$WATER_PREP is showing mineral additions or acid adjustments. Once those are added as ingredients, they show up in the reports.
Water Prep is an area that doesn't automate very well in this program. it doesn't dynamically track with your recipe changes. So, it's best to do as a separate task and then import the profile after everything else is firm.
It's cumbersome, but here's how.
1) Select the target city from the water ingredient list. Copy & paste it, then rename one of the duplicate profiles. I use the city name and "target" after to show it has mineral additions in it. I may add detail like "Target from RO" or "Target from City Well" to differentiate which source was used. You'll create the additions in the following steps.
*NOTE: if you just use the listed water profile, it will always be relative to whatever source water you started the calculations with. I like to leave the default profiles alone.
2) Open the Water Profile tool and add your local source water. I like to use a small volume for the quantity but you can use your total water volume here, too. When you add it to the recipe, it will calculate for the volume you have in that recipe.
3) Select the target profile you just created for the target water field.
4) Click the "Calculate Best Additions" button. You can adjust minerals manually, too.
5) Click the "Save Additions to Target" button.
Your water additions are now just another water profile and can be treated as an ingredient.
In the recipe, you'll select that target profile and the program will prompt you that it includes mineral additions. If you confirm it, the additions will show up for the total water volume needed.
Once added to your recipe, the minerals won't automatically change. If you change the water profile, you'll see BOTH amounts of minerals. You have to manually delete the minerals, just like any other ingredient before adding a changed profile.