Integrated ecotoxicological and chemical approach for the assessment of pesticide pollution in the Ebro River delta (Spain)

Application of pesticides in the Ebro River delta (NE Spain) during the rice growing season is suspected to be one of the major causes behind the shellfish mortality episodes that occur yearly in this area at spring time. In an attempt to shed light on this suspicion, a monitoring study combining ecotoxicity measurements in water using three different bioassays and pesticides analysis in both water and shellfish has been carried out in this area in April-June 2008. Water and shellfish samples have been collected at six selected sites, two of them located in the bays where seafood (mussels and oysters) are grown, and four located in the main draining channels discharging the output water from the rice fields into the bays. Toxicity of the water samples has been evaluated using three standardized bioassays: 24-48h immobilization of Daphnia magna, growth inhibition of Pseudokirchneriella subcapitata (formerly known as Selenastrum capricornutum) and bioluminescence inhibition of Vibrio fischeri. Analysis of pesticides (six triazines, four phenylureas, four organophosphorous, one anilide, two chloroacetanilides, one thiocarbamate and four acid herbicides) in water has been carried out by on-line solid-phase extraction-liquid chromatography-tandem mass spectrometry (SPE-LC-MS/MS). Analysis of pesticides in shellfish has been performed by pressurized liquid extraction (ASE), followed by SPE clean-up and analysis by gas chromatography-mass spectrometry (GC/MS). Results have shown individual pesticides concentrations in water above 100ng/L for about 50% of the compounds investigated, and total pesticides levels above 5μg/L in the draining channels some days. The most ubiquitous compounds have been bentazone and MCPA and the highest levels have been observed for malathion (up to 5825ng/L) and MCPA (up to 4197ng/L). In shellfish, malathion has shown the highest concentration (53mg/kg). A reasonable coherence has been observed between pesticide concentration (in water and shellfish), toxicity, and mortality episodes of shellfish, for the different locations studied. Taking into consideration the pesticides concentrations found, the toxicity values for the individual compounds, and the toxicity data measured on real samples, the pesticides suspected to be the main contributors to the total ecotoxicity are malathion, and to a lesser extent diazinon and molinate.