CPV code 90733100 designates Surface water pollution monitoring or control services in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
90733100 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 90733000 (Services related to water pollution).