Sources and exposure of the New Hampshire population toarsenic in public and private drinking water supplies
Stephen C. Peters a,b,⁎, Joel D. Blum b, Margaret R. Karagas c,C. Page Chamberlain d,e, Derek J. Sjostrom f,e
a Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA, United Statesb Department of Geological Sciences, University of Michigan, Ann Arbor, MI, United States
c Department of Community and Family Medicine, Dartmouth College, Hanover, NH, United Statesd Department of Geological and Environmental Studies, Stanford University, Stanford, CA, United States
e Department of Earth Sciences, Dartmouth College, Hanover, NH, United Statesf Department of Earth and Environmental Sciences, Rocky Mountain College, Billings, MT, United States
An initial survey of the statewide occurrence of ar-senic focused on determining for the first time wherearsenic occurred in New Hampshire groundwater and
Fig. 1. Site location map. Major terranes are delineated by the dashedline and labeled: GRE = Grenville, CMT = Central Maine, NCM =Nashoba–Casco–Miramichi. Shaded areas are Concord age granites(Late Devonian). The towns of (A) Hudson, (B) Bow, and (C) Bristolthat were studied in previous work are labeled for reference.
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suggesting possible sources (Peters et al., 1999). Twosubsequent papers focused in detail on a small area in thesouth central part of the state where the highest arsenicconcentrations were observed in order to understand thegeochemistry of the aquifers (Peters and Blum, 2003)and the characteristics of the arsenic source minerals(Utsunomiya et al., 2003). The goal of this paper is topresent the most recent statewide dataset of arsenic indrinking water, with nearly 1300 new analyses, and topresent an arsenic dataset from 307 bedrock samples.With these new analyses, we evaluate spatial patternsof arsenic concentrations in groundwater statewide, ex-amine arsenic occurrence in bedrock statewide usinghypotheses formulated during the two focused studies,examine the relationship between bedrock geology andelevated arsenic in groundwater, and finally estimate thedistribution of drinking water arsenic concentrations forhouseholds throughout New Hampshire. Throughoutthis paper, we develop a technique to account for thevariable patterns of water sources that can be broadlyapplied to more accurately calculate exposure rates inother regions.
1.1. Arsenic and New Hampshire geology
Mining of base metals including gold, copper, andnative arsenic along with many other minerals com-menced in New England after European settlement andcontinued to the early part of the twentieth century(Myers and Stewart, 1956). Mines at which native ar-senic, arsenopyrite, and other arsenic-rich minerals werethe primary product or occurred within the ore rock werelocated throughout New Hampshire in a variety of li-thologies (Hitchcock, 1878; Myers and Stewart, 1956).Arsenic-rich minerals are abundant enough throughoutthe state that New Hampshire was the largest producer ofarsenic in the US before 1900 (Myers and Stewart,1956). In an 1878 statewide survey of economic min-eralizations, the state geologist wrote “The arsenicalpyrites—mispickel or arsenopyrite— are very commonin our state. They are most abundant along theConnecticut valley, both massive and crystallized….Should the manufacture of arsenic ever be called for,New Hampshire can afford a plentiful supply” (Hitch-cock, 1878).
The bedrock geology of New Hampshire (Fig. 1) iscomprised of individual units within three accretionaryterranes (Hatch et al., 1983; Lyons et al., 1997). TheCentral Maine terrane is the largest, and extends northinto Maine and south into Massachusetts and Connecti-cut. The rocks of the Central Maine terrane consistprimarily of anoxic sediments of the Silurian Rangeley
Formation, which were deposited in a restricted basinand capped by turbidites of the Littleton Formation in theEarly Devonian. The Littleton and Rangeley were thenfolded into both west and east verging nappe structureslater in the Early Devonian. During this time these rockswere intruded by the New Hampshire Plutonic Series,which include the Kinsman, Bethlehem and Spaulding
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intrusive suites. Associated with these intrusions is ev-idence for mobility of volatiles including abundant peg-matites and hydrothermal quartz veins, some of whichcontain graphite (Rumble and Hoering, 1986). Aftertectonic thickening, deeply buried portions of theRangeley and Littleton Formations partially melted,resulting in the emplacement of the post-tectonic Con-cord Granites. During the late stages of the emplace-ment of the Concord granites, incompatible elementsmobilized by metamorphism and melting of theRangeley Formation were deposited as a later genera-tion of pegmatites and veins, some of which containgraphite.
1.2. Arsenic in New Hampshire groundwater
Groundwater in New Hampshire is primarily drawnfrom either drilled bedrock wells or wells tapping un-consolidated valley fill deposits of sands, gravels, andtills (Flanagan et al., 1998). Valley fill surficial aquifersare typically shallow (b30 m deep), and exist as smalldiscontinuous units across the landscape (Medalie andMoore, 1995). Bedrock aquifers are located at depththroughout the state, are variably contiguous, and haveyields that depend on the degree of fracturing whichcontrols the ability to convey and store water (Olcott,1995).
The general geochemical behavior of arsenic in nat-ural waters has recently been extensively reviewed bySmedley and Kinniburgh (2002), with specific oc-currences in the United States also summarized byWelch et al. (2000) and health effects statisticallyevaluated in Bangladesh by Yu et al. (2003). In NewEngland, elevated arsenic in groundwater has beenobserved in Massachusetts (Zuena and Keane, 1985;Aurillo et al., 1994), Maine (Marvinney et al., 1994;Sidle et al., 2001; Sidle, 2003), and New Hampshire(Ayotte et al., 1999; Peters et al., 1999; Ayotte et al.,2003; Peters and Blum, 2003). We therefore willsupplement this general overview only with back-ground information relevant to the specific researcharea in New Hampshire.
In the 1980's two studies examined anomalouslyelevated arsenic concentrations in groundwaters nearthe small south-central New Hampshire town ofHudson. These studies came to the contrasting conclu-sions that either landfill leachate and/or sewage outfall(Boudette et al., 1985) or bedrock geology (US-EPA,1981) were the most likely sources of arsenic ingroundwater. In an overview of statewide arsenic oc-currence in groundwater, Peters et al. (1999) showedthat elevated groundwater arsenic concentrations ap-
peared to be a problem predominantly in the central partof the state. The authors suggested a relationship be-tween elevated groundwater arsenic concentrations andpegmatites associated with the nearby DevonianConcord granite. On a larger scale, correlations havebeen observed across Massachusetts, New Hampshire,and Maine between elevated arsenic concentrations ingroundwater and a lithogeochemical grouping of calc-silicate metamorphic rocks (Ayotte et al., 1999). Mostrecently, two detailed studies of the geochemistry of thecentral part of the state most affected by arseniccontamination showed that 1) the mineralogy of thesource materials is likely to be a mixture of arsenopyrite(FeAsS), and nanocrystalline forms of westerveldite(FeAs) and magnetite (Fe3O4) (Utsunomiya et al.,2003) and 2) that arsenic concentrations in groundwaterwere controlled primarily through adsorption/desorp-tion reactions with iron oxyhydroxides (Peters andBlum, 2003).
2. Methods
2.1. Water samples
Water samples from 2273 randomly selected house-holds throughout the state of New Hampshire (Fig. 2A)were collected as part of an ongoing epidemiologicalstudy (Karagas et al., 1998). Acid-cleaned 125 mL low-density polyethylene bottles were used to collect waterfrom the kitchen tap using strict trace element protocols.Total arsenic concentration was determined by contin-uous flow hydride generation using a magnetic sectorinductively coupled plasma mass spectrometer (ICP-MS) as an element specific detector. Online hydridegeneration eliminates the interference between ArCl+
and 75As+ and allows for rapid analysis (b5 min/sample) of small samples (b1 mL). Elimination of theArCl+ interference allows operation of the massspectrometer at low mass resolution (m /Δm=300)thus maximizing signal intensities. Optimization ofthe instrumental parameters and reagents for this tech-nique are discussed in detail elsewhere (Klaue andBlum, 1999). Briefly, the instrument was linearly cal-ibrated from 0.010 to 10 μg/L with six standardsyielding a minimum R2 of 0.9999. Higher concentrationsamples were diluted into this operating range, andreanalyzed. Typical analytical uncertainty for all anal-yses was less than 5%, with a mean uncertainty of∼3%.Field blanks measured regularly during the study wereall at or below the quantification limit of 0.005 μg/L.Randomized blind replicate sampling of about 10% ofthe households agreed with an intraclass correlation of
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0.91 throughout the range of results (Karagas et al.,1998).
2.2. Rock samples
Total arsenic concentration was measured in 307 rocksamples collected from bedrock outcrops throughout thecentral, western, and northern regions of New Hampshire(Fig. 2B). Samples were not obtained from the southeast-ern region of New Hampshire where previous studieshave already demonstrated a relationship between arsenicin groundwater and calcareous metasedimentary geologicunits (Ayotte et al., 2003). Hand samples were obtainedfrom outcrops in an attempt to survey typical samplesfrom the major rock types (n=245), and also to samplerocks that seemed most likely to contain high concentra-tions of arsenic (n=62). The rock samples that were mostlikely to contain arsenic were selected because of oneor more of the following critera: presence of graphite(n=15), abundant sulfide minerals (n=22), located in anhistoric base metal mine (n=29), located in or near ahydrothermal vein (n=21). These parameters wereselected because of the previously hypothesized relation-ship between both pegmatite formation and hydrothermalcirculation and arsenicmineralization (Peters et al., 1999).Hand samples were sawed into slabs and then pulverizedinto a fine powder using a shatterbox. Powders were
Fig. 2. Location map for (A) water samples and (B) rock samples. Water sampwith roadcuts along major and minor roads.
digested in aqua regia and arsenic concentration deter-mined using atomic absorption by a commercial labora-tory (Chemex) with a quantification limit of 1 mg/kg.
3. Results and discussion
In the following sections, we first discuss the generalstatistical parameters of the arsenic data in both drinkingwater and in bedrock samples. Then, a spatial database isgenerated and presented inmap form, fromwhich we canillustrate the relationship (or lack thereof) between ar-senic in drinking water and arsenic in bedrock samples.Finally, we can use this spatial database to estimate thenumber of households in New Hampshire exposed atvarious arsenic concentrations.
In drinking water, arsenic concentrations ranged frombelow quantification (b0.005 μg/L) to 180 μg/L, with amedian value of 0.23 μg/L (Table 1). For the purposes ofelucidating the relationship between geology, arsenicconcentration in drinking water, and public exposure, thewater sources were categorized into one of three cate-gories: 1) municipal/public 2) domestic drilled bedrockand 3) domestic surficial wells. The water sample dataare plotted on a cumulative frequency distribution graphin Fig. 3A along with previous data for drilled bedrockand surficial wells from Peters et al. (1999). All curvesfrom this study are significantly different from each other
le locations follow population density. Rock sample locations coincide
Table 1Statistical summary of data presented in this study
n Maximum(μg/L)
n BQL % BQL Median(μg/L)
All water data 2273 180 16 0.70% 0.23Private bedrockwells
794 180 7 0.88% 0.56
Private surficialwells
666 49.5 8 1.2% 0.16
Public watersupplies
813 52.7 1 0.12% 0.22
n Maximum(mg/kg)
n BQL % BQL Median(mg/kg)
All rock samples 307 10,000 89 29% 1.9From a vein 21 622 4 19% 2.2Contains sulfide 22 150 5 23% 2.25From a mine 59 10,000 4 6.8% 13.5Contains graphite 15 622 1 6.7% 13.5
BQL refers to sample points below the quantification limit.
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at Pb0.001 using a Mann–Whitney test. Water frombedrock wells had concentrations ranging from belowquantification to 180 μg/L with a median concentrationof 0.56 μg/L, while water from domestic surficial wellsranged from below quantification to 49 μg/L, with amedian of 0.16 μg/L. Drinking water obtained fromdomestic bedrock wells contained higher concentrationsof arsenic than water from domestic surficial wells(Pb0.0001, Mann–Whitney). This result generally agreeswith previously published data, though domestic bed-rockwells are slightly lower, and domestic surficial wellsare slightly higher in overall arsenic concentration thanprevious estimates (Peters et al., 1999). This slightchange in concentrations between the two datasets isillustrated on Fig. 3A by the small displacement betweenthe previous data curves and those plotted from the datafor this study. The slight differences in the curves aremost likely due to the increase in sample size from theprevious study. Sampling rates were calculated bydividing the total number of households sampled bythe total number of households within the state usingcensus data (n=474,606 in 2000 and n=422,653 in1990). In this study, an average of 1 sample was taken forevery ∼200 households, compared with 1 sample for∼426 households in the previous study (Peters et al.,1999). The current U.S. EPA MCL of 10 μg/L is plottedto illustrate the approximate percentage of samples witharsenic concentrations in excess of this regulatory limit.For domestic wells, 12% of bedrock well water samplesand less than 1% of surficial well water samples exceedthis limit. Approximately 2% of municipal water sam-ples exceed this regulatory limit.
Arsenic concentrations determined from the rocksamples were plotted as a cumulative distribution func-tion in Fig. 3B and selected statistics are presented inTable 1. Samples classified as typical for a majorlithology are plotted as a series of filled circles witharsenic concentrations ranging from the quantificationlimit of 1 mg/kg to 10,000 mg/kg, with a medianconcentration of 1.9 mg/kg, as shown by the arrow inFig. 3B. The median arsenic concentration of all of therock samples was close to the mean crustal arsenicconcentration of 2 mg/kg. Samples taken frommine sitesand samples with graphite both had a median concen-tration of ∼13.5 mg/kg which is approximately seventimes greater than both the entire population of samplesand the crustal average arsenic concentration and issignificant at Pb0.0025 (Mann–Whitney). Most of thegraphite samples were obtained from mine sites,however, the mine samples without graphite also had amedian concentration of 9.5 mg/kg — approximately 5times the crustal average. Samples taken from hydro-thermal veins and samples with visible sulfides did nothave appreciably higher concentrations than the remain-der of the samples.
Overall, rocks in New Hampshire do not appear tohave elevated arsenic concentrations compared to con-tinental crust, though some samples from historic minesdo contain as much as 1% arsenic by weight. Whilearsenic was typically observed to be present as sulfideminerals, the presence of sulfides does not alone increasethe likelihood of finding elevated arsenic concentrations,as the dominant sulfide mineral is pyrrhotite, which wasnot observed to contain appreciable concentrations ofarsenic. Hydrothermal vein formation alone also did notresult in elevated arsenic concentrations. Not surpris-ingly, samples taken from mine sites contained the high-est concentrations of arsenic, most likely because thegeologic processes that formed economic concentrationsof base metals also had the same effect on arsenic con-centrations. Some of the high arsenic values are as-sociated with historic graphite mines, particularly nearBristol New Hampshire (Fig. 1). In this area, it has beensuggested that these graphite veins formed as a result ofthe mixing and focusing of metamorphic fluids andhydrothermal activity during metamorphism (Chamber-lain and Rumble, 1988; Zeitler et al., 1990). It is likelythat arsenic was mobilized during dewatering associatedwith metamorphism and subsequently concentrated inthese veins.
To help understand the spatial relationship betweenbedrock geology and the arsenic concentration in drink-ing water, we constructed maps of arsenic concentrationin bedrock samples and arsenic concentration in water
Fig. 3. (A) Plot of cumulative distribution function for all the water samples in this study, plottedwith two curves (⁎) from a previous study (Peters et al.,1999). Water from domestic surficial wells and public water sources had the lowest median arsenic concentrations, while water from drilled bedrockwells had the highest median arsenic concentration. (B) Plot of the cumulative distribution function for rock samples typical of a lithology (●). Samplesfrom mines (+) and samples containing visible graphite (×) have a median arsenic concentration above the crustal average, illustrated by the dashedvertical line. All other sample types, including samples from hydrothermal veins (□) and samples with visible sulfides (○) have a median arsenicconcentration similar to the crustal average.
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for each of the water source categories discussed above:bedrock, surficial, and municipal (Fig. 4A–F). In rocksamples typical of formations, arsenic concentrationsappear to be highest in the western regions of the state,with the highest values located along the western edgeand south central areas of the state (Fig. 4A). In nearly alllocations where very high rock arsenic concentrationsare observed, very low concentrations are also presentnearby. Rock samples from mine sites and samples with
graphite (Fig. 4B) are almost all above the crustal av-erage of 2 mg/kg and occur in several distinct clustersbecause of the development of mines at those sites. Rocksamples from veins or with abundant sulfides also occurthroughout the areas sampled with no distinct pattern ofhigh concentrations (Fig. 4C).
In drinking water samples, the highest arsenic con-centrations were from bedrock well water sources (Fig. 4D),particularly in the south central area of the state. High
Fig. 4. Map of arsenic concentration in all rock samples (A–B) and water samples (D–F). Rock samples are classified as typical for a lithology (A),rock samples taken from mines or containing graphite (B), and rock samples taken from sulfide veins or with visible sulfides (C). Water samples areclassified as either from bedrock well water sources (D), surficial well water sources (E), or municipal water supplies (F).
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arsenic concentrations in bedrock wells were not con-fined to clusters located in the populated zones in centralNew Hampshire as previously thought (Peters et al.,1999), but rather extend throughout the Central Maineterrane. While the highest concentrations of arsenic inbedrock well water are still located in the south centralregion, considerably more values in the 2–10 μg/L rangeare now known to occur throughout the state. Waterfrom domestic surficial wells and municipal systems(Fig. 4E,F) had lower arsenic concentrations than waterfrom domestic bedrock wells, with arsenic concen-trations in domestic surficial well water generally muchless than 2 μg/L. Municipal water sources have morevariability than the other two types of samples, whichmay be due to a combination of water sources that utilizeboth surficial and bedrock aquifers, and the varioustreatment techniques in place within each system. Thelack of data in the north central area of the state is due tothe low population density in the White MountainNational Forest, where well samples are not available.
In many regions where the arsenic concentration inrocks is above the crustal average, we do not observe aconcomitant increase in the median arsenic concentra-tion in drinking water samples. To illustrate this moreclearly, a map of bedrock geology was keyed to themedian arsenic concentration both in bedrock wellsdrilled within that formation (Fig. 5A) and in rock sam-
Fig. 5. Map of generalized bedrock geology (Lyons et al., 1997), coded by ageneralized rock samples (B). White units are those with less than 3 sample
ples of that formation (Fig. 5B). In Fig. 5A, the highestconcentrations are located in the south central region ofthe state, while the arsenic concentration in rock samplesappears to be most elevated in the western half of thestate and in select units on the eastern border. Thereappears to be no simple geographic relationship betweenhigh arsenic concentrations in bedrock samples and higharsenic concentrations in water from bedrock wells. Thiscould be due to a variety of factors, the twomost likely ofwhich include: 1) the Eh–pH conditions and interactionswith aquifer materials, which could retard arsenicmobility, and 2) geological heterogeneity, which maygreatly minimize the area of contamination associatedwith the occurrence of high arsenic rocks.
The aqueous chemistry of the aquifer system has beenshown to be critically important in controlling the mo-bility of arsenic. While arsenic may be present in thebedrock materials, the water may have been either toooxidizing or too reducing to allow any significant trans-port of arsenic in the subsurface. Conditions that are toooxidizing might allow iron oxides to precipitate, whichwould then adsorb arsenic strongly, subsequently re-tarding arsenic transport from the zone of dissolution.Highly reducing conditions would prevent the initialsolubilization of arsenic minerals and retain the metalsin the sulfide phases in the rocks. Only a relativelynarrow band of pH and Eh conditions allow arsenic to be
verage arsenic concentration in bedrock well water samples (A) and ins.
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mobilized from the rocks and transported to drinkingwater wells. While the transport mechanisms have onlybeen studied in detail in one region of central NewHampshire (Peters and Blum, 2003), it is expected thatvariations in the redox conditions in groundwater will bean extremely important determinant of dissolved arsenicconcentrations throughout the state.
Geological heterogeneity arises from the highly lo-calized nature of ore deposits where metals have beenleached from surrounding rocks and deposited in highlyenriched mineralized zones. This creates a relativelysmall area with high metal concentrations surroundedby rocks with considerably lower metal concentrations.Thus, high arsenic concentrations in ore deposits wouldimpact drinking water only in very small areas wherewater had been transported through an arsenic enrichedmineralized zone. The volume of water flowing throughthese deposits is probably very small, and the watersmay also be contaminated by other displeasing odorsand tastes that would make it unfit for drinking water. Ingeneral, waters that may have interacted with aban-doned mine areas are avoided as sources of water whenalternate well locations are available.
3.1. Population impacted by elevated arsenic
In an earlier study, we assumed that the percentage ofsamples in a sample set was directly related to the samepercentage of total households within the state of NewHampshire (Peters et al., 1999). For example, if 5% ofthe water samples were above a certain concentration,then we assumed that 5% of all households in the statewere above that concentration. Embedded in this are theassumptions that water source and arsenic concentra-tions are evenly distributed across the sampling area.Other recent studies have used similar assumptions at thecounty level (Ayotte et al., 2003). In this study, we usedspatial analysis techniques at a finer scale to explicitlytest this hypothesis and to more accurately calculate thepercentages of households in each concentration range.The number of homes at specific arsenic concentrationswas calculated by combining the spatial arsenicconcentration data presented above with water sourcedata which is readily available at the census–tract level(U.S.Census, 1990). In New Hampshire, census tractsare spatial units with no more than ∼3750 homes perunit, and range in size from less than 1 km2 in urbansettings to many tens of square kilometers in rural areas,particularly in northernNewHampshire. For each censustract, the number of households obtaining drinking waterfrom drilled bedrock wells, surficial wells, andmunicipalwater was calculated and plotted in the first row of maps
in Fig. 6. This calculation was performed using the totalnumber of households in each tract from the 2000 census(U.S.Census, 2000) along with the drinking water sourcedata (question H023) from the 1990 census (U.S.Census,1990). This calculation is complicated by the fact thatnew census tracts were created as a result of populationincreases, particularly in urban areas. For this reason,county average water source figures were used in 45census tracts where water source data were not available.However, none of these census tracts with estimatedwater source information were found to have elevatedarsenic concentrations. Drilled bedrock wells appeared tobe the primary source of drinking water for much of therural and suburban areas throughout the state (Fig. 6A). Inparticular, the southern half of the state has a majority ofhouses that utilize drilled bedrock wells for drinkingwater. Surficial wells service only a small percentage ofthe households, and account for less than 10% of thewells in the entire state (Fig. 6B). Municipal wells areused primarily in the urban areas of the southern half ofthe state, particularly the cities of Concord andManchester, which can be identified by the small, darkshaded tracts in Fig. 6C.
The median arsenic concentration was calculated foreach census tract with more than 3 water samples, andplotted in the second row of maps in Fig. 6D–F. Threesamples were considered the minimum number of sam-ples necessary to maintain a minimum sampling rateconsistent with the rest of the dataset. The use offewer than 3 samples would not have includedsignificantly more tracts. Census tracts with less than 3water samples are not shaded. The highest average arsenicconcentrations within each census tract are predominantlyfrom drilled bedrock wells, followed by municipal watersystems, and then surficial water wells. In drilled bedrockwells, the highest average concentrations are located inthe southern half of the state. Surficial wells have thelowest arsenic concentrations statewide, though wells intwo tracts, one located in south central and one located insouth eastern New Hampshire contain remarkably highaverage arsenic concentrations. There are only 3 samplesin each of these two tracts, so they are possibly biased bysampling artifacts and should be verified with furthersampling. Interestingly, the south central New Hampshiretract coincides with the highest concentrations of arsenicin bedrock wells (Fig. 6D,E) and may indicate commu-nication between bedrock and surficial groundwatersystems, or that the surficial deposits are derived fromlocal bedrock that may have anomalously high arsenicconcentrations. Municipal water systems have the lowestarsenic concentrations, with only 14 tracts having be-tween 2 and 10 μg/L and only one tract, located
Fig. 6. Map of census tracts illustrating the drinking water source and average arsenic concentration for that source within that tract. Plots a–c showthe number of households obtaining drinking water from drilled bedrock wells (A), surficial wells (B), or municipal water systems (C). Plots D–Fshow the average arsenic concentrations in water samples from bedrock wells (D), surficial wells (E), and municipal systems (F).
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along the southeastern border with Massachusetts,exceeding the 10 μg/L regulatory standard.
The spatial analysis presented here was not previous-ly possible because of the scarcity of data availableacross such a wide geographic area for all three watersource types. Although our original study had 992 totalwater samples (Peters et al., 1999), compared to 2273 in
this study, far less than 1 /3 of the census tracts within thestate would have had enough samples to allow analysesand interpretation. It is important to point out that thisanalysis eliminates any bias that may have existed in thewater sampling towards known high arsenic areas andany population density dependence. Eliminating thesetwo sampling artifacts using spatial techniques can be
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widely applied to improve the predictive capability ofnon-randomized datasets.
Integrating all of the households in each censustract, we can estimate the number of households atvarious arsenic concentrations across the state (Fig. 7).For each census tract the average arsenic concentrationfor a given water source within that census tract wasmatched to the number of households in the tract thatuse that type of water source. Each census tract isrepresented by a single point on each curve in Fig. 7.When fewer than 3 samples were available for a givencensus tract, the average value was not calculated, andthe census tract was omitted from the graph. Theseomitted tracts amount to 57,000 households, compris-ing approximately 12% of the total households withinthe state. Domestic surficial wells comprise the leastabundant water source, with less than 40,000 house-holds served with a median arsenic concentration of0.15 μg/L. Domestic bedrock wells provide water to∼120,000 households, and have the highest medianarsenic concentration of 1.9 μg/L, with approximately23,000 households above 10 μg/L. Municipal wellsserve ∼265,000 households, the largest subset ofthe population, and have a median concentration of0.41 μg/L.
It is important to note that the median concentrationsof all water samples analyzed (Fig. 3A) do not always
Fig. 7. Plot of the cumulative distribution function of the number of householdin drinking water is primarily from bedrock well sources. Generally speakingfrom domestic bedrock wells affects the largest population, while at concentramore important.
match the median estimated concentrations for all house-holds by census tract (Fig. 7). For example, the medianarsenic concentration from the domestic surficial wellwater samples (0.15 μg/L) is equal to the median house-hold concentration given above. However, the medianmunicipal well sample is 0.22 μg/L while the medianhousehold concentration presented above is 0.41 μg/L.The largest discrepancy is with bedrock wells, where themedian sample concentration is 0.56 μg/L, while themedian household concentration is 1.9 μg/L. This sug-gests that our previous estimates of exposure (Peterset al., 1999) were too low, and need to be revised toinclude more of the population.
A similar calculation was performed by Ayotte et al.(2003) in which the population consuming drinkingwater above 10 μg/L at the county-wide scale was cal-culated using lithogeochemical geologic units whichwere first assigned a drinking water arsenic concentra-tion and then combined with county-wide populationand water source data. A direct comparison is compli-cated by the fact that the study area examined by Ayotteoverlaps with this study in only seven of the ten countiesstatewide. However, the seven New Hampshire countiesin common comprise 88% of the state population (U.S.Census, 2000) and include the geographic areas mostimpacted by elevated arsenic concentrations, thereforeallowing the Ayotte data to serve as a reasonable
s at various arsenic concentrations. Above the MCL of 10 μg/L, arsenic, at arsenic concentrations greater than 0.6 μg/L (vertical arrow), watertions less than 0.6 μg/L, water from municipal water systems becomes
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minimum estimate. Population figures from Ayotte wereconverted to the number of households by dividing bythe county average of the number of individuals perhousehold, and then plotted on Fig. 7. For both domesticsurficial wells and municipal water sources, estimatesfrom this study agree with estimates from Ayotte et al.(2003). For domestic bedrock wells, estimates fromAyotte are slightly lower than those presented in thisstudy. This discrepancy may be due to either the threeadditional counties covered in this study that are notincluded in Ayotte's estimates or the smaller samplesize (n=17) compared to this study (n=794).
The discrepancies in exposure described above mayalso be due to the variability of both water source usageand arsenic concentration within individual censustracts and the statewide average, and points to the im-portance of quantifying these variables at small geo-graphic scales. The lack of sampling density oftenrequires scant water quality data to be applied acrossrelatively large geographic areas. While this may befeasible for municipal water sources, where the watersource is relatively constant for all households, cautionshould be exercised when applying regional estimatesto water from bedrock wells.
The primary route for drinking water arsenicexposure in New Hampshire has typically been thoughtto be through bedrock wells (Ayotte et al., 1999; Peterset al., 1999). Above 0.6 μg/L, bedrock wells supply themost households with the highest arsenic concentra-tions. However, when the arsenic concentration dropsbelow 0.6 μg/L, more households are supplied witharsenic concentrations at this concentration frommunicipal systems than from any other source. Thischangeover is illustrated by the arrow in Fig. 7, whichshows where these two curves cross. If chronic exposureto low levels of arsenic (b0.6 μg/L) is determined tohave an effect on human health, then municipal watersystems become a more important arsenic source thanpreviously thought.
Acknowledgements
The authors would like to thank B. Klaue, A. Klaue,and A. Jacobson for their assistance and support. T.Douglas, T. Horton, K. Keller, and M. Hren helpedwith the bedrock sampling. We express our appreci-ation to the anonymous reviewers who helped improvethis manuscript during review, and to R. Ford, guesteditor of this special issue. Funding was provided byNSF EAR-0000539 to SCP and NIEHS ES-07373 toJDB. [DR]
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