Drainage from steel slag disposal sites can be extremely alkaline and a source of pollution to surface and ground waters. Data is presented detailing the hydrogeochemistry of seven highly alkaline (pH > 10) steel slag surface discharges in the UK. While there is the consistent presence of Ca-OH type groundwater in all the discharges, there are clear disparities in hydrochemical facies within and between sites, reflecting native hydrochemistry, source material and hydrogeological setting. The longevity of the pollution problem from steel slag disposal sites is highlighted at one site where the water quality records date back three decades. The consistent presence of Al, B, Ba, Fe, Sr, V and occasional presence of Cr, Mo, Ni, Pb were found at concentrations typically below surface water quality standards in the leachates. Some of the monitored metals (Al, Fe, Ni, V) were found to be lost from solution downstream of emergence in calcite-dominated precipitates which rapidly form at all sites at rates up to 100 g m(-2) day(-1). The low concentrations of potentially problematic trace elements in both solution and the sediments are discussed with regard development of economically viable passive treatment wetlands for highly alkaline industrial discharges.