The installation of one-way tidal restricting floodgates in regions containing acid sulphate soils has increased the problems associated with acid drainage in many areas of coastal Australia. In southeastern NSW, one-way flap gates installed in deep flood mitigation drains maintain a low groundwater table and increase acid production and transport. Moreover, by restricting the intrusion of brackish water into the drainage system, the floodgates create a reservoir of acidic water. Recent studies suggest that by allowing a controlled amount of brackish water into the drains, via modified floodgates, buffering agents can neutralize the acidic water. A fifteen-month study was undertaken to examine the impact of modifying floodgates on water quality in flood mitigation drains that discharge into Broughton Creek, a major tributary of the Shoalhaven River, NSW. During the pre-modification period, drain water was predominately acidic (pH< 4.5) with high concentrations of aluminium and iron (50mg F1). Brief periods of floodgate leakage permitted saline buffering and neutralised acidic drain water. After a ten-month monitoring period, twoway floodgates were installed that allowed for controlled tidal intrusion into the flood mitigation drain. Following floodgate modifications, average pH increased above 6.0 and aluminium and iron concentrations decreased below < lmg T1. Saline buffering was most effective during the prolonged dry periods with elevated salinity levels (> 5000/jS/cm). Two-way floodgates also decreased the difference between upstream and downstream water chemistry, thereby eliminating aluminium flocculation and the damaging effects ofFe2+ iron oxidation.