For example, soils considered homogenous often have subtle differences in layering that can cause a DNAPL to run and drop many times, creating a complex of thin horizontal and vertical ganglia. Both DNAPL soil residuals, which are the most common form of contamination/spill encountered, and pools become slowly dissolving sources of groundwater and soil vapor contamination. In addition, low conductivity areas into which the DNAPL mass/or and the dissolved-phase plume have diffused or migrated can in turn become sources of low-level contamination after the DNAPL mass has disappeared. Wolfe et al. (USGS 1997) contains a diagram showing the distribution of potential DNAPL-accumulation sites in a hypothetical karst setting (Figure 12Adobe PDF Logo). While the solubilities of these chemicals are very low (often hundreds to low thousands of parts per million), the level at which they can present a human health or ecological risk is considerably lower (few to hundreds of parts per billion). A fuller discussion of the fate and transport of DNAPLs is found in the Chemistry and Behavior section.