Accumulation of Arsenic in Drinking Water Distribution Systems

Accumulation of Arsenic in DrinkingWater Distribution SystemsD A R R E N A . L Y T L E , *T H O M A S J . S O R G , A N DC H R I S T Y F R I E T C H

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 iswell-known and has become the basis for several drinkingwater treatment approaches that remove arsenic. It isreasonable to assume that iron-based solids, such as corrosiondeposits present in drinking water distribution systems,have similar adsorptive properties and could thereforeconcentrate arsenic and potentially re-release it into thedistribution system. The arsenic composition of solids collectedfrom drinking water distribution systems (pipe sectionsand hydrant flush solids), where the waters had measurableamounts of arsenic in their treated water, were determined.The elemental composition and mineralogy of 67 solidsamples collected from 15 drinking water utilities locatedin Ohio (7), Michigan (7), and Indiana (1) were alsodetermined. The arsenic content of these solids rangedfrom 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. Significantamounts of arsenic were even found in solids fromsystems that were exposed to relatively low concentrationsof arsenic (<10 µg/L) in the water.

IntroductionArsenic exists in water primarily as oxyanions with oxidationstates of +III and +V. Arsenate (HnAsO4

n-3) is generally thedominant form in oxic water, while arsenite (HnAsO3

n-3)dominates in sulfidic, methanic, and deeply circulatinggeothermal waters. Sorption, coprecipitation, and oxidation-reduction reactions of arsenic at the sorbent-water interfaceare important factors that affect the fate and transport ofarsenic in aqueous systems (1-4). Numerous studies haveconcluded that arsenite (As[III]) is more soluble and mobilethan arsenate (As[V]), although there are differences in thereactivity and stability of As(III) and As(V) at different solid-water interfaces (5, 6). Iron surfaces are particularly effectiveat adsorbing arsenic in aqueous systems. As a result, thestructure of arsenate and arsenite sorption and coprecipi-tation on iron oxide surfaces has been investigated extensively(5, 7-9). Results have shown that the sorption of arsenic isaffected by many factors including pH, water chemistry,amount and form of iron present, and existence of competingions such as phosphate and silicate (10-13).

The removal of arsenic from drinking water with iron-based treatment processes (such as chemical coagulationwith iron salts, iron removal by oxidation and filtration, andiron-based adsorption media) has been reported (14-20).The effectiveness of iron to remove arsenic is due to the

strong affinity of iron solid surfaces to adsorb arsenic.Therefore, it is reasonable to assume that iron-based solidsaccumulated in water distribution systems, such as corrosionbyproducts and sediment, could adsorb and concentratearsenic. Corrosion byproducts are defined as solids that formdirectly from the corroding metal pipe wall (e.g., R-FeOOH,Fe3O4, CuO, PbO2). Sediment may consist of corrosionbyproducts and precipitated solids (e.g., MnO2, Al(OH)3, Fe-(OH)3, CaCO3) as well as solids that carry over from watertreatment plants. Changes in water chemistry and physicaldisturbances to distribution system materials could releasearsenic back into the bulk water resulting in elevated arseniclevels at the consumer’s tap.

The release of arsenic-containing solids has been docu-mented in a Midwestern drinking water distribution system(21). Chlorination of a previously non-disinfected ground-water released large amounts of iron (>300 mg of Fe/L) andcopper (>200 mg of Cu/L) from the distribution system,resulting in red-colored water complaints from the consum-ers. An investigation found that very high concentrations ofarsenic had sorbed onto solids responsible for the coloredwater. This event raises the question of whether a similarsituation has the potential of occurring at other locations.

To determine whether the potential for “arsenic releases”exist in distribution systems, a field study was undertakento characterize the solids found in distribution systemsexposed to arsenic in the distributed water. The mainobjective was to determine whether these solids, corrosionbyproducts and precipitated solids (sediment) commonlyfound in distribution systems, contain arsenic. The solidsconsisted of (i) the surfaces of drinking water distributionsystem pipe sections and (ii) fire hydrant flush solids. Duringa 2-yr period, pipe sections and hydrant flush solids werecollected from 15 water systems in the Midwest located inOhio (7), Indiana (1), and Michigan (7). All systems hadarsenic in their distributed water, except for one in Ohio andthis system was used as a control.

Materials and MethodsStudy Sites. The 15 utilities in the study were chosen froma group of candidate sites based upon finished water arsenicconcentration, water treatment process, fire hydrant flushingschedules, and utility interest and cooperation. All utilitieswere requested to provide pipe, hydrant flush, and watersamples and available water treatment history and waterquality records. The location of the utilities and their watertreatment practices at the time of sample collection are listedin Table 1.

Solid Samples. Five utilities provided both pipe andhydrant flush samples, two provided only hydrant flushsamples, and eight provided only pipe samples. Utilities wereencouraged to send iron-based pipe sections of any reason-able diameter or length, although any pipe material wasaccepted. The internal surface of corroded metal pipe sectionswere thought to represent corrosion byproduct solids,although iron hydroxide, calcium carbonate, and otherprecipitated solids could also be incorporated into the solidmatrix or the surface of the corrosion solids.

Hydrant flushed samples were requested from distributionzones that historically had colored water complaints. Hydrantflushed water generally contains loosely bound solids thatare susceptible to transport through the distribution systemby hydraulic forces. Sixty-seven solid samples (30 hydrantflush solids and 37 pipe sections solids) were obtained andanalyzed (Table 1).

* Corresponding author phone: (513)569-7432; e-mail: lytle.darren@epa.gov.

Environ. Sci. Technol. 2004, 38, 5365-5372

10.1021/es049850v Not subject to U.S. Copyright. Publ. 2004 Am. Chem. Soc. VOL. 38, NO. 20, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5365Published on Web 09/15/2004

Sample Collection. Pipe sections were normally shippedto the U.S. EPA’s research center (ERC) (Cincinnati, OH) bythe utilities as they became available. Utilities were requestedto ship newly cut pipe sections filled with water to maintain

the mineralogy of the solids but rarely was this possible. Theutility was asked to provide information on the history (age,type, extraction date, etc.) of pipe specimens to the extentknown.

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, sample 1-1 hydrant flush cast ironfiltration, chlorination, and fluoridation sample 1-2 hydrant flush asbestos cement

sample 1-3 hydrant flush naa

sample 1-4 hydrant flush nasample 1-5 hydrant flush na

utility 2, OH, aeration, prechlorination, filtration, fluoridation, and sample 2-1 hydrant flush cast ironpost-chlorination (Fe and Mn removal) sample 2-2 hydrant flush cast iron

sample 2-3 hydrant flush cast ironsample 2-4 pipe cement-lined ironsample 2-5 pipe cement-lined ironsample 2-6 hydrant flush nasample 2-7 hydrant flush nasample 2-8 hydrant flush nasample 2-9 hydrant flush nasample 2-10 hydrant flush na

utility 3, OH, potassium permanganate, and greensand filtration sample 3-1 pipe PVC(Fe and Mn removal sample 3-2 pipe PVC

utility 4, IN, aeration, prechlorination, filtration, and sample 4-1 pipe cement-lined ironpost-chlorination (Fe removal) sample 4-2 pipe cast-iron

utility 5, OH, chlorination sample 5-1 hydrant flush nasample 5-2 hydrant flush nasample 5-3 hydrant flush nasample 5-4 hydrant flush nasample 5-5 hydrant flush nasample 5-6 hydrant flush na

utility 6, MI, chlorination and blended phosphate sample 6-1 hydrant flush nasample 6-2 hydrant flush nasample 6-3 hydrant flush nasample 6-4 hydrant flush nasample 6-5 pipe cast iron

utility 7, MI, aeration, prechlorination, filtration, fluoridation, sample 7-1 pipe cast ironand post-chlorination (Fe removal) sample 7-2 hydrant flush na

utility 8, MI, chlorination, fluoridation, and blended phosphate sample 8-1 pipe cast ironsample 8-2 pipe cast ironsample 8-3 pipe cast iron

utility 9, OH, alum coagulation, filtration, GAC filtration,chlorination, fluoridation

sample 9-1 pipe cast iron

utility 10, MI, chlorination and blended phostphate sample 10-1 pipe cast ironsample 10-2 pipe cast ironsample 10-3 pipe cast ironsample 10-4 pipe cast ironsample 10-5 pipe cast ironsample 10-6 pipe cast ironsample 10-7 pipe cast ironsample 10-8 pipe cast ironsample 10-9 pipe cast ironsample 10-10 pipe cast ironsample 10-11 pipe cast ironsample 10-12 pipe cast ironsample 10-13 pipe cast ironsample 10-14 pipe cast ironsample 10-15 pipe cast ironsample 10-16 pipe cast ironsample 10-17 pipe na

utility 11, OH, chlorination sample 11-1 pipe nasample 11-2 pipe nasample 11-3 pipe na

utility 12, MI, no treatment sample 12-1 pipe nautility 13, MI, chlorination sample 13-1 pipe cement

sample 13-2 pipe PVCsample 13-3 pipe PVC

utility 14, MI, chlorination and blended phosphate sample 14-1 pipe asbestos cementutility 15, OH, chlorination and blended phosphate sample 15-1 hydrant flush na

sample 15-2 hydrant flush nasample 15-3 hydrant flush nasample 15-4 hydrant flush nasample 15-5 hydrant flush nasample 15-6 pipe plastic

a In the case of hydrant flush samples, pipe material refers to the type of pipe used to distribute water in the flushing zone. b na, informationwas not available.

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Hydrant flushed water samples were normally collectedduring routine fire hydrant flush events by placing a 5-Lbottle into the flowing water stream when colored water wasobserved. Information was requested on the samplinglocation, water color, pipe material, and time into flushingwhen the sample was collected.

Solid Sample Preparation. All pipe samples were pro-cessed for analyses at the ERC. Each pipe section was givenan identification number and photographed, and one solidsample was collected for analysis. The solid sample wascollected by scraping the pipe surface, ground to pass througha 75-µm mesh sieve, and stored in a desiccator.

All hydrant flush samples were given identificationnumbers and gravity settled for at least 24 h. The supernatantwas removed, and the remaining sample was concentratedby centrifugation in separate 250-mL bottles at 45 rpm for30 min. This process was repeated until the concentratevolume of particles was reduced to 30-60 mL. The solidswere air-dried and stored in a desiccator.

In a few cases, hydrant flush samples were sent directlyto Battelle Memorial Institute (BMI), an EPA contractor, forprocessing and analysis. The sample was first shaken toensure homogeneity and filtered through a Versapor-450 0.45-µm membrane filter (Gelman Sciences Lot 8759) using atabletop vacuum pump. The membrane filters were taredprior to filtering and also weighed 24 h after filtering todetermine the weight of the particles collected and recorded.The filters were digested and analyzed by inductively coupled-argon plasma spectrometer (IC-APS). Filter blanks wereprepared and analyzed to ensure there was no contamination.

Solids Analysis. All solids were analyzed for their elementalcomposition by BMI using an inductively coupled plasmamass-spectrometer (ICP-MS) and for their crystalline phasesby U.S. EPA using X-ray diffraction (XRD). For ICP-MSanalysis, the available amount of solid sample (approximately1 g) was weighed and digested as per U.S. EPA Method 3050B(Acid Digestion of Sediments, Sludges and Soils). Afterpreparation, the solids and filters were placed separately into250-mL beakers. One method blank was prepared for the 26samples. Ten milliliters of 1:1 nitric acid:deionized (DI) waterwas added to each beaker; the beakers were covered withwatch glasses and then heated (approximately 95 °C) for 30min for refluxing. An additional 5 mL of concentrated nitricacid was added to each of the beakers and heated for another2 h. After cooling, 2 mL of water and 3 mL of 30% H2O2 wereadded to each of the beakers. After the resulting effervescencehad subsided, the beakers were heated for another 2 h.Thereafter, samples were cooled, transferred with numerouswashes, filtered through Whatman No. 41 filters (WhatmanLot A576937), conditioned with 1% HNO3, and poured into100-mL volumetric flasks. Samples were subsequently diluted1:10 and 1:100 via serial dilutions with 1% HNO3. Thesesamples were quantitatively analyzed on the Perkin-Elmer-Sciex 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 groundsolids. XRD analyses were performed using a Scintag (Scintag,Inc., Santa Clara, CA) XDS-2000 θ-θ diffractometer with acopper X-ray. The tube was operated at 30 kV and 40 mA,and scans were typically over the range of 5-60° 2θ, with0.03° step sizes that were held for 3 s each. Pattern analysiswas performed using the computer software provided by themanufacturer, which generally followed ASTM procedures(22).

Water Chemistry Analyses. Samples of the source waterand distribution water were collected on-site by either EPA,BMI, or the utility. When collected by the U.S. EPA, the pHof water samples was measured on-site immediately aftersampling with a Hach Company (Loveland, CO) EC40benchtop pH/ISE meter (model 50125) and a Hach Company

(Loveland, CO) combination pH electrode (model 48600) withtemperature corrections. Otherwise, historical utility mea-surements were obtained and reported.

The analysis conducted by the U.S. EPA consisted ofinductively coupled argon plasma atomic emission spec-trophotometer (ICP-AES) (Thermo Jarrel Ash, Franklin, MA,model 61E) for Ca, Fe, Mg, Mn, Na, P, Si, and S in watersamples (U.S. EPA Method 200.7) and the atomic adsorptiongraphic furnace method (AAGF) (U.S. EPA Method 7060A)for arsenic. Samples were also analyzed for alkalinity andchloride analysis (potentiometric titration). If a field site visitwas not made, historical water quality records were used.

Arsenic Speciation. The source water from 12 utilitiesand the finished water from 7 utilities were speciated by BMIfor As(III) and As(V) using the modified anion-exchangeseparation method of Edwards et al. (23). The speciationprocedure 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 speciationsamples were also analyzed for iron, manganese, andaluminum in order to determine the particulate and dissolvedfraction of these elements (ICP-MS, U.S. EPA Method 200.7).

Results and DiscussionWater Chemistry. Chemical analyses of the source watersamples collected from the 15 water utility systems wereperformed. Except for two utilities (nos. 9 and 11), at leastone source water sample from each utility had an arsenicmeasurement above the U.S. EPA revised arsenic maximumcontaminant level (MCL) of 0.010 mg/L (10 µg/L). The highestarsenic level measured was 69 µg/L in the well water of utility3.

Except for only one utility (no. 13), all of the utilities hadat least one source water sample with an iron level above theEPA secondary MCL (SMCL) of 0.3 mg/L with several above2.5 mg/L. All of the utilities with the exception of utility 13,had at least one source with high hardness levels above 250mg/L (as CaCO3). The pH of most of the waters was in themid 7’s. The source water used by utility 9 (control site) wasriver water and was not expected to contain significant levelsof iron or be very hard.

Six utilities had either an iron removal or softening(precipitation) process, both of which are capable of removingarsenic. Four of the six systems had their arsenic reduced tobelow the MCL (6-8 µg/L) in the finished water, and theother two reduced to 11 and 15 µg/L. All of the finished watersfrom these same systems had their iron concentrationreduced to below the SMCL except for one (utility 5) that hada measured concentration of 0.38 mg/L of iron.

One or more distribution system samples were collectedand analyzed from 14 water systems (Table 2). Except forutility 8, the distribution water samples had arsenic levelsthat were less than the companion source water sample(s).Furthermore, except for this same utility, the iron levels ofthe distribution samples were less than the companion sourcewater sample, with the majority being below the SMCL of 0.3mg/L. Utility 9 (control site) did not have detectable arsenicor 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 resultsshowed that arsenic in the majority of the source waters waspredominately 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 becauseall of the utilities except one (no. 12) had some form ofchemical oxidation, either chlorination of potassium per-manganate, that will oxidize As(III) to As(V) (24). Utility 12had no oxidation process; therefore, the form of the arsenicin the distribution system is unknown.

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The speciation test results indicated that iron andmanganese in the sources were generally in the reduced form(dissolved), which is typical for arsenic-containing ground-water in the Midwest. Speciation tests were also conductedon one finished water sample at six utilities. The test resultsshowed that the arsenic was predominately in the particulateform (attached to the oxidized iron) and that the dissolvedarsenic fraction was predominately As(V).

The water quality test data showed that the distributionsystem water of all of the utilities, except of no. 9 (control),were exposed to water containing arsenic (likely As[V]) inthe range of 1 µg/L to as high as 30 µg/L or more. The dataalso showed that any measured iron in the distribution waterswas predominately ferric iron (particulate) that containedsome arsenic.

Elemental Composition of Solids. The results of the 67elemental analyses (µg/g) of the 30 hydrant flush and 37pipe section solids are presented in Tables 3 and 4,respectively. Except for five samples (four pipe solids andone hydrant flush), iron was the largest component in all ofthe 67 samples analyzed. The only exception was calcium(three samples) and zinc (two samples), and the secondhighest element in these five samples was iron. All three ofthe high calcium results were of solids from cement-linedpipes that would be a second source of calcium to thenaturally hard water. The two high zinc results were fromtwo PVC pipe sections from utility 13.

The second most abundant element following iron wasusually calcium (39 samples). Seven samples had phosphateas the element second-most abundant to iron, four hadaluminum, and two had zinc. The samples with the highphosphate were from utilities 6 and 8, both of which addphosphorus-based (ortho-, poly-, or blended-phosphate)chemicals for iron and/or corrosion control. Systems thatused a phosphorus-based chemical accounted for the 13highest phosphate-containing samples.

Excluding the one pipe solid sample from the controlsystem (utility 9), the arsenic content ranged from a low of10 µg/g to a high of 13 650 µg/g and averaged 897 µg/g ((2562 µg/g SD). The arsenic of the hydrant flush solids averaged2309 µg/g (( 2757 µg/g SD), nearly 3 times the arsenic of thepipe solids (897 µg/g). Eleven of the highest 13 arseniccontaining solids were hydrant flush samples.

Because arsenic is generally considered to be associatedwith iron solids, the elemental arsenic and iron data werecalculated as a ratio of As:Fe in units of micrograms of As/milligrams of Fe (Tables 4 and 5). The As:Fe ratio rangedfrom 0.07 to 58.2 µg of As/mg of Fe and averaged 5.2 µg ofAs/mg of Fe (( 7.6 µg of As/mg of Fe). The average hydrantflush samples ratio (7.4 ( 6.9) was more than double theaverage pipe sample ratio (3.4 ( 7.8).

Arsenic: Hydrant Flush Solids. The principal source ofthe solids in the hydrant flush water is likely to be carryoveriron solids from the source water (nontreated) or the solidsthat make their way through the treatment plant. This isparticularly the case with systems that have waters containingsignificant levels of iron and do not remove it. While the pipesolids may also contain these iron carryover solids andprecipitates from the distributed water, the surface mass ofthese pipe solids is likely dominated by corrosion byproducts(metal pipes). Furthermore, the solids that are looselydeposited at the pipe surfaces can become re-suspended byhydraulic flow that occurs during hydrant flushing. Theamount of iron solids in the hydrant flush water is likelydependent upon the iron content of the source water andthe amount that makes its way into the distribution system.Because of the strong adsorption tendency of iron for arsenic,the iron in the solids is the likely source of arsenic.

One example that illustrates the assumption that most ofthe solids found in hydrant flush water is carryover from thesource water is data from utility 5. The top five solids withthe highest amount of arsenic (6204-9936 µg/g) were all

TABLE 2. Summary of Water Quality of Utility Distribution System Waters

location descriptionAS

(µg/L)Ca

(mg/L)Cl

(mg/L)Fe

(mg/L)Mg

(mg/L)Mn

(mg/L)Na

(mg/L)total PO4

(mg/L)SiO2

(mg/L)SO4

(mg/L)

totalalkalinity(mg/L ofCaCO3)

hardness(mg of

CaCO3/L) pH

utility 1, OH distribution 1 9 11 38 0.53 16 0.002 58 naa 7 63 na 95 nadistribution 2 17 16 35 1.15 19 0.009 55 na 9 76 na 119 na

utility 2, OH distribution 1 107 21 0.00 33 <0.01 9 na 16 36 335 406 nautility 3, OH distribution 5 111 na 0.07 57 0.292 11 na na 1 213 514 7.7utility 4, IN distribution 1 11 63 24 0.08 26 <0.01 28 1.95 16 1 290 na na

distribution 2 11 56 9 0.01 25 <0.01 20 1.69 14 0 292 243 nautility 5, OHb well 1 12 na 14 0.33 na 0.010 na na na 21 220 na 7.68

well 2 13 na 14 0.35 na 0.014 na na na 30 220 na 7.81utility 6, MI distribution 8 76 41 0.71 29 0.019 25 0.51 15 54 267 311 nautility 7, MI distribution 1 9 80 na 0.03 31 <0.01 60 na 14 51 na 330 7.42

distribution 2 16 72 na 0.28 29 0.007 58 na 13 53 na 299 nadistribution 3 10 77 na 0.12 31 0.005 63 na 13 50 na 320 nadistribution 4 6 69 na 0.02 28 0.006 57 na 13 55 na 288 na

utility 8, MI distribution 1 31 93 25 2.77 26 0.11 10 0.01 14 59 289 343 nadistribution 2 25 51 12 1.25 16 0.06 15 0.03 11 15 217 196 na

utility 9, OH distribution <1 42 40 <0.05 14 <0.01 33 <0.2 <0.2 3 60 163 8.53utility 10, MI distribution 2 98 na <0.05 27 0.024 95 na 12 81 na 358 nautility 11, OH building 2 9 84 na <0.05 34 0.03 9 <0.2 16 40 na 352 na

building 55 12 83 na 0.17 34 0.036 10 <0.2 16 40 na 349 nabuilding 45 11 83 na 0.01 34 0.034 9 na 16 40 na 349 nabuilding 30 3 74 na 0.94 35 0.077 9 <0.2 12 43 na 331 nabuilding 17 1 83 na 0.03 33 0.03 9 <0.2 16 40 na 345 nabuilding 24 13 84 na 0.02 34 0.038 8 <0.2 16 40 na 352 na

utility 12, MI distribution 1 5 4 na 0.03 1 0.005 na <0.2 13 63 na 14 nadistribution 2 6 4 na 0.03 1 0.005 na <0.2 13 63 na 14 na

utility 13, MI distribution 10 na na na na na na na na na na na nautility 14, MI distribution 16 90 na 0.06 28 0.05 22 na 15 43 na 342 nautility 15, MI distribution 23 73 na 0.01 34 0.02 12 na 18 31 na 324 na

a na, not analyzed. b Distribution system samples were not available; raw water quality is reported.

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hydrant flush samples from utility 5. The five solids haveAs:Fe ratios ranging from 17.6 to 20.1 µg of As/mg of Fe; theaverage being 19.7 µg of As/mg of Fe. The source water ofthis system is not treated (except for chlorination that will

oxidize the iron and As [III]) and has an iron level ofapproximately 0.35 mg/L and arsenic level of 13 µg/L. TheAs:Fe ratio in the particulate fraction of the speciation testwas 21 µg of As/mg of Fe, which is approximately the same

TABLE 3. Elemental Composition of Hydrant Flush Solids (µg/g)

sample ID Al As Ca Cu Cd Fe Mg Mn Ni P Pb Si Zn As:Fe

sample 2-9 790 107 4394 302 10 113280 624 360 20 220 223 1234 153 0.9sample 2-2 263 109 32871 1383 <4.6 516711 1257 981 58 718 171 4412 381 0.2sample 2-8 96 137 45625 125 <2 90304 1456 679 7 328 197 1737 140 1.5sample 5-1 naa 205 23375 na na 44872 2456 1611 na <477 na 4288 na 0.5sample 6-4 899 211 43159 1273 35 348528 5812 434 13 54715 828 1705 791 0.6sample 1-2 11708 237 9139 3515 <11 577349 1482 2295 90 557 1030 6601 914 0.4sample 6-2 138 348 48031 27 <0.2 337030 4266 330 5 73865 25 3947 81 1.0sample 2-7 227 455 22060 37 <4 70335 2324 1612 <4 625 26 5787 46 6.5sample 6-1 165 469 68785 102 0.6 339758 8752 523 11 104681 59 2709 125 1.4sample 2-10 161 493 11040 116 0.7 84728 942 1958 4 875 138 5762 129 5.8sample 6-3 164 498 52462 14 <0.2 353743 5517 698 5 80728 3 4938 86 1.4sample 2-6 402 529 14665 65 <4 69722 3719 1591 <4 657 39 5616 49 7.6sample 1-3 144265 554 11454 381 0.2 232272 3457 94 6 301 140 1469 89 2.4sample 1-4 79648 613 10988 133 0.3 357950 7312 1061 2 211 10 1136 23 1.7sample 7-2 5534 747 34727 189 2.9 378256 7715 682 48 1254 224 11562 271 2.0sample 1-1 na 785 10321 na na 167995 5013 89 na <491 na <1227 na 4.7sample 1-5 127555 929 10957 11 0.2 223649 5434 139 2 299 8 1923 25 4.2sample 15-1 7512 1508 79610 132 0.2 178182 28383 526 20 22522 19 3808 510 8.5sample 2-3 1261 1718 183074 335 <25 178742 2758 3576 44 3102 138 14573 <980 9.6sample 10-17 101 1978 43360 809 <0.7 497525 4256 884 10 49203 148 1357 376 4.0sample 15-2 3022 2841 27339 171 0.1 244037 6469 460 16 8221 119 662 121 11.6sample 2-1 1855 2935 48255 733 15 304863 1790 10579 28 3786 238 22106 <429 9.6sample 15-3 515 4352 51245 193 0.1 353350 8263 1237 4 28935 58 702 81 12.3sample 15-4 664 4467 60946 1618 0.1 348411 11409 776 8 15451 539 1134 297 12.8sample 15-5 579 4469 56708 210 0.1 352362 8328 1254 6 29485 61 1312 87 12.7sample 5-2 na 6204 28779 na na 352631 4502 9876 na 1741 na <1230 na 17.6sample 5-6 209 6289 33963 22 0.1 312587 4297 10833 5 1204 7 641 5721 20.1sample 5-3 na 7343 30528 na na 345536 4677 12028 na 1228 na <1244 na 21.3sample 5-5 228 7789 41685 23 0.1 398866 5356 13521 4 1576 8 975 2430 20.0sample 5-4 348 9936 49184 29 0.1 512298 6346 17881 5 1950 9 1005 1259 19.4

a na, not analyzed.

TABLE 4. Elemental Composition of Pipe Section Solids (µg/g)

sample id Al As Ca Cd Cu Fe Mg Mn Ni P Pb Si Zn As:Fe

sample 9-1 1324 3 7169 0.2 13 582983 195 287 10 967 5 601 27 0.01sample 10-6 31 10 4421 0.0 5 518759 96 1838 5 2621 6 448 12 0.02sample 10-1 718 20 25066 0.1 21 505369 1091 1804 8 810 6 1959 127 0.04sample 10-2 29 20 30692 0.1 80 530346 495 1090 13 488 1 1162 271 0.04sample 10-3 70 25 40559 0.1 54 469196 729 744 14 587 2 510 49 0.05sample 10-4 28 26 48340 0.2 58 456537 827 585 11 859 2 1972 401 0.06sample 10-5 85 30 7563 0.3 40 496196 485 219 8 584 2 577 198 0.06sample 6-5 158 32 9636 <0.2 33 459909 1120 553 7 2984 81 1306 <49 0.07sample 10-7 1391 32 17686 0.4 146 430987 3611 469 31 671 320 2041 541 0.07sample 10-8 853 33 10436 0.1 136 540558 567 1417 37 1766 6 1362 177 0.06sample 10-9 145 41 4596 0.3 25 474550 458 318 4 974 2 582 98 0.09sample 10-101 132 44 22724 0.3 274 518640 624 936 48 1002 4 416 507 0.08sample 10-11 121 44 7791 0.3 203 533938 758 415 32 687 2 1282 1140 0.08sample 10-12 135 45 23058 0.3 272 536531 637 860 47 1071 7 1047 523 0.08sample 10-13 168 48 6676 0.2 53 533811 403 312 10 2496 5 960 401 0.09sample 11-1 311 54 343521 0.6 27 79647 8620 312 27 150 12 5223 1162 0.68sample 2-5 2361 75 79544 <6.6 330 248592 15718 1112 109 530 694 3743 <263 0.30sample 7-1 116 75 151766 101 245 359449 2539 1242 52 395 44 1627 <50 0.21sample 4-1 1053 84 106475 <1.2 129 168757 6023 638 51 949 17 1286 50 0.50sample 2-4 952 108 308085 <8.5 270 40039 2869 10187 51 513 31 3090 747 2.70sample 8-3 760 109 22659 <0.3 19 55452 3648 6897 59 19208 10 2321 99 1.96sample 11-3 7286 112 11773 3.9 69 260851 4552 2083 39 894 365 3038 4768 0.43sample 10-15 351 178 5128 0.2 70 398163 747 186 5 1894 4 716 222 0.45sample 10-14 3259 260 146414 0.1 414 191037 7402 627 51 18618 17 3000 176 1.36sample 11-2 410 294 173830 1.5 40 300792 5023 660 32 279 34 3042 1842 0.98sample 12-1 33 383 2523 17.5 79 592987 119 88 2 251 156 348 29351 0.65sample 8-1 785 412 50206 0.6 43 244341 15414 18591 7 98831 18 1624 188 1.68sample 8-2 995 414 47294 0.5 47 227691 12617 20585 12 85696 23 1669 238 1.82sample 10-16 2381 554 130110 0.1 233 304775 3580 324 21 44152 14 4409 243 1.82sample 13-1 3061 719 181478 0.5 65 102865 13407 1454 28 652 91 6238 1182 6.99sample 14-1 1364 825 74555 0.7 142 397287 2620 632 12 3165 73 3218 1348 2.08sample 4-2 naa 1033 27927 na na 361893 3587 609 na 13885 na 3239 na 2.85sample 15-6 4955 1230 57790 0.3 208 144200 21156 882 18 10001 114 649 206 8.53sample 13-2 1025 1416 28859 36 391 77030 1736 290 110 850 4667 8719 535783 18.4sample 13-3 574 2008 3541 19 355 460137 371 1143 30 678 2009 5420 84002 4.4sample 3-2 2329 7842 4455 375 1145 237293 442 1267 137 1410 9681 4074 541564 33.0sample 3-1 1906 13650 22939 <4.8 406 442528 1492 5141 6 15410 210 10452 8915 30.8

a na, not analyzed.

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ratio (19.7) of the hydrant flush solids. The similarity suggeststhat the primary source of the solids in the hydrant flushwater is source water iron.

Because of the high iron and arsenic contents in thehydrant flush solids of utility 5, a series of distribution watersamples were collected and labeled 1-4 based upon theirdistance from the treatment plant (i.e., tap 1 was closest andtap 4 was the furthest away). These samples were analyzedfor arsenic and iron, and the results are shown in Figure 1.These data show that the iron and arsenic levels decreasedwith distance traveled in the distribution system, indicatingthat the iron solids with the adsorbed arsenic settle out inthe distribution system as they flow away from the plant.

Arsenic: Pipe Solids. The source of arsenic in the pipesolids is likely 2-fold; disposition of iron carryover solids fromthe source water onto the pipe surfaces and the adsorptionof arsenic from the distribution water onto the corrosionbyproducts. While some of the arsenic in the pipe solids maybe iron carryover solids and precipitates from the distributedwater, the surface mass of these pipe solids is likely dominatedby corrosion byproducts. The solids that are loosely depositedat the pipe surfaces can become re-suspended by hydraulicflow. Because the mass of the corrosion-byproducts canproduce a “dilution effect”, the amount of arsenic in thepipe material was expected to be lower than that found inhydrant flush solids in some cases.

An example of the proposed dilution effect can be foundby comparing the arsenic content of pipe and hydrant flushsolids collected at utility 10. Although two of the source waterwells contained over 24 µg of As/L (no form of arsenic removalwas practiced), the 16 solid samples analyzed from heavilycorroded cast iron pipes had a relatively low arsenic content(Table 4), only averaging 83 µg of As/g of solid. The hydrantflushed solid (sample 10-17), however, contained nearly 2000µg of As/g of solid. The differences in arsenic content suggeststhat most of the arsenic concentrated in carryover orprecipitated solids and that the solids did not bind to thepipe surface.

The solids data of Table 4 indicate that nature and contentof the pipe solids are also pipe material dependent. Forexample, Table 4 shows that the top five pipe sections withthe highest amount of arsenic were all the PVC/plastic pipes.The solids collected from these pipes consisted entirely ofdeposited films of iron colored material. Due to the type ofmaterial, these pipes do not form corrosion byproducts;therefore, the source of the iron/arsenic material must bethe carryover or precipitated solids of the source water. Thetwo pipes (3-1 and 3-2) having solids with the highest amountsof arsenic and As:Fe ratios were from utility 3. Both pipesections were collected from the distribution line betweenthe well and the iron removal treatment system. The higharsenic levels and As:Fe ratios can be explained by the factthat the pipe line carried the highest level of arsenic wellwater (69 µg/L) of any of the source waters.

Other Elements: Solids. The arsenic compositions of thesolids were compared to the concentration of other elementsin the solids. No obvious relationships were found betweenarsenic and any other elements including iron and manga-nese.

Phosphate and silicate are known to compete with arsenicfor adsorption sites on the surface of iron solids (10-13),which suggests that As concentration should be lower insolids having high phosphate and silica concentrations. Thiswas the case for the hydrant solids at utilities 6, 10, and 15that added phosphate. In all three cases, the phosphorus (P)content of the solid samples was higher than the arsenic, butnevertheless, it did not prevent the arsenic from adsorbingeither onto the pipe or hydrant solids.

X-ray Diffraction Analysis. XRD analysis was performedon only 48 of the 67 solids collected during the study becausein some cases, all of the solids were used to determine ele-mental composition (Table 5). Goethite (R-FeOOH), lepido-

TABLE 5. Crystal Phase Identification Pipe Section Solids

sample ID X-ray diffraction results

Pipe Section Samplessample 4-2 goethite, calcite, quartzsample 6-5 sulfursample 7-1 goethite, calcitesample 8-1 calcite, bukovskyitesample 9-1 goethite, magnetite, lepidocrocitesample 10-1 sideritesample 10-2 goethite, siderite, sulfur, calcitesample 10-3 goethite, sulfur, calcitesample 10-4 goethite, siderite, sulfursample 10-5 goethite, sideritesample 10-6 goethite, siderite, sulfursample 10-7 calcite, lepidocrocite, pyrite, quartzsample 10-8 goethite, siderite, calcite, quartzsample 10-9 goethite, siderite, sulfur, quartzsample 10-10 goethite, siderite, sulfur, lepidocrocitesample 10-11 goethite, sulfur, lepidocrocitesample 10-13 goethite, sulfur, lepidocrocitesample 10-14 goethite, calcite, quartzsample 10-15 lepidocrocite, sulfur, quartzsample 10-16 quartz, calcitesample 11-1 calcitesample 11-2 goethite, calcitesample 11-3 goethite, quartzsample 12-1 goethitesample 13-1 calcite, quartzsample 13-2 quartz, calcite, hydrozincitesample 13-3 goethite, quartzsample 14-1 calcite, quartz, goethitesample 15-6 dolomite, quartz

Hydrant Flush Samplessample 1-3 goethite, lepidocrocitesample 1-4 goethitesample 1-5 quartz, goethitesample 5-1 calcite, quartz, goethitesample 5-2 calcitesample 5-3 ferrihydritesample 5-4 calcite, ferrihydritesample 5-5 calcite, ferrihydritesample 5-6 calcite, ferrihydritesample 6-1 goethitesample 6-2 ferrihydritesample 6-3 amorphoussample 7-2 calcite, quartz, goethite, halitesample 15-1 calcite, quartz, dolomitesample 15-2 quartz, dolomite

FIGURE 1. Arsenic and iron concentration of samples collectedfrom distribution water of utility 5.

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crocite (γ-FeOOH), magnetite (Fe3O4), and/or siderite (Fe-CO3) were identified in nearly all of the samples analyzed.These iron minerals are known iron corrosion byproducts(25-27). Calcium carbonate (CaCO3) was also identified inmany of the solids, which most likely precipitated from thebulk distributed water. Quartz was also frequently found,and its presence was attributed to carryover from the treat-ment plant (e.g., filter media, sand, etc.). Elemental sulfurwas identified in pipe sections removed from utility 10 andwas attributed to the microbiological reduction of sulfate(28).

The probable presence of the arsenic-containing mineralbukovskyite [Fe2(AsO4)(SO4)(OH)‚7H2O] in pipe section solidscollected from utility 8 (solid 8-1) was of greatest interest.Bukovskyite contains oxidized iron, Fe(III), and arsenic, As-(V), and is reported to be an alteration product of arsenopyrite,FeAsS. Very little additional information is available aboutthis mineral. Its discovery in the examined samples supportsthe conclusion that mechanisms other than adsorption maybe responsible for the accumulation of arsenic in drinkingwater distribution system solids.

Engineering Significance. All of the solids analyzed werecollected from drinking water distribution systems whosesource and distribution waters contained arsenic and iron.These waters were typical Midwest groundwaters having highiron and hardness levels. Of the 13 elements measured, ironwas the most abundant in all of the samples, except five. Thesolids arsenic content varied from 10 to 13 650 µg/g of sample,and the As:Fe ratio (µg of As/mg of Fe) varied from a low of0.01 to a high of 31. A review of the solids data indicated thatthe amount of arsenic in the solids did not correlate with thearsenic concentration of the source or treated waters or thecontent of the major elements in the solids. Most significantis the fact that these solids containing relatively high amountsof arsenic were exposed to distribution water having onlylow levels of arsenic (generally less than 10 µg/L). The studyresults showed that the arsenic associated with distributionsystem solids varies widely and is difficult to predict andlikely depends on a combination of many factors, such aswater chemistry, pipe material and age, flushing proceduresand frequency, and solids retention and exposure time.

The results of the study suggest that any distributionsystem transporting water containing arsenic could be apotential source of arsenic in consumer’s tap water becausethe arsenic that accumulates in the distribution system solidsover time could be released back into the water. Changes inthe distribution water brought about by modifications to thetreatment practice or source water variability could lead toarsenic desorption. Or the solids (fine particles) themselvescould make their way to consumer’s taps (without detection)by the resuspension of the material caused by changes inhydraulic flow patterns.

Monitoring requirements under the U.S. EPA’s chemicalcontaminant regulations (except for lead and copper) specifycompliance monitoring only at the entry point into thedistribution system and not at the consumer’s tap. Conse-quently, most utilities do not routinely sample consumer’stap water; therefore, elevated arsenic concentrations at theconsumer’s tap caused by the distribution solids would notnormally be discovered. The results of this study suggestthat utilities with any amount of arsenic in their distributionwater should examine the distribution solids for arsenic toassess the potential for arsenic release from the solids. Thisresearch also reinforces the need for utilities to be diligentin maintaining their distribution system flushing programsand to create water quality maintenance programs to reducesolids buildup in their distribution systems.

The study results indicated a need for follow-up researchto address several relevant issues. First, research is neededto determine the predominant form(s) of arsenic in drinking

water distribution systems (e.g., adsorbed, precipitated, etc.).Second, the mechanism(s) (e.g., particle transport, desorp-tion, dissolution, etc.) that control(s) the release of arsenicfrom distribution system solids and the factors (e.g., waterquality, hydraulic, etc.) that are most influential on the releaseof arsenic back into the distributed water must be betterunderstood. A need also exists to identify what water qualityparameters (e.g., turbidity, color, redox potential, etc.) mightbe useful in predicting the re-release of arsenic and the arseniclevels into the distributed water. From a prevention stand-point, research is needed to determine what utilities can doto effectively remove arsenic from their distribution systemsolids and what can be done to prevent further solidsaccumulation. And finally, studies are needed to identify theexistence of, or the potential for, accumulation of other tracecontaminants such as radium, uranium, and antimony indrinking water distribution systems.

AcknowledgmentsThe authors thank U.S. EPA co-workers Barbara Francis andKeith Kelty for analytical support, Ian Laseke and JeremyPayne for their assistance with solids preparation, MikeSchock for his valuable thoughts and opinions, and ElissaWhitacre for her manuscript and editorial suggestions.Abraham Chen, Lili Wang, Bruce Sass, and Sandip Chatto-padhyay of BMI are acknowledged for efforts in coordinatingsolids and data collection, arsenic speciation, and solidsanalysis. Finally, the authors thank participating water utilitiesfor supplying information, water samples, and solid samples.Any opinions expressed in this paper are those of the authorsand do not necessarily reflect the official position and policiesof the U.S. EPA. Any mention of products or trade namesdoes not constitute recommendation for use by the U.S. EPA.

Supporting Information AvailableFour additional tables. This material is available free of chargevia the Internet at http://pubs.acs.org.

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Received for review January 29, 2004. Revised manuscriptreceived July 30, 2004. Accepted July 30, 2004.

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