There is provided according to the present invention a sustained release reactant blend and methods for making the sustained release reactant blends. There is further provided methods for using such blends in the in situ or ex situ remediation of contaminated groundwater and in the treatment of water and wastewater. There is further provided, according to the present invention, methods for controllably coating reactant blends whereby such coated blends exhibit sustained release properties and have an open structure.