Water, reactions and isotopes in the Critical Zone

A long-standing observation in hydrochemistry is that the concentrations of dissolved solutes in streams often change as stream discharge (flow) changes.  The behavior is often complex – in the same stream an increase in water flow may result in some solutes becoming more dilute, others changing concentration very little, and other increasing in concentration as discharge increases.  These variations affect what we refer to as “water quality”.  The complex patterns imply a combination of control by hydrologic factors, such as subsurface flow paths that deliver water to the stream and the travel time of water in the subsurface, and geochemical factors, including the spatial distribution of reactive minerals and the rate of reaction between percolating water and the soil and bedrock substrate. In the CZTOP project we combined high resolution time series data from Critical Zone observatories in France and Luxembourg with novel isotopic tracers and new hydrologic modeling to help disentangle the competing effects of time, transport pathway, and reactivity.  We also examined how the hydrologic and geochemical properties of volcanic landscapes evolve over time in Guadeloupe and Hawaii to develop better understanding of why these changes occur and what they imply for water supply and chemistry.  Rivers provide solutes to the ocean where they are critical for ocean ecosystems.  The estuarine zone where rivers mix with salt water is a critical interface and reactions there strongly influence how  metals are delivered to the oceans.  We developed new techniques for measuring the estuarine behavior of critical metals that can also be applied to river transport processes.  Critical zone processes could contribute negative emissions of CO₂ to help stabilize climate, and we investigated some potential tools and the limitations and requirements necessary to facilitate and evaluate implementation strategies.