The optical properties of water impounded in reservoirs such as large tanks with closed covers and in uncovered reservoirs are monitored to detect harmful substances. The system and method comprises illuminating the water with an artificial light source within the enclosure for monitoring of the reservoir. Radiance and irradiance fluxes are measured by one or more multiband radiometers. Changes in radiance and irradiance are used to calculate spectral attenuation coefficients of the water. Fluorescence spectra are measured. A computer look-up table of known absorption and emission spectra is compared to the spectral attenuation to determine if a substance has been detected.