FLOW AND REACTIVITY EFFECTS ON DISSOLVED ORGANIC-MATTER TRANSPORT IN SOIL COLUMNS

Dissolved organic matter (DOM) plays a prominent role in the transport of contaminants in porous media. As DOM has to be considered as a rea ctive component, now regime and sorbent reactivity should affect overa ll DOM transport in an important way. We focused on DOM transport in u nsaturated column experiments using quartz sand (QS) and goethite-coat ed quartz sand (GS). Rate constrictions to DOM sorption were investiga ted by varying the volumetric now rate, while extent and reversibility of sorption were studied in consecutive adsorption and desorption ste ps. In the QS, DOM retention was low and unaffected by changes in now rate. Desorption-step breakthrough curves (BTCs) and mass balances sho w full reversibility of the sorption process. However, DOM retention i n GS was significant and sensitive to now variation, indicative of non equilibrium sorption. At lower now rates, DOM breakthrough exhibited a change in curvature (shoulder) due to the superimposition of two BTCs representing reactive and nonreactive DOM fractions. Transport was su ccessfully modeled assuming these two fractions governed overall DOM m obility. At higher flow rates, the BTC shoulder vanished due to reduce d contact time between the DOM and the solid phase (rate-limited sorpt ion). Sorption of DOM on GS is accompanied by a marked rise in effluen t pH, indicative of a ligand-exchange mechanism. Recovery of DOM durin g desorption was incomplete due to either partially irreversible sorpt ion or strongly rate-limited desorption. Increased DOM mobility in the consecutive adsorption step resulted from partial blocking of sorptio n sites by the initial pulse of DOM.