Light backscattering in turbid freshwater: a laboratory investigation

The relationship between backscattering and inorganic sediment concentration was examined in a controlled laboratory setting. Different concentrations of two sediment sizes (clay/silt and medium sand) of marine clay deposition were added separately in an opaque black tank containing 500 l of distilled water. The experiment used two optical Environmental Characterization Optics-Backscattering sensors to quantify backscattering in m−1 and NTU. Increasing backscatter coefficients (maximum 5.3 m−1) were observed with increasing suspended particulate concentrations up to 272 and 412 mg l−1 for clay/silt and medium sand, respectively. This demonstrates that a greater range of backscattering values can be measured for highly turbid waters than previously measured in situ. Mass-specific backscattering values for clay/silt (0.02 m2 g−1) are on average higher than for medium sand (0.014 m2 g−1). Although the backscattering ratio [b˜bp(λ)] and refractive index calculated are in accordance with the literature, it is worth noting that the difference in particle size does not exclude differences in matter composition, shape, and other physical factors that contribute to b˜bp variation. The results suggest that further investigation of inorganic optical properties affecting particle size as well as matter composition is required in a controlled environment.