Turbidity correction applied to in-situ monitoring of natural fluorescence

Karst aquifers are vulnerable to contamination due to rapid transfer of infiltrated water. These aquifers are however often recognized as strategic water resources for drinking water, particularly in Mediterranean climatic conditions. In this context, the implementation of monitoring strategies to anticipate as early as possible the transfer of contaminants through the infiltration zone requires a well understanding of their hydrological functioning. Natural organic matter (NOM) has been the subject of numerous studies based on the fluorescence properties of organic compounds, revealing the value of monitoring this proxy as a tracer of karstic flow dynamics through the infiltration zone. According to wavelength, humic-like NOM is an indicator of rapid infiltration dynamics, while proteic-like compounds can in some cases reveal anthropogenic contaminant fluxes. Changes in NOM signals can be very rapid in karst systems, particularly during flood event. Thus, as part of the ANR-funded PEPR Onewater K3 project, this study aims to propose and test different protocols for high-frequency optical monitoring of karst spring or well outflows that could be used to set up an early warning system for water contamination (Frank et al., 2018), specifically designed for karst groundwater.First results obtained with GGUN fluorometers (Albillia Sàrl) show the importance of correcting fluorescence monitoring for spurious signals due to water turbidity. The aim of this study is to describe the correction method developed by Schnegg (2002), and to test it using standard turbidity solutions. Limitations and improvements to this method are presented, in particular by comparing it with a conventional regression method. These methods are also applied with fluorescence time-series from two monitored karstic springs in southern France: the Lez spring and Fontaine de Nîmes (SNO KARST network observatories), to produce a natural fluorescence time series. Frank S., Goeppert N., Goldscheider N. (2018). Fluorescence-based multi-parameter approach to characterize dynamics of organic carbon, faecal bacteria and particles at alpine karst springs, Science of The Total Environment, 615.Schnegg P.A. (2002). An inexpensive field fluorometer for hydrogeological tracer tests with three tracers and turbidity measurement. Articles of the Geomagnetism Group, University of Neuchâtel.