Water is a fundamental resource for human society and natural ecosystems.  Ensuring good water quality in the natural waters that provide water for drinking and other uses around the world is of forefront importance for management of this resource. Even after implementing substantial management strategies and measures for mitigating water pollution, water quality still remains too deteriorated in many freshwater and coastal water environments. A possible reason for this lack of quality improvements may be the formation of legacy sources, due to various physical and biogeochemical processes that delay (up to several decades or more) the waterborne pollutant propagation through hydrological catchments and the effects of efforts to mitigate this propagation. The resulting legacy source contributions of pollutants after such delaying processes contrast to the contributions of active sources, characterized by considerably shorter times (of around a year) of waterborne pollutant transport and associated propagation of mitigation effects through catchments.

This thesis aims at quantitative differentiation of the contributions of active and legacy pollutant sources to currently measured stream water quality. The research uses different types of data for various spatial and temporal scales, to explore variations and their dependence on monitoring resolution both in space and time. To this end, the thesis considers different types of pollutants, including nutrients, metals, dissolved carbon, and chloride, as well as common water quality variables measured by automatic sensors.

The theoretical framework used in this thesis hypothesizes, on mechanistic grounds, a significant linear relationship between pollutant loads and water discharges, which is here tested against data. Based on the support that data provide for the hypothesis, the obtained linear relationships are further used to quantify and differentiate between active and legacy source contributions to observed water quality conditions.  A main result from this work is significant indication that today’s measured water quality is strongly influenced by pollution history, with legacy source contributions dominating current concentration and load levels for all investigated pollutants in different catchments, of various scales, around Sweden and the world. This result can explain why recent water quality improvement efforts have been relatively unsuccessful in improving water quality in natural water environments of various countries around the world, as such efforts so far have mainly considered and targeted active sources. Furthermore, the thesis has quantified important relationships between legacy nutrient contributions and agricultural land, with widely consistent relationship parameters for nitrogen and more location-specific ones for phosphorous. For shorter-term water quality monitoring with fine (sub-daily) time resolution, the thesis shows that and why legacy source contributions are most influential under high flow conditions, while the active contributions are increasingly important for lower flows, which in turn drives distinct seasonal variation patterns in the water quality dynamics.

The methodological and result contributions of this thesis provide valuable tools and insights for distinction and quantification of legacy and active source contributions to water quality in natural water environments. The contributions apply across various scales, and for different pollutants and water quality variables, as basis for improved and more efficient water quality management.