Research Similarities in the Chemical Composition of Carbonate Groundwaters and Seawater V E R N O N F . H O D G E , †,* KLAUS J. STETZENBACH,‡ AND K E V I N H . J O H A N N E S S O N ‡,§ Department of Chemistry, University of NevadasLas Vegas, Las Vegas, Nevada 89154-4003, and Harry Reid Center for Environmental Studies, University of NevadasLas Vegas, Las Vegas, Nevada 89154-4009
Fifty-four elements were quantified in spring waters emanating from carbonate rock in Ash Meadows, in southern Nevada, and in Death Valley, CA. The results show that the concentrations of many of the trace elements found in these groundwaters are remarkably close to those found in modern seawater. The concentrations of 26 of the elements in the spring waters and seawater are within a factor of 2; 14 more are within a factor of 10; 8 elements are enriched in the groundwater by more than a factor of 10; and 6 elements are depleted by more than a factor of 10. Similarities in the trace chemical composition of ancient seawater and modern seawater can be inferred from the fingerprint of trace elements found in these carbonate rock-source spring waters.
Introduction Investigations focusing on the concentrations and to a lesser extent the speciation of many trace elements in seawater have been numerous and spanned many decades (1-3), and the major components such as sodium have been studied for well over a century (4). In contrast, far less information on the concentrations of trace elements in groundwater is available. However, with newer analytical instruments, such as the inductively coupled plasma-mass spectrometer (ICPMS), many elements can be “simultaneously” quantified during a single analysis of a water sample (5, 6), and it is possible that, as the number of investigations of trace elements in groundwater increases, the gap in our knowledge of the behavior of trace metals in groundwater and that in seawater will narrow. Using an ICP-MS with ultrasonic nebulization for the determination of the trace elements (2 × 10-5-5 × 10-13 mol/kg of water), an atomic absorption spectrophotometer for the quantitation of the major cations (5 × 10-3-3 × 10-4 mol/kg of water) and an ion chromatograph for the major anions (1 × 10-3-1 × 10-5 mol/kg of water), we attempted to measure 65 elements in waters from “carbonate” springs located in southern Nevada and Death Valley National Park, CA. Of these, 54 were quantified, and the remaining 11 elements were at or below the detection limits for the ICP-MS. * Corresponding author phone: (702)895-3510; fax: (702)895-4072; e-mail:
[email protected]. † Department of Chemistry. ‡ Harry Reid Center for Environmental Studies. § Present address: Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Norfolk, VA 23529-0276. S0013-936X(98)00080-7 CCC: $15.00 Published on Web 07/18/1998
1998 American Chemical Society
The study area is within the south-central part of the Great Basin in the western United States (see Figure 1). The Ash Meadows subbasin covers approximately 12 000 km2 and is the largest aquifer system in southern Nevada (7-11). The geology consists of late Precambrian and Paleozoic marine carbonate and clastic sedimentary rocks and overlying intrusive and extrusive Tertiary igneous rocks (10). The regional aquifer is composed of extensively fractured Paleozoic carbonate rocks that range from 9 000 to 12 000 m thick, overlain by block-faulted Cenozoic volcanic and sedimentary strata (7, 10, 12-19). The best estimate of the direction of groundwater flow in the study region is from north to south, with a component of the flow diverted southwesterly toward Death Valley, CA (7, 10, 11). Bore holes drilled for oil exploration have documented freshwater circulation at depths of more than 3000 m in some areas (20). In addition, there is evidence that groundwater in the study area is well oxygenated, even at depth (20). The age of the water evolving from the sampled springs is in question, but current estimates place it at several thousands of years (21). No perennial streams or rivers are found in the study area. However, approximately 2 × 107 m3 of water per year is discharged from the artesian springs in Ash Meadows, and about 6 × 106 m3 of water discharges from the springs in the arid Furnace Creek region of southern Death Valley, at the western edge of the study area (Figure 1) (10). With less than 4 cm of precipitation a year, Death Valley is among the most arid spots in the world (10, 22). Moreover, spring discharge in the Furnace Creek region of western Death Valley exceeds any reasonable estimate of local recharge, and it is suggested that a large portion of the groundwater discharging in southern Death Valley comes from the surrounding areas, possibly in part, from the aquifer system supplying the water to springs in Ash Meadows (10). As part of a larger effort to gather data to help elucidate the groundwater circulation in southern Nevada using trace metal fingerprinting, groundwaters were collected, for this study, from 19 “carbonate” springs in Ash Meadows and Death Valley (see Table 1 and Figure 1 for locations). Twelve of the springs are located in the Ash Meadows National Wildlife Refuge in southern Nevada, approximately 100 km to the northwest of Las Vegas, and the remaining springs are located in Death Valley, CA, about 50 km west of Ash Meadows. These springs were sampled either four or five times over a 2-year period from June 1992 to March 1994.
Experimental Section The methods of sample collection and analysis are described in detail elsewhere (6, 23) and are abstracted below. Sample Collection. Water samples were collected by pumping groundwater from each spring’s source with a peristaltic pump through acid-washed Teflon tubing directly into acid-washed polyethylene bottles. The samples passed through acid-washed in-line polysulfone filters (0.45 micron, Gelman Sciences, Ann Arbor, MI). The sample bottles were rinsed three times with the filtered sample water before they were filled. The bottles were rebagged in acid-washed polyethylene bags and returned to clean plastic ice chests for transport back to the laboratory. Samples for trace element analysis were acidified to a pH of less than 2 with 10 mL/L of ultrapure nitric acid (SeaStar, Seattle, WA). An additional 125 mL of filtered samples was collected and acidified for the determination of the major cations (Ca2+, VOL. 32, NO. 17, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. General locations of the springs (groundwater sampling locations) in Ash Meadows, NV, and Death Valley, CA (modified from ref 45).
TABLE 1. Sampling Site Names and Locations spring name Big Spring Bradford Cold Spring Crystal Pool Fairbanks Jackrabbit King’s Pool Longstreet Point of Rocks, NE Point of Rocks, NW Rogers Scruggs Grapevine Middle Grapevine Upper Grapevine Nevares Texas Travertine, A Travertine, B
latitude
longitude
Ash Meadows 36°22.41′ N 36°24.07′ N 36°27.65′ N 36°25.20′ N 36°29.45′ N 36°23.39′ N 36°24.09′ N 36°28.05′ N 36°24.12′ N 36°24.15′ N 36°28.86′ N 36°26.05′ N
116°16.48′ W 116°18.18′ W 116°20.74′ W 116°19.37′ W 116°20.49′ W 116°16.71′ W 116°16.42′ W 116°19.58′ W 116°16.28′ W 116°16.37′ W 116°19.69′ W 116°18.61′ W
Death Valley 37°01.22′ N 37°01.26′ N 37°01.45′ N 36°30.75′ N 36°27.45′ N 36°26.65′ N 36°26.57′ N
117°23.29′ W 117°23.02′ W 117°23.04′ W 116°49.27′ W 116°50.30′ W 116°49.81′ W 116°49.67′ W
Mg2+, Na+, and K+, denoted as Ca, Mg, Na and K in the text). Another set of 125 mL filtered samples was collected from each spring for the analysis of the major anions (Cl- , SO42-, F-, Br-, and NO3-, denoted as Cl and SO4, etc.). Sample Analysis. The major cations were determined with a Varian Analytical Instruments (San Fernando, CA) model AA-20 atomic absorption spectrophotometer within 1 week of sampling. The anions were measured with a Dionex Corp. (Sunnyvale, CA) ion chromatograph system within 48 h. Trace elements were quantified using a Perkin-Elmer 2482
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(Norwalk, CT), Elan model 5000 ICP-MS within 2 weeks of sampling. The samples were introduced to the ICP-MS with a Cetac Technologies, Inc. (Omaha, NB) model U-5000 ultrasonic nebulizer. Ultrasonic nebulization of the samples increased the sensitivity of the analysis as compared to crossflow nebulization and also reduced the oxide formation (thereby reducing potential false positives). Our QA/QC required that the oxide level (CeO/Ce) be less than 3% prior to the beginning of an analysis. The ICP-MS was equipped with an active film multiplier ion detector (ETP Scientific Inc., Auburn, MA) that also increased its sensitivity. The ICP-MS was calibrated with multi-element solutions whose concentrations ranged from 25 ppt to 10 ppb. They were prepared from multi-element solutions that were purchased from Perkin-Elmer (Norwalk, CT). Calibration and sample solutions were spiked with 10 ppb Tb (mono-isotopic, at mass 159) as an internal standard (in the early analyses (1992), In was used as the internal standard). An initial calibration check was performed by measuring the elements in a 10fold dilution of the NIST (Gaithersburg, MD) Standard Reference Material (SRM), “Trace Elements in Water” No. 1643b, which contains 20 of the elements of interest. The masses of the elements in this reference material range from Li-3 to Pb-204, 206, 207, 208, and ionization potentials range from 5.4 eV for Li to 9.8 eV for As. Absolute concentrations of all 20 elements in this reference material were required to be within 10% of the certified values, suggesting that the single internal standard was satisfactory. (The use of three internal standards to span the mass range and the use of the method of standard additions, for the complete suite of elements, did not improve the accuracy or reproducibility of the results; therefore, the use of one internal standard was adopted.) For the elements not in the NIST-SRM No. 1643b, several multi-element solutions that were NIST Traceable were purchased from High Purity Standards (Charleston, SC). Whenever possible, two or more isotopes were monitored to evaluate the likelihood of isobaric interferences. As additional checks on the overall quality assurance of our analytical methods, the laboratory participates in the U.S. Geological Survey’s Analytical Evaluation Program, analyzing (1) trace constituents, (2) major constituents, (3) nutrients, and (4) low ionic strength water samples twice a year and is certified by the State of Nevada’s Drinking Water Program, which requires the analysis of the U.S. Environmental Protection Agency’s WS and WP water samples twice a year. It was necessary to dilute the groundwater samples for analysis when the elements were present at concentrations above the highest calibration concentration. Elements whose concentrations were commonly above 10 ppb were Li, Ti, As, Rb, Sr, and Ba. Although the concentration of La could be determined directly in most of the groundwater samples, the remaining rare earth elements (REEs) could not. Therefore, the REEs were concentrated 50-fold by ion exchange (6). However, some Ba remained in the rare earth fraction, and the barium oxides interfered with the accurate determination of Eu. An extraction with diethylhexylphosphoric acid was used to remove the Ba (24). Platinum served as the internal standard for ICP-MS analysis of the REEs (in the early analyses, Cs was used).
Results and Discussion The results for the groundwater analysis and a comparison of the concentrations of the 54 constituents in groundwater with the average concentrations of the elements in modern seawater (1-3, 25-39) are presented in Table 2. Although attempts were made to measure the concentrations of 65 elements, 11 were found to have concentrations at or below the detection limits of the ICP-MS; therefore, only upper limits for these elements are presented in Table 2. In general, the groundwater concentrations obtained in this study are
TABLE 2. Comparison of Trace Elements in Groundwater from Carbonate Springs in Ash Meadows, NV, and Death Valley, CA, with Seawatera (Listed in Descending Concentration in Groundwater) concentration (mol/kg)
elementb,c
Ash Meadows + Death Valley (mean ( SD)
seawatera (mean)
Na (11) C Ca (20) Cl (17) C Mg (12) C S (16) C K (19) C F (9) C Li (3) C Sr (38) Br (35) C N (7) Ba (56) As (33) Rb (37) C Mo (42) C Cu (29) V (23) Cr (24) Cs (55) C Al (13) Ti (22) U (92) C Zn (30) Se (34) Ni (28) Ge (32) Mn (25) W (74) Sb (51) Sn (50) Tl (81) C Co (27) Zr (40)
(5.1 ( 2.0) × 10-3 (1.2 ( 0.2) × 10-3 (9.2 ( 0.4) × 10-4 (9.1 ( 2.1) × 10-4 (4.3 ( 1.7) × 10-4 (2.6 ( 0.7) × 10-4 (1.1 ( 0.4) × 10-4 (1.7 ( 0.7) × 10-5 (1.2 ( 0.3) × 10-5 (2.9 ( 1.4) × 10-6 (2.9 ( 1.8) × 10-6 (3.9 ( 1.2) × 10-7 (2.7 ( 1.0) × 10-7 (2.6 ( 1.4) × 10-7 (1.1 ( 0.4) × 10-7 (3.9 ( 1.9) × 10-8 (3.3 ( 1.7) × 10-8 (3.3 ( 3.1) × 10-8 (3.0 ( 1.9) × 10-8 (2.2 ( 1.3) × 10-8 (1.9 ( 0.8) × 10-8 (1.3 ( 0.5) × 10-8 (1.3 ( 0.7) × 10-8 (1.3 ( 1.2) × 10-8 (1.0 ( 0.8) × 10-8 (9.0 ( 7.0) × 10-9 (4.8 ( 9.9) × 10-9 (2.9 ( 3.5) × 10-9 (2.9 ( 2.7) × 10-9 (1.3 ( 2.3) × 10-9 (1.2 ( 0.6) × 10-9 (6.3 ( 3.7) × 10-10 (5.2 ( 2.6) × 10-10
0.468 1.03 × 10-2 0.546 5.32 × 10-2 2.82 × 10-2 1.02 × 10-2 6.8 × 10-5 2.5 × 10-5 9 × 10-5 8.4 × 10-4 3 × 10-5 1.0 × 10-7 2.3 × 10-8 1.4 × 10-6 1.1 × 10-7 4 × 10-9 3 × 10-8 4 × 10-9 2.2 × 10-9 2 × 10-8 d 2 × 10-10 (3, 26) 1.4 × 10-8 (27) 6 × 10-9 1.7 × 10-9 8 × 10-9 7 × 10-11 5 × 10-10 5 × 10-10 1.2 × 10-9 4 × 10-12 e 6 × 10-11 2 × 10-11 3 × 10-10 (28, 29)
concentration (mol/kg) groundwater/ seawater elementb,c 0.011 0.12 0.0017 0.017 0.015 0.025 1.6 0.7 0.13 0.0035 0.1 3.9 12 0.19 1.0 10 1.1 8 14 1.1 100 0.9 2.2 8 1.3 130 10 6 2.4 330 20 32 1.7
Cd (48) Te (52) Y (39) Ga (31) Re (75) C Ta (73) La (57) Ce (58) Nd (60) Sm (62) Gd (64) Pr (59) Dy (66) Th (90) Er (68) Yb (70) Ho (67) Eu (63) Tb (65) Lu (71) Tm (69) Be (4) Pb (82) Nb (41) Ag (47) In (49) Hf (72) Bi (83) Au (79) Ru (44) Pt (78) Ir (77)
Ash Meadows + Death Valley (mean ( SD)
seawatera (mean)
(2.2 ( 1.9) × 10-10 7 × 10-10 (1.3 ( 1.0) × 10-10 1 × 10-12 (3) (1.1 ( 0.6) × 10-10 1.5 × 10-10 (1.0 ( 0.8) × 10-10 1 × 10-11 (30) (3.7 ( 1.4) × 10-11 4.0 × 10-11 (31) (4.1 ( 4.5) × 10-11
