Determination of sampling time and frequency In order to make the collected water samples representative and reflect the temporal and spatial variation of water quality, it is necessary to determine the reasonable sampling time and frequency according to different water body functions, hydrological factors, pollution sources, pollutant discharge and other actual conditions (1) The monitoring sections in drinking water source areas and provincial boundary sections that need to be controlled should be monitored at least 12 times a year; (2) For the monitoring sections of large 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, generally from the 5th to the 10th of each month; (3) The sampling of monitoring section affected by tide is carried out in spring tide and neap tide respectively. Water samples of rising and falling tide were collected for determination. The rising tide water sample should be taken at the rising water level of the section, and the falling tide water sample should be taken at the falling water level; (4) The background section of water system is sampled once a year; (5) If a required project has not been detected for three consecutive years, and it is determined that there is no new emission near the section, but the emission of existing pollution sources has not increased, it can be sampled once a year for determination. Once it is detected, or there is a new emission source near the section, or there is a new emission from the existing pollution source, the normal sampling will be resumed; (6) In case of special natural conditions or pollution accidents, emergency monitoring scheme shall be adopted in time to increase sampling frequency; The sampling frequency should be increased in a certain period of time when the river course of local flow area is renovated, and the specific scheme should be formulated by the local administrative department of environmental protection where the renovation project is located. Result expression, quality assurance and implementation plan Many chemical, physical and biological monitoring data measured by water quality monitoring are the basic basis for describing and evaluating water environment quality and carrying out environmental management. It must be calculated and processed scientifically and expressed in the monitoring report according to the required form. Quality assurance summarizes all activities and measures to ensure the accuracy and reliability of water quality monitoring data. Quality assurance runs through the whole process of monitoring. See Chapter 2 for details. The implementation plan is the specific arrangement for the implementation of the monitoring scheme, which should be practical and feasible, so that all aspects of the work can be carried out in an orderly and coordinated manner.