First of all, you want to identify the
constituents of acid rain so that you can prepare a synthetic solution of it. What acids (sulfuric acid, nitric acid, etc.) will be in the water and what pH will the water be? You can research which acids and pH values are most common in acid rainfall to help you with this. Then you have to decide which type(s) of soil you would like to test. Do you want to test the filtration of various types of soil or just one type?
You could probably set up your soil in a planting pot with rocks below the soil so that the water you pour on the soil will run off below the soil and settle with the rocks at the bottom. You can then remove the soil and pour the run off water out to measure the pH. This is just one possible experimental set-up, but it really depends on the questions you want to ask.
For example, do you want to know
whether the soil causes the water to become
more/less acidic? Would you like to test the
effects over time (maybe a few days)? One
interesting thing you could do would be to plant
some seeds in maybe 5 different pots of soil (the
same soil) and see if acid rain has a negative
effect on the ability of the seeds to mature and
grow into a plant. Of course, one of your pots
would be your control where you would just use regular water. There are a lot of interesting questions you could ask. If you need more assistance I will be glad to help. Good luck!