Research Areas > Information Management > Data Visualization > San Diego Mapping Project

Project: Cost-Efficient Surveys for Mapping Chemical Contaminants and Benthic Infauna in Oceanic Sediments Surrounding Two Wastewater Outfalls in San Diego

Background and Objectives

Maps are useful tools for understanding, managing and protecting the marine environment. Maps are also effective and efficient media for communicating information to the public. Current environmental management efforts, such as ocean monitoring by wastewater dischargers, involve routine sampling of areas of potential impact using relatively sparse sampling grids (e.g., fewer than 30 sites) to create maps of environmental conditions. Variability in oceanic conditions often makes extrapolation from the sampled grids to the non-sampled locations questionable. Ideally, scientists would like to have a dense sampling grid that covers the entire study area; however, budget considerations usually limit the total number of sites that can be visited, as well as the number of samples that can be collected and processed. Therefore, determining the most appropriate sample locations requires a selective and strategic approach.

The goal of this project is to provide practical sampling design guidelines that local monitoring agencies can use to construct statistically-defensible maps of environmental conditions. In particular, this project focuses on a two-phased sampling approach for creating maps of sediment condition in the area surrounding two ocean outfalls that discharge treated wastewater to the Pacific Ocean offshore of San Diego. The purpose of the first phase is to develop and implement a survey for capturing spatial variability at multiple spatial scales for a host of benthic community and chemical indicators of sediment condition. This information can then be used to perform a cost-efficiency analysis with spatial statistics to determine optimal grid spacing, and to locate areas of interest/concern for sampling intensification. The purpose of the second phase is to apply this optimal sampling design in practice and use the resulting data to generate maps that include statistical confidence measures of benthic sediment condition surrounding each outfall.

Status

This project was initiated in 2005. Phase II will be initiated in 2009.

Methods

Kriging is a statistical approach that uses information observed at sampled locations to improve predictions at non-sampled locations. The precision and accuracy of those predictions rely entirely on the ability to capture spatial variability of the response. One measure of spatial variability is a variogram, calculated as half the variance of paired sample differences as a function of distances between them. To estimate the variogram, a multi-lag cluster design is introduced where clusters of sites that are spaced at various lag distances are placed around fixed locations on an existing sampling grid (Fig. 1). The variogram can then be used to create a map of prediction error.

Kriging utilizes the variogram to perform a cost-efficiency analysis, examining the tradeoffs between prediction error and resource allocation as adjustments are made to the sampling intensity, spatial coverage, and sample configuration (e.g., the pattern in which samples are placed around grid sites) (Fig.2). Kriging estimates values at non-sampled locations using the weighted average of neighboring sites, where weights are based on the strength of the spatial correlation in the variogram.

Findings

Two primary findings demonstrated by Phase I are:

• Multi-lag cluster designs offered a simple approach for augmenting existing grids, and greatly improved variogram parameter estimation.

• Variograms differed greatly depending on the type of indicator assessed, and which outfall region was being monitored.

Partners

Collaborators on this project included the City of San Diego Marine Biology Laboratory and the City of San Diego Ocean Monitoring Program (Metropolitan Wastewater Department, Environmental Monitoring and Technical Services Division).

Presentation

Cost-Efficient Mapping of Sediment Condition Offshore in San Diego - January 2009 presentation to SCCWRP member agencies describing statistical mapping procedures and summary findings from the San Diego study.