Down but not straight down: significance of lateral flow in the vadose zone of karst terrains

Abstract

Karst terrains exhibit some of the most heterogeneous and anisotropic subsurface properties of any geologic media. The vadose zone, and especially the epikarstic portion of that zone, is often important in detaining and laterally transporting contaminants in the subsurface. The epikarst, which is the interval between the regolith and rock in soluble rock landscapes, is routinely poorly characterized due to its vast heterogeneity and generally inadequate understanding of its importance in the subsurface movement of water and contaminants. At waste sites in karst, the traditional approach of measuring water table elevations at several wells, contouring these data, and inferring groundwater flow directions often fails to adequately characterize the subsurface movement of contaminants. This simplistic view of the subsurface presumes that infiltrating water and contaminants will move vertically from the surface to the water table. Lateral flow within the vadose zone, and especially in the epikarst, can produce radically different infiltration pathways than anticipated under the traditional vertical infiltration conceptual model. The horizontal extent of flow in the vadose zone is summarized from four case histories. The complicated connectivity of openings in the vadose zone, coupled with often variable infiltration from the surface, not only creates an unpredictable groundwater recharge regime at any one point in time but also a flow regime that can change under varying hydrologic conditions. Smith (Tenth multidisciplinary conference on sinkholes and the engineering and environmental impacts of karst, ASCE Geotechnical Special Publication, vol 17, pp 154–160, 2005) studied two landfills in karst portions of Tennessee. He found that landfill contaminants were distributed roughly radially around the landfills and even up-gradient monitoring wells were impacted. Smith (Tenth multidisciplinary conference on sinkholes and the engineering and environmental impacts of karst, ASCE Geotechnical Special Publication, vol 17, pp 154–160, 2005) suggested groundwater mounding beneath the landfills, but a more likely explanation is lateral flow in the vadose zone. A case history of an Alabama (Aley in Hydrogeologic assessment of ground water and surface water contamination resulting from the Florence, Ala., Municipal Landfill, p 35, 2010) landfill illustrates the practical implications of substantial lateral transport through the vadose zone. An additional case history of petroleum leakage contaminating a nearby water supply well in Arkansas (Aley in The engineering geology and hydrogeology of Karst Terranes, Balkema, Rotterdam, pp 207–211,1997) illustrates rapid transport of contaminants through a vadose zone with thick residuum.