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Among the biggest threats to coastal water quality are freshwater discharges. It is difficult to predict the spatial extent of freshwater plumes at marine beaches because processes governing mass transport in the surf zone are complex. Participatory science approaches could facilitate collecting shoreline data, although volunteer sampling campaigns can be challenged by data quality and volunteer retention. The goals of this study were to (1) work with volunteers to estimate safe swimming distances at beaches that receive polluted discharges, and (2) test whether informational feedback to volunteers increased retention. Forty-six volunteers participated over 12 weeks in 2019 by collecting 1452 salinity measurements at beaches near the mouths of two Central California freshwater discharges and completing participation surveys. These measurements resulted in 145 distinct estimates of safe swimming distances ( D 90 ), spanning a range of environmental conditions during rainy and dry periods. Median D 90 s were 150 and 100 m at San Pedro Creek south and north, and 490 and 330 m at San Lorenzo River west and east, respectively. D 90 was significantly associated with adjacent freshwater discharge rate at both discharges and tide level at one discharge. On average, the odds of volunteers conducting sampling decreased by 4% (95% CI: 1%, 7%) with each successive week. A randomized intervention providing repeated data feedback via email to volunteers did not affect their retention in the study.