What usually happens to a Soldier who fails urinalysis?

Keep in mind that there are soft rules, like always be respectful of superiors, and hard rules, like never be found to have drugs in your system. The difference? A hard rule is easily measured and the tolerance for violation is generally minimal. Soft rules tend to be subjective and open to interpretation and take far more effort to nail someone for it.

Army National Guard, or Army in General. If you are notified that you are positive on a drug screen, the first thing to expect is a notification of Article 15 charges, usually at Field level, and a flag on re-enlistment. At your rank, if the Commander finds you guilty, aside for the article 15, you will be referred to the Army Substance Abuse Program, or if you are Guard and no military program with reasonable distance, then to one of the Community Programs, and the process for Administrative Discharge started. If you test positive, you will be found guilty unless you have some extraordinary extenuating circumstances that can be proved.

In the Army, at ranks under E5, this doesn't always mean the end of your career. That will depend on how good of a soldier you were up to this incident, what your chain of command thinks of you, and the recommendations from either ASAP, or the community program if you are Guard, when you complete the program. Then it is up to the Commander to decide whether to stop the discharge process and lift the re-enlistment flag or let the process complete. If you are near the end of you enlistment and want to stay, but won't have the program completed, you can request an extension to complete the program, and then hope for the best.

If you are E5 or above, or any W or O rank, I would start working on my Resume' and looking for civilian employment.

If the command decides to give you a chance, be prepared to piss in a cup on a regular basis.

Also, self enrollment in ASAP before the Article 15 can show the Command that your mind is in the right place. Doesn't always matter, but it doesn't hurt.

The drug tester will get in contact with you and set up another urinalysis.

It would depend on what you view as a heavy punishment. I've seen this happen in both the Active duty and National Guard. For the guard and marijuana, typically, it was the soldiers having to much fun finding themselves in college parties. All of there education benefits would be ended, and any outstanding payments may need to be paid back in full. That alone could be thousands. When it comes to discharge or UCMJ, that's handled on a case by case.

The best advice I can give would be to have a conversation with your leadership about what happened and why it happened as soon as possible.