Osmium tetroxide is indeed wonderful stuff. Osmium is a rare element, so disposal of used solutions should consist of recycling, not dumping, even though osmium compounds are not considered environmentally hazardous (Smith et al., 1978 Trace Metal in the Environment, vol 4. Ann Arbor Science Publishers). The colorless and soluble toxic tetroxide. is rapidly reduced by almost any kind of dirt to a black, insoluble dioxide, usually in a colloidal form that's readily dispersed by moving water if it isn't firmly stuck to the solid organic matter that brought about the reduction.