SAN DIEGO -- Chemical facilities are vulnerable to accidents and sabotage. Terrorism is a real concern, as the I-Team discovered when shooting video for this story outside the Escondido-Vista Water Treatment Facility.

Down the street from the facility, police officers in squad cars and motorcycles surrounded the reporter and photographer, wanting to know if they were a threat.

"Is that standard procedure?" asked reporter Mitch Blacher. "Do the water employees do that regularly?"

"The call us anytime they have a concern," said one officer.

Security is important at the Escondido facility, and at the Alvarado Water Treatment Plant in La Mesa. Besides the importance of keeping the water supply safe, both store large amounts of chlorine gas to treat drinking water. Chlorine itself is not the issue of concern – it’s the form of the chlorine.

"When the chlorine is used as a gas, there's the danger of an emergency release forming a dense ground-hugging plume and drifting off site for many miles, and being very harmful to people," said Paul Orum, an advocate for chemical safety.

That’s exactly what happened in Graniteville, S.C., in 2005. A train accident that ruptured a tank of chlorine created a dense cloud of chlorine gas. Nine people were killed and 500 were treated in emergency rooms.

In Henderson, Nev., in 1991, a massive accidental chlorine gas leak at a chlorine production plant affected thousands of residents.

The Department of Homeland Security estimated that a major chorine gas spill in an urban area could kill 17,500 people.

Orum has authored reports on chemical security for the Center for American Progress and testified before Congress. He's been on a crusade for two decades asking for federal laws to reduce chemical hazards that put communities in danger.

"When there are safer alternatives that are cost effective, it only makes sense to use them," said Orum.

The two local water treatment plants are cited in his latest report as high-hazard facilities that could be made safer.

"At water utilities, it is typically switching from chlorine gas to chlorine bleach," he said.

Other methods of treating water include generating bleach on-site, ozone or ultraviolet light.

In the report "Chemical Security 101," Orum cited chlorine gas as a "preventable security concern."

"What you don't have can't leak or be blown up by terrorists," he said.

Not everyone agrees that it’s easy for all facilities to replace chlorine gas.

"These are issues that are not simple. They have to be engineered and done properly," said Neal Langerman, a San Diego chemist.

He said that in some cases, replacing chlorine gas with an alternate disinfectant could actually increase overall risk.

Langerman testified in June on chemical security issues before the House Committee on Homeland Security on behalf of the American Chemical Society.

"The chemical industry's position is if it's feasible and safer, we'll do it. We may not do it tomorrow, we may have to program it in," he said.

But Congress may be getting impatient. Although water treatment plants have been exempt from existing Chemical Facility Anti-Terrorism Standards, new federal legislation is in the works that would toughen the requirements and include water plants.

Last week, the House passed the Chemical and Water Security Act of 2009. Now the action moves to the Senate.

"It would require facilities to look at alternatives," said Orum of the proposed law.

Like Valley Center Municipal Water District was able to do -- they replaced chlorine gas with a tablet form of chlorine and eliminated the danger of a toxic gas leak.

"We have water quality responsibilities, but we also have a responsibility to be good neighbors and to have a safe operation for our community," said Gary Arant, general manager of Valley Center Municipal Water District.

Arant said they were largely able to make the switch when they began receiving filtered water and their chlorine needs were reduced.

Every agency has to look at its own needs, its own circumstances and make those decisions, said Arant.

The city of San Diego will add ozone treatment of water to the Alvarado plant next year, but will still store large amounts of chlorine gas on site.

Jim Fisher, deputy director of the water department operations division, told the I-Team that chlorine gas will be used as the secondary disinfectant, and stored in bulk as the backup disinfectant. He said ozone is generated with electricity, so a backup is needed in case of an electrical outage. An onsite power generator is an option for backup, but Fisher said it would be costly to install and maintain.

In 2003-2004, a consultant looked into the feasibility of replacing chlorine gas with sodium hypochlorite – liquid bleach – at the Alvarado plant. Fisher said several factors, including cost and other risks, led the department to reject liquid bleach.

Lori Vereker, Escondido’s director of utilities, said they plan to replace chlorine gas with something safer, but they're not saying just when that will happen.