These barriers are usually made out of zero-valent iron (ZVI) but can also be made with any other zero-valent metal. The most common way they are made is by filling a trench with ZVI, nanoscale iron, or palladium. Nanoscale iron particles can also be injected directly into the subsurface to treat plumes, and they have large surface areas and, therefore, high reactivities and can be distributed more evenly in the contamination site. Palladium's reaction rates are rapid. The main advantages of PRBs are that it can reduce many a variety of contaminants and it has no above-ground structure. Problems with PRBs include that even with well constructed barriers, there might be the problem of hydraulic short-circuiting.