New Potentiometric Wireless Chloride Sensors Provide High Resolution Information on Chemical Transport Processes in Streams

Keith Smettem,Laurent Pfister,Nick Harris,Julian Klaus

doi:10.3390/w9070542

Keith Smettem, Laurent Pfister + Show 2 more

Open Access

https://doi.org/10.3390/w9070542

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Abstract

Quantifying the travel times, pathways, and dispersion of solutes moving through stream environments is critical for understanding the biogeochemical cycling processes that control ecosystem functioning. Validation of stream solute transport and exchange process models requires data obtained from in-stream measurement of chemical concentration changes through time. This can be expensive and time consuming, leading to a need for cheap distributed sensor arrays that respond instantly and record chemical transport at points of interest on timescales of seconds. To meet this need we apply new, low-cost (in the order of a euro per sensor) potentiometric chloride sensors used in a distributed array to obtain data with high spatial and temporal resolution. The application here is to monitoring in-stream hydrodynamic transport and dispersive mixing of an injected chemical, in this case NaCl. We present data obtained from the distributed sensor array under baseflow conditions for stream reaches in Luxembourg and Western Australia. The reaches were selected to provide a range of increasingly complex in-channel flow patterns. Mid-channel sensor results are comparable to data obtained from more expensive electrical conductivity meters, but simultaneous acquisition of tracer data at several positions across the channel allows far greater spatial resolution of hydrodynamic mixing processes and identification of chemical ‘dead zones’ in the study reaches.

Highlights

There is ongoing interest in understanding and quantifying the travel times and dispersion of solutes moving through stream environments, including the hyporheic zone and/or in-channel dead zones, where retention affects biogeochemical cycling processes that are critical to stream ecosystem functioning [1,2]
We have shown for the first time in a field deployment that the calibrated potentiometric sensors record similar tracer curves to calibrated Electrical Conductivity (EC) probes when installed at adjacent locations
The advantage of deploying potentiometric sensor arrays is that many more measurements can be taken at a fraction of the cost associated with the use of multiple EC probes

Summary

Introduction

There is ongoing interest in understanding and quantifying the travel times and dispersion of solutes moving through stream environments, including the hyporheic zone and/or in-channel dead zones, where retention affects biogeochemical cycling processes that are critical to stream ecosystem functioning [1,2]. Information on chemical transport processes in streams can be obtained from introduced tracers that are usually injected instantaneously or at constant rate [4] at an upstream location under constant flow conditions and measured at one or more downstream locations in the main channel [5]. The acquisition of such data can be expensive and time consuming, so many modelling studies of chemical transport in streams have tended to rely on relatively few well documented. The data are limited to individual mid-channel breakthrough curves and provide no information on cross-channel variability of chemical transport

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