Episode:Purity

is the twentieth episode, in the fourth season of NCIS: Los Angeles, and the ninety-second in the series as a whole excluding the back door pilot episodes and any crossovers. 
==Synopsis==
When cyanide in a communal water jug lethally poisons a lieutenant and leaves a Navy pilot in serious condition, the NCIS: LA team investigates whether this is an isolated incident or an early test for large-scale water contamination as the search for the source of the cyanide.
==Cast==
===Series Regulars===
===Recurring Cast=== 
===Other Cast===
==Production Notes==