Testing subterranean water for a hazardous waste material repository

1. A method, comprising:

identifying a drillhole from a terranean surface through a first rock layer at a first depth below the terranean surface and a second rock layer at a second depth below the terranean surface, the second depth being deeper than the first depth;

determining a first concentration of a radioisotope in a first fluid sample from the first rock layer;

determining a second concentration of the radioisotope in a second fluid sample from the second rock layer;

determining that the radioisotope in the second fluid sample is at a secular equilibrium based on a ratio of the first concentration to the second concentration;

based at least in part on determining that the radioisotope in the second fluid sample is at the secular equilibrium, determining that the second rock layer is suitable as a hazardous waste repository; and

based on the determination that the second rock layer is suitable as the hazardous waste repository, forming an access drillhole of the hazardous waste repository from the terranean surface toward a depth in or under the second rock layer.

2. The method of claim 1 , wherein the radioisotope comprises at least one of uranium, helium, I-129, or Cl-36.

3. The method of claim 1 , comprising:

determining, using the first concentration and the second concentration, a vertical profile of the radioisotope;

determining, based on the vertical profile of the radioisotope, a rate of vertical flow of brines below the terranean surface; and

determining that the second rock layer is suitable as the hazardous waste repository based at least in part on the rate of vertical flow of brines below the terranean surface.

4. The method of claim 3 , wherein determining that the second rock layer is suitable as the hazardous waste repository based at least in part on the rate of vertical flow of brines below the terranean surface comprises determining that the rate of vertical flow of brines below the terranean surface is less than a threshold rate of vertical flow.

5. The method of claim 1 , wherein the radioisotope comprises uranium, and determining that the radioisotope in the second fluid sample is at the secular equilibrium based on the ratio of the first concentration to the second concentration comprises determining that the ratio of the first concentration to the second concentration is at or close to one.

6. The method of claim 1 , wherein the radioisotope comprises I-129, the method comprising:

determining a gradient of iodine concentration versus depth;

determining, based on the gradient of iodine concentration versus depth, that a flow velocity of iodine is less than a threshold velocity; and

determining that the second rock layer is suitable as a hazardous waste repository based at least in part on determining that the flow velocity of iodine is less than a threshold velocity.

7. The method of claim 1 , wherein the radioisotope comprises Cl-36, the method comprising:

measuring neutron flux in the first rock layer;

measuring a concentration of Cl-35 in the first fluid sample; and

determining the first concentration of Cl-36 in the first fluid sample based on the neutron flux in the first rock layer and the concentration of Cl-35 in the first fluid sample.

8. The method of claim 7 , comprising:

measuring neutron flux in the second rock layer;

determining a concentration of Cl-35 in the second fluid sample; and

determining the second concentration of Cl-36 in the second fluid sample based on the neutron flux in the second rock layer and the concentration of Cl-35 in the second fluid sample.

9. The method of claim 1 , wherein the radioisotope comprises I-129, the method comprising:

determining a concentration of 1-127 in the second fluid sample;

determining, using the concentration of 1-127 in the second fluid sample and the second concentration of I-129 in the second fluid sample, an age of the second rock layer; and

determining that the second rock layer is suitable as the hazardous waste repository based at least in part on the age of the second rock layer.

10. The method of claim 9 , wherein determining the age of the second rock layer comprises:

determining an iodine signature in the second rock layer;

based on the iodine signature in the second rock layer, determining that an iodine pulse is present the second rock layer; and

determining the age of the second rock layer based on determining that the iodine pulse is present in the second rock layer.

11. The method of claim 1 , wherein the radioisotope comprises helium, the method comprising:

determining a production rate of helium in the second rock layer based on a density of uranium and thorium isotopes in the second rock layer;

determining, based on the production rate of helium and the second concentration of helium in the second fluid sample, a velocity of helium gas flow in the second rock layer; and

determining that the second rock layer is suitable as a hazardous waste repository based at least in part on the velocity of helium gas flow in the second rock layer.

12. The method of claim 1 , comprising:

determining a flow velocity of the radioisotope based on the first concentration and the second concentration; and

determining that the second rock layer is suitable as the hazardous waste repository based at least in part on the flow velocity of the radioisotope being less than a threshold flow velocity.

13. The method of claim 1 , comprising:

determining that the first rock layer is formed from impermeable rock; and

determining that the second rock layer is suitable as the hazardous waste repository based at least in part on determining that the first rock layer is formed from impermeable rock.

14. The method of claim 13 , wherein the radioisotope comprises helium, and determining that the first rock layer is formed from impermeable rock comprises determining, based on the first concentration and the second concentration, that an upward helium mobility is stopped at the first rock layer.

15. The method of claim 1 , comprising:

determining a third concentration of the radioisotope in a third fluid sample from a surface water source;

determining a second ratio between the second concentration of the radioisotope in the second fluid sample and the third concentration of the radioisotope in the third fluid sample; and

determining that the second rock layer is suitable as the hazardous waste repository based at least in part on the second ratio being less than a threshold value.

16. The method of claim 1 , wherein determining that the radioisotope in the second fluid sample is at the secular equilibrium comprises determining that a production rate of the radioisotope is equal to a decay rate of the radioisotope in the second fluid sample.

17. The method of claim 16 , comprising:

calculating a secular equilibrium concentration for the radioisotope in the second rock layer based on a chemical and isotopic constitution of the second rock layer; and

determining that the radioisotope in the second fluid sample is at the secular equilibrium based at least in part on comparing the second concentration of the radioisotope in the second fluid sample to the calculated secular equilibrium concentration.

18. The method of claim 1 , wherein the first rock layer is adjacent to the second rock layer and is formed from the same type of rock as the second rock layer.