Figures (4)  Tables (1)

    • Figure 1. 

      Assessment logic for surface water status under the WFD.

    • Figure 2. 

      Execution logic linking surveillance monitoring (SM), operational monitoring (OM), and investigative monitoring (IM) within a six-year management cycle.

    • Figure 3. 

      Institutional architecture of the US water quality governance and monitoring system.

    • Figure 4. 

      Comparison of surface water monitoring frameworks under the WFD and the CWA across four dimensions: management objective, monitoring network, site selection, and oversight and accountability.

    • Type Objectives Site selection strategy Key parameters
      and elements      		 Frequency and timing Typical outputs
      Surveillance monitoring
      (SM)      		 Assessment of baseline status and long-term trends; Validation of pressure and impact analyses. Located at the main river stems, key control sections, representative lakes, and estuaries to cover major water body types. EQR; Priority substances and river basin specific pollutants; Multi-media matrices where applicable. Cross-seasonal sampling over multi-year cycles; Integration with automated stations where feasible. Establishment of ecological and chemical baselines; Identification of long-term trends; Risk screening.
      Operational monitoring
      (OM)      		 Compliance diagnosis for at-risk water bodies; Evaluation of the effectiveness of management measures. Targeted deployment at sensitive reaches subject
      to dominant pressures; Locations requiring increased temporal resolution.      		 Pressure-specific indicator combinations (e.g., nutrients, dissolved oxygen, salinity, hydromorphology, and specific substances). Frequencies adapted to parameter variability; Intensified sampling during critical periods or for event monitoring. Trajectories of compliance status; Assessment of measure efficacy; Prioritization of management actions.
      Investigative monitoring
      (IM)      		 Diagnosis of causes for status deterioration; Source tracing for anomalies or accidental pollution events. Flexible and intensified deployment; Situated upstream or downstream
      of suspected sources, tributaries, or point source perimeters.      		 Expanded screening, including suspended solids, sediments, and biota; Application of source tracing and fingerprinting techniques. Temporary, high-frequency sampling campaigns. Determination of causes; Recommendations for intervention; Evidence base for network optimization.

      Table 1. 

      Comparison of the EU surface water monitoring types