Variations of Bromide in Potable Ground Water in the ...

Variations of Bromide in Potable
Ground Water in the United States

by Stanley N. Davis1, June T. Fabryka-Martin2, and Laura E. Wolfsberg3

Abstract

Concentrations of bromide in potable ground water that has < 10 mg/L chloride range from 0.0032 to 0.058 mg/L
with a median value of 0.016 mg/L. The chloride/bromide mass ratio for the same water ranges from 43 to 285 with
a median value of 101. The ratios, which resulted from screening ~165 analyses of water from 32 locations in 24
states in the United States, show a distinct geographic variation with highest values near the coast and trending toward
a value of ~50 in the continental interior.

Introduction 2002) and on mineral surfaces in water having a low pH
(Seaman et al. 1996). (5) Human activity has introduced a
Bromine is one of the halogen elements found in trace large number of compounds of bromine into aquifers. On a
amounts in all ground water. The general geochemistry of global scale, biomass burning of agricultural fields, forests,
bromine in ground water has been covered by Rittenhouse and savannahs may be the single largest human source of
(1967), Carpenter (1978), Edmunds (1996), Davis et al. bromide to the atmosphere (Andreae et al. 1996). Only
(1998), Custodio and Herrera (2000), and others. Salient ~10% of today’s fires are thought to be entirely natural in
characteristics of bromine as reported by these authors origin (Lobert et al. 1999). In the recent past, the oxidation
include the following. (1) Almost all bromine from natural of ethylene dibromide, an antiknock additive to gasoline,
sources found in ground water is in the form of the simple constituted a major artificial source of bromide in the envi-
negative monovalent ion, bromide. (2) The most common ronment (Thomas et al. 1997). Locally, agricultural chemi-
ratios of chloride/bromide in potable ground water are cals have also been an important source (Flury and Papritz
between 40 and 400 on the basis of mass. (3) Inorganic 1993; Vengosh and Pankratov 1998).
compounds of both chloride and bromide are highly solu-
ble. However, because those of bromide are most soluble, Interest in the concentration of natural levels of bro-
evaporation to partial dryness will leave a residual brine mide in ground water is due to at least three reasons. First,
rich in bromide after chloride solids start to precipitate. (4) it is useful in helping to determine the history of the water
Bromide, like chloride, is normally conservative in a (Rittenhouse 1967; Whittemore 1995; Edmunds 1996;
ground water system; however, some of the bromide will Andreasen and Fleck 1997; Custodio and Herrera 2000;
sorb on organic solids (Gerritse and George 1988; Reeve Davis et al. 2001; Goni et al. 2001; Mills et al. 2002; Shade
et al. 2002). Second, bromide has proved to be a useful arti-
1Department of Hydrology and Water Resources, University of ficial tracer for ground water studies (Leap 1992; Davis et
Arizona, Tucson, AZ 85721; [email protected] al. 1985; Poletika et al. 1995). Third, water purification for
municipalities can use chemicals that react with bromide
2Hydrology, Geochemistry, and Geology Group (EES–6), Mail and dissolved organic compounds and form carcinogens
Stop T003, Los Alamos National Laboratory, Los Alamos, NM 87545 (Cooper et al. 1986; Edmunds 1996; Flury and Papritz
1993; Nabukawa and Sanukida 2001; Song et al. 1997). For
3Isotope and Nuclear Chemistry Group (C-INC), Mail Stop each of the given points of interest, it is useful, and some-
J514, Los Alamos National Laboratory, Los Alamos, NM 87545 times essential, to establish the probable natural back-
ground values of bromide in ground water in the region of
Received May 2003, accepted March 2004. study. The present paper is a preliminary step toward defin-
ing the natural geographic distribution of these background
Copyright © 2004 by the National Ground Water Association.

902 Vol. 42, No. 6—GROUND WATER—November–December 2004 (pages 902–909)

values and sketching the large-scale natural processes that rion eliminated 43 of the 165 analyses from further consid-
shape it. eration.

Our paper is based primarily on analyses of ground Nitrate Concentrations
water samples that were collected for an evaluation of the The association of nitrate with chemical fertilizers,
geographic variation of 36Cl. Only after studying the results
was it clear that a distinct regional variation of choride/bro- animal waste, and many other human-related sources is
mide ratios was present across the United States. Unfortu- well known. Although entirely natural sources of signifi-
nately, because the primary objective of the study was not cant concentrations of nitrate exist, they are probably not
bromide per se, we have had the task of selecting relevant common in most of the areas sampled. In a chemically
data from ~165 analyses that were made of potable ground reducing environment, the nitrate in ground water will not
water. The selection process, as outlined later, was be stable. Therefore, the absence of nitrate in ground water
designed to eliminate analyses of water affected by human does not necessarily mean the absence of contamination
activity, as well as water containing significant amounts of from human sources. For the purpose of this report, never-
ancient subsurface brine. With the exception of 10 samples theless, water with > 5.0 mg/L of nitrate was not used. A
from Tucson, Arizona, that are discussed by Davis et al. total of eight samples with concentrations of < 10 mg/L
(2003), analytical results for the 36Cl project are given in a chloride were eliminated on the basis of high nitrate con-
thesis by Moysey (1999). Portions of the dataset are also centrations.
presented in Davis et al. (2000), Davis et al. (2001), and
Davis et al. (2003). Relevant data from the selection Radionuclide Content
process are given in Table 1. In addition, two datum points Several radionuclides originating from human activity
shown in Figure 1 derive from previous studies in southern
Alberta, Canada (Fabryka-Martin et al. 1991), and Yucca can be well above natural background concentrations in
Mountain, Nevada (Fabryka-Martin et al. unpublished). ground water and will identify ground water potentially
These data were screened by the same criteria as were altered from its original state. Only water with < 2.0 TU,
applied to the 36Cl project dataset. 80% modern 14C, and a (36Cl/total chlorine) × 1015 ratio
< 2000 was used. Seventeen samples that passed other cri-
The purpose of this paper is primarily to show results teria were eliminated owing to high radionuclide content.
of our bromide analyses and to explain how the values were
selected that are shown in Figure 1. Although we speculate Clusters of Wells
on the possible causes of the geographic variations of bro- Where several ground water samples came from the
mide and chloride/bromide values, the subject involves
complex physical and chemical processes, and a detailed same general locality (within a radius of ~50 km), only the
discussion is not attempted. single analysis having the lowest chloride concentration
was used. Like the 10 mg/L cutoff, the choice of the lowest
Criteria for Selecting Bromide Analyses chloride concentration in water from a cluster is intended to
select analyses of preanthropogenic recharge. The use of
Concentration of Chloride only one analysis out of a cluster served to combine 62 of
Only analyses of water containing < 10 mg/L of chlo- the total number of analyses that met the selection criteria.

ride were used. The assumption was made that higher con- The selection of the chloride/bromide ratio of the sam-
centrations might reflect contamination from human ple with the lowest chloride concentration may seem
sources or from natural sources unrelated to the original inferior to the use of the median or mean values of the clus-
composition of meteoric recharge water. This sorting crite- ter. The median or mean values, nevertheless, can be influ-
enced strongly by the total chloride as already demonstrated
by our data from both North Dakota and Minnesota (Davis
et al. 2000). In North Dakota, for example, the chloride/
bromide ratio is only 63 when the chloride content was 0.62
mg/L, but the ratio increased to 211 when the chloride con-
tent reached 10 mg/L. Regardless of this problem, most of
the points plotted that represent clusters are reasonably close
to the median and mean values (Table 2).

Figure 1. Chloride/bromide mass ratios in potable ground Analyses of Samples
water in the United States.
Table 1 shows the results of electrical conductivity,
chloride, bromide, nitrate, and isotopic analyses of the rel-
evant water samples. The electrical conductivity was mea-
sured in the field with a precision of ~ ± 5 µS. Nitrate was
measured in the Soil, Water, and Plant Analysis Laboratory
of the University of Arizona with a precision of ± 0.3 mg/L.
Tritium concentrations were measured at the U.S. Geolog-
ical Survey in Menlo Park, California. 14C values were
measured at the Accelerator Mass Spectrometry (AMS)
Laboratory at the University of Arizona. 36Cl values were

S.N. Davis et al. GROUND WATER 42, no. 6: 902–909 903

Table 1

Probable Preanthropogenic Bromide in Ground Water

No. in Continentality Conductivity Cl– Br– NO3 3H 14C 36Cl/Cl
Cluster mg/L mg/L TU % mod. ‫ ן‬1015
Item State Latitude Longitude Index µS mg/L Cl–/Br–

1 GA 1 32º28Ј 83º44Ј 32 50 1.67 0.0118 141 0.54 — — 156
2 GA 1 33º11Ј 82º31Ј 30
3 FL 1 27º28Ј 81º28Ј 17 20 1.66 0.0087 190 < 0.5 — — 97
4 FL 4 29º11Ј 81º42Ј 20
5 AR 2 35º15Ј 90º33Ј 38 330 8.06 0.032 253 1.8 — — 45
6 MS 2 31º09Ј 89º25Ј 31
7 OK 2 35º37Ј 97º27Ј 47 110 4.42 0.018 252 4.1 — — —
8 AR 1 34º04Ј 92º14Ј 38
9 ND 2 45º59Ј 97º50Ј 54 210 2.87 0.023 123 < 0.5 — — 70
10 SD 3 44º41Ј 103º51Ј 49
11 WV 2 39º37Ј 78º13Ј 40 30 2.23 0.010 228 < 0.5 — — 50
12 AZ 1 31º53Ј 110º12Ј 37
13 UT 2 37º11Ј 113º30Ј 46 470 5.54 0.033 166 1.0 — — 300
14 UT 2 41º53Ј 111º38Ј 47
15 ID 1 44º05Ј 111º26Ј 46 20 2.93 0.029 101 < 0.5 — — 1040
16 OR 4 44º29Ј 121º17Ј 28
17 OR 6 44º45Ј 122º55Ј 20 499 0.62 0.0098 63 1.2 — — 830
18 WA 2 45º43Ј 121º32Ј 25
19 CO 4 39º00Ј 104º44Ј 39 610 0.87 0.011 81 1.1 — — 1190
20 CA 4 39º10Ј 120º07Ј 32
21 WY 2 43º50Ј 104º11Ј 45 310 2.04 0.021 100 < 0.3 < 0.9 — 385
22 AZ 1 33º49Ј 109º07Ј 43
23 NE 1 41º30Ј 101º20Ј 48 240 3.52 0.038 93 3.1 — — 364
24 IN 4 40º57Ј 85º15Ј 43
25 KY 1 37º29Ј 83º08Ј 34 455 4.81 0.035 136 0.9 — — 359
26 AL 6 31º58Ј 85º01Ј 31
27 AL 5 32º58Ј 87º38Ј 33 30 0.58 0.0066 88 < 0.3 — — 130
28 MT 8 48º12Ј 114º04Ј 35
29 IA 3 42º07Ј 93º32Ј 52 170 3.07 0.018 168 < 0.3 < 1 47 133
30 WI 2 43º07Ј 89º30Ј 48
31 MN 6 45º20Ј 93º00Ј 53 120 1.83 0.0066 277 0.6 — — 409
32 AZ 5 32º11Ј 110º50Ј 37
105 1.78 0.0093 191 1.8 — 74 289

130 1.65 0.0058 285 2.0 — — 1116

218 1.12 0.022 51 < 0.3 — 2 1672

169 0.62 0.0032 194 < 0.3 — — 1257

547 1.37 0.0164 84 1.0 — — 1095

276 1.97 0.0385 51 < 0.3 — — 1842

180 1.02 0.0233 44 < 0.3 — — —

377 1.65 0.0380 43 3.2 — — 166

729 6.27 0.0576 109 < 0.3 — 5 117

293 2.83 0.0162 175 < 0.3 0.1 4 100

28 1.44 0.0148 97 < 0.3 — — 122

254 0.47 0.0080 59 < 0.3 0.3 4 1180

520 0.52 0.0102 51 < 0.3 < 0.7 46 473

520 0.66 0.0116 57 < 0.3 0.9 62 368

350 0.69 0.0095 73 < 0.3 < 0.9 — 657

242 4.64 0.0457 102 3.7 — 73 373

measured in the AMS laboratory (PRIME Lab) at Purdue affected materially by evapotranspiration. As a conse-
University. Chloride and bromide were measured at the Los quence, we studied this ratio as well as the absolute con-
Alamos National Laboratory in New Mexico. centration of bromide.

Chloride and bromide concentrations were determined The reduction of statistical noise by considering the
using an ion chromatography system with a conductivity chloride ratio rather than the bromide concentration results
detector. All samples were filtered prior to analysis using a in a moderately well-defined geographic distribution. As
0.2 µm nylon filter. Detection limits were 4 ppb bromide might be expected from data on chloride in precipitation,
and 0.1 ppm chloride with precisions of better than ± 5%. isopleths generally follow the contours of the continental
Samples below the bromide detection limit were concen- coastline (Figure 1). A notable exception is the zone of high
trated by evaporation before analysis. ratios extending from southern Idaho eastward into South
Dakota (Figure 1). This zone may possibly be caused by
Bromide in Ground Water salt-ladened fallout from the Bonneville Salt Flat and nearby
playas. The chloride/bromide ratios in brine from the Bon-
Concentrations of bromide in potable ground water neville Salt Flat range from ~2000 to 7800 (Turk et al.
that passed our screening criteria (Table 1) range from 1973), which could account for the anomalously high zone.
0.0032 to 0.058 mg/L with a median value of 0.016 mg/L.
The arithmetic mean of the same values is 0.019 mg/L with Discussion
a standard deviation of 0.013 mg/L. The rather large statis-
tical scatter was assumed to have been caused in large mea- At least three critical assumptions underlie the position
sure by the locally variable concentration of bromide by of the isopleths shown in Figure 1. (1) The chloride/bro-
evapotranspiration during the process of ground water mide ratio measured for the plotted ground water sample
recharge. Inasmuch as almost all natural inorganic com- reflects the soluble inorganic component of atmospheric
pounds of both bromide and chloride are highly soluble, we chloride/bromide. Complicating factors in the transport of
assumed that the chloride/bromide ratio would not be halogens from the atmosphere to the ground water are

904 S.N. Davis et al. GROUND WATER 42, no. 6: 902–909

Table 2 Several factors without doubt control the general distri-
bution of preanthropogenic chloride/bromide ratios in
Sample Clusters of Cl–/Br– Ratios ground water. The most important is probably the distance
from the recharge area to the nearest source of particulate
Latitude Longitude Cl– Br– Cl–/Br– chloride, which most commonly is the shoreline of the
mg/L mg/L ocean. This regional source of chloride was recognized as
early as 1905 (Jackson 1905). Chloride in pristine streams
A. Montana (Cluster 28 in Table 1) and ponds in New York and New England was found in
concentrations of ~6 mg/L near the shoreline. The concen-
48º12Ј 114º22Ј 3.41 0.0585 58 trations decreased inland to values of ~0.2 mg/L in western
New York State. Subsequent measurements of regional
48º15Ј 114º29Ј 2.49 0.0490 51 samples of precipitation showed similar chloride distribu-
tions in the United States as a whole (Junge and Werby
48º21Ј 114º24Ј 0.66 0.0166 40 1958; National Atmospheric Deposition Program/National
Trends Network 2003), as well as in other parts of the world
48º24Ј 114º15Ј 0.83 0.0194 43 (Eriksson 1957, 1960; Junge 1963; Reimann et al. 1997).

48º15Ј 114º16Ј 0.52 0.0139 37 The particles carrying chloride are entrained in the
atmosphere primarily from bursting bubbles due to the sur-
48º06Ј 114º11Ј 1.10 0.007 157 face turbulence that produces oceanic foam. Sea salt
aerosol production by this mechanism is the dominant nat-
48º20Ј 114º10Ј 1.48 0.0254 58 ural source of bromide (Sander et al. 2003). Ocean water,
however, has a chloride/bromide mass ratio of 290, and
48º12Ј 114º04Ј 0.47 0.0080 59 almost all samples of water that we have measured have
ratios that are significantly lower. Thus, an explanation for
Value plotted on Figure 1 = 59; median = 54; mean = 63; δ = 36 this chemical fractionation of the original sea salt composi-
tion is needed.
B. Western Oregon (Cluster 17 in Table 1)
Considerable chemical fractionation takes place at or
44º03Ј 123º19Ј 4.91 0.021 232 near the ocean surface through the generation of sea salt
aerosols by bubble bursting (Bloch et al. 1966; Baier 1972;
44º38Ј 122º45Ј 4.40 0.021 210 Blanchard and Syzdek 1972). Many marine organisms con-
centrate bromine, and microorganisms caught in the bubble
45º08Ј 123º14Ј 3.42 0.022 155 film are propelled into the atmosphere when the bubble
bursts. Each bubble can generate as many as 10 jet drops
45º29Ј 123º04Ј 2.61 0.015 173 with a typical size of 1 to 2 µm radius, but extending to
more than 10 µm, and up to several hundred film drops in
45º24Ј 122º45Ј 2.49 0.0152 164 the submicron range (O’Dowd et al. 1997). Large jet drops
have chloride/bromide ratios similar to that of sea water,
44º45Ј 122º55Ј 1.78 0.0093 191 but the smallest particles are greatly enriched in bromine
relative to chlorine (Zhou et al. 1990). For example, studies
Value plotted on Figure 1 = 191; median = 182; mean = 188; δ = 27 by Virkkula et al. (1999) showed a chloride/bromide ratio
in fine particles (diameter < 2.5 µm) of 1.43. In contrast,
C. Southern Arizona (Cluster 32 in Table 1) coarse particles (diameter > 2.5 µm, but less than 15 µm)
had a chloride/bromide ratio of 144. An additional frac-
32º15Ј 110º56Ј 9.26 0.0633 146 tionation mechanism leading to enrichment of bromine
over chlorine in the smallest aerosols may arise from the
32º14Ј 110º54Ј 5.06 0.0498 102 recently documented finding that halide anions not only
have a propensity to cluster at the air-water interface, but
32º13Ј 110º52Ј 7.68 0.0482 159 that they do so in proportion to the polarizability of the
anion (Jungwirth and Tobias 2002). The polarizability of
32º13Ј 110º50Ј 5.85 0.0549 107 bromine is ~30% higher than that of chlorine, causing
bromine to have a higher attraction for the air-water inter-
32º11Ј 110º50Ј 4.64 0.0457 102 face than does chlorine, and hence is more likely to be
enriched in the smaller film drops.
Value plotted on Figure 1 = 102; median = 107; mean = 123; δ = 24
The results of these fractionation processes are
D. Eastern Alabama (Cluster 26 in Table 1) obscured in low-lying coastal areas where halide deposition
is dominated by the contribution from large sea spray par-
32º09Ј 85º43Ј 4.01 0.0262 153 ticles, with chloride/bromide ratios on the order of 250 to
285, only slightly less than that of sea water (290) (Figure
32º08Ј 85º43Ј 5.14 0.0310 166 1). However, the influence of large sea spray particles
diminishes rapidly with increasing distance as well as ele-
32º05Ј 85º31Ј 7.63 0.0689 111 vation (Lodge 1955). In a profile measured in Hawaii, for

32º04Ј 85º32Ј 9.98 0.0554 180 S.N. Davis et al. GROUND WATER 42, no. 6: 902–909 905

31º50Ј 85º57Ј 4.52 0.0539 84

31º58Ј 85º01Ј 2.83 0.0162 175

Value plotted on Figure 1 = 175; median = 159; mean = 145; δ = 35

assumed to be negligible. Such factors observed in field
and laboratory studies include chemical sinks and sources
during soil transit (Shorter et al. 1995) and recycling by
vegetation (Cornett et al. 1997; Gan et al. 1998). (2) A char-
acteristic chloride/bromide ratio can be defined for a geo-
graphic area, and this value has been constant for the past
~10,000 yr or so covered by the ground water samples.
Complicating factors are volcanic activity, climatic
changes, shifting jet stream, presence and extent of conti-
nental halide sources such as dry playas, and shifting coast-
lines caused by changes of sea level. (3) Datum points are
plotted at the location of the sampled well. We assume that
the characteristic chloride/bromide ratio of the recharge
area also pertains to the well location. A complicating fac-
tor would be the case of samples from an extensive aquifer
where the point of recharge of water would be a great dis-
tance from the sampling point.

example, the sea salt concentration at 19 m was ~22 µg/m3, Figure 2. Continentality index vs. chloride/bromide mass
decreasing to about half that value (11 µg/m3) at 30 m. ratios of potable ground water in the United States.
Above 30 m, the sea salt concentration slowly decreased to
~5 µg/m3 at 600 m a.s.l. (Blanchard et al. 1984). The steep The continentality index values for each of the chlo-
gradient is probably due to the settling of the largest salt ride/bromide samples plotted in Figure 2 were taken from
particles (> 10 µm radius), which are controlled by gravita- the map published by Trewartha (1961) and are listed in
tional forces. Chlorine/bromine ratios in rain water likewise Table 1. The plot qualitatively shows the trends observed in
decrease with increasing altitude, from ~250 at 305 m to 50 Figure 1, with chloride/bromide ratios slightly below sea
at 2130 m (Duce et al. 1965). In general, the lower or water values in coastal states, decreasing rapidly with dis-
marine regime extends upward ~1 or 2 km. tance inland concomitant with increasing annual tempera-
ture ranges, until reaching a plateau value of ~50 in the
Sea salt concentrations also decrease steadily with dis- continental interior where the greatest temperature range is
tance from the shore due to dispersion by intensive convec- also experienced due to minimal buffering of local climates
tive mixing in the lower troposphere. This process results in by marine air masses.
a near-constant chloride level in precipitation for distances
> 500 km from the coast (Junge and Gustafson 1957; Blif- A correlation with sea salt deposition rates also sug-
ford 1970; Delany et al. 1973; Patterson et al. 1980). gests a strong relationship of chloride/bromide ratios with a
marine aerosol source. Guelle et al. (2001) calculated wet
The middle and upper troposphere is thus character- deposition fluxes of sea salt over the continental United
ized by a consistent trend toward uniformity in aerosol con- States, using weekly precipitation chemistry data collected
centrations and in chloride/bromide ratios with increasing by a large number of stations in the National Atmospheric
altitude (Delany et al. 1973). Therefore, although sea salt Deposition Program network in 1987. The stations were
dominates the chloride/bromide component of aerosols at separated into two groups in order to represent zonal trans-
lower altitudes over oceans, above a few kilometers, the port of marine air from the North Pacific in a west-to-east
chloride/bromide ratios of aerosols even over the oceans direction across the United States, and from the Gulf of
are similar to those over the midcontinental United States Mexico in a south-to-north direction (Figure 3). The calcu-
(Blifford 1970). Ratios of chloride/bromide are reduced to lated deposition fluxes decrease by an order of magnitude
values < 50 in the interior of the United States. The most from the coast to the continental interior, a trend which is
probable mechanism causing the reduction of the chlo- mirrored by parallel drops in chloride/bromide ratios along
ride/bromide ratio as air masses move toward the continen- each transect (Figure 4). The extent to which data plotted in
tal interior may thus relate to the relative size of aerosol Figures 2 and 4 deviate from smooth trends may be related
particles. Inasmuch as the coarse particles are removed in part to the degree to which relative humidity varies spa-
much more rapidly by gravity than the fine particles, the tially and temporally at the sample locations, which could
chloride/bromide ratio would decrease as air masses move be expected to cause variability in the local aerosol deposi-
from the coast toward the interior of the United States. Rel- tion rates as well.
ative humidity also plays a role in the rate of aerosol depo-
sition. An aerosol with a diameter of 10 µm when The explanation of the observed fractionation may be
equilibrated at a relative humidity of 70% is about twice compatible with studies of present-day aerosol particles;
that size when it leaves the sea as a droplet (Blanchard et al. however, the modern atmospheric content of bromide has
1984), but shrinks as the relative humidity diminishes. increased significantly owing to pollution by industry, agri-
culture, and transportation (Butler et al. 1999). Conclusions
The trend in decreasing chloride/bromide ratios as one
moves away from the ocean roughly correlates with an
increase in the continentality index, which is a measure of
the extent to which the local climate is influenced by the
land mass as opposed to the ocean (Figure 2). Although
there have been many versions of this index over the past
century, the continentality index is almost always a func-
tion of the temperature range of the site, such as the differ-
ence between the average temperatures of its warmest and
coldest months, and is normalized by the latitude of the site
(Huschke 1959). We use here the formulation of Conrad
(1946), who added arbitrary constants to the index formula
so that it could be calculated even for locations on the equa-
tor, and that served to set the limits of the index to 0 (for a
marine climate, represented by Thorshavn, Denmark) and
100 (for a continental climate, represented by Verkhoyansk
in northeastern Siberia):

k = 1.7A/sin φ ‫מ‬20.4

where k is the continentality index in percent, A is the aver-
age annual range of temperature in °C at a given place, and
φ is the geographical latitude.

906 S.N. Davis et al. GROUND WATER 42, no. 6: 902–909

Figure 3. Boundaries of continental transects for which the same samples is 129 ± 73, and the median value is 101.
Guelle et al. (2001) calculated sea salt deposition rates (plotted Virtually all samples of potable water have chloride/bro-
in Figure 4). Dots mark the location of ground water sampling mide ratios lower than sea water, which has a ratio of 290.
sites as per this study.
Owing to the short-term and seasonal variations of the
bromide input to the atmosphere (Montzka et al. 2003;
Scott et al. 2001), the bromide in ground water may be a
better reflection of the average bromide content of recharge
water than all but the longest monitoring of precipitation
could provide.

A distinct geographic variation of chloride/bromide
ratios exists in the United States with highest values near
the coast, trending toward a value of ~50 in the continen-
tal interior. More studies are needed in order to understand
this variation, but differences in the chloride/bromide
ratios in different sizes of aerosol particles may provide
the explanation.

based on today’s data cited previously, therefore, may not Acknowledgments
be an entirely accurate explanation of the preanthropogenic
distribution of bromide that we are trying to understand. Research reported here was supported by National Sci-
ence Foundation Grant EAR9526881. Numerous individu-
Conclusions als assisted with the fieldwork, particularly Stephen
Moysey, who helped process samples and who also col-
The mean concentration of bromide in potable ground lected samples from Colorado and California, and Augusta
water having < 10 mg/L of chloride is 0.019 ± 0.013 mg/L Davis, who assisted with the collection of all other samples.
for 32 water samples from the United States, and the median 36Cl samples were analyzed by the PRIME Laboratory at
value is 0.016 mg/L. The mean chloride/bromide ratio for Purdue University, tritium was determined by the U.S.
Geological Survey laboratory in Menlo Park, California,
and 14C analyses were made at the Isotope Laboratory of
the University of Arizona. The present manuscript was
improved materially by our reviewers, F.M. Phillips and
M.A. Plummer. To these and many other individuals and
organizations, we are very grateful.

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