Development of FBG salinity sensor coated with lamellar polyimide and experimental study on salinity measurement of gravel aquifer

Under the influence of climate warming, sea level rise and human activities, seawater intrusion has become a significant problem. Seawater intrusion salinates groundwater and impacts on ecological environment and society. It needs to be monitored to scale to environment in order to develop and implement preventive or remedial measures. In-situ large-scale monitoring of groundwater salinity is a key step in the prevention of seawater intrusion. However, current monitoring technologies are either too labor-intensive or excessively expensive when used in large-scale in-situ monitoring. Optical fiber techniques that are presently used in measurements of salinity are mostly limited to laboratory experiments. Herein, a fiber Bragg grating (FBG) salinity sensor for in-situ monitoring of groundwater salinity is proposed. The sensor is composed of a FBG coated with polyimide (PI) and an uncoated FBG for salinity sensing and temperature compensation. On the basis of feasibility test and analyses using water diffusion model, a mathematical matrix for calculating salinity and temperature is proposed and applied to salinity measurement in gravel aquifers. The results indicate that the sensitivity coefficients of FBG coated with lamellar polyimide, to water salinity and temperature, are −0.0358 nm/%, 0.0321 nm/°C, respectively. The uncoated FBG is only sensitive to temperature, and the sensitivity coefficient is 0.0095 nm/%. The results of gravel aquifer measurements show that the FBG salinity sensor has good repeatability and consistency with measurements obtained with an electronic salinity meter. The absolute and RMS errors of the salinity measurements are −0.16 to 0.21%, 0.1048%, respectively, and the absolute and RMS errors of temperature are −0.31 to 0.33 °C, 0.1917 °C, respectively. The FBG salinity sensor provides a new means for in-situ monitoring of seawater intrusion.