The problem is that you have a large volume of radioactive fluid. The radioactivity is "low-level" but still poses a danger to health and life. If you store it

The problem is that you have a large volume of radioactive fluid. The radioactivity is "low-level" but still poses a danger to health and life. If you store it somewhere, you have to be sure it will stay there. Additionally, if it turns out you stored it somewhere silly, you'd want to be able to get it back. The alternative is to dilute it down so that it's less harmful, but then you have a larger volume, and so containing it is a larger problem. If you dilute it to the point that its volume is the entirety of water on earth, then it's at a safe level (and a fraction of the natural level), and so isn't a containment problem anymore. Sand will not help the problem; the radioactive component is tritium, which is in the water molecules themselves. The sand will just mix with the water and give you a larger volume that has handling challenges of wet sand and evaporates and sweats radioactive water.