barcelona

Murky waters: millions of cubic metres of acidic water were pumped from the area affected by the spill. Credit: GREENPEACE/BELTRA

The Spanish government was overoptimistic when it initially evaluated the consequences of a toxic spill from the Swedish company Boliden's pyrite mines into the Guadiamar River and a small part of the Doñana National Park last April, according to the latest analysis.

The Higher Council of Scientific Research (CSIC), which was asked, in collaboration with universities, to monitor the clean-up operation, produced a map last week showing the current level of contamination in the 4,000 hectares of agricultural land that was covered by toxic sludge.

CSIC researchers estimate that, even after an intensive clean-up, half the soil contains acidity or heavy metals — especially arsenic and zinc — at concentrations that César Nombela, head of the CSIC, says “should still be called contaminated”. Average pollution levels have been found to exceed 50 to 100 parts per million (p.p.m.) of arsenic, and 15 per cent of the area may contain more than 275 p.p.m. of arsenic. International standards suggest a maximum of 50 p.p.m.

CSIC's report conflicts with an announcement in February by the Ministry of Agriculture that 84 per cent of the agricultural soil affected by the toxic spill is now suitable for cultivation. According to CSIC, government scientists appear to have based their study on only three elements — zinc, copper and lead — and to have ignored arsenic.

The accident led to the removal of 4 million tons of waste, in the form of sludge, and 4 million cubic metres of acidic water.

The clean-up work ended in January, having cost US$100 million. During the five months of intensive work, however, it was not possible to prevent the drying up and chemical oxidation of the sludge covering the river bed and 4,000 hectares of agricultural soils.

The oxidation increased the acidity of the soil, which was also penetrated by pyrite-containing metals. The challenge now is to find ways of cleaning up the soil. Some chemical procedures are already being applied to reduce the acidity and to immobilize metals in an attempt to prevent them from penetrating into the groundwater.

CSIC's researchers are also considering using the technique of phytobioremediation to remove arsenic from the soil. This involves the use of certain plants, including Brassica species indigenous to Spain, that can take up arsenic in large quantities through the roots.

Despite the lack of experimental data on the usefulness of such methods, Nombela says CSIC's scientists “have demonstrated the absorptive capacity of arsenic by indigenous plants” and argues that this will “soon lead to field tests to evaluate practical applications”. Work on achieving this by developing genetically modified plants, containing a bacterial gene, is said to be in its preliminary stages.

An international meeting in January in Seville discussed remediation schemes. But, rather than agreeing on the use of phytobioremediation, it was concluded that, partly as a result of the high concentrations of arsenic, there is no short- or medium-term solution to the pollution in Doñana's wetlands. Participants agreed that the only feasible strategy was to immobilize the toxic metals as an interim measure.

CSIC's strategy is controversial. Luis E. Santamaria-Galdón, an ecologist at the Netherlands Institute of Ecology, says he cannot understand why CSIC insists on using transgenic plants and microorganisms: “The problem is too urgent to start developing previously unknown techniques.”

But Nombela argues that “the molecular analysis of genes and biochemical mechanisms of absorption of pollutants offers an obvious path for scientific exploration that may also contribute to the development of new technologies”.