Daily oxygen summaries bound the mean share below 5 mg/L to 7.6–18.7% at 131 US sites under a 0.05 mg/L error model, 2000–2025

A low daily oxygen minimum does not establish the share of a day with low oxygen. We applied mean-and-support probability bounds to 348,034 selected USGS site-days at 131 stream and tidal stream sites in June–September of 2000–2025. With an absolute error allowance of 0.05 mg/L for each daily summary, the equal-site, equal-year and equal-month bounds on the mean share of readings below 5 mg/L are 7.65–18.72%. This result assumes a common underlying record, consistent mean weights, and true summaries inside the allowed error range. With no error allowance, the bounds are 7.90–17.64%, and a share of at least 25% below the limit is unresolved on 52.31% of days with reported minima below 5 mg/L. The latter fraction is 55.45% with the 0.05 mg/L allowance. Complete regular readings on 1,267 days at 12 fixed sites in summer 2024 satisfy the bounds after exact source-series matching. An initial different-depth join gave two failures; both are preserved as an audit. The package has the bound proof, original within-day responses, daily source inputs, code, figures, all planned checks and an offline repeat. These bounds concern shares of readings. A time-exposure claim also requires readings and weights that represent time. The selected network does not establish national prevalence or biological damage.

dissolved oxygen · exposure bounds · daily summaries · USGS · river monitoring · partial identification · source series · reproducible data