Sampling and Sampling Equipment for Water, Soil and Sediment

It is always nearly impossible to investigate and analysis the whole population. Thus, it is important to know how to obtain a representative sample for analysis.

A representative sample is one that truly reflects the composition of the material to be analyzed within the context of a defined analytical problem. Without a representative sample, results may be meaningless or even grossly misleading.

Wrong selection of sample introduces error that cannot be corrected during the analysis. This section is therefore focused on best and standard practice in the collection of sample for environmental monitoring with particular reference to oil pollution monitoring.

Pre-Sampling Activities

Before setting out to collect samples, it is very vital that you understand the aims of the analysis and know the appropriate sampling procedure to be adopted for field sample collection.

In a number of situations, a sampling plan or strategy may need to be devised so as to optimize the value of the analytical information collected. This is necessary particularly if the environmental samples of soil, water or the atmosphere are to be collected for a complex industrial process being monitored.

In some situations, legal/regulatory requirements may determine the sampling strategy to be adopted like in the food and drug industries or in dispute cases.

Sampling Plan Design

As previously mention, there must be a strategy/plan for your sampling and such sampling plan must support the goals of an analysis. Factors that must be considered vary with the sampling goals.

For instance, in a qualitative analysis the sample’s composition does not need to be identical to that of the substance being analyzed.

What is important in this case is having enough samples taken to ensure that all components can be detected. In a quantitative analysis, however, the sample’s composition must accurately represent the target population.

Read Also : Waste Sampling Equipment and Waste Sampling Procedures

In sampling plan design you must ask yourself the following four questions and provide appropriate answers to them to have a correctly designed sampling plan:

From where within the target population should samples be collected?

What type of samples should be collected?

What is the minimum amount of sample needed for each analysis?

How many samples should be analyzed?

The first question deals with the sampling location within the populations. This can be addressed by the selection of sample using random sampling or selective sampling or systematic sampling or stratified sampling.

Random sampling protocol involves an unbiased estimate/division of the target population’s properties and the collection of samples at random from the target population. This is not a haphazard collection but the use of random table (a statistical table) to select randomly.

The opposite of random sampling is selective or judgmental sampling. In this case available information about the target population is used to help select samples.

There are assumptions about the target population included in the sampling plan. Selective sampling is more biased than random sampling and often fewer samples are required.

Systematic sampling falls in between the two extremes of random and selective sampling. In systematic sampling the sampling site/population (e.g. a lake) is divided as in the figure 6.1 below and samples may be collected from either the centre of each unit or at the intersection of grid lines.

Figure: Systematic sampling of a lake (Harvey, 2000)

Figure: Illustration of Stratified sampling

After determining where to collect samples, the second question to answer is what type of sample to collect. There are three (3) common types of samples:

• Grab samples,
• Composite samples, and
• In-situ sampling.

The most common type of sample is a grab sample, in which a portion of the target population is removed at a given time and location in space. A grab sample, therefore, provides a “snapshot” of the target population.

A composite sample on the other hand consists of a set of grab samples that are combined together to form a single sample. After thoroughly mixing, the composite sample is analyzed as representative of the whole population.

In-situ sampling,uses an analytical sensor placed directly in the target population to collect necessary information about the population. It allows continuous monitoring without removing individual grab samples.

For example, the pH of an effluent polluted river moving can be continually monitored by immersing a pH electrode into the flowing river.

The third question is how much sample to be collected based on how much sample is needed for the analysis. To minimize sampling errors sample must be of an appropriate size. If the sample is too small its composition may differ substantially from that of the target population, resulting in a significant sampling error.

You may also not have enough for the analysis thus hindering the goal of the analysis by limiting information available. Samples that are too large, however, may require more time and money to collect and analyze, without providing a significant improvement in information obtained from sampling.

The last question is how many samples to collect and in what different matrix (surface water, groundwater, soil, sediment, plant, fish etc.). The essence of this is to provide detailed information about the sampled population with minimal error. This is an important because inappropriate number of samples will introduce maximum sampling error.

Sample Collection

The collection of sample for environmental analysis is often carried out with specialized equipment based on the media of the environment of interest. The section is focused on the different sampling equipment for the various environmental media.

Sampling Equipment for Water, Soil and Sediment

Soil: The equipment used for soil sample collection is soil auger (Figure 6.3) especially when the sampling is for profiling at different depth. The soil augers are of different design due to different soil types.

The auger is marked 0-15 cm, 15-30 cm. Soil samples collected in each portion of the mark is labelled as top soil (0-15 cm) and bottom soil (15-30 cm).

The samples are then analysed and used for the profiling. If profiling is not the aim of the analysis, a hand trowel can be used to collect the top soil sample alone.

Figure: Image of Various Designs of Soil Auger (Online)

Sediment: Collection of sediment samples are often undertaken with different designs of grab samplers. The samplers are used to collect sediment samples is river beds. The variation in the design is due to the depth and the speed of the water current in the water.

A Van- veen grab sampler is often used in deep, fast moving water. It has a rope tied to it that is used to close it after the sediment sample has been collected and to pull it up. The sediment in the collection chamber of the grab sampler is emptied into aluminum foil or suitable containers.

The Erkman grab sampler is often used for shallow, slow moving river and can be deployed by applying pressure which closes the sampler and collects the sediment in the process. Erkman can also be adapted for use in deep, fast moving water.

Figure: Van-veen Grab Sampler

Sample Collection General Requirements and Quality Assurance

The following general requirements should be observed during sampling to ensure quality of your results:

Ensure that samples meet the requirements of the purpose of the sampling;

Handle samples such that they do not deteriorate or become contaminated or become compromised before they are analysed;

Ensure that all sampling equipment is cleaned and quality- assured before use;

Ensure that sample containers are cleaned and free of contaminants to avoid false results due to contamination;

Bake all bottles to be used for organic-analysis sampling at 450°C to ensure removal of organic contaminants;

Fill sample containers without prerinsing with sample especially if preservative is pre-added to the container. Prerinsing can sometimes cause bias higher results when certain components adhere to the sides of the container.

Depending on determinations to be performed, fill the container full (most organic compound determinations) or leave space for aeration, mixing, etc. (microbiological and inorganic analyses).

If a bottle already contains preservative, take care not to overfill the bottle, as preservative may be lost or diluted.

When sampling for analysis of volatile organic compounds, leave an air space equivalent to approximately 1% of the container volume to allow for thermal expansion during shipment.

You may obtain composite samples by collecting over a period of time, depth, or at many different sampling points depending on your sampling purpose and design.

Carefully consider the technique for collecting a representative sample and define it in the sampling plan.

Add appropriate preservative when needed and in correct quantity. Use ultrapure preservative to prevent contamination.

Keep a sampling note with details of sample such as: record of sampling location; identity of sample as written on every sampling bottle; sufficient information about sample identification; date of sampling, hour, exact location (use Global positioning systems – GPS); sample type (e.g., grab or composite); any other data especially those collected at the field such as water temperature, pH, weather conditions, water level, stream flow.

State details of sample handling in your sampling note. Information such as whether or not the sample has been filtered; preservatives used etc;

Make sure you properly label sample immediately after collection. Use waterproof ink to record all information.

Read Also : Checklist for Waste Sampling

In summary, the quality of your analysis results cannot be better than the quality of your samples collected. It is therefore very important that you have an excellent pre-sampling plan or design by correctly answering the questions of: from where within the target population should the samples be collected? What type of samples should be collected? What is the minimum amount of sample needed for each analysis? How many samples should be analyzed?

You should ensure that you use appropriate sampling equipment and containers, appropriately pre-treat the sampling vessels; collect the sample with adequate quality assurance steps; preserve and handle the sample properly.