Researchers have found that human-made marshes or wetlands do not work as water filters. Studying why Huntington Beach in Southern California, USA, was closed in 1999, a University of California research team found the persistently high levels of faecal bacteria content to blame. The team found that water from the nearby 10 hectare Talbert Marsh, picks up bacteria from bird droppings as it flows. Better retention of water within the marsh allows salt water and sunlight to kill the bacteria. In the case of Talbert Marsh, the short retention time was however found to be the cause for high bacterial levels ( Nature , June 18, 2001).