Ok, lets take this at face value. Does any, and I do mean literally any country on this planet, dump their tritium water in the local lake?

Problem 1 : Water extraction

Extracting large volumes of water, is naturally not a ecologically friendly practice. It's actually one of the large barriers stopping us from solving water issues via large scale desalination. Ecological damage includes animals being sucked in, to higher water temperatures causing algae blooms. (Moving all that water, does overall increase the water temperature) Furthermore, it may actually case over-oxygenation of the water, which can cause it's own set of problems. But I'm not a specialist so I will leave that topic as "I have heard of this thing".

Problem 2 : Actual dilution

That's not all though. You need to ensure local PH balances are stable. Since we are starting off by using ocean water (it was the closest and most viable option), you now have to do an ecological analysis of the lake, and make the water match the lake's overall water profile. That requires a whole other set of equipment, and imported materials to balance it out. Of course, "it's relatively speaking, a drop in an lake", but that's the thing, it's a lake. It doesn't have ocean currents.

The Lake is overall a enclosed environment, when compared to the ocean. And it is one hell of a lot more expensive than the ocean, which is right next door!

Now lets take your number, 120 becquerels of tritium per liter. Lets ignore the WHO safe level for a moment. Since our goal is diluting it, preferably, we won't appreciably change the per-liter value of tritium in fresh water. But that's not the case is it? The natural background amount of tritium in 1cubic liter in surface water is around 0.4–1.2 Bq/L. So your proposal, increases the amount by 100x. That misses the entire point of "dilution", which is why I said it doesn't count. When you dilute something like this, the whole point is to dilute in a manner that makes environmental effects negligible. Your proposal, misses the entire point.

Further more, they are only releasing ~28becquerels (officially it is 22becquerels) per year into the ocean, to ensure proper dilution. Divide 28 by 365 days, that is 0.077becquerels per cubic liter (rounded up), in an entire 24 hour cycle! Which would bring it very close to the natural background! This is dilution! This is the bloody point of it all!

Dumping it all in a lake, makes this completely pointless!

Problem 3 : Logistics (AKA, why are we even transporting this?)

This one is simple. Why in the actual hell, would you spend time and risk a truck flipping, rather than just use the pipe you already have and release into the ocean? And this ties into the two previous comments. You have to extract water from the lake, bring it to the water storage facility, mix it, then transport it back.

Or take sea water, desalinate it, then get it to the correct profile, then transport... The amount of extra steps, and chances for human error are significantly higher. It's impractical, and a waste of time.

Problem 4 : Local Resistance (Or domestic politics)

Naturally, locals don't like it. It doesn't matter if it's safe, or checked by independent auditors or international organizations. Humans are overall, emotional, social creatures. Not logical ones. Local pressure is naturally easier to evade if you say "It won't be your problem in the future", which is unfortunately how humans work. Hell, if I had the money, I'd buy the lake (which isn't possible), and let them dump it there (Which I cant, because the fresh-water underground water table is legally protected). Of course, I would tell them "It won't be a problem in 14 years, due to the half-life", but they won't care.

Non-salt water is also significantly more valuable than salt water. It's the difference between incredibly energy intensive and expensive desalination and, just not needing it. People care more about non-salt water than salt water.

I'd happily drink it, assuming it's boiled and de-salted. (And I am not being facetious)