S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 8 9 ( 2 0 0 8 ) 2 4 5 – 2 5 2

ava i l ab l e a t www.sc i enced i rec t . com

www.e l sev i e r. com/ loca te / sc i to tenv

Effectiveness of household reverse-osmosis systems in aWestern U.S. region with high arsenic in groundwater

Mark Walkera,⁎, Ralph L. Seilerb, Michael Meinertc

aMS 370, FA 132, University of Nevada, 1664 N. Virginia St., Reno, NV 89557, United StatesbResearch hydrologist, U.S. Geological Survey, 2730N, Deer Run Road, Carson City, NV 89701, United StatescHydrologic Sciences Graduate Program, University of Nevada, 1664 N, Virginia St., Reno, NV 89557, United States

A R T I C L E I N F O

⁎ Corresponding author. Tel.: +1 775 784 1938E-mail address: mwalker@cabnr.unr.edu (M

0048-9697/$ – see front matter © 2007 Elsevidoi:10.1016/j.scitotenv.2007.08.061

A B S T R A C T

Article history:Received 1 February 2007Received in revised form27 August 2007Accepted 29 August 2007Available online 4 October 2007

It is well known to the public in Lahontan Valley in rural Nevada, USA, that local aquifersproduce water with varied, but sometimes very high concentrations of arsenic (N4 ppm). Asa result, many residents of the area have installed household reverse-osmosis (RO) systemsto produce drinking water. We examined performance of RO systems and factors associatedwith arsenic removal efficiency in 59 households in Lahontan Valley. The sampling resultsindicated that RO systems removed an average of 80.2% of arsenic from well water. In 18 ofthe 59 households, arsenic concentrations exceeded 10 ppb in treated water, with amaximum in treated water of 180 ppb. In 3 of the 59 households, RO treatment had littleeffect on specific conductance, indicating that the RO systemwas not working properly. Twomain factors lead to arsenic levels in treated water exceeding drinking-water standards inthe study area. First, arsenic concentrations were high enough in some Lahontan Valleywells that arsenic levels exceeded 10 ppb even though RO treatment removed more than95% of the arsenic. Second, trivalent As+3 was the dominant arsenic species inapproximately 15% of the wells, which significantly reduced treatment efficiency.Measurements of specific conductance indicated that efficiency in reducing arsenic levelsdid not always correlate with reductions in total dissolved solids. As a consequence,improvements in taste of the water or simple measurements of specific conductance madeby technicians to test RO systems can mislead the public into assuming the water meetssafety standards. Actual measurements of treated water are necessary to assure thathousehold RO systems are reducing arsenic concentrations to safe levels, particularly inareas where groundwater has high arsenic concentrations or where As+3 is the dominantspecies.

© 2007 Elsevier B.V. All rights reserved.

Keywords:ArsenicReverse osmosisPrivate wells

1. Introduction

1.1. Background

Approximately 15% of the population of the United States (∼45million people) relies on privately owned wells for watersupplies (USEPA, 2007; Focazio et al., 2006). Such wells oftenserve as the sole source of drinking water for a single

(off); fax: +1 775 784 4789. Walker).

er B.V. All rights reserved

household. Standards for chemical, microbiological andphysical characteristics of water, codified in the Safe DrinkingWater Act and amendments, are not enforced for privatewater supplies, although some state and local governmentshave regulations that mandate occasional testing — forexample, as part of real estate sales.

In many rural areas, more people use private wells than areserved by public drinking-water systems. Public water suppliers

.

.

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mayhave the financial resources to invest in treatment facilitiesor blending strategies. Private well owners have few options toavoid exposure to contaminants other than treatment that theyinstall andmanage, or alternativesupplies suchasbottledwater(Focazio et al., 2000).

In theWestern United States, arsenic is one of the principalnaturally-occurring contaminants in groundwater. The cur-rent drinking-water standard for arsenic (10 ppb) became fullyenforceable for public water supplies in 2006. Elevated arsenicconcentrations (N10 ppb) in groundwater have been docu-mented in large areas of the United States, particularly inareas with aquifers influenced by geothermal sources andevapoconcentration of surface waters (Welch et al., 2000). Insome areas, groundwater from private wells has arsenicconcentrations more than 100 times greater than the drink-ing-water standard (Shaw et al., 2005; Shiber, 2005; Focazioet al., 2006; Walker et al., 2006).

Lahontan Valley, in Churchill County, Nevada, is one sucharea where naturally-occurring arsenic concentrations com-monly exceed 100 ppb in groundwater (Fitzgerald, 2004; Seiler,2004). In 2001, arsenic concentrations in 89 of 100 wellssampled in Lahontan Valley exceeded 10 ppb (Seiler, 2004).The median arsenic concentration in eight municipal wellssampled in the valley was 103 ppb (range 90 to 110 ppb) andwas 61 ppb (range 2 to 2910 ppb) in the 92 domestic wells.Arsenic concentrations exceeding 1000 ppb in LahontanValleyhave been documented by several authors (Welch and Lico,1998; Seiler, 2004; Shaw et al., 2005; Walker et al., 2006), and achild in a household served by a well with 2750 ppb of arsenicwas poisoned in the late 1960s (Donahue, 2001). The concen-trations of arsenic in the Lahontan Valley have been reportedin newspapers, public meetings and radio and televisionbroadcasts (Benson, 2003) and many homeowners withprivate wells as water sources have installed systems totreat drinking water. In fact, a recent study showed that 33% ofrespondents in Lahontan Valley were highly concerned aboutarsenic and had installed household RO systems (Walker et al.,2006). However, because RO-treated water had arsenic con-centrations that exceeded the drinking-water standard, Walk-er et al. (2006) concluded that investment in a RO system couldlead to a false sense of security because contaminants such asarsenic cannot be readily sensed by taste or odor. Field trialsby Fox (1989) demonstrated that RO systems episodically failand must be regularly maintained to ensure that rejectionefficiency remains at a maximum.

The purposes of the research described in this paper wereto characterize concentrations of arsenic in drinking waterfrom wells treated with RO in an area with high groundwaterarsenic, and to evaluate the factors affecting the efficiency ofarsenic removal by RO systems. A secondary goal was toidentify reasons users of RO systems may mistakenlyconclude that treated water meets drinking-water standards.

1.2. Reverse osmosis

Contaminatedwatermay be treated by a variety of small-scaledevices that substantially reduce concentrations of arsenicand other contaminants (Fox and Sorg, 1987; Fox, 1989;Slotnick et al., 2006). Small-scale RO systems are a popularway to treat arsenic contaminated well water because they are

often more than 90% efficient in removing arsenic and arerelatively simple to install and maintain.

In RO systems, water is forced under high pressure throughsynthetic membranes with extremely small pores. Themajority of solutes in the feed water are rejected and do notpass through the membrane and in properly functioning ROsystems, treated water contains lower solute concentrationsthan feed water. RO systems treat water fairly slowly, and insystems commonly available for home use treated water isstored in small tanks (1–3 gallons) until used.

The efficiency of RO systems in removing contaminantsvaries with the specific contaminant of concern. Large ions areremoved more efficiently than small ions, and divalent ionsare removed more efficiently than monovalent ions (Cliffordet al., 1986). Efficiency also depends upon the type ofmembrane used, chemistry of the feed water (Shih, 2005),age of devices (Slotnick et al., 2006) and length of time inservice (Fox, 1989). RO systems typically remove nearly 100%of arsenate (As+5) and can be significantly less efficient inremoving arsenite (As+3). At concentrations between 40 and1900 ppb, efficiency of five RO membranes varied between 98and 99% removal for As+5 but only 46–75% for As+3 (Cliffordet al., 1986). pH affects efficiency by affecting the charge ofboth the RO membrane and the arsenic species.

High pH water can make the overall charge of themembrane more negative by deprotonating membrane func-tional groups (Elimelech et al., 1994). Furthermore, as pHincreases, As+5 and As+3 become negatively charged. Stabilitydiagrams for arsenic species (e.g. Smedley and Kinniburgh,2002) show that at the high pH typical of groundwater in thestudy area, As+5 occurs primarily as the negatively chargedspecies HAsO4

−2, whereas As+3 is largely the neutral speciesH3AsO3

0. The negative charges on both the membrane andHAsO4

−2 result in As+5 being repulsed by themembrane, whichlowers the arsenic concentration near the membrane andtherefore lowers arsenic passage through the membrane. ROmembranes remove the neutral As+3 species H3AsO3

0 lessefficiently because charge repulsion does not lower arsenicconcentration near the membrane.

1.3. Study area

The study area, Lahontan Valley (Fig. 1), is centered on thetown of Fallon in Churchill County in western Nevada.Lahontan Valley is in the southern end of the Carson Desert,a hydrologically closed basin at the terminus of the CarsonRiver. The study area is in the rain shadow of the SierraNevada and annually receives ∼12.7 cm of precipitation(Maurer et al., 1994). The principal land use in the LahontanValley is agriculture. However, the area is rapidly urbanizingand agricultural land is being converted to residential areas(Seiler, 2004). The recent occurrence of a childhood leukemiacluster in the Lahontan Valley (CDC, 2007) led to an extensiveinvestigation of groundwater quality in the study area insupport of the CDC investigation of the cluster (Seiler, 2004;Seiler et al., 2005).

All drinking water in the Lahontan Valley is drawn fromgroundwater. In 2002, at the time of this study, approximately25,000 people resided in Churchill County and an estimated11,500 were served by 16 public water supplies primarily in the

Fig. 1 – Approximate location of the Lahontan Valley studyarea in Churchill County, Nevada.

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city of Fallon. The remainder, an estimated 5500 households,relied on privately managed wells (Churchill County PlanningDepartment, 2002).

The groundwater flow system and water quality in theLahontan Valley have been well described (Glancy, 1986). Alldomestic wells tap unconsolidated and semi-consolidatedbasin-fill sediments of Quaternary and Tertiary ages. All wellsproviding water for the city of Fallon, Naval Air Station Fallon,and the Fallon Paiute-Shoshone Indian Reservation tap abasalt aquifer that was formed from volcanic activity duringthe Quaternary and is mostly buried in lacustrine sedimentsdeposited in ancient Lake Lahontan. The basalt aquifer iscomposed of consolidated rock of widely ranging permeabil-ity, ranging from dense fractured lava flows to basalt rubbleand cinders in zones between flows (Maurer et al., 1994).

The principal source of recharge to the basalt and basin-fillaquifers is infiltration of water from the Carson River. Isotopicdata indicate that the shallow basin-fill aquifer (b50 ft) isrecharged by applied irrigation water and leakage fromunlined canals (Seiler et al., 2005). Three hundred and seventymiles of lined and unlined canals supply water to irrigatedfields in the Carson Desert, primarily for alfalfa production(Lico and Seiler, 1994). Isotopic data indicated that water fromthe basalt and deeper basin-fill aquifer (N50 ft) is older,predating construction in 1915 of an irrigation reservoirupstream of Lahontan Valley (Seiler et al., 2005).

Groundwater quality in Lahontan Valley is highly variablein composition and quality and ranges from a dilute calcium-bicarbonate type to a saline sodium-chloride type (Seiler,2004). Typically, water from shallowwells (b50 ft) in the basin-fill aquifer is moderately hard (N70 ppm as CaCO3) with pH

values commonly near 7.4 (Seiler, 2004). Water from the basaltaquifer and deeper wells (N50 ft) in the basin-fill aquifer is soft(b25 ppm as CaCO3) with pH values commonly around 8.3 and9.3, respectively. Dissolved-oxygen concentrations in theaquifer system typically are b0.5 ppm and the water com-monly has an odor of hydrogen sulfide (Seiler, 2004).

Arsenic concentrations in the City of Fallon's public water-supply wells exceeded 100 ppb (Seiler, 2004) until a newtreatment facility began operation in April 2004 and loweredconcentrations to less than 10 ppb. Churchill County, whichcontains Lahontan Valley, was identified in a previousnational-level study of groundwater quality as having highconcentrations of arsenic in groundwater, such that 25% ofsamples obtained from the county exceeded 50 ppb As.Dissolved arsenic in groundwater was derived primarilyfrom arsenic-bearing sediments (Welch and Lico, 1998; Seileret al., 2005), with possible localized contributions fromgeothermal sources (Fitzgerald, 2004). Previous studies ofoccurrence of arsenic indicated broad spatial trends inconcentrations, generally increasing along the groundwaterflow path from the western to the eastern margins of thebasin. Arsenic concentrations often differ significantly be-tween wells in close proximity (Fitzgerald, 2004).

Arsenic-speciation data from 78 wells sampled in 2001 inthe study area (Seiler et al., 2005) indicated that arsenic in sixmunicipal wells in the basalt aquifer was N98% As+5, butranged from 9.5% to 100% As+5 in 72 domestic wells in thebasin-fill aquifers. As+3 was the predominant species inapproximately 25% of the Lahontan Valley wells.

2. Methods

The study examined 59 RO systems in households inLahontan Valley. This included samples from 51 householdsserved by private domestic wells and 8 households served by amunicipal water-supply system. Water samples were collect-ed in 2002 by the University of Nevada, Reno (UNR) from 44households that reported applying RO for water treatment.The households had participated in an earlier survey of tap-water quality in Churchill County in 2002 that included 351households (Shaw et al., 2005). The 351 households wererecruited in a convenience sample by direct solicitation, flyersand newspaper advertisements (Benson, 2003). Of the 351households included in the previous study, 62 applied RO forwater treatment. The data discussed in the present studywerecollected in a follow-up sampling survey carried out in 2003that focused only on households from the first survey thatreported applying RO for drinking-water treatment. Amongthose with RO systems, respondents reported that they drankRO-treated tap water.

Unfiltered water samples for trace element analysis,including arsenic, were collected after approximately 1 L ranfrom small under-sink tanks containing RO-treated water.Unfiltered water samples were also collected directly fromwells, usually at an access point between the well head andpressure tank after pumping until pH, temperature, dissolved-oxygen concentration and conductivity (continuously mea-sured by a portable Yellow Springs Instruments (YSI) model556MPS) stabilized. Arsenic samples were acidified in the field

Table 1 – Summary data for groundwater and RO-treated water quality in sampled households

Arsenic Specific conductance

Groundwater(ppb)

Treated water(ppb)

Removalefficiency (%)

Species(% As+3)

Groundwater(μS/cm)

Treated water(μS/cm)

Reductionefficiency (%)

Number 59 59 59 52 58 58 58Minimum 4 b3 0 0 251 5 0Maximum 4100 180 100 98 8260 1727 9810th percentile 12 b3 37 0 322 22 7825th percentile 40 b3 73 0 435 34 8750th percentile 103 4 95 1 861 67 9175th percentile 165 16 98 31 1087 143 9590th percentile 386 53 99 57 2336 336 96

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to pHb2 using nitric acid provided by the analytical laboratory.The sampler administered a survey about maintenance andinspected each RO system. When legible, the samplerrecorded information about brand and model.

Samples for pH, specific conductance, arsenic speciationand unfiltered arsenic concentrations were analyzed at theNevada State Health Laboratory, a certified drinking-wateranalysis facility. In samples for arsenic speciation, theproportion of arsenic occurring as As+3 was maintained byon-site treatment using chelation with solid-phase separa-tion (Bednar et al., 2002). Arsenic concentrations weredetermined by EPA method 200.8 (ICP-MS) for samples withturbidity less than 1 NTU and ASTM method D2972-93B(hydride-generation AA) for samples with turbidity greaterthan or equal to 1 NTU.

As part of a cross-sectional investigation of a suspectedleukemia cluster (CDC, 2007), tap-water samples were col-lected in 2002 by theU.S. Geological Survey'sWater ResourcesDivision in Carson City, NV (USGS) at 15 households using RO.This included seven households that relied on private waterwells and eight supplied by amunicipal system. Unfiltered ROwater samples for trace element analysis, including arsenic,were collected and pH and specific conductance weremeasured after approximately 1 L had run through the ROsystem. To evaluate RO efficiency, groundwater samplesfrom spigots near the well head were collected from theprivate water wells after 5–10 min of pumping. The pH andspecific conductance were measured from a spigot near thewell head while the well was pumping. To evaluate ROefficiency at the eight households receiving water frommunicipal wells, water quality data collected in 2001 fromthemunicipalwellswere used. All themunicipalwells tap thesame basalt aquifer. Water from the municipal wells ischemically homogeneous and temporally stable, with anaverage specific conductance of 1010 μS/cm (standard devi-ation 53 μS/cm) and an average arsenic concentration of103 ppb (standard deviation 6 ppb)(Seiler, 2004).

Unfiltered USGS samples were acidified in the field topHb2 using 7.7 N ultrapure nitric acid. Unfiltered samples forarsenic concentrations were analyzed following digestion atthe USGS National Water Quality Laboratory using graphitefurnace atomic-absorption spectrometry. Arsenic speciationwas not determined for the USGS groundwater samplescollected in 2002. In 2001 however, USGS determined arsenicspeciation in Lahontan Valley groundwater using chelationwith solid-phase separation (Bednar et al., 2002).

3. Results

3.1. Reverse-osmosis efficiency

Summary statistics describing water quality and treatmentefficiency are presented in Table 1. RO systems operated atvarying degrees of efficiency in removing arsenic, rangingfrom no removal to N99% removal (Fig. 2A) with a median of95% (average 80.2%, standard deviation 28.1%). Arsenicconcentrations in untreated water exceeded the drinking-water standard of 10 ppb in 54 of the 59 residences, but ROreduced arsenic concentrations to less than 10 ppb in 36 ofthe 54. In the 18 households where RO-treated watercontained N10 ppb arsenic, the maximum concentration inRO-treated water was 180 ppb (Fig. 2A). In those 18 house-holds, removal efficiency ranged from 0% (in one case) toN99% (average 57.8%). In four cases removal efficiencies wereinadequate to reduce even relatively low concentrations ofarsenic (b50 ppb to b10 ppb), and in four cases removalefficiencies were insufficient to reduce moderate concentra-tions (50 to b100 ppb) to b10 ppb. In nine cases, removalefficiencies were inadequate to reduce high concentrationsof arsenic (100 to b1000 ppb) to b10 ppb, and in two cases evenvery high removal efficiencies (∼99%) were inadequate toreduce excessively high concentrations (1220 and 4100 ppb)to b10 ppb.

Specific conductance measurements in 58 systemsshowed RO systems lowered specific conductance from 0to 98.4% (Fig. 2B), with a median of 91% (average 86.7%,standard deviation 18.2%). All but three RO systems reducedspecific conductance by N70%. However, in those threesystems specific conductance was reduced by b30%. Thevery low efficiency in reducing specific conductance at thesesites was likely the result of poor maintenance or damagedmembranes. Differences in efficiency among the moreefficient sites are likely due to differences in the major-ioncomposition of the feed water, and the maintenance his-tory, membrane type, and age of the RO systems.

3.2. Relation between water chemistry and efficiency ofarsenic removal

Consistent with what many previous investigators havepreviously shown, there is a significant decrease in arsenicremoval efficiency as the percentage of arsenic occurring in

Fig. 2 – Reductions in arsenic concentration (A) and specific conductance (B) in groundwater and reverse-osmosis treated tapwater. Diagonal lines represent RO system effectiveness in reducing arsenic and specific conductance in water. The MCL andSMCL refer to the primary and secondary drinking-water standards for arsenic and total dissolved solids (0.010 ppm and500 ppm, respectively).

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the As+3 form ([As+3]/[As]) increases (Fisher's Exact Test Pbb0.001, Fig. 3). Even though there is considerable scatter inthe data, in all nine cases where As+3 was the dominantspecies, arsenic efficiencywas less than 60% andwas less than45% in eight of the cases. A simple linear regression of ROrejection efficiency as a function of [As+3]/[As] (not shown) isstatistically significant and, based on the adjusted R2 value of0.57, arsenic speciation is the most important factor affectingefficiency. Factors not considered here, including mainte-nance, use habits, and other chemical characteristics ofgroundwater, are of lesser importance.

Fig. 3 – Relation between reduction in arsenic concentration by reAs+3 data were not available for 7 sites.

Arsenic rejection efficiency and changes in specific con-ductance were not correlated significantly (Fisher's Exact TestPN0.10, Fig. 4). At eight of the 50 sites where RO reducedspecific conductance by more than 80%, efficiency of arsenicremoval was less than 50%.

3.3. Relation between perceived improvements in waterquality and arsenic

Users of RO systems may assume that water meets stan-dards because the taste of the water has improved. One

verse osmosis and the percentage of total arsenic that is As+3.

Fig. 4 – Relation between reduction in arsenic concentration by reverse osmosis and reduction in specific conductance.Numbers by selected data points are As concentrations in RO-treated water.

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determinant of taste related to perceived salinity is totaldissolved solids (TDS), and on the basis of taste USEPA hasestablished 500 ppm as the secondary maximum conta-minant level (SMCL) for TDS. Total dissolved solids (in ppm)and specific conductance (in μS/cm) were strongly cor-related in the Lahontan Valley in 2001 according to therelation:

SC¼1:094TTDS1:052 n¼100; r2¼0:99;USGS;Unpublished data� �

:

Using the above relationship, the SMCL corresponds to aspecific conductance of approximately 750 μS/cm. At one site(site A, Fig. 5) RO treatment lowered arsenic concentrationsfrom 350 ppb to 73 ppb but reduced specific conductance from2210 μS/cm to 150 μS/cm. This corresponds to a TDS reduction

Fig. 5 – Relation between specific conductance and arsenic concosmosis. The grey area corresponds to water which exceeds thespecific site discussed in the text.

from about 1400 mg/L to about 110 mg/L. As a consequence,treated water at site A would taste better than untreatedgroundwater even though the arsenic concentration followingtreatment exceeded the drinking-water standard sevenfold.This effect may be common. Prior to treatment specificconductance exceeded 750 μS/cm at 30 of the sites andtreatment reduced specific conductance to b750 μS/cm at 28of those sites. However, arsenic concentrations in RO-treatedwater at 7 of the 28 sites exceeded 10 ppb.

4. Discussion and conclusions

Reverse osmosis is a robust technique for removing arsenic andother contaminants from water and is well-adapted for small

entrations in water before and after treatment by reverseMCL for arsenic but meets the SMCL for TDS. Letter refers to

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volume applications. It has been applied extensively fordesalination of sea water and performs well under a widerange of chemical conditions. USEPA lists RO as a Small SystemCompliance Technology for arsenic (USEPA, 2000). However, inthis study, arsenic concentrations in samples of treated waterfrom 18 of the 59 RO systems exceeded the drinking-waterstandard of 10 ppb. Presumably, residents of Lahontan Valleyinvested in household RO systems to lower contaminantconcentrations in their drinking water, principally arsenic, tosafe levels. It is therefore important to identify why RO-treatedwater failed to meet drinking-water standards and whetherusers of RO systemsmay bemislead into assuming that treatedwater meets drinking-water standards (b10 ppb) and is safe todrink as suggested by Walker, et al (2006).

Contaminants bound to particulate matter will be efficientlyremoved by RO because the small size of pores in ROmembraneswill restrict passage of the particles. It is unlikely, however, thatdifferences in arsenic rejection efficiency observed in this studyresult from the arsenic at some sites being associated withparticulate matter. In Lahontan Valley groundwater almost allarsenic is dissolved rather than associated with the particulate(N0.45 μm) fraction inmunicipal anddomesticwells (Seiler, 2004).

In this study, the most efficient RO systems tended to bethose using treated municipal water, which contains smallamounts of residual free chlorine. Chlorination of feed waterimproved the efficiency of arsenic removal by RO from 58% to91% in Utah groundwater by oxidizing the As+3 to As+5

(Schneiter and Middlebrooks, 1983). However, RO membranescan also be damaged by exposure to chlorine in feed water,eventually resulting in membrane failure, which enhancespassage of both salt and water (Glater et al., 1994). Manyowners of domestic wells in the Lahontan Valley shockchlorinate their wells (Seiler, 2004), and this practice couldimprove efficiency by oxidizing As+3 to As+5, or alternativelyreduce efficiency by damaging the membranes. Data are notavailable, however, to evaluate the actual significance ofchlorine exposure on the efficiency of the RO systems tested.

Efficiencyof arsenic removalwasb60%at thenine siteswheremore than 50% of the arsenic was As+3, andwas b45% at eight ofthenine sites. At pH typical of LahontanValley groundwaterAs+3

is largely the neutral species H3AsO30 whereas As+5 is largely the

negatively charged species HAsO4−2. The neutral As+3 species

H3AsO30 is more likely to pass through the membrane than the

As+5 species HAsO4−2 because charge repulsion does not lower

As+3 concentrations near the membrane.Data from Lahontan Valley show that household RO

systems can be an effective method to treat arsenic contam-inated well water, with the majority of the RO systemsremoving more than 90% of the arsenic. Nonetheless, treat-ment with RO failed to lower arsenic concentrations to safelevels when arsenic in the well water was very high. Even if aRO system removes 95% of the arsenic, treated water will stillexceed the drinking-water standard for arsenic if the feedwater contains more than 200 ppb of arsenic.

This study demonstrated that the proportion of As+3 presentin groundwater was the most important factor associated withthe efficiency of arsenic removal. Several RO systems in thisstudy removed less than 50% of the arsenic when As+3 was thedominant arsenic species. A limitation of this study is thatinadequate data were available to determine the importance of

other potential chemical factors, membrane type, and systemage and maintenance history on rejection efficiency.

The lack of a correlation between efficiency in reducingarsenic and specific conductance has important public healthimplications. Simple field measurements, such as specificconductance, are commonly used by technicians to evaluatewhether RO systems are functioning properly. Furthermore,some RO systems use an indicator light that illuminates whenspecific conductance reaches an upper threshold, indicatingthat rejection efficiency is declining and the RO membraneshould be replaced. Since efficiencies in reducing specificconductance and arsenic are not correlated, reliance onspecific conductance as a measure of RO systems' perfor-mance can lead to the erroneous conclusion that the treatedwater meets drinking-water standards. The data also indicatethat r users of RO systems can be mislead on the basis ofimproved aesthetic qualities, including taste, that their ROsystem is removing arsenic. Arsenic concentrations stillexceeded 10 ppb at 25% of the sites where RO treatmentreduced specific conductance to levels where the taste of thewater likely would have improved substantially.

For users of RO systems, actual measurements of treatedwater are necessary to assure that arsenic concentrations areindeed being reduced to safe levels. This is particularlyimportant in areas where groundwater has high arsenicconcentrations, or where As+3 is the dominant species.

Acknowledgements

The support of the Lahontan Valley residents who allowedUniversity andGovernment scientists to come into their kitchensto collect samples is appreciated. Thisworkwas partly supportedby a grant to UNR from the U.S. Department of Agriculture'sCooperative State Research, Education andExtension Service, theUniversity of Nevada's Agricultural Experiment Station and ajoint grant from the University of Nevada's Agricultural Experi-ment Station and the College of Cooperative Extension.

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