Sanders Beach Community Association

Education - Chemicals of ACW - Part 2 - DNAPLs

Mar 15, 2005

What is DNAPL

By definition, dense non-aqueous phase liquids (DNAPLs) are liquids that are heavier than water and have a low aqueous solubility. DNAPLs, such as creosotes, coal tar, and the chlorinated solvents perchloroethylene (PCE), trichloroethylene (TCE) and dichloro-ethylene (DCE), are the most frequent sources of contamination at Superfund sites. The main reasons for the high incidence rate and the magnitude of groundwater contamination by DNAPLs are: 1) for decades these compounds were widely used across industrial, commercial, and service sectors 2) both intentional and accidental releases of these compounds into the subsurface environment occurred routinely
3) DNAPLs are long-lived in the subsurface environment. The removal of DNAPL may lower the estimated time for site remediation. . . and lower estimated remediation costs.

Why are DNAPLS of concern? What are the health effects of DNAPLS on humans?
Extensive contamination of groundwater occurs as a result of significant dissolved plumes from DNAPL source zones, the groundwater regions in which DNAPL is present as a separate phase. DNAPLs often contain compounds that cause a significant risk to human and ecological health. Once in the subsurface DNAPLs tend to migrate rapidly downward toward the bottom of the aquifer. A pool is formed when a migrating DNAPL is prevented from making further progress through the porous medium by an increase in capillary resistance. Remediation of DNAPL-contaminated sites has proven to be one of the most difficult problems facing the Superfund program. Common remediation practices such as pump-and-treat, vapor extraction, gas stripping, and in situ biodegradation have been shown to be time-consuming, expensive and inefficient when applied to these sites.

DNAPLs can cause kidney and liver damage and may affect the central nervous system causing headaches, dizziness, nausea and even unconsciousness.

What are methods of cleanup for DNAPLS?
The current [EPA] strategy of source-zone containment has generally proven reliable for limiting routes of human and ecological exposure to chemical contaminants from DNAPL-impacted sites, provided that the containment system (e.g., pump-and-treat or permeable barriers) has been properly designed and maintained. However, this strategy poses long-term risks, transfers the burden of site management to future generations, and is generally difficult or impossible to quantify accurately. Commonly used pump-and-treat remediation systems have not been effective in removing DNAPL from subsurface source areas or in restoring down-gradient contaminated groundwater to desired levels of cleanliness. However, field-scale research has demonstrated that a high percentage of the DNAPL mass can be removed by implementing in-situ technologies such as thermal or chemical flooding.

Sources: 1) http://toxics.usgs.gov/highlights/dnapl_removal.html
2) http://www-apps.niehs.nih.gov/sbrp/research/research4_s3_s1.cfm
3)http://www-apps.niehs.nih.gov/sbrp/researchbriefs/view.cfm?Brief_ID=85