A reliable quantification of fresh submarine groundwater discharge (SGD) is key to understanding and managing water quality and habitat of coastal ecosystems. However, current radon tracer-based SGD quantifications suffer greatly from uncertainty and subjectivity in tracer signal processing and the indirect estimation of non-SGD related tracer sources and sinks. In this study we provide a new procedure and model SGD rates as upper envelopes in radon and inverted salinity vs. water depth regressions over tidal cycles. We use non-linear quantile regression to model these envelopes and account for signal-lag correction and noise suppression via optimization towards matched radon and inverted salinity envelope data, and a monotonic (i.e., non-oscillating) trend for the water depth. We then quantify the SGD rate based on the temporal change of the modeled radon envelope inventory and define the radon signature of the fresh groundwater endmember (which is critically important for the calculation of volumetric discharge rates) as the zero-salinity intercept in the envelope regression. Our results indicate that the traditional radon mass balance, if applied at a data aggregation interval of < 1 h and if based on a subjective mixing loss estimation, may lead to a substantial overestimate of SGD. This limitation should be carefully considered in the calibration of lager scale, model-based SGD analyses.