Miljøprojekt, 1094 – Poreluftprojekt - Styrende parametre for tidslige variationer af indholdet af klorerede opløsningsmidler i sand- og lerjorde - Hovedrapport

Poreluftprojekt - Styrende parametre for tidslige variationer af indholdet af klorerede opløsningsmidler i sand- og lerjorde - Hovedrapport

Summary & Conclusions

Experience from many years’ measurement of soil gas and indoor air concentrations shows that variations in concentrations over time are significant. This project was initiated by the Danish EPA in cooperation with the County of Funen and “Amternes Videncenter for Jordforurening” (Danish counties’ knowledge centre for soil contamination) with the aim of acquiring further knowledge of these time-related variations at a sand locality (a site with a sandy geology) as well as at 2 clay localities (sites with clay geology).

Literature study

Initially, a literature study was carried out. The study showed that soil gas concentrations vary naturally over time by several orders of magnitude. The literature study was published by ”Amternes Videncenter for Jordforurening”. The literature study has shown time-related concentration variations at a given sampling point of up to 2-4 orders of magnitude, whereas concentration in sampling points placed in the same area showed variation of up to 6 orders of magnitude (area variations).

Sampling

The project included extensive sampling and measurements at the sand locality and the two clay localities.

At the sand locality, the measurement set-up included:

Establishment of an outdoor sampling field with 7 soil gas probes
Establishment of an indoor sampling field in a cellar with 8 soil gas probes for sub-slab sampling
Establishment of an indoor measuring station in the cellar for sampling of indoor air
Establishment of an outdoor measuring station for sampling of outdoor air and measurement of the meteorological conditions
3 rounds of 2-3 weeks’ duration with continuous measurement of tetrachloroethylene, radon, barometric pressure, differential pressure (above/below floor level, outdoors/indoors, soil gas/atmosphere), temperature and relative humidity. Additionally, periodical measurement of air exchange, moisture content in the soil, ground water potential, meteorological conditions (atmospheric precipitation, sunny hours, wind direction, wind force)
Preliminary test pumping on selected outdoor soil gas probes and probes below floor level, to investigate whether the soil gas concentration in a probe changes, when significant air volumes are drawn.

At the two clay localities, the measurement set-up included:

Establishment of probes and subsequent draw test, including development of new sampling probe for use in clay
5 measuring rounds distributed to different times of the year with momentary measurement of differential pressure, flow and counter pressure in the probes, content of trichloroethylene and tetrachloroethylene in the soil gas/outdoor air and ground-water sounding in selected borings
Intensified measuring round with continuous measurement of differential pressure and ground-water level on selected probes during a 3-week period. Simultaneously, 4 measuring rounds with measurement of trichloroethylene and tetrachloroethylene in the soil gas from the probes
Preliminary test pumping on selected probes, to investigate whether the soil gas concentration in a probe changes when significant air volumes are drawn
Ventilation tests on selected probes, where it was measured whether an open probe or a recently established probe affects the contamination concentration and the pressure conditions of the soil gas
Collection of data on temperature, atmospheric pressure and precipitation of the period covering all tests and measuring rounds.

Results

Results from the sand locality showed:

Generally, only minor variations in the PCE content of the soil gas were found in the three measuring rounds. Annual variations of up to a factor of 2 have been found in the same probe. With respect to area, variations of up to a factor of 3 over a distance of 4-5 m were seen. If previous results from the sand locality are included, long-term variations of the soil gas of up to 2 orders of magnitude are seen
The sub-slab soil gas measurement showed small time-related variations – both short-term and annually. The short-term variations are typically within 5-10 % of the mean value. The concentrations between the individual measuring rounds vary up to a factor of 2. However, previous measurements during the period from 1996 to 2002 at the locality showed variations of up to 2 orders of magnitude
At the sand locality variations of the indoor air concentrations of up to a factor 20 were seen, i.e. a range of a decade. Previous measurements during the period 1996 to 2002 showed variation of a range of up to a factor of 100
The radon measurement provided a fairly good description of the attenuation factor between concentrations of PCE below/above floor level. Radon can consequently be applied for assessment of the attenuation of concrete floors
The preliminary pumping tests on sub-slab soil gas probes and outdoor soil gas probes showed that sampling at the sand locality was very robust in relation to various pump flows. When sampling at a sand locality, a flow of 0.1-1 l/min. and a maximum counter pressure of 150 mbar in the probe are recommended. A reasonable preliminary pumping volume is estimated at 5-10 times the established pore volume in the formation and in the probe and hoses up to the sampling spot
Investigations at the sand locality showed that vapour intrusion to indoor air is controlled by many factors and that it is not possible to identify cause–and-effect relationships between PCE in soil gas and indoor climate unambiguously. Driving forces like changes in barometric pressure, temperature differences in indoor/outdoor air and pressure difference above/below floor level can explain some of the variations in the indoor air, but not all. Inflow of atmospheric air via the impact zone, evaporation from ground water/soil contamination, variations in ventilation conditions and air exchange above the terrain surface may explain other variations. What makes the interpretation difficult is that the various driving forces are acting simultaneously and are mutually dependant
A fall in the barometric pressure is the most significant driving force for vapour intrusion to indoor air at the locality. In general, the pressure difference above/below floor level and the temperature difference between outdoor and indoor air seem to have no significant influence on the PCE content in indoor air at the specific locality in the city of Aalborg. It should be noted that these results are not in accordance with the results from Risø (1997), in which the temperature difference in outdoor/indoor air was a significant driving force for soil air intrusion.

Results from the two clay localities show the following:

At the 2 clay localities, area variations were found in soil gas concentrations of up to 3 orders of magnitude and time-related concentration variations during the measuring period (months) of typically 2 orders of magnitude – however for a few probes up to 3 orders of magnitude
Based on the investigations previously carried out at the localities it cannot be rejected that significant time-related variations in the soil gas concentration might occur, especially as long-term variations (annual variations). Based on the literature study it is assessed that realistic concentration variations will be between 3 and 4 orders of magnitude
Preliminary pumping and sampling should be carried out at the lowest possible flow – preferably 0.1-0.5 l/min. Immediately after establishment of probes - or for probes that have been closed for some time - the preliminary pumping should not exceed twice the artificial volume of the probe (the volume of probe, hoses and placement of gravel, if any)
In the event of short-time leaks in the measuring arrangement (a few minutes), re-establishment of stable conditions in the formation can be achieved by a preliminary pumping of 2-3 times the artificial volume of the probe
Pressure changes in the atmosphere have significant importance to short-term variations in the soil gas concentration. Generally, a pressure rise results in a fall in the concentration, whereas a pressure fall results in a rise in the concentration. As regards the long-term variations in the soil gas concentration (annual variations) it is assessed that also precipitation and changes in the ground-water level, including uncovering of a smear zone, have significant importance
Precipitation also shows significant importance to the short-term variations. Precipitation generally results in a concentration fall because of leaching. Especially the impact by heavy precipitation overshadows the simultaneous impact by pressure fall in the atmosphere
At interpretation of the measured concentration in the soil gas - and as a rule-of-thumb – the impacts by pressure changes and precipitation during a period of time prior to the measurement should be taken into consideration. The length of the period depends on the geology and the ground-water level. The investigations have shown that the response time of the pressure extension in the unsaturated zone is a good guide for the length of the period
According to the results there is a faint tendency that a rise in the ground-water level results in a rise in the soil gas concentration, whereas a fall in the ground-water level results in a fall in the soil gas concentration
Apparently the outdoor temperature has no significant importance as regards the concentration in the soil gas in the measurement carried out.
Based on the available data and the theoretical considerations it is assumed that the highest soil gas concentrations at the clay localities occur in late summer/early autumn. This assumption is based on the fact that this period is generally characterized by:
Low ground-water level resulting in higher source evaporation
Low water content in the unsaturated zone because of low precipitation during the summertime resulting in an increase of the vertical gas permeability
Incipient saturation of the top layer because of autumn rain diminishing the exchange with the atmosphere

It should be noted that similar physical conditions may occur at other times of the year.

Discussion and recommendations

Although it is possible to explain some of the stated concentration variations at both the sand and the clay localities, the investigations have however shown clearly that it is not possible at present to explain all the found variations in the concentrations. Thus there are still a series of unrevealed contexts affecting the concentration level.

Risk assessments, decisions concerning charting in conformity with the soil contamination law or protective measures based on sampling concentrations are subject to a considerable uncertainty due to the considerable time-related variations.

Consequently at future soil gas investigations it is recommended to carry out repeated measurement and be aware that the sensitivity of decisions must be assessed on the basis of both the area and the time-related variations.

Furthermore it is recommended to initiate work with the purpose of achieving a better understanding of the natural variations, including development of new risk assessment tools.