FIELD TRACER TESTS ON THE MOBILITY OF NATURAL ORGANIC-MATTER IN A SANDY AQUIFER

The field-scale transport of natural organic matter (NOM) was examined in a two-well forced gradient injection experiment in a sandy, coasta l plain aquifer in Georgetown, South Carolina. Spatial moments describ ed the migration of the center of mass of NOM and conservative tracer. Temporal moments were used to estimate mass loss and retardation of t he NOM along a transect of six sampling locations at two depths and at the withdrawal well. Large differences were observed in transport beh avior of different subcomponents of NOM. Larger and more strongly bind ing NOM components in the injection solution are postulated to adsorb and displace weakly binding, low-molecular weight NOM in groundwater. Conversely, NOM components that were similar to the groundwater NOM we re transported almost conservatively, presumably due to ''passivation' ' of the aquifer by previously adsorbed components of the groundwater NOM. NOM may thus exhibit two types of effects on contaminant dynamics in the subsurface. When the equilibria between solution and solid pha se NOM is disrupted by introduction of a novel source of NOM, descript ions of the multicomponent transport process are complex and predictiv e modeling is problematic. Because of the differences in transport beh avior of NOM subcomponents, the chemical properties and, more importan tly, the functional behavior of NOM with respect to contaminant migrat ion will vary with time and distance along a flow path. However, when groundwater NOM exists at a steady state with respect to adsorption on aquifer surfaces, the migration of NOM, and the contaminant-NOM compl ex, may be approximated as the transport of a conservative solute.