Constructed wetlands (CWs) are a promising technique for groundwater remediation, but field-scale application is still seldom. In this pilot-scale study for treating groundwater contaminated with benzene, MTBE, and ammonia-N, the performance of two types of CWs (a wetland with gravel matrix and a plant root mat) was investigated. Hypothesized stimulative effects of filter material additives (charcoal, iron(III)) on pollutant removal were also tested. Increased contaminant loss was found during summer; the best treatment performance was achieved by the root mat. Removal efficiencies of the planted gravel filter / root mat, respectively, amounted to 81 / 99% for benzene, 17 /82% for MTBE, and 54 / 41% for ammonia-N. Filter additives did not improve contaminant depletion, although sorption processes were observed and elevated iron(II) formation indicated iron reduction. Bacterial and stable isotope analysis provided evidence for microbial benzene degradation in the CW, emphasizing the promising potential of this treatment technique.