If all the world's water were represented in a half-gallon milk cartons, the amount of fresh water that people can get at would be about half a teaspoonful. And only a single droplet of that would not have to be pulled out of the earth.

Documents prepared for the United Nations Water conference opening Monday in Mar del Plata, Argentina, are a fountain of such information. The worldwide scarcity of fresh water mens that women in rural Burma regularly walk 15 miles a day to get some and bring home, a six-hour trip. What they get is often polluted: 25,000 persons died every day from water-borne disease.

There are floods, droughts, waste, water rights and irrigation to discuss at the two-week conference, which is expected to draw more than 2,000 representatives from most countries on earth on the seaside resort south of here.

"This will be a conference on policy and management," said Secretary General Yahia Abdel Mageed of Sudan in a briefing her. "The accomplishment of the conference should be looked on as creating awareness of the magnitude and importance of the problems of water."

One problem sure to arise is political, United Nations meetings traditionally have provided forums for conflicting nations to denounce one another, and this one could see several such disputes. India and Bangladesh are quarreling over rights to the Ganges River, while Israel and the Arab countries have all prepared position papers on the Jordan River question in case the other side raises it, according to diplomatic sources.

Conference organizers hope, however, that the main focus will be on worldwide fresh water supply crisis looming by the year 2000, and that some action will result "through idscussion which leads to agreement at the highest government levels," according to a background booklet.

"The perception of water as a finite resource to be preserved and protected is relatively new and not universally shared," the National Audubon Society reports.Only a fifth of the world's population has access to processed drinking water, and more than a third of them get it from an outside source and not in their homes.

Half of all people supplied with water get it only intermittently, and a quarter to half of all water that leaves the world's treatment plants is listed as unaccounted for - lost through leaks, unauthorized use or unmetered faucets.

"Rich countries are often well-endowed with water but they, too, can be faced with severe water crisis," Mageed said, noting the severe drought in the western United States. "One way or another, all nations are affected, developing and developed, rich and poor."

The human body demands from a pint and a half to five gallons of water a day, depending on the climate and exercise involved, U.N. documents show. But human use ranges from a subsitence-level three pints in some places to 165 gallons per person where lawns are watered and golf courses are maintained.

Growing industrialization is making it worse. Africulture now accounts for 80 per cent of the world's fresh water use, and it takes 1,000 tons of water to grow a ton of grain. But industrial water use is more than agricultural, and household use together in some developed areas. Processing, cleaning, steam heating and cooling are 'contributing to the progressive and chronic degration of the quality of available water," Mageed said.

Only 18 per cent of the world's cultivated land is irrigated, according to surveys, but it produces 40 to 50 per cent of all world's food. "The solution to it all seems to lie not in exotic production methods, which are still too expensive, but in expansion of traditional means," Mageed said.