A Water Quality Index Applied to an International Shared River Basin: The Case of the Douro River |
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Authors: | Adriano A Bordalo Rita Teixeira William J Wiebe |
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Institution: | (1) Laboratory of Hydrobiology, Institute of Biomedical Sciences (ICBAS) and Ciimar, Lg. Abel Salazar, 2, 4099-003 Porto, Portugal;(2) Department of Marine Sciences, University of Georgia, Athens, Georgia 30602, USA;(3) Present address: Ciimar, Rua dos Bragas, 289, 4050-123 Porto, Portugal |
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Abstract: | A Water Quality Index (WQI) is a numeric expression used to evaluate the quality of a given water body and to be easily understood
by managers. In this study, a modified nine-parameter Scottish WQI was used to assess the monthly water quality of the Douro
River during a 10-year period (1992–2001), scaled from zero (lowest) to 100% (highest). The 98,000 km2 of the Douro River international watershed is the largest in the Iberian Peninsula, split between upstream Spain (80%) and
downstream Portugal (20%). Three locations were surveyed: at the Portuguese–Spanish border, 350 km from the river mouth; 180
km from the mouth, where the river becomes exclusively Portuguese; and 21 km from the mouth. The water received by Portugal
from Spain showed the poorest quality (WQI 47.3 ± 0.7%); quality increased steadily downstream, up to 61.7 ± 0.7%. In general,
the water quality at all three sites was medium to poor. Seasonally, water quality decreased from winter to summer, but no
statistical relationship between quality and discharge rate could be established. Depending on the location, different parameters
were responsible for the episodic decline of quality: high conductivity and low oxygen content in the uppermost reservoir,
and fecal coliform contamination downstream. This study shows the need to enforce the existing international bilateral agreements
and to implement the European Water Quality Directive in order to improve the water quantity and quality received by the downstream
country of a shared watershed, especially because two million inhabitants use the water from the last river location as their
only source of drinking water. |
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Keywords: | Water quality index Discharge Residence time Reservoir Dam Douro River |
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