TUEN MUN is the only temporary site available for boats which unload cargo from ocean-going vessels, according to Secretary for Economic Services Gordon Siu Kwing-chue. Replying to a question from Alfred Tso Shiu-wai, Mr Siu said: ''If the administration could find other suitable sites in the west New Territories, we would not have looked to Tuen Mun for help.'' The site, in Area 16 of Tuen Mun, will accommodate boats which are now based in Tsing Yi, where the land is to be cleared for Container Terminal 9. Mr Siu said the operator would be allowed the site for a period of one year, and from then on could seek government approval to extend their operation for three months at a time. District Board members have severely criticised the project. They say container trucks entering and leaving the site would worsen congestion on the Tuen Mun Highway. Mr Siu, however, said the Government would strictly monitor the work of the operators to make sure the site would not generate more traffic on already congested roads in Tuen Mun. Direct land access to and from the site by vehicles would be prohibited. ''Cargo will only be transported by sea, keeping additional loads off the existing highway,'' he said. As for noise pollution, Mr Siu said operators would have to adopt noise reduction measures. Work at night would be strictly prohibited. He said the Government was studying whether islands near the west New Territories could be used as a permanent site for the boats, known as mid-stream operators. An additional 20 hectares of land would eventually need to be found for mid-stream operators.