1.1.02 Keeping tabs on pollutants – recording and assessing natural degradation and retention processes

The former premises of a dry cleaner’s for leather work clothing in the Harburg region of Lower Saxony was contaminated with perchlorethylenSee: Tetrachloroethylene. over a period of several decades. As the groundwater is extremely low down and the contamination is extremely difficult to access, conventional surveying and remediation procedures are not suitable for this abandoned waste. One of the projects within the KORA principle (retentionIn the field of water management, this term refers to the balancing effect of reservoirs on the outflow of flowing waters; see also ->Retention basins. and degradation processes to reduce contaminations in groundwater and soil) therefore aimed to use suitable monitoring and forecasting procedures to estimate the degree to which natural degradation and retentionIn the field of water management, this term refers to the balancing effect of reservoirs on the outflow of flowing waters; see also ->Retention basins. processes are sufficient in ruling out any risk to a neighbouring water conservation area.

PerchlorethylenSee: Tetrachloroethylene. (PCESee: Tetrachloroethylene.) was and still is used in trade and industry to remove paint, as a solvent and to degrease materials – as it was in a former specialist dry cleaner’s for leather work clothing in Rosengarten-Ehestorf in Harburg. After use, the chemical along with the improperly cleaned wastewater filtered underground over the decades below the company’s premises measuring approx. 3000 sqm.

An inaccessible aquifer

The surface of the affected aquiferIn the field of hydrogeology, an aquifer (from the Latin words aqua = water, and ferre = to bear or carry) is an underground layer carrying groundwater. Aquifers are also referred to as water-bearing strata. Different types of aquifer are distinguished depending on the individual rock type (and thus the rock’s porosity). is 30 to 40 metres underground – unusually low down. With one stretch up to 230 metres deep, it is also extremely powerful. These factors – combined with the heavily built-up area – would make it extremely difficult and expensive to carry out remediation using conventional methods (pump-and-treat procedure). The Harburg authorities therefore looked at alternatives and opted to take part in the BMBF KORA principle. Run from 2003 to 2006, the project entitled “Field-scale quantification of the potential of NA in deep large-scale aquifers – example: VOC contamination from a dry cleaners in Rosengarten-Ehestorf”involved project planners determining the extent of natural attenuation (NA) in the soil since the introduction of contamination and estimating its subsequent course. The project partners included the Harburg authorities, the Institut für Gewässerschutz und Umgebungsüberwachung (institute for the protection of natural waters and environmental monitoring) in Kiel, the State Authority for Mining, Energy and Geology in Hanover and the Tübinger Grundwasser-Forschungsinstitut (Tübingen groundwater research institute).

In order to quantify the nature of the contaminant plume and the natural degradation and retentionIn the field of water management, this term refers to the balancing effect of reservoirs on the outflow of flowing waters; see also ->Retention basins. process underground, the PCE introduction and its distribution had to be recorded first. Existing measuring points were upgraded and new ones set up in order to locate the precise location of the contamination source and the ensuing discharge.

Innovative systems for taking soil gas and groundwater samples

The site conditions made it extremely expensive to sample the groundwater using conventional wells, which is why only five were installed. The experts secured additional samples, but above all site surveys, using what is known as the direct-push procedure. This innovative drilling method is a faster, more flexible and considerably more cost-effective alternative to the conventional procedure. The direct-push procedureProcedure used for depthoriented groundwater sampling. A special filter probe is advanced directly into the aquifer at the required depth, thus enabling groundwater sampling from pre-defined depth ranges. was further developed during the project so that the required underground depths could also be reached. The scientists also expanded the wells to create multilevel measuring points to enable permanent sampling of soil gas and groundwater at various depths. This innovative procedure enabled a precise analysis of the vertical distribution of the contamination. Conversely, it proved difficult to determine the horizontal spread of the contamination by attempting to pump at the wells. The impenetrable soil did not permit any conclusions to be drawn regarding the overall breadth of the contaminant plume.

Steady introduction of PCE

Key results of the survey: the scientists measured the highest concentrations of PCE at a soil depth of five to ten metres below the surface. They determined that the soil continuously feeds PCE into the groundwater. However, the quantity continually drops over time until the discharge finally comes to a halt. Because the contaminant is primarily accessing the groundwater with the drainage water, the current and future amount of contaminant delivered and thus the lifetime of the pollution source is significantly affected by the amount of precipitation. On average, the total delivery over the non-sealed surface area of the company premises amounts to nine grams a day, of which over seven grams can get into the groundwater. The rest evaporates into the atmosphere, where it is broken down into harmless substances.

Pollution transport models

The project team made use of what are known as transport models to obtain a quantitative estimate of the future introduction of pollution into the groundwater. With their help the team ran through various versions of pollution spread since the assumed introduction of the contamination some 40 years ago. According to the most likely scenarios, which were determined by a comparison with the results measured on site, the PCE contamination plume in the aquiferIn the field of hydrogeology, an aquifer (from the Latin words aqua = water, and ferre = to bear or carry) is an underground layer carrying groundwater. Aquifers are also referred to as water-bearing strata. Different types of aquifer are distinguished depending on the individual rock type (and thus the rock’s porosity). has been stable and virtually unchanged for around 20 to 25 years. Calculations suggest that it is between 400 and 500 metres long. The results from the measuring points, which are extremely heterogeneous in part, lead to the conclusion that there may not be just one contaminant plumeThe spread of dissolved contaminants in the ►aquifer. but it could have split into two or more branches.

Pollution source will be “clean” in around 40 years

Scientists expect that the source of the pollution – i.e. the PCE still in the soil above the groundwater – will have completely disappeared in around 40 years’ time. The PCE plume in the groundwater will completely break down over time due to the existing natural degradation processes.

These statements are indispensable for a legal assessment of using natural attenuation. They enable the authorities to dispense with the extremely expensive and frequently only partly efficient measures of active remediation without any risk. Overall, the procedure developed at the Rosengarten site for surveying and assessing the contamination situation has shown that decisions on whether and to what extent natural pollution reduction is taking place can also be made with sufficient certainty for deep aquifers.

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1.1.03 Permeable treatment walls

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1.1.01 In-situ Remediation of abandoned waste