<p>Dissolved organic carbon (DOC) is a critical parameter for assessing metal bioavailability and toxicity in aquatic systems, but data from routine measurements in Japan are limited to a small number of specific sites. The goal of this study was to develop a statistical model to estimate DOC concentrations in Japanese rivers using biochemical oxygen demand (BOD) as a proxy. Because the relationship between BOD and DOC was expected to be highly variable, we focused on obtaining conservative (i.e., lower bound) rather than central tendency estimates of DOC concentrations to support “safe-side” screening-level ecological risk assessments. Based on BOD and DOC measurements from 30 river sites across Japan, we developed a quantile regression model at the 0.1 quantile to provide conservative estimates of DOC. Validation with additional monitoring datasets, including original field surveys in Kanagawa and Osaka Prefectures, demonstrated that the developed model provided reasonably conservative estimates of DOC and hence supported its use for “safe-side” screening-level ecological risk assessment. Because of the variability of the BOD-DOC relationship across sites, direct DOC measurements may be appropriate where screening-level assessments indicate potential ecological risks, guiding limited monitoring efforts to priority sites.</p>