Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
bĭtūmĭno, āre, 1, v. a. [bitumen], to cover or impregnate with bitumen (late Lat.), Ambros. Hexaëm. 6, 9, 72; cf.: bitumino, ἀσφαλτόω, Gloss. Lab.
Hence, P. a.: bĭtū-mĭnătus, a, um, impregnated with bitumen, bituminous: aqua, Plin. 31, 6, 32, § 59.