Treating Disinfectant By-products as Mixtures: Using Total Organic Halogen Content and the Growth Inhibition Assay as Potential Indicators of Toxicity in Drinking Water
Since the 1970s when the first disinfection by-products (DBFs) were discovered in drinking water, over 600 of them have been reported in literature, however, the U. S. Environmental Protection Agency currently regulates only 11 of these species. Many toxicological studies have been performed to assess the health and exposure impacts of a subset of DBFs on various living systems including rodents, bacteria, and mammalian cells. Epidemiologists have found weak linkages between individual DBFs and incidences of bladder cancer, spontaneous abortions, and reproductive problems in humans. While these studies have provided valuable information on possible health endpoints associated with single chemical species, what is more relevant to public health are the exposure effects from DBF mixtures in drinking water which consumers come in contact with on a daily basis. The challenge with the study of mixtures is that they are complex and difficult to study leading many toxicologists to avoid incorporating them into their research and, as a result, little is known about their applicability to toxicity assays. Our present understanding of DBFs is that the bromine- and iodine-containing species are more geno- and cytotoxic than their chlorine-containing counterparts, but for now we only have individual measures of toxicity for these compounds. In an effort to evaluate the exposure effects of mixtures of DBFs in real drinking waters, the study described in this report took two approaches. The first was to chemically analyze drinking waters that underwent different treatment scenarios as part of a pilot plant study, including UV irradiance. Individual concentrations of the most commonly occurring DBFs were measured, along with total organic halogen (TOX), total organic chlorine (TOCl), and total organic bromine (TOBr) content. The proportions of known and unknown TOX, TOCl, and TOBr in the water samples were also determined. The second approach aimed to tie together the chemical analysis of real drinking waters with their inherent toxicities. The first attempt at using a mixture of DBFs with the growth inhibition assay (GIA) involved spiking two very toxic DBFs, bromoacetic acid (BrAA) and iodoacetic acid (IAA), into the same matrix to evaluate whether the combination of the two caused a synergistic or antagonistic effect on the growth inhibition of human colon cells. Several preliminary experiments were also performed to evaluate the toxicity (by GIA) of water concentrated 10-fold by reverse osmosis from different disinfection scenarios. Results from both approaches included but were not limited to the following findings. In the pilot plant study samples, the percentage of TOX in real drinking waters that was not accounted for by the measurement of 9 DBPs was between 40% and 80%. UV irradiation followed by post- disinfection treatments created higher TOBr concentrations than in samples that were not UV irradiated, while ozonation (O3) contributed to formation of a greater proportion of unknown TOBr than TOCl in chloraminated waters. This suggests that in waters containing bromide, the use of UV or O3 can potentially increase the toxicity of the water. The combination of BrAA and IAA in the same matrix produced an apparent toxicity that was greater than the sum of the effects of the two individual species, leading to the postulate that studies of the toxicity (and by inference concentrations) of individual DBFs cannot accurately predict the potential impact of a mixture. This work demonstrates that current techniques for determining exposure effects of individual DBFs do not truly reflect the toxicity of the drinking water matrix. Thus, drinking water should be treated as a mixture of DBFs, not as individual compounds. Measuring TOX, TOCl, and TOBr content in addition to individual THM and HAA species chemically characterized mixtures, while the GIA evaluated their toxic potency. Applying both methods to the same types of waters would provide a better evaluation of the drinking water quality resulting from different source waters and treatment processes.Master of Science in Public Healt
