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1.
The depth profiles of electrical conductance, δ18O,210Pb and cosmogenic radio isotopes10Be and36Cl have been measured in a 30 m ice core from east Antarctica near the Indian station, Dakshin Gangotri. Using210Pb and δ18O, the mean annual accumulation rates have been calculated to be 20 and 21 cm of ice equivalent per year during the past ∼ 150 years. Using these acumulation rates, the volcanic event that occurred in 1815 AD, has been identified based on electrical conductance measurements. Based on δ18O measurements, the mean annual surface air temperatures (MASAT) data observed during the last 150 years indicates that the beginning of the 19th century was cooler by about 2‡ C than the recent past and the middle of 18th century. The fallout of cosmogenic radio isotope10Be compares reasonably well with those obtained on other stations (73‡ S to 90‡ S) from Antarctica and higher latitudes beyond 77‡N. The fallout of36Cl calculated based on the present work agrees well with the mean global production rate estimated earlier by Lal and Peters (1967). The bomb pulse of36Cl observed in Greenland is not observed in the present studies – a result which is puzzling and needs to be studied on neighbouring ice cores from the same region.  相似文献   

2.
《Quaternary Science Reviews》2007,26(11-12):1547-1556
A new method of permafrost dating with the cosmogenic radionuclide 36Cl is presented. In the first application, syngenetic ice wedges are dated using the ratio of 36Cl and Cl concentrations in ice as the signal. 36Cl is produced in the atmosphere by nuclear reactions of cosmic rays on argon. Stable chlorine enters the atmosphere from the oceans. Their ratio does not depend on chloride concentration in precipitations and on sublimation of snow. In situ production of 36Cl in permafrost ice via cosmic ray-induced reactions and neutron capture are calculated and the dating age limit is estimated as 3 million years. 36Cl/Cl ratios in permafrost samples from cape Svyatoy Nos (Laptev Sea coast), North-Eastern Siberia, are measured by accelerator mass spectrometry. Analysis of the first results and the calculated dates support the feasibility of the 36Cl permafrost dating method  相似文献   

3.
The use of 14C (half-life?=?5,730 years) in modeling the evolution of the 36Cl/Cl ratios in groundwater is reported for the first time. The complexity of the Cl–36Cl system due to the occurrence of different Cl and 36Cl sources and the difficulty of the determination of the initial groundwater 36Cl/Cl ratios have raised concerns about the reliability of using 36Cl (half-life?=?301 thousand years, a) as a groundwater-dating tool. This work uses groundwater 14C age as a calibrating parameter of the Cl–36Cl/Cl decay-mixing models of three wells from the southwestern Great Artesian Basin (GAB), Australia. It aims to allow for the different sources of Cl and 36Cl in the southwestern GAB aquifer. The results show that the initial Cl concentrations range from 245 to 320 mg/l and stable Cl is added to groundwater along flowpaths at rates ranging from 1.4 to 3.5 mg/l/ka. The 36Cl content of the groundwater is assumed to be completely of atmospheric origin. The samples have different Cl–36Cl/Cl mixing-decay models reflecting recharge under different conditions as well as the heterogeneity of the aquifer.  相似文献   

4.
《Applied Geochemistry》2005,20(3):599-609
36Cl produced by thermonuclear bomb testing has been proposed as an additional tool to date or at least to identify recent groundwater components. In order to investigate the behaviour of 36Cl in shallow groundwater a multi-tracer approach (3H/3He, SF6, CFC-12 and 85Kr) was used to characterise and date the groundwater of a quaternary sands aquifer which is located on the Island of Funen near the city of Odense, Denmark. Recharge to the semi-confined shallow aquifer occurs through permeable sand windows and fractured tills at the surface. Locally, however, mixing with older pre-bomb water from the underlying limestone aquifer may occur. The integrated analyses of the available tracer data allowed a well constrained age structure determination of the investigated water system.The 36Cl/Cl ratios measured in groundwater were used to reconstruct the fallout rates for radioactive 36Cl at Odense. The calculated fallout values exceeded the fallout estimated based on data from the Dye-3 ice core in Greenland. Recycling of the bomb peak fallout seems to be the most probable reason of the high values measured. The local extent of this process is difficult to quantify, which impedes the use of 36Cl for dating.  相似文献   

5.
Natural production of the radionuclide chlorine-36 (36Cl) has provided a valuable tracer for groundwater studies. The nuclear industry, especially the testing of thermonuclear weapons, has also produced large amounts of 36Cl that can be detected in many samples of groundwater. In order to be most useful in hydrologic studies, the natural production prior to 1952 should be distinguished from more recent artificial sources. The object of this study was to reconstruct the probable preanthropogenic levels of 36Cl in groundwater in the United States. Although significant local variations exist, they are superimposed on a broad regional pattern of 36Cl/Cl ratios in the United States. Owing to the influence of atmospherically transported ocean salt, natural ratios of 36Cl/total Cl are lowest near the coast and increase to a maximum in the central Rocky Mountains of the United States. Electronic Publication  相似文献   

6.
 The sustainability of community water supplies drawn from shallow aquifers in the arid southwest of the Northern Territory has been evaluated using the radioactive isotope chlorine-36 (36Cl). These aquifers include fractured sandstones of the Ngalia Basin, fractured metamorphic rocks and Cainozoic sands and gravels. 36Cl/Cl ratios for these shallow, regional groundwaters exhibit a bimodal distribution with peaks at 205 (±7) and 170 (±7)×10–15. The higher ratio probably represents modern (Holocene) recharge, diluted with windblown salts from local playa lakes, and occurs mostly around the margin of the basin. The lower ratio corresponds to a 36Cl "age", or mean residence time, of 80–100 ka, implying that the last major recharge occurred during the last interglacial interval (Oxygen Isotope Stage 5). These values are mainly observed in the interior of the Ngalia Basin. Lower values of the 36Cl/Cl ratio measured near playa lakes are affected by addition of chloride from remobilised salts. Finite carbon-14 (14C) data for the groundwaters are at variance with the 36Cl results, but a depth profile suggests low recharge, allowing diffusion of recent atmospheric carbon to the water table. The 36Cl results have important implications for groundwater management in this region, with substantial recharge only occurring during favourable, wet, interglacial climatic regimes; most community water supplies are dependent on these "old" waters. Received, September 1997 · Revised, August 1998, March 1999 · Accepted, March 1999  相似文献   

7.
《Applied Geochemistry》1991,6(4):435-445
The36Cl/Cl ratios of 12 groundwater samples from the Milk River aquifer were determined by accelerator mass spectrometry. Using known Cl concentrations,36Cl concentrations were deduced. Approximately linear relations were observed between the logarithm of the36Cl concentration, the Cl concentration, and the distance from the recharge area along two flow paths. The results are discussed in two approaches:
  • (1)in an interpretation of the linear relation between logarithm of the36Cl concentration and Cl concentration excluding and includingin situ production of36Cl;
  • (2)in a diffusion model. The increase of the Cl concentration with the distance from the recharge area is considered to be due to diffusion of Cl from the underlying confining Colorado shale to the aquifer.
Flow velocities ranging between 0.04 and 0.14 m/a, and ages of the groundwater between 0.6 and 2 Ma are obtained at a distance of 80 km from the recharge area.  相似文献   

8.
When using 36Cl to date very old groundwater in regional aquifer systems, knowledge of the subsurface 36Cl input into the aquifer system is essential. Although 36Cl can be produced through nuclear reactions in the subsurface, in many situations, the input of 36Cl into sedimentary aquifer systems by this avenue of production can be neglected. This is a valid assumption when investigating long-flowpath groundwater systems composed of sandstones, limestones, and shales of typical composition. These rock types are not sufficiently enriched in radioactive elements to produce significant 36Cl in the deep subsurface. Carbonaceous shales, on the other hand, can concentrate the radioactive elements necessary to produce significant 36Cl in the deep subsurface. Chlorine-36 ratios (36Cl/Cl) for a suite of Late Devonian and Pennsylvanian carbonaceous shales were calculated from bulk-rock chemistry as well as measured using accelerator mass spectrometry. The poor agreement between calculated and measured ratios is the result of the assumption of chemical homogeneity used by the calculation algorithm, an assumption that was not satisfied by the carbonaceous shales. In these shales, organic matter, clay minerals, and accessory minerals are heterogeneously distributed and are physically distinct on a micron-order scale. Although organic matter and clay minerals constitute the overwhelming bulk of the shales, it is the phosphate minerals that are most important in enhancing, and suppressing, 36Cl production. Minerals such as apatite and carbonate-apatite (francolite)—by including uranium, rare earth elements (REEs), and halogens—have an important impact on both neutron production and thermal neutron absorption. By incorporating both uranium and fluorine, phosphate minerals act as neutron production centers in the shale, increasing the probability of 36Cl production. By incorporating REEs and chlorine, phosphate minerals also act to shield 35Cl from the thermal neutron flux, effectively suppressing the production of 36Cl. To reconcile the measured 36Cl ratios with the ratios calculated assuming chemical homogeneity, the shales were artificially split into three fractions: organic, clay mineral, and phosphate mineral. Neutron production was calculated separately for each fraction, and the calculation results demonstrated that the phosphate fraction exerted much more control on the 36Cl ratio than the organic or clay mineral fractions. By varying the uranium and chlorine contents in the phosphate fraction, a new, heterogeneous 36Cl ratio was calculated that agreed with the measured ratio for the overwhelming majority of the carbonaceous shales. When using rock chemistry to calculate the 36Cl ratio, rock types that show mineralogical heterogeneity on a micron scale can be divided into bulk fractions and accessory fractions for separate calculations of neutron production and neutron absorption. In this manner, a more accurate, heterogeneous 36Cl ratio can be calculated for the rock as a whole.  相似文献   

9.
Trimlines separating glacially abraded lower slopes from blockfield‐covered summits on Irish mountains have traditionally been interpreted as representing the upper limit of the last ice sheet during the Last Glacial Maximum (LGM). Cosmogenic 10Be exposure ages obtained for samples from glacially deposited perched boulders resting on blockfield debris on the summit area of Slievenamon (721 m a.s.l.) in southern Ireland demonstrate emplacement by the last Irish Ice Sheet (IIS), implying preservation of the blockfield under cold‐based ice during the LGM, and supporting the view that trimlines throughout the British Isles represent former englacial thermal regime boundaries between a lower zone of warm‐based sliding ice and an upper zone of cold‐based ice. The youngest exposure age (22.6±1.1 or 21.0±0.9 ka, depending on the 10Be production rate employed) is statistically indistinguishable from the mean age (23.4±1.2 or 21.8±0.9 ka) obtained for two samples from ice‐abraded bedrock at high ground on Blackstairs Mountain, 51 km to the east, and with published cosmogenic 36Cl ages. Collectively, these ages imply (i) early (24–21 ka) thinning of the last IIS and emergence of high ground in SE Ireland; (ii) relatively brief (1–3 ka) glacial occupation of southernmost Ireland during the LGM; (iii) decoupling of the Irish Sea Ice Stream and ice from the Irish midlands within a similar time frame; and (iv) that the southern fringe of Ireland was deglaciated before western and northern Ireland.  相似文献   

10.
A number of chemical and physical processes inside and outside a sedimentary basin (e.g. evaporite dissolution and topographic drive, respectively) affect groundwater flow near the basin’s margin. Contrasting formations at the margin, typically basinal sedimentary rocks and basement, are host to the interplay between these processes so that groundwater flows and compositions change within a relatively small volume. To interpret how groundwater flow and geochemistry have evolved, interactions between these processes must be understood. Such interactions were investigated near the margin of the East Irish Sea Basin in NW England, by sampling deep groundwaters (to 1500 m below sea level) from Ordovician volcanic basement rocks and Carboniferous to Triassic sedimentary cover rocks. Variable Br/Cl ratios and Cl concentrations in deep saline waters and brines indicate mixing patterns. Variations in 36Cl/Cl constrain the timing of mixing. Relatively low Br/Cl ratios (ca. 1 × 10−3 by mass) characterise brine from the western sedimentary cover and reflect halite dissolution further west. Saline water with relatively high Br/Cl ratios (ca. 2 × 10−3 by mass) of uncertain origin occupies the eastern basement. These two waters mix across the area. However, mixing alone cannot explain variable 36Cl/Cl ratios, which partly reflect differing in situ36Cl production rates in different rock formations. Most 36Cl/Cl ratios in groundwater sampled from the eastern metavolcanic basement (mean = 25 × 10−15) and western sedimentary cover (mean = 10 × 10−15) are at or close to equilibrium with in situ36Cl production. These variations in 36Cl/Cl across the site possibly took >1.5 Ma to be attained, implying that deep groundwater flow responded only slowly to the Quaternary glaciation of the site. Interplay between varied processes in basin marginal settings does not necessarily imply flow instability.  相似文献   

11.
The volcanic island of Jan Mayen, remotely located in the Norwegian-Greenland Sea, was covered by a contiguous ice cap during the Late Weichselian. Until now, it has been disputed whether parts of the island south of the presently glaciated Mount Beerenberg area were ever glaciated. Based on extensive field mapping we demonstrate that an ice cap covered all land areas and likely also extended onto the shallow shelf areas southeast and east of the island. Chronological interpretations are based on K-Ar and 40Ar/39Ar dating of volcanic rocks, cosmogenic nuclide (36Cl) surface exposure dating of bedrock and glacial erratics, and radiocarbon dating. We argue that ice growth started after 34 ka and that an initial deglaciation started some 21.5–19.5 ka in the southern and middle parts of the island. In the northern parts, closer to the present glaciers, the deglaciation might have started later, as evidenced by the establishment of vegetation 17–16 cal. ka BP. During full glaciation, the ice cap was likely thickest over the southern part of the island. This may explain a seemingly delayed deglaciation compared with the northern parts despite earlier initial deglaciation. In a broader context, the new knowledge of the Late Weichselian of the island contributes to the understanding of glaciations surrounding the North Atlantic and its climate history.  相似文献   

12.
New cosmogenic surface-exposure ages of moraine-crest boulders from southwestern Colorado are compared with published surface-exposure ages of boulders from moraine complexes in north-central Colorado and in west-central (Fremont Lake basin) Wyoming. 10Be data sets from the three areas were scaled to a single 10Be production rate of 5.4 at/g/yr at sea level and high latitude (SLHL), which represents the average 10Be production rate for two high-altitude, mid-latitude sites in the western United States (US) and Austria. Multiple nuclide ages on single boulders indicate that this 10Be production rate yields ages comparable to those calculated with a commonly used 36Cl production scheme. The average age and age range of moraine-crest boulders on terminal moraines at the southwestern Colorado and Wyoming sites are similar, indicating a retreat from their positions ∼16.8 36Cl ka (Cosmogenic ages in this paper are labeled 10Be or 36Cl ka or just ka when both 10Be or 36Cl ages are being discussed; radiocarbon ages are labeled 14C ka, calibrated radiocarbon are labeled cal ka, and calendar ages are labeled calendar ka. Errors (±1σ) associated with ages are shown in tables. Radiocarbon ages were calibrated using the data of Hughen et al. (Science 303 (2004) 202). This suggests a near-synchronous retreat of Pinedale glaciers across a 470-km latitudinal range in the Middle and Southern Rocky Mountains. Hypothetical corrections for snow shielding and rock-surface erosion shifts the time of retreat to between 17.2 and 17.5 10Be ka at Pinedale, Wyoming, and between 16.3 and 17.3 36Cl ka at Hogback Mountain, Colorado.  相似文献   

13.
Published cosmogenic 36Cl production rates from Ca and K spallation differ by almost a factor of 2. In this paper we determine production rates of 36Cl from Ca and K in samples of known age containing little Cl. Ca-rich plagioclases and K-feldspars were separated from a total of 13 samples collected on the surfaces of four basaltic lava flows at Mt. Etna (38°N, Italy) and from a trachyte lava flow at Payun Matru volcano (36°S, Argentina). Eruption ages, determined by independent methods, range between 0.4 and 32 ka. Sample site elevations range between 500 and 2500 m. Corresponding scaling factors were calculated using five different published scaling models, four of which consider paleo-geomagnetic field variations integrated over the exposure durations. The resulting five data sets were then analyzed using a Bayesian statistical model that incorporates the major inherent uncertainties in a consistent way. Spallation production rates from Ca and K, considering all major uncertainties, are 42.2 ± 4.8 atoms 36Cl (g Ca)−1 a−1 and 124.9 ± 8.1 atoms 36Cl (g K)−1 a−1 normalized to sea level and high latitude using the scaling method of Stone (2000). Scaling models that account for paleo-geomagnetic intensity changes yield very similar mean values (at most +4%). If the uncertainties in the independent ages are neglected in the Bayesian model, the calculated element specific production rates would be about 12% higher. Our results are in agreement with previous production rate estimations both for Ca and K if only low Cl (i.e. ?20 ppm) samples are considered.  相似文献   

14.
Tunnel excavation at Äspö Island, Sweden, has caused severe groundwater disturbance, gradually extending deeper into the tunnel as present-day Baltic seawater intrudes through fractures connecting to the surface. However, the paleo-hydrogeochemical conditions have remained in the deep highly saline waters that have avoided mixing. A correlation has been observed between dissolved 4He concentration and Cl− ion concentration, measured every two years from 1995 to 2001 at Äspö. Groundwater mixing conditions can be examined by the correlations between 1/Cl, 36Cl/Cl, and 3H concentrations. Subsurface production is responsible for the majority of the 36Cl and excess dissolved 4He of interstitial groundwater in fractures. The secular equilibrium ratio of 36Cl/Cl in rock was theoretically estimated to be (5.05 ± 0.82) × 10−14 based on the neutron flux intensity, a value comparable to the measured 36Cl/Cl ratio in rock and groundwater. The degassing crustal 4He flux was estimated to be 2.9 × 10−8  1.3 × 10−6 (ccSTP/cm2a) using the HTO diffusion coefficient for the Äspö diorite. The 4He accumulation rate ranges from 6.8 × 10−10 (for the in situ accumulation rate) to 7.0 × 10−9 (ccSTP/(gwater · a) considering both 4He in situ production and the degassing flux, assuming 4He is accumulated constantly in groundwater. By comparing the subsurface 36Cl increase with 4He concentrations in groundwater, the 4He accumulation rate was determined from data for groundwater arriving at the secular equilibrium of 36Cl/Cl. The 4He accumulation rate was found to be (1.83 ± 0.72) × 10−8 ccSTP/(gwater · a) without determining the magnitude of degassing 4He flux.  相似文献   

15.
The presence of large amounts of atacamite in oxide zones from ore deposits in the Atacama Desert of northern Chile requires saline solutions for its formation and hyperarid climate conditions for its preservation. We investigated the nature and origin of atacamite-forming solutions by means of coupling groundwater geochemical analyses with fluid inclusion data, high-resolution mineralogical observations, and chlorine-36 (36Cl) data in atacamite from the Mantos Blancos and Spence Cu deposits. In both deposits, the salinities of fluid inclusions in atacamite are comparable to those measured in saline groundwaters sampled from drill holes. The average salinity of fluid inclusions in atacamite for the Mantos Blancos and Spence deposits (~7–9 and 2–3 wt.% NaCleq, respectively) are strongly correlated to the salinities at which gypsum supersaturates from groundwaters in both deposits (total dissolved solids ~5–9 and 1–3 wt.% NaCleq, respectively). This correlation is confirmed by transmission electron microscopy observations of atacamite-bearing samples, revealing an intimate association between atacamite and gypsum that can be traced down to the nanometer scale. 36Cl data in atacamite provide new lines of evidence concerning the origin and age of the saline waters that formed atacamite in various stratabound and porphyry Cu deposits from the Atacama Desert. All atacamite samples show very low 36Cl-to-Cl ratios (11 × 10−15 to 28 × 10−15 at at−1), comparable to previously reported 36Cl-to-Cl ratios of deep formation waters and old groundwaters. In addition, 36Cl-to-Cl ratios in atacamite correlate with U and Th concentration in the host rocks but are independent from distance to the ocean. This trend supports an interpretation of the low 36Cl-to-Cl ratios in atacamite as representing subsurface production of fissiogenic 36Cl in secular equilibrium with the solutions involved in atacamite formation. Therefore, 36Cl in atacamite strongly suggest that the chlorine in saline waters related to atacamite formation is old (>1.5 Ma) but that atacamite formation occurred more recently (<1.5 Ma) than suggested in previous interpretations. Our data provide new constraints on the origin of atacamite in Cu deposits from the Atacama Desert and support the recent notion that the formation of atacamite in hyperarid climates such as the Atacama Desert is an ongoing process that has occurred intermittently since the onset of hyperaridity.  相似文献   

16.

Recently collected naturally occurring geochemical and isotopic groundwater tracers were combined with historic data from the Pahute Mesa area of the Nevada National Security Site (NNSS), Nevada, USA, to provide insights into long-term regional groundwater flow patterns, mixing and recharge. Pahute Mesa was the site of 85 nuclear detonations between 1965 and 1992, many of them deeply buried devices that introduced radionuclides directly into groundwater. The dataset examined included major ions and field measurements, stable isotopes of hydrogen (δ2H), oxygen (δ18O), carbon (δ13C) and sulfur (δ34S), and radioisotopes of carbon (14C) and chloride (36Cl). Analysis of the patterns of groundwater 14C data and the δ2H and δ18O signatures indicates that groundwater recharge is predominantly of Pleistocene age, except for a few localized areas near major ephemeral drainages. Steep gradients in sulfate (SO4) and chloride (Cl) define a region near the western edge of the NNSS where high-concentration groundwater flowing south from north of the NNSS merges with dilute groundwater flowing west from eastern Pahute Mesa in a mixing zone that coincides with a groundwater trough associated with major faults. The 36Cl/Cl and δ34S data suggest that the source of the high Cl and SO4 in the groundwater was a now-dry, pluvial-age playa lake north of the NNSS. Patterns of groundwater flow indicated by the combined data sets show that groundwater is flowing around the northwest margin of the now extinct Timber Mountain Caldera Complex toward regional discharge areas in Oasis Valley.

  相似文献   

17.
36Cl断代法   总被引：5，自引：0，他引：5       下载免费PDF全文
黄麒 《第四纪研究》1997,17(3):240-247
本文讨论了36Cl的成因、36Cl断代法的原理及其在第四纪年代学中的应用范围和测试方法，重点论述了大陆蒸发盐的36Cl断代法。作者等曾应用该法测得了柴达木盆地1．4MaB．P．以来的石盐年龄。该法也已在冰碛物、火山岩等第四纪地质体的测年中获得了成功。有理由可以相信36Cl断代法将在第四纪年代学研究中得到更广泛的应用。  相似文献   

18.
We present results of measurements of cosmogenic 10Be, 26Al and 36Cl, and the indigenous (intrinsic) concentrations of the stable elements Be, Al and Cl in 120-200 kyr old corals from Barbados and Puerto Rico. The concentration levels of these radionuclides in the corals lie in the range 104 to 108 atoms/g. A comparison of the measured nuclide concentrations with those expected to be produced in the corals by nuclear interactions of energetic cosmic radiation shows that (i) the radionuclides 26Al and 36Cl are derived from in situ cosmic ray interactions in the corals after their formation, but that (ii) the radionuclide 10Be owes its provenance in the coralline lattice primarily due to incorporation of dissolved beryllium in seawater in the lattice structure of the corals.  相似文献   

19.
Pollution of groundwater by seawater intrusion poses a threat to sustainable agriculture in the coastal areas of Korea. Therefore, seawater intrusion monitoring stations were installed in eastern, western, and southern coastal areas and have been operated since 1998. In this study, groundwater chemistry data obtained from the seawater intrusion monitoring stations during the period from 2007 to 2009 were analyzed and evaluated. Groundwater was classified into fresh (<1,500 μS/cm), brackish (1,500–3,000 μS/cm), and saline (>3,000 μS/cm) according to EC levels. Among groundwater samples (n = 233), 56, 7, and 37% were classified as the fresh, brackish, and saline, respectively. The major dissolved components of the brackish and saline groundwaters were enriched compared with those of the fresh groundwater. The enrichment of Na+ and Cl− was especially noticeable due to seawater intrusion. Thus, the brackish and saline groundwaters were classified as Ca–Cl and Na–Cl types, while the fresh groundwater was classified as Na–HCO3 and Ca–HCO3 types. The groundwater included in the Na–Cl types indicated the effects of seawater mixing. Ca2+, Mg2+, Na+, K+, SO4 2−, and Br− showed good correlations with Cl− of over r = 0.624. Of these components, the strong correlations of Mg2+, SO4 2−, and Br− with Cl− (r ≥ 0.823) indicated a distinct mixing between fresh groundwater and seawater. The Ca/Cl and HCO3/Cl ratios of the groundwaters gradually decreased and approached those of seawater. The Mg/Cl, Na/Cl, K/Cl, SO4/Cl, and Br/Cl ratios of the groundwaters gradually decreased, and were similar to or lower than those of seawater, indicating that Mg2+, Na+, K+, SO4 2−, and Br−, as well as Cl− in the saline groundwater can be enriched by seawater mixing, while Ca2+ and HCO3 − are mainly released by weathering processes. The influence of seawater intrusion was evaluated using threshold values of Cl− and Br−, which were estimated as 80.5 and 0.54 mg/L, respectively. According to these criteria, 41–50% of the groundwaters were affected by seawater mixing.  相似文献   

20.
Corrected groundwater 14C ages from the carbonate aquifer in Yucca Flat at the former Nevada Test Site (now the Nevada National Security Site), USA, were evaluated by comparing temporal variations of groundwater 36Cl/Cl estimated with these 14C ages with published records of meteoric 36Cl/Cl variations preserved in packrat middens (piles of plant fragments, fecal matter and urine). Good agreement between these records indicates that the groundwater 14C ages are reasonable and that 14C is moving with chloride without sorbing to the carbonate rock matrix or fracture coatings, despite opposing evidence from laboratory experiments. The groundwater 14C ages are consistent with other hydrologic evidence that indicates significant basin infiltration ceased 8,000 to 10,000 years ago, and that recharge to the carbonate aquifer is from paleowater draining through overlying tuff confining units along major faults. This interpretation is supported by the relative age differences as well as hydraulic head differences between the alluvial and volcanic aquifers and the carbonate aquifer. The carbonate aquifer 14C ages suggest that groundwater velocities throughout much of Yucca Flat are about 2 m/yr, consistent with the long-held conceptual model that blocking ridges of low-permeability rock hydrologically isolate the carbonate aquifer in Yucca Flat from the outlying regional carbonate flow system.  相似文献   

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