I live in an area where we all have septic tanks and huge forest trees with root systems that will always find the drainage fields and piping and yet for some reason the roots clogging things don't seem to be that prevalent a problem. However this may be for reasons specific to my region. Or maybe I just don't know the victims of this particular problem.

I had done some research when we put in a new drainfield on a formerly wooded, sandy hillside to see what I dared to plant over it. I learned that trees with a taproot aren't as bad nearby as the ones with spreading-type roots. Elms are particularly bad. Oaks aren't as bad. They still recommend keeping trees 100' from the field, which isn't very practical for most of us treelovers. I chose to plant blueberry bushes over the drainfield, since they have very shallow roots. After one year, so far, so good. I must add that my parents and a sister in California have had lots of problems from roots in their drainpipes and have given Rotorooter lots of business over the years, so roots can be a serious, chronic problem. Northwoodswis

wertach,
I'm one of the resident 'nuts' here; I wouldn't remove the pecan - and not just because I'm a pecan/hickory enthusiast. A willow, poplar, or maple - I'd say yes unequivocally - but not a pecan or oak; their roots are just not as 'invasive'.
If you google up 'planting on septic leach field' and 'controlling root invasion into septic field lines', etc., you'll find that the wholesale call for removal of any tree in the leach field vicinity is not supported by evidence.

In support of harvestman's response, I have friends in IL who operates a private arboretum. Their home is located in a heavily wooded area of the property, with the septic field lines running out through mature forest(likely few to no maples, and almost certainly no poplars or willows in that vicinity). They flush CuSO4 down the toilet once or twice a year, and have had no issues with root invasion over a period of several decades.

Here is a link that might be useful: Controlling root invasion with CuSO4

I have huge sugar maples with roots all over my land and the largest one is only 30 feet from my septic tank and the field immediately below. I'm an idiot who never even has my septic emptied and I've been an idiot for the 22 years I've been on this land.

The only septic expenses I've had so far is the one time I had the tank pumped for maintenance. Never added any copper formulas to my water either. However this is only one case and my soil is very porous. Still, all my neighbors have similar scenarios as far as proximity of huge old maples, tulips, beech, birch and oak. Think I better go outside and knock on one of my trees now.

First house the bride and I bought - in '86 - was built in '51, and the owners planted sugar maples all around it - but two red maples in the back yard, where the septic tank and leach field were.
When we had a major 'stoppage' in the bathroom within the first 2-3 years after moving in, and the plumber opened the septic tank, there was a huge mat, 6-12" thick, of red maple roots which had grown under the tank lid, covered the area of the tank and grown 30 ft up the inflow pipe, into the basement and up into the bathroom.

Just last week I observed a plumber repair a 4" pvc sewer line that was about 20' from a maple with about a 2' diameter trunk. The maple had grown a root along side the line and proceeded over time to enlarge (in diameter) to the point that it had basically taken the place of the line. Where once there was a section of pvc there was now a section of root about the same diameter of the line, it wasn't draining too well ;) Copper sulfate might not have helped any in that circumstance.

An old plumber around here, now retired, told me he always dumped sacks of salt around sewer lines, particularly the joints to keep roots out, his theory was that the feeder roots would die when they encountered the spiked soil. In climates dry enough to not leach the salt over many years' time I think his theory would likely hold up.