The two types of water samples typically taken are grab samples and composite samples. Grab samples are usually taken when you want information specific to a particular sampling location, time or distinct areas within a sampling location. Composite samples are usually taken when we want an average representation of a sampling location or time. For example, if we want to know how water chemistry varies within a lake, then we will take a number of grab samples. If we don't care how the chemistry varies within the lake, but just want to know the "average" water quality of the lake, we can take a composite sample from several locations.
Grab samples
A properly taken grab sample is a snap shot of the quality of the water at the exact time and place the sample was taken. Depending on the water body, grab samples may be taken by simply dipping a sample bottle in the water body, or they may require the use of specific sampling devices.
There are two types of grab samples that are used for sampling water matrices: discrete and depth-integrated. The discreet grab sample is one that is taken at a selected location, depth, and time and then analysed for the constituents of interest.
A Depth-Integrated Grab Sample is collected over a predetermined part or the entire depth of the water column, at a selected location and time, in a given body of water, and then analysed for the constituents of interest.
The primary advantage of grab samples is that sometimes very little equipment is required for sample collection and there is flexibility in sampling location selection. However, this method sacrifices data resolution because of the smaller number of samples that are usually collected
Composite samples
A composite sample is a mixture of grab samples taken at different times or locations and pooled together to provide one sample. The advantage of composite sampling is that it gives you an idea of the average condition of a water body over time, (samples taken at different times and mixed together) or space, (samples taken at different locations within the water body). This is particularly useful in water bodies that have a lot of chemical variability either over space or over short time periods. Composite samples are often used to reduce the cost of analyzing a large number of samples.
This method also has its limitations. Individual sample information is lost and you lose the ability to mathematically assess the variability. Also, some constituents like pH or dissolved oxygen tests are not stable enough for the mixing process, and others like bacteria require sterile sampling containers, a condition difficult to maintain over the composite sampling process.
Things to remember about composite sampling
- When considering multiple analytes in a composite, information regarding analyte relationships in individual samples will be lost.
- When the objective of the monitoring program is a preliminary evaluation or classification, compositing may dilute the analyte to a level below the detection limit if that particular analyte was present in one location only or in one time period only. This could produce a false negative.
- If sampling costs are greater than analytical costs, analyzing each sample individually may be more cost effective.
- If compositing reduces the number of samples collected below the required statistical need of the Data Quality Objectives, then those objectives will be compromised.