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Erfaringsopsamling på udviklingen i poreluftkoncentrationer på villatanksager Summary
BackgroundBased on experience from several soil pollution cases from private heating oil storage tanks, reduction of mass as caused by evaporation and biological degradation etc. is relatively fast immediately after a leak has taken place – especially for the relatively volatile components in the heating oil. So far, data which quantify the decrease in hydrocarbon concentration in soil air over time, or the time required before the overall level of soil air hydrocarbons has decreased to a level where it no longer poses a risk, have not been presented. ScopeTherefore, the primary scope of this report is to review the data from both ongoing and already finished pollution cases at private heating oil sites, in order to investigate whether there is a general tendency for the soil air concentrations of TVOC, benzene, sum of BTEX and C9/C10 aromatic compounds to decrease over time, and to investigate how fast the soil air concentrations decrease to a level below which the risk posed to the indoor air climate is very low or where a new concrete floor would be sufficient to eliminate risk for the indoor climate – see discussion below. The secondary scope is to investigate weather the necessary time for reduction of the hydrocarbon concentration in soil air is dependent on site specific data like remediation status, size of the oil spill, soil concentrations, geological layers and distance between soil pollution and sampling point for soil air. Considerations on general riskIn the Danish EPA guidelines no. 6, 1998, page 57, covering public financed remediation at polluted sites /3/, a dilution factor of 100 is considered to be a conservative estimate of the contribution of soil air concentrations to the indoor air in the majority of Danish housings provided that the building has a concrete floor without visible cracks. Furthermore, it is presumed that the floor is without other leaks around e.g. pipes, and that the building has passive ventilation on the order of 0.3 per hour. The considerations above are presumed when it is merely stated that “the concentrations in the soil air is below a factor of 100 times the Danish EPA regulatory limits for indoor air”, since this level often is considered a practical starting point for an assessment of whether a measured soil air concentration pose a potential indoor air risk, as there is (or is planned constructed) a concrete floor, which meets the demands regarding attenuation characteristics. The data generally encompass the largest private heating oil spillsInitially, a screening has been performed on all 353 ongoing and finished cases from Fyn and Northern Jutland to pinpoint cases with measurements of soil air sampled over time. Based on the screening, data from 24 well documented cases (135 measurements from 53 samplings points) were included in the study. Note that the 24 cases, due to the screening criteria applied, are expected to be among the largest and the most problematic cases, since smaller cases are discarded because of lack of time series (no need to measure in the soil air or no need to measure over time) or because all measured concentrations are below a chosen level of significance of 10 times the detection limit. A notable tendency toward decreasing soil air concentrationsThe data support a conclusion that the soil air concentrations in general are reduced over time. The reduction is most notable in the initial 2 years after a spill has taken place, after which the concentrations generally stabilize at levels below 100 times the regulatory limits for all heating oil components. The reduction encompasses all components (TVOC, benzene, BTEX and C9/C10 aromatics). The half life of the highest concentration in the typical sampling point (the median) is 7 months for TVOC; 2.2 months for benzene; 3.5 months for BTEX and 3.4 months for C9/C10 aromatics. Significant temporary increases are rareHowever, in 3-4 % of the measurements temporary significant concentration increases are observed for TVOC (after more than one year), which could lead to a change in the risk assessment, based on increases from a level below to a level above 100 times the regulatory limit. Under practical conditions, an increase from one measuring round to the next will often require that a new sample is collected and analyzed. Increases to a level above 20 times above the regulatory limits (still a factor of 5 below 100 times the limit) such temporary elevations are found in approx. 5 % of the measurements (after more than one year). In all instances, significant increases are followed by significant reductions in the subsequent measurement. For the other components (benzene and C9/C10 aromatics), no such significant increases are observed one year after the spill, and the increases are not observed in measuring points with larger distance from the soil pollution. The temporary increases in TVOC concentrations are observed in instances where the distance between sampling point and severe soil pollution is short (approx. 0.5 meters) or when the pollution is dissolved and the water level elevates to 0.3-0.4 meters below the floor – i.e. when the source is brought closer to the sampling point. Concentration decreases are fastest for relatively small spillsThere is a clear tendency for the concentrations to decrease to a lower level within the first 1-2 years when the pollution is of moderate size; <100 tons of polluted soil or <500 kg (residual) product, compared to cases with more severe pollution. Fraction of data below 100 times the regulatory limit after 1 year for all dataFor benzene and C9/C10 aromatics there are no incidents where the concentration is above a 100 times the criteria after more than one year. The concentration of TVOC is below 100 times the regulatory limit in 75% of all measurements after one year. TVOC is the primary risk component in soil air from private heating oil tanksAssessing data overall, TVOC is the primary risk component for soil air pollution at private heating oil storage tank cases. This assessment is based partly on the fact that the concentration level for TVOC generally is higher than for the other components; benzene and C9/C10 aromatics respectively (compared to the regulatory limit), and partly on the fact that the TVOC concentration has a slower rate of decrease than the other components. No risk after 6 months when the distance to the soil pollution is >1mWhen the distance between the sampling point and the soil pollution is = 1 meter (e.g. corresponding to more than 1 meter unpolluted soil underneath the floor) all measured concentrations for all components (including TVOC) are below 100 times the regulatory limits after 6 months. This is regardless of the size of the spill, geological layers, contaminant location under the floor etc. When the (residual) spill is also below 500 kg oil, the soil air concentrations decreases to a maximum of 8 times the regulatory limits after one year. Hence, the study implies that 6 months after an oil spill there is generally no unacceptable influence on the indoor air of housings if the soil pollution is at least 1 meter below a concrete floor. The risk is further reduced 1 year after the spill, if residual pollution is of moderate size (< 500 kg heating oil). Data analyses without clear conclusionsNo significant trends were observed, regarding the concentration levels or the rate of reduction over time, related to the remediation status, the soil concentrations or the geological layers in or above the soil pollution. This is presumably due to several opposite factors which affects the overall picture.
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