Recommendations for expansion tank?

I have a submersible well with a buried expansion tank (not sure if I'm using the correct terminology) and last night I noticed it cycled very short intervals even when the pressure gauge was at 55lbs, but it seemed to settle down...

Today I got brown water coming through the pipes and inspected the well area outside. There was muck and water all around the area and when I shut it down there were noticable seepage voids where I could put a stick down 2-3'. I called a well company who is coming out tomorrow to evaluate the problem, but he prewarned me that the service manager is probably going to recommend digging up the old tank and replacing it with an indoor tank. The house is a 1973 and to my knowledge (I'm the second owner) it is all original. I shut down the power to the well (as recommended) but am a little concerned with freezing; I'm in MN and temps will be around 10 tonight.

I'm just trying to do some research so I'm not going in blind on this... It sounds resonable that the tank may need to be replaced, but I have to believe it may just be a leak in a line. ...hopefully... Nonetheless, maybe it should be replaced anyway since they'll have to dig it up to fix a leak.

Also, what should I know about sizing the tank? The house has 4 people, 3 baths, sprinkler system, and other common uses.

Thanks.

I agree with putting a good bladder tank in the house and not burying another one. I have had nothing but bad experiences with buried tanks.

Just be sure to get a good Bladder Tank. Either Flexcon or Well-X-Trol.

bob...

If you use a Cycle Stop Valve to handle your variable flow requirements, you don't need a very big tank. See this link;

http://www.cyclestopvalves.com/images/psidekick-ad.jpg