MERCURY TOXICITY AND TRANSPORTATION IN A WILDFIRE AFFECTED WATERSHED AND LAGOON SYSTEM

Malibu Lagoon is the terminal endpoint of a 285 km² coastal watershed that contains a mix of urban and undeveloped land. During the dry season, a permeable sand berm physically separates the lagoon from surface water exchange with the ocean, but during low tide, a mix of recirculated seawater, terrestrial groundwater, and lagoon water discharge to the ocean (i.e., tidal pumping). In 2012, the lagoon was remediated to reduce nutrient and sediment inputs and improve circulation. Post-remediation monitoring indicates lagoon conditions have improved, but heavy metal concentrations have not been evaluated. This is a concern because a 2009 study found that bioaccumulative organic mercury (monomethylmercury, MeHg) concentrations were 20 times higher in the lagoon relative to coastal seawater. Furthermore, tidal pumping resulted in an 8-fold increase in seawater MeHg concentrations, suggesting Malibu Lagoon was a key source of bioaccumulative mercury to the nearshore marine system. We are building upon past work to assess current post-remediation conditions in the Malibu Lagoon water column and sediment, with a focus on mercury species (total mercury and MeHg) and other heavy metals (e.g., Pb, Cu, Fe, Zn). Additionally, our lagoon investigation is part of a broader study to evaluate contaminant transport and fate following the 2018 Woolsey Fire which burned almost 400 km², including much of the Malibu Lagoon watershed. Preliminary results of the wildfire study indicate total mercury concentrations within Malibu Creek are elevated downstream from burned areas relative to unimpacted areas with significant increases during rain events when the suspended load increases because metals readily adsorb to particulate matter. Overall, the Malibu Lagoon study will (1) contribute to the ongoing investigation to assess the land-to-sea transport of contaminants in a wildfire impacted coastal ecosystem, (2) evaluate Hg cycling in an ephemeral coastal lagoon system over tidal and seasonal timescales, and (3) assess the benefits of coastal restoration on Hg toxicity.