Making ‘Chemical Cocktails’ in Streams across the Periodic Table of Elements

Abstract:

Human-dominated land use can contribute to the formation of distinct and diverse elemental combinations and signatures in terrestrial and aquatic watersheds, also known as 'chemical cocktails.' The composition of chemical cocktails evolves across space and time due to: (1) elevated concentrations from anthropogenic sources, (2) accelerated weathering and corrosion of the built environment, (3) hydrologic modifications to drainage networks, and (4) enhanced rates of geochemical and biogeochemical transformations. Characterizing chemical cocktails and underlying geochemical and biogeochemical processes can be used in: (1) tracking pollution sources using chemical mixtures in addition to individual isotopes, elements, or compounds; (2) developing new strategies for co-managing groups of contaminants based on geochemical and biogeochemical potentials for coupled transport and/or transformation in watersheds; (3) identifying proxies from continuous environmental sensor data for predicting transport, transformation, and legacies of diverse chemical mixtures; and (4) determining whether interactive effects of chemical cocktails produce ecosystem-scale impacts greater than the sum of individual chemical stressors. We synthesize global patterns in concentrations of major ions, carbon and nutrients, and trace metals in human-dominated streams across different world regions and make comparisons with reference conditions. We highlight examples from the Baltimore-Washington DC region, which show increased transport of major ions, trace metals, and nutrients across streams draining an urban to rural land-use gradient. Chemical cocktails of major and trace elements are formed over diurnal cycles coinciding with changes in streamflow, dissolved oxygen, pH, and other variables measured by high-frequency sensors. We also present new data showing formation of emerging chemical cocktails in larger rivers draining the Chesapeake Bay watershed over decades within the context of legacies in management of nitrogen loads and sulfur emissions. Our analysis shows that chemical cocktails form over diurnal cycles and decades and can show lag times and legacies in accumulation and transport within river basins (e.g., salt ions, nutrients, and organic matter) due to changing atmospheric deposition, watershed management, and land development. A chemical cocktail approach targeting sources, transport, and transformations of different and distinct elemental combinations is necessary to more holistically monitor and manage the emerging impacts of chemical mixtures in the world’s fresh waters. We conclude by presenting new research questions related to analyzing and managing the emerging impacts of environmental legacies of diverse chemical cocktails of salts, nutrients, and metals in freshwaters.