Selected water quality data from La Plata River Basin shared by 5 SouthAmerican countries (Argentina, Bolivia, Brazil, Paraguay and Uruguay) was employed to evaluate advantages and constraints for global water quality indicators development. Water quality state indicators from UNSD/UNEP Questionnaire (2004) were considered at 5 sampling stations located on the Paraná (3), Pilcomayo and La Plata Rivers. Water pollution pressure indicators, also called driving forces, were estimated for the main emissions from the Metropolitan Area of Buenos Aires into the La Plata River. Multiple and different monitoring programs and operating agencies limited the availability of continuous time series of water quality indicators for this large river basin. Further development of water pollution pressure and technological indicators is required to improve cause-effect water quality follow up. Attention was given to the need to develop criteria for censored data statistics (mostly related with toxics constituents), and to establish tools to assess, periodically, the state of water environments through water quality guidelines, standards and risks to the biota through the usage of potency factors.
The Biotic Ligand Model (BLM) was used to assess copper bioavailability, toxicity and water-effect ratios (WER) for the Yacyretá Reservoir on the Paraná River. Copper in the Yacyretá Reservoir watershed is ubiquitous (median dissolved concentration = 4.4 μg L -1 ), being contributed mainly by non-point surface run-off from citrus fields and urban sewage point sources. BLM simulations indicated that copper bioavailability is negligible, as almost 100% of the dissolved copper is complexed by dissolved organic carbon (DOC). Copper bioavailability in the Paraná River, which has a mean hardness concentration of 16.5 mg CaCO 3 L -1 , is controlled by the presence of humic substances (≈60% of humic acids) in DOC. BLM estimates of acute toxicity for a cyprinid fish species were of the same order of magnitude as those corresponding to daphnids (EC 50 = 63.54 μg L -1 ). However, zooplankton species were chosen to represent the aquatic biota for water quality regulatory purposes because of their slightly higher sensitivity to copper toxicity. Based on the corresponding minimum WER (14.6) for copper in Yacyretá Reservoir on the Paraná River, a site-specific copper criterion for the protection of the aquatic biota of 24 μg L -1 was derived. All measured dissolved copper concentrations for Yacyretá Reservoir during the 2001-2002 period were in compliance with this criterion. Development and application of the BLM provides a useful tool for managing metal loads in reservoir environments.
In order to determine copper toxicity (LC50) to a local species (Cnesterodon decemmaculatus) in the South American Pilcomayo River water and evaluate a cross-fish-species extrapolation of Biotic Ligand Model, a 96 h acute copper toxicity test was performed. The dissolved copper concentrations tested were 0.05, 0.19, 0.39, 0.61, 0.73, 1.01, and 1.42 mg Cu L−1. The 96 h Cu LC50 calculated was 0.655 mg L−1 (0.823 − 0.488). 96-h Cu LC50 predicted by BLM for Pimephales promelas was 0.722 mg L−1. Analysis of the inter-seasonal variation of the main water quality parameters indicates that a higher protective effect of calcium, magnesium, sodium, sulphate, and chloride is expected during the dry season. The very high load of total suspended solids in this river might be a key factor in determining copper distribution between solid and solution phases. A cross-fish-species extrapolation of copper BLM is valid within the water quality parameters and experimental conditions of this toxicity test.
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