Development of a water quality index as a management tool to Sado coastal estuarine areas

This work was developed within the scope of creating an integrated management model to the Sado estuary (Portugal) that will provide decision-makers and citizens a tool, regarding water quality, that will be easy to interpret and use, allowing to determine high, medium and low zones of anthropogenic pressures on Sado estuary identified by colors, blue (excellent), green, yellow, orange and red (very bad). To fulfil this objective, an estuarine water quality index (EWQI) was created and applied to the Sado estuary by establishing a criteria-basis for the water quality index. A total of 27 samples were collected in 9 different points of the estuary, and 16 parameters were measured. To define the EWQI, the relevance of the parameters were analyzed and combined with indicators in previous literature, noting that proposed WQI for saline waters are rather scarce, literature is mainly focused in evaluating freshwater systems. A new EWQI index for saline and transition waters is now proposed based on 7 parameters: Dissolved Oxygen, Biochemical Oxygen Demand BOD, Nitrite + Nitrate, Phosphate, Total Suspended Solids, Chlorophyll-a and Ammoniacal Nitrogen; with respectively weighs of 0.20, 0.15, 0.15, 0.15, 0.10, 0,10, 0.15. The application of EWQI to Sado estuary data allowed to observe similar water quality patterns, amongst zones with same coastal activities, like industrial areas, and identify seasonal variability, with winter clearly showing a better water quality. This new tool are the basis to the development of an integrated management model for the Sado estuary.