Testing subterranean water for a hazardous waste material repository

1. A method, comprising:

determining a concentration of at least one noble gas isotope of a plurality of noble gas isotopes in a fluid sample from a subterranean formation below a terranean surface, the plurality of noble gas isotopes comprising helium (He), neon (Ne), argon (Ar), or xenon (Xe);

determining a produced amount of the at least one noble gas isotope in the subterranean formation based on a production rate of the at least one noble gas isotope and a minimum residence time, the production rate based on one or more properties of rock in the subterranean formation;

calculating a ratio of the determined concentration of the at least one noble gas isotope in the fluid sample to the determined produced amount of the at least one noble gas isotope;

based on the calculated ratio being at or near a threshold value, determining that the subterranean formation is suitable as a hazardous waste repository; and

based on the determination that the subterranean formation is suitable as the hazardous waste storage repository, forming an access drillhole from the terranean surface toward the subterranean formation.

2. The method of claim 1 , wherein the one or more properties comprise at least one of:

a bulk rock chemistry of the subterranean formation;

an amount of uranium per unit volume in the subterranean formation;

an amount of thorium per unit volume in the subterranean formation;

a decay rate of uranium in the subterranean formation; or

a decay rate of thorium in the subterranean formation.

3. The method of claim 1 , further comprising:

determining a concentration of another noble gas isotope of the plurality of noble gas isotopes in a fluid sample from a subterranean formation below a terranean surface;

determining a produced amount of the another noble gas isotope in the subterranean formation based on a production rate of the another noble gas isotope and a minimum residence time that is sufficient to show that the subterranean formation is suitable as a hazardous waste repository;

calculating another ratio of the determined concentration of the another noble gas isotope in the water sample to the determined produced amount of the another noble gas isotope; and

based on the another calculated ratio being at or near a threshold value, determining that the subterranean formation is suitable as the hazardous waste repository.

4. The method of claim 3 , further comprising:

comparing the calculated ratio with the another calculated ratio; and

based on the comparison, determining that the subterranean formation is suitable as the hazardous waste repository.

5. The method of claim 4 , wherein the at least one noble gas isotope is He, and the first production mode comprises alpha decay of at least one of uranium 238, uranium 235, or thorium 232.

6. The method of claim 3 , wherein the at least one noble gas isotope is produced from a first production mode, and the another noble gas isotope is produced from a second production mode different than the first production mode.

7. The method of claim 1 , further comprising collecting the water sample from a drillhole formed into the subterranean formation.

8. The method of claim 7 , wherein collecting the water sample comprises:

operating a downhole tool in the drillhole to collect a core sample from the subterranean formation;

retrieving the core sample to the terranean surface; and

removing the water sample from the core sample.

9. The method of claim 8 , wherein the drillhole comprises a portion of the access drillhole.

10. The method of claim 1 , further comprising forming the drillhole from the terranean surface to the subterranean formation.

11. The method of claim 1 , wherein the threshold value is one.

12. The method of claim 1 , further comprising initiating creation of the hazardous waste repository in or under the subterranean formation.

13. The method of claim 12 , wherein initiating creation of the hazardous waste repository in or under the subterranean formation comprises:

forming a storage drillhole coupled to the access drillhole in or under the subterranean formation, the storage drillhole comprising a hazardous waste storage area.

14. The method of claim 13 , wherein the access drillhole comprises a vertical drillhole.

15. The method of claim 13 , wherein the storage drillhole comprises a curved portion and a horizontal portion.

16. The method of claim 13 , further comprising storing hazardous waste material in the hazardous waste storage area.

17. The method of claim 1 , wherein the fluid sample comprises liquid brine.

18. The method of claim 1 , wherein the minimum residence time is sufficient to show that the subterranean formation is suitable as the hazardous waste repository.

19. The method of claim 1 , further comprising:

monitoring at least one variable associated with the storage canister or the hazardous waste material from a sensor positioned proximate the hazardous waste storage area;

based on the monitored variable exceeding a threshold value:

removing the seal, and

retrieving the storage canister from the hazardous waste storage area to the terranean surface.

20. The method of claim 1 , wherein the at least one noble gas isotope is He.

21. A method, comprising:

determining a concentration of a krypton isotope in a subterranean water sample collected from a subterranean formation;

determining that the concentration of the krypton isotope is less than a threshold value that is based on a concentration of the krypton isotope in a surface water sample;

based on the determination, determining that the subterranean formation is suitable as a hazardous waste repository; and

based on the determination that the subterranean formation is suitable as the hazardous waste storage repository, forming an access drillhole from the terranean surface toward the subterranean formation.

22. The method of claim 21 , wherein the krypton isotope comprises Kr-81.

23. The method of claim 21 , wherein determining the concentration of the krypton isotope in the subterranean water sample comprises using Atom Trap Trace Analysis to calculate the concentration of the krypton isotope.

24. The method of claim 23 , wherein the threshold value is based on the concentration of the krypton isotope in the surface water sample.

25. The method of claim 24 , wherein the threshold value is between 16-20 atoms of the krypton isotope in the subterranean water sample.

26. The method of claim 21 , wherein the threshold value is between 16-20 atoms of the krypton isotope in the subterranean water sample.

27. The method of claim 21 , further comprising collecting the subterranean water sample from a drillhole formed into the subterranean formation.

28. The method of claim 27 , wherein collecting the subterranean water sample comprises:

operating a downhole tool in the drillhole to collect a core sample from the subterranean formation;

retrieving the core sample to the terranean surface; and

removing the water sample from the core sample.

29. The method of claim 27 , wherein the drillhole comprises a portion of the access drillhole.

30. The method of claim 21 , further comprising forming the drillhole from the terranean surface to the subterranean formation.

31. The method of claim 21 , further comprising initiating creation of the hazardous waste repository in or under the subterranean formation.

32. The method of claim 31 , wherein initiating creation of the hazardous waste repository in or under the subterranean formation comprises:

forming a storage drillhole coupled to the access drillhole in or under the subterranean formation, the storage drillhole comprising a hazardous waste storage area.

33. The method of claim 32 , wherein the access drillhole comprises a vertical drillhole.

34. The method of claim 32 , wherein the storage drillhole comprises a curved portion and a horizontal portion.

35. The method of claim 34 , further comprising storing hazardous waste material in the hazardous waste storage area.

36. The method of claim 35 , wherein storing hazardous waste material in the hazardous waste storage area comprises:

moving a storage canister through an entry of the access drillhole that extends into the terranean surface, the entry at least proximate the terranean surface, the storage canister including an inner cavity sized to enclose the hazardous waste material;

moving the storage canister through the access drillhole and into the storage drillhole; and

moving the storage canister through the storage drillhole to the hazardous waste storage area.

37. The method of claim 36 , further comprising forming a seal in at least one of the access drillhole or the storage drillhole that isolates the hazardous waste storage area from the entry of the access drillhole subsequent to moving the storage canister through the storage drillhole to the hazardous waste storage area.

38. The method of claim 21 , wherein the subterranean sample comprises liquid brine.

39. The method of claim 21 , further comprising:

monitoring at least one variable associated with the storage canister or the hazardous waste material from a sensor positioned proximate the hazardous waste storage area;

based on the monitored variable exceeding a threshold value:

removing the seal, and

retrieving the storage canister from the hazardous waste storage area to the terranean surface.

40. The method of claim 21 , wherein the subterranean water sample is between 10-100 liters.