Results revealed that almost 90% of 2,4-D (70 mu g L-1), present in simulated groundwater containing 0.3 mg L-1 of iron at p(H) 7.0, was degraded after 320 min (60 min t(30w)) of natural sunlight irradiation while the viability of Escherichia coli cells (followed by DVC-FISH) was completely reduced after 220 min (40 min t(30w)) by simple addition of 10 mg L-1 of hydrogen peroxide. Klebsiella pneumoniae exhibited an especial behavior since its viability was only reduced in 3.5 logs after 320 min of sunlight irradiation (60min t(30w)). Dark experiment (in presence 10 mg L-1 of H2O2) showed that Fenton processes may also play an important role reducing the 40% of 2,4-D after 320 min (60 min t(30w)) while viability of E. coli and K. pneumoniae underwent a reduction of 2.5 and 2 logs respectively. Photolysis experiments were not able to degrade 2,4-D and E. coli and K. pneumoniae viability was partially reduced (2 logs). Results showed that high 2,4-D abatement could be due to photo-induced and/or dark processes such as photo-Fenton and Fenton (dissolved and colloidal iron), photocatalysis (colloidal iron) and UV-B photolysis of H2O2. Viability reduction of microorganisms should be related to combined effects of UV-A + B irradiation, rising of temperature (44 degrees C), photo-Fenton, Fenton and photocatalytic processes. (C) 2017 Elsevier Ltd. All rights reserved.