Human-made marshes that filter water naturally are gaining in popularity as an alternative to high-tech water purifying systems, according to experts speaking at a National Marketplace for the Environment meeting in Anaheim, Calif. Partially treated wastewater and stormwater runoff are sent to constructed wetlands, where plants and microbes purify the water of silt and some pollutants so that it can be reused for farming and other non-drinking purposes. In Davis, Calif., a 400-acre, $6 million wetlands project should be open to the public by late summer. There are some 800 human-made wetlands in the U.S., and about 2,400 worldwide.