Evaluation of Transparency and Light Attenuation by Aquashade™

ABSTRACTAquashade™, a colorant and dye registered by the US Environmental Protection Agency (USEPA) for use in aquatic environments to manage plant growth, does not reduce the visibility of water that otherwise would meet safe swimmming standards. This product was examined for its potential impact on underwater visibility in surface waters used for recreational swimming. Aquashade™ treatments of 0, 1, and 5 mg L−1were evaluated in 3-m deep outdoor tanks containing reservoir water. Transparency and light attenuation were examined using a Secchi disk and an underwater photo synthetically active radiation (PAR) meter. In the first experiment, filtered lake water was used, while in the second, bentonite clay was added to provide a light transparency of approximately 1 m. In filtered lake water, Aquashade™ treatments (1 mg L−1and 5 mg L−1) reduced light transparency by approximately 10% and 30%, respectively, but the transparency (272 and 194 cm, respectively) was well above typical international (100 cm) and state (33 cm) standards for safe swimming. In the trial with clay turbidity, the 1.0 mg L−1Aquashade™ treatment was not significantly different in transparency when compared with the untreated reference, but the 5 mg L−1treatment reduced transparency to 67 cm (31%). Aquashade™ absorbed light from 550 nm to 650 nm, with peak absorbance in the 630 nm range. Field measurements of Aquashade spectral attenuation matched that observed in the laboratory. While treatments at the allowable level of 1 mg L−1will reduce transparency, the effect is small compared to inorganic or algal-induced turbidity that can occur in surface waters.