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PROPERTIES OF MTBE

	MTBE		BENZENE
Solubility, mg/l	48,000		1750
Koc	12		97
Diffusion Coefficient water, cm²/sec		0.78E-05	1.10E-5
Vapor pressure, mm Hg	249		95.2
Henry's Law Constant	0.024		0.220

All of the following properties show why MTBE is more likely to create a ground water problem than is Benzene.

Solubility in Water

MTBE is 28 times more soluble than Benzene.

Koc

Koc = organic carbon coefficient = concentration in carbon / concentration in water

The transport of hydrocarbons by groundwater is retarded by the presence of organic carbon in the soil.

The retardation factor = 1 + (R/q) x Kd.

R= Dry Bulk Density

q = Volumetric water content

Kd = Koc x organic soil content

Assuming a soil with the following properties: P= 1.8, q = 0.2 and organic soil content = 0.01 or 1%.

MTBE Retardation factor = 1+(1.8/0.2) x (0.01 x 12) = 2.08

Benzene Retardation factor = 1+(1.8/0.2) x (0.01 x 97) = 9.73

The groundwater transportation velocity of a compound: vr = the groundwater velocity/ Retardation factor.

vr MTBE = v/2.08

vr Benzene = v/9.73

Therefore, if the above site has a groundwater velocity of 200 feet per year, Benzene will move 20.5 in one year, while MTBE will move 96 feet.

Vapor Pressure

MTBE is 2.6 times more likely to vaporize than Benzene.

Henry's Law Constant

MTBE is 9.2 times more likely to stay in water or to enter water from the atmosphere than Benzene. MTBE prefers to be in water 42 times more than it does air.

The Henry's Law Constant is the ratio of the concentration in air divided by the concentration in water. MTBE's low Henry's Law Constant (0.024) indicates that it greatly prefers water to air and any vapor that contacts water will preferably move into the water.

CONCLUSIONS

All of the above properties make MTBE an difficult groundwater pollutant.

If a leak occurs of a gasoline containing MTBE and the gasoline encounters water a large part of the MTBE will enter the water.
Once in water, the MTBE will spread through groundwater transport and diffusion.
Even if the spill does not reach groundwater, MTBE is more likely to vaporize. Once that vapor reaches water it will enter the water. Often diffusion through soil vapor is much faster than groundwater transport or leaching.
Once in water, most of the MTBE will pass through carbon canister water filters, thereby making water disposal a much more expensive endeavor.