This report presents methods and data for a Lagrangian sampling investigation into chemical loading and in-stream attenuation of inorganic and organic contaminants in two wastewater treatmentplant effluent-dominated streams: Boulder Creek, Colorado, and Fourmile Creek, Iowa. Water-quality sampling was timed to coincide with low-flow conditions when dilution of the wastewater treatmentplant effluent by stream water was at a minimum. Sample-collection times corresponded to estimated travel times (based on tracer tests) to allow the same "parcel" of water to reach downstream sampling locations.