A mechanistic model of runoff-associated fecal coliform fate and transport through a coastal lagoon.

Fecal coliform (FC) contamination in coastal waters is an ongoing public health problem worldwide. Coastal wetlands and lagoons are typically expected to protect coastal waters by attenuating watershed pollutants including FC bacteria. However, new evidence suggests that coastal lagoons or marshes can also be a source of high indicator organism concentrations in coastal waters. We asked for a Mediterranean-type climate, what is the fate of runoff-associated FC through a coastal lagoon? To address this question, we developed a mass balance-based, mechanistic model of FC concentration through a coastal lagoon and simulated, for summer and winter conditions, FC within the lagoon water column, lagoon sediments, and in the ocean water just downstream of the lagoon mouth. Our model accounts for advective flow and dispersion, decay and sedimentation and resuspension of FC-laden sediments during high flow, erosional conditions. Under low flow conditions that occur in the summer, net FC decay and FC storage in lagoon sediments are predicted. Under high flow conditions that occur in the winter, FC-laden sediments are predicted to erode, resuspend and flow out of the lagoon where they elevate FC concentrations in the coastal ocean. For both seasonal conditions, the predicted water column FC concentrations were within an order of magnitude of field measurements for a reference site in southern California. Our results suggest that there are seasonally varying roles for coastal lagoons in mediating FC contamination to coastal waters.