Quick Post: What the Heck?

I went to the store yesterday and picked up a Brita water filter:  I’ve been drinking a lot of water lately, trying to get up to 8 cups (I used to usually drink 4).  For the installation of the filter, I had to remove the aerator from the tap, the aerator being a little screw-on that makes the water that comes out more of a tap water look than a pouring out look.  I took out the aerator which had a little screen on the top.  Full of little rocks and sand.  What the heck?  There’s freaking rocks in my drinking water?  Granted, the larger (i.e. largish grain of sand) ones are filtered out, but what about, say, the smaller ones?  Like dust or smallish grains of sand?  Blech.