Assessment of risks of MTBE contamination of groundwater aquifers
Summary and conclusions
MTBE (methyl-tertiary-buthyl-ether) has since 1985 been added to non-leaded gasoline, primarily 98 octane, but is also found in 95 octane. MTBE is readily soluble in water and will only slightly be biodegraded or withheld in soil and groundwater. Surface spills or leakages of MTBE from gas stations therefore present a threat to groundwater quality. This is confirmed by an increasing number of MTBE-contaminations found in Danish groundwater.
Investigations of gas stations on the Island of Funen have shown a high probability of finding MTBE in secondary groundwater aquifers. In several cases MTBE has also been found in primary groundwater aquifers. In 2001 the Danish Environmental Protection Agency and the County of Funen therefore initiated a number of projects concerning contamination with MTBE. The present project is one of them. The primary purpose of the project is to assess the risk of groundwater contamination from 43 MTBE-contaminated sites on Funen. Risk assessment is carried out towards groundwater resources, well fields and the total abstraction rate on Funen. Secondarily the purpose of the project is to scale this risk assessment to cover all Denmark.
The risk assessment is primarily carried out towards MTBE and secondarily towards benzene. The assessments are based on the future groundwater limit value (5 µg/l) for MTBE and the existing limit value for benzene (1 µg/l).
The project is based on the results of investigations carried out at 43 sites on Funen where MTBE contaminations exceeding the present groundwater limit value for MTBE (30 µg/l) have been found either in the secondary groundwater aquifers or the primary groundwater aquifers or in nearby drinking water wells.
A review of degradation constants for MTBE under different redox conditions has been carried out in order to create input data for the risk assessments.
Furthermore the risk assessments have been carried out based on a number of assumptions about the following;
 | point source concentrations |
| extent of contamination |
| rate of infiltration |
| groundwater gradient |
| hydraulic conductivity |
| thickness of low permeable layers between secondary and primary aquifers |
| degradation rates and redox conditions |
Risk calculations show that these parameters have varying implications for the calculations and their uncertainties. The point source concentrations and the extent of contamination are of major importance for the calculations.
The risk of groundwater contamination under the 43 sites is based on calculations in a worksheet prepared by the Danish EPA (JAGG version 1.5).
Based on a number of assumptions the calculations show that the contents of MTBE in the primary aquifer exceed the groundwater limit value under 39 % of the MTBE contaminated sites (after a transport time of 1 year). The corresponding percentage for benzene is 44 %. A major assumption is that the highest measured concentration corresponds to the point source concentration.
However it is estimated that the highest measured concentrations of MTBE and benzene only on 20 % of the investigated sites correspond to the concentrations measured in the middle of the point source area. Calculations on these sites show, that a larger number of sites than above mentioned can lead to contamination of the primary aquifer in concentrations exceeding groundwater limit values for MTBE and benzene.
The risk of contaminating groundwater wells in catchment areas covering the 43 MTBE contaminated sites has been assessed by comparing the amount of contaminant with the amount of abstracted water. This method of risk assessment has been supplemented with a numerical model that simulates mass transport under 3 sites located within the same catchment area.
Mass calculations show that 25 % of the catchment areas may be contaminated with MTBE above the limit values. The corresponding percentage for benzene is 38 %. In these calculations it is assumed that the contamination within one year is washed out and transported to the drinking water well without being degraded or withheld.
As expected these calculations as well as the JAGG calculations show that the content of MTBE and benzene is reduced under transport to the drinking water well. The reduction for benzene is less than expected due to the fact that the mass calculations do not include degradation, an important process for benzene.
Simulation of groundwater flow and mass transport under the mentioned 3 sites indicate that the model in comparison to mass calculations gives a more precise indication of groundwater flow direction, extent of the catchment area and the contamination plume.
Based on mass calculations and data collected from investigations on the 43 sites a rough estimate of the risk of contaminating all groundwater abstracted on Funen and of the risk of contaminating the groundwater resource has been carried out. This risk assessment has furthermore roughly been scaled to cover the whole country.
Calculations show that 8-26 % of the total amount of abstracted water on Funen may be contaminated with MTBE above the limit values. Compared to the total potential drinking water resource this means that 630 million cubic meters of water may be contaminated with MTBE above the limit value. This amount corresponds to 5 times the yearly recharge.
For the country as a whole it is roughly estimated that 36-117 million cubic meters of water out of a total abstracted amount of 450 million cubic meters can be contaminated with MTBE exceeding the limit value. This estimation is assessed to overestimate the size of the risk in areas where the groundwater aquifer is oxidized and MTBE therefore to some degree will be biodegraded (as for example in the west of Jutland). On the other hand the risk is underestimated for the Copenhagen area where large amounts of water are abstracted compared to the potential resource.