Summary

Erfaringsopsamling på udbredelsen af forureningsfaner i grundvand på villatanksager

An experience gathering has been carried out, covering all approx.1.400 cases under the Danish domestic heating oil insurance policy, between March 2000 and the fall 2008.

The study shows that groundwater pollution plumes from private heating oil storage tanks in general are limited in size, and that pollution plumes larger >40-50 meters can only be expected in extraordinary situations. Thus, on the 14 cases matching the objective screening criteria applied, the median pollution plume length was 13 meters and the largest plume was 48 meters.

Data from the selected cases furthermore shows that the pollution plumes decreases in length over time. After 2-3 years of monitoring, the expected maximum plume length is 25-30 meters. The largest reduction in plume length is found in cases where the maximum soil concentration is <5.000 mg/kg dry matter and in cases without presence of Non-Aqueous Phase Liquid (NAPL).

Additionally, the results show that plumes, in cases where the contaminant mass is <500 kg product, are <10 meters after plume stabilization.

The primary parameters controlling the plume propagation is documented to be the size of the spill and presence of NAPL in the source area.

In addition, there is a depletion of oxygen, nitrite and sulfate within the plume compared to both up- and down steam conditions, indicating that that aerobic degradation of heating oil components are occurring within the plume.

The length of measured pollution plumes and the length of plumes calculated by applying Miljøstyrelsens JAGG model were compared. It must be noted, that in some cases the JAGG model domain was extrapolated beyond the prescribed limits for plume length calculations. As a consequence, calculated plume lengths beyond 100 meters should be considered with caution and are only of orienting character. The results show, that when degradation and sorption are disregarded, plumes calculated with the JAGG model are more than 20 times longer than the measured plumes. If sorption and aerobic degradation are included in the JAGG model (JAGG step III), using standard values for benzene as a model component, it also results in conservative estimates of the calculated plumes lengths compared to the measured plume lengths in all 14 cases.

As cases with deposits of clay under the groundwater table were shown to result in shorter plume lengths, pore water velocity was indirectly shown to influence on the length of the pollution plumes.

The horizontal propagation of heating oil in the groundwater was generally found to exceed vertical propagation by far. The maximum concentration of total hydrocarbons measured in primary groundwater was 65 µg/L and the concentration decreased to <5 µg/L within two years of monitoring. BTEX was never measured in concentrations above the regulatory limits.

Data regarding NAPL plume length was relevant in 9 of the 14 cases. The maximum propagation of NAPL was measured to approx. 30 meters with a median plume propagation of 4 meters. The largest NAPL plumes were found in cases with the largest amount of heating oil. Furthermore, there is a tendency to a reduction of the NAPL plume length over time. NAPL-plumes are found to stabilize during the first year of monitoring in cases without source zone remediation.

The cases considered in the study represent some of the largest cases with spills are from private heating oil tanks both regarding the spill size and the extent of the groundwater pollution. This is because the objective screening criteria include documented groundwater concentrations significantly exceeding the regulatory limits and that groundwater monitoring has been carried out over a relatively long period either before or after remediation. Furthermore, the screening criteria ensure that the pollution has reached the groundwater. Thus, the remaining cases are smaller than the ones applied in the study. Hence, the cases included in the study are presumed to have larger plumes than the average spill from private heating oil storage tanks.