Location of soil borings
Soil borings are usually located as follows:
- In areas where contamination is expected (hot spots) on the basis of preliminary studies (soil gas, geophysical methods, etc.) and the knowledge of the location of previous existing production plants, landfills, or other land use (preliminary site investigation).
- Near the boundaries of already known contaminated areas in order to determine the extent of contamination.
- In areas, which have or are suspected to be highly sensitive to contamination.
- At the rest of the site, since it is not always possible to localize all hot spots during an initial investigation.
In order to reveal unknown contamination and to achieve a strategic coverage of the site, it may be beneficial to define sample grids or nets which include a specific number of soil borings. Sampling densities and location of sampling points will depend on the site and contamination characteristics. The most frequently used distribution models are based on a random or a systematic location of sampling points. Within systematic distribution, different model types can be distinguished.
Download figure: Distribution of sampling points
Generally, boreholes can be distributed uniformly over the whole contaminated site, in case contaminated areas within the site are unknown. However it is important to know that such approach is rather costly and it is only recommended if results of Phase I Site Assessment indicate that soil/groundwater contamination is extended over the whole site.
The distance between soil borings should not exceed 450-500 m and recommended working scale for an intrusive investigation is 1:50.000 - 1:25.000. For small sites or sites with high expected soil/groundwater contamination level the optimal scale is 1:10.000 - 1:5.000.
On the contrary, if only a few potential contamination sources are identified and results of Phase I Site Assessment indicate a limited soil/groundwater contamination, intrusive investigation should be considered only within contaminated areas. In this case, the number of soil borings can be limited to one-three borings per each identified source of contamination and should be located mainly downgradient of the sources of contamination.
If results obtained during Phase I Site Assessment show a potential risk of nearby areas causing impacts on soil and groundwater quality in the study area, some soil borings should be located close to or along the site boundaries. This is especially important, in case potential sources of soil and groundwater contamination in the nearby areas are located upgradient of the site.
Generally, the exact location of soil borings on-site must be identified before starting with drilling operations and marked with painting/spray directly on the ground.
Soil boring depths generally depend on specific site geological conditions. However, since the aim of investigation works is to define impacted soil volumes, the depth of soil borings may be extended to vertically delineate any soil impacts observed visually during drilling operations. If risk of contamination of deep aquifers is negligible, drilling of soil borings until the first confining layer is recommended. Nevertheless, if this risk can’t be excluded taking water samples from deeper aquifers will be required.