The word for 'garden' in the LXX (which as we know from Gmirkin is close to the original text) is a Greek reproduction of the Persian pardes.Now the Lord God had planted a garden in the east, in Eden; and there he put the man he had formed. 9 The Lord God made all kinds of trees grow out of the ground—trees that were pleasing to the eye and good for food. In the middle of the garden were the tree of life and the tree of the knowledge of good and evil.
10 A river watering the garden flowed from Eden; from there it was separated into four headwaters. 11 The name of the first is the Pishon; it winds through the entire land of Havilah, where there is gold. 12 (The gold of that land is good; aromatic resin[d] and onyx are also there.) 13 The name of the second river is the Gihon; it winds through the entire land of Cush.[e] 14 The name of the third river is the Tigris; it runs along the east side of Ashur. And the fourth river is the Euphrates.
15 The Lord God took the man and put him in the Garden of Eden to work it and take care of it. 16 And the Lord God commanded the man, “You are free to eat from any tree in the garden; 17 but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die.”
Secondly Philo generally arranges his answers where the initial explanation is discountable. In Questions and Answers on Genesis he strongly implies that this paradise is in heaven:
His next words seem to be inspired by the Samaritan understanding. I cite in full:For some say that the tree of life is the earth, for it causes all things to grow for the life of both man and all other things. Wherefore He apportioned a central place to this plant; and the centre of all is the earth. And some say that the tree of life is a name for the seven circlese which are in heaven. And some say it is the sun because it is, in a sense, in the midst of the planets and is the cause of the seasons, by which all things are produced. And some have said that the tree of life is the government of the soul.
What is the river that went out from Eden, by which Paradise is watered; and four rivers separated, the Pishon, Gihon, Tigris and Euphrates For the sources of the Tigris (Arm. Dktat‘) and Euphrates (Arm. Aracani) are said to rise in the Armenian mountains. And in that place there is no Paradise, nor are there the two other sources of the river. Unless perhaps Paradise is in some distant place far from our inhabited world, and has a river flowing under the earth, which waters many great veins so that these rising send (water) to other recipient veins, and so become diffused. And as these are forced by the rush of water, the force which is in them makes its way out to the surface, both in the Armenian mountains and elsewhere.
So the understanding is clearly that the sources of rivers are mountain tops. Of course he avoids explaining what is the ultimate source of all four rivers mentioned but clearly whether Gerizim or another mountain his discussion confirms the Samaritan idea that Paradise is located on a mountain top and from that Paradise the source of all four rivers are to be found.
The point of the thread was to say that with rivers starting in Ethiopia, Egypt and Mesopotamia there are really only two possibilities for the ultimate source (a) underground or (b) above ground.