Evaluation of sampling frequency for the water quality monitoring network of coastal Galicia (northwest Spain)

To contribute to the efficiency of the water quality monitoring network of the hydrographical area of coastal Galicia (Northwest Spain), a sampling frequency that would ensure reliable verification of compliance with European Union standards, and a preliminary trend estimation of water quality are calculated. The criterion applied was that for each of four quality determinants (flow rate, chemical oxygen demand, conductivity, and percent dissolved oxygen), the sampling frequency should ensure a 95% confidence interval for the annual mean whose upper limit would lie below the corresponding legal threshold. During the study, all water tested complied with the more stringent standards for drinking water. The confidence interval and trend for flow rare were also calculated.