Sampling protocols for soil geochemical surveys

Abstract

The protocols used for the field collection of soil samples in Canada for the North American Soil Geochemical Landscapes Project are described in detail in Geological Survey of Canada Open File 6282. This document is available on-line at http://geopub.nrcan.gc.ca/moreinfo_e.php?id=261633&_h=6282 [accessed March 14, 2010]. The collection of soil samples for geochemical analysis involves preparation in the office as well as work in the field. If the purpose of the survey is to provide an estimate of the background range of element concentrations in soil, use of a spatially random and statistically defensible sampling design is required. In the office, target and alternative sites are determined based on the sample design and study of existing maps showing the distribution of bedrock, surficial materials and soils, and also topographic maps showing road access and the lay of the land. It is also necessary to design a field card that includes information on site location and description, the underlying types of surficial materials and bedrock, descriptions of the soil pit and soil materials collected from the pit. The field card can be modified as needed to meet field conditions. Site and pit photographs are important for archiving and referencing to complement the field card information. Horizon based sampling is recommended, otherwise varying amounts of organic matter, Fe- and Mnsesquioxides, and clays will influence trace element concentrations due to the varying amounts of their presence. Sampling by horizon allows for an easier interpretation of geochemical variation if samples are collected from similar pedologic horizons. Horizon-based sampling requires familiarization of the soils of the study region through review of the literature. When beginning work in a new study area it can prove to be quite helpful to have a field meeting with soil scientist familiar with local soils and learn what to expect and how to discriminate between horizons and sub-horizons. To provide information on the parent materials, a sample is collected from the C-horizon which at most sites in Canada is reached by a depth of 1 m. Depending on the project purpose, samples will be collected from other horizons (O, A or B). It may also be desirable to collect a sample from the 0-5 cm depth interval referred to as the Public Health (PH) layer. Although the PH layer is not a pedologic horizon, it is the depth that most strongly affects human exposure. Samples should be collected using a standard set of equipment. A list of suggested equipment is given in Open File 6282. Stainless steel should not be used to avoid contamination and if the sampling tool is covered by paint, have it removed possibly by sand. Once in the field, a pit is excavated to allow visible distinction of different horizons and the collection of samples from the desired horizons for geochemical analysis and other types of tests such as bulk density and carbonate content determinations. Sampling begins with the lowermost horizon and sampling upwards through the profile minimizes cross-contamination of the horizons. Kraft paper bags are recommended for sampling as canvas sample bags are often treated with elements such as As or Sb to guard against mildew. To prevent loss of volatile elements such as Hg and As, samples should be stored, shipped and dried in areas where the temperature remains below 30o C.