Assessing the Representativeness and Sufficiency of Chemical Data from Water Samples Collected During an Oil Spill

Abstract

ABSTRACT Traditionally, the representativeness and sufficiency of data in environmental monitoring efforts are judged against an external standard, such as a pre-determined statistically-based survey design intended to achieve specified data quality objectives. However, given the nature of oil spill responses, where the primary focus is usually on finding the oil and documenting exposure related to the release, samples collected from oil spill studies rarely follow a statistically-based, pre-determined sampling design. Using water chemistry data from the Deepwater Horizon oil spill, we have developed statistical, observational, and forensic approaches to evaluate the representativeness and sufficiency of field-collected chemistry data to characterize exposure in the water column. Three complementary approaches were selected for evaluating water chemistry data. The first “Zone” approach evaluates properties of the data within defined spatial-temporal exposure zones. The second “Probability” approach examines the data independent of predetermined zones using three dimensional (3D; i.e., latitude, longitude, depth) modeling methods (interpolation, contouring) to assess whether the field-collected water chemistry data alone provide sufficient information to model chemical exposure in the water column. The final “Design” approach compares the field-collected sample data to a theoretical sampling design that could have been developed at the beginning of the incident. Integral to all of these approaches is a pre-analysis screening that considers the original objective of each sample collection and the method of sample collection. Review of the chemical forensics of samples can provide further refinement. In this way, samples that were collected as part of a targeted effort based on visual or sensor observations can be considered in light of the context in which they were collected. The results of this analysis can be used to inform future oil spill sample collections to provide sufficient and representative samples that meet the immediate needs of the response as well as longer-term damage assessment determinations.