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Floating a New Theory on Foul-Ocean Mystery

April 21, 2002

Re "A Sewage Plume Comeback," April 4:

Thanks for shining light on one solution to Huntington Beach's 1999 "mystery." We now have evidence from scientists at UC Irvine and Scripps Institute of Oceanography that could explain how the Orange County Sanitation District's partially treated sewage plume reached the beach. The recent paper in Environmental Science and Technology examines shoreward transport of bacteria driven by internal waves and tides.

No one sees the bugs directly, but it's clear that cold, subthermocline water layers harboring the offshore plume are connected with the same temperature layers running up and breaking near the beach. A second point made by the authors is that although urban runoff and marsh birds also have been linked in some studies, "it is difficult to explain the observed spatial and temporal variability based on these sources."

Looking for urban runoff to explain all contamination is a diversion from the apparent facts. A preliminary plume-modeling report done for the district in 1997 predicted the presence of a plume in shallow, near-shore waters, a fact demonstrated by district monitoring on several occasions.

The study pointed to the possibility of wind-driven upwelling and downwelling that could serve as important transport processes. As the Environmental Science and Technology article notes, upwelling reduces water-column stability and could explain some of the contamination events. This process has not yet been studied, but it should be pointed out that the summer of 1999 was part of a La Nina year, in which surface temperatures were unusually cool (lowering stability) and strong westerlies blew surface waters offshore (at a 45-degree angle to the wind), enhancing coastal upwelling. Hmm....Could further study help to resolve the "mystery"?

Irwin Haydock

Fountain Valley

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