International audienceIn electrolyte-saturated sands, the reversible storage of electrical charges is responsible for a phase lag between the current (injected and retrieved by two current electrodes) and the electrical field recorded by two voltage electrodes. This phenomenon is called 'spectral induced polarization' in geophysics and can potentially be used to monitor salt tracer tests in shallow aquifers to infer their permeability and dispersivity tensors. We demonstrate analytically that the polarization of the inner part of the electrical triple layer coating the surface of the grains (named the Stern layer in electrochemistry) is consistent with available data. We also perform new experiments using silica sands saturated by NaCl and CaCl2 pore water solutions. The salinity dependence of quadrature conductivity can be modelled using an analytical solution of the triple layer model, which offers a simple way to interpret laboratory and field data. This analytical solution depends on the total site density of the mineral surface, the pH value and the sorption coefficient of the cation in the Stern layer. This model shows that both the specific surface conductivity of the Stern layer and the quadrature conductivity of the porous material depend on the conductivity of the pore water. The quadrature conductivity is becoming independent of the salinity above 1 S m−1. The parameters entering the analytical model are consistent with independent estimates from titration data and zeta potential measurements, which are two classical methods to characterize the electrical triple layer at the pore water mineral interface

Tremendous opportunities exist for enhancing water quality and improving aquatic habitat by actively managing urban water infrastructure to operate in conjunction with natural systems. The hyporheic zone (HZ) of streams, which is the area of active mixing between surface water and groundwater, is one such system that is overlooked by many water professionals, because the state of the science on this topic has not been transferred into practice. As a biogeochemically active zone, the HZ offers great potential to provide natural treatment of organic compounds, nutrients, and pathogens in urban streams, which are often strongly impacted by flow modifications and water pollution. Reliable treatment is most likely in streams in which the majority of flow occurs through the HZ, the flow is aerated, and sufficient residence times occur, which may be limited to specific channel morphologies and seasons. Integration of the HZ into stream management plans could also provide quality habitat in a landscape with increasingly depauperate biodiversity. Here, we review current knowledge on hydrological, chemical, and biological aspects of the HZ, with a focus on urban settings, and include a set of examples drawn from the literature of low-flow, effluent-dominated streams in which there is significant hyporheic flow and potential for contaminant attenuation. The HZ can be incorporated much more effectively into urban water management, including stream restoration efforts, by understanding the surface and subsurface features conducive to HZ flow and the water-quality and biodiversity improvements that can be gained in the HZ without posing unreasonable risk. The main barriers to implementation of HZ considerations include lack of information, absence of established metrics for evaluating success, small number of controlled HZ experiments in urban settings, and concern over risks to both public health and aquatic organisms. A combination of field studies, laboratory experiments, and model development that consider hydrological, chemical, and biological interactions in the HZ can overcome these barriers.

6p.International audienceIn electrolyte-saturated sands, the storage of electrical charges under an alternating electrical field (called "induced polarization") is responsible for a phase lag between the applied current and the resulting electrical field. Because a variety of polarization mechanisms exists in porous materials, the underlying physics of induced polarization is somehow unclear and the field data difficult to interpret quantitatively. Measurements at various pHs and salinities can be used to discriminate between different competing mechanisms at low frequencies (1 mHz-1 kHz) in porous media in the absence of electronic conductors. New experimental data point out that, in addition to the polarization of the Stern layer (the inner part of the electrical double layer coating the surface of the silica grains), there is another polarization mechanism possibly associated with a hopping process of the protons on the silica surface. We propose that such a process could follow a Grotthuss cooperation mechanism (as in ice) involving the bound water of the silica surface. Our data also rule out a mechanism based on the diffuse layer. The new polarization mechanism may be applied to quantifying induced-polarization data collected over acidic contaminant plumes

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