Hydrochemical constituents in streams may originate from currently active sources at the surface and/or legacy sources retained in soil, slow-flowing groundwater and sediments from earlier surface inputs, waste deposits, and land contamination. These source contributions need to be distinguished for effective pollution mitigation and water quality improvement. This study outlines a methodology for such distinction based on some general mechanistic differences in stream concentration and load behavior versus discharge between the contributions from these different types of sources. The methodology is applied to and tested on stream concentration data for chloride, nitrogen, phosphorous, copper, lead, and zinc, and corresponding data for water discharge, measured over recent decades (time series of 1-3 decades) in multiple Swedish hydrological catchments of different scales (10-37 catchments depending on substance, chloride/nutrient/metal, and its monitoring). Mixed sources are indicated in most (18 of 19) catchments for chloride (with average 19% active-source contribution to total load), but only in 3-4 (of 37) catchments for total nitrogen and total phosphorus (32-59% active), and 1-3 (of 11) catchments for copper, lead, and zinc (1-3% active). Only 1 catchment (of 37) is indicated to have dominant active sources for total nitrogen, and most catchments thus have dominant legacy source contributions for all substances. The legacy contributions correlate well with human activity indicators in the catchments (urban areas for chloride, agricultural land share for nitrogen, population density for phosphorus, historic mining and mine waste areas for the metals), indicating that they are largely anthropogenic. The developed and tested methodology is relatively simple and can be used to screen commonly available stream monitoring data for distinction of active and legacy contributions of any hydrochemical constituent in and across various hydrological catchment settings.