Guidance for Performing Site Inspections Under CERCLA (540-R-92-021), Page 76
"Special precautions should be taken to ensure the sample represents the surface at that location, and that the sample is not altered or contaminated by sampling and handling procedures. Soil samples collected for comparison should be the same soil type and from the same soil horizon. Considerable variability may occur between soil types as well as within a single soil type because of grain size, mineralogy, composition, soil horizons, and lateral heterogeneity. Soil type should be identified and delineated. For metals analysis, background, and observed contamination, soils should have similar texture, color, and grain size."