The application of aluminium salts such as polyaluminium chloride and aluminium sulphate has been widely used to assist in the remediation of eutrophic freshwater ecosystems. Aluminium binds to phosphorus, reducing its availability for phytoplankton growth, with the additional benefit that sediment phosphorus is reduced under anoxic lake conditions. Since 2013, alum (aluminium sulphate) has been applied twice per year to Lake Okaro to reduce dissolved reactive phosphorus availability. The lake’s annual Trophic Lake Index has decreased from 5.5 in the early 2000s to 4.5 in 2020. Alum dosing has resulted in the addition of 3.8 t of aluminium to the lake, however, the spatial distribution and sediment accumulation rate of alum derived aluminium has not been investigated. The University of Waikato was contracted by the Bay of Plenty Regional Council to survey sediment aluminium concentrations in Lake Okaro to determine the distribution and accumulation of alum derived aluminium. In addition to sediment total aluminium content, the proportion of amorphous (non-crystalline) aluminium was assessed. Amorphous aluminium is the fraction of total aluminium able to adsorb dissolved phosphorus, sequestering it from uptake by phytoplankton. It was assumed that increased proportions of amorphous aluminium were primarily derived from alum dosing.