Evaluation of the water quality in the Guadarrama river at the section of Las Rozas‐Madrid, Spain

The variation of the water quality of the Guadarrama river and its tributaries in a section of Las Rozas-Madrid, Spain, was studied during the time period between January 2003 and January 2008. The parameter water quality index (WQI) was used to determine the water quality based on the conventional parameters of pollution. It was found that the WQI was slightly affected in the section evaluated. The value of the WQI was in the range of 56–64, which corresponded to the classification of ‘good quality’. It was determined that 64.3% of the organic matter present in the river was removed in the section of Las Rozas-Madrid. The river acted as a plug flow reactor and a first-order kinetics governed the ultimate biological oxygen demand (BOD5) (BODU) decay. The value of the first-order constant demonstrated the river's high self-purification capacity. In addition, a high linear relationship was found between the WQI and the dissolved oxygen deficit (D). Therefore, a quick determination of WQI may be carried out if the values of D are known. These are easily obtainable by field measurements.