Sorbent Wicking Device for Sampling Hydrophobic Organic Compounds in Unsaturated Soil Pore Water. I: Design and Hydraulic Characteristics

The hydraulic characteristics of horizontally installed sorbent wick sampling devices were evaluated through wick tracer studies and laboratory soil column experiments to assess the influence of horizontal wick length and sampler interface design on sampling pore water in unsaturated soils. The nominal sampler design consisted of a cylindrical porous metal interface packed with granular-activated carbon encapsulating the end of a fiberglass wick that extended 100 cm horizontally from the interface before dropping 100 cm vertically to a collection vessel. The maximum sampling rate of horizontally installed wick systems declines exponentially with increasing horizontal wick length, while the vertical length influences the range of soilwater pressures that may be sampled. The nominal design sampled pore water from clay loam laboratory columns at 8 to 14 mL  h-1 under steady-state infiltration conditions and 2 to 5 mL  h-1 under draining conditions across a -10 to -45 cm H2O soil-water pressure range. Sampling rates in medium-grained sand under similar flow conditions were less than that of the clay loam due to reduced water content and reduced interface/soil contact area. An analysis of observed sampling velocities versus calculated soil water contents and hydraulic conductivities indicated that the design performs best when the soil water content is greater than 0.15 and unsaturated hydraulic conductivity is greater than 0.2 cm  h-1. A hydraulic model was developed that estimates the sampling velocity of the nominal design based on sampler interface pressure, which was linearly correlated with soil pressure.