Mercury Speciation and Cycling in an Ephemeral Coastal Lagoon System

Abstract

Malibu Lagoon, located in Los Angeles County, CA, is impacted by urban contaminants, such as nutrients and heavy metals. In 2012, the lagoon was remediated to reduce nutrient and sediment inputs and improve circulation. Post-remediation monitoring of Malibu Lagoon indicates conditions have improved, but heavy metal concentrations have not been evaluated. This is a concern because a 2009 study found that bioaccumulative organic mercury (Hg) concentrations were 20 times higher in the lagoon relative to coastal seawater. Hg is a neurotoxin that impairs neurological functions. Monomethylmercury (CH3Hg+ or MeHg), produced by anaerobic bacteria, is the form that has the greatest impact on human and ecosystem health because it bioaccumulates in organisms and biomagnifies through the food web. This project will: (1) be the first to evaluate Hg cycling in an ephemeral coastal lagoon system over tidal and seasonal timescales, (2) assess the benefits of coastal restoration on Hg toxicity, (3) contribute to an ongoing investigation to assess the land-to-sea transport of contaminants in a wildfire impacted coastal ecosystem, and (4) provide insight on the importance of coastal lagoons as MeHg hotspots. To accomplish this, I will conduct two water column and sediment sampling events to assess wet and dry season variability in Hg concentrations and toxicity. During the dry season, sand builds up at the mouth of the lagoon, barring direct exchange with the ocean. During the wet season, increased runoff breaches the berm and the system functions as an estuary. I will also conduct a 12-hour tidal study, collecting lagoon water, shallow groundwater (beneath the berm), and nearshore seawater hourly during a falling tide to evaluate the land-to-sea transport of Hg. To provide context for the Hg results, I will quantify a range of other parameters (e.g., temperature, salinity, pH, dissolved oxygen, suspended particulate matter (SPM), nutrients, dissolved organic carbon (DOC)). By comparing my results to the 2009 study and post-remediation monitoring reports, I will evaluate the impact of the remediation on Hg cycling in Malibu Lagoon. Relative to 2009, I expect to find: (1) Inorganic Hg (HgII¬¬) has decreased along with the reduction in sediment because HgII¬¬ is typically transported on particles; (2) Organic Hg has decreased in response to increased oxygen concentrations in the water column because it is produced by anaerobic bacteria in low oxygen environments.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)