Does your water enter the house on metal pipes?  It comes in, goes through a water meter, and then does it exit onto PVC or metal pipes?  Magnetic fields are created whenever current flows through wiring, by running motors, wiring errors which estimates say over 90% of homes have - my house had many and the magnetic field went to 6 mG every time a light was turned on, power lines, and from the electric utility using earth for a neutral instead of beefing up their own neutral wires to return the electric signal back to the substation - almost all do it regardless if there are laws against it.  There are probably other things which create magnetic fields, but these are what come to mind.

We had a high magnetic field on our copper water pipes which BruceM on this list instructed us how to fix by splicing in pieces of PVC wherever water pipes entered and exited the house.  We disconnected the ground to the water pipes also.  You might consider asking Bruce about this.  He may no longer be on the list, which would be a great loss since he's an electrical engineer who is also ES.  If you can't contact him here, email me off list and I'll try and connect you.

An after thought, have you ever gone through your house room by room when nothing was turned on, done a magnetic field read and then switched on a light to see if the magnetic field rises?  http://emwatch.com/house-wiring-radiation/

Poor House Wiring Causes Strong Magnetic Fields | EM Watch emwatch.com In houses where magnetic field radiation is higher than usual, poor house wiring is often the cause. What kind of wiring issues give rise to a high EMF?