MONITORING OF A FIELD-SCALE TRANSPORT EXPERIMENT WITH GASOLINE COMPOUNDS: GROUNDWATER SAMPLING VS. PIEZOMETRIC HEAD MEASUREMENTS

As a result of a railway accident at the Elsterwerda railway station (Brandenburg, Germany) in 1997 a spill of about 470 t of unleaded fuel contaminated the quaternary aquifer with benzene, toluene, ethylbenzene and xylene (BTEX). Active remediation (soil excavation, soil vapor extraction, pump & treat) yielded a recovery of gasoline and dissolved gasoline compounds of about 260 t. The development of a plume was prevented due to the pumping activity. In April 2006, the pumping activity was stopped at the field site. The aim of the shut-down of pump & treat measurements was to determine piezometric heads as well as biodegradation rates under natural conditions. Due to the appearance of nonaqueous phase liquid (NAPL) hydrocarbons especially beneath the former spill location, dissolved BTEX is being continuously released from the NAPL pool into the groundwater.

To investigate the impact on the gain of knowledge during remediation investigations, the data of a two year monitoring of hydraulics and hydrochemistry at the Elsterwerda site was analyzed. While about 65 observation wells were sampled every third month, piezometric heads were measured monthly with water level meters. Additionally, several pressure transducers were installed in selected observation wells not only at the contaminated site, but also in the wider surroundings of the site. With these data, groundwater flow velocities and flow directions were calculated.

After two years of monitoring, the BTEX plume spread approximately 30 m in groundwater flow direction. The first-order decay rate for the BTEX of 0.0013 d^-1 was determined by balancing the electron acceptors and donators of the first sampling campaign after the shut-down of pump & treat measurements in comparison to the sampling campaign two years later.

Summarizing the two years of monitoring, only the first and the last sampling event was necessary to evaluate the spread of the BTEX plume and the biodegradation rate for the BTEX group. However, much greater importance was inherent in the piezometric head measurements. With these easy to achieve data the hydraulic system could be characterized over the whole time and with this, the maximum spreading of the dissolved compounds was known.