Uranium and radium mobility in groundwaters and brines within the delaware basin, Southeastern New Mexico, U.S.A.

Chemical Geology (Isotope Geoscience Section), 72 (1988) 181-196 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

181

URANIUM AND RADIUM MOBILITY IN GROUNDWATERS AND BRINES WITHIN THE DELAWARE BASIN, SOUTHEASTERN NEW MEXICO, U.S.A.*I ANDREW L. HERCZEG 1'.2, H. JAMES SIMPSON 1'2, ROBERT F. ANDERSON 1, R O B E R T M. T F J E R 1, GUY G. M A T H I E U 1 and BRUCE L. DECK 1 ~Lamont-Doherty Geological 9bservatory o[ Columbia University, Palisades, N Y 10964-0190 (U.S.A.) 2Department of Geological Sciences, Columbia University, Palisades, N Y 10964-0190 (U.S.A.) (Received October 9, 1986; revised and accepted August 11, 1987)

Abstract Herczeg, A.L., Simpson, H.J., Anderson, R.F., Trier, R.M., Mathieu, G.G. and Deck, B.L., 1988. Uranium and radium mobility in groundwaters and brir.es within the Delaware Basin, southeastern New Mexico, U.S.A. Chem. Geol. ( Isot. Geosci. Sect. ), 72:181 - 196. Concentrations of naturally occurring isotopes of U and Ra were measured in fresh waters, groundwaters and in sodium chloride brines within and near the Delaware Basin of southeastern New Mexico. We sought to determine the effect of high chloride concentrations in a wide range of redox conditions on the mobility of U and Ra in natural waters. Two important features of radionuclide mobility are evident from our results: (1) There is a slight tendency for U and Ra concentrations to correlate with the chloride content of the water samples. Thermodynamic speciation calculations suggest that this may result from complexation of these elements by C1 - 1ions. (2) Much more dramatic than the correlation with C1- concentration is the effect of the redox state of the waters on U and Ra concentrations. Chemically reducing groundwaters contained much lower U concentrations and much higher Ra concentrations than were ~leasured in oxic and suboxic samples. The low U values are consistent with the expected reduction of U (VI) to insoluble U (IV). We suggest that the increased Ra concentrations result from mobilization of Mn which eliminates the strongest adsorption sites for Ra thereby greatly enhancing its concentration in the anoxic waters.

1. I n t r o d u c t i o n

The investigation of the solubility and transport of radionuclides in natural waters has important application to the siting and management for long-term storage of nuclear *ILDGO contribution No. 4233. *2Present address: Environmental Geochemistry Group, Research School of Earth Sciences, The Australian National University, G.P.O. Box 4, Canberra, A.C.T. 2601, Australia.

0168-9622/88/$03.50

waste, the safe exploitation of U deposits and the understanding of the genesis of U ore deposits. Field studies are necessary to augment laboratory-scale experiments and thermodynamic stability calculations because the time scale involved in natural systems far exceed those possible in the laboratory, and the thermodynamic calculations cannot completely define adsorption-desorption processes or nonequilibrium behaviour. We present here the results of measured concentrations of the long-

© 1988 Elsevier Science Publishers B.V.

182


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et al., 1982, 1984; Anderson et al., 1982). This is in accord with the predicted stability of the uranyl-carbonate complex (Hostetler and Garrels, 1962; Langmuir, 1978). Dissolved U concentrations are typically very low in reducing groundwaters such as oil-/gas field brines (Osmond and Cowart, 1976 ) because of the reduction of the relatively soluble U (VI) to highly insoluble U (IV). No correlation with carbonate ion has been observed for Ra though there is some documentation of a linear relationship between Ra concentration and salinity (e.g., Gutsalo, 1964; Kraemer and Reid, 1984). Exceptionally high Ra activities have been documented in brines associated with hydrocarbon deposits (e.g., Armbrust and Kuroda, 1955; Alekseev et al., 1958; Bloch and Key, 1981; Kraemer and Reid, 1984).

2. General geology and hydrogeology

1'Skm

Fig. 1. Location of surface waters and groundwaterssampled during this study. The sampleidentificationnumbers correspondto those givenin Tables I, III and IV. lived isotopes 23sU, 2~4U and 226Ra in some surface waters and groundwaters in southeastern New Mexico. The aim is to determine the effect of very high chloride concentration and the effect of various redox conditions on the mobility of these nuclides. The rivers, lakes and groundwaters in southeastern New Mexico cover a wide range of salinity (freshwater to NaC1 saturation) and redox conditions (atmospheric 02 saturation to anoxic). Water samples were collected within the region covered by the Delaware Basin located near Carlsbad, New Mexico (Fig. 1) which comprises a portion of the Permian Basin of the southwestern U.S.A. The formations from which each water sample was taken are given in Table I and a discussion of their geological relationships follows in the next section. Previous studies showed strong enhancement of U mobility (by about two orders of magnitude) in CO~--rich lakes (e.g., Simpson

The Delaware Basin is a structurally downwarped basin of ~ 3-104 km 2 bounded to the north and west by the Capitan Reef, an extensive basin-margin reef deposit (Mercer and Orr, 1977; Ward et al., 1986). Within the upper portion of the Delaware Basin there are thick sections of evaporites (halite-anhydrite) that include the Ochoan Group (Fig. 2). The Castile, Salado and Rustler Formations comprise 1250 m of the main portion of the upper evap9rite sequence. The site of the U.S. Department of Energy Waste Isolation Pilot Plant ( W I P P ) is located in the lower part of the Salado ~ 655 m below ground level. The Rustler is the most important water-bearing unit of the three and consists of interbedded dolomite-anhydrite (Anonymous, 1980). Carbonate oil- and gasbearing strata of Late Permian age (YatesSeven Rivers-Queens Formations) directly underly the evaporites which in many instances act as a hydrocarbon trap. Below the thick Permian sequence lies a sequence of Carboniferous, Devonian, Silurian and Ordovician carbonates and shales which also contain hydrocarbons (Ward et al., 1986).

183 TABLE I Sample description and identification. Location of wells and surface waters are shown in Fig. 1 Sample

Description

Formation

Comments

WIPP site well H-6 WIPP site well H-4 Potash Co. of America mine well No. 3 salt lake Carlsbad City Water WIPP site well DOE-/ salt lake Laguna Grande (salt lake) windmill bore No. 1 windmill bore No. 2 Surprise Springs Pecos River Carlsbad Caverns Rattlesnake Springs Conoco recirculated brine private bore Conoco oil well State H-22 Conoco gas well Anderson Ranch Conoco gas well Wolfcamp Cities Service gas well AA No. 1 Cities Service gas well Govt. T No. 1 Cities Service gas well Govt. Z No. 1 Cities Service gas well Tracy B No. 1

Rustler Rustler Salado

salt (anhydrite/halite) salt (anhydrite/halite) salt (halite/sylvite/anhydrite)

Capitan Castile

carbonate salt (anhydrite/halite)

Capitan Capitan Salado

carbonate carbonate salt (halite/sylvite/anhydrite)

Capitan Capitan Capitan + Rustler Ogallala Y-SR-Q Devonian Wolfcamp Y-SR-Q Y-SR-Q Y-SR-Q Wolfcamp

carbonate carbonate

NO. ~'

1 2 5 6 7 8 9 10 11 12 13 14 15 17 18 19 20 21 22 23 24 25 26

sandstone dolomite sandstone cherty limestone shaley limestone dolomite/sandstone dolomite/sandstone dolomite/sandstone shaley limestone

Y-SR-Q = Yates-Seven Rivers-Queens Formation. All formations are of Upper Permian age except Wolfcamp Fro. (Lower Permian), Ogallala (?) and Devonian Fm. (Devonian). *New Mexico Survey No.

SW

NE

SANTA ROSA FEET

.............(

.........................DEwe~ L ~ E RE~SE~S

METERS [000

::2;} S~L:IDo====================== II

WIPP REPOSITORY

500

I000 M.S,L,

M.S.L.

-IOOO

-500

-20005

IO km

[ ] W A T E R - B E A R I N G UNITS

(HORIZONTAL SCALE)

Fig. 2. Generalized stratigraphy of the Upper Permian evaporite beds in the Delaware Basin. Castile, Salado, Rustler (evaporites) and Dewey Lake Redbeds Formations belong to the Ochoan Series. The section is a line running S E - N W across the plan map depicted in Fig. 1. (Adapted from Chaturvedi and Rehfeldt, 1984. )

The most dilute water-bearing unit within the Permian Basin is the Capitan Reef which provides much of the drinking and irrigation water for southeastern New Mexico. Wells drilled at the WIPP site have intersected water in the Rustler and Castile Formations that have total dissolved solids contents that range from 3-103 to > 3" 105 mg l - 1 ( Lambert, 1978; Anonymous, 1983). Pressurized brine has been encountered in the Castile Formation in several wells near the WIPP site (Chaturvedi and Rehfeldt, 1984). Ephemeral and perennial lakes that derive their salt from surface expressions of Permian evaporite deposits are essentially saturated sodium chloride solutions. Brines are also associated with hydrocarbon deposits (oil and gas ) in the Delaware Mountain Group that underlies the evaporite sequence. Groundwater

184 movement within the evaporite sequence is considered to be via fractures with water discharging into the Pecos River (Chaturvedi and Rehfeldt, 1984). The major source of concern cited by Chaturvedi and Rehfeldt regarding a potential breach in the W I P P site lies with the groundwater transport of high-level waste radionuclides through the Rustler Formation to the Pecos River.

lowed an accurate determination of absorbed C02 and thus the total C02 concentration of the water sample. Partial pressure of CO2 was determined by equilibrating ~ 2.4 1 of water with 400 ml of atmospheric air using an air pump that recirculated the air through the water. A 200-ml aliquot of the gas was returned to the laboratory for C02 analysis on a Perkin-E1mer ® gas chromatograph.

3. A n a l y t i c a l m e t h o d s

3.2. Radiochemistry

Water samples for chemical and radiochemical analysis were collected in the field by direct pumping from surface water bodies or from the well head of the groundwater bores. Groundwater samples were collected after the water had been flowing freely for several minutes to minimize contamination from the metal well head and maintain integrity of the dissolved gaseous phase. Water from the oil-/gas field brines was tapped from the bottom of separating tanks. Several water samples were collected at each site in polyethylene or glass bottles for return to the laboratory for chemical analysis of anions, total C02, and some cation species. 2.5 liters were collected for Pco2 equilibration. A further 20 1 of filtered water were collected for U and Ra determination and acidified to pH ~ 2 to prevent adsorption onto the walls of the plastic containers.

The 20-1 acidified water samples returned to the laboratory were analyzed first for 226Ra using the 222Rn in-growth technique described by Mathieu (1977). Briefly, the water was purged of 222Rn using He, then the sample bottle sealed. After about one week the 222Rn accumulated from decay of dissolved 22~Rawas extracted and its activity determined on a scintillation detector. After analysis of water samples for 226Ra, 250ml subsamples of water were taken for analysis of U isotopes using a procedure similar to that of Ku (1966). The 250-ml samples were made up to 8 N HC1, a 236U yield tracer added, and the solution (now ~ 500 ml) passed through an anion-exchange column (Bio-Rad ®, AG 1 X 8, 100-200 mesh) on which U is retained. The U was eluted offthe column with 0.1 N HC1. Very saline samples were purified by solvent extraction using saturated A1 (NO3)3 and ethyl acetate in order to remove excess salts. Samples were further purified by solvent extraction with isopropyl ether (to remove Fe) and the U was extracted onto thenoyl trifluoroacetone ( T T A ) - b e n z e n e which was evaporated onto stainless-steel planchets. The thin sources were counted on an Ortec ® Si surface barrier detector equipped with a pulse height analyzer.

3.1. Water chemistry The standard methods used to analyze for certain pertinent chemical species are given in Table II. Total carbon dioxide (CO2(aq) + HCOa + CO~- ) was determined by coulometry as described by Johnson et al. (1985). Briefly, the procedure involved acidification of 50 ml of water to pH~,2 and subsequent gas stripping of the evolved C02 with nitrogen gas into a solution that is color sensitive to absorbed CO2. Subsequent electrochemical titration of this organic solvent and colorimetric determination of the titration end-point al-

4. G e o c h e m i s t r y of u r a n i u m a n d r a d i u m The radionuclides investigated during this study (238U, 2~4Uand 226Ra) are part of the 238U decay series, the relevant portions of which are

185 TABLE II Chemical species measured, and analytical methods employed, to provide supplemental data for the study of radionuclide mobility in Delaware Basin waters Species

Precision

Method

Instrument

Chloride

_+2 %

Metrohm ® C1 titrator

Alkalinity pH ~COz

_4-1% _+0.1 pH unit

electrometric titration to AgC1 endpoint gran titration combination electrode acidification and titration of evolved

pco~

_+2% _+10% (anoxic brines) _+20% _ 5% _+5%

_+3%

CH4 SO4 Feand Mn

COz equilibration and gas chromatography Perkin-Elmer®gas chromatograph measuredin Pcoz flasks ion chromatography atomic absorption

23SUSeries U po Th

Ac RO

Fr Rn

Radiometer ® pH meter Radiometer ® pH meter Coulometrics ® coulometer

258U* 234U~k 4.51 x 109y ~f~'2.48 x 105y al 254pe ~a ~/~'1.18m 254Th 250Th 24.1d Z52 xlO4y

Ia

226R: 160J y

Ia

222Rn 5.825 d

Fig. 3. A portion of the 238U decay series ~ith the isotopes measured for this study indicated by an a ~terisk.

shown in Fig. 3. U occurs in the + 4 oxidation state in rock-forming minerals and is immobile in that state at low temperatures and pressures (Gascoyne, 1982). In oxidizing environments U is easily oxidized to the more mobile + 6 state and its solubility further enhanced by formation of carbonate, phosphate or chloride complexes in the pH range of most natural waters (Langmuir, 1978). The importance of organic complexes of U is considered to be small in the presence of greater than a few ppm of any of the above-mentioned inorganic ligands (Lang-

PerkimElmer®gas chromatograph Dionex®IC Perkin-Elmer®AA

muir, 1978; Gascoyne, 1982 ). U concentrations in groundwater are strongly correlated with redox conditions. Decreases in 23sU concentration by at least two orders of magnitude have been observed across oxic-anoxic interfaces in some aquifer systems (e.g., Osmond and Cowart, 1976; Andrews and Kay, 1984) due to reduction of U (VI) to U (IV). The distribution of Ra in natural waters is controlled primarily by its production from the parent isotopes and its removal from solution by adsorption or cation exchange. Although Ra salts of sulfate and carbonate are extremely insoluble, it is unlikely that natural Ra concentrations would ever be high enough to exceed the solubility product of RaSO4 or RaC03 (Langmuir and Riese, 1985; Langmuir and Melchior, 1985 ). However, solid solutions with celestite, barite or other alkaline-earth sulfates and carbonates may control soluble Ra concentrations in some cases (Langmuir and Melchior, 1985). Complexes of Ra with SO~- or C1- may be important in saline brines based on thermodynamic data calculated by Langmuir and Riese (1985) and may thereby enhance Ra mobility by reducing the cation-exchange affinity of Ra.

186

5. Results and discussion

5.1. Water chemistry Surface and groundwaters in southeastern New Mexico cover a wide range in salinity from very fresh ( Capitan aquifer, Rio Grande River: [C1-] <50 ppm) to hypersaline surface and subsurface brines (Laguna Grande, waters in the Castile and Salado Formations: [ C1- ] > 1.8.105 ppm) (Table III). Total alkalinity was relatively low ( < 2.5 meq 1-1) although several samples from deeper brines exceeded 5 meq l-1. pH's were always between 6 and 8. Because we know from thermodynamic data (Hostetler and Garrels, 1962; Langmuir, 1978) and field measurements ( Simpson et al.,

1982, 1984) that carbonate ion strongly complexes U, we need to know the alkalinity in order to distinguish CO~- complexing from a potential C1- ion influence on radionuclide mobility. None of the waters or brines contained significant amounts of dissolved hydrogen sulfide. Many of the wells that were sampled contained very high concentrations of dissolved Fe, therefore the sulfide was probably sequestered by an Fe solid phase (FeS?) thereby inhibiting the buildup of H S - . High concentrations of methane, Fe and Mn in all of the oilfield brines indicate anoxic conditions there and probably also in the W I P P brines. Two of the hypersaline salt lakes exhibited very high Mn concentrations and we speculate that high chloride concentra-

TABLE III Anion chemistry, carbonate chemistry, methane, iron and manganese data for Delaware Basin surface and groundwaters Sample No.* 1 2 5 6 7 8 9 10

11 12 13 14 15

17 18 19 20 21 22 23 24 25 26 31

C1(meq1-1 )

SO24(mM)

~Alk (meq 1-1 )

EC02 (raM)

Pco2 (]tatm.)

pH

CH4 (ppm)

Fe (ppb)

Mn (ppb)

893 217 4,280 5,130 0.42 2,130 5,010 5,200 16.9 1.15 3,130 54 0.03 0.03 143 4.8 1,692 65 63 2,776 2,525 2,215 127 1.15

39.8 61.3 39.4 212.1 N.D. 72.6 N.D. N.D. N.D. N.D. 29.2 N.D. N.D. N.D. N.D. N.D. 28.8 2.9 30.3 7.5 2.9 3.2 < 0.3 N.D.

1.5 1.4 2.2 5.5 4.2 1.1 4.2 7.7 1.4 2.3 3.7 1.6 4.0 4.2 4.6 4.2 5.1 6.0 0.5 1.0 0.1 N.D. 0.2 2.3

1.5 1.0 1.9 4.0 2.2 0.7 2.5 4.6 1.4 0.9 1.6 1.7 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

2,640 447 2,617 488 5,166 770 N.D. 544 N.D. N.D. N.D. 405 1,129 3,820 5,970 N.D. 6,090 6,582 5,800 N.D. 3,230 2,060 N.D. 634

7.2 8.0 7.0 7.3 7.4 7.2 7.6 7.4 8.0 7.9 7.2 7.4 8.5 7.9 7.0 7.5 7.0 7.3 7.0 6.7 7.1 6.2 6.8 7.8

0.3 2.0 2 1.5

300 150 250 200 N.D. 975 N.D. 110 45 N.D. N.D. N.D. 58 N.D. 150 N.D. 750 323 234 39,000 50,000 31,750 N.D. N.D.

165 163 2,875 180 N.D. 360 N.D. 1,800 66 N.D. 720 N.D. N.D. 3 650 45 200 230 85 825 2,450 775 600 50

n.d. = not detected; N.D. = not determined. *For sample identification refer to Fig. 1 and Table I.

n.d. 19 N.D. 0.4 N.D. N.D. N.D. n.d. n.d. n.d. 10 N.D. 543 480 1,000 N.D. 740 540 N.D. N.D.

187

tions may stabilize Mn (II) with respect to air oxidation in these brines. This phenomenon has implications for Ra mobility to be discussed further below.

5.2. Uranium mobility U concentrations (expressed as 23sU activity) range over two orders of magnitude for our entire suite of samples (Table IV). 238U activities of > 20 pCi 1-1 were measured in some of the salt lakes while the anoxic oilfield brines had values < 0.3 pCi 1-1. In oxidizing environments, the high-chloride brines had 23sU activities up to twenty times that of the dilute waters. A plot of U activity vs. the logarithm of chloride concentration (Fig. 4) shows no discernible trend except toward the upper e~treme of chloride values. Only as the waters approach halite saturation do we observe elevated U concentrations. The increase in apparent U mobility can be explained in a number of ways. We first explore the possibility that C1- comple~:es are important at very high chloride concer;£rations. Calculations using the Gibbs free energy tabulated by Langmuir (1978) for the pure system U Q - C 1 - show that ~ 90% of U should exist in solution as a UQC1 + complex at NaC1 saturation ( ~ 6 eq l-1) under normal E h - p H conditions of surface waters or oxygenated groundwaters (Table V; Fig. 5A). Therefore, the solubility of U may be appreciably enhanced in pure NaC1 brines. However, calculations involving chloride and carbonate ion as complexing ligands (Table V; Fig. 5B) show that carbonate complexes should dominate even at relatively low alkalinities ( > 0,02 meq 1-1). The effect of carbonate ion comp[exation is so strong that even if 99% of HCO~ were tied up as Na complexes, the U-CO~- complexes would still overshadow the U-C1 complexes for alkalinities of > 2 meq 1-1. A plot of 23sU activity vs. alkalinity (Fig. 6) shows that the three samples that have the highest U content also have relatively high alkalinity (i.e. high car-

bonate content). Carbonate complexes probably account for most of the enhancement in U concentration in brines where oxygen is present. Increased competition with other ions for ion-exchange sites on clay minerals may also contribute to the higher concentration of U in solution in these brines. 23sU activities in all of the oil-/gas field brines were extremely low ( <0.3 pCi 1-1; Table IV). Samples from these anoxic brines had nearly two orders of magnitude less 23sU than nearby surface waters of equivalent salinity or alkalinity. These anoxic brines had the highest 22~Ra concentrations of the entire sample suite (see Table IV) therefore the low U concentrations do not simply reflect a lack of U in the source regions of the brines. U must be present to supply the 22~Ra. Reduced mobility of U is consistent with the thermodynamically predicted much lower solubility of U ( + 4) compared to U ( + 6 ) (Hostetler and Garrels, 1962; Langmuir, 1978) in chemically reducing systems. The concentration of dissolved U predicted to be in equilibrium with uraninite at pH = 7 is ~ 10 -3 pCi 1-1 (Langmuir, 1978). Only upper limits can be given for U concentrations in the oilfield brines (Table IV), however, some of these values are within an order of magnitude of the theoretical solubility. Therefore, we conclude that high chloride concentrations (up to 103 meq 1-1 ) do not greatly increase the mobility of U in chemically reducing environments. The activity ratio (AR) of 234U/2~sU for the entire sample suite range from 1.9 to > 13 (Table IV). Oxygenated surface and groundwaters tended to have lower AR's (1.9-3.5) and higher U concentration ( > 1 pCi 1-1) than samples from the oil-/gas fields (AR's > 5; 23su < 0.3 pCi 1-1). The W I P P brines were intermediate within this range having AR's from 3.4 to 5.3 and 23su activity ~ 2 pCi 1-1. The variability of the 234Uff3sU ratio as a function of 23sU activity (Fig. 7) shows a trend of decreasing AR with increasing U concentration. This relationship has been observed in many other groundwater systems [see Osmond and Cowart (1982) for

6 7 8 9 10 11 12 13 14 15 17 18 19 20 21 22 23 24 25 26 30

1 2 5

Sample No. W I P P H-6 W I P P H-4 P o t a s h Co. of America well No. 3 salt lake - 3.5 mile Carlsbad city water WIPP DOE-/ salt lake - 4.8 mile Laguna Grande windmill No. 1 windmill No. 2 Surprise Springs Pecos River Carlsbad Caverns Rattlesnake Springs Conoco brine well Ogallala Well oil well - State H 22 gas well - Devonian gas well - Wolfcamp oil well - AA No. 1 oil well - T No. 1 oil well - Z No. 1 gas well - Tracy B Roaring Spring

Location

5,130 0.42 2,130 5,010 5,270 16.9 1.2 3,130 54 0.03 0.03 143 4.8 1,692 65 63 2,776 2,525 2,215 127

893 217 4,280

C1(meq 1-1)

23.2 ± 0.7 2.0 ± 0.2 20.6_+0.5 13.5 -+ 0.4 9.5_+0.3 2.5 -+ 0.1 6.0 _+0.2 2.3 _+0.1 0.6 ± 0.1 0.43 -+ 0.03 1.0 _+0.1 2.4 _+0.1 < 0.04 < 0.03 < 0.01 < 0.03 < 0.12 < 0.3 < 0.2 1.2 ± 0.1

49.8 ± 1.6 1.07 ± 0.05 9.7 -+ 0.4 39.2_+0.8 29.8 -+ 0.7 23.1-+0.5 8.4 -+ 0.3 15.5 _+0.4 5.9 _+0.1 1.5 -+ 0.1 2.3 ± 0.1 2.1 _+0.1 4.9 ± 0.1 0.3 ± 0.01 0.21 _+0.01 0.13 + 0.02 0.15 ± 0.1 0.8 -+ 0.1 1.4 ± 0.1 1.4 _+0.1 3.1 _+0.1

7.1_+ 0.1 10.0_+0.4 15.5_+0.3

(pC± 1-1 )

(pC± 1-1) 2.1 _+0.1 1.9_+0.2 7.4_+0.3

234U

23sU

Delaware Basin water sample uranium and radium concentrations and isotope activity ratios

T A B L E IV

2.1 4.9 1.9 2.2 2.4 3.4 2.6 2.6 2.5 5.3 2.1 2.0 > 7 >8 > 13 >6 > 7 >5 > 7 2.6

-

3.4 5.3 2.1

2'~4U/~3sU

1.73 -+ 0.2 0.45 -+ 0.01 1.2 _+0.1 0.21 _+0.01 420 2 6 152 ± 2 71.4 -+ 1.0 261 ± 2 634 _+12 1,352 2 10 9.3 ± 0.1 0.81 ± 0.03

3.3 _+0.1

9.0 ± 0.2 0.62 ± 0.02 132 -+ 2 12.2_+0.4 3.1 _+0.1

3.1_+0.1 81 __ 1 9.6_+0.1

226Ra (pC± 1-1)

0.2 1.2 0.2 0.6 0.04 1,400 760 546 1,740 794 966 7 0.3

-

-

0.2 0.6 14 0.3 0.1

0.4 8 0.6

226Ra/234U

Oo

189

o • • L

20

15

IO

Fresh water Brine (0 z present) WlPP brine Oiifield brine (O 2 absent)

+ o

o --

2

+~ IC o 5 0

o@

In

o

I

o

t

o

,

2

- _ a e_

3

I

4

I |

,,AA~-5

I

I

I

3

2

I

4

i.

5

6

a-CI- [ e q / I )

Log C I - ( l o g p p m )

~°°II

Fig. 4. Concentration of dissolved uranium plotted against the logarithm of chloride concentration for fresh waters and brines in the New Mexico survey area. Absence of 02 was inferred from presence of measurable CHt.

B

o~ v

summary]. The presently accepted explanation for high AR's is that of direct recoil injection of 234Th into solution (e.g., Kigoshi, 1971 ) and/or enhanced solubility of 234U due to its + 6 oxidation state induced by decay of the parent isotopes and emplacement in radiation-damaged lattice sites vulnerable to leaching (Rosholt et al., 1963). TABLE V Uranium speciation calculations in chlo:ide and sulfate solutions

(A ) The system containing H20, Cl- and UO~2+:

o.I

o.O,.o .&

.& .&

A,

2,0 o.',~ o.~

a-HCO~ (meq/I)

Fig. 5. A. The ratio aco2c,+/avo~+ as a function of C1- concentration in the system U O ~ + - C 1 - - H 2 0 . The plotted curve is derived from equation (A) in Table IV. B. The ratio avo~cl+/avo2(co3)~- as a function of H C O [ concentration at a C1- concentration of 6 eq l - a ( i.e. NaC1 saturation . The curve is derived from equation (B) in Table IV.

UO~ + + Cl- ~- U02 Cl + ZIG= - 0 . 3 2 kcal. = - R T ln K

20

The ratio of avo2c,+/auo~+ is plotted against C1- activity in Fig. 5a

UO~(C03)~- + C 1 - + 2 H + ~ U 0 2 C I + + 2 H C O ~

ziG= - 5 . 3 4 1 kcal. = - R T l n K K=8.254.103 =

auo2cl+(aHC0,£)2

auo2(co3)~-ac]- (as+) 2

i

i

0 Fresh water • Brine {02 present)

K = 1.717 = auo2cl/auo~+acl-

( B ) The system as in (A ) except C02 added:

I

25

i

I

I

6-

7

g @

• WIPP brine • O i l f i e l d brine ( 0 z absent)

O_

2 10

O

5i • ipO I

0 2

0 3

4

5

Total Alkalinity (meq/I)

For acl- = 5 eq l - ~and pH = 7, the ratio auo2cl+/auo2(co~)~-as a function of aHco¢ is plotted in Fig. 5b

Thermodynamic data obtained from Langmuir (1978).

Fig. 6. Concentration of dissolved uranium plotted against total alkalinity in fresh waters and brines in the New Mexico survey area.

190 i

7

i

i

,

l

3,5

i

o Fresh wQter • Brine (02 present] • WIPP brine • Oilfield brine (02 abseni)

l

t

i

1

i

i

i

i

3.0 • •

2.5

•

4 2

~,OL~ o_

I

i

i

0 Fresh w a t e r • Brine IOz present) • WIPP brine • O i l f i e l d brine OZ a b s e n t

•

AI

1.5

o 1.0

o •

o

I 2

cu

L 4

t 6

Q

Q

0

I 8

t I I ~0 12 14 238U i p C i / I )

I 16

I IB

I 20

212

24

Fig. 7. 2~4U/23sU activity ratio plotted against uranium concentration.

5.3. Radium mobility Very high concentrations of 226Ra and 22SRa have been documented in saline and oilfield brines (e.g., Armbrust and Kuroda, 1955; Alekseev et al., 1958; Bloch and Key, 1981; Kraemer and Reid, 1984). Kraemer and Reid show a covariance of Ra activity with C1- concentration and all of the above-mentioned workers observed anomalously high Ra activities in waters associated with hydrocarbons. We analyzed 226Ra in all of the waters to confirm the results o f o t h e r studies and explain the apparent enhanced 226Ra solubility in the high-chloride and oilfield brines. 226Ra activities range over three orders of magnitude with the highest concentrations occurring in the oilfield brines (Table IV). Several samples had 226Ra activities of > 100 pCi l- 1 and one of the oil well brines had an activity of > 1300 pCi 1-1 (oil well Govt. Z No. 1 ). The hypersaline surface brines had higher 226Ra activities than fresh waters but an order of magnitude less than the anoxic oilfield brines. The logarithm of 226Ra activity is plotted as a function of chloride concentration in Fig. 8 and it can be seen that the data fall into two groups. The fresh waters and oxygenated brines have low to moderate 226Ra activity and show a general trend of increasing Ra content with chloride concentration. The Ra activities in-

% o1-.

0 0 - O. 5 l ' -

g

- I.O!

I 2

I 4

I S

I 8 Chloride

I I0

I 12

I 14

I 16

I 18

20

( p p m x 10 4 )

Fig. 8. The logarithm of ~26Ra activity vs. chloride concentration for fresh waters and brines in the New Mexico survey area.

crease by about an order of magnitude from dilute to saline waters within this group. The oilfield brines and two WIPP brines have about two orders of magnitude more 226Ra than any of the surface waters and a less well-defined correlation with chloride concentration. The delineation of these two groups is important and is discussed further below. The increase in Ra content with C1- concentration may be explained by: (1) formation of soluble Ra complexes with chloride (or associated sulfate or carbonate) ions, (2) competition with Na + or other cations for ion-exchange sites, or (3) higher concentrations of 23sU at increasing C1- concentrations which supports more 226Ra. Calculation of Ra speciation (Table VI) in pure Ra2+-C1 - or Ra2+-S024 - systems from thermodynamic data given by Langmuir and Riese (1985) shows that chloride and especially sulfate complexes can significantly enhance Ra mobility. More than 50% of Ra exists as RaSO ° even at concentrations of sulfate as low as I m M [Table VI; equation (B) ]. Thermodynamic speciation calculations imply that RaSO ° complexes are more important than RaCl + even when SO~- concentration is as low as 0.5% of C1- concentration

191 I 5.5~-

T A B L E VI Radium speciation calculations in chloride and sulfate solutions

I 0 • • •

5"0 t 2,5~-

(A ) The system consisting of Ra 2÷, Cl- and RaCl+: Ra 2+ + C I - ~ R a C I +

3 2.o[ cJ

A G r - 0.139 kcal.; K = 0.789 aRaCl+/ae. . . .

0.789aclj

At ac~ = 5 eq 1 1, 80% of Ra exists as RaCl +

(B) The system consisting of Ra 2+, ~0~- and RaSO°4:

1,5• •

oI"

0,5

O•

-0.5

Ra 2+ + S O ~ - ~ R a S O °

-I.O

AG~- - 3 . 7 5 kcal.; K= 5.62" 102

-I.5

aRaso~/a~ + .=Kaso~ - = 5.62" 102 asoiAt aso~- > 10-2M, >80% of Ra is in the form of RaS04°

( C) The system consisting of RaCl +, RaSO~, SO~and Cl-:

Fresh water Brine (02 present) WIPP brine Oilfield brine (0 z absent)

0

~ I 5

I

I

I0 15 258U (pCi/I)

I

20

25

Fig. 9. The logarithm of 226Ra/23tU activity ratio vs. uranium concentration (expressed as pCi l - ~of 234U) for fresh waters and brines in the New Mexico survey area.

RaC1 + + SO~- ~ R a S O ° + C1~G~ = - 3.89 kcal.; K = 7.11-102 aR~so~/aR~cl- = K ( aso~-/ac,- ) = 7.11" 102 (aso~ - / a c l - ) Thermodynamic data obtained from Langmuir and Riese (1985).

[Table VI, equation (C)]. However, in hypersaline brines where SO42- may be removed by gypsum precipitation or in reducing groundwaters where S042- is depleted by sulfate reduction, C1- may be the mo,,;t important inorganic ligand influencing Ra mobility. Displacement of Ra from ion-exchange sites by increasing ionic strength also occurs (Benes, 1983; Dickson, 1985) and it has been observed that Ra adsorbed to river particles is desorbed under conditions of higher salinity as rivers empty into the ocean (Li et al., 1977; Li and Chan, 1979). Langmuir and Melchior (1985) further speculate that both H + and Ca 2+ displace exchangeable Ra into solution in Texas brines at low pH. Both complex formation and ion-exchange competition should contribute to enhanced Ra mobility in saline waters. This is consistent with

our observed increase in Ra concentration by more than an order of magnitude over the entire salinity range of our oxic samples (Fig. 8 ). However, there is no apparent correlation of Ra activity with sulfate concentration or SO427/C1- ratio for the Delaware Basin sample suite. This lack of correlation may be caused by the complicating redox effects discussed below. All of the brines had sulfate concentrations and SO~-/C1- ratios sufficiently high to make sulfate the most important complexing ligand for Ra. The observation in this study and that of other workers ( Pierce et al., 1955; Gutsalo, 1964; Bloch and Key, 1981; Kraemer and Reid, 1984 ) of Ra concentrations in oilfield brines far in excess of that predicted from a Ra-salinity relationship of the oxygenated samples (Fig. 8) leads us to suspect that something other than complexing or ion exchange is the dominant control on Ra concentrations in these anoxic groundwaters. Fig. 9 is a plot of the logarithm of the activity ratio of eesRa to its grandparent z34U vs. total U concentration (expressed as 288U activity) from the data in Table IV. Activ-

192 3.5 5.C 2.5

i 0 • • •

water | Brine (02 present) Fresh

/

W IPP brine Oilfield brine (Ozabsent)

] | J

2.0 ¢J~ 1.5

¢q

o.s

J 0 O

-0.5

-[.OI

I

I

I

I

2 5 Log Fe (log ppb)

I

4

5

Fig. 10. The logarithmof 226Raactivity vs. logarithm of iron concentrationfor water samples in the New Mexico surveyarea. Iron concentrationis usedhereas an indicator of redoxconditionswithin the DelawareBasin. ity ratios of 226Ra/234Ureach values in excess of 3000 at low U concentrations. Essentially all of the dissolved 226Ra in the oilfield brines and two WIPP brines is unsupported by dissolved 234U. We cannot therefore ascribe the high levels of dissolved Ra to higher U concentrations nor is there any evidence to suggest that the formations containing the reducing brines have orders of magnitude more U associated with particles than those associated with oxygenated brines. The oxygenated hypersaline brines have 226Ra/234U ratios close to or less than unity, suggesting that Ra may be less mobile than U in oxidizing environments, regardless of the salinity. 226Ra activity correlates reasonably well with dissolved Fe concentration (Fig. 10). Because Fe concentration in waters generally increases with more reducing conditions, it serves as a useful indicator of the redox state of the system and further demonstrates the dependence of 226Ra concentration within redox state. Ra can only exist in the + 2 oxidation state under normal environmental conditions and therefore its chemical properties are insensitive to redox conditions. We suggest that redox conditions indirectly control the mobility of Ra

through the reduction of insoluble Mn (IV) -oxides to the soluble + 2 state in oxygen-deficient environments. Scavenging of Ra by Mn-oxides has been shown to occur in marine sediments (Berelson et al., 1987 ) and MnO2-impregnated fibers are used routinely to efficiently remove Ra from seawater (Moore and Reid, 1973; Reid et al., 1979 ) and groundwater (W. Moore, pets. commun., 1984). Even in suboxic conditions appreciable amounts of Mn 2+ may coexist metastably with small amounts of oxygen because of the slow kinetics of Mn (II) oxidation in the absence of bacteria (Emerson et al., 1982 ). The increase in apparent Ra solubility in the oxygen-deficient waters from the WIPP site and oilfield brines (Figs. 8 and 9) is considered to result from dissolution of Mn-oxyhydroxides. Because Mn is essentially immobile under welloxygenated conditions, weathering will not readily cause Mn to be transported in the hydrosphere. Thus, Mn-oxyhydroxides may constitute >~0.5% of the highly weathered aquifer minerals, especially in carbonate-rich sediments (Wedepohl, 1978). Since these oxides generally occur as coatings on the surfaces of mineral grains, Mn may account for > 0.5% of the exposed surface area of mineral grains. Given the extremely strong affinity of Ra for Mn02, such an amount of solid-phase Mn may easily control Ra activity in oxidizing groundwaters. A decrease in Eh and concomitant release of Mn 2+ into solution will eliminate the strongest adsorption sites for Ra thereby increasing its concentration in groundwaters where in highly reducing groundwaters it may move unretarded in solution.

6. Summary and conclusions Concentrations of 23sU, 226Ra and the ratio of e26Ra/2~4U show a wide variability in surface and groundwaters in southeastern New Mexico. Oxic and suboxic saline brines had U concentrations an order of magnitude higher than dilute waters, indicating a slight enhancement of U mobility by chloride ion. This cannot be dem-

193

onstrated unequivocally since the increased 2asU activities may be due to alkalinity increase or differences in the relative magnitude for the U sources in the fresh and saline waters. U concentrations in the chemically reducing oilfield brines approach the theoretical limit for uraninite solubility. High chloride concentrations in these brines had no apparem~ effect on U mobility. 226Ra concentrations showed a slight correlation with chloride ion concentration that may be caused by chloride or sulfate complexes. The most striking feature of the 226Ra results was the extremely high Ra concentrations in reducing brines. These high activities were almost completely unsupported by dissolved U. We hypothesise that reduction of Mn from solid oxyhydroxides to more soluble forms appears to remove the strongest adsorption sites for Ra thereby increasing its apparent solubility. A comparison of U and Ra concentrations measured in the New Mexico survey area with other environments is given in Table VII. U and Ra concentrations are greatly reduced and enhanced in anoxic waters, respectively. For oxygenated waters, high-alkaline environments

(e.g., Mono Lake, California) enhance U toobility while high-sulfate environments may enhance Ra mobility. Chloride ion has an apparent second-order effect on both of these nuclides. Based on these observations, we can make some first-order predictions about the fate of high-level nuclear waste should they ever be released into these or other groundwaters. The redox state (Eh) of the waters appears to be the most important factor controlling radionuclide mobility; complexing by inorganic or organic iigands are of secondary importance in most situations. Actinides with multiple oxidation states (U, Np, Pu) tend to be less soluble in their reduced forms ( + 3, + 4 ) than their oxidized forms ( + 5, + 6; Nelson and Lovett, 1978, 1981; Cleveland et al., 1983). Consequently, the chemically reducing conditions of some of the brines sampled would serve as an additional barrier to these nuclides. By analogy with Th and Pa which have a very strong affinity for adsorption to Fe- and Mnoxyhydroxide surfaces (e.g., Bacon and Anderson, 1982; Anderson et al., 1983 ), we would pred i c t a behaviour similar to that of Ra for transplutonium actinides which exist primarily

TABLE VII Comparison of 2asU and 226Ra data from the New Mexico sample survey with sample from other environments analyzed by the LDGO laboratory

Carlsbad Caverns Conoco RS brine WIPP H-6 WIPP DOE-1.1 Gas well Z No. 1.I Laguna Grande ELA lakes .2 Seawater *a Mono Lake *a Whiteshore Lake .4

C1(meq1-1)

S042(mM)

0.03 143 893 2,130 2,215 5,200 < 0.1 540 600 676

ND ND 40 73 3.2 < 0.1 134 3170

~Alk (meql -~) 4.0 4.6 1.5 1.1 7.7 0.1 2.3 700 18.7

2asu (pCi1-1)

226Ra (pCi1-1)

0.6 1.0 2.0 2.0 < 0.3 14 ~ 0.02 1.2 185

1.7 1.2 3.1 132 1350 3.1 <1 0.04 0.5 0.2

*1Anoxic brines. *201igotrophic soft-water lakes at the Experimental Lakes Area, northwest Ontario, Canada (R.F. Anderson and S.L. Schiff, unpublished results, 1982, from the LDGO laboratory). *aSee Simpson et al. (1984). *4See Simpson et al. (1985a).

194 in t h e + 3 o x i d a t i o n state. T h e s e e l e m e n t s s h o u l d be m o r e m o b i l e in r e d u c i n g g r o u n d w a t e r s w h e r e Fe- a n d M n - o x i d e s u r f a c e s are absent, a l t h o u g h t h e e f f e c t m a y n o t be as d r a m a t i c as for Ra.

Acknowledgements We t h a n k S. M a t h i e u , G. G o v e a n d E. R a m a g e for a s s i s t a n c e in t h e field a n d l a b o r a t o r y . Access to v a r i o u s wells a n d a s s i s t a n c e w e r e p r o v i d e d b y R. K e r b o , W. D u n m i r e a n d J. W a l t e r s of N a t i o n a l P a r k s Service ( C a r l s b a d C a v e r n s ) , H . I n g r a i n , C o n o c o Oil Co., a n d Cities Service for access to t h e o i l - / g a s field b r i n e s , W. S t e n s r u d of S a n d i a N a t i o n a l L a b o r a t o r y ( W I P P w e l l s ) , J. E y e r of P o t a s h C o m p a n y o f A m e r i c a , a n d J. R a m e y , Oil C o n s e r v a t i o n Division, S a n t a Fe. Lee W i l s o n a n d A s s o c i a t e s ( S a n t a Fe ) p r o v i d e d c o n s i d e r a b l e logistical s u p p o r t d u r i n g t h e o r g a n i z a t i o n a n d p r e p a r a t i o n of t h e field p r o gram. Financial support provided by a grant f r o m t h e U.S. N u c l e a r R e g u l a t o r y C o m m i s s i o n (NRC-04-81-217).

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