Some­times one can hear adver­tise­ments from com­pa­nies sell­ing drink­ing water, in which they jus­ti­fy the need to pur­chase their prod­uct specif­i­cal­ly from them by claim­ing that the water is extract­ed from a depth of 300 m and that it sup­pos­ed­ly formed dur­ing the Juras­sic peri­od, when dinosaurs still roamed the Earth and humans did not yet exist, not to men­tion any anthro­pogenic impact. But is this true, and what is the actu­al age of ground­wa­ter?

Inter­est in deter­min­ing the age of ground­wa­ter has emerged over recent decades in devel­oped coun­tries. This is dri­ven by the need to assess the rate and direc­tion of changes in the chem­i­cal com­po­si­tion of ground­wa­ter used for water sup­ply. Deter­min­ing ground­wa­ter age is cru­cial for pre­dict­ing which types of con­t­a­m­i­na­tion it may con­tain.

What exact­ly is the age of ground­wa­ter? For infil­tra­tion-type ground­wa­ter (water that enters an aquifer through the per­co­la­tion of atmos­pher­ic precipitation—these waters are most often of high qual­i­ty and are used as drink­ing water), the “age” refers to the time dur­ing which the water trav­els from recharge areas to dis­charge areas. Ground­wa­ter moves through rock for­ma­tions quite slow­ly. Accord­ing to esti­mates by the U.S. Geo­log­i­cal Sur­vey, a flow veloc­i­ty of 0.3 m per day for ground­wa­ter is con­sid­ered high; depend­ing on the rock com­po­si­tion, this veloc­i­ty may decrease to 0.3 m per year or even per decade. There­fore, the time required for water to pass through rock for­ma­tions and reach an aquifer may range from tens to hun­dreds or even thou­sands of years.

Why is ground­wa­ter age so impor­tant? “Young” ground­wa­ter is more often affect­ed by anthro­pogenic con­t­a­m­i­na­tion (for exam­ple, com­po­nents of fer­til­iz­ers and pes­ti­cides) than “old” ground­wa­ter. On the oth­er hand, “old” ground­wa­ter more fre­quent­ly con­tains nat­u­ral­ly occur­ring con­t­a­m­i­nants (such as met­als or nat­ur­al radionu­clides), which enter the water due to long-term inter­ac­tion with water-bear­ing rocks enriched in these ele­ments.

Ground­wa­ter — the waters of the first aquifers from the sur­face are, as a rule, young, they have a high rate of water exchange. Deep aquifers of sig­nif­i­cant thick­ness, con­tain­ing water­proof lay­ers in the roof, are old — their age is mea­sured in thou­sands of years. The youngest are the waters of moun­tain springs.

The age of ground­wa­ter is deter­mined by mea­sur­ing age mark­ers, most com­mon­ly radioac­tive iso­topes that decay at a known rate. These may include both nat­ur­al iso­topes and arti­fi­cial ones intro­duced into the envi­ron­ment through nuclear test­ing or nuclear acci­dents, that is, at a spe­cif­ic known point in time. In Ukraine, such mark­ers may include radionu­clides of Chornobyl ori­gin, par­tic­u­lar­ly cesium-137 and stron­tium-90.

Tri­tium is often used to deter­mine ground­wa­ter age. It is a nat­ur­al radioac­tive iso­tope of hydro­gen incor­po­rat­ed into water mol­e­cules. Each water mol­e­cule con­sists of two hydro­gen atoms and one oxy­gen atom. Three iso­topes of hydro­gen are known: the light­est pro­tium, heavy deu­teri­um, and super­heavy radioac­tive tri­tium. When sea­wa­ter evap­o­rates, clouds with a spe­cif­ic iso­topic com­po­si­tion are formed. Sub­se­quent­ly, water mol­e­cules con­tain­ing heav­ier iso­topes pre­cip­i­tate first as rain, while lighter mol­e­cules trav­el far­ther inland. There, young water with a lighter iso­topic com­po­si­tion falls as pre­cip­i­ta­tion and enters the hydro­log­i­cal cycle, includ­ing the recharge of ground­wa­ter. By mea­sur­ing the ratio between heavy and light iso­topes in ground­wa­ter, sci­en­tists can deter­mine the ori­gin of the water and the char­ac­ter­is­tics of its move­ment.

In Ukraine, only a lim­it­ed num­ber of stud­ies on ground­wa­ter age have been con­duct­ed. These stud­ies were main­ly car­ried out using hydro­ge­o­log­i­cal mod­el­ing meth­ods, and the age of ground­wa­ter in Cre­ta­ceous and Juras­sic deposits was esti­mat­ed to range from 1,000 to 15,000 years.

Thus, water con­tained with­in Juras­sic deposits is indeed old. How­ev­er, it is nowhere near as old as the water-bear­ing rocks them­selves, which were formed dur­ing the Juras­sic peri­od approx­i­mate­ly 145–200 mil­lion years ago. There­fore, the water cer­tain­ly does not “remem­ber” the dinosaurs. Most like­ly, if the water were tru­ly 200 mil­lion years old, pro­longed inter­ac­tion with the sur­round­ing rocks would have made it high­ly saline and unsuit­able as a source of drink­ing water sup­ply.

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