Many Shanghai residents take fresh water supplied to their homes for granted; the tap is turned on and the water begins to flow.

But few are aware that a group of dedicated armed police officers safeguard the Qingcaosha Reservoir, the city's largest drinking water source, 24 hours a day, seven days a week.

"We are always on alert," said Lu Xiang, head of the police substation in the Qingcaosha area, west of Changxing Island at the mouth of the Yangtze River.

The surveillance from land, water and air ensures the safety of the reservoir, which supplies 70 percent of the tap water for Shanghai, he said. "For us, no news is good news."

Completed in 2010 and formally starting operations the following year, Qingcaosha Reservoir now provides about 7 million metric tons of water to nearly 14 million residents in areas including Pudong, Huangpu, Changning, Jing'an and Xuhui every day.

It covers an area of 70 square kilometers, the size of 9,800 soccer pitches.

The city once relied on the Huangpu River, a tributary of the Yangtze River running through Shanghai. But ammonia and nitrogen arising from heavy industrial pollution on the upper reaches meant a new supply was needed.

With a capacity of 430 million cubic meters, Qingcaosha was designed to provide 68 days of water without refilling from the Yangtze River.

Thanks to the reservoir, the city's tap water supply has been steady despite the lingering drought and the consequent intrusion of saltwater.

Now that the reservoir has been put into operation, patrolling it and keeping it safe is a priority.

Before police took over the job of safeguarding it 18 months ago, fishermen often sneaked into the reservoir.

"Diesel oil from some of these fishing boats is the biggest threat, and their fishing hindered efforts to reduce algae by raising hundreds of thousands of carp and other fish in the reservoir," said officer Yang Liuqing, who has guarded Qingcaosha since April last year.

Although the reservoir is surrounded by barbed wire, intruders still manage to sneak in, concealing their boats under foliage along the shore to fish at night. Official figures show that in 2012 alone, about 50 people were caught for illegally entering the reservoir to fish.

In response, armed police used helicopters, speedboats and foot patrols to launch surveillance from land, water and air. Cameras and sniffer dogs support the 24-hour surveillance effort.

Qingcaosha police are now training 10 Labradors and German shepherd dogs to detect dangerous items and tackle violent suspects. These are all trained animals that in turn help officers safeguard entrances to the reservoir, said Wang Bin, head of the police canine unit.

Flourishing fauna

With illegal intruders mostly a thing of the past, Qingcaosha is now a haven for wild poultry.

According to the city's greening administration, 75 categories of bird can be found in the area, among them seven raptors, including the common kestrel, peregrine, amur falcon, merlin, osprey and eastern marsh harrier.

Bird experts said the suitable environment, premium water quality, thick green coverage and minimal human disturbance offer wildlife a favorable environment.

They also benefit from an abundant food supply. Biologists said birds of prey, at the top of the food chain, will only appear where the supply of vertebrates, including other birds, is ample.

Yet the reservoir is not free from problems. Rising levels of nitrogen, phosphor and other nutrients in the Yangtze River, the water source of Qingcaosha, have encouraged the growth of plant life, especially oxygen-sapping algae.

Water lingering in the reservoir has aggravated the problem. On a typical day in early September, blue-green algae can be seen at scattered corners of the reservoir as laborers work to remove it.

The microscopic organism is naturally present in lakes and streams. But in 2007, a massive outbreak on the Taihu Lake, left millions of residents in Wuxi, Jiangsu province, without tap water for a week.

Shanghai Chengtou Raw Water Co, which manages the supply of tap water at Qingcaosha, took measures to guarantee the quality of tap water by enhancing supervision of the water body and restraining the excessive growth of algae.

To get real-time information about the water's condition, the company collected samples at 34 spots in and around the reservoir, and analyzed 39 key indicators, including the algae content, salinity, heavy metals and nutrients, the company said.

By 2015, the company aims to be able to collect 42,000 water samples a day from the reservoir, providing a more comprehensive analysis on the quality of the water.

To control the water's eutrophication, a process whereby bodies of water receive excess nutrients that stimulate excessive plant growth, the company focused on the wetlands near the reservoir. The wetlands account for about 29 percent of the reservoir.

Its plants and microorganisms play an effective role in intercepting silt, taking in nitrogen, phosphorous and heavy metals, while biodegrading other pollutants.

Further measures include breeding fish to restrain the growth of algae and building an ecological shelterbelt surrounding the reservoir.

Police dog handlers Wang Bin (left) and Han Jicun from the Qingcaosha canine unit patrol the reservoir's levee. Gao Erqiang / China Daily