San Diego

Palaeontologists and hydrologists are being called in to help the US government decide what to do with a disused, $400-million desalination plant on the banks of the Colorado River.

The researchers will study tree rings and clam shells, among other things, in a bid to establish long-term historical climate patterns in the river basin. These will help the US Bureau of Reclamation to determine whether it is worth reopening the Yuma Desalting Plant, which was designed to remove salt from the river before it reaches Mexico.

The plant was completed in Arizona in 1989, after salty water from the Colorado River had upset agricultural and natural ecosystems in Mexico. The salty water had entered the river from irrigation run-off from agricultural fields, particularly in western Arizona. Other steps taken to address the problem included building a canal to divert agricultural run-off to a marsh near the Gulf of California.

The desalination plant was operated for a test period of about six months in 1992. But in January 1993, heavy rains diluted the river, making desalination operations unnecessary. The plant was subsequently criticized in media reports as a waste of taxpayers' money, and environmental groups complained about the potential impact of brine extracted by the plant that was sent to the Mexican marshland.

But a four-year drought in Arizona has led officials to try to find new ways of securing more usable water, while meeting the requirements of the complex laws and treaties covering the river. A treaty signed in 1944 requires the United States to send 1.85 billion cubic metres of water to Mexico via the river, with the salinity level not exceeding set limits. The desalination plant could make a partial contribution to this target by removing salt from part of the river's water flow — it can desalinate 92.5 million cubic metres of water per year.

Reopening the plant would cost about $25 million, and annual operating costs are also about $25 million, so bureau officials only want to do this if it will be needed for a sustained period. In January, they convened a group of researchers at the University of Arizona in Tucson to develop a model for predicting droughts and higher-precipitation periods for the next 50 years.

University of Arizona palaeontologist Karl Flessa told the meeting that studying tree-ring records in forests in the Colorado River basin could allow him to track droughts, but not short periods of high precipitation, for the past 500 years.

Flessa and his colleagues are also examining the ratio of two oxygen isotopes in clam shells in the river delta to estimate historical higher-precipitation periods. The isotope ratio reflects the degree of salt water and fresh water to which a clam was exposed, as one isotope is more abundant in fresh water.

If the scientific studies show that it would be cost-effective to reopen the desalination plant, bureau officials expect such a move to face legal challenges from environmental groups. Just keeping the facility on stand-by costs about $3 million a year.