Concerns over contamination of ground water (GW) and its subsequent effect on surface water quality underscore the need for an improved understanding of the fate and transport of the contaminants in the subsurface. Among the contaminants that are harmful to humans and the environment are nutrient pollutants [e.g., nitrogen (N) and phosphorus (P)] and microbes.The general goal of this research was to evaluate the subsurface fates and transport of contaminants in a vadose zone-GW continuum under various simulated hydrologic conditions through a series of laboratory-scale studies. The first study, which aimed to visually evaluate the effects of GW velocity and water table (WT) fluctuation on the fate and extent of horizontal transport of solutes and microbes in the capillary fringe (CF) and GW, was conducted in a sand-packed flow cell. Subsurface transport of surface-applied solutes and microbes tended to be isolated in the CF at a higher pore-water velocity.A rise in WT resulting from surface recharge of contaminated water occurred without the contaminants reaching the GW. Subsequent drainage did not effectively leach contaminants that were initially in the CF into the GW. The second study assessed the effect of pore-water velocity on the development of reduced conditions in a vadose zone-GW continuum. Reduction potential (Eh) was monitored at various locations in flow cells packed with Ponzer (Terric Haplosaprists), Lynchburg (Aeric Paleaquult), and Leon (Aeric Alaquod) soil materials that were subjected to different lateral pore-water velocities. Regardless of organic carbon (OC) content of the soil materials (12.4 to 195 g kg-1), locations close to the WT became reduced within 14 days. In contrast, the upper portions of the CF remained oxic. Increasing the pore-water velocity also slowed the development of reducing conditions especially in soils with low OC content. The third study was conducted to evaluate the effect of pore-water velocity on the fate and transport of nitrate (NO3-) in a simulated vadose zone-GW continuum. This was conducted in flow cells packed with soils of various OC content (0.3 to 35 g kg-1)that were subjected to different horizontal-water velocities. Nitrate and bromide (Br) concentrations as well as Eh at various locations along the flow path of an applied NO3- and Br- solution were monitored.Results show that in the presence of sufficient OC, NO3- was lost under reducing conditions below the WT but persisted while in transport in aerobic regions in the CF. Increasing GW flow pore-water velocity from 3.5 to 28 cm d-1 reduced the degree of NO3- removal from solution. High flow velocity also tended to limit the horizontal transport of surface-applied NO3- only in the upper regions of the CF. The fourth study was conducted to evaluate the dissolution of phosphorus (P) in pore-water flowing through the vadose zone-GW continuum. Distilled water was allowed to flow horizontally at different pore-water velocities through flow cells packed with an organic soil material (from Ponzer series). Extensive P dissolution was detected below and just above the WT. Phosphorus dissolution at the upper portion of the CF was relatively limited. These results suggest the following: a) the non-detection of contaminants below the WT down-gradient from a source does not definitively indicate that contaminants are not being transported horizontally in the subsurface as they can be transported in the CF, b) collection of samples from the CF should be considered when monitoring the subsurface transport of contaminants, and c) the hydrology of a system could be managed to improve nitrate removal from solution or to limit P dissolution.
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Hydrologic Effects on Subsurface Fates and Transport of Contaminants
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