Dense non-aqueous phase liquid (DNAPL) are mixtures of organic compounds that when combined have a specific density greater than water, which has a specific density of 1.00. They sink through water and form a separate liquid phase below the water. DNAPL mixtures can include compounds such as chlorinated solvents, PCB oils, coal tars, and creosotes (complex mixtures consisting of polycyclic aromatic hydrocarbons and other aromatic hydrocarbons). Since these compounds sink through water they have the tendency to migrate deep into the subsurface. As the DNAPL migrates through the subsurface, it leaves behind residuals as it sinks and will displace water from the soil pores. Upon reaching an impermeable layer, the DNAPL will pool until the depression is filled. If cracks and fissures are present in the soil, DNAPL will spill and infiltrate the fissures. DNAPL is typically sparingly soluble. Biodegradation of DNAPL is limited in the environment due to the pure phase creating a hostile environment for microbes.