Auto populate cells based on drop down list selection

I'm trying to auto populate sample numbers from one location based on the selection of a drop down list in another location.

See attached: in the MONDAY group (X4:AG21), I want to have sample numbers populate for Ditch #1, #2, and #3 (AA12:AA14) when the drop down under Ditches (AG7) is set to YES. These numbers would pull from D11:D13. Is that possible?

Any help is appreciated!

Thank you for the quick response! I've edited the sheet a bit since adding your suggestion to make the sample numbers list in B7:B21 match the analyte list in Y6:Y21. Please see the new attachment.

It seems to work well, except that I'm getting 0's in AA17:AA21. Why is that?

Also, with the info in B4:I21 - we don't run those tests every day, hence the yes/no drop down lists in AG. Is there a way to make those numbers auto populate in C12:I21 based on the selection of the drop downs in AG? For example, on Mondays, if I select Yes on the ditches in AG7, I would want the numbers in D12:D21 to be the next in the sequence from those in D7:D11. Does that make sense?

Thanks for the response! Yeah, the second part is confusing. Let me try again:

* In B7:I21, I want these numbers to auto populate based on a drop down, or even based on a cell having a character in it to designate that that sample was taken that day.
-----Example: For column D (in the 2nd attachment), the first sample taken for the day is the influent, which is formatted as yymmdd"PT01" (based on date in D6), the next one down is the effluent, and is formatted as yymmdd"PT02". You can see that there are no entries in D9:D11 for pH, E.coli, or Clarifier 3; so D12 (Ditch 1) picks up with "PT03".
-----I probably need to designate which sample is taken each day, and then have the column fill the sample numbers in the format mentioned above, which is based on the dates in row 6. If the sample is not taken that day, I need the formula to skip those cells and pick up the series with the next populated cell going down the column. Is that a little clearer?
-----I want the cells in AA6:AA21 to populate based on the data in D7:D21.

* I would also like the cells in AF6:AF21 to populate based on the cells being populated in AA.
-----Example: If there is an entry that populates in AA6, I want AF6 to populate a specific text, in the case of MONDAY - TSS, NH3. For Rows 6 and 7, the analytes in AF for influent and effluent will differ based on the day. I will eventually copy these formulas into the groups below (Y24:AF39, Y42:AF57, etc.), so the day will change the tests run on the influent and effluent. All the other tests always have the same analytes regardless of the day. So if there is a number that populates in the cell for pH, the analyte listed in AF will always be pH; E.coli will always populate Total Coliform, E.coli; etc.
-----Is there a formula I can use to accomplish this?