Formatted Title

Role of Sequence Stratigraphy in In Situ Remediation of LNAPL Contamination: A Case Study from the Los Angeles Basin, California

Background/Objectives

Spatial distribution of preferential migration pathways and capillary barriers largely controlled by subsurface heterogeneity, play a dominant role on LNAPL distribution in porous systems. However, in traditional LNAPL remedial investigations, the interpretation of the aquifer hydrostratigraphy largely relies on ‘layer cake stratigraphy’ and broad regional hydrogeological contours, which may oversimplify the complex geometry and distribution of aquifer and aquitard sediments underneath. Sequence stratigraphy, on the other hand, is a powerful subsurface investigation approach that can capture the true internal heterogeneity of aquifers and aquitards. We demonstrate with a case-study how LNAPL investigations and remediation strategies can significantly benefit from a sequence stratigraphic approach.

Approach/Activities

Previously, a stratigraphic correlation was undertaken at a geologically complex, LNAPL-impacted industrial site in the Los Angeles Basin, California, where the main purpose was to provide geological input for developing a cost-effective and efficient in situ bioremediation workplan for the site. However, the initial correlation was conducted using a lithostratigraphic approach, which did not provide sufficient resolution for identifying high permeability target zones.

In this study, a high-resolution sequence stratigraphic framework using the same preexisting dataset was developed, taking account of the true heterogeneity of depositional environments. The distribution of various depositional units beneath the site and their variable transmissivities were determined within this stratigraphic framework for optimal placement of in situ bioremediation tools.

Results/Lessons Learned

Results of the sequence stratigraphic investigation at the site led to strategic placing of 60 biosparging locations with precise screening interval based on identified high-permeability zones. The biosparging tools are presently functioning at the site with optimal biodegradation of LNAPL because of their strategic positioning. Since no new geological data acquisition was required, sequence stratigraphy proved to be a cost effective and efficient way of guiding the remediation workplan for the site. This case study emphasizes the need for conducting sequence stratigraphic investigations and adequately addressing subsurface heterogeneity for achieving effective in situ remediation of petroleum-contaminated sites.