POM higher in runoff water?

Great answes from the users below.

whoops.. pH or Ppm.. same process.

As the water 'rolls' through the substrate, it carries out some of what was in the substrate before. in a soilless context this is the underlying factor of the "always have 10% runoff waste water" .. it maintains a roughly consistent level in the substrate over time. Mitigates effect of evaporation on concetration etc. Same effect is occuring in soil, but context is slightly different. You don't want to leach out pre-amended nutes if you don't have to with excess runoff. Be consistent about how much runoff you get and when you test the runoff. Form a baseline of what is "good" while the plant is happy... this may take some notes and hindsight. plants can look healthy while something is building up or slowly depleting. But, you learn what is normal and even if it's an offset of what you feed, that's what you base any concern off of.. high/low from that offset understanding.

It's not uncommon for runoff pH to be higher. A plant doesn't get to use up everything you give and you're getting a little nutrient buildup in the media which devolves into your runoff. If you aren't experiencing any detrimental effects you could probably get by with just plain pHed water every now and then. If you think it's creating issues you'll want to flush.