Determination of Sampling Time and Sampling Frequency
In order to make the collected water samples representative and reflect the temporal and spatial changes in water quality, it is necessary to determine reasonable sampling time and frequency based on different water body functions, hydrological elements, pollution sources, pollutant emissions, and other actual situations

Determination of Sampling Time and Sampling FrequencyThe general principle is:
(1) Monitoring sections that require key control in drinking water sources and provincial boundary sections should be monitored at least 12 times a year;
(2) For monitoring sections on larger water systems, rivers, lakes, and reservoirs, samples are taken once a month and monitored six times a year; The national monitoring section (or vertical line) is sampled once a month, usually from the 5th to the 10th of each month;
(3) The sampling of monitoring sections affected by tides is carried out during the high tide and low tide periods respectively. Collect water samples for each rising and falling tide and measure them separately. The water sample for high tide should be taken at the level of rising water at the cross-section, and the water sample for low tide should be taken at the level of falling water at the cross-section;
(4) The background section of the water system is sampled once a year;
(5) If a certain mandatory test item has not been detected for three consecutive years and no new emissions have been confirmed near the section, and the existing pollution source emissions have not increased, it can be sampled once a year for measurement. Once detected, or when there are new emission sources or new pollution sources with increased discharge near the cross-section, normal sampling will be resumed;

（6） In case of special natural situations or pollution accidents, emergency monitoring plans should be adopted in a timely manner to increase sampling frequency; When renovating rivers in local water flow areas, the sampling frequency should be increased within a certain period of time, and the specific plan should be formulated by the local environmental protection administrative department where the renovation project is located.

2. Result expression, quality assurance, and implementation plan

The numerous chemical, physical, and biological monitoring data obtained from water quality monitoring are the basic basis for describing and evaluating water environment quality and conducting environmental management. They must be scientifically calculated and processed, and expressed in the required form in the monitoring report.
Quality assurance encompasses all activities and measures to ensure the accuracy and reliability of water quality monitoring data. Quality assurance runs through the entire process of monitoring work. Please refer to Chapter 2 for detailed information.
The implementation plan is the specific arrangement for implementing the monitoring plan, which should be practical and feasible to ensure that all aspects of the work are carried out in an orderly and coordinated manner.