Summary and conclusions

Vurdering af naturlig nedbrydning af tjærestoffer i grundvand. Ringe Tjære- og Asfaltfabrik, Ringe, Fyn

Ringe Tar and Asphalt Factory’s (RTA) activities have led to severe contamination of soil and groundwater with coal-tar or coal-tar components at the site. The investigation at the site has revealed widespread contamination with coal-tar in fill/topsoil and the clayey till subsurface. Particularly strong coal-tar contamination occurs at the southern end of the site, and a concentrated plume of coal-tar compounds spreads more than 100 m to the southwest in the lower part of a sand groundwater reservoir.

A long series of investigations and evaluations of the contamination and risks associated with it have been performed. Among these several with relation to evaluation of natural attenuation of coal-tar compounds. In all very thorough evaluations have been performed based on new/innovative as well as traditional methods.

The purpose of this report is, to give a summarized description of:

distribution, transport and natural attenuation of coal-tar compounds
at RTA based on a literature review, results of field investigations, laboratory tests and contaminant transport modeling
risk evaluation for the groundwater reservoir
possibilities for the stimulation of natural degradation
the use of natural attenuation as a remediation technology
evaluation of the methods used and of the line of evidence.

As it is of great importance in this case to gain certainty in the evaluation of natural attenuation, several methods have been used in order to gain documentation by ”line of evidence” for the degradation of the coal-tar compounds:

Literature review of research projects concerning transport and degradation of coal-tar compounds, focusing on experiments
conducted at or in relation to the RTA site
Mapping and characterization of soil contamination with coal-tar in
the source area in order to evaluate the dissolution of coal-tar compounds from the source (particularly with respect to compound composition)
Mapping of the plume and characterization of the groundwater contamination (compound composition) and redox conditions for evaluation of the degradation potential
Analysis of stable isotope fractionation for specific compounds in groundwater samples for documentation of compound degradation
Analysis for specific degradation products in groundwater samples
for documentation of compound degradation
Anaerobic degradation batch experiments for compounds of special concern (with and without stimulation) for evaluation of the
degradability of these compounds, the possibility for stimulation of degradation and determination of degradation rates
Contaminant transport modeling of different scenarios with respect
to natural degradation for calibration of degradation rates in relation
to the observed spreading of contaminants and in order to evaluate
the future development of the contaminant plume.

Severe coal-tar contamination has been observed in the southern part of the RTA site, at more than 17 m bgs. The coal-tar has penetrated the water table of the primary groundwater as free phase (NAPL) in an area, where the clayey till dips down under the water table in the sand groundwater reservoir.

The NAPL coal-tar found near and below the water table has resulted in severe local contamination of the groundwater with dissolved coal-tar compounds. The dominant compound groups in the groundwater are aromatic hydrocarbons (BTEX and naphthalenes), phenols and NSO-compounds. The concentration level in the groundwater in the source area for these groups is 1-100 mg/l. A narrow contaminant plume with very high concentrations of coal-tar compounds migrates downstream from the source area in the primary groundwater aquifer – at first in a westerly direction concomitantly with the plume diving deeper into the reservoir - and then in a southerly direction. The plume follows a north-southward erosion channel in the underlying clayey till deposit. The plume can be followed approximately 150 m downgradient of the source area, and it is defined by drillings conducted in an ongoing investigation about 170 m from the source area.

Natural attenuation can be illustrated by traditional analyses for contaminants and redox-sensitive parameters. However, it may not be feasible to evaluate, in particular, for the least degradable compounds if they are degraded, based only on concentration data for these and other compounds. The influence of redox-sensitive parameters may be related to the degradation of the more easily degradable compounds, and in the plume, where compound concentrations are diluted, the effect of redox is often negligible.

Isotopic fractionation and determination of specific degradation products are methods, which can document degradation of specific compounds in the field. These techniques, in addition to traditional techniques, have been used for the field evaluation of the degradation of coal-tar compounds at RTA.

Finally, the potential for natural attenuation of specific compounds of concern can be investigated in laboratory experiments specific to the site/aquifer. In such experiments, the effect on compound concentrations of degradation can be separated from effects from other processes such as dissolution and dilution.

Contaminant transport modeling provides an opportunity to compare simulated contaminant distribution and concentrations for different scenarios including different zones (e.g., redox-based) with or without degradation or with different degradation rates with actual contaminant distribution and concentrations measured in the plume. Finally, modeling provides an opportunity to evaluate the extent of the influence on the groundwater resource now and in the future for different relevant scenarios.

All of the compounds from the BTEXN, phenols and NSO-compounds groups are present in the center of the source area at significant concentration levels and in a composition reflecting the composition of the coal-tar phase. But even in the source area, significant changes in composition are observed as a result of degradation. Thus, an increase is observed in the fraction of benzene in the BTEXNs, 2,4-, 2,5- og 2,6-xylenol in the phenols, and thiphene, benzothiophene and methylbenzofurans in the NSO-compounds. This development is strengthened a short distance downgradient in the plume where 2,6-xylenol completely dominates the phenols. In the most distant part of the plume, thiophene and benzene fractions of the total dissolved coal-tar compounds decrease, and 2,6-xylenol, benzothiophene and methylbenzofurans dominate. The changes in compound composition correspond well with an increase in the fraction of the anaerobically most slowly degradable (or persistent) compounds, as the other compounds are degraded. The results indicate that a large portion of the coal-tar compounds are degraded in the source area and the initial part of the plume.

This is confirmed for toluene, ethylbenzene and xylenes by the occurrence of specific anaerobic degradation products. Significant isotopic fractionation for these compounds is also observed in the initial part of the plume, whereas no significant isotopic fractionation is observed for benzene. The degradation of compounds in the source area may have contributed to increased dissolution of the most easily degradable compounds in the coal-tar.

At the far end of the plume, isotopic fractionation of xylene increases, whereas the thiophene and benzene fraction of coal-tar dissolved compounds decreases. However, no isotopic fractionation of benzene is observed in the plume. This indicates that benzene is not degraded within about 90 m downgradient of the source area. At a greater distance from the source area, benzene concentrations are too low to determine isotopic fractionation.

In summary, the results of the field investigations indicate that the problematic compounds, 2,6-xylenol, benzothiophene and methylbenzofurans, are persistent (or very slowly degradable) in the strongly reduced part of the aquifer and downgradient of this zone. There are indications that the two problematic compounds, benzene and thiophene, are degraded at the far end of the plume, i.e. downgradient of the strongly reduced area (more than 90 m downgradient of the site). The ongoing investigation confirms this and furthermore shows degradation of benzothiophene and methylbenzofuran about 130-150 m downgradient of the site and of 2,6-xylenol between 130 and 170 m downgradient of the site, where the plume is forced upward, and conditions become nitrate-reducing.

Over time, the concentrations of problematic compounds in the source area will decrease, as they are depleted from the source by dissolution. This will eventually lead to decreasing concentrations in the plume. This development is already being observed, as the concentration level of 2,6-xylenol in the plume about 130 m downgradient is higher than in the source area, and the concentration level of benzothiophene og methylbenzofurans here are comparable to the level in the source area, despite presumed dilution in the plume.

Laboratory degradation experiments were conducted for the problematic compounds benzene, 2,6-xylenol, thiophene, benzothiophene and methylbenzofurans (in spiked experiments 2-methylbenzofuran) under natural and stimulated conditions. The experiments comprised four series: One series (# 3) with high concentrations of coal-tar compounds and strongly iron-reducing conditions is assumed representative for degradation in the plume, while another series (# 1) with intermediate concentrations of coal-tar compounds and weakly iron-reducing conditions is assumed to represent degradation in the periphery of the plume. One series (# 2) investigates if NSO compounds have an inhibitory effect on degradation. Finally, nitrate was added to one series (# 4). This series (# 4) is assumed representative of the degradation in nitrate-reducing parts of the aquifer.

The results of the anaerobic biodegradation experiments indicate that there is a good potential for degradation of the problematic compounds outside the most reducing central part of the plume, and that it is possible to stimulate degradation by adding nitrate. The NSO-compounds do not appear to inhibit the degradation of benzene but possibly inhibit degradation of 2,6-xylenol in the aquifer.

A reactive transportmodel has been set up, and simulations have been conducted for 3 of the problematic compounds (benzene, 2,6-xylenol and benzothiophene) observed in the plume from RTA. The selected problematic compounds represent the least degradable compounds under anaerobic conditions from their respective groups of coal-tar compounds. Based on observed redox conditions in the source area, the plume and the aquifer, the area has been divided into 3 zones with different degradation potential. A strongly reducing zone in the source area and the central plume, an upper zone with nitrate-reducing conditions and a lower zone with weak iron-reducing conditions.

Based on the degradation experiments, the most realistic scenarios for the compounds are believed to be the following scenarios: without degradation in the plume until the zone boundary (about 100 m downgradient of the site) and with degradation rates corresponding to the lowest degradation rates observed under weak iron-reducing (lower part) and nitrate-reducing conditions (upper part) after the boundary.

Model simulations of the expected most realistic degradation rates for the 3 problematic compounds (2,6-xylenol, benzothiophen og methylbenzofurans) indicate that the concentration level for these compounds will be within an acceptable level within about 200 m downgradient of the site. Model simulations imply that it is unlikely that benzene and thiophene are present at more than approximately 100 m downgradient of the source area. These simulations correspond well with the results of the ongoing investigation. The contamination is not expected to pose any risk for the contamination of the Ringe Waterworks situated 1.4 km downgradient of RTA.

The complete investigation shows that as a result of natural degradation, the plume is less than 200 m long and can be expected to decrease in length over time. Therefore, monitored natural degradation is found to be a suitable strategy to ensure that contamination from this site does not result in unacceptable effects on the drinking water resource. The size of the plume length can be reduced faster by stimulation of the degradation in the plume.

Each of the methods used for evaluation of natural degradation at this site constitutes an important card in the evaluation. None of these can stand alone, the strength lies in the combined interactive use of all of them. Particularly for a complex contamination and geology, as found at RTA, all methods are needed in order to obtain a line of evidence for the correct evaluation of the effect of natural attenuation, on which the risk assessment and the evaluation of the need for additional measures is based.