Hydrological characteristics and water quality of anthropogenic subsurface drainage and brook runoff were observed in a small agricultural catchment near Basel (NW Switzerland). A two year monitoring programme provided concentration data for soluble Molybdate Reactive Phosphorus (MRP = Dissolved Reactive Phosphorus) for both base flow and storm events. Recurrence frequency and correlation analyses were conducted to evaluate the samples with regard to time–space variability as well as observance of eutrophication limits. Results suggest substantial MRP input into the brook system via subsurface drains, partly originating from private wastewater treatment facilities. Runoff, particularly in upstream brook sections, clearly exceeds eutrophication limits already at MRP concentrations (CMRP) of low recurrence frequency. During non-event periods a decrease in CMRP is obvious from upstream brook sections towards the catchment outlet as a result of dilution as well as hydrobiological and chemical processes.