The first step in a site investigation should be the gathering of background information. information concerning the history of activity at a site (including locations and age of buildings, drainage pathways, contours, building layout, foundations, septic systems, tanks, etc.; processes and materials for manufacture, storage and disposal both past and present, or historical spills) can be extremely useful in planning sampling events.
A file search may reveal areas of a site used for specific processes (aerial site history, site plans, area land use may also be useful) and will help in the logical placement of sampling locations.
Data from the Nigerian Geographic Information System (GIS) are a valuable resource that can provide additional background information to investigators, enabling the ability to analyse mapped datasets on computer. GIS datasets relevant to the history of activity at a site include statewide land use, soils, geology, and digital aerial orthophotography.
By revealing what materials were handled on site, a file search may provide guidance in choosing which parameters to include for analysis.
Additionally, while caution must still be used, judgments regarding health and safety requirements can be made.
When no information is available, field personnel must consider that worst case conditions may exist and take proper precautions to insure safety.
Defining the Physical Environment
Equal in importance to finding out what may be on-site is determining where it is most likely to be located. A pre-sampling site visit should be conducted to gather additional background information.
Labels and dot numbers on drums and tanks may be useful. Files found on-site may include information about materials that were manufactured, stored or disposed of on-site. Product names may be determined from shipping labels or manifests.
Any and all information will be useful in sampling plan preparation, and in formulating a site- specific health and safety plan.
The fate of environmental contaminants is dictated by the source, the characteristics of the contaminant itself, (i.e., persistency and toxicity) and perhaps most importantly, by the physical environmental system into which it is released. Contaminants move at varying rates and to varying degrees when released into different kinds of matrices.
Defining what kind of environmental system the site is a part of is extremely important to the success of achieving the sampling objectives.
An investigation into the local geology, hydrology (including flow rates of nearby surface waters, average depth to ground water and flow direction, identification of areas of recharge, etc.), and climatology is necessary.
The biological system should also be assessed. The flora and fauna of the area (including identification of sensitive environments and/or species, stressed vegetation, potential for bioaccumulation and biotransformation in the plant and animal life, especially agricultural) are definite factors to be taken into account.
Stressed vegetation may serve as an indicator for contaminant migration to a particular area. A GIS system and GIS data can assist investigators in defining both the environmental and biological systems.
These data elements include soil type, hydrograph, land use, wetland delineation, surface contours and more.
Overall, by defining the physical environment, the fate of contaminants can be predicted. Migration pathways should also be identified assuring that samples will be collected in the most appropriate area.
The factors addressed above offer an overview of considerations that must be evaluated for a sampling plan to be complete.
The more information is obtained, the more that will be known about the source, movement, and concentrations of contaminants in the media to be sampled. With this knowledge, it will be easier to write a complete, site specific sampling plan.
Along with the historical and physical information needed prior to sampling plan development, the following topical areas of basic information are necessary components for an inclusive sampling plan.
Sample Locations and Numbers
The objective of the sampling event is important when choosing the location of sampling points. Samples are sometimes collected to characterize a site for which limited background information is available and/or obvious contaminated areas do not exist.
In such a case, a random sampling scheme may be useful. Random sampling depends on the theory of random chance probabilities to choose the most representative sample. This process is utilized when there are numerous available sampling locations and there are no satisfactory reasons for choosing one location over another.
Tables of random numbers are readily available from many sources and should be used to eliminate any possible bias generated by those collecting the sample, assuming a random approach is used.
Also important when choosing sample locations is consideration of the site’s physical environmental setting and how these factors can influence the concentration and movement of the material of concern.
Sampling at hazardous waste sites is usually conducted in an attempt to discover contamination and to define its extent and variability. With such an objective, it is most logical to choose sample locations that will yield the most information about site conditions.
Here, judgment (or biased) sampling should be employed. Biased samples are those collected at locations that were chosen based on historical information, knowledge about the location and behavior of the contaminant(s), and/or knowledge about the effects of the physical system on the contaminants’ fate.
Both biased and random sampling techniques can be used together to thoroughly address an entire site. Some samples may be biased to potentially contaminated areas (e.g., stained soil, former process or disposal areas) or potentially impacted areas (e.g., areas of stressed vegetation, sediment downstream from discharge pipe).
In areas less likely to be contaminated or areas with little available background information, random samples may be used to allow adequate assessment of the entire site.
Sample Size Parameters
There are seven factors that determine the number of samples required for site characterization:
Statistical performance objectives
Data quality objectives
Quality assurance objectives
Site specific conditions.
For example, if the objective of the event is to determine whether the site is contaminated, a limited number of samples, from properly chosen locations, will yield useful information. A greater number of samples may be needed however, if the site is known to be contaminated and delineation of the contamination is the objective. In many cases statistical considerations can be helpful in determining sampling strategy.
Sample Methodology and Matrix
Once the appropriate numbers and locations have been chosen, consideration must be given to what collection method will be used to assure that representative samples of site conditions are obtained.
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The selected sampling methodology will be matrix dependent. In some instances, there may be several acceptable options available for collecting a sample. In other instances, site-specific conditions may dictate that only one approach will work, even though that method may not be the preferred method.
In all cases, the construction material of the sampling device, its design, decontamination, and proper use are critical factors and should be included in the proposed sampling plan.