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Remediation of contaminated groundwater using permable reactive barriers (RESET)

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Basic project information

Shortcut: http://syke.fi/projects/reset

Research objectives

The aim of the project was to gain knowledge on innovative technologies feasible for the remediation of contaminated aquifers where utilization of traditional pump-and-treat technology is limited. The specific objectives of the project were:

  1. to study the performance of permeable reactive barriers (PRB) for the remediation of contaminated groundwater in Finnish climatic and geological conditions by installing and monitoring a granular iron barrier;
  2. to test the reactivity of Finnish granular iron materials with chlorinated solvents at a range of temperatures and under various geochemical conditions and to create test methods for PRB materials;
  3. to develop criteria for potentially feasible applications of PRBs.

Background

In Finland, new cases of groundwater contamination that poses risks to the environment and human health are frequently found. Risk assessment of contaminated areas in Finland has focused on excavating or remediating the soil above the groundwater table.

Complete removal of the sources of the groundwater contamination from the soil is often not possible. The mass of contaminated soil is usually large and for example sources of chemicals heavier than water are often found deep in the saturated zone, in fractured bedrock or underneath a building. At such sites, groundwater contamination may continue even decades after soil remediation. Hence, groundwater remediation is the only way to prevent further migration of the contaminants and to reduce the risks they pose.

In the past, groundwater remediation usually involved application of pump and treat methods. These approaches, however, appeared to be inefficient and costly in most cases. In treating contaminated groundwater, in situ techniques in which the remediation takes place under ground at site are increasingly used in many countries. In the North America and in the Central Europe PRBs have gained ground in challenging contamination cases. There are over hundred PRB applications around the world.

PRB technology

PRB technology is applicable under most hydrogeological and geochemical conditions, thus offering a cost-effective and sustainable treatment solution. In PRB technology, a barrier containing granular reactive material is placed in a contaminated aquifer usually as a continuous wall throughout the saturated zone.

RESET Layout
Layout of the Orivesi PRB

When contaminated groundwater flows through the reactive barrier, adsorption and chemical reactions between the contaminants and the reactive material take place, resulting in the removal of the harmful substances. PRB is a passive configuration that does not require external source of energy or regular maintenance. Therefore, in PRB technology the long-term treatment costs are markedly lower than in the conventional pump-and-treat methods.

Based on earlier field studies at a range of PRB sites, permeable reactive barriers may remain functional for several decades. Anyhow, Finnish climate and hydrogeological conditions often differ from those in which PRBs have been installed and investigated before. The long-term performance of a PRB in Finnish conditions has been monitored since summer 2006.

PRB material tests

RESET location
Location of the field site

Granular iron (Fe0) PRB is an in-situ method applicable for treating groundwater contaminated with chlorinated solvents.

A range of Finnish granular iron materials were tested with batch and column experiments. These laboratory tests were conducted at a range of temperature and at various geochemical conditions.

The dimensions of the pilot-scale barrier at the Orivesi field site were designed on the basis of these laboratory experiments to ensure the removal of chlorinated solvents and their degradation products.

Hydrogeological investigations at the Orivesi field site

 
RESET ivestigations
Investigations at the study area

Based on preliminary hydrogeological studies, the Asemanseutu aquifer at Orivesi was chosen for testing a pilot-scale granular iron PRB for remediation of chlorinated solvents. The aquifer had been contaminated with trichloroethene (TCE) and tetrachloroethene (PCE), released from a dry cleaner that operated from 1959 until 1989. Due to the presence of a range of contaminants, the groundwater in the area has not been used for drinking water purposes since 1981 and the aquifer has been withdrawn from the list of classified aquifers since 2000.

The extent of the contamination in Asemanseutu aquifer was studied in 1999–2003. For planning  the PRB location and dimensions, thorough hydrogeological investigations were made in 2003–2006 including drilling and soil sampling, video filming of the bedrock wells, in situ measurements of hydraulic conductivity and groundwater quality, water consumption measurements, tracer tests, groundwater level monitoring, groundwater sampling and a variety of geophysical surveys.

Reset kaivu
An open pit extending to the groundwater level was made in the course of the PRB installation

PRB application

The PRB construct was placed across the path of groundwater flow based on site investigations and groundwater modeling. The PRB is a funnel and gate configuration with an additional control well.

In addition to hydrogeological site information, results from material and column tests were also used in dimensioning the PRB. The permeable barrier and the funnels were installed in summer 2006 at the south-east part of a ballpark situated 200 m south of the dry cleaner.

The performance of the PRB application and the removal of the contaminants in the aquifer were observed by the monitoring of the groundwater quality.

Reset tuettu kaivanto
The part under groundwater was excavated as steel cleat supported excavation

Groundwater samples were taken from several observation wells once a month. Samples were also taken via the control well from the observation tubes placed in the iron fill.

The fracture zones in bedrock were filled with injection material to eliminate the contaminated groundwater's bypass below the PRB.

 

Experience from the Orivesi PRB

It is possible to make a PRB work both hydraulically and remedially in Finnish hydrogeological conditions found in aquifers that are important for municipal water supply.

Reset injektointi
Grouting bedrock

Though tetrachloroethene and trichloroethene degrade completely already in the beginning of the residence time in the iron fill and no remarkable bypass of contaminated groundwater from any side of the PRB has occurred, the influence of the remediated water on the groundwater quality in the downstream observation wells have appeared slowly by summer 2011.

The effect of temperature on the degradation of contaminants proved to be less than expected. The mean and median values of temperature in the PRB were 8–7 °C. In Finland the average temperature of groundwater is 5–7 °C, so the chemical degradation can be assumed to occur efficiently enough also in Finnish climatic conditions.

In addition to the chemical remediation also microbial activity affects the quality of the groundwater flowing through the PRB. The changes in water quality in the PRB show that at least nitrate and sulphate reducing bacteria and methanogens were active.

The field investigations, material studies and installation work comprise the main expenses of a PRB. The extent of the hydrogeological studies needed depend on the complexity of the area and the coverage of earlier investigations. At a site geotechnically and hydrogeologically suitable for using PRB technology the installation costs according to a suggestive estimate range from 80 000 to 400 000 €.

Reset rauta
280,7 t of granular iron was used in the PRB

In eskers and end moraine formations with thick soil layers like those at the Orivesi field site, installation of a PRB is demanding and expensive. However, use of conventional groundwater treatment methods is ineffective at sites contaminated with chloroethenes. Even if a PRB was be the most cost-efficient method to treat the contaminated groundwater, due to the high costs of earthworks, a continuous reactive barrier would probably not come into question at areas with soil layers thicker than 10–15 m. The PRB method is well suited for groundwater remediation in smaller-scale formations where hydraulic conductivity is 10-4–10-6 m s-1 in order of magnitude and where both total soil layer and saturated zone thicknesses are less than 10–15 m.

Reset kaivo
Sampling tubes from the iron fill to the control well

Temperature is a factor to be taken into account upon testing the reactivity of Finnish materials. The correction factor describing the retardation of the reactions in cold water should be determined by making at least part of the material tests at the temperature of 5–10 °C.

For dimensioning a PRB, groundwater from the actual field site should also be used in the material tests to gain realistic estimates of the need for residence time and to assess the potential material passivation or mineral precipitation that might affect the long-term performance of the barrier.

In the Orivesi project, traditional open pit and cleat-supported excavation techniques were practically the only available earthwork methods. Suitable modifications to grouting equipment are probably the easiest way to enable passive methods using reactive material to treat contaminated groundwater flowing deep in the ground. Moreover, applying grouting, deep stabilization or various pile driving methods would enable the installation of PRBs in areas with limited access due to housing etc.

Practical applications

The results of the project can be used by environmental authorities and consultants in planning remediation procedures at the contaminated aquifers. The results are also applicable in other countries with similar climatic and geological conditions.

The Finnish granular iron material was supplied by Metso Paper.

Further information

LicSc(Tech), Senior researcher Sirkku Tuominen, Finnish Environment Institute,
tel. +358 400 148619,
firstname.lastname@ymparisto.fi
Published 2013-10-16 at 18:11, updated 2014-10-03 at 21:25

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