Pore-water extraction from the unsaturated and saturated zones

Abstract

A simple, inexpensive method for extracting pore water from unsaturated soils, which uses a combination of immiscible fluid displacement, suction, and mechanical compression, is described. This method uses a squeezing chamber to contain the unsaturated sediments in aluminum core tubing to prevent exposure of the sediments and pore water to atmospheric O2 and subsequent oxidation reactions. Geochemical artifacts resulting from high-compression squeezing are prevented by maintaining the pressure applied during the squeezing at a minimum. Comparison of squeezed profiles with field measured profiles of pH, Eh (oxidation–reduction potential), alkalinity, and metal speciation indicates good agreement between these parameters using this method. Depending on the water content of the sediments, a range of water volumes (several millilitres to excess of 100 mL) can be extracted from a single core section 7.6 cm in diameter by 20 cm long. Pore water was produced immediately from sediments at near-saturated conditions, whereas several hours were required to obtain pore water from sediment with low moisture contents. Pore water was extracted from unsaturated sediments with as little as 6% volumetric moisture content. The squeezing technique provided water samples from both unsaturated and saturated sediments ranging from silt- to sand-sized particles. The method was applied successfully to collect pore waters from unsaturated sediments in neutral and acidic mine tailings; processed sands from oil sand operations, septic beds, agriculturally impacted sediments; and saturated soils from peat bogs, from wetlands, and at groundwater–surface interfaces.