest cost. The suite of maps was also studied by the Environmental Protection Agency as a guide to evaluating cleanup options at the site.

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