The protection of karst type landscapes and the water resources contained within, are a fundamental objective of any overseeing authority of these areas entirely covered by limestone formations.
A great deal of drinking water is supplied by karst springs that could be close to rivers, lakes or even salt water and the relationship between a restitution karst and associated surface water is a key element in the management of the water catchment area. Depending on the relative water levels, the circulation of water within the karst can be reversed, a phenomenon known as “estavelle” or “inversac”, resulting in water returning to the karst compartments having spent time outside in the open.
Estavelle events represent a major risk of contamination; they must be identified and qualified in order to manage the drinking water supply. The different chemical characteristics of surface water and groundwater can be used to identify estavelle events through hydrochemical analysis. However, surface water and groundwater often have similar characteristics, which requires the use of other parameters to differentiate them.
In order to characterise these types of mass transfer, CPGF Horizon has developed, over many years, a new approach based on the study of chlorophyll. As a natural parameter presenting a characteristic signal for one of the water masses, surface water, and almost zero background noise for the other, groundwater, it is an environmentally and scientifically sound parameter to characterise. Algae, which contain chlorophyll-a, live only in surface water exposed to sunlight. Chlorophyll-a, therefore, represents a natural biological tracer which makes it possible to distinguish surface water from groundwater, where no algae can survive.
So, continuous monitoring of chlorophyll-a makes it possible to study river-karst mass transfer and the risks of estavelle events. As part of one of its studies, CPGF Horizon installed a chlorophyll-a monitoring solution provided by Valeport Water. This system utilised chlorophyll-a sensing Hyperion fluorometers with a detection limit of 0.025 µg/L. Associated with the fluorometers was an EnviroLog display and logging unit or a more sophisticated EnviroLog 4G datalogger and telemetry unit that can provide not only data on a web based data hosting site at a predetermined upload schedule, but two way communication allowing changes to that observation and upload schedule. These solutions makes it possible to characterise the exchanges of chlorophyll-a concentrations within the karst compartments, in real time, without the need to visit the site regularly or carry out time consuming chemical analysis.