Environ. Sci. Technol. 2003, 37, 1331-1336
Precursors of N-Nitrosodimethylamine in Natural Waters ANDREAS C. GERECKE† AND DAVID L. SEDLAK* Department of Civil and Environmental Engineering, University of California at Berkeley, 609 Davis Hall, Berkeley, California 94720
Monochloramine disinfection of drinking water can result in the formation of the potent carcinogen N-nitrosodimethylamine (NDMA). To assess NDMA precursor concentrations in natural waters, samples from lakes, reservoirs, groundwaters, and isolated natural organic matter were exposed to relatively high concentrations of monochloramine. After 1 week, the concentration of NDMA ranged from approximately 0.1 to 0.8 nM. Application of a sensitive new GC/MS/MS technique and amendment of a sample with the known NDMA precursor dimethylamine (DMA) prior to chloramination indicated that only a small fraction of the NDMA produced during chloramination could be attributable to DMA. Chloramination of isolated natural organic matter and application of solid-phase extraction prior to chloramination suggested that natural organic matter accounts for a significant fraction of the precursors. However, the concentrations of NDMA precursors were higher in the epilimnion than the hypolimnion of two stratified reservoirs despite similar concentrations of dissolved organic carbon throughout the water column. Experiments performed to simulate the formation of NDMA during conditions comparable to those encountered in water treatment and distribution systems that use chloramine indicated that the concentration of NDMA produced from naturally occurring precursors usually will be less than 0.1 nM. The results indicate that chloramination of natural organic matter alone is unlikely to produce NDMA levels exceeding the current regulations.
Introduction In recent years, N-nitrosodimethylamine (NDMA) has been detected in groundwater (1), in recycled water (1), and in treated surface water (2). Because of suspected adverse health effects of NDMA, these findings have led to concern among drinking water providers about the sources of NDMA and strategies for minimizing concentrations in drinking water (3). NDMA is a N-nitrosamine, which is a family of compounds found in many food products and in tobacco smoke. Many of the N-nitrosamines are potent carcinogens, mutagens, and teratogens (4). Using the U.S. EPA’s IRIS database and standard risk assessment assumptions (Weibull extrapolation * Corresponding author e-mail:
[email protected]; telephone: (510)643-0256; fax: (510)642-7483. † Present address: Swiss Federal Laboratories for Materials Testing and Research, Laboratory of Organic Chemistry, Ueberlandstrasse 133, 8600 Duebendorf, Switzerland. 10.1021/es026070i CCC: $25.00 Published on Web 03/07/2003
2003 American Chemical Society
technique) yields an estimated 10-6 lifetime cancer risk level for NDMA in drinking water of 0.7 ng/L (0.009 nM) (5). The Ontario (Canada) Ministry of the Environment (MOE) has set an interim maximum acceptable concentration (IMAC) for NDMA at 9 ng/L (0.12 nM) (6), and the State of California has set an action level of 10 ng/L (0.14 nM) (1). Although the formation of NDMA in chlorinated water was first described in 1981 (7, 8), insight into the mechanism of NDMA formation has only become available with the widespread availability of sensitive analytical techniques (9-11). Use of these techniques has indicated the presence of NDMA in drinking water near certain industrial sites where hydrazine-based rocket fuel was manufactured or used (12, 13) and in waters subjected to chloramine disinfection (14-16). It has been demonstrated that the formation of NDMA can occur during chloramine disinfection through a reaction between monochloramine and an organic nitrogen-containing precursor, such as dimethylamine (DMA) (13-15). Other potential NDMA precursors include dithiocarbamates (e.g., dimethyldithiocarbamate) and dithiocarbamate-based fungicides (e.g., thiram), which readily hydrolyze to form DMA (17). In addition, certain nitrogencontaining cationic polyelectrolytes used as flocculation aids also can serve as NDMA precursors (13, 18). Surveys of NDMA concentrations following chloramine disinfection suggest that NDMA precursor concentrations in municipal wastewater effluent and in recycled water are significantly higher than those detected in surface water and in groundwater that is not affected by the discharge of wastewater effluent (16). The higher concentrations of NDMA precursors in municipal wastewater effluent can be explained by the excretion of dimethylamine and related compounds by humans (19, 20) and the use of dithiocarbamates for metal treatment and control of tree roots in sewer systems. In contrast to the situation encountered in municipal wastewater effluent, there is no obvious source of NDMA precursors in natural waters. Although it has been presumed that DMA in natural waters is related to animal wastes and microbial respiration (20), little evidence has been offered to support this conclusion because amines are difficult to quantify at the low concentrations expected in natural waters. In one of the few reported measurements of DMA in surface waters, Sacher et al. (21) detected DMA at concentrations up to 67 nM in several German rivers. However, it is not clear if this DMA was of natural origin or if it originated from the discharge of wastewater effluent, which is known to contain DMA (e.g., ref 22). The total pool of dissolved organic nitrogen (DON) in natural waters, which may theoretically serve as NDMA precursors, ranges from 1 to 100 µM (23). Although the chemical nature of DON is not well-understood, only a small fraction consists of amino acids and amino sugars or other well-defined compounds (24, 25). It is likely that most of the nitrogen is associated with high molecular weight polymers such as humic substances and biopolymers. To identify and quantify the NDMA precursors present in natural waters, we conducted a series of experiments with natural water samples collected from reservoirs, lakes, groundwaters, and natural organic matter isolates. To determine if DMA was responsible for NDMA formation, we measured DMA with a sensitive new analytical technique and amended a natural water sample with different concentrations of DMA prior to chloramination. To quantify other NDMA precursors, we used the NDMA precursor test (26), which is a procedure for converting NDMA precursors into NDMA under controlled conditions. For several samples, filtration and solid-phase extraction were employed prior to VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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chloramination to characterize the chemical properties of the precursors. Results of this research provide a better understanding of the importance of natural waters as a source of NDMA precursors and can be used to prioritize future efforts to minimize human exposure to NDMA.
Experimental Section Materials. Chemicals were obtained from the following sources at the specified purities: NDMA, 99+ (Acros, Pittsburgh, PA); NDMA-d6 (Cambridge Isotope Laboratories, Andover, MA); dimethylamine hydrochloride, 99% (Acros, Pittsbrugh, PA); dimethylamine hydrochloride-d6 (CDN Isotopes, Pointe-Claire, Canada); water and dichloromethane (both Ultra Resi-analyzed (J.T. Baker, Phillipsburg, NJ); Na2HPO4, NaH2PO4, NaBr (all ACS certified), ascorbic acid (USP powder), NH4Cl (USP/FCC), and 0.7-µm pore size borosilicate microfiber filters (Environmental Express, binderfree filters) (all from Fisher Scientific, Fair Lawn, NJ); 4-methoxybenzenesulfonyl chloride (98+%) (Avocado Research Chemicals Ltd., Heysham, England); Suwannee River humic acid and Suwannee River natural organic matter (International Humic Substance Society, St. Paul, MN). Sources for other chemicals used for analytical procedures are described in refs 14 and 26. Solid-phase extraction resins were obtained from the following sources: Carbopack X 120/ 400 mesh and C18 Supelclean Envi C18 were both obtained from Supelco (Bellefonte, PA); Bio-Beads SM-2, a macroreticular styrene-divinylbenzene copolymer (SDVB), was obtained from Bio-Rad Laboratories (Richmond, CA). NDMA Precursors. To quantify the total concentration of NDMA precursors, water samples were exposed to relatively high concentrations of monochloramine for an extended period. The concentration of NDMA formed serves as a surrogate for all compounds that can be converted into NDMA during chloramination (i.e., the measured NDMA concentration in the formation tests is a surrogate for the NDMA precursor concentration.). This NDMA precursor test is based on a similar idea as the trihalomethane (THM) formation potential test (27), which has been used to investigate sources of THM precursors in natural waters. Details of the procedure are given elsewhere (26), and a brief summary is included below. Samples (960 mL) were buffered at pH 7.2 ((0.2) with 20 mL of 0.5 M phosphate buffer; 20 mL of a 50 ( 5 mM stock solution of monochloramine (Cl/N ratio ) 0.7) was added to achieve an initial NH2Cl concentration of 1 mM. All reagents and blank experiments were prepared with Ultra Resi-analyzed water from J.T. Baker. The reaction containers were stored protected from light at 23 ( 2 °C. The reaction was halted after 7 d by the addition of 5 mmol of ascorbic acid. Under these conditions, monochloramine decayed mainly by disproportionation and other autodecomposition reactions (28, 29). As a result, the monochloramine doses were very consistent (66 ( 9 mM h) for the samples included in this study. Residual concentrations of monochloramine at the end of the experiments were 0.18 ( 0.04 mM. The coefficient of variation for the test is 10%. Analytical Techniques. NDMA was extracted from water in a separatory funnel with dichloromethane. The compound was analyzed using GC/MS/MS with methanol as the chemical ionization gas, as described previously (14) except 10 µL of a 0.14 nM solution of NDMA-d6 was added to the samples as a surrogate standard prior to extraction and the extracts were concentrated to 200 µL prior to analysis. Although recoveries from liquid/liquid extraction were relatively low (22%), the use of the deuterated internal standard (i.e., NDMA-d6) allowed us to correct the data for recoveries with a high degree of precision. The reproducibility of the analytical procedure was (0.01 nM, and blanks were consistently below the detection limit of 0.03 nM. 1332
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DMA was analyzed as reported elsewhere (26). The method consisted of an aqueous derivatization with 4-methoxybenzosulfonyl chloride and GC/MS/MS detection using methanol as chemical ionization gas. The detection limit of this method was 4 nM. Total nitrogen was analyzed using the persulfate oxidation method followed by detection of nitrate with the cadmium reduction method (30). Ammonia was measured using the colorometric phenate-method (30), and DON was calculated by subtracting the nitrate and ammonia concentration from the total nitrogen concentration. Dissolved organic carbon (DOC) was determined by passing samples through baked glass fiber filters followed by measurement of DOC with a Shimadzu 5000A TOC analyzer. The concentration of monochloramine was determined using the colorometric N,N-diethyl-p-phenylenediamine (DPD) method (30), which measures the sum of monochloramine, free chlorine, and forms of combined chlorine other than dichloramine. Under the conditions employed in the NDMA precursor test, the concentration of NH2Cl should have been much greater than the concentrations of the other chlorine species. Solid-Phase Extraction (SPE). Extractions were performed in 6-mL glass cartridges filled with varying amounts of SPE material. The cartridges were conditioned by rinsing them with 20 mL of acetone followed by 20 mL of water. For the Carbopack cartridges, an additional conditioning step consisting of the addition of 20 mL of dichloromethane/ acetone (80:20 v:v) was employed before the acetone rinse, and 20 mL of ascorbic acid solution (10 g/L) was added after the acetone rinse. These steps were necessary to ensure good wetting and to reduce quinone structures to hydroquinones (31). Flow rates during extraction were maintained at 6 mL/ min ((4 mL/min), and the cartridges were kept wet during the whole procedure. Environmental Samples. Reservoir samples were collected in Teflon Niskin bottles. Other environmental samples were transferred directly either into Teflon-lined PET containers or into glass bottles. After collection, samples were stored at 4 °C until further processing. Sampling Sites. Samples were collected from three surface waters located near Berkeley, CA: Upper San Leandro Reservoir (Alameda County, CA), a 0.047 km3 drinking water reservoir that receives water from the local undeveloped watershed and imported water from the Sierra Nevada Mountains; San Pablo Reservoir (Alameda County, CA), a 0.048 km3 reservoir that receives mainly imported water from the Sierra Nevada Mountains; Lake Anza (Berkeley, CA), a small (0.5 km2), manmade eutrophic lake that receives runoff from the local undeveloped watershed. Samples also were collected from the shoreline of Lake Mendota (Madison, WI), a moderate sized lake (39.8 km2) on February 20, 2002, and from a groundwater production well in Florida (Dade County, FL) on February 4, 2002. NDMA was not detected in any sample prior to chloramination.
Results and Discussion Application of the NDMA precursor test on the natural water samples resulted in the formation of between approximately 0.1 and 0.8 nM NDMA (Table 1). Measurement of DMA in the same samples indicated that the concentration of DMA always was less than 4 nM (i.e., the method detection limit). Because the DMA detection limit is greater than the concentration of NDMA precursors, it is theoretically possible that DMA could have been responsible for NDMA formation if NDMA was formed from DMA with a relatively high yield. To determine the yield of NDMA from the reaction between DMA and monochloramine under the conditions used in the precursor test, NDMA precursor tests were performed in separate aliquots of samples from Lake Anza
TABLE 1. NDMA Precursor Concentrations in Isolated NOM Solutions and in Representative Natural Water Samples
sample 1 2 3 4d 5d 6d 7d 8 9 10 11
blank Suwannee River humic acids solution Suwannee River natural organic matter solution San Pablo Reservoir
date and collection depth
description
DOCa (mg/L)
DONb (µM)
NDMA precursorsc (nM)
acid precipitation extract
0.2 4.9
na 8
