Determining suspended solids and total phosphorus from turbidity: comparison of high-frequency sampling with conventional monitoring methods

Suspended solids (SS) are important carriers of pollutants such as phosphorus (P) in streams, but the sampling frequency in monitoring programs is usually insufficiently frequent to capture episodic SS and total P (TP) peaks. The suitability of turbidity and conductivity as a surrogate for SS and TP was studied using 108 monitoring stations located in catchments of different sizes, land uses, and pollution levels. The use of high-frequency turbidity measurements to estimate SS and TP loads was compared with the use of two sampling methods (grab, flow-proportional sampling) in a case study. When all samples were considered, turbidity was a good predictor of SS (r² = 0.76) and TP (r² = 0.75). For single sites, there was a large range in how well turbidity could predict the two variables. The site-specific turbidity-SS relationship was significant at 87% of sites (mean r² = 0.72). The site turbidity and conductivity-TP relationship was significant at 78% of sites (mean r² = 0.62). A stronger turbidity-SS relationship was found in catchments with a higher percentage of agricultural land. The turbidity and conductivity-TP relationship was stronger when the TP concentration was high. In the case study, TP loads were smallest when estimated with grab sampling, which missed several discharge peaks. Loads estimated with high-frequency turbidity measurements were 19-51% smaller than with flow-proportional sampling, probably due to differences in sampling points. High-frequency turbidity measurements can be a viable alternative to conventional sampling methods in studies on concentration dynamics and load estimates.

Organic carbon causes interference with nitrate and nitrite measurements by UV/Vis spectrometers: the importance of local calibration

Compared with sporadic conventional water sampling, continuous water-quality monitoring with optical sensors has improved our understanding of freshwater dynamics. The basic principle in photometric measurements is the incident light at a given wavelength that is either reflected, scattered, or transmitted in the body of water. Here, we discuss the transmittance measurements. The amount of transmittance is inversely proportional to the concentration of the substance measured. However, the transmittance is subject to interference, because it can be affected by factors other than the substance targeted in the water. In this study, interference with the UV/Vis sensor nitrate plus nitrite measurements caused by organic carbon was evaluated. Total or dissolved organic carbon as well as nitrate plus nitrite concentrations were measured in various boreal waters with two UV/Vis sensors (5-mm and 35-mm pathlengths), using conventional laboratory analysis results as references. Organic carbon increased the sensor nitrate plus nitrite results, not only in waters with high organic carbon concentrations, but also at the lower concentrations (< 10 mg C L^-1) typical of boreal stream, river, and lake waters. Our results demonstrated that local calibration with multiple linear regression, including both nitrate plus nitrite and dissolved organic carbon, can correct the error caused by organic carbon. However, high-frequency optical sensors continue to be excellent tools for environmental monitoring when they are properly calibrated for the local water matrix.