Steel slag wastes are large volume residues generated in increasing quantities globally during steel production. While there are many afteruses for slag, roughly a quarter produced globally is stockpiled or landfilled where it may pose environmental risks. Furthermore as resource pressures increase there is a growing interest in recovering valuable metals from industrial by-products. Given the uncertainties in environmental risks and opportunities for further valorisation of wastes, an improved understanding of leaching processes from steel slag would help inform long term management of these industrial by-products. This thesis aims to investigate a series of alkaline disposal sites (both steel slag and limespoil) to improve our understanding of the geochemical nature and fate of notable contaminants, as well as valuable metals of interest, in highly alkaline settings. The results of the field investigations show that leachates are characterised by high pH ( > 11) and negative redox potential, excess deposition of secondary precipitates, and increased mobility of lithium ( > 800 ppb), strontium ( > 2500 ppb) and vanadium ( > 50 ppb), present in concentrations greater than those typically encountered in natural surface waters. Furthermore, these slag deposits were enriched with less mobile elements such as molybdenum (60 ppb) and nickel (61 ppb) with associated low environmental concern, but high resource value. Laboratory batch tests showed that acid leaching promotes the leaching of the elements of interest particularly vanadium. However, such approach may not be viable at legacy sites due to cost. On the other hand, compost amendment of slag enhanced the leaching of molybdenum and vanadium by a factor of 3.6 and 2.5 respectively above water leaching alone. Column experiments reinforced these patterns in showing enhanced leaching of vanadium, molybdenum, and lithium when organic amendment is in contact with hyperalkaline leachate under aerobic conditions. This is most likely due to alkaline hydrolysis of organics within the system and subsequent metal complexation. Analysis of secondary precipitates (notably calcium and magnesium carbonates) forming around the slag suggest these are key in controlling solubility of contaminants and metals of potential resource value (e.g. Ni).