SIEM REAP, Cambodia — Nineteen-year-old Ra Pheap is a garbage sweeper at Cambodia's world-famous Angkor Wat archaeological site and is keenly grateful for the influx of tourists to the centuries-old monuments. It's because of them she has her $50 a month job.

Suos Samnang, a 17-year-old souvenir vendor, also knows that her livelihood is closely linked to the busloads of camera-toting foreign visitors that arrive every day.

But as they witness the frenzied construction of hotels and guest houses to tap the flow of visitors' dollars in this once-quiet town, even these two poor country girls realize that the blessings of tourism are mixed ones.

"I am worried that this will cause more pollution and migration to the town. The number of people living here just keeps growing. The streets are getting more crowded now," Samnang said.

And some experts are even more concerned than that. They fear the unregulated development — specifically, unrestricted local pumping of underground water to meet rapidly rising demand — may literally be undermining Angkor's foundations, destabilizing the earth beneath the famous centuries-old temples so much that they might sink and collapse.

Tourism is a key moneymaker for cash-strapped Cambodia, about one-third of whose 14 million people earn less than 56 cents a day.

Last year, about half of the 1.4 million visitors who came to Cambodia went to see the Angkor monuments, architectural masterpieces built at the height of the Khmer empire from the 9th to the 15th centuries. Total tourist arrivals for Cambodia in 2005 were an impressive 34.7 percent above 2004's figures.

The steady boom has already transformed Siem Reap into a bustling town filled with luxury hotels and vehicles. Its streets are adorned with billboards promoting the latest mobile phones, pizza and burger joints. Several notable old buildings have been razed to make way for visitors' lodgings, and honky-tonk strips have sprung up catering to low-budget travelers.

"The identity Siem Reap had for centuries is gradually disappearing, or maybe almost disappeared," said Teruo Jinnai, director in Cambodia of the U.N. cultural organization UNESCO, and a 10-year resident of the country. "You have restaurants, massage parlors, hotels, and it's very sad to see that."

Culture shock aside, the health and quality of life of many of its 120,000 residents is imperiled by the boom, as is plain to see when traffic snarls the roads and streets get flooded by rain because of clogged sewers.

"This tremendous growth added to population increase has been exacerbating pressure on infrastructure," said a World Bank report on Cambodia's tourism sector last year. "Energy, water, sewage and waste are all significant problems."

It noted that hotels are not legally required to have sewage treatment facilities, though larger ones do have their own plants.

"But most guesthouses reportedly dump used water directly into the river, causing noticeable river pollution," it said, adding that E. coli, the bacteria found in human feces, has reportedly begun seeping into local wells.

At least as threatening over the long run is the uptake of water, with unrestricted pumping from the water table underlying the area.

"Water is being drawn from 70 to 80 meters (230 to 260 feet) underground by hotels and treated for use," warned the World Bank, noting that no one was quite certain how this affects the aquifers, or underground layers of rocks and sand, from which it is pumped.

Already though, "one of Angkor's temples is reportedly falling into a sinkhole, suggesting that the underground aquifers may be rapidly disappearing," said the report.

Japanese Ambassador Fumiaki Takahashi, whose country has drawn up a development master plan for Siem Reap to deal with the tourism boom, said most of its hotels are pumping underground water for their own use, "and there is no control."

It is the Cambodian government's "urgent task" to control the practice, he said, because "if you take too much water, it might affect the Angkor site. In the long run, the underground water will go down and the site would sink."

The plan of the Japan International Cooperation Agency calls for tapping underground water from near Phnom Kraom, a hill near the edge of the Tonle Sap lake about 7.4 miles south of the town, to avoid depletion of Siem Reap's underground water and reduce the risk of endangering the fragile temples, he said.

Deputy Tourism Minister Thong Khon said the government is ready to accept the master plan to address existing problems and accommodate future growth.

He sees a bright future for Siem Reap, in which the province won't just be a destination for touring the temples but will also become a hub providing air links for tourists to enjoy the sandy beaches of southwestern Cambodia and ecotourism in the jungles of the northeast.

He envisions that by promoting a diversity of destinations, the crowds will be distributed around the country, and the Angkor temples won't get "too jammed up."

Meanwhile, the tourist hordes continue to tramp through fabled Angkor Wat and its satellite temples of Angkor Thom, Bayon, Ta Prohm and Bakheng. Even at the lesser-known 10th-century Bakheng temple, an average of 3,000 tourists climb the 223 feet just in the two hours before dusk each day to view the spectacular sunset.

ON THE WEB: JICA Siem Reap Planning Study: http://www.jica.go.jp/english/about/policy/envi/profile/cam02.html