Monochloramine is typically used in water distribution systems as a secondary disinfectant but it also forms in wastewater treatment systems by the reaction of ammonia and chlorine. As a result of this, monochloramine is expected to be found in rivers and lakes that receive treated wastewater effluent. Once there, as an oxidant and disinfectant, monochloramine can disrupt algal and bacterial communities. Monochloramine has also been linked to the formation of chlorinated byproducts that may be harmful to aquatic ecosystems. While many efforts have been made to understand the stability of monochloramine in water distribution systems, few studies have been dedicated to understanding the fate and transport of monochloramine in surface waters. A deeper knowledge of the environmental fate and transport of monochloramine is needed to understand monochloramine’s persistence in surface waters and to predict its potential impacts to aquatic environments. This study assessed loss mechanisms that affect the stability of monochloramine and developed a comprehensive model describing its persistence, fate and transport in surface waters.

The partitioning of monochloramine between the aqueous and gas phase has not been extensively studied. To better understand monochloramine’s potential for volatilization, the dimensionless Henry’s law constant of monochloramine was determined using an equilibrium headspace technique. The resulting values ranged from 8x10-3 to 4x10-2 over a temperature range of 11-32 °C, indicating a semi-volatile compound, and were found to be consistent with quantitative structure activity relationship predictions. The Henry’s constant values for monochloramine suggests that volatilization could be a relevant loss process in open systems such as in rivers and lakes.

The stability of monochloramine in the presence of DOC from different surface waters was assessed implementing the specific UV absorption at 280nm normalized to DOC concentration (SUVA280) as a proxy of the reactivity of DOC towards monochloramine. Results confirmed that monochloramine reacts with DOC in surface waters via two pathways: a direct oxidation of DOC by monochloramine resulting in NH4+, Cl- and oxidized carbon species followed by chlorination of dissolved organic matter by the hypochlorous acid formed during monochloramine auto-decomposition resulting in the formation of chlorinated organic compounds. Chlorination was found to be more predominant in samples with lower SUVA280, while oxidation was found to be more extensive in samples with higher SUVA280.

In a separate experiment, the concentration of monochloramine solution in contact with bottom sediment was found to decrease rapidly (t1/2 values of 0.1-13 days), with rate constants increasing exponentially with the total oxidant demand of the sediments. Considering that the reaction between monochloramine and sediments will be limited by the transport across the benthic boundary layer, the effect of rapid mixing on the rate of reaction was considered. Monochloramine concentration was found to decrease at a greater rate in a rapidly mixed sample than in a sample with minimum periodic mixing, with reaction rate constants of 1.46x10-1 hr-1 and 1.00x10-1 hr-1 respectively. This indicates that a correction factor for transport should be included with the rate constant expression originally presented. Monochloramine was also found to decrease more rapidly than dissolved oxygen, suggesting that monochloramine is a more reactive oxidant than oxygen.

The results determined in this study for the Henry’s constant of monochloramine and its reaction with DOC from surface waters and with bottom sediments were combined with rate constants for the auto-decomposition of monochloramine and related reactions (Jafvert and Valentine, 1992; Vikesland et al., 2001) to develop a model for the fate and transport of monochloramine in surface waters. The model was found to be in good agreement with field data collected from the Santa Ana River, near Riverside, CA and the New River, near Calexico, CA, during the spring of 2016 in Southern California with relative root-mean-square error values between predicted and observed concentrations below 0.05. Monochloramine was rapidly lost in the New River, decreasing from 72 µg/L at the Mexican border to <1.0 µg/L 5.63 km downstream (corresponding to a travel time of 1.5 hrs). Monochloramine was more persistent in the Santa Ana River, decreasing from 16 µg/L at the discharge of the Riverside Water Quality Control Plant to 15 µg/L 4.8 km downstream (travel time of 2.5 hrs).

Results showed that auto-decomposition accounted for approximately 10% monochloramine lost in both rivers. Volatilization was found to be more important in the Santa Ana River than in the New River, accounting for 30% and 5% monochloramine loss respectively. Monochloramine losses due to reactions with DOC accounted for 15% in the Santa Ana River and 42% in the New River. Monochloramine interactions with sediments were also an important loss process, accounting for 45% loss in the Santa Ana River and 43% of monochloramine loss in the New River. The model demonstrates that monochloramine will be more persistent in the Santa Ana River than in the New suggesting that the effect of chlorinated organic compounds would be more of a concern in the Santa Ana River than the New River and the longer persistence of monochloramine may result in greater downstream impacts to bacterial and algal communities.