Old Location

Here is a link that might be useful: Hydrangea in old location

You would have no problems with the septic 9' away - the hydrangea roots will extend approx as far as its widest stems, they don't have an extensive root system.

My experience is in a much cooler Z8, but you may find the period if full sun between 11-2PM a little hot for best foliage and length of bloom on hydrangeas in Dallas is my guess.

I agree with Mor - I think that you may find that location a little too intense for these shrubs to be happy. Is there any filtered shade available? Hydrangeas are really woodland understory plants, so grow well under the canopies of larger deciduous trees -- even more so in hot climates.

An alternative would be on the north side of the residence or anywhere out of direct afternoon sun.