Consider the use of naturally occurring isotopes in ground water studies. a) One

Question

Consider the use of naturally occurring isotopes in ground water studies. a) One of the uses is for ground water dating. Discuss how you can do this, what age ranges are possible with various pairs of isotopes, what you need to consider when interpreting the results, and what use this information could be to us in environmental work. b) Now consider any other use of isotopes in ground water investigations and discuss it. Be sure and explain how it “works”, the types of investigations where its use might be appropriate, and the problems you might have in using it.

Explanation / Answer

Environmental isotopes are used to study hydrological systems and it is mainly useful for ground water studies. Commonly used natural isotopes are heavy isotopes of the elements of water molecule, oxygen and of the element of carbon (13C and 14C). Use of these in ground water studies are based mainly upon isotopic variations in atmospheric precipitation.

For determining the groundwater age, the radioactive decay of environmental radioisotopes and the temporary nature of some of these (bomb tritium, anthropogenic krypton-85, bomb carbon-14 and bomb chlorine-36) are considered. Residence time or groundwater age is the time taken for the water to isolate from the atmosphere. Recharge of unconfined aquifers mainly results in a vertical gradient of groundwater ages that is increasing age with depth, but in these confined aquifers the controlling feature is a horizontal or lateral gradient that is age increasing with distance from area of recharge. In the first case the gradient is roughly proportional to the inverse of the recharge rate (volume/time), but in the second case the gradient is roughly proportional to the inverse of the flow velocity. Therefore, the hydro geologically significant parameters for groundwater dating with radioactive isotopes are the recharge rate and flow velocity of groundwater in unconfined and confined aquifers, respectively.

One of the methods to calculate groundwater flow velocity is by evaluating the decrease in radioisotope concentration along the flow path. If we know the mean porosity value of the aquifer, groundwater flow rate can be calculated. This method needs at least two wells along the flow path of an aquifer and the initial radioisotope concentration in the recharge area. Naturally, groundwater movement is generally very slow, in the order of a few metres per year. Water which has moved a few kilometres along the flow path under natural conditions is estimated that hundreds or thousands of years old which is an age greater than the dating range of tritium, tritium and helium-3, and chlorofluorocarbons. Because of this, in large aquifers with long flow paths, commonly used radiometric method to determine groundwater residence times has been carbon-14. Its half-life is 5730 years and this makes a suitable tool for the dating of groundwater in a long age range of about 2000 to 40 000 years.

Very slow moving groundwater in deep confined aquifers has an age of tens or hundreds of thousands of years. These ages are far from the dating range of carbon-14 and so it requires very long-lived radioisotopes. Among the three long lived radioisotopes used in water studies, krypton-81, chlorine-36 and iodine-129, only chlorine-36 has been used in wider practical use.

b)

Environmental isotopes are used for ground water recharge studies. The characterization of groundwater recharge is necessary for the development and management of groundwater resources. Aquifers with little recharge show only small fluctuations in groundwater levels; so we cannot easily obtained a consistent estimate of recharge rate cannot through classical approaches such as water level monitoring alone.

The isotope methods are practically the main tools that can be used to estimate present day groundwater recharge under different conditions. The isotopic composition of groundwater is calculated by the isotopic composition of recharge. If most of the recharge is from direct penetration of precipitation, the groundwater will reveal the isotopic composition of that precipitation. However, if most of the recharge is through surface water, such as rivers or lakes, the groundwater will show the average isotopic composition of the surface water.

Hydrological settings of the very old (>10 000 years) groundwater, the regional climatic conditions at the time of recharge may have been different from those existing today, and this is showed in the isotope composition of the groundwater. It is possible to estimate the modern recharge, within 40 to 50 years by measuring the isotopes and dissolved gases in soil water, in an unsaturated zone or in groundwater from shallow and unconfined aquifers and springs.

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