## 1 Introduction  
Trihalomethanes (THMs) are normally considered to be of primary concern when naturally occurring organic compounds react with disinfection compounds in a water treatment process. In such cases, the compounds are referred to as disinfection byproducts (DBPs), of which the most prevalent are haloacetic acids (HAAs), haloacetonitrile (HANs), haloketones, chloral hydrate and chloropicrin [1]. However, THMs and substituted haloalkanes can be naturally produced in soils through the interaction of hydrogen peroxide decay products, if iron (III) is present [2], and then transported by rainfall or runoff, to the water column whereby they may continue to react to form THM precursors in natural waters [3]. Further, THMs can be formed naturally in the water column itself, through the interaction of natural bromide and/or chloride concentrations, especially when high dissolved organic matter (DOM) content is present. In North America, organic input from leaf foliage and other decay materials entering surface waters can be significant during the fall of the year, where THM concentrations have been found as high as 300 ug: L\({}^{-1}\) (ppb) without disinfection treatment [4]. In unpolluted natural waters, bromine and the bromide ion concentrations are generally less the 1 ppm, and about 100 ppb in rainwater [5; 6]. Typical chloride concentrations in surface waters are in the 10-30 ppm range, but due to component volatility, concentration variance from one water system to another is quite possible. Hydrogen peroxide has been found at concentrations from 1-30 ppb in surface waters. For rainwater, the hydrogen peroxide concentration can vary from 50 ppb to low parts per trillion (ppt) depending upon amount of sunlight exposure [6; 7]. Brominated THMs are produced through bromination of humic acids in the same way chlorinated THMs are produced; usually from the reaction of HOBr or HOCl (respectively) with acetyl groups in humic acids, which are contained in breakdown products of natural organic matter like lignin [8]. The bromide ion can undergo a one-electron transfer from chlorine to form aqueous bromine, as demonstrated in Equations (1)-(3). \[\mathrm{Cl}_{2(\mathrm{aq})}\,+\,2\mathrm{Br}^{-}{}_{(\mathrm{aq})}\,\to \,2\mathrm{Cl}^{-}{}_{(\mathrm{aq})}\,+\,\mathrm{Br}_{2(\mathrm{aq})} \tag{1}\]  
\[\mathrm{H}_{2}\mathrm{O}\,+\,\mathrm{Br}_{2(\mathrm{aq})}\,\leftrightarrow \,\mathrm{HOBr}_{(\mathrm{aq})}\,+\,\mathrm{HBr}_{(\mathrm{aq})} \tag{2}\]  
\[\mathrm{H}_{2}\mathrm{O}\,+\,\mathrm{HBr}_{(\mathrm{aq})}\,\leftrightarrow \,\mathrm{BrO}^{-}{}_{(\mathrm{aq})}\,+\,3\mathrm{H}^{+}{}_{(\mathrm{aq})}\,+ \,3\mathrm{e}^{\prime} \tag{3}\]  
The bromite ion being more reactive than the chlorite, suggests that hypobromite will be a better brominating reagent than hypochlorite for chlorination [9]. Hence, brominated THM production might be preferred over their chloro adduct in unpolluted waters [10]. The exact reaction mechanism is not understood, mainly due to the complexity of the DOM involved. Use of humic model compounds, interpreted from fractions that have been isolated through ion exchange separations of humic acids [10], suggest that the reaction may follow the haloform reaction route, as demonstrated by 3,5-hydrobenzoic acid under basic conditions of pH > 6.8 in Figure 1[9; 11]. The original THM classification for bromochloromethane, bromoform, chlorodibromomethane and chloroform has been extended to include a variety of different substituted halogenated compounds. Given the variety of THM precursors possible, the type of THM production may be altered by changes in pH; high pH favors THM adducts produced by way of Figure 1, low pH favors halonitrile THM adduct production. An increase in temperature also tends to increase the rate of THM production [12]. The interest in THMs in the water column are generally due to their deleterious health aspects, where many of the THMs are suspected carcinogens belonging to Group B2 or C of the International Agency for Research on Cancer (IARC) [13].