Determination of sampling points: after setting the monitoring section, the sampling vertical line on the section shall be determined according to the width of the water surface (see table 3-4), and then the position and number of sampling points shall be determined according to the depth of the sampling vertical line (see table 3-5). Table 3-4 setting of sampling vertical line number Water surface width Number of vertical lines explain ≤50m One (thalweg) 1The vertical line should be set away from the pollution zone, and additional vertical line should be added to the pollution zone to be measured. 50-100m Two (near left and right bank with obvious water flow) 2If it can be proved that the water quality of the section is uniform, only the thalweg vertical line can be set.3. When the pollutant flux is to be calculated in this section, it must be set according to this table ＞100m Three (left, middle, right) Set the vertical line.   Table 3-5 sampling points setting on sampling vertical line Water depth Sampling points explain ≤5m A little bit up there 1The upper layer refers to the place 0.5m below the water surface and 1 / 2 of the water depth when the water depth is less than 0.5m. 2The lower layer refers to 0.5m above the river bottom. 5-10m Upper and lower two points 3Middle layer refers to 1 / 2 depth of water. ＞10m Upper, middle and lower levels 45 m under the ice, and 1 / 2 of the water depth when the water depth is less than 0.5 m. 5If the pollutant content is to be calculated in this section, the sampling points must be set according to this table. When there is temperature stratification in the water body of lakes and reservoirs, the number and location of sampling points can be determined according to the relationship between temperature distribution layer and sampling points (table 3-6).   Table 3-6 setting of vertical sampling points for Lake (reservoir) monitoring Water depth Stratification Sampling points explain ≤5m   One point (0.5m below water) 1Stratification refers to the stratification of lake water temperature.2. When the water depth is less than 1m, the measuring point is set at 1 / 2 of the water depth.3. When there is sufficient data to prove that the water quality of the vertical line is uniform, the number of measuring points can be reduced as appropriate. 5m-10m No stratification Two points (0.5m below water surface and 0.5m above water bottom) 5m-10m layered Three points (0.5m below water surface, 1 / 2 inclined temperature layer, 0.5m above water bottom) ＞10m   Except 0.5m below the water surface and 0.5m above the water bottom, 1 / 2 of each inclined layer is set   Fig. 3-2 schematic diagram of sampling point setting in intermediate temperature layer A1: in the surface temperature layer;A2: under the interlayer;A3: in the subthermal layer;A4: about 1m above the interface between sediment and water medium;h: After the location of water depth monitoring section and sampling point is determined, there should be fixed and obvious natural signs on the bank.If there are no natural markers, artificial markers should be set up, such as vertical stone columns, piling, etc.Each sampling should be strictly based on the markers, so that the collected samples are taken from the same location, so as to ensure the representativeness and comparability of the samples.