This assessment is based on data on contaminants, monitored in transitional, coastal and marine waters in the context of the WFD and the MSFD. The part of reported data from water, sediment and biota, as well as the information about biological effects, are derived from the DOME data portal of the International Council for the Exploration of the Sea (ICES). Other key data sources are data reported under the European Environment Information and Observation Network (Eionet), EMODnet Chemistry (the Baltic Sea and the Black Sea) and the EMBLAS project (Black Sea). In addition, France and Portugal have made new data sets available. For this analysis, Europe's seas were divided into grid cells of 20 × 20 km2 in coastal waters and 100 × 100 km2 in offshore areas. The CHASE+ methodology is a simple five-step procedure applied in every assessment unit. The five steps are: Step (1): substances/indicators are grouped into four categories (C1: water; C2: sediment; C3: biota, C4: biological effects). Step (2): for each individual substance/indicator, a contaminant ratio (CR = Cstatus/Cthreshold) is calculated. Step (3): for categories C1-3, a contamination score (CS) is calculated. Step (4): each category is subdivided into five status classes with class boundaries: 0.0-0.5 (NPAhigh), 0.5-1.0 (NPAgood), 1.0-5.0 (PAmoderate), 5.0-10.0 (PApoor) and > 10.0 (PAbad). Step (5): category-specific classifications are subsequently combined for each assessment unit into an integrated classification of 'non-problem area' (NPA) or 'problem area' (PA) by using the worst classification — the 'one-out, all-out' principle.

Based on the EEA reference grid, two grids have been developed covering the Marine Regions and Sub-regions of Europe. The first grid 100x100 km cell is used in offshore areas (> 20 km from the coastline); the second grid 20x20 km covers the coastal areas (<= 20 km from the coastline). The grid sizes were chosen after an evaluation of data availability versus the need for sufficient detail in the resulting assessment. Each cell in the grids has a unique identification defined from the lower left UTM coordinates. The CHASE+ results are associated to the unique cells in the grids.

More information can be found in the report "Contaminants in Europe's seas" and online material on https://www.eea.europa.eu/publications/contaminants-in-europes-seas/.