Hydrogen peroxide measurements in recreational marine bathing waters in Southern California, USA

Hydrogen peroxide (H2O2) was measured in the surf zone at 13 bathing beaches in Southern California, USA. Summer dry season concentrations averaged 122 ± 38 nM with beaches with tide pools having lower levels (50–90 nM). No significant differences were observed for ebb waters at a salt marsh outlet vs. a beach (179 ± 20 vs. 163 ± 26 nM), and between ebb and flood tides at one site (171 ± 24 vs. 146 ± 42 nM). H2O2 levels showed little annual variation. Diel cycling was followed over short (30 min; 24 h study) and long (d) time scales, with maximum afternoon concentration = 370 nM and estimated photochemical production rate of 44 nM h−1. There was no correlation between the absorbance coefficient at 300 nm (used as a measure of chromophoric dissolved organic matter (CDOM) levels) and H2O2. H2O2 concentrations measured in this study are likely sufficient to inhibit fecal indicator bacteria in marine recreational waters through indirect photoinactivation.