I am away from home for awhile or I would take a picture of what I am trying to describe. It consists of three short pieces (about a foot each), and a tee fitting. You put bait poison into it, they eat the bait and go back underground and die. Here is a link to a similar set-up.
http://www.pesteducation.com/cdfiles/newmaster09/html/OutdoorVertPests_files/image036.jpgChances are your county agent has plans for these.
As far as the poison, I cannot remember the name of it, but it is a green colored bait and is more deadly than your normal rat/mouse poison, your local farm supply store should have it though.
I had the same problem with gophers (not ground squirrels), I built a bunch of these and put throughout the problem area near any where that was sign of gopher activity. It took a couple of years but I am gopher free now.
p.s. - I used 1-1/2 inch pvc rather that 4 inch as it shows in these plans.
drover