The environmental fate of nitroguanidine (NQ) in surface waters is dominated by photolysis with surface half‐lives at 40°N ranging from 0.6 d in summer to 2.3 d in winter. The environmental quantum yield is 0.01. The NQ is initially photolyzed to nitrite and hydroxyguanidine; nitrite is photochemically converted to nitrate and hydroxyguanidine undergoes sensitized photolysis to unknown products. The photooxidation of nitrite is assisted by organic material in a process not involving H2O2 or singlet oxygen. Nitroguanidine biotransforms cometabolically; in the absence of extra organic nutrients the second‐order rate constant was (3.8 ± 0.9) × 10−10 ml cell−1 h−1. Half‐life estimates for aerobic, aquatic biotransformation range from 1 to 100 d. Cyanamide appears to be an end product of NQ use and no intermediate biotransformation products were observed. Nitroguanidine is expected to move readily through soils (soil sorption coefficient Kp < 0.1); however, anaerobic biotransformation occurs readily in soil, with an estimated half‐life of 4 d. Other fate parameters measured at 25°C are a water solubility of 2,600 ± 100 ppm, octanol/water partition coefficient of 0.148 ± 0.001 (dimensionless), Henry's constant of < 7 × 10−6 (dimension‐less), base hydrolysis constant of (3 ± 1) × 10−4 M−1 S−1, neutral hydrolysis constant ≤ 2 × 10−8 s−1, and biouptake constants of 110 g dry cells/g water for Anabena flos‐aquae and 150 g dry cells/g water for Selenastrum capricornutum.

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