Sampling the water directly is historically the most common monitoring approach. This type of sampling provides a direct measure of the concentrations of pollutants and other properties of the water such as temperature. These values also link to many water quality criteria. When coupled with flow measurements, this approach allows the calculation of the load (mass) of a particular pollutant found in a water body for a given period.
Water column samples are often easy to collect although some lab analyses may be quite costly. Most of the analytic methods for water chemistry and physical properties are standardized, simplifying comparisons between different monitoring sites and sampling events.
Water samples are often collected as “grab” samples by dipping a bottle or bucket into the water. In a well-mixed water body, a single grab sample may be representative, particularly for dissolved constituents. In a stratified water body, consider sampling the entire water column using vertically integrated samplers, or collect at distinct depths using specialized sample buckets. Particulate constituents may also be unevenly distributed across the width of a stream or river. In these cases, a series of samples should be collected across the width of the stream.
A significant disadvantage of collecting water samples at regular intervals is the loss of any information about the water body between sampling events. Often it is assumed that conditions do not change between sampling events. This may be a reasonable assumption (although one that should be verified) in relatively stable situations, such as deep water in stratified lakes or groundwater. In many situations, especially in rivers and streams, these “snapshots” in time may miss significant changes in water quality conditions between sampling events.
Integrating over time requires additional special equipment. “Automatic” water samplers with pump intakes can take samples at established intervals ranging from every few minutes to weekly. Take care that samples sitting in the collection bay of these devices do not exceed analytical holding times. Probes, coupled with data logging capabilities, are also increasingly available for water quality measurements. Probes must be calibrated at regular intervals, but this approach can provide essentially continuous data at a relatively low cost.
Some monitoring equipment or approaches, such as properly setting up data loggers or automatic samplers, may require specialized technical expertise. It is best to get adequate training and do “dry runs” ahead of time and/or hire or partner with people who have the requisite skills.