Will it be a negative mark?
Yes. Getting arrested, no matter the crime/reason, is always negative. Don't believe me...go ask the guy who got arrested for shooting that writer in Montana how getting falsely accused and arrested is working out for him.
Why is it negative?
You are an adult and you are expected to not have placed yourself in a situation to get arrested in the first place.
How negative will it be?
Well that is partially up to you to decide.
You can just not talk about it and hope they never look into it...(this would be the bad idea).
You can also be proactive about it and try to show you have increased in maturity since the events occured. Although with it being just this past April it is hard to just blow it off as an "imature act," especially with you being a senior level student(I assume this due to you mentioning graduating in the Fall).
My advice to you would be to look at how you can use this as a learning experience. Working with the university is a good idea. Maybe look into any opportunities to volunteer with the police/fire dept.. You can use this to show that you have moved on and are trying to give back .
Also in your LoR make sure you get your writers to mention specifics about your character and judgement(ie that you are not a bad person and these events were the exception, rather than the rule).
Crap happens, but this is a very slipperly slope you are climbing. I would talk to professors and others who have atteneded any form of graduate school. Maybe they can give you advice on how to handle this.
Either way, best of luck.