Abstract: A runoff water monitoring system for sampling sheet-flow from a generally flat surface having an edge comprises an elongated recess contiguous to the edge and depending below the generally flat surface. An insert in the recess has an upper end and an open frame plate member. A grate member is secured to the open frame and has a downwardly depending sample receptacle attachment sleeve and a grate plate having a predetermined number of sheet-flow metering apertures therein. A sample collection receptacle is releasably secured on the attachment sleeve to receive water flowing through the sheet-flow metering apertures. The sheet-flow metering apertures are preferably rectangular and have a rounded leading or upstream edge and square side and back edges. The sample receptacle over the attachment sleeve which has an endless groove in the sleeve and an endless elastic member is seated in the groove over the open end of the sample receptacle.