Environ. Sci. Technol. 2004, 38, 5365-5372
Accumulation of Arsenic in Drinking Water Distribution Systems DARREN A. LYTLE,* THOMAS J. SORG, AND CHRISTY FRIETCH U.S. Environmental Protection Agency, ORD, NRMRL, WSWRD, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268
The tendency for iron solid surfaces to adsorb arsenic is well-known and has become the basis for several drinking water treatment approaches that remove arsenic. It is reasonable to assume that iron-based solids, such as corrosion deposits present in drinking water distribution systems, have similar adsorptive properties and could therefore concentrate arsenic and potentially re-release it into the distribution system. The arsenic composition of solids collected from drinking water distribution systems (pipe sections and hydrant flush solids), where the waters had measurable amounts of arsenic in their treated water, were determined. The elemental composition and mineralogy of 67 solid samples collected from 15 drinking water utilities located in Ohio (7), Michigan (7), and Indiana (1) were also determined. The arsenic content of these solids ranged from 10 to 13 650 µg of As/g of solid (as high as 1.37 wt %), and the major element of most solids was iron. Significant amounts of arsenic were even found in solids from systems that were exposed to relatively low concentrations of arsenic (300 mg of Fe/L) and copper (>200 mg of Cu/L) from the distribution system, resulting in red-colored water complaints from the consumers. An investigation found that very high concentrations of arsenic had sorbed onto solids responsible for the colored water. This event raises the question of whether a similar situation has the potential of occurring at other locations. To determine whether the potential for “arsenic releases” exist in distribution systems, a field study was undertaken to characterize the solids found in distribution systems exposed to arsenic in the distributed water. The main objective was to determine whether these solids, corrosion byproducts and precipitated solids (sediment) commonly found in distribution systems, contain arsenic. The solids consisted of (i) the surfaces of drinking water distribution system pipe sections and (ii) fire hydrant flush solids. During a 2-yr period, pipe sections and hydrant flush solids were collected from 15 water systems in the Midwest located in Ohio (7), Indiana (1), and Michigan (7). All systems had arsenic in their distributed water, except for one in Ohio and this system was used as a control.
Materials and Methods Study Sites. The 15 utilities in the study were chosen from a group of candidate sites based upon finished water arsenic concentration, water treatment process, fire hydrant flushing schedules, and utility interest and cooperation. All utilities were requested to provide pipe, hydrant flush, and water samples and available water treatment history and water quality records. The location of the utilities and their water treatment practices at the time of sample collection are listed in Table 1. Solid Samples. Five utilities provided both pipe and hydrant flush samples, two provided only hydrant flush samples, and eight provided only pipe samples. Utilities were encouraged to send iron-based pipe sections of any reasonable diameter or length, although any pipe material was accepted. The internal surface of corroded metal pipe sections were thought to represent corrosion byproduct solids, although iron hydroxide, calcium carbonate, and other precipitated solids could also be incorporated into the solid matrix or the surface of the corrosion solids. Hydrant flushed samples were requested from distribution zones that historically had colored water complaints. Hydrant flushed water generally contains loosely bound solids that are susceptible to transport through the distribution system by hydraulic forces. Sixty-seven solid samples (30 hydrant flush solids and 37 pipe sections solids) were obtained and analyzed (Table 1). VOL. 38, NO. 20, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Utility Water Treatment Practices and Description of Solid Samples Examined in This Study location and treatment practice
sample ID
flush or pipe
pipe matertiala
utility 1, OH, aeration, lime softening, alum addition, re-carbonation, filtration, chlorination, and fluoridation
sample 1-1 sample 1-2 sample 1-3 sample 1-4 sample 1-5 sample 2-1 sample 2-2 sample 2-3 sample 2-4 sample 2-5 sample 2-6 sample 2-7 sample 2-8 sample 2-9 sample 2-10 sample 3-1 sample 3-2 sample 4-1 sample 4-2 sample 5-1 sample 5-2 sample 5-3 sample 5-4 sample 5-5 sample 5-6 sample 6-1 sample 6-2 sample 6-3 sample 6-4 sample 6-5 sample 7-1 sample 7-2 sample 8-1 sample 8-2 sample 8-3 sample 9-1
hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush pipe pipe hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush pipe pipe pipe pipe hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush pipe pipe hydrant flush pipe pipe pipe pipe
cast iron asbestos cement naa na na cast iron cast iron cast iron cement-lined iron cement-lined iron na na na na na PVC PVC cement-lined iron cast-iron na na na na na na na na na na cast iron cast iron na cast iron cast iron cast iron cast iron
sample 10-1 sample 10-2 sample 10-3 sample 10-4 sample 10-5 sample 10-6 sample 10-7 sample 10-8 sample 10-9 sample 10-10 sample 10-11 sample 10-12 sample 10-13 sample 10-14 sample 10-15 sample 10-16 sample 10-17 sample 11-1 sample 11-2 sample 11-3 sample 12-1 sample 13-1 sample 13-2 sample 13-3 sample 14-1 sample 15-1 sample 15-2 sample 15-3 sample 15-4 sample 15-5 sample 15-6
pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe pipe hydrant flush hydrant flush hydrant flush hydrant flush hydrant flush pipe
cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron cast iron na na na na na cement PVC PVC asbestos cement na na na na na plastic
utility 2, OH, aeration, prechlorination, filtration, fluoridation, and post-chlorination (Fe and Mn removal)
utility 3, OH, potassium permanganate, and greensand filtration (Fe and Mn removal utility 4, IN, aeration, prechlorination, filtration, and post-chlorination (Fe removal) utility 5, OH, chlorination
utility 6, MI, chlorination and blended phosphate
utility 7, MI, aeration, prechlorination, filtration, fluoridation, and post-chlorination (Fe removal) utility 8, MI, chlorination, fluoridation, and blended phosphate
utility 9, OH, alum coagulation, filtration, GAC filtration, chlorination, fluoridation utility 10, MI, chlorination and blended phostphate
utility 11, OH, chlorination
utility 12, MI, no treatment utility 13, MI, chlorination
utility 14, MI, chlorination and blended phosphate utility 15, OH, chlorination and blended phosphate
a In the case of hydrant flush samples, pipe material refers to the type of pipe used to distribute water in the flushing zone. was not available.
Sample Collection. Pipe sections were normally shipped to the U.S. EPA’s research center (ERC) (Cincinnati, OH) by the utilities as they became available. Utilities were requested to ship newly cut pipe sections filled with water to maintain 5366
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b
na, information
the mineralogy of the solids but rarely was this possible. The utility was asked to provide information on the history (age, type, extraction date, etc.) of pipe specimens to the extent known.
Hydrant flushed water samples were normally collected during routine fire hydrant flush events by placing a 5-L bottle into the flowing water stream when colored water was observed. Information was requested on the sampling location, water color, pipe material, and time into flushing when the sample was collected. Solid Sample Preparation. All pipe samples were processed for analyses at the ERC. Each pipe section was given an identification number and photographed, and one solid sample was collected for analysis. The solid sample was collected by scraping the pipe surface, ground to pass through a 75-µm mesh sieve, and stored in a desiccator. All hydrant flush samples were given identification numbers and gravity settled for at least 24 h. The supernatant was removed, and the remaining sample was concentrated by centrifugation in separate 250-mL bottles at 45 rpm for 30 min. This process was repeated until the concentrate volume of particles was reduced to 30-60 mL. The solids were air-dried and stored in a desiccator. In a few cases, hydrant flush samples were sent directly to Battelle Memorial Institute (BMI), an EPA contractor, for processing and analysis. The sample was first shaken to ensure homogeneity and filtered through a Versapor-450 0.45µm membrane filter (Gelman Sciences Lot 8759) using a tabletop vacuum pump. The membrane filters were tared prior to filtering and also weighed 24 h after filtering to determine the weight of the particles collected and recorded. The filters were digested and analyzed by inductively coupledargon plasma spectrometer (IC-APS). Filter blanks were prepared and analyzed to ensure there was no contamination. Solids Analysis. All solids were analyzed for their elemental composition by BMI using an inductively coupled plasma mass-spectrometer (ICP-MS) and for their crystalline phases by U.S. EPA using X-ray diffraction (XRD). For ICP-MS analysis, the available amount of solid sample (approximately 1 g) was weighed and digested as per U.S. EPA Method 3050B (Acid Digestion of Sediments, Sludges and Soils). After preparation, the solids and filters were placed separately into 250-mL beakers. One method blank was prepared for the 26 samples. Ten milliliters of 1:1 nitric acid:deionized (DI) water was added to each beaker; the beakers were covered with watch glasses and then heated (approximately 95 °C) for 30 min for refluxing. An additional 5 mL of concentrated nitric acid was added to each of the beakers and heated for another 2 h. After cooling, 2 mL of water and 3 mL of 30% H2O2 were added to each of the beakers. After the resulting effervescence had subsided, the beakers were heated for another 2 h. Thereafter, samples were cooled, transferred with numerous washes, filtered through Whatman No. 41 filters (Whatman Lot A576937), conditioned with 1% HNO3, and poured into 100-mL volumetric flasks. Samples were subsequently diluted 1:10 and 1:100 via serial dilutions with 1% HNO3. These samples were quantitatively analyzed on the Perkin-ElmerSciex Elan 6000 ICP-MS (U.S. EPA Method 200.8) for Mg, Si, P, Ca, Mn, Fe, and As using Sc, Y, and Tb as internal standards. XRD was used to identify crystalline phases of ground solids. XRD analyses were performed using a Scintag (Scintag, Inc., Santa Clara, CA) XDS-2000 θ-θ diffractometer with a copper X-ray. The tube was operated at 30 kV and 40 mA, and scans were typically over the range of 5-60° 2θ, with 0.03° step sizes that were held for 3 s each. Pattern analysis was performed using the computer software provided by the manufacturer, which generally followed ASTM procedures (22). Water Chemistry Analyses. Samples of the source water and distribution water were collected on-site by either EPA, BMI, or the utility. When collected by the U.S. EPA, the pH of water samples was measured on-site immediately after sampling with a Hach Company (Loveland, CO) EC40 benchtop pH/ISE meter (model 50125) and a Hach Company
(Loveland, CO) combination pH electrode (model 48600) with temperature corrections. Otherwise, historical utility measurements were obtained and reported. The analysis conducted by the U.S. EPA consisted of inductively coupled argon plasma atomic emission spectrophotometer (ICP-AES) (Thermo Jarrel Ash, Franklin, MA, model 61E) for Ca, Fe, Mg, Mn, Na, P, Si, and S in water samples (U.S. EPA Method 200.7) and the atomic adsorption graphic furnace method (AAGF) (U.S. EPA Method 7060A) for arsenic. Samples were also analyzed for alkalinity and chloride analysis (potentiometric titration). If a field site visit was not made, historical water quality records were used. Arsenic Speciation. The source water from 12 utilities and the finished water from 7 utilities were speciated by BMI for As(III) and As(V) using the modified anion-exchange separation method of Edwards et al. (23). The speciation procedure also includes a filtration step (0.45 µm disk filter) to determine total, particulate, and dissolved arsenic, As(III), and As (V) of the dissolved fraction. The speciation samples were also analyzed for iron, manganese, and aluminum in order to determine the particulate and dissolved fraction of these elements (ICP-MS, U.S. EPA Method 200.7).
Results and Discussion Water Chemistry. Chemical analyses of the source water samples collected from the 15 water utility systems were performed. Except for two utilities (nos. 9 and 11), at least one source water sample from each utility had an arsenic measurement above the U.S. EPA revised arsenic maximum contaminant level (MCL) of 0.010 mg/L (10 µg/L). The highest arsenic level measured was 69 µg/L in the well water of utility 3. Except for only one utility (no. 13), all of the utilities had at least one source water sample with an iron level above the EPA secondary MCL (SMCL) of 0.3 mg/L with several above 2.5 mg/L. All of the utilities with the exception of utility 13, had at least one source with high hardness levels above 250 mg/L (as CaCO3). The pH of most of the waters was in the mid 7’s. The source water used by utility 9 (control site) was river water and was not expected to contain significant levels of iron or be very hard. Six utilities had either an iron removal or softening (precipitation) process, both of which are capable of removing arsenic. Four of the six systems had their arsenic reduced to below the MCL (6-8 µg/L) in the finished water, and the other two reduced to 11 and 15 µg/L. All of the finished waters from these same systems had their iron concentration reduced to below the SMCL except for one (utility 5) that had a measured concentration of 0.38 mg/L of iron. One or more distribution system samples were collected and analyzed from 14 water systems (Table 2). Except for utility 8, the distribution water samples had arsenic levels that were less than the companion source water sample(s). Furthermore, except for this same utility, the iron levels of the distribution samples were less than the companion source water sample, with the majority being below the SMCL of 0.3 mg/L. Utility 9 (control site) did not have detectable arsenic or iron in the distributed water. The source waters of all utilities were speciated for As(III) and As(V) except for utilities 9, 11, and 15. The test results showed that arsenic in the majority of the source waters was predominately As(III). Three utilities (nos. 10, 12, and 14) had their arsenic to be an approximately 50/50 mix of As(III) and As(V). Distribution waters were not speciated because all of the utilities except one (no. 12) had some form of chemical oxidation, either chlorination of potassium permanganate, that will oxidize As(III) to As(V) (24). Utility 12 had no oxidation process; therefore, the form of the arsenic in the distribution system is unknown. VOL. 38, NO. 20, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Summary of Water Quality of Utility Distribution System Waters
location
description
utility 1, OH
distribution 1 distribution 2 utility 2, OH distribution utility 3, OH distribution utility 4, IN distribution 1 distribution 2 utility 5, OHb well 1 well 2 utility 6, MI distribution utility 7, MI distribution 1 distribution 2 distribution 3 distribution 4 utility 8, MI distribution 1 distribution 2 utility 9, OH distribution utility 10, MI distribution utility 11, OH building 2 building 55 building 45 building 30 building 17 building 24 utility 12, MI distribution 1 distribution 2 utility 13, MI distribution utility 14, MI distribution utility 15, MI distribution a
na, not analyzed.
b
AS Ca Cl Fe Mg Mn Na total PO4 (µg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 9 17 1 5 11 11 12 13 8 9 16 10 6 31 25
