**Q. I still don't think I completely understand the characteristics of non-aqueous phase liquids (NAPLs). I understand that it refers to contaminants in soil but I don't get how it interacts with the soil particles, water, and the vapors. You did say that it was complex so I don't feel too badly about it.
A. Complex it is. Petroleum engineers study this quite a bit in relation to oil extraction. It is a 4-way contest between sand, water, air or gas, and liquid hydrocarbons, and three phases, liquid, gas, and solid. Lots of equations and stuff.

**Q. I was surprised to see that contaminate in the soil is referred to as NAPL. I am familiar with LNAPL and DNAPL as substances that do not dissolve into the groundwater. I was also familiar with the pysiochemical process that you diagramed for us. But, I had never heard of contaminates in the soil being referred to as NAPL.
Just a generic term, which could be either D or L. Usually if were not talking about groundwater contamination, we don't use that acronym.

** Q. Several modules ago we looked at pump and treat system for biological remediation of groundwater contaminated with pentachlorophenol, (BioTrol, Inc.'s fixed film treatment process). In the paper it stated that the contaminate was mineralized to CO2. What happens to the other components of the original molecule?
A. Not sure what happens to the "chloro-," but I believe it goes to HCl. The hydrogen of course goes to water.

** Q. TCE solvent vapors are very volatile. The solvents in the shallow groundwater or soil may enter buildings and affect indoor air quality.
I 'm not sure about the pathway here. TCE would escape into the atmosphere if it is so volatile.
A. Excellent question. If you put some TCE in a pan and left it in a warm room, the next day it would be all gone. It has a fairly high vapor pressure. It's behavior in soil may be quite different. It may bind to soil particles and organics and chemicals in the soil. It is in a 4-way equilibrium with water, air, soils and itself in a matrix of soil, water, and air. I have found benzene, which has similar physical properties to TCE, in very high concentration in soil 20 or 30 years after a spill. In addition, the source may still be present. That is, it may be leaking from buried drums or tanks. From the soil, rainwater leaches the chemicals into the groundwater.

* Q. I didn't know that until relatively recently it was legal to have injection well least one quarter mile from a drinking water aquifer. I wonder how they came up with that measurement and what was the reasoning behind it.
A. The reasoning was that contamination did not travel that far. Today we know some contamination does travel that far. So new wells are not being permitted. But some old ones are still in operation. Presumably some risk assessment was done before the permit was renewed. Or they are in remote areas and there has never been a problem with them.