Contaminant plumes and pseudoplumes in karst aquifers

摘要

Contaminant plumes in the strictest sense only occur in granular media. Defined as contaminated groundwater masses spreading by diffusion and hydrodynamic dispersion, plumes take the shape of a feather, from which the name derives. While plume-like masses may appear in a uniformly and densely fractured medium, they rarely develop in karst aquifers—though they are frequently alleged to exist. Unfortunately, a karst pseudoplume is often incorrectly interpreted to mean that monitoring wells are functioning appropriately; this misinterpretation can divert attention from gross contaminant movement to more distant locations. The highly heterogeneous and anisotropic character of karst aquifers, and the convergent nature of the conduit flow systems within them severely limit plume formation. What is often mistaken for a plume in these rocks may derive from three processes which will be discussed in this paper. First, the contaminant may move as a true plume in a granular overburden and find entry into the solution modified fractures of the bedrock at numerous points. There may be groundwater flow connections between these modified fractures, but such flow is most likely to be convergent rather than divergent. Second, when volatile contaminant vapors spread in an unsaturated epikarst, they may dissolve in percolating vadose groundwater and contaminate phreatic groundwater below. Third, solution conduits in karst are subject to regular and sometimes extreme increases in hydraulic head caused by concentrated quick-flow from precipitation events. Contaminated water in a conduit may be forced by these increases into the surrounding permeability structure, similar to temporary bank storage adjacent to a stream. When the head in the conduit declines, water returns to the conduit, leaving a fraction of the dissolved contaminants behind. With each successive rain event, the contaminant may be driven further into the surrounding rock.