Soil and Groundwater Sampling Methods

Abstract

Abstract Groundwater assessment for environmental projects involves significant soil sampling to evaluate hydrologic and lithologic conditions. Soil samples in environmental investigations are typically collected at a minimum of 1–2 m and at changes of lithology or obvious signs of contamination, based on soil staining or discoloration of the soil or in the case of volatile organic compounds, organic vapor readings. Continuous coring is recommended for a minimum of at least one soil boring per site to characterize the subsurface adequately. Soil samples collected in the vadose zone above the water table are in the unsaturated zone. Groundwater is moving generally vertically, and some horizontal movement is caused by rootlets, faults, impermeable layers, or other conduits. Soil samples collected within the capillary fringe or below the groundwater table are in the saturated zone. The capillary fringe moves up and down with seasonal variations of the groundwater table. Petroleum hydrocarbons move vertically from the source, such as an underground storage tank, through the vadose zone. As the hydrocarbons move vertically through the lithologic section, in general, the finer grained the soil, the greater the lateral spreading of the contaminant. The capillary fringe, within the saturated zone, is the zone immediately above the water table, where water is drawn upward by capillary attraction. Owing to irregularities in pore sizes among the grains of sediment, the capillary water does not rise to an even height above the water table but rather forms an irregular capillary fringe. The capillary fringe is higher in fine‐grained soils such as a fine silt than in coarse‐grained soil such as a gravel due to greater tensions created by the smaller pore openings. The capillary fringe can be the zone of direct evaporation of groundwater if the water table is close enough to the surface. This article is an updated version of an earlier article from 2005.