Most pressure treated wood sold now contains copper compounds which increase its resistance to decay, however, this does not protect it from termites. Some copper may leach into soil from the wood. Copper also will be deposited in the soil when the wood eventually decays. Wood treated with chromated copper arsenate (CCA) was withdrawn for use in residential applications in January 2004 although it is still available for industrial use.
Copper from treated wood quickly binds to clay particles in the soil and does not move very far. Most soils naturally contain some copper and tree roots growing near treated wood could absorb more copper than usual. I don’t know how much additional copper the fruit from your trees may contain but it would not be very much and it should not pose a health hazard.
For more information on pressure treated wood, see UC Agriculture and Natural Resources Publication 8144, Selecting Lumber and Lumber Substitutes for Outdoor Exposures and Publication 8128, Using CCA Preservative-Treated Lumber in Gardens and Landscaping.
By V. Lazaneo, Urban Horticultural Advisor, Emeritus, UC Cooperative Extension, October 2012
If your fruit trees are 25 years old you probably need to water a larger area than just inside the 4' x 4' wood border. Roots of most trees grow out to and often beyond the edge of the trees canopy, which is called the "drip line". All of the soil under the trees' canopy should be irrigated when the soil begins to dry. Enough water should be applied to penetrate at least 2' deep. Water should not be allowed to stand next to the tree's trunk as this could cause crown rot or gummosis, a fungus disease which attacks citrus.