Using Aerial Imaging and History to Evaluate Sediment Impacts in the Context of a Sedimentation TMDL
Sedimentation total maximum daily loads (TMDLs) typically aim to address impairments related to habitat conversion and/or benthic community degradation. In a special study completed for compliance with the Total Maximum Daily Load for Organochlorine Pesticides, Polychlorinated Biphenyls, and Siltation in the Calleguas Creek, Its Tributaries, and Mugu Lagoon (OCs and Sediment TMDL), a combination of direct and indirect measurements were used to evaluate sediment-driven habitat conversion as well as benthic community effects. This presentation will focus on the unique approach used to evaluate habitat conversion due to sedimentation, which was performed using three lines of evidence:
• Historic characterization – digitized aerial images from 1938 to 2010. Images were selected on a decadal scale (to evaluate long-term sedimentation impacts) as well as pre- and post-El Nino years (to evaluate the impact of large, episodic events).
• Habitat dynamics – generated from digitized aerial images from 1938 to 2010. Using the digitized images, wetland types were classified based on a modified version of the National Wetlands Inventory Cowardin classification system. Statistics were generated for each image to compare changes in wetland extent and wetland habitats.
• Sedimentation rates – obtained, where feasible, from radioisotope dating of sediment cores collected from different locations in Mugu Lagoon.
The specific results of this study are engaging and visually interesting. Rather than relying solely on in-stream measurements, this study shows that an understanding of history and land development can provide important context to the current state of the environment and be used to influence regulatory outcomes. Using the previously described lines of evidence, this study concluded that Mugu Lagoon is unimpaired due to sedimentation. Additionally, the original basis for requiring a siltation TMDL were explained and put into context. The study conclusion necessitates regulatory actions, as there are existing TMDL and NPDES permit/conditional waiver requirements in place to implement the TMDL. Responsible parties are working with the Los Angeles Regional Water Quality Control Board to carry out appropriate regulatory actions. Those recommended in the special study include: delisting Mugu Lagoon for sedimentation, striking the sedimentation portion of the TMDL, and modifying existing NPDES permits/conditional waivers. An exploration of these and other regulatory options will be discussed. Additional information related to the progress of working with the regulators to make such changes will be available at the time of the presentation.
The approach taken in this special study to evaluate and assess potential sedimentation impacts can be applied to other lagoon and estuarine systems. Aerial imagery analysis as well as sediment core dating may be useful separately or together in a variety of regulatory contexts: as part of a special study, to gain evidence to support TMDL avoidance, or to implement long-term monitoring strategies while avoiding having to execute an on-going, in-stream monitoring program.