A recent carbon-14 study dating the tunnel to about 700 B.C.E. is not precise enough to distinguish between the reigns of Hezekiah and Yehoash. See Amos Frumkin and Aryeh Shimron, “Tunnel Engineering in the Iron Age: Geoarchaeology of the Siloam Tunnel,” Journal of Archaeological Science 33 (2006), p. 227.