The scientific literature presents conflicting assessments of whether tidal saltwater wetlands reduce or increase fecal indicator bacteria (FIB) impairment of marine bathing waters. In this paper we describe the use of a two end-member salinity-mixing model to calculate FIB treatment efficiencies for the Talbert Marsh, a tidal saltwater wetland in Orange County, California. The mixing model utilized FIB and salinity measurements (n = 10 716) collected during a three-year longitudinal study of the Talbert Marsh. Over the course of the study the marsh received progressively less dry weather surface water runoff from the surrounding urban landscape due to the implementation of a runoff interception and treatment program. As the volume of dry-weather runoff entering the marsh declined, the Talbert Marsh more efficiently removed one FIB group (total coliform) and became a significantly smaller source of two other FIB groups (Escherichia coli and enterococci bacteria). Hence, there may be a maximum volume of dry weather urban runoff (in this case < 1% of the average tidal prism of 2.35 x 10(5) m3/day) that a tidal saltwater wetland can receive, above which the wetland is a net source of FIB to coastal waters.