Wastewater and treatment byproducts from fossil fuel extraction (produced waters from oil and gas extraction) and combustion (flue gas desulfurization brines from coal power generation facilities) contain metals that can enter the environment and affect human health if not properly contained in disposal. Solidification/stabilization (S/S) is a common method of disposal and involves addition of cementitious materials and other additives with the produced waters or brines to form solid monoliths that can be landfilled. The samples’ low permeability combined with appropriate hydration products (e.g. C-S-H, portlandite, and AFt/AFm) prevents leaching of contaminants over time. The hydration phases that form are dependent on chemical and physical properties of the mixture components, so characterization of components is critical. This work proposes new methods for determining an appropriate mixture based on material properties and for testing whether S/S of the mixtures is sufficient for containing contaminants in landfill leaching conditions. The method for determining mixture composition involves small-scale reaction analysis (SSRA) in a slipstream approach. Mixtures with varying additives (e.g. cement, lime, ground granulated blast furnace slag) and varying pH are analyzed after 48 hours, 28 days, and 90 days of reaction on a laboratory rotisserie. Analysis involves x-ray diffraction (XRD) to determine the presence of crystalline and amorphous phases and ICP-MS and IC-ICP-MS to determine contaminants present in the solids and leaching fluids, respectively. Based on this data, mixtures that have appropriate phase content and stabilization are selected for long-term leaching tests. The method for determining the S/S efficacy involves preparing hollow-core cylindrical samples of the mixtures selected in the SSRA method. The samples are tested with a radial flow through (RFT) test in which pressurized leaching fluid with specified ionic strength and pH is allowed to permeate the sample’s hollow core, simulating long-term leaching in a landfill, and analyzed using IC-ICP-MS to determine the presence and speciation of contaminants in the leaching fluid. This test also allows for calculation of sample permeability. These methods allow for time-efficient and affordable determination of mixtures for S/S of harmful wastewater and treatment byproducts.