There are two main methods of isotopic dating. One detects and counts radioactive atoms through their radiation. The pioneers of radiocarbon dating used this method because carbon, the radioactive isotope of carbon, is very active, decaying with a half-life of just years. Tracing and Dating Young Ground Water. In the early s, USGS scientists (Busenberg and Plummer, ) developed a method to date ground water on the basis of chlorofluorocarbon (CFC) content of the water that is practical, cost-effective, and applicable to most shallow ground-water systems.
Discussion The samples collected in the bypass channel show the importance of the evaporation process for surface water in the semiarid northeast of Brazil. There are 2 explanations for the enrichment of the light isotopes signal. Either it is related to ancient recharge by paleo rains that occurred in different climatic conditions, or it is due to the water-rock interaction process, or both of them.
Frape et al. However the saline ones are plotted above the GMWL. Finally, the authors concluded that the saline waters have undergone an intense process of water-rock interaction and that the isotopic sign observed can reflect either a recharge in a different climate or the process of water-rock interaction Frape et al.
The first hypothesis suggests the aquifer recharge occurred at lower temperatures to result in enhanced rainfall in light isotopes. Groundwater dating shows approximate ages ranging from to 5, years before present. It is known that the Earth is in an interglacial period that started after the end of the last glacial period approximately 10, years to 12, years, which can corroborate this explanation.
The second hypothesis is that this is due to water-rock interaction during the hydration of silicate minerals in the groundwater contact with rocks, where the fractured rocks would preferentially use heavy isotopes, causing the fractionation of the groundwater and residual water enriched in light isotopes.
In the study area, as in the northeastern region of Brazil, the aquifers occur on interconnected fractures with low flow or with many stagnation centers resulting in water being a long time in the aquifer, and allowing the development of water-rock interaction processes that take place during an extended time.
In the fieldwork, it was possible to observe that wells located relatively close to each other, can show hydraulic characteristics yield, static level, specific capacity and water characteristics ages, electrical conductivity quite distinct.
This indicates that the aquifers in the area occur in isolated systems of fractures and that there is a not significant regional flow. These systems of fractures would be interconnected by large sub horizontal fractures that serve as major paths to conduct and store water.
Due to little Strontium Sr concentration in the groundwater, the water-rock interaction can result after a long time in a isotopic equilibrium with respect to Sr. The data analysis for strontium in 3 pairs of samples allows observing that the process of water-rock interaction in groundwater exists in different intensities.
However, in Sample , they are. The results confirm the process of water-rock interaction into the isotopic equilibrium with respect to strontium in different intensities. The water dating results show that there is no relationship between the electrical conductivity and the groundwater ages even considering a small data set, just 4 analyses. The differences in the fracture characteristics such as geometry and connectivity and in the dissolution rates of minerals that filled the fractures explain the lack of correlation.
In areas where the flow is restricted and has the presence of more soluble minerals, the salinity of the waters is high but not necessarily is it older. In the region of Petrolina, a set of factors are involved in the high salinity of groundwater, such as: The enrichment in salts in the soils can be a major controller in the groundwater salinization when compared to the water-rock interaction process, which explains the absence of relationship between the E.
The lack of correlation of the E. Conclusions Groundwater salinization is observed in several fractured aquifers areas of the semiarid region in northeastern Brazil.
It is believed that it can be the result of a joint action of several factors and is not being caused by just one aspect alone. The dating data prove there is no correlation between the electrical conductivity and the groundwater ages.
As the water-rock interaction process is slow, the soil's salt enrichment is considered as the most important factor for the water salinization. The majority of the soils in the region are poorly evolved, shallow and enriched in alkali and alkaline earth elements. Precipitation waters percolating in the soils reach the crystalline substrate and flow through open fractures transporting the solute.
During this process, these may carry soluble elements present in the soils. Such processes can enrich the waters and increase its salinity before infiltration into the fractures in the rock mass.
The plan relief pattern, the crystalline terrain nature, and the thin soil coverage result in high volumes of precipitation waters to remain on the surface as depression storage for a long time where intense direct evaporation as can be verified by the isotopic signal of the bypass channel causes the salt enrichment in the recharge water. Subsequently rains can carry the salts accumulated as depression storage into the fractured aquifers.
The low connectivity of the fractures, together with the relief pattern showing shallow and open valleys , restricts the water velocity circulation, due to the low gradient of the system as a whole. The longer the time the water stays in the fractures increases the water-rock interaction increasing the salinity. This can be caused by water-rock interaction, or by recharge of paleo rains.
The first option occurs by the feldspar hydration processes to form clay minerals e. Historic reports of Petrolina residents demonstrate that groundwater quality can be enhanced after continuous pumping for some decades.
The pumping activates the water circulation in the aquifer and open space in the depression cones so that new recharge water can mix with the old stagnated water or move slowly in the fractures pore space. February 07, ; Accepted: August 15, This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Biotite and hornblende are also commonly used for K-Ar dating. A clastic sedimentary rock is made up of older rock and mineral fragments, and when the rock forms it is almost certain that all of the fragments already have daughter isotopes in them.
Furthermore, in almost all cases, the fragments have come from a range of source rocks that all formed at different times. If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock. It might be possible to date some chemical sedimentary rocks isotopically, but there are no useful isotopes that can be used on old chemical sedimentary rocks.
Radiocarbon dating can be used on sediments or sedimentary rocks that contain carbon, but it cannot be used on materials older than about 60 ka.
The proportion of 40K remaining is 0. Using the decay curve shown on this graph, estimate the age of the rock. Solution An example is provided in blue for a 40K proportion of 0. This is determined by drawing a horizontal line from 0. Some of the other important pairs are listed in Table 8. When radiometric techniques are applied to metamorphic rocks, the results normally tell us the date of metamorphism, not the date when the parent rock formed.
Table 8. These phenomena are important to those who study the climate, because isotopic compositions can be used to trace the global flows of water and other compounds. Because of these effects, fresh waters on Earth vary relatively widely in their isotopic composition. Despite this variation, it is important to have a "standard" water which is easily reproducible and against which other waters can be compared.
Fortunately, ocean water can provide such a standard. The isotopic composition of deep offshore ocean water is remarkably uniform across the Earth. VSMOW is the standard against which isotopic compositions of both hydrogen and oxygen are reported. For the most precise scientific work, it may be necessary to convert some values between VSMOW and the water actually used for example, the difference in density due to isotopic composition between VSMOW and purified tap water can be a few parts per million.
The isotopic composition is also important in converting between mass-based and molar units; the relative molar mass of VSMOW is Finally, we can mention "heavy water.
The relative molar mass of heavy water by this definition is