http://www.postandcourier.com/article/20140515/PC16/140519525/1177/dredging-might-be-reason-for-folly-beach-crystal-blue-water-scientist-says

Bo PetersenPosted: Thursday, May 15, 2014 4:24 p.m., Updated: Thursday, May 15, 2014 6:28 p.m.

FOLLY BEACH – Well, the reason the seas are so blue might just be the beach renourishment underway here after all.

People have been remarking for a few weeks how spectacularly light blue the normally sand-browned Folly Beach surf waters have become. On a Wednesday flight, College of Charleston geology professor Scott Harris suspects he found out why – mud. The renourishment dredging evidently is digging up lime mud that is making the water it collects in denser than surrounding water, he said. That water sinks, clearer water flows over it and light reflected through that clear water reflects again off the light-toned mud layer. The double refraction is making the light blue hue of it more intense, he said.

On top of that, as noted by Shea Gibson, a local surf wind conditions forecaster for WeatherFlow, an eddy in the Gulf Stream has moved clearer offshore flow into Folly waters, enhancing the effect.

Earlier speculation put the unusual blue down to the renourishment work, or a drop in estuary rain flow and calmer tides. But on Thursday, as more rain came and tides became stronger, the water was still blue, an observer reported.