| Front page | | Contents | | Previous | | Next |
Airsparging og jordventilation med vandrette boringer
This project has been carried out under The Danish Environmental Protection
Agency’s Technology programme for soil and groundwater contamination in co-operation
with The Environmental Control Department of the Municipality of Copenhagen. The project
concerns the remediation of contaminated groundwater by combined air sparging and vacuum
ventilation from horizontal wells at the locality at Drejøgade 3-5, Østerbro,
Copenhagen. At this site, a former dyeing and dry cleaning site has caused contamination
of soil, water and air due to spills of solvents based on mineral oil (turpentine, petrol
and kerosene) and chlorinated solvents (perchloroethylene). The geology of the site is
dominated by fine sands with a free groundwater level at approx. 8.5 m under ground level
thereby allowing the use of both air sparging and vacuum ventilation cleanup techniques.
The area is approx. 7,000 m2 and is partly covered by a building (a nursing
home) with a cellar.
The horizontal wells have been installed using the "blind-hole" technique,
whereby filters up to 130 m can be installed in the underground from an excavation pit
outside the site. A total of 4 air sparging filters have been installed 2.5 m below
groundwater level and 5 vacuum ventilation filters 2.5 m above groundwater level, a total
of approx. 375 m filter. Due to impermeable stony horizons in certain areas, one of the
filters was installed significantly shorter than planned. This indicates the importance of
accurate underground geological characterisation before planning and installation of the
filters. The use of filters with pre glued screens was abandoned due to problems caused by
the stony horizons, In future projects, filters with slits can be recommended.
The treatment system has been build up in a 20 ft container with a vacuum ventilation
capacity of 1,000 m3/hour, an air sparging capacity of 375 m3/hour
and 2 x 260 kg active carbon filters. An on-line gas chromatograph (GC) monitored the soil
gas concentration from the vacuum ventilation system, but the on-line GC-monitoring was
unsatisfactory and cannot be recommended for similar projects. In general, the treatment
system has functioned satisfactorily over the course of approx. 80% of the 2-year
operation period.
In the operation period, the effect of pulsating and continuous air sparging operation
cycles has been evaluated. There is a general tendency that the pulsating operation cycle
maintains a higher overall soil gas concentration, and furthermore higher removal rates
are obtained by this form of operation as compared to continuous air sparging. During the
operation period, approx. 500 m3 soil gas/hour has been extracted,
corresponding to 900 times (instead of the expected 1,500 times) the volume of soil gas at
the southern area predominantly polluted with chlorinated solvents. At the northern area
predominantly plotted with petroleum contaminants the soil gas volume has been changed
approx. 600 times in comparison to approx. 1,500 - 5,000 times as expected. The
calculations of the airflow and the extent of vacuum around the horizontal borings in the
soil throughout the design phase have been verified by tracer tests and vacuum
measurements.
The concentration of chlorinated solvents in the unsaturated zone has been reduced
significantly (75-98%), however after termination of the treatment, some rebound has been
observed in one of the "hot-spot" areas.
In the area with free phase turpentine on the ground water table, no significant
reduction in the content of total hydrocarbons in the soil air just above the groundwater
level has been observed. This is due to the fact that the turpentine is caught in the
capillary zone, which is not sufficiently ventilated due to an almost 100% saturation of
the pores by water and turpentine. Outside this area, treatment has lowered the content of
total hydrocarbons to a level below the detection limit.
The effect of air sparging is most apparent in the area with free phase turpentine
trapped on the ground water table, as evaporation rates and soilgas concentrations of
volatile hydrocarbons in the capillary zone are high.
A similar marked increase in the removal rate for the chlorinated solvents has not been
observed. Thus, there are no indications that free phase chlorinated solvents are present
within the area affected by the air sparging. In the 3 separate filter levels in the
saturated zone, a general reduction of 60-95% of the volatile compounds such as
chlorinated solvents and their degradation products as well as BTEX is observed, while
reduction of the less volatile components of the turpentine hydrocarbons is less apparent.
In the area with free phase turpentine, reduction in the concentration of the less
volatile turpentine components in the filter levels in the capillary zone is not observed,
but a significant increase has been observed in the deeper filter levels. In all filter
levels - but most significantly in the upper level – an increase in the oxygen
concentration was measured during operation of the air sparging system.
As an effect of start/stop of the air sparging, water samples taken at specific depths
in the soil profile indicate that a vertical transport and mixing of ground water from
near the groundwater table to the deepest filter level approx. 5 m below groundwater level
occurs. This effect is only significant for the less volatile turpentine components and
for oxygen, while the most volatile components such as the chlorinated solvents and BTEX
apparently evaporate before the groundwater penetrates to this depth. Thus, the effective
radius of effect is extremely dependent on the contaminant, but for chlorinated solvents
the radius of effect is 5-6 m in a horizontal plane from the filter and extending to a
depth of approx. 2.5 m below the level of injection (5 m below groundwater level).
Since the main objective of the cleanup is to achieve an acceptable indoor climate for
the buildings above the contaminated zone, the system was stopped after 2 years operation,
as the soil gas concentrations in the unsaturated zone showed a stable and reduced content
of contaminants. The concentration of PCE in the capillary layer under the cellar floor
was immediately after shut down of the system (24 hours) reduced 1,000 times compared to
the initial concentrations before treatment (from approx. 5-100 mg/m3 to
0.004-0.018 mg/m3). Several measurements carried out over the 1st year after
the shut down show a slight increase to a level approx. 0.1-0.2 mg/m3, and thus
the net effect is a 100 times reduction of the initial concentrations.
In general, the concentration of xenobiotic contaminants after treatment does not
comply with the existing groundwater quality criteria – in spite of the significant
reductions of especially the most volatile components.
The total costs of the project are approx. 8 mio. kr., and a treatment price for the
total soil volume at the locality (saturated and unsaturated zones) would be approx. 50-75
kr./ton. A similar commercial clean up is estimated at approx. 5 mio. kr.
| Front page | | Contents | | Previous | | Next | | Top
|