Left water-radical 氵 - the indexing radical. Anchors 渣 in the watery-residue family: dregs, sediment, what settles at the bottom of a liquid (汤渣 broth dregs, 茶渣 tea grounds). Same radical anchors 汁 (juice), 浆 (slurry), 泥 (mud).
Right 查 supplies the sound (chá → zhā, a regular sibilant alternation). Semantically empty here - 查 just lends its phonology. Phonetic mnemonic: the stuff that sinks to the bottom is what an inspector would 查 (inspect).
