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Drinking water is taken from an underground lake. The water contains small quantities of radioactive isotopes such as tritium (³H). The decay of tritium has a half-life of 12.3 years. 1Water is unsafe to drink when its activity due to tritium decay is greater than 7.6 x 107 Bq per cubic metre of water. 2.50 104 m³ of water from the lake contains 1.1 x 10¹0 tritium atoms.X Deduce whether the amount of tritium in water from this lake makes it unsafe to drink. No more tritium can enter the underground lake. The activity due to tritium decay can be measured to estimate the time for which the water has been in the lake. This time is called the residence time. Suggest why this method is only used when the residence time is less than approximately 150 years. ]Kr-81 is a radioactive isotope of krypton that is found in water. The amount of Kr-81 dissolved in the water can also be used to find the residence time. An amount of water from another underground lake contains 2.2X106 atoms of Kr-81 When the water was originally trapped, the same amount of water contained 3.7 x 106 atoms of Kr-81 No more krypton entered the lake after the water was trapped. Calculate, in years, the residence time for this lake. decay constant of Kr-81 = 3.03 x 10-6 year

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