Roberto Gonfiantini

The altitude effect on the isotopic composition of tropical rains

Abstract Data on the Isotopic composition of yearly and monthly precipitation samples collected at various altitudes on Mount Cameroon, Africa, and in two transects from the Amazon to the Altiplano in Bolivia, South America, are presented. In Bolivia, the 2H/1H and 18O/16O ratios show seasonal variations, with lower values in the summer rainy months with respects to the winter dry ones. The δ2H and δ18O values are linearly correlated with a slope of 7.5 in all seasons, but the intercept is higher in winter than in summer. The δ-gradient vs. altitude is larger in rainy periods. The isotopic data are fitted by using a numerical model based on Rayleigh adiabatic condensation process. Model simulations show that the δ–altitude relationship slightly deviates from linearity, because the slope increases with altitude due to the lowering of temperature and the consequent increase of the condensation rate of atmospheric vapour. The parameters which most affect the shape of δ–altitude relationships are the temperature vertical gradient (lapse rate) and the initial relative humidity of the ascending air masses, while a change of the initial isotopic composition of water vapour determines a shift of the curve along the δ-axis. In addition, the model explains the observed increase of the deuterium excess with altitude.

Isotopic and chemical composition of parbati valley geothermal discharges, North-West Himalaya, India

The isotopic compositions of the waters discharged from Parbati Valley geothermal areas indicate a higher altitude meteoric origin, with discharge temperatures reflecting variations in the depth of penetration of the waters to levels heated by the existence of a ‘normal’ geothermal gradient. On the basis of mixing models involving silica, tritium, discharge temperatures and chloride contents, deep equilibration temperatures of 120–140°C were obtained for Manikaran, possibly reaching 160°C at even greater depth. Geothermometers based on sulfate-water 18O exchange and gas reactions point to similar temperatures. Exceptionally high helium contents of the discharges correspond to apparent crustal residence times of the waters in the order of 10–100 Ma; relative nitrogen-argon contents support a largely meteoric origin of the waters with a possible fossil brine, but no detectable magmatic component.

Isotope effects accompanying vacuum extraction of soil water for stable isotope analyses

Abstract The vacuum distillation method of extracting soil water for stable isotope analysis was tested for three different types of soil characterized by high water content: (1) pure sand, (2) cambisol with high organic matter content, developed on calcareous sandstone under temperate climatic conditions (Austria), and (3) tropical latosol poor in organic matter, developed on sandy clay sediment (Brazil). The method yields accurate and reproducible results for sand, provided that more than 98% of the original soil water is extracted. The time required for complete extraction is a function of sample size and the applied extraction temperature. Column experiments with the clayey soils revealed existence of a weakly bound, easily exchangeable pool of water which is isotopically different from the mobile water. The experiments showed that the extracted soil water is depleted in both deuterium and oxygen-18 by 5–10% and 0.3–0.5%, respectively, when compared with the percolate (mobile water). This depletion depends strongly on the soil type. The reproducibility for replicate extractions of soil water from clayey soils is around ±3% and 0.3% for δD and δ 18 O, respectively.

Carbon isotope exchange rate of DIC in karst groundwater

Abstract The kinetics of isotopic exchange between dissolved inorganic carbon (DIC) of groundwater and calcite of the matrix of karst aquifers of Cyrenaica, Libya, can be deduced from 13 C and 14 C data. The aquifers are mostly confined, and the majority of the wells do not show any occurrence of modern recharge: in 1976–1980, in fact, the tritium content was below 1 tritium unit (TU) in most sites. Assuming that the isotopic exchange takes place through a first order reaction such as 14 C radioactive decay, it can be shown that a linear correlation occurs between ln A and ln( δ M − δ − e P ), where A is the 14 C activity, δ M and δ are the 13 C contents of matrix calcite and DIC, respectively, and e P is the 13 C enrichment in CaCO 3 precipitation. The slope of the correlation provides the half-life of the isotopic exchange process. For Cyrenaica karst groundwater, a half-life of about 11,000 years is obtained, i.e. about double that of 14 C radioactive decay. The isotopic exchange kinetics also depends on the ratio between groundwater volume and the calcite surface exposed to the exchange process. Thus, other aquifers will show different exchange half-life values. The Cretaceous chalk aquifers of the Paris Basin, France and Lagerdorf, Germany give a half-life of about 4000 years, much shorter than that of Cyrenaica, which may be due to the high porosity, i.e. to the large surface available for the isotope exchange process. The Berkshire Chalk aquifer, UK, gives a half-life of about 10,000 years. Much higher half-lives, above 20,000 years, are obtained for two sandy aquifers in Flevoland, The Netherlands, and Texas, USA, which could be explained by the low CaCO 3 content of the aquifer matrix. The highest half-life value, about 40,000 years, is obtained in an artesian limestone aquifer in Florida, USA.

Chapter 7 – Isotopes in Groundwater Hydrology

Publisher Summary The concentration of stable isotopes in groundwater depends mainly on the origin of the water. The concentration of radioactive isotopes and dissolved compounds in groundwater depends on the initial concentration and residence time of groundwater in the aquifer. These two factors determine the amount of radioactive isotope(s) decayed, removed, or added during water-rock processes. As geochemical tools, stable and radioactive environmental isotopes provide information on the geochemical processes operating on groundwater and on the hydrogeological characteristics of aquifers. Information provided by environmental isotopes is also useful in modeling groundwater systems. This chapter discusses the scientific background of applications of environmental isotope techniques to groundwater hydrology. The stable isotope composition of groundwater reflects that of the precipitation in the recharge area that seeps through the soil and the unsaturated zone to reach the water table. Stable isotopes are often used to identify groundwater recharge by rivers and lakes. These water bodies frequently have isotopic compositions different from that of precipitation over the study area.

Composition isotopique et origine de la vapeur d'eau atmospherique dans la region du lac Leman

Abstract The isotopic composition of the atmospheric water vapour over the Leman Lake changes regularly with the distance from the water surface. This fact indicates that the vapour results from a mixing of the vapour generated by the lake with the regional atmospheric vapour. The fractions of these two vapours of different origin are evaluated in each sample on the basis of the isotopic data. The method allows to estimate the climatic zone of influence of the lake in the atmosphere.

Environmental isotopes in geothermal studies

Abstract The variations in the 18 O/ 16 O, D/H, 13 C/ 12 C, 34 S/ 32 S isotopic ratios and in the tritium concentrations of several components of geothermal fluids have been used widely in the hydrological study of thermal systems. The recharge areas, as well as the mixing phenomena between different fluids and the circulation times, can all be evaluated satisfactorily with the help of these techniques. Of great diagnostic value are the effects of the high temperatures at depth in the geothermal fields on the isotopic exchange reactions between the fluids and the reservoir rocks and on the fractionation factors between the compounds that coexist in the fluid. The latter phenomenon in particular has permitted us to use many isotopic geothermometers that provide useful indications on the temperatures of the main reservoirs. This paper gives a detailed description of their main applications and present limits, together with some indications as to future development of field research with regard to isotopic methods applied to exploration and study of geothermal areas.

Hydrology of the Greater Tongonan geothermal system, Philippines, as deduced from geochemical and isotopic data

Fluids in the Greater Tongonan geothermal system exhibit a large positive {sup 18}O shift from the Leyte meteoric water line. However, there is also a significant shift in {sup 2}H. The {delta}{sup 2}H-{delta}{sup 18}O plot shows that the geothermal fluids may be derived by the mixing of meteoric water with local magmatic water. The most enriched water in the Greater Tongonan system, in terms of {delta}{sup 18}O, {delta}{sup 2}H and Cl, is comprised of approximately 40% magmatic water. Baseline isotope results support a hydrogeochemical model in which there is increasing meteoric water dilution to the southeast, from Mahiao to Sambaloran and towards Malitbog. The Cl-{delta}{sup 18}O plot confirms that the geothermal fluid in Mahanagdong, further southeast, is distinct from that of the Mahiao-Sambaloran-Malitbog system.

On the isotopic composition of precipitation in tropical stations (

Sao discutidas algumas tendencias e caracteristicas da composicao isotopica das precipitacoes nas estacoes tropicais. Em ilhas pequenas do Pacifico, as estacoes apresentam variacao de acordo com a latitude, com valores-8 menores entre 15°N e 15°S e variacoes maiores a latitudes mais elevadas. Os isotopos pesados, nas estacoes interioranas tias, apresentam-se empobrecidos com relacao as estacoes costeiras, porem algumas vezes esse efeito continental e bastante complexo, como por exemplo na Africa. A media mensal de valores -8 indica uma sensivel correlacao com a quantidade de precipitacao, porem as variacoes em declive nao indicam uma dependencia clara da media a longo prazo do valor-8 como era de se esperar teoricamente. Nas estacoes da America do Sul, de acordo com as estacoes do ano, ha uma correlacao entre as variacoes das medias mensais dos valores-8 os quais sao maiores nas estacoes mais para o interior devido a continentalidade. Sao tambem discutidos os possiveis efeitos da reciclagem do vapor dagua proveniente da evapotranspiracao.

Isotopic evidence for magmatic and meteoric water recharge and the processes affecting reservoir fluids in the Palinpinon geothermal system, Philippines

Abstract Stable isotopic compositions of meteoric and geothermal waters indicate that the Palinpinon geothermal system of Southern Negros is fed by a parent water that originated from a mixture of local meteoric (80%) and magmatic (20%) waters. The meteoric water has an isotopic concentration of −8.5‰ and −54‰ in 18O and 2H, respectively, which corresponds to an average infiltration altitude of about 1000 m above sea level. With exploitation of the system and injection of wastewaters to the reservoir, the stable isotopic composition became heavier due to significant mixing of geothermal fluids with injection waters. Incursion of cooler meteoric waters, which is confirmed by the presence of tritium, also leads to the formation of acid-sulfate waters. Stable isotopes are effective as “natural tracers” to determine the origin and mixing of different fluids in the reservoir.