Rivers play a significant role in assimilating or carrying off municipal and industrial wastewater, as well as run-off from agricultural land. To collect a representative water sample, it is best to collect from the middle of the water body, about 6 inches below the surface. Hold the container near its base, submerge the neck below the surface, and face the opening upstream (against the current) approximately 6 inches inside the water. Slowly tilt the container to allow the sample to flow in, then remove and seal the bottle immediately.

If direct access to the sampling point is not possible, you may use another container to collect the sample and transfer it to the official laboratory bottle. However, it is crucial to avoid contamination during the transfer. Fill the sample container in one go without rinsing it, leaving about ¼ of the container as air space. Bottles should be thoroughly rinsed with the sample water before collection, even if pre-cleaned.

The choice of sampling method depends on the nature of the water body and the specific requirements of the analysis. can be choose as follows:

Grab Sampling- A single sample is collected from a specific location and time, representing the water quality at that moment. This method is suitable when the water quality does not change significantly over short periods.

Composite Sampling- Multiple samples are collected over time from the same location and combined to represent average conditions. This is useful when water quality fluctuates over time or depth.

Integrated Sampling- A mixture of grab samples from different points is collected simultaneously to account for horizontal or vertical variations in water quality.

Drone Sampling- For inaccessible sites, drones equipped with sampling systems can be used to collect water samples autonomously. This method enables precise sampling in hard-to-reach areas and is controlled via ground software that manages the drone's flight and sampling process. you can go through these links for more detail.

https://cpcb.nic.in/displaypdf.php?id=em9iZW5nYWx1cnUvV2F0ZXJfc2FtcGxpbmdfYW5kX3ByZXNlcnZhdGlvbl90ZWNobmlxdWVzLnBkZg==

https://www.usgs.gov/special-topics/water-science-school/science/water-quality-sampling-techniques

https://www.fs.usda.gov/rm/pubs/rmrs_rn049.pdf

In one of my research regarding water sampling, though its a lake, not a running water, i chose three areas for sampling'; [1] the area where I know that the sample will be worst, [2] area where water is very much accessible to the community and [3] the area nearest to the source. Then of course, take precautionary measures on handling the sample. it should reach the testing laboratory within 8 hours, properly capped and do not expose to sun.

Yes there are. At United Nation Environmental Programme (UNEP). https://www.unep.org/explore-topics/water/monitoring-water-quality

When sampling water from rivers, collect samples from the middle of the water body at a depth of 20-30 cm, with the container facing upstream to minimize contamination. In shallow areas, using another container for initial collection is acceptable before transferring to the official bottle. These practices help ensure the sample is representative and reliable.