Dampoprensning af klorerede opløsningsmidler på tidligere industrigrund i Hedehusene

Vapour Extraction of Solvents in Old Industrial Site at Hedehusene

Summary and conclusions

Remediation of a PCE contaminated site was conducted at Teglstenen and Industrivej in Hedehusene using a combination of steam injection, soil vapor extraction and groundwater extraction during July 1999 to April 2002. From April to December 2002 the site was reestablished after removal of installations from the remediation, adding new pavements etc.

The site was split in three different sub-sections, each characterized by its own hot-spot. The areas named I, II and III covered approximately 2.000 m², 1.200 m², and 5000 m², respectively. In order to optimize the use of the treatment system and the boiler, the remediation was designed to be conducted in two separate clean-up events. Remediation of area III took place first, followed by areas I+II.

The remediation can be described for different event or time blocks according to the following list:

Fall 1998. Design and competitive tendering. Two rounds of tendering were carried out. After the first round the county (the building owner of the remediation) and the consultant scrutinized the project in order to redesign to reduce the cost, then the second round was performed.
Spring-summer 1999. Treatment system and wells were established
July 1st 1999. Steam injection was started in area III.
November 15th 1999. Steam injection stopped due to different problems in the treatment system.
November 1999 to June 2000. Negotiations between contractor, consultant and county regarding technical solutions and liability.
Summer 2000. Treatment system repair and enlargement, renovation of wells.
September 4th 2000. Restart of steam injection in area III. During the following year of operation period minor rearrangement and maintenance work of the treatment system was carried out.
October 1st 2001. Operation of steam injection and extraction in area III was shut down.
October-December 2001. Parts of the treatment train were renovated and moved to the new location for treatment of area I+II. A new and better system to measure the rate of steam injection in the individual wells was installed.
December 1st 2001. Steam injection started in area I+II.
March 1st 2002. Soil throughout Area I+II had warmed up and cyclic operations was initiated.
March 6th 2002. Stop of steam injection in area I due to a major leakage in a rainwater runoff sewer. Cold melting water entered the treatment area rapidly and thereby cooled down the area and caused flooding of the wells. Treatment of area II continued.
March 21st 2002. Shutdown of soil vapor extraction in area I+II. Remediation of the entire area completed.
Spring and summer 2002. Samples collected from all 3 areas for documentation of the remediation efficiency.
Summer to fall 2002. Repairs of damaged sewer lines, minor cracks in buildings, pavements etc..
October 2002. The area was reestablished after removal of remediation installations.
December 2002. The documentation report was prepared and submitted.

The remediation is considered successful with removal of approximately 97 % of the chlorinated solvents in the vadose zone and 93 % in the saturated zone (secondary aquifer). The solvent remaining in the treated area does not violate the site specific clean up criteria determined by a risk assessment for the land use as well as for the underlying primary aquifer.

The primary goals of the remediation was to reduce the amount of chlorinated solvents in the glacial deposits at the site to a level that did not lead to concentrations over the MCA (1 μg/l PCE) in the underlying primary aquifer. Also, the contribution of PCE from the contamination to the indoor climate in residential houses was not allowed to exceed 0,25 μg/m³.

Based on the existing knowledge of the geology, sediment permeability, infiltration, etc. for the site it was estimated that the site specific clean up criteria corresponded to values of a maximum of 10 μg PCE/l in the secondary aquifer and 25 μg PCE/m³ in the soil gas directly below the slab. Criteria should be met on a statistical basis with more than 90 % of the data obeying the criteria.

Subsequent to the determination of the site specific criteria in 1998, the Danish EPA has reevaluated the toxicology of PCE and raised the acceptance level for PCE contribution to the indoor climate from 0,25 to 6 μg/m³. This led to a reevaluation of the site specific criteria at the end of the remediation. The risk assessment model had also been improved after 1998.

During the operation of the remediation a number of pumping tests in the primary aquifer were carried out. The tests showed that water transport in the aquifer is transported preferentially in fractures etc. in the limestone and that the permeability of the aquifer is extremely high, about 1-10^-2 m/s or higher.

Using measured values of permeability and gradient obtained during operation in the underlying aquifer, a site specific clean up criteria for the secondary aquifer was estimated to approximately 40 μg PCE/l, raising the initial criteria by a factor of 4.

Using the Danish EPA model for vapor transport to indoor air in buildings and the revised acceptance criteria lead to an acceptance level of 1 mg PCE/m³ directly below slab or 4 mg/m² 1 meter below grade. Therefore, acceptance level in the soil was 40 times less strict than initially expected.

Samples collected to evaluate the remaining PCE concentration in the water from the secondary aquifer after remediation was found to follow a log normal distribution. More than 90 % of the data in the distribution were below 40 μg PCE/l and approximately 2/3 of the data were below 10 μg PCE/l.

Samples collected to evaluate the remaining PCE concentration in the soil gas were subject to a geostatistical analysis using Krieging. The data were corrected to average soil temperature (10 °C) due to elevated temperatures at the sampling time (30-40°), using the fugacity principle. Based on this, the expected concentrations was 50 % of those measured at 30-40 °C. The data shows that 96 % of the area was below the acceptance criteria 4 mg PCE/m³, and that only 16 % obeyed the initial 25 μg PCE/m³ criteria.

As a conclusion the primary goal of the remediation was achieved cleaning more than 90 % of the area to site specific values, resulting in an acceptable risk of the remaining contamination.

As a secondary goal we had initially hoped that the technology would be able to clean up the uppermost part of the clayey till deposits underlying the alluvial, which was the primary target of the remediation. The clayey till separates the alluvial and the primary aquifer in the limestone. Unfortunately, it must me concluded that the secondary goal was not achieved during the remediation, primarily due to a cooling effect caused by small amounts of water running on the top of the clayey till. However, in some spots the goal was achieved removing the PCE approximately 0,5 m down into the till.

During operation of the remediation a number of experiences considering technical issues of a steam injection were gathered, including choice of components for the treatment systems, choice of different materials etc. The most important conclusions with respect to this are listed beneath:

The filter sand and backfilling materials for the wells should be selected very carefully with respect to grain size distribution since materials penetrating into the filter eventually will cause problems in the piping, valves and other places in the treatment system. The wellheads are to be finished in a way that facilitates easy access to the filter, to enable descaling or flushing of the well.
Bentonite (both slurry and pellets) is inadequate as backfilling in wells used for both extraction and injection in steam stripping applications. It craks up resulting in leakage to the surface. Instead, cement stabilized bentonite should be used, e.g. Storebælt-mixture, see recipe in chapter 3.2.1.
Liquid ring pumps of sufficient capacity are available for steam stripping applications. They can deliver the necessary flow as well as vacuum for an extended period of operation. The temperature of the pumps inlet should be as low as possible in order to maximize the pump capacity.
Its crucial to obtain effective liquid/air separation before air is led to the liquid ring pump. If separation is ineffective, there is a high risk of lime precipitation in the pumps with following break down.
If recirculated softened water is used in a liquid ring pump application biological fouling easily occurs. This leads to higher viscosity and greater friction in the pump thereby, causing higher power consumption. It can be avoided by occasionally adding a biocide.
Cooling of air containing oil mist potentially leads to quickly reduced cooling effiency due to oil film on the fins in the heat exchanger. The heat transfer coefficient drops dramatically when the film is formed. During the remediation, this problem occurred, when the steam front in the soil moved through an area with heavy fuel oil. Different attempts to clean the heat exchanger were only of limited success. No conclusions were drawn as to which remedy was the most effective apart from changing the fins in the heat exchanger.
Low capacity of GAC filters was observed in this application. The explanation is the high relative humidity in the air leaving the heat exchanger, even after reheating the gas 3-5 °C. Water blocks pores in the GAC filters, thereby decreasing the ability to absorb solvents. Breakthrough was observed at ¼-? of the time expected based on information in isotherms supplied by the GAC vendor.
Hot air and water mixtures containing chlorinated solvents are very reactive towards sealers, gaskets etc. Take this into account when selecting materials and design.
The nominal yield of a boiler is only a relative measure for the actual production under field conditions. In the actual project we were not able to obtain more than 60 % of the nominal yield of the boiler, continually. Therefore, we recommend use of boilers with at least twice the nominal yield of that aimed for in the design for stream stripping applications.

Monitoring of the steam front in the soil and the progress of the remediation was performed by a combination of measuring soil temperatures by means of specially designed wells with thermocouples spaced with 0,5 m interval in depth and measurements of steam flow, water flow, and air flow in the treatment system as well as measurements of chlorinated solvents, oxygen, and carbon dioxide in the treatment system and in the soil. In addition, measurements were carried out in the injection and extraction wells during the operation period using a specially designed temperature measurement system. Also, temperature measurements were carried out using either a direct IR thermometer or IR thermography on the soil surface and on building floors.

Some overall conclusions on the monitoring systems were drawn during the operation period. The most important are listed beneath:

The special designed wells for temperature measurement performed well delineating temperatures in the vertical plane. At first heavy bentonite slurry was used as backfilling around the thermocouples and guide. However, it cracked thereby leading steam to the surface. The backfilling was later changed to cement stabilized bentonite with much better results. Wells using factory sealed Pt100 sensors showed much better survival rate than wells using thermocouples (Type K) sealed by the contractor on site. We believe, that failure of the thermocouples was caused by the sealant not being resistant enough for the combination of high temperature, moisture and solvents, thereby enabling water intrusion in the cabling resulting in shortcuts.
Temperature measurements by the means of lowering a set of thermocouples into the wells was found to be a good supplement to the dedicated temperature measuring wells although the vertical spacing was less detailed than in the dedicated wells and on-line monitoring was not possible using this method.
Temperature measurement in the piping going into the treatment system from the individual wells did not work satisfactorily during operation. Instead measurement at the wellhead is preferred.
Precise measurements of steam and air flow to and from individual wells did not work satisfactorily during the clean up of area III, although a lot of attempts to correct the problems were made. The original system consisted of pitot tubes inserted in the pipes going to/from the wells connected through plastic tubing to a single and a differential pressure transmitter, respectively. Due to condensation in the plastic tubing and water droplets and small particles in the flow stream, the pressure readings were not precise. Our recommendation is not to use such a system in other steam stripping applications.
Relatively precise measurements of the steam and airflow were obtained after the rearrangement of the plant before clean up of area I+II using a new system with rotameters.
Reliable pressure readings in the soil were hard to obtain due to condensation etc. In general relatively small deviations from the atmospheric pressure is expected unless readings are conducted close to either an injection or extraction well. In the pressure zones, temperature is recommended instead as it shows almost the same pattern as pressure readings.
Concentration of various components can be measured in different ways. To get a clear picture of what is going on pretreatment of the gas stream was carried out before the actual measurements. The sample stream was cooled to 5 °C and atmospheric pressure before measurements were carried out. The employed system yielded reproducible results. Therefore, the concentrations shown in the report almost correspond to unit/Normal m³.

The primary results were given in general earlier, but is went more thoroughly in the following:

The total power consumption for the remediation has been 336 kWh/m³ and 214 kWh/m³ for area III and area I+II respectively. The theoretical power consumption for heating soil to steam temperatures is in the range of 60-80 kWh/m³, depending on the water content of the soil. The difference between actual and theoretical consumption was caused by loss to the surroundings by thermal conductance and to a lesser extent the energy removed by the extracted hot air and water from the soil. The lower consumption in the second stage of the clean up was a result of optimization of the steam injection rate (doubled in area I+II) and of the treatment system.
Soil gas concentration were reduced from up to 7500 mg PCE/m³ to a maximum of 18 mg PCE/m³. Its expected that when the soil is cooled to background temperatures the maximum concentration will not exceed 10 mg/m³. Concentration in 96 % of the area are below the Danish acceptance level for the most sensitive land use i.e. residential areas, kindergartens etc. None of the buildings used for residential purposes had unacceptable contributions of PCE to the indoor climate after the clean up.
Before clean up, PCE concentration in the secondary aquifer were in the range of 20-4.000 μg/l, with an average of 800 μg/l. After clean up the arithmetic average concentration was less than 30 μg/l, with a highest value of 480 μg/l. It should be noted that after the clean up, a much larger number of samples were collected in areas where high concentrations were expected compared to the number of samples taken before clean up. Based on a statistical analysis more than 90 % of the area shows PCE concentrations less than 40 μg/l in the secondary aquifer located in the alluvial, which based on the Danish EPA risk assessment model, is the local acceptance level at the site.
Initial PCE soil concentrations measured in the alluvial deposits showed values in the interval of 1-120 mg/kg with an average of 12 mg/kg. After clean up all areas, where the temperature had been above 90 °C, had virtually no remaining PCE. The resulting removal rate of the uppermost 6 meters of the soil was 98 %. In areas where heating was inadequate (primarily at the interface between the alluvial and the clayey till deposits) the removal rate was 60-70 %. In the underlying clayey till only very limited areas had been cleaned to any depth. In general we conclude that small amounts of water limited the heating of the underlying till.
All in all 370 kg of PCE was removed, 10 kg from pumping in the primary aquifer and the rest from the steam stripping process. Of the 360 kg only 0,5 % originated from the pumped water phase, the rest came from the soil vapor extraction system. Hydrous pyrolysis oxidation is not considered to be of any importance for removal of PCE under the prevailing circumstances during the clean up, however, a large amount of carbon (possibly natural occurring compounds) was transformed into carbon dioxide, which was detected in the extracted air in percentage concentrations.

Some side effects of economic consequence were observed during the remediation. Also, the daily use of the area was influenced by the remediation activity during operation. The most important side effects and influences are listed below:

In the design phase of the project it was decided to move the gas and power lines. During the operation it was found necessary move the water supply lines to the users of the buildings in the area too. Damage on a telephone line was also observed during the clean-up.
After the operation period damage to the sewer system was observed. The damage were observed on pipes made of plastic as well as cement. Old glazed pipes didn’t show any sign of damage. Damages were repaired after cooling down of the soil.
During the operation period in area III comfort problems were observed, especially in area III. The temperature rose to unacceptable levels within the buildings. Active ventilation in the buildings was tried as a remedy, but since the operation period was in the summer it only had a limited effect. In two cases the residents in the buildings had to move out for a shorter period during the operation.
Minor damage due to penetration of steam/condensate into the floor and walls was observed to the buildings in area III. Repairs were carried out after the clean up.
In area I+II the problems with moist and heat were handled by heavy active ventilation below building slabs. This solution effectively prevented any problems.
Lowering as well as rising of buildings in the range of 10 mm was observed during the remediation. These changes in the foundation conditions activated already formed cracks in the walls and led to minor damage like fall down of tiles etc. Repairs were carried out after the clean up.

The total economy of the remediation including funding to special documentation from the Danish EPA was in the range of 35 mill. DKK (4,7 mill. Euro). The treated amount of soil is approximately 90.000 tons leading to a price of 400 DKK/ton (54 Euro). The total is distributed on the following sub elements:

18,5 mill. DKK to the building contractor incl. building and repair of the treatment system, re lay of piping, cables etc.
4 mill. DKK to the operating contractor including maintenance and minor repairs in the operation period.
4 mill. DKK to make good of the area after the clean up, including 0,5 mill. DKK to building repairs.
4,5 mill. DKK to consumables (gas, fuel oil and electricity, chemicals etc.)
4 mill. to the consultants, including preliminary design tests, design, supervision of the building and operating contractor, analyses cost during the operation period as well as for the end documentation, report writing etc.