The potamochemical symphony: new progress in the high-frequency acquisition of stream chemical data

Abstract

Abstract. Our understanding of hydrological and chemical processes at the catchment scale is limited by our capacity to record the full breadth of the information carried by river chemistry, both in terms of sampling frequency and precision. Here, we present a proof-of-concept study of a "lab in the field" called the "River Lab" (RL), based on the idea of permanently installing a suite of laboratory instruments in the field next to a river. Housed in a small shed, this set of instruments performs analyses at a frequency of one every 40 min for major dissolved species (Na + , K + , Mg 2+ , Ca 2+ , Cl − , SO 2− 4 , NO − 3 ) through continuous sampling and filtration of the river water using automated ion chromatographs. The RL was deployed in the Orgeval Critical Zone Observatory, France for over a year of continuous analyses. Results show that the RL is able to capture long-term fine chemical variations with no drift and a precision significantly better than conventionally achieved in the laboratory (up to ±0.5 % for all major species for over a day and up to 1.7 % over 2 months). The RL is able to capture the abrupt changes in dissolved species concentrations during a typical 6-day rain event, as well as daily oscillations during a hydrological low-flow period of summer drought. Using the measured signals as a benchmark, we numerically assess the effects of a lower sampling frequency (typical of conventional field sampling campaigns) and of a lower precision (typically reached in the laboratory) on the hydrochemical signal. The highresolution, high-precision measurements made possible by the RL open new perspectives for understanding critical zone hydro-bio-geochemical cycles. Finally, the RL also offers a solution for management agencies to monitor water quality in quasi-real time.

Technical note: A two-sided affine power scaling relationship to represent the concentration–discharge relationship

“…All the technical qualities, calibration of the equipment, comparison with laboratory measurements, degree of accuracy, etc. have been well described in a publication by Floury et al (2017).…”

“…We used the half-hourly (every 30 min) hydrochemical dataset collected by the in situ River Lab laboratory at the Orgeval-ORACLE observatory (Floury et al, 2017;Tallec et al, 2015). A short description of the study site is given in Appendix A1.…”

“…Figure 1 shows an example from our own highfrequency dataset: the 17 500 data points (which correspond to the calibration period of Table 1) represent half-hourly measurements collected over a 2-year period, during which the catchment was exposed to a variety of high-and low-flow events, thus providing a great opportunity for exploring the shape of the C-Q relationship. This being said, we do not wish to imply that a similar behavior could not been identified in medium-and low-frequency datasets, which remain essential tools with which to analyze and understand longterm hydrochemical processes (e.g., Godsey et al, 2009;Moatar et al, 2017). Figure 1 illustrates the inadequateness of the power law for this dataset: the C-Q relationship evolves from a welldefined concave shape on the left to a slightly convex shape on the right in the log-log space.…”

“…In June 2015, the "River Lab" was deployed on the bank of the Avenelles River (within the limits of the Orgeval-ORACLE observatory, see Fig. A1) to measure the concentration of all major dissolved species at high frequency (Floury et al, 2017). The River Lab's concept is to "permanently" install a series of laboratory instruments in the field in a confined bungalow next to the river.…”

Freshwater pearl mussels as a stream water stable isotope recorder

“…A critical challenge going forward is the collection of stream water isotope data sets across a wide variety of headwater streams and for long durations. Despite the advent of field deployable laser spectrometers (Berman, Gupta, Gabrielli, Garland, & McDonnell, 2009) and the deployment of compact environmental laboratories in the field (Floury et al, 2017;Von Freyberg, Studer, & Kirchner, 2017), collection of such data remains prohibitively labour and time intensive. Here, we build on prior work where oxygen stable isotope ratios (δ 18 O) obtained from mechanically drilled freshwater bivalve shell material have been extensively used for palaeoenvironmental reconstructions (e.g., Helama & Nielsen, 2008;Versteegh, Troelstra, Vonhof, & Kroon, 2009;Versteegh, Vonhof, Troelstra, Kaandorp, & Kroon, 2010), including hydroclimate variables (Kelemen et al, 2017).…”