Water Resources Collections and Archives

Rodeo Lagoon is an estuarine lake dominated by freshwater, with maximum measured salinity levels less than one-third that of seawater. This project identifies three possible sources for salinity beyond the seasonal input of seawater from the adjacent Pacific Ocean, and evaluates their significance in the early spring season using two data sets: a groundwater study on the barrier beach between the ocean and the lagoon to determine rate and direction of subsurface flow; and a salinity profile in the lagoon at depths of 0, 0.5, 1.5 and 2.5 meters. Groundwater flows through the barrier beach toward the ocean at a Darcy velocity of about 5 meters per day. Salinity declines steeply with proximity to the inlet of Rodeo Creek into Rodeo Lagoon indicating that the effluent is fresh water with low dissolved solid content. Leachate from sediments at the bottom of Rodeo Lagoon are a possible salinity source. The validity of this source could be determined by sampling and testing the bottom sediments. The most significant source of dissolved solids is the seasonal input from the Pacific Ocean when a channel connects the lagoon and the ocean.