You might consider running a soil slurry test for pH and ec if you haven’t already. Runoff is not representative as it has little contact time with the soil. A soil slurry test allows the water to be in contact long enough to leach the soil.

Just a suggestion.

Per Vernier:

To Prepare the water-soil mixture.

a. Place 50 g of Soil into a Beaker.
b. Add 100 mL of distilled water to the beaker.
c. Stir the mixture thoroughly.
d. Stir once every three minutes for 15 minutes.
e. After the final stirring, let the mixtures settle for about five minutes. This allows the soil to settle out, leaving a layer of water on top for you to take your measurement