with highly alkaline soil like that, your best bet is to mix it with a more acid soil, so they stabilize. adding vinegar or any other acid solution can neutralize your alkalinity in the short term but youll be making salt, and salt is plant poison.
all the PH up and down solutions are caustic, and roots dont like caustic materials, your soultion should go into the soil between 6.8 and 7.2 or root damage will occur..
this PH 8 soil, is it your native back yard soil? if so, go find soil thats not toxic. caustic or acidic. dont buy the stuff at a buider's merchant, get it from a local farmer, a plant nursery or other garden store (walmart and home depot are NOT garden stores)
how did you test it? if you tested the runoff then your soil could be markedly different then the runoff. you have to know what your water is like before going through the soil, and then youll still only know what the water picked up form the dirt. test the soil, not the runoff. if you only have PH strips or a solution tester, dissolve some of your soil in a large volume of PH neutral water and let it settle out. then test the water on top.
municipal water ranging between 6.8 and 7.4 is unacceptable. you shouldnt need a chemist to prepare your mac-n-cheese! get a reverse osmosis system, or buy bottled water.
in your situation i would amend my soil with peat moss and tannin rich mulches (oak leaves and bark are best) to normalize the PH then keep a close eye on it. if you live near an oak forest go get some loam from under those trees and till it in.
with a wild fluctuation like that i would keep a PH tester on hand and test the water before and after preparing my nutrient solutions, then use PH up and down solutions to normalize the fertilized water's PH before putting it on the plants