Assessing changes in streamwater chemistry along a rural–urban gradient

Urban expansion is altering hydrological processes and water quality dynamics in many catchments. At a regional scale, catchments are often only partially urbanized, with land cover combining both natural and developed areas. A common configuration is a predominantly rural upper part, where forests, agricultural land, and natural landscapes prevail, which transitions to increasingly urbanized areas downstream. This land cover arrangement creates a rural–urban gradient along the stream network, providing a natural framework to assess how land cover influences hydrochemical dynamics.

Here, we investigate the influence of this land-cover gradient on streamwater chemistry in two parallel catchments in the Lausanne area (Switzerland). Five stream gauges monitor nested sub-catchments (5–22 km²) spanning urban land cover fractions from 5% to 40%. Stream gauges continuously measure streamflow and water quality parameters (electrical conductivity, temperature, turbidity, and fDOM), complemented by weekly and event-based streamwater sampling for major ions and trace metals.

Results from the first year of measurements confirm that urbanization strongly alters hydrochemical dynamics. We find that individual solutes respond differently along the land-cover gradient, reflecting contrasting dominant sources (geogenic, agricultural, and urban). This results in distinct downstream patterns in their mean concentrations. Despite these differences, most solutes exhibit increasingly dilution-dominated concentration–discharge (C–Q) relationships in the more urbanized downstream sub-catchments. This behavior is consistent with relatively constant solute inputs (from point sources or spatially diffuse sources) that become rapidly diluted during high-flow conditions by low-solute runoff generated from impervious surfaces. Together, these observations provide new insights into how urbanization influences the storage and release of water and solutes. The approaches developed and insights gained will support a more holistic understanding of water and solute dynamics across diverse catchment types, as urban areas represent an ever-growing proportion of landscapes worldwide.