The USGS Water Science School
Checking the water quality of the Nation's streams, rivers, and lakes is one of the main responsibilities of the U.S. Geological Survey (USGS). Physical water measurements and streamflow are almost always taken, but often water samples are needed for chemical analyses. Generally, it is imperative that water samples be representative of the whole stream, and so, sampling a stream means more than just dipping a coffee cup in at the stream bank and sending it to the laboratory. The USGS uses strict scientific methodology in taking samples of any water body.
The USGS has to utilize different methods and equipment when taking a sample of water from a stream—it all depends on the size of the stream, how deep the water is, and how fast the water is moving. Also, I should add, on the ability of the water scientist to be able to access the water. As the left-side pictures below show, often a hydrologist can simply step out into a small stream and dip a bottle in at the appropriate place, but on larger rivers, it might be necessary to build a cableway and take water samples from high above the water surface. Sampling methodology also depends on the type of water sample needed.
For a small stream where the water is well mixed, it is sometimes possible to take a single "grab sample", where the hydrologist just dips a bottle in the stream at one location, still trying to move the bottle up and down to sample the entire vertical column of water. Note how the sampler always stands downstream from the sampling point—don't want to stir up any sediment that could alter the chemical analysis of the water sample.
Quite often it is important to take a water sample that represents the stream as a whole. That entails taking small amounts of water from numerous horizontal sections across the stream, at regular intervals, as the middle picture shows. There is a bottle inside the white container at the end of the pole (bottom picture). The bottle has a small tube in it that allows only a small amount of flow into the bottle, and thus, the hydrologist can regulate how much water is sampled at various points in the stream. She can sample different horizontal sections separately by using a different bottle for each vertical section or use a single bottle for the whole stream.
It takes a lot more work to get a water sample from a larger river, as this picture shows. In larger rivers, there is more chance of variability in the water characteristics and quality across the river. There may be a tributary coming in from the left side above the sampling point or there may a wastewater-treatment outflow pipe a mile upstream on the right bank.
It takes longer for all the water in large rivers to mix together. So, to understand the water properties of the whole river it is necessary to obtain individual samples at set increments across the river. Bridges make this task very convenient, although samples can be taken using a boat, if no bridge is available.
If the water is moving fast or if the depth is too deep, then a crane with an electric motor (or hand crank for especially hardy hydrologists) is used to obtain the water sample (above picture). The heavy metal "fish" which holds the sampling bottle is needed to keep the sampler from being pushed downstream, as it is important to representatively sample the vertical column of water at each sampling point across the river. The hydrologist has to move the sampler up and down at a steady rate until the bottle is filled, while at the same time being sure not to smash the nozzle into the mud on the stream bed!
USGS can't always count on a nice, wide bridge being available for hydrologists to sample from, and sometimes it is too dangerous (due to high flows or floating debris) to use a boat for sampling. In these cases, a cable can be strung across the river, from which a hydrologist can move across and sample and measure the river as needed.