Several, though not too many, technically feasible and economically viable new options are available for increasing fresh-water supplies. Of the non-conventional ways, such as seeding clouds to induce precipitation, towing icebergs, desalting sea and brackish water and transporting water by tankers, the latter two appear to hold the greatest near-term potential. As a matter of fact, with the oceans holding 97% of all the water on earth, desalination of sea water might eventually offer the solution to a limited renewable supply of fresh water. Several desalination technologies, such as distillation, electrodialysis and reverse osmosis, have been developed, but since they are highly energy-intensive, they are so far too expensive for use except by countries that have non-marketable supplies of natural gas or islands that depend on tourism for a large share of their income. Transporting large quantities of water by tanker has become more common in recent years, as the unit cost of such transport has declined. Moreover, the current glut in the oil market has put a good proportion of the tanker fleet out of operation. These developments have provided an incentive for oil shippers to transport water to arid areas on their return voyages in tankers especially equipped with ballast tanks, but this has still not been done on a long-term or large scale basis.
Besides augmenting water supplies indirectly through conservation and by direct means, as discussed above, another major field of action must be stricter pollution control. Pollution control goes hand in hand, to a large extent, with water conservation in industry, agriculture and municipal water management. However, additional measures are needed, particularly to avoid eutrophication of surface and ground water through careless use of fertilizers and other chemicals in agriculture and to prevent pollution of water through long-range, transboundary air pollutants. Avoiding water pollution obviously means preventing hazardous pollutants from getting into bodies of water. In developing countries, controlling pollution caused by polluted municipal water and industrial waste water will pose a major challenge, in part because of the general lack or inefficient functioning of sewerage systems. Ground-water pollution is also becoming more serious, and poses serious risks to human health. These problems will have to be tackled, however, in order to prevent an already bad situation from getting worse.