somehow i feel its a trick, but being groundwater fed, and having the groundwater reside in the limestone, i vote the obvious as well.......greater than 7.
i might be able to understand a pH <7 if it is a fast moving aquifer, recharged by highly draining soils, such that the "groundwater" has a low residence time in the limestone, in this case you could have pH's less than 7 in a limestone pit.
