Are you sure they're moles & not voles? Moles eat grubs in the soil & will have small tunnels while voles are vegitarians & make the large tunnels (quarter size hole & larger) TOMCAT rodent block broken into small pieces & placed in active tunnels will kill voles. The easiest way to get rid of moles is to kill their food source. Diazinon will kill the grubs but be sure there's no runoff into the water. Neither animal tunnels very deep but I suppose an infestation could cause erosion problems.
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