Miljøprojekt, 1059; Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Forceret udvaskning på Hjørring Gasværk - Afslutningsrapport

Forceret udvaskning på Hjørring Gasværk - Afslutningsrapport

Summary and conclusions

At Hjørring Gasworks, experiments with enhanced leaching have been carried out in order to develop an uncomplicated in-situ technique to reduce pollution of groundwater due to diffuse large-scale, low-level soil contamination with coal tars - a situation which is common at many former coal gasworks throughout the country.

The main concept for technology is as follows:

To introduce groundwater along the same flow pathways as those travelled by the pollution under transport from the surface and downward
To enhance leaching of pollutants from the residual soil contamination in the unsaturated zone (primarily tar components)
To improve conditions for natural degradation in the unsaturated and saturated zone by infiltrating treated groundwater as the carrier for oxygen, alternative oxidative agents, nutrients, etc.

This report concerns the period of extended operation for the experimental project with enhanced leaching carried out in the period 1993 - 1996. The original project formed part of the Danish Environmental Protection Agency's programme for innovation of clean-up of gasworks sites ("Gasværkspakken"). The objective of the experimental project was t° Clarify and document the possibilities for in-situ clean-up by enhanced leaching. The results and conclusions are reported in the final project report "Hjørring Gasværk, In-situ oprensning ved biovanding" /7/.

The objective for the continuation of the project under the technology programme (phase 2) is to verify the following observations made during the first 2 ½ years of operation under phase 1:

To verify documentation for degradation processes in the upper part of the groundwater zone by examining the developments in groundwater chemistry with time and distance.
To document that the dominating pollutants degrade microbiologically in the groundwater zone within the transport distance from the pollution source to the outer monitoring ring.
To document transport and dilution of pollutants in the groundwater zone by tracer experiments with measurements of tracer and pollutant concentrations at different distances from the source.
To document, using laboratory experiments, the degradation potential in the groundwater zone for selected pollutants under different redox conditions.
To test and calibrate a re-oxygenation model that can predict the time required to achieve re-oxygenation in a given depth in the soil profile.
On the basis of these observations, to prepare recommendations which can be used for other projects.

Under phase 2, the following main activities were carried out in the unsaturated and saturated zones respectively:

Activities in the unsaturated zone

Measurement of oxygen diffusion in intact and packed soils.
Draining experiments on soil columns.
Development of an oxygen diffusion model.
Measurement of sorption and desorption of ammonium and naphthalene in soil sediments from the gasworks site.
Assessment of aerobic and anaerobic degradation potential in different soil types with high and low contaminant concentrations (BTX, phenol and methyl phenols).

Activities in the saturated zone

In-situ contaminant transport experiments with bromide as tracer.
Assessment of water and contaminant transport.
Assessment of aerobe and anaerobe degradation potential in different sediment types with high, moderate and low pollutant concentrations (BTX, phenol and methyl phenols).
Monitoring of the developments in groundwater chemistry during enhanced leaching operations.

Based on these activities, the following conclusions can be summarised concerning laboratory experiments in the unsaturated and saturated zones, groundwater monitoring and enhanced leaching as clean-up technology:

Summary of conclusions concerning the unsaturated zone

Experimental determinations of oxygen diffusion coefficients require measurements on intact soil samples with a minimum sample volume of 100 cm³. Contrary to the general opinion, measurements on packed soil samples are not satisfactory.
Oxygen diffusion coefficients (D_P) are dependent on both water content and soil type. The almost universally used Millington & Quirk (1961) equation (also used in the Danish Environmental Protection Agency's JAGG model for soil, vaporisation, gas and groundwater) is imprecise and will typically underestimate D_P with a factor 10, especially at high water contents. Therefore, the calculated oxygen and gas flux will also be underestimated. A better equation for D_P is proposed in Chapter 4. (D_P is equivalent to the expression N x D_L in the JAGG model as applied by the Danish Environmental Protection Agency in risk assessments /8/).
Assessment of oxygen diffusion coefficients and re-oxygenation times in a natural, intact soil system requires a calculation model that is independent of the soil type. A simple model to estimate the oxygen penetration times and achieve an estimate of the time-scale for diffusion-controlled re-oxygenation for a number of soil types and soil depths has been derived and tested.
Model calculations and experiments demonstrate that water drainage through the soil layers at Hjørring gasworks occurs slowly, with drainage times of at least 5-6 weeks per meter soil before the natural soil re-oxygenation is sufficient to provide air-filled pore voids.
The high drainage and re-oxygenation times for the soil layers at Hjørring Gasworks are due to absence of coarse sand (>500 μm) in the soil. The dominating soil type is fine sand (< 500 μm) with inserts of silt and clay, which give lower rates of oxygen diffusion and water drainage than is normal in a typical sandy soil
Since the Hjørring soil profile mostly comprises very fine sand practically without a content of coarse sand, the soil typically has
1. a relatively low content of air-filled pores with a field capacity of less than 15 vol.% (∼ 0.15 cm³ cm^-3), which gives a slow oxygen diffusion in the soil, and
2. a high water retention ability which leads to slow drainage of the soil profile.
Since the criteria for enhanced leaching requires a reasonably rapid re-oxygenation, the technique is only suitable on soils that have an air-filled pore volume equivalent to a field capacity of > 20 vol. % (e.g. soils with an appreciable content of coarse sand and a content of clay and silt of less than 10%). Detailed texture (clay, fine silt, coarse silt, fine sand, coarse sand, gravel, organic content) as well as air-filled pore volumes with a field capacity or a drainage time sufficient to achieve a 20 vol. % air-filled pore volume need to be confirmed for all soil types /soil layers present at a locality before the locality can be classified as suitable for enhanced leaching.
Sorption and desorption measurements have determined that soil retention of ammonium and naphthalene is low due to the low content of organics in the Hjørring soil. Sorption of naphthalene demonstrated a decided hysteresis indicating that release of adsorbed naphthalene will be limited.
Under aerobic conditions, rapid microbiological degradation of phenol and methyl phenols is observed in the different soil layers in the unsaturated zone, demonstrating transformation of pollutants during transport towards the saturated zone.
Under denitrifying conditions, the degradation rate decreases markedly (typically a factor 5 -10). At higher infiltration rates, the unsaturated zone becomes saturated with water, and the dissolved oxygen in the percolating water is rapidly consumed by the transformation of tar substances, resulting in the rapid establishment of denitrifying conditions.
After termination of infiltration, a reasonable draining and re-oxygenation of the unsaturated zone (approx. 10 m) at Hjørring can be expected to be achieved after about 9-12 months. With the applied infiltration strategy at Hjørring Gasworks, it is therefore probable that degradation in the unsaturated zone has occurred under denitrifying conditions for most of the duration of infiltration.

Summary of conclusions; the saturated zone

Rapid degradation of BTX and phenols is also observed in the groundwater zone under oxygenated conditions > 1 mg/l at Hjørring Gasworks. Phenols degrade more rapidly than BTX. At low concentrations (< 200 μg/l), the degradation rate is still high for benzene, but the rate falls markedly for the methylated phenols and is around a factor 10 lower than for benzene.
Benzene and toluene do not degrade or degrade only at a very slow rate under conditions of denitrification. The degradation potential for phenols is reduced about a factor 5-15 compared to conditions with oxygen concentrations of more than 1 mg/l, and falls to very low values at oxygen concentrations of less than 0.2 mg/l.
In the contaminated groundwater zone, denitrifying conditions are present. Oxygen concentrations are less than 0.5 mg O₂/l, and nitrate concentrations vary from <1 to 250 mg NO₃-N/l. In the upper part of the uncontaminated groundwater zone, the oxygen concentrations are greater than 2 mg O₂/l.
Experiments indicate that degradation is expected to proceed slowly in the anaerobe zones, and benzene will not be degraded. Groundwater in the top of the magazine re-oxygenates before it reaches the wells in the boundary to the gasworks site, and under these aerobe conditions the degradation of BTX and phenols will be completed within a transport time of a few meters.
The relatively high aerobic degradation potential at Hjørring Gasworks combined with an overall aerobic groundwater aquifer can therefore be expected to provide good conditions for in-situ degradation of the residual pollution in the saturated zone.

Summary of conclusions; groundwater monitoring

The changes in pollutants concentration and groundwater chemistry in the top of the groundwater magazine at the start of the enhanced infiltration in phase 2 can be compared with the changes in composition development seen at the start of phase 1 in 1993. After resumption of infiltration, groundwater concentrations decreased over the first 12 months, followed by a significant increase for all pollutants in the upper groundwater zone.
Deeper in the groundwater magazine, a significant penetration of especially benzene was observed after only three months of operation in phase 2.
In the downstream wells, no organic pollutants (BTX, phenols, or NS° Compounds) were detected in the top of the groundwater magazine in phase 1 or 2, although more downstream wells have been established in phase 2.
The development in pollutants levels determined in the most severely contaminated area indicates that a stabile relationship between enhanced leaching of tar components and degradation in the unsaturated zone has not been achieved during the project period.
However, no organic pollution was found in the monitoring wells downstream of the contaminated zone. Elevated levels of inorganic constituents such as sulphate - a typical indicator parameter for gasworks pollutants – demonstrates that the downstream wells receive water from the gasworks.
The results indicate that infiltration of groundwater has lead to a markedly enhanced leaching of both tar and inorganic pollutants such as sulphate from the unsaturated zone, but that the groundwater contamination with tar constituents is probably limited to a zone around the most polluted well.
Since organic pollution is not found in the downstream wells, there are clear indications that pollutants are degraded in the unsaturated zone before the groundwater table is reached or in the top of the saturated zone within a relatively short distance of the pollution source.
The data documentation concerning benzene contamination in the deeper anaerobic part of the aquifer is, however, insufficient to allow conclusions to be drawn concerning the transport and degradation of this compound at greater depths.

Summary of conclusions concerning enhanced leaching as a clean-up technology at gasworks sites

The clean-up project at Hjørring Gasworks shows that a traditional excavation of gasworks related surface hotspots combined with enhanced leaching of the water-soluble and degradable pollutants present in the residual contaminated soil can be a suitable technology to reduce groundwater contamination at decommissioned gasworks.

Since it is probable that the infiltration water is introduced along the same transport pathways as the original pathways travelled by pollution, the water-soluble components residing in residual pollution in the unsaturated zone are mobilised to an important extent as compared to a situation with natural infiltration.

However, the method has a fundamental weak spot, since the transport capacity of water for oxygen is very limited compared to the oxygen demand due to degradation of the mobilised contaminants released during enhanced leaching.

The technology is therefore dependent on effective and natural re-oxygenation/diffusion of oxygen to maintain or re-establish aerobic conditions in the unsaturated zone down through the soil profile after each infiltration period.

At sites with a thick unsaturated zone and relatively low permeability of the geological layers, it is not always possible to maintain aerobic conditions in the entire unsaturated zone under enhanced leaching. This inhibits the natural degradation of the leached substances, leading to undesirable breakthrough of substances in the saturated zone.

At the actual Hjørring site, the results indicate that the time-period necessary to ensure natural oxygen diffusion in depth in the contaminated soil profile can be unrealistically long. Full re-oxygenation of a 10 m soil profile with alternating layers of low permeability in practise will require unrealistically long periods without infiltration (9-12 months).

To achieve a reasonable relationship between leaching and re-oxygenation after infiltration, it is assessed that enhanced leaching is appropriate primarily at sites with sandy soils and with a relatively homogenous geology without inserts of low permeability in the unsaturated zone.

The technology is therefore unsuitable for sites with overall geology comprising low permeable layers.

At gasworks, the enhanced leaching is primarily of interest for substances cause groundwater pollution, i.e. substances with relatively high water solubility such as BTEX's, phenols etc.

Enhanced leaching is not suitable if a total cleanup is required since typical tar pollution als° Contains significant amounts of recalcitrant (little or no degradation) compounds. Therefore, a gasworks site will often still be polluted with PAH compounds after clean up by enhanced leaching.

Based on the limitations due to geology and degradation potential, it is concluded that enhanced leaching combined with a natural re-oxygenation has only limited application as a clean-up technology at Danish gasworks or at other localities contaminated with tar contaminants.

Benefits of the technique

Enhanced leaching has the following benefits:

Simple and low technological clean-up technique.
Infiltration water is introduced along the same pathways as those travelled by the pollutant in the soil matrix. The infiltration water and the substances carried in the water have a good chance of reaching the actual contaminated areas in the unsaturated zone.
The technique can also be used in areas with thick unsaturated zones, if the geological conditions are suitable.
A smaller treatment plant building/container and a number of manhole covers are the only visible parts of the plant, and therefore enhanced leaching can be carried out at the same time as other activities. The physical installations can with advantage be incorporated in combination with cover systems comprising clean materials.
Low inspection requirements and fully automatic operation.

Disadvantages of the technique

Enhanced leaching has the flowing disadvantages:

Not unlike a number of other clean-up technologies, the technique is not suitable for heavily contaminated soils since massive pollution with coal tar often results in free-phase tar globules which encapsulate the water soluble constituents with an impermeable layer. Furthermore, the area of application for the technology is relatively water soluble and aerobically degradable substances
Enhanced leaching does not provide complete clean-up of the treated area. Coal tar pollution will usually contain large amounts of recalcitrant substances with low water solubility and it must be expected that treatment with enhanced leaching will leave a residual PAH pollution.
Since the rate of leaching of contaminants from tar polluted soils is typically limited by diffusion, it must be expected that relatively long treatment times are required as compared to more radical clean-up technologies.
Enhanced leaching provides only a limited potential for introduction of oxygen to the soil via natural re-oxygenation or by as transported by the infiltrating water. A criterion for application of the technology is that a reasonably rapid re-oxygenation must occur after infiltration. Enhanced leaching is therefore primarily suited to sites with sandy soils, a considerable content of coarse sand and a low content of clay and silt.
The technology is not suitable at sites with mainly low permeable geological layers
If the method is used in water abstraction areas vulnerable to pollution, it is important to establish a closed hydraulic circulation system to avoid unforeseen spreading of pollution
Extensive soil excavations are necessary to establish the plant will there affect land end-use.