The occurrence of antibiotics in subsurface environments is increasingly reported recently in addition to the widespread distribution in surface waters. However, the mechanisms governing the transport of antibiotics from surface into subsurface are not well understood. In this study, the transport of tetracycline in saturated sand columns were investigated in the presence and absence of humic acid (HA) at different ionic strengths and cationic types. NaCl (0-0.1 M) and CaCl2 (0-0.1 M) were used as representatives of mono- and divalent cations in subsurface. The presence of HA moderately facilitated the transport of tetracycline in sand columns. The presence of Na+ inhibited the transport of tetracycline and HA through increasing the deposition of HA aggregates on the sands. The inhibition increased slightly with the increase in Na+ concentration from 0 to 0.1 M. In contrast, the presence of Ca2+ dramatically inhibited the transport of tetracycline and HA even at a low concentration of 0.001 M, which was attributed to the increased aggregation of HA and the greater electrostatic attractive interactions. Results from this study improve our understanding on the penetration of antibiotics from surface into subsurface waters and the transport of antibiotics in saturated aquifers at environmentally relevant conditions. (C) 2015 Elsevier B.V. All rights reserved.