James B. Cowart

Radioelements, radiogenic helium and age relationships for groundwaters from the granites at Stripa, Sweden

Abstract The solution of radioelements and radiogenic 4He by groundwaters in fractured rocks is dependent upon the radioelement distribution in the rock matrix and the extent of the rock-water interface. The 234 U 238 U activity ratio and the dissolved U, Rn and He contents of such groundwaters respond to changes in the flow regime with time. Although 234 U 238 U activity ratios change with groundwater residence time as a consequence of 234Th-recoil induced solution of 234U, the activity ratio is strongly influenced by the U distribution within fractures, by the extent of the rock-water interface and by the amount of 238U in solution. A model for the quantitative evaluation of these effects is presented. Groundwaters from depths up to 880 m in the Stripa granite have variable dissolved uranium contents and 234 U 238 U activity ratios. The uranium geochemistry is primarily determined by variations in flow path rather than by groundwater age. Dissolved radiogenic 4He in the groundwaters increases with their depth of origin, and is dependent upon the U content of the granite and upon its fracture porosity. It increases with groundwater residence time but movement of 4He by diffusion and transport processes make the actual groundwater age indeterminate.

Polonium in Florida groundwater and its possible relationship to the sulfur cycle and bacteria

The last radioactive member of the {sup 238}U natural decay-series, {sup 210}Po is normally considered a very particle-reactive isotope. Analysis of most natural waters shows that {sup 210}Po is present at very low activities, usually even lower than its insoluble precursor, {sup 210}Pb. The authors have recently discovered, however, that {sup 210}Po exists at very high concentrations in groundwaters of some shallow aquifers in west central Florida. These waters tend to be fairly acidic (pH < 5), sulfide-bearing, and relatively high in {sup 222}Rn. Detailed study of one well with extraordinary levels of {sup 210}Po ({approximately} 1000 dpm/l) indicates that: (1) {sup 210}Po in this water is in great excess of radioactive equilibrium with its predecessors {sup 210}Pb and {sup 210}Bi; (2) most Po in this water exists in a form which does not coprecipitate with an iron hydroxide scavenge; and (3) the conversion of soluble (0.2 {mu}m filter) to particulate Po occurs over a time scale of a few days during sulfide oxidation. The authors suspect that Po cycling in this environment is related to the sulfur cycle and may, therefore, be influenced by sulfur bacteria.

Desert paleoenvironmental data from cave speleothems with examples from the Chihuahuan, Somali-Chalbi, and Kalahari deserts

Abstract Pollen-bearing lake, bog, and spring sediments are relatively scarce in many arid and semiarid regions of the world, and few are dateable beyond the 14C range. We have obtained pollen spectra from speleothems collected from caves in the Somali-Chalbi and Kalahari deserts suggesting that these deposits may be an important future source of desert paleovegetation data. As cave speleothems can be dated by the 230Th/234U method to c. 350,000 yr B.P., and by the TL and ESR methods potentially to 1 m.y. B.P., and can sometimes give paleotemperature and paleohydrologic data, they could provide a first glimpse of desert paleoenvironments during isotope stages 4–9. Ages of speleothems from the Chihuahuan, Kalahari, and Somali-Chalbi deserts suggest that there was more available moisture in the southwestern U.S.A. and in northwestern Bostwana during glacials and interstadials of the last c. 300,000 years, but that wetter conditions in the Horn of Africa corresponded with interglacials and perhaps to a lesser extent with interstadials. Pollen from three northern Somalia speleothems indicate more mesic conditions in the Horn of Africa at 10,000, 11,800, and 176,500 yr B.P., while speleothem pollen spectra from Matupi Cave in northeastern Zaire, presently surrounded by tropical rainforest, suggest a savanna grassland at this cave c. 14,000 yr B.P.

A Holocene pollen record for the Kalahari Desert of Botswana from a U-series dated speleothem

A 61 cm core through a speleothem column in Drotskys Cave, Botswana, has yielded a U-series dated pollen record of Holocene vegetation changes in the Kalahari Desert. Between c.10000-7000BP, the site was surrounded by an arid grassland with dry-adapted trees and shrubs such as Acacia and Commiphora. An increase in pollen of Combretaceae and Cyperaceae, and the appearance of pollen of such mesic savanna plants as Grewia between c. 7000—6000 BP, may indicate the onset of slightly wetter conditions. Combretaceae and other mesic pollen types increase between c. 6000-3000 BP, but the late Holocene record appears variable, with the period c. 5000-4000 BP being somewhat dryer than the millennia before and after. It would appear that Kalahari vegetation has changed less during the Holocene than many other African environments previously investigated. Despite the limitations imposed by pollen analysis of cave sites, the record from Drotskys Cave shows good agreement with regional trends derived from other studies. The use of U-series dated pollen spectra derived from speleothems holds potential for expanding the geographic scope of palynological studies to areas lacking suitable lakes and bogs, and for extending the dated pollen record back an order of magnitude beyond the 14C timescale.

Mixing volume calculations, sources and aging trends of Floridan aquifer water by uranium isotopic methods☆

Abstract The activity ratio (A.R.) of two naturally occurring isotopes of uranium, U-238 and U-234, varies significantly in aqueous environments. The A.R., used in conjunction with the uranium concentration, provides a ‘fingerprint’ of groundwater masses and, by use of these parameters, mixing volumes and flow patterns can be determined. This method was applied to two large limestone springs in central Florida; the mixing volumes and flow patterns calculated are notably similar to those derived by standard hydrologic methods. In deeper parts of the aquifer, the waters are found to have low uranium concentration and a pattern of higher A.R.s suggestive of an aging effect.

Radium in the Suwannee River and estuary

A two-year study of radium in the Suwannee River has shown that groundwater discharge, via springs, is a very important source of radium both to the river and to offshore Gulf of Mexico waters. Dissolved radium is maintained within relatively narrow limits in the river by uptake into suspended particles. In the estuary, dissolved radium versus salinity profiles show distinctive nonconservative behavior with radium in significant excess of its linear mixing value at mid-salinities. Unlike the situation in many other estuaries, however, desorption of radium from particles cannot account for most of the observed excess. Thus, the anomalously high radium characteristic of much of the west Florida shelf apparently does not have a riverine source. Direct effusion of high-radium groundwater into these coastal waters is thought to be the major supplier of radium, and perhaps other elements as well.

The relationship of uranium isotopes to oxidation/reduction in the Edwards carbonate aquifer of Texas

Abstract The concentration of dissolved uranium and 234 U/ 238 U alpha activity ratio (“A.R.”) were determined in water samples from 23 locations in the Edwards carbonate aquifer of south central Texas by isotope dilution methods and alpha spectrometry. This aquifer consists of two parts, an updip oxidized portion and a downdip reduced portion. At some places the boundary is associated with faulting and at others it is not. The boundary between the two portions of the aquifer can be located by an abrupt change in chemical properties of the water such as a large increase in concentration of Cl − , SO 4 2− and total dissolved solids, the presence of H 2 S and a decrease in Eh in moving from updip to downdip. Compared with the oxidized samples, the uranium concentration is much lower and the A.R. higher in the reduced samples so that the uranium from each portion falls in a distinct field. The oxidized aquifer samples show very little variation in the measured uranium parameters even though, in some cases, there is evidence that the water has flowed through some tens of kilometers of aquifer. Samples collected near the boundary at those places not associated with faulting yield dissolved uranium values which fall in neither field and which, for the most part, cannot result from mixing of the oxidized and reduced waters. These samples probably result from changes in location of the oxidation-reduction boundary.

U-Series Nuclides as Tracers in Groundwater Hydrology

The occurrence of the heavy radionuclides in the hydrosphere has become increasingly important in the context of today’s emphasis on the measurement of quality and quantity of water resources. The study of the natural aqueous behaviour of uranium, radium, and the shorter-lived daughters serves both as a background for radioactivity pollution studies, and also as a widely applicable method of tracing movements of the groundwater itself.

Uranium-isotope variations in groundwaters of the Floridan aquifer and Boulder Zone of south Florida☆

Water samples from four wells from the main Floridan aquifer (300–400 m below mean sea level) in southeast Florida exhibit 234U233U activity ratios that are significantly lower than the secular equilibrium value of 1.00. Such anomalous values have been observed previously only in waters from sedimentary aquifers in the near-surface oxidizing environments. These four wells differ from six others, all producing from the same general horizon, in being located in cavernous highly transmissive zones. We hypothesize that the low activity ratios are indicative of a relic circulation pattern whereby water from the surface aquifer was channelled to lower levels when sea level was much lower. At a deeper cavernous level, known as the Boulder Zone (800–1,000 m below mean sea level), the U isotopes, along with other chemical constituents, show progressive changes with increasing distance from an inferred flow source in the Straits of Florida. This tends to support the hypothesized landward flow (though with a more northerly component) of cold seawater in the extensively transmissive Boulder Zone.

Strontium isotopes in brines and associated rocks from Cretaceous strata in the Mississippi Salt Dome Basin (southeastern Mississippi, U.S.A.)

Abstract Sr concentrations and 87 Sr 86 Sr ratios have been determined for brines from formations spanning most of the Cretaceous in two oilfields in the Mississippi Salt Dome Basin and from local salt domes, chalk, sands and shales. The Selma Group is predominantly a fossiliferous chalk whereas the remainder of the stratigraphic sequence consists of sands and shales with only minor carbonate phases. The sampled interval ranges from 2000 to just over 4000 m below land surface. Sr concentrations range from 443 to 1718 mg kg −1 for brines from Baxterville field and from 528 to 1762 mg kg −1 for brines from Soso field. Brine 87 Sr 86 Sr ratios range from 0.7071 to 0.7116 at Baxterville field and from 0.7083 to 0.7119 at Soso field. With the exception of brines from the Selma Chalk, all 87 Sr 86 Sr ratios are more radiogenic than Cretaceous seawater. On plots of 87 Sr 86 Sr vs. Sr concentration, brines from individual formations within a producing field occupy distinct non-overlapping areas with the exception of a single sample. In some cases, individual reservoirs within formations can be distinguished. Brines from formations that are present in both fields, which are ∼ 85 km apart, do not have the same 87 Sr 86 Sr ratio. In each field, brine Sr concentration increases with depth. 87 Sr 86 Sr ratios increase with depth to ∼ 3000 m and then appear to level off. These trends are interpreted to be caused by dissolution or recrystallization of carbonate minerals which contain significant amounts of non-radiogenic Sr, and decomposition of Rb-rich detrital micas and K-feldspar, which are sources of radiogenic 87 Sr. Salt and cap rock from domes in southeastern Mississippi have an average 87 Sr 86 Sr ratio of 0.70707, consistent with a Middle Jurassic age for the Louann Salt. In Baxterville field, chalk from the Upper Cretaceous Selma Group has a Cretaceous seawater 87 Sr 86 Sr ratio of 0.7076. 87 Sr 86 Sr ratios range from 0.7091 to 0.7111 for Sr leached from sands and shales from both Baxterville and Soso fields. If these values reflect the 87 Sr 86 Sr values of the easily dissolved interstitial carbonate phases, they indicate that the calcite cements precipitated from sedimentary fluids which were more radiogenic than the seawater of that time, but which were not as evolved as the brines which are now present. Alternatively, they may reflect leaching of radiogenic sources.