2 Methods
2.8 Isotope Hydrology
54 2 Methods cussion. The number of radioactive decay events is measured at the air sample by an a-scintillometer or an ionization chamber.
The emittance of radon by contaminated waste or by natural rock is based upon geochemical and geodynamical processes. Radon enrichment of the soil indicates the presence of preferred ascent paths. This can be leaking seals at dumps or natural steep dipping structures, like fracture or fissure zones. In any case, gases contaminated by radon will follow the path that is paved by higher permeabilities of rocks or materials. Radon surveys are therefore well adapted to investigate planned waste disposal sites in hard rock for potential transport paths of conta- minants.
-4r---~----+_----+_~~4_----4_----~
o -8r-+-~~~~++~--~~~~~~~~-4~~
~ ~
",0
~vO -10r4----~----~~---=+_----_H--~~_+--_4
-12r---+---~~----_+---~----~--~
Mean 196[,- 69
196[, 1965 1966 1967 1968 1969 January Fig.2.30. a-180 content and mean yearly temperature in GroningeniHolland
t
~ 15 ~
::J
10 "'§ W
5 E 0.
~
Such isotopes are tools of the environmental geophysics that are used to estimate the amount of ground water regeneration and its dynamics, to differen- tiate ground water horizons by their genesis, and to estimate the share and the origin of toxic constituents in ground water. The determination of the portions of hydrogen and oxygen isotopes in ground water can be done fast and cheaply. The 180/160 relation of a water sample is called its a-value.
Stable isotopes are components of water molecules with different atomic weights: IH 2160, IH2H I60, 2H 2160 and IH/80. In these molecules, the atomic masses 18,19 and 20 are predominant. Their environmental importance stems from their different steam pressures, which lead to the enrichment of the lighter molecules in the volatile phase by evaporation, condensation or sublimation of water. This effect is called isotope fractioning.
The relation between hydrogen and oxygen isotopes is measured mostly not by its absolute value but comparatively, by its VSMOW-delta-value. It comes from Vienna Standard Mean Ocean Water a-value, scaled in %0 (parts per thou- sand). If ground water is accumulated by precipitation, it is called meteoric water.
Its a l80 and a2H rates have a linear relation, which is called MWL or meteoric water line. With open and stagnant water, this relation is no longer linear, since evaporation leads to isotope fractioning.
The heavier water, molecules evaporate faster at higher temperature. Therefo- re the isotope configuration of the precipitation shows a yearly variation, which is distinctly different for summer and winter. This effect can be observed only in ground water that is not older than 4 years. It is, for example, possible to assess whether contaminated ground water is older than 4 years (Fig. 2.30).
Moreover, this temperature dependence allows the distinction of ground water, that was formed during the cold Pliocene period from the more recent ground water of the warmer Holocene.
56 2 Methods The reduction of air temperature with growing altitude produces lower delta values of the rain. This is the key to determining the topographic altitude of the catchment area of spring water with an accuracy of ± 50 m.
The age assignment of hundreds to thousands of years-old ground water is sub- stantially based on the radiocarbon isotope 14C, which is created by the omni- present cosmic radiation. The radiocarbon is inhaled as the gas CO2 by living beings. After their death, the process of decay creates new CO2 , which is dis- solved as hydrogen carbonate in rainwater and later in ground water.
During the aging of ground water, its radioactivity decreases regularly, ac- cording to the half-life time of radiocarbon. This means that by measuring the radiocarbon content, the age of the ground water could be calculated. However, not all carbon in the water comes from the sky. Other carbon molecules stem from the weathering and dissolving of limestone. They are the culprits for the unwan- ted permanent hardness of ground water.
This extra carbon may lead to ages of ground water that can be several thousand years too high. Therefore these ages have to be corrected. The differ- ence between corrected and non-corrected ages is constant and is used to identify ground waters of equal origin or to follow-up movements of special ground water.
Lesser water ages are dealt with by the tritium eH) method. This has been caused by nuclear testing in the atmosphere and has later seeped into the ground.
The nuclear test program reached its peak during the years 1963/64 (Fig. 2.29).
Today there is none of this tritium left and any age determinations can operate only by the qualitative analysis of ground water that has been formed ever since.
The contents of tritium in water are scaled in tritium units = TV. 1 TU corre- sponds to the very weak concentration of one tritium atom to 10 18 molecules of water. In many countries, the International Atomic Agency of the United Nations in Vienna performs continuous 3H monitoring to study the short-term ground water circulation and to control the adherence to the nuclear test ban treaty.
In the environment, especially in the exploration of hazardous waste sites, the isotope hydrology may be employed in many ways. Three examples illustrate this:
A) It can be decided whether the salinity of ground water originates from great depths, or from dumps of salt mines.
B) At old watered mines, it can be established when its water reserves are ex- hausted by pumping.
C) The age of contaminated leachate plumes can be determined.