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

Our understanding of hydrological and chemical processes at a 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 in precision. Here, we present the proof-of-concept of a new system of water quality monitoring that we called the River Lab (RL), based on the idea of permanently installing a suite of laboratory instruments in the field. Confined in a bungalow next to the river, this set of instruments performs analyses at a frequency of 40-minutes for major dissolved species (Na p , K p , Mg 2p , Ca 2p , Cl − , SO 4 2− , 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 a 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 two months). Using chemical signals obtained as a benchmark, we assess the effects of a lower sampling frequency (typical of traditional field sampling campaigns) and of a lower precision (typically reached in the laboratory) on the chemical river signal. The RL is able to capture the abrupt changes in dissolved species concentrations during a typical 6-days flood event, as well as unexpected daily oscillations during a hydrological boring period of summer drought. The unprecedented, high-resolution, high precision measurements made possible by the RL open new perspectives for understanding critical zone hydro-bio-geochemical cycles. This approach also offers a solution for operational agencies to monitor the water quality in quasi real-time.