New technology of In-Situ-Alcohol-Flushing (ISAF) for mobilizing residual LNAPL in the subsurface by using swelling alcohol

The Infiltration of liquid hydrocarbons, the so-called non aqueous phase liquids (NAPL), into the subsurface is a common problem. Our research is focused on light NAPL (LNAPL), which are often trapped as a residual immobile phase (residuals) on the soil matrix. Due to the low solubility of NAPL components in water these residuals form long-term sources of pollution in groundwater. During the last years surfactants (surface active agents) were usually used to increase the efficiency of pump-and-treat aquifer remediation of those contaminated sites. Surfactants increase the solubility of NAPL- components in the aqueous phase, so that the these NAPL-components and surfactants highly contaminate the groundwater. Therefore their application leads to significant costs for treatment of the extracts. Above the critical concentration (critical micelle concentration) surfactants assemble into dynamic clusters called micelles, which are described as droplets of oil with an ionic or polar coating. At concentrations below the CMC surfactants form ad-micelles or hemi-micelles, which are adsorbed on the solid soil matrix. In this way also surfactant-NAPL compounds can be re-adsorbed on the soil matrix. Based on these disadvantages a new technology was developed with the research project 'Investigation for LNAPL - mobilization / solubilization in the subsurface'. This technology for mobilizing residual LNAPL in the subsurface by using swelling alcohol avoids the solution of LNAPL components in the groundwater. Using this new technology the problems arising with the CMC are not relevant. As part of this project, this paper reports results of a field test in Schwarze Pumpe, a former centre of carbo-chemical industry, located in Germany on the border between Saxony and Brandenburg and which covers an area of about 4.5 km2. There are three primary contaminated locations and some hot spots, causing considerable groundwater contamination with large plumes. In the centre of pollution high concentrations of phenols (1000 mg/L), BTEX (200 mg/L) and petroleum hydrocarbons (200 mg/L) were measured in the groundwater. During the field test, 20.15 t of 1-propanol was infiltrated into the depth of 8 m bgs, using 12 infiltration lances. After injecting the 1-propanol within 48 hours, water was infiltrated by using the same lances over a period of about two years. For the monitoring of the field test 14 observation wells, two observation points at the extraction well (one for each screen) and 5 gas sampling systems (GASSYS) were continuously observed. A hydraulic groundwater model (MODFLOW) was running for determining the necessary pumping rates at the two screens of the extraction well and for the arrangement of the 12 infiltration lances. Furthermore, a special predictive model was built to determine the concentration of 1- propanol in the groundwater depending on the time. The effect of mobilization, caused by the alcohol-infiltration, could be seen shortly after the infiltration. During the first four months about 6 t of residuals were mobilized and extracted as floatings. The concentration of Propanol in the extracted floatings rose from 10 g/l up to the maximum of 90 g/l after two months, continuously decreasing afterwards. Therefore, this period is called mobilization phase. During the subsequent remediation the concentration of phenols decreased from values of 1000 mg/l to about 500 mg/l. During a period of 2 years no significant solution of residual LNAPL-components could be observed in the groundwater. The aquifer was only influenced by the infiltrated Propanol. In opposition to the LNAPL contaminants the Propanol was dissolved into the upper groundwater where its concentration rose up to 4500 mg/L for a short time and decreased continuously afterwards. The calculated decrease of the 1-propanol concentration by means of the predictive model compared to the measured values are in good agreement. The In-situ-Alcohol-Flushing (ISAF) is a promising technology to clean the subsurface, contaminated by residual immobile LNAPL-products. In the field test the specific costs for the alcohol of 3.5 e/kg mobilized residuals were relatively low compared to the costs that would arise applying surfactant technologies where the residuals are dissolved in the groundwater. Furthermore, the risk to contaminate the aquifer by the dissolved LNAPL is much lower. The aquifer is only influenced by the alcohol, which is characterized by very good bio-degradability. For detecting the alcohol standard analytical methods can be used easily, which makes the monitoring of this technology much better. The predictive model proven to be a useful tool for optimising the remediation procedures