Urbanization and Water Management Control Stream Water Quality Along a Mountain to Plains Transition.

Urbanization can have substantial effects on water quality due to altered hydrology and introduction of constituents to water bodies. In arid and semi‐arid environments, streams are further stressed by dewatering as a result of diversions. We conducted a high‐resolution synoptic survey of two streams in Colorado, USA that transition abruptly from granitic/metamorphic forested mountains to sedimentary urbanized plains and are both highly managed for water supply, yet differ in degree of urbanization. A synoptic mass balance approach developed for mine drainage applications was adapted to elucidate effects of urbanization, geology, and diversions on stream chemistry during baseflow conditions. Urbanization was a more important driver of stream concentrations than geology. The urban area was a strong source of bromide, calcium, chloride, and manganese, while lanthanum and dissolved organic carbon were primarily sourced from the mountains. A majority of streamflow was removed by diversions near the mountains/plains interface. Groundwater accounted for 31% of the subsequent flow increase to the urbanized stream, and delivered at least 33% of chloride loading. Constituents that were primarily urban‐derived (bromide, calcium, chloride, and manganese) were 2–3 times higher in the urban region due to diversions; without diversions, stream water quality would have largely retained characteristics of forested streams through the urban reach. This study provides insights into processes that affect water quality in highly managed streams of the semi‐arid western USA. Plain Language Summary: Water supplies in the western U.S. are under pressure from increasing population growth, which results in both urban development and more water management to maximize irrigation and public supply use. Urbanization substantially alters hydrology and can introduce an array of contaminants and nutrients to water bodies. Water diversions decrease the amount of water available to dilute urban inputs. A mass balance approach, originally developed for mine drainage sites, was adapted to elucidate effects of nonpoint source urban pollution, geology, and water management in two streams in the Colorado Front Range. The urban environment introduced bromide, calcium, chloride, and manganese to streams, while lanthanum and dissolved organic carbon arose from forested, mountainous inputs. Urbanization was a more important driver of stream concentrations than changes in geology. Water diversions removed a majority of the water from streams, and impacted water quality by limiting the dilution of urban‐derived constituents. This work provides insights into processes that affect water quality in highly managed streams of the semi‐arid western U.S. Key Points: A synoptic mass balance approach was adapted to elucidate effects of urbanization, geology, and water management on stream chemistryA majority of bromide, calcium, chloride, and manganese loading occurred in the urban area, and was partly sourced from groundwaterDiversions removed the majority of water from streams, leading to increased downstream concentrations of urban‐derived constituents [ABSTRACT FROM AUTHOR]