Airsparging og jordventilation med vandrette boringer

Summary

Background

The Danish Environmental Protection Agency has allocated funds designated for use by the Counties for technology development projects for chemical waste sites. The purpose of the fund is to help finance development projects to clarify different cleanup technologies. This project is one of the projects initiated by the Copenhagen Environmental Inspectorate and jointly financed by Copenhagen City Council and the Danish Environmental Protection Agency in accordance with this fund. This report gives an account of the technology where air sparging and vacuum ventilation techniques are combined by use of horizontal bore holes established by the "blind hole " technique; a technique which to date has not been tested in a relevant size scale or with respect to the real objectives in Denmark.

This report is the first report in a series of reports, which will constitute the collective description of project. In addition to this report, a number of status reports for the operating phase will be prepared and finally a compendium will be produced giving an account of the accumulated operating data and the economics for the operation phase.

Objectives

The objective for the technology development project is to establish and define at a detailed technical level, the dimensions and operating parameters for the utilisation of air sparging and vacuum ventilation. Furthermore, to define the necessary extent of the monitoring program for the different phases of the project, such as the test adjustment phase, the operation phase and the final documentation phase. On the basis of the experience gained, guidelines for the dimensions and operating parameters for the plant and the criteria for termination of the remedial project are to be prepared. An assessment of the suitability of the blind hole technique at the actual locality; Drejøgade in Copenhagen, is to be evaluated.

Report

The report gives an account of the following aspects;

	A description of the project locality.
	A baseline study comprising a supplementary description of the extent of contamination before cleanup is started.
	Definition of the criteria for the dimensions and operating parameters for the plant.
	The detailed project description and a statement concerning experience gained during the establishment of the plant.
	The economic key figures.

Project locality

At the project locality, various dry cleaning and dyeing activities have been carried out in the period from 1900 to 1975. The contaminants present at the site are turpentine and chlorinated solvents. The site is approximately 7000 m² and the two former factory buildings are now demolished and replaced by a nursing home. In the initial investigations, elevated contents of tetrachloroethylene (perchloroethylene - PCE) were measured the indoor air and in the soil air immediately under the floors. On the basis of a risk assessment, it was conclude that remedial actions were required.

Baseline Study

A detailed hydrogeological investigation and chemical characterisation of contamination at the locality was carried out before remediation. 15 permanent monitoring wells with screens at different levels were established in the unsaturated and groundwater zones. Predominately glacial melt sand is found to at least 15 m´s under ground level with solitary layers of moraine clays close to the ground level. An unconfined aquifer is located at about 8,5 m´s under ground level. In the soil air, 15 - 20 mg/m³ of both turpentine and chlorinated solvents have been found. Respiration tests to determine the aerobic degradation rates for the hydrocarbons in turpentine have been determined in the unsaturated zone with rates of about 5 - 10 mg/kg soil /day. In the ground water, concentrations of turpentine hydrocarbons have been determined at levels of about 18 mg/l and a total content of chlorinated solvents of up to 4 mg/l. The main component among the chlorinated solvents is PCE (tetrachloroethylene) and to a lessor extent trichloroethylene (TCE). High concentrations of TCE´s degradation products are found in samples from some of the ground water well screens and the geochemical data indicate therefore that there is an appreciable transformation of especially the chlorinated solvents.

Dimensioning of the plant

Based on the results of the vacuum ventilation tests, the horizontal air permeability is calculated to be about 10 Darcy in the glacial sands, and by use of numeric models the resulting flow patterns around the abstraction wells can be calculated. The dimensions for the horizontal wells are derived by numerical models based on the data from the vacuum ventilation tests carried out for the vertical wells. Since the distribution of contamination varies in different areas at the site, it was necessary to section each well water screen in two parts. In the area dominated by the turpentine contamination, the plant is dimensioned to an exchange capacity of 5000 pore volumes (PV) over a cleanup period of about 2 years. Similarly, the area contaminated primarily with chlorinated solvents is dimensioned to an exchange capacity of 1500 pore volumes over two years.

On the basis of the two air sparging tests, the geological formation resistance pressure is estimated to be 0,5 bar and the effective radius of effect is assessed to about 3 - 5 m. The number of vertical air sparging wells necessary for treatment is calculated, and based on this figure the number of equivalent horizontal wells is estimated. To take account of the distribution of contamination, the screens in the air sparging wells are sectioned in the same manner as for the vacuum ventilation wells. On the basis of tests, it is assessed that an operating cycle of about 8 hours comprising a 4 hour injection period followed by 4 hours to reestablishment, will possibly increase cleanup efficiency and give a reduction of operating costs.

Detail project and experience during establishment

The report focuses primarily on the experiences gained during the installation of the horizontal filters.

The 5 horizontal filters were constructed by specially designed sectioning, and had to be installed about 6 m´s under ground level for the vacuum ventilation- (SVE) installation; and about 12 m´s under ground level for the air sparging installations (IAS).

The installation processes needed to be carried out in two steps, whereby the first step was establishment of the 9" pilot hole, followed by introduction by intrusion of the 165mm diameter PVC special screen in the open pilot hole.

An unforeseen complication occurred since the sandy formation also comprised many stones and the drilling of each pilot hole typically took 10 - 15 hours to complete instead of the 5 - 7 hours as expected. As filter introduction needed to be carried out immediately after completion of the pilot holes, partly to minimise the risk of collapse of the open pilot hole, and partly to ensure that the introduction of the screens was carried out in the fresh low friction drilling mud, all wells had to be re-drilled. To further reduce the friction resistance during intrusion of the screens, the planned SVE rods with pre-welded screen materials were replaced to take advantage of the smooth slotted PVC rods without screen materials.

Another reason to replace the pre-welded screens was that these, during a failed attempt at introduction in a drilled, but stony pilot hole, were torn to pieces. IAS screens with special sparging inserts glued to each of the drilled 3 mm diameter holes were installed as planned.

Navigation of the pilot holes during the drilling operations was carried out with a combined wireline system and a walk over locator. But even with this special equipment, the signal reception was poor or lost for wells close to the reinforced concrete foundations. Due to the natural rigidity of the drill rods, the location of pilot holes did not however deviate from the planed locations by more than 1 m in the horizontal plane and less than 0,1 m in the vertical plane. These divergences were confirmed by pressure transducer tests in the installed screens.

384 meters of screens were installed in the 2.5 months long construction phase. A SVE screen section of about 30 meters was however never installed, as it was not possible to drill through the stony horizons in the projected well line.

The treatment plant was installed in a 30-foot container, which was dealt in 2 separate sections to separate the noise generating and heat producing processes from the main control panel, the gas chromatographs and the two serial-coupled active carbon filters.

Both the SVE and the IAS process lines were constructed traditionally. The side channel blowers were chosen instead of the capsule blowers due to a collective assessment of the costs for the establishment of the plant and operational costs. For the SVE process line, a heat exchanger to cool the extracted soil air to < 40 °C was installed. Cooling of the soil air ensured a better utilisation of the activated carbon adsorption capacity for the contaminants. All process lines were constructed with appropriate instrumentation to enable on-line and manual measurements of the relevant operating parameters such as pressure, vacuum, temperature, flow, CO_2, CH₄, O₂, PID etc.

As part of the extensive documentation of the effectiveness of remedial project concept, on-line analyses of the concentration of the extracted soil air, both at the inlet and outlet of the treatment plant, as well as before, between and after the two carbon filters were carried out. The analyses were carried out on gas chromatographs with both ECD and FID detectors.

Economics

The economical key figures for the detailed project planning and construction phase as well as an estimate for the operating costs for the plant are presented in the report conclusions.