Natural waters with a blue color are occasionally observed in hot springs, ponds, and lakes. Although water appears clear in shallows, the blue color becomes visible in deeper waters (like deep lakes or the sea), because water absorbs light at the red end of the visible spectrum (BROUN & SMIRNOV 1993). If water contains solute such as Fe2+ or Cu2+, it absorbs light and one sees greenish-blue or blue, the complementary color of red or orange. It is also well known that the particles in water can scatter light. Some glacial lakes containing finely ground rock called rock flour (e.g., Peyto Lake and Lake Louise in Canada, Gjend Lake in Norway, and Lake Pukaki & Lake Tekapo in New Zealand) have a characteristic blue color. We have reported that the blue color of some hydrothermal waters of Beppu (Oita-ken) in Japan are explained by Rayleigh scattering of the short wavelength visible light by the colloidal silica (OHSAWA et al. 2002).
We have observed 3 blue natural waters generated by the mixture of sulfuric acid water (containing Al) and Na-Cl type hot spring water (containing Si) in Japan. We attempted to determine the factor responsible for the color of Lake Shima (36 degrees 37'N;138 degrees 47'E) in Gunma-Ken, Pond Aonuma (37 degrees 39'N;140 degrees 4'E) of Goshikinuma in Fukushima-Ken, and Pond Umijigoku (33 degrees 19'N;131 degrees 28'E) in Oita-Ken.