Permeable Reactive Barriers in Europe - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 294

iron shavings mixed with gravel to prevent precipitation of secondary phases
in the pore space. The treatment of the contaminants takes place both within
the large diameter cyclindrical boreholes loaded with ZVI and in the dis-
solved Fe(II)-plume generated downstream of the barrier. Monitoring over 3
years provided evidence of the mobilization, transport, and behavior of the
contaminants in the aquifer. Tracer experiments revealed a rather complex
hydrological regime at different scales, complicating the PRB's performance.
Results from the large 3D hydrogeochemical dataset show that the double
row of cylinders successfully treated the Cr VI contamination. Remediation
by the single row was not effective enough due to insufficient lateral overlap
of the cylinders and dissolving Fe(II)-plumes. The low amount of precipi-
tated secondary phases observed in the pore space of the reactive material
reduced the risk of clogging the system and suggested a favorable longevity
of the barrier. Limiting factors to long-term operation are the availability and
accessibility of Fe(II) within the cylinders and the concentration within the
generated Fe(II)-plume (Flury et al., 2009).
13.3.3 Thun
A PRB for Cr VI reduction by gray-cast iron was installed in May 2008. It is
composed of a double array of vertical piles containing iron shavings and
gravel. The aquifer in Thun is almost saturated with dissolved oxygen and
the groundwater flow velocities are 10-15 m/day. Two years after the PRB's
installation, Cr VI concentrations were found to exceed the Swiss threshold
value downstream of the barrier. Cr isotope measurements indicated that
part of the Cr VI plume is bypassing the barrier. Using a Rayleigh fraction-
ation model, a minimum overall Cr VI reduction efficiency of about 15% was
estimated. A series of two-dimensional (2D) model simulations, including
the fractionation of Cr isotopes, confirmed that the malfunction of the PRB
was due to Cr VI contaminated groundwater partly bypassing the PRB. This
might be probably due to insufficient permeability of the PRB piles. It was
concluded that with such a special PRB design/construction, a complete and
long-lasting Cr VI reduction was extremely difficult to achieve for Cr VI con-
taminations of oxygen- and calcium carbonate-saturated aquifers character-
ized by high groundwater velocities in addition (Wanner et al., 2012).
13.4 PRB Sites in Denmark
13.4.1 Vapokon
Set up in 1999, type: F&G (additional drainage system upstream to reduce
groundwater flow rate through PRB), funnel length: 122 m, gate length:
15.2 m, depth: 0.6 m, and thickness: 9.1 m; a full-scale; ZVI (type/brand: