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Journal of Lake and Reservoir Management 15 (2): 142-147, 1999.

"Evaluation of Transparency and Light Attenuation by Aquashade"

John D. Madsen, Kurt D. Getsinger, R. Michael Stewart, John G. Skogerboe, David R. Honnell, and Chetta S. Owens

ABSTRACT

Aquashade, 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 swimming 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^-1 were evaluated in 3-m deep outdoor tanks containing reservoir water. Transparency and light attenuation were examined using a Secchi disk and an underwater photosynthetically 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 1m. In filtered lake water, Aquashade treatments (1 mg L^-1 and 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^-1 Aquashade treatment was not significantly different in transparency when compared to the untreated reference, but the 5 mg L^-1 treatment 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^-1 will reduce transparency, the effect is small compared to inorganic or algal-induced turbidity that can occur in surface waters.

For more information:

Madsen, John D.
Minnesota State University
Biological Sciences Department
S-242 Trafton Science Center
Mankato, MN 56001
Email: john.madsen@mnsu.edu