I have a well that is shared with the house next door. It has a submersible pump, and the PVC pipe from the pump comes through the basement foundation and runs to a Well Mate WM-9 pressure tank. After the tank, I have a whole-house filter that it seems many people here hate, followed by a Kenmore water softener. Sorry, I don't have details on the well pump with me at the moment.

I have not been good about changing the filters in the whole-house filter unit as recommended, and instead usually end up changing them when the water pressure gets low. This happened last night, and when I pulled out the filter, I had a ton of this black, gritty/sandy sediment packed in there. I've seen this before, but this is the most I've ever seen. It was so packed in that I had a hard time getting the filter out of the housing. What is this stuff??