Sorption and transformation of the reactive tracers resazurin and resorufin in natural river sediments

Resazurin (Raz) and its reaction product resorufin (Rru) have increasingly been used as reactive tracers to quantify metabolic activity and hyporheic exchange in streams. Previous work has indicated that these compounds undergo sorption in stream sediments. We present laboratory experiments on Raz and Rru transport, sorption, and transformation, consisting of 4 column and 72 batch tests using 2 sediments with different physicochemical properties under neutral (pH=7) and alkaline (pH=9) conditions. The study aimed at identifying the key processes of reactive transport of Raz and Rru in streambed sediments and the experimental setup best suited for their determination. Data from column experiments were simulated by a travel-time-based model accounting for physical transport, equilibrium and kinetic sorption, and three first-order reactions. We derived the travel-time distributions directly from the breakthrough curve (BTC) of the conservative tracer, fluorescein, rather than from fitting an advective-dispersive transport model, and inferred from those distributions the transfer functions of Raz and Rru, which provided conclusive approximations of the measured BTCs. The most likely reactive transport parameters and their uncertainty were determined by a Markov chain–Monte Carlo approach. Sorption isotherms of both compounds were obtained from batch experiments. We found that kinetic sorption dominates sorption of both Raz and Rru, with characteristic timescales of sorption in the order of 12 to 298 min. Linear sorption models for both Raz and Rru appeared adequate for concentrations that are typically applied in field tracer tests. The proposed two-site sorption model helps to interpret transient tracer tests using the Raz–Rru system.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Copernicus Publications on behalf of the European Geosciences Union. The published article can be found at: http://www.hydrology-and-earth-system-sciences.net/home.html

Temporal responses of groundwater-surface water exchange to successive storm events

Groundwater-surface water exchange within the hyporheic zone is widely recognized as a key mechanism controlling the fate of nutrients within catchments. In gaining river systems, groundwater surface water interactions are constrained by upwelling groundwater but there is increasing evidence that a rapid rise in river stage during storm events can result in a temporary reversal of vertical hydraulic gradients, leading to surface water infiltration into the subsurface and supply of surface-borne reactive solutes to this biogeochemically active interface. At a UK study site, using logged hydraulic heads in the surface water, riverbed, and riverbanks and logged electrical conductivity at multiple depths in the riverbed we show that storm events can lead to a temporary reversal of vertical hydraulic gradient with mixing evident up to 30 cm beneath the riverbed. Cross-channel variability is evident, with the center of the channel consistently having shorter reversals of hydraulic gradient, compared to the channel margins. The direction of shallow subsurface riverbank flow at the site is also reactive to storm events, temporarily aligning with the surface flow direction and then reverting back to pre-event conditions. Such a transition of flow paths during events is also likely to lead to expansion of lateral hyporheic exchange. This study provides evidence that storm events can be a key driver of enhanced hyporheic exchange in gaining river systems, which may support nutrient reactions beyond the duration of event-driven change. Our observations demonstrate the dynamic nature of the hyporheic zone, which should be considered when evaluating its biogeochemical function

Managed Aquifer Recharge as a Tool to Enhance Sustainable Groundwater Management in California

A growing population and an increased demand for water resources have resulted in a global trend of groundwater depletion. Arid and semi-arid climates are particularly susceptible, often relying on groundwater to support large population centers or irrigated agriculture in the absence of sufficient surface water resources. In an effort to increase the security of groundwater resources, managed aquifer recharge (MAR) programs have been developed and implemented globally. MAR is the approach of intentionally harvesting and infiltrating water to recharge depleted aquifer storage. California is a prime example of this growing problem, with three cities that have over a million residents and an agricultural industry that was valued at 47 billion dollars in 2015. The present-day groundwater overdraft of over 100 km3 (since 1962) indicates a clear disparity between surface water supply and water demand within the state. In the face of groundwater overdraft and the anticipated effects of climate change, many new MAR projects are being constructed or investigated throughout California, adding to those that have existed for decades. Some common MAR types utilized in California include injection wells, infiltration basins (also known as spreading basins, percolation basins, or recharge basins), and low-impact development. An emerging MAR type that is actively being investigated is the winter flooding of agricultural fields using existing irrigation infrastructure and excess surface water resources, known as agricultural MAR. California therefore provides an excellent case study to look at the historical use and performance of MAR, ongoing and emerging challenges, novel MAR applications, and the potential for expansion of MAR. Effective MAR projects are an essential tool for increasing groundwater security, both in California and on a global scale. This chapter aims to provide an overview of the most common MAR types and applications within the State of California and neighboring semi-arid regions