Distinguishing active and legacy source contributions to stream water quality: Comparative quantification for chloride and metals

• Published in: Hydrological processes 2021-07, Vol.35 (7), p.n/a
• Main Authors: Destouni, Georgia, Cantoni, Jacopo, Kalantari, Zahra
• Format: Article
• Language: English
• Subjects: Catchment area; Catchments; chloride; Chlorides; Comparative analysis; Constituents; Contamination; Copper; Data; Heavy metals; Hydrochemicals; Hydrology; Lakes; Land pollution; Lead; legacy sources; Metal concentrations; Metals; Methodology; multi-catchment analysis; Pollutants; Rivers; source attribution; Tracers; Water discharge; Water quality; Zinc
• ISSN: 0885-6087; 1099-1085; 1099-1085
• DOI: 10.1002/hyp.14280

Hydrochemical constituents in streams may originate from currently active sources at the surface and/or legacy sources from earlier surface inputs, waste deposits and land contamination. Distinction and quantification of these source contributions are needed for improved interpretation of tracer data and effective reduction of waterborne environmental pollutants. This article develops a methodology that recognizes and quantifies some general mechanistic differences in stream concentration and load behavior versus discharge between such source contributions. The methodology is applied to comparative analysis of stream concentration data for chloride (Cl−), copper (Cu), lead (Pb), and zinc (Zn), and corresponding data for water discharge, measured over the period 1990–2018 in multiple hydrological catchments (19 for Cl−, 11 for Cu and Zn, 10 for Pb) around the major Lake Mälaren in Sweden. For Cl−, the average load fraction of active sources is quantified to be 19%, and the average active and legacy concentration contributions as 2.9 and 11 mg/L, respectively. For the metals, the average active load fractions at outlets are 1%–3% over all catchments and 9%–14% in the relatively few catchments with mixed metal sources. Average active and legacy concentration contributions are 0.14 and 3.2 μg/L for Cu, 0.05 and 1.5 μg/L for Pb, and 1.4 and 12 μg/L for Zn, respectively. This multi‐catchment analysis thus indicates a widespread prevalence of legacy sources, with greater legacy than active concentration contributions for both Cl− and the metals, and active contributions playing a greater role for chloride than for the metals. The relatively simple first‐order methodology developed and applied in the study can be used to screen commonly available stream monitoring data for possible distinction of active and legacy contributions of any hydrochemical constituent in and across various hydrological catchment settings. A data‐driven methodology is developed to distinguish between currently active and legacy source contributions to stream water quality. Comparative multi‐catchment analysis shows widespread legacy contributions for both chloride and metals. Mixed sources determine the chloride concentrations while legacy sources determine the metal concentrations. Overall the legacy concentration contributions emerge as more significant than those from active sources.