Quantification of dissolved organic matter in pore water of the vadose zone using a new ex-situ positive displacement extraction

Quantification of dissolved organic carbon (DOC) in pore water is vital for understanding the transport of solutes and the fate of contaminants. However, the extraction of pore water from saturated soils, and even more so, from unsaturated soils without an artificial increase in the DOC content is challenging. To this end, we have established a methodology for pore water extraction from unsaturated and saturated soils based on the displacement of the pore water from cores withdrawn from the ground by direct-push drilling machines. The displacement extraction is performed by introducing distilled water at the top of a liner filled with soil core and collecting the displaced eluent from the bottom. This positive displacement extraction (PDE) method was compared to pore water extraction by centrifugation for the analysis of DOC. Method validation was carried out with various DOC levels using different flow rates and three different soil types: sandy, sandy-clayey and clayey. The limit of detection for the suggested method was < 0.22 mg L^{− 1} for all soil types, and for all studied saturation levels (> 15%) under flow rates lower than 1 mL min^{− 1}. Extraction by centrifugation gave biased, unacceptable results throughout the studied DOC range (0–25 mg L^{− 1}). Detailed parametric dependence by mobile – immobile modeling showed that for most soils and saturation levels collection of the first 10% of the pore water initially present in the core will introduce a bias of less 8% in DOC. For clayey soils with an exceedingly high immobile water fraction, a pedotransfer parameter estimation or comprehensive column tests should be carried out in order to estimate accurately the collectible water fraction and assure accurate prediction of the DOC level. Field studies that were carried out in a Soil Aquifer Treatment system provided 30 m deep DOC profiles with < 1.5 m spatial resolution, a resolution that is unattainable by alternative techniques.