Inside out: A Comparison of Entrainment Sampling in-Plant Vs Offshore

Entrainment by the San Onofre Nuclear Generating Station Units 2 and 3 cooling water systems was monitored over 12 months in 2006–2007. Both units withdraw seawater through intake risers situated 960 m offshore at a depth of 9 m and 198 m apart from each other. Simultaneous sampling was performed within the plant and offshore (at a station located between the intake risers) during 13 surveys. Plankton cropping was a concern because prior the intake conduits are colonized with filter-feeding invertebrates. Prior studies in the 1980s found higher densities in-plant than offshore, a result that puzzled the researchers. This was not the case at San Onofre where mean larval concentrations collected in-plant were 629/1000 m3 (Unit 2) and 841/1000 m3 (Unit 3). Meanwhile, densities offshore averaged 1,965/1000 m3, which exceeds both in-plant estimates alone, and the combination of the two. Periodic heat treatments of the cooling water system denuded the intake conduits of most filter-feeding invertebrates, providing gradients of colonization that could alter the cropping rate. This study can provide empirical information to guide future entrainment studies at sites prone to filter-feeding invertebrate colonization within the cooling water intake.