Last week I noticed some white powder residue in my patio and along
the patio wall (one inch above the base). The backyard has been moist
for the last month with the sprinklers off. We did have a lot of
rain, but I think that is not the reason. I have checked the outside
water meter and there is no leakage (meter is not running when the
water is not turned inside the house).

There is no damp spot inside the house yet. I am seeking advice as to
who to call for an inspection or advice - should it be a plumber or
some other expert?

Any advice would be greatly appreciated.