Geochemistry of Great Salt Lake, Utah I: Hydrochemistry since 1850

0016703?/8S/f3.00 + .OD

Gfwhimti et Cosm&imica Acta hi. 49. pp. 727-731 0 Ibpmon Rcss Ltd. 1985. Printi in U.S.A.

Geochemistry of Great Salt Lake, Utah I: Hydrochemistry since 1850 R. J. SPENCER,*H. P. EUGSTER,’ B. F. JONES~ and S. L. RESIGN LDepartment of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada T2N 1N4 2 Department of Earth and Planetary Sciences, Johns Hopkins Unive~~ty, Baltimore, Maryland 2 I2 18 3 U.S. Geological Survey, Reston, Virginia 22092 (Received April 25, 1983; accepted in revisedfirm December 3, 1984) Abstract-The hydrochemistry of Great Salt Lake, Utah, has been defined for the historic period, 1850 through 1982, from published data combined with new observations. The water balance depends largely on river inflow, atmospheric precipitation onto the lake surface and evaporation. Input of the major solutes can best be accounted for by mixing dilute calcium-bicarbonate type river waters with NaCIdominated hydrothermal springs. Prior to 1930, lake concentrations fluctuated inversely with lake volume in response to small climatic variations. Since then, salt precipitation and dissolution have signiticantly modified lake brine compositions and have led to density stratification and the formation of brine pockets of differing composition. Brine mixing has become an important component of brine evolution. We have used calculated evaporation curves with mineral p~ipi~tion and ~iution to clarify these processes. Pore fiuids represent important storage for solutes. Solute profiles can be modeled by simple onedimensional diffusion calcuiations. Short-term historic variations in lake composition affect shallow pore Buids in the upper 2 metres of sediment. INTRODUCIION

The general relationship between hydrology and salinity of closed-basin systems has been illuminated by LANGBEIN ( 1961), and the role of pore fluids deduced by JONES and VAN DENBURGH ( 1966). Studies of solute fluxes in and out of pore fluids have been summarized by LERMAN (1979) and BERNER (1980) for lacustrine and marine settings. Such fluxes are particularly large in hydrol~~ly closed basins which, through climatic effects, may be subjected to substantial short-term variations in their solute concentration (see LERMANand JONES, 1973). Our work on Great Salt Lake adds new insights to these aspects, particularly with respect to the importance of pore fluids for salt budgets.

THE FIRST chemical analyses of the waters of Great Salt Lake, Utah, were carried out in 1850 (see TALMAGE,1890; RICHARDSON,1906; CLARKE, 1924). Much has been written since that time on the origin of the salts of this lake and on its salt budget (GILBERT, 1890, BROECKER and WALTON, 1959; EARDLEY et al., 1957), and yet we stili do not have a satisfactory model to account for the concentration and composition of its waters. A large body of contemporary data has been collected, including inflow compositions (HAHL and MITCHELL, 1963: HAHL and LANGFORD, 1964), hydrologic parameters (ARNOW and JENSEN, 1977; ARNOW, 1980), and pore

fluid data (SPENCER,1982). We now are in a position to provide a more quantitative account of the hydrochemistry of Great Salt Lake and its short-term variations. Subsequent contributions in this series will deal with the Pleistocene-Holocene hydrochemistry and with diienetic reactions. A summary of other aspects of our work on Great Salt Lake can be found in SPENCERet al. ( 1984) and SPENCER (1982). We begin with a discussion of the presentday hydrology and proceed to the composition of the various inflow sources and their relative importance. Next we interpret the lake chemistry throughout historic time and its response to changes in inflow, restriction of circulation and salt precipitation. Shortterm fluctuations in brine chemistry are reflected in the uppermost portion of the pore fluid profile; we find that diffusion of solutes into and out of pore fluids represents a significant part of the solute balance. Pore fluids more than 2 m below the sediment-water interface respond only to longer-term variations, on the order of hundreds or thousands of years.

HYDROLOGY

ARNOWand JENSEN(1977) and ARNOW(1980) have defined the present hydrologic system in terms of inflow, outflow and changes in storage. The inflow is derived primarily from the Bear, Weber, and Jordan River basins, and from atmospheric precipitation on the lake surf&e, white outflow is in the form of evaporation. Changes in storage are manifested in lake volume and hence lake elevation fhictuations. Small climatic variations during the historic period have caused adjustments in the relative proportions of inflow and outflow, reelected in the elevation fluctuations (ARNOW, 1980). It is generally recognized that the construction of the railway causeway from Promontory Point to Lakeside has greatly restricted circulation between the North and South Arms; the fact that dilute inflow primarily enters the South Arm (see Figs. 1 and 5) has led to substantial salinity differences between the two water bodies. An analogous division occurred at the end of the Pleistocene (SPENCER, $982; SPENCERet al., 1984), when a North Basin was separatedFroma South &&I along a ridge running northeast from Gnington Island toward Promontory Point. Restrictad circulation during that period is clearly re&cted in the sedimentary record.

727

c) Increased evapotranspiration and Input d p&. lutants from urban. mining, and agricultural areas. d) Hydrothermal spring waters. e) Mixing of inflow waters with one another or with lake waters or salts. f) Recycling of lake salts by wind.

KILOMEYRES

FIG. I, Locations of inflow water sample sites used in this study shown within the present configuration of the Great Salt Lake Drainage basin after HANDY and HAHL (1966). The dashed curve running North-South is the trace of the Wasatch Fault. COMPOSITION OF THE INFLOW HANDY and HAHL (1966) noted that several compositionally distinct inflow waters are present within the basin. EUG~TERand JONES(1979) used data from HAHL and MITCHELL(1963), HAHL and LANGFORD (1964), HAHL (1968), and WADDELL and PRICE ( 1972) to illustrate major solute loss due to mineral precipitation during evaporative concentration. More recently COLE (1982) has described hydrothermal waters in the basin. Re-examination of these data within the hydrologic context indicates that evaporative concentration and major element loss from solution during inflow are not dominant. On the contrary, mixing of distinctly different inflow waters can resuft in the observed behavior. Figure 1, taken from HANDY and HAHL (1966). shows the configuration of the drainage basin and sample locations. Inflow compositions are listed in Table I. Compositions of samples from locations I. 2, 3 and 4 collected on January 1I, 1961 by HAHL and MITCHELL( 1963) illustrate that mechanical mixing of 1 (Bear River) and 2 (Malad River) nicely account for compositions 3 and 4, except for a distinct loss of silica, perhaps due to diatoms. The following factors will affect the total solute input into the lake:

a) The compositions of atmosphere precipitation. b) Interaction of dilute pr~ipi~tion with rocks within the watershed.

Of these types only a. b. and d are primary natural sources for the geochemicaf evolution of the system. The ~om~sition of atmospheric p~ipitation (type a) has been estimated on the basis of data presented by FETH cr ul. (1964). The dilute Ca”-HCO% river waters described by HAWDY and HAHL (1966) appear to be the result of process b above, with a minimum of man-induced pollutants or recycled salts. The analyses of waters derived from hydrothermal springs (the Malad River area) are used to estimate the composition of type d waters in the basin. The lower portions of the major river drainages are interpreted to be strongly influenced by urban, mining and a~cuituml activity (type c inflow) and are more susceptible to contamination by recycling (type f inflow). The lake margin springs represent interaction of lake salts and brines with dilute groundwater similar in composition to the rivers (type e waters). The data of Table I together with estimates of the proportions from various inflow sources by ARNOW and JENSEN ( 1977) and ARNOW (1980) allow us to develop a hydrochemical model for the present system. Missing only is an estimate of hydrothermal spring water input. since spring discharge is difficult to measure. We can monitor this input at Station 4 (Fig. 1) Iocated below the confluence of the Bear River and the Malad River, the latter essentialty repre~nting hydrothe~al effluent. This is station 39 of HAHt and MITCHELL( 1963). for which they report

rable Location

1

Inflov Comppoaition

Date

CL-

01-61~

soz- HCOJ‘

N.+

1

Bear

84

58

2 2 2 2 2 3 3 4 4 5 6 7 8 9 9 10 11

ml.d River Mald River Spring Spring Spring Sear River Sear River Sear River Sear River See.= River SprinS Plea* River l'h.,lasFork Seer RIYer Scar River "ebcr River Ucber River 11 Weber River 12 "eber River

126 135 102 480 91 64 44 66 46 51 52 54 so 38 48 24 IS 21 28

284 456 1170 362 2800 480 15000 320 2960 400 190 324 105 400 179 320 120 298 29 938 30 304 31 z&lo 73 16 212 252 21 256 38 128 9.1 218 13 228 9

1, 14 15 15 16 17 17 A S

01-619 980 05-79 1950 05-79 5025 79b 26000 08-83 5120 Ol-61a 295 08-83 L48 Ol-616 276 06-83 175 08-83 32 OS-83 13 OS-83 33 08-83 98 OS-79 15 08-83 21 08-83 52 10 05-79 08-83 19 6.7 Me 61.628 148 Jordan River OS-83 146 J.xd." River 08-83 15 Provo River 05-79 4.8 Provo River 08-83 8.3 Provo Riv+r 08-83 14 Spmish Fork 05-79 34 Spmish Fork OS-83 0.5 Prtcipitiltion c 1.0 Prccipit~~fon c

171 182 21 30 27 29 61 1.1 2.2

220 240 148 125 164 264 272 3.6 6.3

River

399

5::

116 117 9.2 3.7 6.5 17 30 0.4 0.6

Y

+

11 43 a7 14 790 I26 19 9.2 17 II 3.6 12 3.3 1.3 1.0 2.0 4.0 1.2 2.1 2.0 12 12 1.1 I11 1.7 1.4 3.1 0.4 0.5

cd+

I@+

85 41 61 54 118 65 220 73 830 230 210 62 80 43 70 34 77 44 69 34 64 34 IO5 116 56 34 69 20 54 lb 59 22 63 18 35 ?.I 59 14 61 13.5

sio, 16 24 28 25 32 27 13 10 13 13 12 67 9.9 1.5 7.6 7.4 10 7 7.7 9.4

64 51 38 12 42

41 43 11 7.8 11

21 22 5.2 10

56 58 0.8 2.2

19 28 0.2 0.3

11 12

10

729

Great Salt Lake geochemistry. I many analyses covering a wide range of flow conditions. In Fig. 2 we have plotted chloride content VS. discharge for single (circles) and multiple (bars) analyses. The solid curve is calculated for a constant chloride contribution by Malad Basin springs of 1.5 x 10” g/yr, diluted by various amounts of Bear River water. Background Cl- was estimated for the Bear River at site 35 of HAHL and MITCHELL(1963) which is above the confluence with the Malad River. The data on Fig. 2 indicate a constant Cl- input by the springs in the Malad drainage, concentration being inversely proportional to discharge as noted by WADDELL and PRICE (1972). The entire flow of the Bear and Malad Rivers does not, however, pass station 4 and actual spring input of Cl- is estimated at 2.8 x 10” g/yr on the basis of fewer time synchronous data points for stations 1, 2, 3, and 4. The input of other dissolved constituents by the springs is estimated from the proportions relative to Clfrom the analyses of the Malad River and springs; total annual input is shown in Table 2. The annual input of Cl- due to human activity and recycling within the basin is estimated at -2 X 10” g annually. This estimate is based on the compositional changes of the rivers as they flow through agricultural, mining and urban areas. This is significant relative to the natural annual input, but the additional input over the period during which the basin has been occupied by man (less than 2 X lOI g) is insignificant relative to the total mass of Cl- within the lake (2.65 X lOI5 g). LAKE COMPOSITION Analyses of lake waters were compiled by HANDY and HAHL (1966). In 1965, a detailed sampling Program was begun by the U.S. Geological Survey and the Utah Water Resources Department. Results are given in HAHL and HANDY (1969) WHELAN (1973), WHELANand PETERSON (1975, 1977) STURM (1980) and BUI-S (1980). Detailed information is

SO”X.3

springs

A*0

cl-

soz-

mxj-

2.80

0.082

0.055 0.03

0.02 0.07 0.882 4.955

Rivers

2.3

0.30

PCZCiP. Tot.1

1.1 3.4

0.01 3.11

0.78

m*

d

cz?+

&?+

siop

1.69

0.095 0.10

0.027 0.004

0.31 0.007 2.007

0.03 1.26 0.006 0.02 0.131 1.38

0.32 0.21 0.004 0.0007 0.351 0.2147

Dimsolved species are prcaenred in Srau 10'2.

x 1O'l; !I20 in litres x

also available on changes in lake elevation through time (ARNOW, 1980). Since the estimated input of salts to the lake during the historic period is insignificant relative

to the mass of salts in solution in the lake, the composition of the lake brines historically can be examined without consideration of salt input. The recorded lake elevation changes are assumed to be the result of fluctuations of water input (dilute inflow) and water output (evaporation). HANDY and HAHL (1966, p. 139) have shown that lake concentration is inversely proportional to lake elevation (volume). The lake elevation is shown on Fig. 3 for 1850 through 1980. Concentration factors for the lake are also shown, relative to the 1930 lake volume, on Fig. 3. The volumes were calculated using the volume/elevation curve of ARNOW (1980) and concentration factors calculated relative to the 1930 volume. Data for Cl- concentration relative to the 1930 concentration are plotted on Fig. 3 and appear to confirm the simple volume-concentration relationship prior to 1955, although large gaps are present in the data. The precipitation of halite in the lake beginning around 1955 controlled Cl- in the lake brines. This resulted in significant deviation of lake brines from the simple volume-concentration relationship (Fig. 3). Precipitation of both halite and mirabilite has

4215 2

I

I

I

I

I

4210

1

41901 1850 2.0,

I 1875 I

I 1900 I

I 1925 I

I 1950 I

I 1975

I

11

8

2000

IOW

DISCHARGE

kfs)

FIG. 2. Concentration of c1- at she 29 (Bear River at the Bear River Bird Refuge) of HAHL and MITCHELL (1963) shown as a function of discharge. Open symbols are single analyses, solid symbols indicate multiple analyses. The curve is drawn for a constant mass of Clinput by the Malad Basin Springs, being diluted by Bear -. (site 4, Fig. 1)

Rover water.

0.0’ 1850

I

I

1875

1900

I

1925 YEAR

I

I

1950

1975

I

FIG. 3. Fluctuations in lake elevation from 1850 through 1980 are shown from data given by HAHL and HANDY ( 1969) and ARNOW(1980).Lower curve is calculated concentration factor (C.F.) relative to 1930 lake volume. Circles represent Cl- concentration of lake brines relative to 1930 concentration. Horizontal line represents halite saturation.

been reported

in the lake during the post- 1930 period

(EARDLEY, 1966: WHELAN. 1973: BUTTS. 1980). and

is also predicted to have occurred between 1903 and 1905 (Fig. 3). Mineral precipitation profoundly affects brine composition and brine evolution (HARDIE and EUGSTER, 1970; EUGSTER and HARDIE. 1978). The data, therefore. will be evaluated in terms of lake concentration in proportion to volume and mineral precipitation as a result of evaporative concentration. The concentration of Cl- in the lake brines will be examined to interpret the effects of halite precipitation. The concentrations of Na’. SO:-, Ca” and HCO; will also be affected by salt precipitation (halite. mirabilite and aragonite). Both Mg” and K’ appear to be for the most part conservative in the water column with only secondary removal through reaction with part&dates. The year 1930 is taken to represent a well-mixed lake and concentration factors are estimated from hydrologic data for subsequent years relative to 1930. Figure 4 contains data for the shallow South Arm and North Arm brines given in Table 3. Assuming a completely mixed system, brines would plot along lines A-B (below halite saturation) or B-C (at halite saturation) on Fig. 4. Between 1930 and 1935 the lake elevation fell and the theoretical concentration factor would be 1.45 (Fig. 3). The lake remained at

low levels until 1945 when 11 began to nsc: to near the 1930 level by 1Y.57. The lake appears to have been nearly mixed. with the halite precipitated prior to 1945 redissolved. b> 1954 (Fig. 4). The lake again dechned until 1963 with the theoretical concentration factor reaching 1.8 (Fig. 3. point D on Fig. 4). The lake was divided by the railway causeway in 1959. The analyses from 1060 and 1963 (Fig. 4) show the effect of this division.

with both arms at halite saturation but higher concentrations of the conservative elements in the North Arm. From 1963 through 1978 the lake elevation increased, resulting in dilution of the brines. The North Arm brines appear to have remained as concentrated even during times of lake elevation increax, while South Arm brines became less concentrated. The difference must lie in the inflow to the two brine bodies. The South Arm received dilute (
for 1964. 1975 and

1978 appear

r----r-- ----r----7

60

80

120

140

160

120

140

Cl (g/kg)

FIG. 4. The predicted behavior of conservative components (K’, M$‘) relative to Cl- is shown by the lines A-B and EC for undersaturation and saturation with respect to halite. Concentration factors are calculated with respect to the 1930 lake values using the data of Table 3. The effect of the causeway construction and halite precipitation may be seen in the North and South Arm brine bodies beginning in 1960. Lkep North Basin and South Basin brines differ markedly in composition; North Basin brines represent a mixture of North Arm and shallow South Arm brines (as illustrated by the dashed line 1965 1) with minor Cl- enrichment due to halite dissolution, and deep South Basin brines are almost entirely the result of halite dissolution in shallow South Arm brines (illustrated by the dashed line 1965 2).

to fall

731

Great Salt Lake geochemistry. 1

Date 1930 Dct 1954 Ott 1960 No" 1961 Dac 1963 July 1964 Ott 1965 by 1966 act 1973 Ott 1975 Ott 1978

nAr

1 2 3 4 s *

South Arm Sallow Source Site Cl- K+ 1 1 1 2 2 2 2 2 3 4 5

123 120 129 143 123 150 lrve 147 *ve 149 be 121 *ve 118 Ave 104 Ave 69.3 Ave 63.8 Ave 72.7

3.38 4.07 4.43 5.25 5.08 4.68 4.75 4.20 3.17 2.88 3.27

I&+ 5.70 6.94 7.64 9.39 8.98 7.34 7.73 7.18 5.31 4.12 4.57

North Basin South &sin D=P+ _D=P+ K IQ*+ Site Cl- K ns*+ Site Cl

123 149 5.57 9.44

20 25

145 4.91 7.68 141 4.40 a.37

15 15 15

145 6.15 10.40 144 6.48 11.30 142 6.66 11.20

lG

112 3.93 6.75

lF

135 4.83

9.10

Ave 154 7.56 12.82 Ave 150 7.02 11.70 Avve 148 7.64 12.38 Ave 147 7.02 11.78 Ave 154 6.19 10.71 Ave 153 7.44 9.83 Ave 150 6.89 9.81

tLh1 and Mitchell(1963) bhl md Nmdy (1969) uhclan (1973) IJhAm and Peterson(1977) sturm (1980) This study

along a simple without

North Am slJa11$u Site Cl K I&*+

dilution line of the 1963 brines halite dissolution, however, the remaining

data for shallow South Arm brines fall along a curve above this (Fig. 4). This indicates that the shallow South Arm brines are a result of mixing dilute river inflow, the previous South Arm brines and North Arm brines, with little halite dissolution. In addition to the causeway, which divides the lake into a North and South Arm, the lake is divided by a natural ridge into a North and South Basin. The ridge lies to the south of the causeway as shown on Fig. 5 and therefore the South Arm includes parts of both the North and South Basin. HANDY (1967) recognized distinctly different brines within the South

Arm, the locations of which are shown for October 1965 on Fig. 6. The position of salt beds, reported by WHELAN (1973), during July and August 1969 are also shown on Fig. 6. Cores collected during I978 through 1981 encountered salts only at location F, Fig. 5; the salts consist of halite, mirabilite and gypsum. HANDY (1967), HAHL and HANDY (1969) and WHELAN (1973) have interpreted the deep North Basin brines of the South Arm to be the result of underflow of dense North Arm brines into the South Arm. HAHL and HANDY (1969) interpreted the deep South Basin brines to be a result of groundwater inflow, while WHELAN(1973) interpreted these brines to be the result of migration of North Arm brines along the bottom of the South Arm dissolving halite. Whelan recognized only 3 brine types between 1966 and 1975: the North Arm, shallow South Arm and

q

NORTH BASIN DEEP BRINE

@

SOUTH SASIN DEEP BRINE

WlJlH

FIG. 5. Great Salt Lake is divided into a north and south arm by the railroad causeway running from Promontory point on the east to Lakeside. Cores were collected from the locations shown in the south am (outlined in heavier lines). A subaqueous ridge extends from Canington Island toward Promontory Point (dotted line) dividing the lake into a north and south basin.

ARM OREAT SALT LAKE

The position of the deep North Basin (horizontally ruled area) and the deep South Basin (stippled area) brines, reported by HANDY (1967) for October 1965, in the South Ann of Great Salt Lake are shown. The position of salt beds (diagonally ruled areas) in the South Arm of Great Salt Lake in July-August 1969 reported by WHELAN (1973) are FIG. 6.

also shown.

732

K .I

Spencer t7 UI

deep South Arm brines (WHELAN. 1973: WHEUN and PETERSEN, 1976. 1977). During this study four distinct brines were recognized in 1978 (Fig. 4). with both deep North Basin and deep South Basin brines

present. Examination of sample locations from WHO:LAN (1973) and WHELANand PETERSEY ( 1975. 1977) indicate no sampling sites in the heart of either the deep South or deep North Basins between 1966 and 1975. The deep South Arm brines located in the North and South Basins differ significantly in composition. The deep North Basin brines appear to be a result of mixing shallow South Arm and North Arm brines with a minor component of salt dissolution. Figure 4 shows a mixing line for 1965 North and South Arm brines; the deep North Basin brines are slightly enriched in Cl- relative to this line. indicating some salt dissolution. Deep North Basin brines for 1964. 1966 and 1978 are offset from the North Arm-South Arm mixing curves for these years in a similar manner to the 1965 brines. From this it appears that the deep North Basin brines are constantly replenished. Deep South Basin brines appear to be the result of halite dissolution in the shallow South Arm brines. The 1965 deep South Basin brines have approximately the same K+ and M2+ concentrations as the shallow South Arm brine, however the Cl- content is much higher in the deep brines (Fig. 4), indicating the halite dissolution. Halite remained in the South Basin at least through 1969 and probably through the early 1970s. The brines probably remained near the 1965 and 1966 values (Fig. 4) until the halite dissolved. The 1978 deep South Basin brines appear to be the result of mixing of the Cl- enriched brines of the mid 1960s with shallow South Arm brines of the mid 1970s. There appears to be little influence of North Arm brine influx into the deep South Basin brine pocket. From this brief account it should be obvious that the composition of individual lake brines is the product of complex interaction of evaporative concentration, mineral precipitation, mineral dissolution and mixing. Because it is close to mineral saturation, the system is dynamic and in flux on a yearly scale, and changes can be rapid and substantial. In a shallow lake such as Great Salt Lake. precipitation of soluble minerals appears to be a major mechanism for initiating and mainlining density stratification. For this reason, it is of interest to estimate the lifetime of a salt layer or stratified brine created by a single low stand. TOTH and LERMAN (1975) have discussed salt movement in several lakes. In lakes with a permanent ice cover, in which molecular diffusion is the dominant mechanism of brine mixing, stratification occurs for periods of thousands to tens of thousands of years. At Great Salt Lake the South Arm salt beds (Fig. 6) dissolved within about ten years. This is in part due to the role of wave energy entraining the

lower dense brines. Simple mixing of water. dnvcn by the propagation of wind derived stress ~rthin thz water column, results in a mixing ratt orders <)f magnitude greater than molecular diffusion. with

diffusion coefficients on thr order of 10 nn’/sec (LERMAN. 1971: RL.ANTO~, 1973). In this case salt dissolution would occur in much less than one year. much faster than the obsened rate at (itcat Salt Lake. TOTH and

LERMAN ( 1975) have interpreted salt in the Great Bitter Lake to be controiled by diffusion through a lower boundary layer. The lower dense brine at Great Salt Lake is approximately one metre in thickness. assuming molecular diffusion through this layer to be the dominant control on salt dissolution. and using a diffusion coefficient of 1.5 X IO-’ cm’/sec (ROBINSON and STOKES. 1970) the rate of salt dissolution is

dissolution

SDa

1.5 X IO-’ cm’s ’ (3.16 x 10’ s yr ’ ) 100 cm = 4.75 cm yr-‘.

This value is close to the estimated dissolution of the 20 cm of salt in the South Basin between 1969 and 1972 (WHELAN, 1973). Following the salt dissolution, deep South Basin brines appear to have continued mixing with shallow South Arm brines through molecular diffusion. It appears that even in a broad, shallow system. such as Great Salt Lake, stratification from salt precipitation and redissolution may persist for tens of years. PORE FLUID CHEMISTRY Pore fluids were obtained from sediment cores of six (C, D, F. G, H and I) of the thirteen stations sampled in the South Arm (Fig. 5). Soon after collection, cores in their plastic liners were cut into 10 cm sections in a glove bag flushed with nitrogen. Pore fluids were extracted anaerobically through a millipore filter in a squeezer, the piston of which was driven either hydraulically or with gas pressure. Density, pH. alkalinity and transient solutes, such as sulfide and ammonia, were measured immediately. The samples were then acidified for later analysis of the major solutes. Complete chemical analyses have been listed in SPENCER( 1982). Within analytical uncertainty, the concent~tion of each species in the uppermost pore fluid sample for each core matched that of the immediately overlying lake brine. Chloride ranged from 90 g/kg to 144 g/kg within the pore fluids, with a maximum within the upper metre of sediment and a minimum at the maximum core depth. Two contrasting profiles are illustrated in Fig. 7. Core C is from a South Basin location. while core F is from the North Basin-South Arm (Fig. 5). At the latter location, halite was encountered at the sediment-water interface, presumably preserved because of its proximity to the causeway and input of North-Arm brines, or transport as

Great Salt Lake geochemistry. I C

F

IO Cl-(g/kg)

FIG. 7. The concentration of c1- in the pore fluids of cores C (squams) and F (circles) shown as a function of the stratigtaphy. Notice that con C penetrates all five major stratigraphic units while core F penetrates only unit I; both cores contain the same tephra layer Tl. The similarity in

20

30

S042-(g/kg) FIG. 8. The concen~tion of Se- in the pore &ids of cores C (squares) and F (circles) shown as a function of the stratigraphy. The maxima in the profile of core F are due to the presence of mirabiiite (NatSO,- 10H20) in the salt beds at the top and bottom of the core.

the profiles indicate that the pore fluids are not controlled by the original depositional fluids.

detrital halite. In any case, the uppermost pore fluid value is compatible with halite saturation. At core location C the chloride maximum is within the sediment column and the South Basin brines obviously have a diluting effect. The SOi- profiles are similar, with the exception of the second maximum at the bottom of core F (Fig. 8). The SOi- range is from 2.69 g/kg to 40.0 g/kg. The Nat, Mg’+, K+, and Br- profiles are comparable either to those for Cl- or SO:-, with Na+ ranging 56. I g/kg to 91.6 g/kg, K” from 2.6 g/kg to 4.4 g/kg, while Mg2+ varies from 2.38 g/kg to 7.16 g/kg and bromide from .03 g/kg to . 10 g/kg. In Fig. 7, the sedimentary units I-V as defined by SPENCER f 1982) have been indicated as well as volcanic tephra layer (T,). While core F is entirely within unit I, core C penetrated the whole sequence, representing over 30,000 years of depositional history (see SPENCER et al., 1984; SPENCER,1982). Unit III was deposited in a deep, fresh lake. Obviously, the pmsentday pore fluid compositions have no relation to the original lake compositions at the time of deposition. Both compositions are a product of processes which have occurred over thousands of years, but the changing hydrologic conditions recorded historically have resulted in significant changes in the upper portion of the pore fluids. What, then, do the pore fluids represent? Unlike the lake water mass, mixing resulting from wind derived stress does not occur in pore water and compositional profiles may be modeled in terms of

one-dimensional diffusion with appropriate boundary conditions. As a first approximation, we have omitted charge balance constraints which must apply during mass transport, and have considered only individual ions. For the chloride profile of core F (Fig. 9), we have assumed that the halite bed at the surface buffers Qnear saturation (I50 s/kg). Considering the history

1 a iF 8

1

I

I20

130

I

J

140

Cl- (Qhg) FIG. 9. The Cl- pore fluid analyses for core F are shown as a function of depth @ircies). The dashed curve is the estimated 1933 profile, &or to halite precipitation. The solid curve is the calculated diffusion profile (see text), assuming the bottom waters remained at or near halite saturation from 1933 to 1979.

of the lake level changes (see Fig. 3). and the low stands of the mid 1930s to 1940s and the late 1950s to 196Os, we may assume that the sediment-water interface at site F has remained near halite saturation from 1933 to 1979. Therefore, a boundary condition at zero depth of 150 g/kg is used from 0 to 47 years (1933-1979). The second boundary condition is that of Clconcentration with depth at time zero ( 1933). At that time. the lake is estimated to have contained 120 g/kg Cl-. Prior to 1933. the lake elevation fluctuated; however, the average composition appears to be near 120 g/kg Cl- during the historic period prior to 1930. The dashed curve is fitted through the data points of core F below 2.5 metres and the 1930 lake composition at the sediment-water interface: a linear profile is used. With these two boundaries. the present profile is fit using a solution for diffusion in a semiinfinite medium with a fixed concentration at depth equal to zero given by CRANK ( 1975. p. 2 1) as: d AC, = AC, erfc 2fi where AC0 = 30 g/kg; the change in Cl- at d (depth) = 0 from I = 0 to t = 47 yrs; AC, = ACl- as j(d); AC’, = 120 - .02d at t = 0: d = depth in cm: I = time (set); D* = apparent diffusion coefficient (cm’/sec). The profile at time equals 47 years is then fit (Fig. 9) and a solution for D* obtained. In the case of core F, D* is approximately 3.8 X 10e6 cm’/ set, as compared with D = 1.52 X 10e5 cm’/sec in 3 molar NaCl solution (ROBINSONand STOKES. 1970). In a porous medium the apparent diffusion coefficient will differ from that in a pure solution by some factor (K) of the medium D* = KD.

Using the values of D and D* above, K is calculated to be 0.26, a reasonable value for a porous sand-mud mixture (see LERMAN and WEILER, 1970: estimated K range 0. I-0.5). Mathematical solutions for the profiles in cores C. G, H, and 1 are obtained in a similar manner. The initial boundary conditions as a function of depth are estimated from the 1930 lake composition and the profile below 2.5 m in the same manner as those for core F. The boundary conditions at the sedimentwater interface have changed with time at these locations. Using the lake composition data discussed previously, it is estimated that at location G halite precipitated in 1933 and bottom waters remained at 150 g/kg Cl- until 1969. From 1969 to 1975 the estimated Cl- concentration was 120 g/kg, and from 1975 to 1979, 115 g/kg. The diffusion solution is therefore: C, = 30 erfcx, - 30 erfcx2 - 5 erfcs, where

d x=m

for _v,. 1 = 47 ys; .x1. : I I yrs: .x1. i 7 ,_(>rs: the fit of this curve (using D* from core F) fhr rhe data points from core G is shown in Fig. IO. The curves for cores C. H. and I are also constructed using the boundac conditions estimated from the past lake composition at these locations, ‘These are 150 g/kg CT from 1933 to 1956. I I5 g/kg from 1956 to 1960. 135 g/kg from 1960 to 1964. 120 g/kg from 1964 to 1975 and I I5 g/kg from 1975 to 1079, The

equation used is AC, = 30 erfc.t-, -- 35 ert‘c.rz t 20 erfcsl -

15 erfcx4 -.- .i erfc.vc.

for x, , 1 = 47 yrs; x2. f = 24 yrs; sl, [ = I9 yrs: _t-*, I = 15 yrs: x5, f = 4 yrs: for the most part the curves on Figure 10 fit the data quite well. We may conclude that the Cl- pore fluid profiles are consistent with a simple diffusion model, the major controls on which are 1) the Cl- concentration of the lake waters over long periods at the site, as manifested in the profile below 2 m which is not very sensitive to short term fluctuations or the original water composition; 2) the Cl- concentration of the lake mass at the site over shorter periods, which in this system is largely controlled by the presence or absence of halite and resulting stratification. Fluctuations occurring over a period of less than 50 years are recorded within the upper 2 m, whereas the lower portion of the profile apparently is defined by a timescale measured in centuries or millenia. The other major ions may be modeled in a manner analogous to the Cl- profiles. In the lake waters Na’ and SOi- behave similarly to Cl-. Their concentrations are controlled largely by salt precipitation and dissolution (halite and mirabilite) resulting in stratification. The pore fluid behavior of Na+ and SO:- is similar to that of Cl-, except that in the North Basin pore fluids, such as those of core F (Fig. 8) a maximum is encountered for these solutes at depth due to dissolution of a fossil mirabiiite layer which accumulated during the rapid drawdown of the lake near the end of the Pleistocene. Judging from the profiles for core C, no such deposition took place in the South Basin (see also SPENCER, 1982; SPENCER et al., 1984). Pore fluid profiles of other solutes conserved in the lake mass may also be modeled by diffusion calculations. The lake water boundary conditions vary relative to those for Cl-, SO:-, or Na+. Calculated profiles for K+, Mg*+, and Br- are consistent with the diffusion model. We recognize that our single ion diffusion model is inadequate. The constraint of electrical neutrality and the heterogeneity of the pore fluids must be considered. and work is in progress on a more sophisticated model. Meanwhile, the preliminary Calculations provided an excellent fit to the data points,

Great Salt Lake geochemistry, 1

735

Es&

T 0

1. :0 I

c

0

0

:

:

t

I

f

0

I

IO

Cl-

(g/kg)

FIG. 10. The Cl- pore fluid analyses for cores C, G, H and I are shown as a function of depth (circles). The estimated 1933 profiles are shown by the dashed lines, the calculated 1979 diffision profiles are

shown by the solid curves. Diffision calculations are explained in the text.

ju~if~ng to some extent the ~mplif~ng and pointing to a diffusion control.

assumptions,

SUMMARY OF THE MAJOR SOLUTE BEHAYIOR The present shallow saline perennial lake within the basin reflects a dynamic system. The level of Great Salt Lake has fluctuated in response to minor climatic changes over the past 150 years of historic record. The major sources of water to the lake are river discharge and atmospheric precipitation onto the lake surface. Significant amounts of dissolved species are also con~but~ by hydrothe~~ springs, such as those of the Malad River basin. As a result of the historic lake elevation changes, salts, mainly halite and mirabilite, have precipitated and redissolved, complicating lake chemistry through the formation of stratified brines. Although the present stratified system is in part an artifact of the construction of the railroad causeway, the lake has become stratified by natural processes in the past. Examination of lake waters and pore fluids has suggested several processes which are occurring in the system. The behavior of the major solutes is summarized below. Chloride. Within the lake system, cl- may be enhanced or depleted in various brine bodies. Pluctuations of the lake volume have caused halite precipitation and dissolution to occur during the historical period. Assuming that a mixed lake represents “normal” brines, Cl- enriched brines may form as a result of halite dissolution whereas halite precipitation results in brines deficient in Cl- relative to “normal” brines. Brines may be separated within the lake through density stratification. The pore fluid Cl- concentrations are a result of mass transport. The upper boundaries are controlled by conditions within the lake and are especially

affected by the presence or absence of halite and/or stratified brines. in the absence of buried halite beds, the major removal of Cl- from the lake is diffusion into the pore fluids. Sulfate. The behavior of SOi- within the lake system is very similar to that of Cl-. The precipitation and dissolution of mirabilite leads to the formation of Sa- depleted and enriched brines respectively, which may be separated through density stratification. Along with lake volume changes, temperature fluctuations lead to mirabilite precipitation and dissoiution. Like the Cl- profiles, the SO$ pore fluid profiles are mass transport controlled and are most strongly influenced by stratified lake brines and/or mirabilite. Pore fluids, gypsum and mirabilite, represent sinks and sources of SOi- in the sediments. Although SOi- has been removed from solution in the past by these salts, at present mirabilite of the North Basin is dissolving. Carbonate. Carbonate alkalinity accounts for a very small portion of the anion content of the lake brines. Although fluctuations in the lake volume have occurred, the alkalinity appears to have remained constant. Aragonite is abundant in the sediment at and near the sediment-water interface and the lake remains near aragonite saturation. Calcium. Historically Ca*+ appears to have remained at relatively constant low levels within the lake. This is probably controlled by aragonite solubility. Within the sediment, gypsum dissolution may cause local maxima in the Ca’+ concentration. Magnesium. Magnesium is a major constituent of the lake brines. Magnesium salts have not precipitated during historic lake fluctuations. Precipitation of salts (halite or mirabilitef result in Me enriched brines, whereas salt dissolution results in Mg*+ deficient brines relative to Cl-. Significant removal of M$ from the lake at the present time occurs through

diffusion into pore fluids and incorporation into diagenetic mineral phases. Sodium. The behavior of Na’ within the lake follows that of both SOi- and Cl . Sodrumdepleted brines form as a result of halite or mirabilite precipitation while Na’ enrichment is caused by dissolution of these salts. The concentration of Na’ in the pore fluids also follows those of Cl- plus SOi The major removal of Na’ from the lake appears to be diffusion into the pore fluids and salt formation. Potassium. The behavior of K’ in the lake is very similar to that of Mg”. Removal of K* from the lake occurs through diffusion into the pore fluids as well as exchange and fixation on clays. The exchange reaction probably occurs both within the lake and pore fluids. of the results presented here were taken from the Ph.D. thesis submitted by R. J. Spencer to Johns Hopkins University and supported by NSF grant EAR-7903875. Acknowledgement is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of this research. Our thanks to Ted Amow, Doyle Stephens, Briant Kimball and Rich Fuller of the Utah District, Water Resources Division, U.S.G.S., and to Paul Sturm of the Utah Mineralogical and Geological Survey for local support. We are grateful also to Mitch Larson and Rangers of the Utah Dept. of Parks and Recreation for help in coring operations. using the R/V Gilbert. Alan Riggs and Ray Kablanov provided diving assistance under difficult conditions. The work of Mary Jo Baedecker, Rick Forester. Marty Goldhaber. Kerry Kelts, Judy Mckenzie, Dave Madsen. Meyer Rubin and Carl Bowser on aspects of the Great Salt Lake core sediments and pore fluids has been essential background. We thank Kidd Waddell and Dan Hahl for review of the manuscript, and Carolyn Moss for assistance with the illustrations. Acknowledgements-Some

CRANK

J. (1975) T/IV Muthemaru.c of Dlttusurn ()xlorc! Univ. Press. EARDLEY A. J. (1966) Sediments of Great Salt Lake. ii: Great Salt Lake. Utah. (;cr~/ .S~JC,G’uidrhook I<) (;vo/ oi L:fah (20). 105-120.

J.. GVOSDETSK~ V. and MARSELLR. 1. ( I957 b Hydrology of Lake Bonneville and sediments and soils 01 its basin. Geol. Sort. .4mcr Buli. 68, 1 141-1201. EUGSTER H. P. and HARDIE L. A. (1978) Saline Lakes. In EARDLE’~.A.

ldzJs-

C‘hemrstr~~Geo/og.K f’h~srcs (ed. A. 1.. Lr RMAN)

pp. 237-294. Springer-Verlag. EUCSTER H. P. and JONES B. F. (1979) Behavior ot maJor solutes during closed basin brine evolution. drncv .I S’cr 279, 609-63 I. FETH J. H.. R%ERS

S. M. and R~BERSONC. t. (1964) Chemical composition of snow in the northern Sierra Nevada and other areas. C’.S. Gcol. Surr. Uhter-Suppb Paper lF3.(-.I GILBERT G. K. (1890) Lake Bonneville. L’.S Grlrl! Sari Monograph 1. 438 pp. HAHL D. C. (I 968) Dissolved-mineral inflow to Great Salt Lake. C’tah Geol. and Mineral Sun.. Wat. Res. Bull. IO. 35 PP. HAHL D. C. and MITCHELLC. G. (1963) Dissolved-mineral inflow to Great Salt Lake and chemical characteristics of the Salt Lake brine; Part I. selected hydrologic data. Utuh Geol. and Mineral Sun,.. Wat. Res. Bull. 3, 40 pp. HAHL D. C. and LANGFORDR. H. (1964) Dissolved-mineral inflow to Great Salt Lake and chemical characteristics of the Salt Lake brine; Part II. Utah Geol. and .Mineral Sun.. Wur. Res. Bull. 3. 40 pp. HAHL D. C. and HANDY A. H. (1969) Great Salt Lake, Utah: a chemical and physical variation of the brine 1963-1966. Utah Geol and Mineral Surv.. U’at Re.s. Bull. I-7. 33 pp. HANDYA. H. (1967) Distinctive brines in Great Salt Lake, Utah. C’S Geol. Sun,. Pro/I Paper 575-B. 225-227. HAND). A. H. and HAHL D. C. (1966) Great Salt Lakechemistry of the water in The Great Salt Lake. C’tah Geol. Sot. Guidebook to Geol. of Utah (20). I35- I5

I.

HARDIEL. A. and EUGSTERH. P. (1970) The evolution of closed-basin brines. Mineral. Sue. Amer. Spec. Publ. 3, 273-290.

Editorial

handling: J. I. Drever

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ARNOW T. and JENSEN L. J. (1977) The historic level of Great Salt Lake, Utah, in Desertic Terminal Lakes (ed. D. C. GREER). Proc. Int. Conf Desertic Terminal Lakes. Mav 2-5. 1977. Ogden. Utah, 73-79. BERN~RR. (1980) E&y Diagenesis: A Theoretical Approach. Princeton Univ. Press, 241 p. BLANTONJ. 0. (1973) Rates of vertical entrainment in stratified lakes. in Hydrology of Lakes Symposium, lnr Assoc. Hydrol. Sci. 109, 301-305. BROECKERW. S. and WALTONA. (1959) Re-evaluation of the salt chronology of several Great Basin Lakes. Gc>ol. Sot. Amer. Bull. 70, 601-618. Buns D. S. (1980) Factors affecting the concentration of Great Salt Lake brines. In Great Salt Lake: A Scientific Historical and Economic Overview (ed. J. W. GWYNN). Utah Geol. and Mineral Surv. Bull. 116. 1 16 168. COLE D. R. (1982) Tracing fluid sources in the east shore area, Utah. Groundwater 20, 586-593. CLARKEF. W. (1924) The data of geochemistry. U.S. Geol Surv. Bull. 770. 783 pp.

JONESB. F. and VAN DENBLJRGHA. S. (1966) Geochemical influences on the chemical character of closed lakes. In1 A.~soc Sci. Hydrol. no. 70. 435-446. LANGBEIN W. B. (1961) Salinity and hydrology of closed lakes. L:.S Geol. Surr Prqf: Paper 412, 20 pp. LERMANA. ( 197 I ) Time to chemical steady-states in lakes and ocean. Adv. Chem. Ser. 106, 30-76. LERMANA. (1979) Geochemical Pr0cesse.s: Water and Scdrmenr Envrronments. Wiley, 48 I p. LERMANA. and JONES B. F. (1973) Transient and steadystate salt transport between sediments and brine in closed lakes. Limnol. and Oceanog. 18, 72-85. LFRMANA. and WEILER R. R. (1970) Diffusion and accumulatron of chloride and sodium in Lake Ontario. Proc. 13th Conf Great Lakes Re,s.. Int. Assoc. Great Lakers Re.s I, 477-489.

MURPHY P. J. and GWVNN J. W. (1979) Geothermal Investigations at Selected Thermal Systems of the Northern Wasatch Front Weber and Box Elder Counties. Utah. Ulali Geol. and Mineral Surv. Invest. 141. 50 pp. RICHARDSONG. B. (1906) Underground water in valleys of Utah Lake and Jordan River, Utah. II.S. Geol. Surv Water-Supply Paper 157. 87 pp. ROBINSON R. A. and STOKES R. H. (1970) Electrolyte Solutions. Buttenvorths. 559 pp. SPENCERR. J. (I 982) The geochemical evolution of Great Salt Lake, Utah. Ph.D. dissertation, The Johns Hopkins Univ.. Baltimore, MD, 308 pp. SPENCERR. J.. BAEDECKER M. J., EUGSTERH. P.. FORESTER

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R. J., EUGSTERH. P. and JONES B. F. (1985) Geochemistry of Great Salt Lake, Utah, II: PleistoceneHolocene evolution. Geochim. Cosmochim. Acta 49,739-

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STURM P. A. (1980) The Great Salt Lake brine system. In Great Salt Lake: A ~c~e~i~c. Historical and Eronomic Overview fed. J. W. G&N). Utah Geoi. and Mineral Surv. Bufl. 116, 147-163. TALMAGEJ. E. (1890) The Great Salt Lake Present and Past. Desert News. I 16 pp.

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WADDELLK. M. and PRICE D. (1972) Quality of surface water in the Bear River Basin, Utah, Wyoming, and Idaho. U.S. Geol. Surv. H.vdrol. Inves. Atlas #A-417. WHELANJ. A. (1973) Great Salt Lake, Utah: chemical and uhvsical variation of the brine. 1966-1972. Utah Geol. anb Mineral Surv., Wat. Res. Bull. 17. 24 pp. WHELANJ. A. and PETERSONC. A. (1975) Great Salt Lake. Utah chemical and physical variation of the brine, wateryear 1973. Utah Geoi. and Mineral Surv.. War. Rex Bull. 20, 29 pp. WHELANJ. A. and PETERSON C. A. (1977) Great Salt Lake. Utah: chemical and physical variation of the brine. wateryears 1974 and i 975. Utah Geol. and Mineral Sun:. U.ur. Res. Bull. 22, 57 pp.