What to test after a flood (bacteria hold times, no Friday drop-off)
Floodwater is not a free rinse for your well. Surface water, septic, manure, and whatever was in the yard can go down the casing. The first job is not a hardware-aisle strip. It is safety, disinfection, then a certified bacteria test on a clock most people miss — and that clock is why Friday drop-off fails.
The short version
- Do not drink or wash from the flooded well. Stay away from the pump while water is still around it — electric shock is a real hazard. Use bottled water, or boil only if local guidance says boiling is enough for germs and you are not dealing with chemical contamination. Boiling does not remove fuel, pesticides, or metals.
- Get a well or pump contractor to inspect, clean, and disinfect before you lean on the pump. EPA’s private-well flood steps and CDC’s post-emergency disinfection page both push professional help after flooding. Shock-chlorinating yourself can be hazardous if the well was submerged or the electrical system was wet.
- Wait 7 to 10 days after disinfection, then sample for bacteria. EPA’s “What to Do After the Flood” and CDC both say to wait until chlorine is flushed. Sampling too early can give a false clear.
- Bacteria is on a short hold time — often about 30 hours, kept cold. Many labs will not accept Friday bacteria samples because weekends break the clock. Collect Monday–Wednesday. Do not leave the bottle in a hot car or a Friday mailbox.
What EPA and CDC say to do first
This is not a treatment sales script. It is the official sequence, paraphrased from EPA’s Protect Your Home’s Water flood response and CDC’s how to disinfect wells after an emergency.
- Stay away from the well pump while the area is flooded.
- Do not drink or wash with that water.
- Have a contractor clean and disinfect the well before turning the pump back on.
- After the pump is on, run until the water is clear to push floodwater out.
- If it will not clear, call county or state health / extension — do not invent a DIY fix from a blog.
- Wait at least 7–10 days after disinfection, then have a state-certified lab test the water. Until then, bottled water or another safe source; boil only when that is the right tool for germs and local advice says so.
EPA’s short PDF What to Do After the Flood is still the handout many counties point to. It also warns that disinfection does not protect against pesticides, heavy metals, and other non-biological contamination — and that a well in a big flood zone can stay unsafe for months. Plan on retesting, not one lucky clear.
What to order from the lab
A flood does not introduce one neat contaminant. The floor is bacteria. Everything else depends on what the water passed through. Ask county environmental health what is common locally — that is still the right first call.
The table scrolls sideways on a phone.
| Test for | Why after a flood | When to prioritize |
|---|---|---|
| Total coliform and E. coli | The core safety check. Shows whether surface water, sewage, or animal waste reached the well. | Always, after flooding or wellhead submersion. CDC and EPA both treat flooding as a reason to test immediately (after the disinfection wait). |
| Nitrate (and often nitrite) | Fertilizer, septic, manure, and flooded sewers ride floodwater. Dangerous for infants and pregnancy. | Rural / farm country, nearby septic, or any time you have never done an annual nitrate. Hold time for nitrate/nitrite is often about 48 hours — still not a weekend souvenir. |
| pH and total dissolved solids | Annual baseline indicators. Not the flood villain, but cheap context on the same trip. | If you are already ordering a county or mail-in panel and they are on the menu. |
| VOCs (fuels / solvents) | Floodwater can pull petroleum and solvents from tanks, garages, stations, and industrial sites into a well. | If you smell fuel, or fuel tanks / gas stations / chemical storage flooded nearby. Ask the lab which VOC list they run — do not invent a method number. |
| Pesticides / other chemicals | Ag runoff and stored chemicals move with floodwater. Shock chlorination does not fix them. | When county health or extension says they are a local flood risk. These panels cost more; ask before you order a kitchen-sink kit. |
Hardware-store color strips are screening-only. They are not a certified bacteria analysis, not what EPA means by “have your water tested,” and not what a lender means by “tested.” Do not treat a strip as the post-flood clear.
The bacteria clock (why Friday fails)
Holding time is the allowed time from collection until the lab starts analysis. For total coliform drinking-water methods, many state programs and labs use about 30 hours. Wisconsin DNR’s public hold-time sheet states 30 hours for coliform and 48 hours for nitrate/nitrite. Other labs quote 24–30 hours for bacteria, kept cold; some private-well desks allow up to about 48 hours — ask yours in writing before you fill the bottle.
That clock is why Friday is a trap:
- Most county and commercial labs are closed Saturday and Sunday, or staffed too thin to start the incubation on time.
- A bottle collected Friday afternoon often sits until Monday — past hold time — and gets rejected. You pay again. You wait again.
- Mail-in kits still need overnight return on the bacteria bottle. A Friday mailbox is not overnight to a Monday lab bench.
- Real desks already say this out loud. EGLE in Michigan stopped accepting Friday total-coliform / E. coli samples. Vermont local-health drop-offs are generally Monday–Thursday. County labs from California to Minnesota commonly cut bacteria off Thursday afternoon or refuse Friday entirely. Check the desk you are using — do not assume.
Practical rule: collect bacteria Monday, Tuesday, or Wednesday. Confirm receiving hours and holiday weeks. Keep the bottle cold, out of sunlight, and on the path the lab named. If they mark it invalid, that is the clock, not a conspiracy.
Where to get the test
The table scrolls sideways on a phone.
| County environmental health | State-certified drop-off lab | Mail-in kit that emails a PDF | |
|---|---|---|---|
| Best for | Fast, cheap bacteria (and often nitrate) after a local flood, when they are open and taking private-well samples. | When you need a named certified method, VOC/chemical scope, or the lender named a local lab. | When county is closed, flooded itself, or you want a broader health PDF this month. |
| Hold-time reality | Ask receiving days. Many are Mon–Thu only for bacteria. Call before you collect. | Same short bacteria clock. Call for Friday rules and after-hours drop boxes (a box does not restart the weekend). | Overnight the bacteria bottle. Read the kit sheet. No Friday ship-and-pray. |
| PDF? | Usually written or emailed. Ask how they send flood-related results. | Yes, if you ask for the report. | Yes — that is the product. We name Tap Score / SimpleLab and National Testing Labs. |
| Catch | Short menu. May not cover VOCs or pesticides. After a big flood, desks get slammed — call early. | Many certified labs only serve utilities. Confirm they take a residential well sample before you drive. | Costs more than county. Still not a softener sales visit. See the cost table. |
State pages with desks we have already verified for homeowners: Michigan, Minnesota, Pennsylvania, Texas. Until your state is listed, start with county environmental health and EPA’s state certification program list.
Calendar for the next two weeks
- Day 0–2 (water still up / just dropped): bottled water. Stay off the pump. Call a well contractor and county health. Do not collect a bacteria bottle yet if you still need disinfection.
- After professional disinfection: flush until clear. Mark the calendar for sample day = disinfection day + 7 to 10 days, landing on a Mon–Wed that the lab accepts.
- Sample day morning: sterile bottles from the lab or kit. Fill to the line. Cap without touching the inside. Keep cold. Deliver or overnight the same day.
- If bacteria is present: stop drinking it. Call county health. You may need another disinfection round and another wait. Retest.
- If bacteria is clear but the flood was chemical-heavy: order the VOC / pesticide panel county recommended. A clear coliform result is not a fuel clear.
- Weeks to months later: EPA warns groundwater can keep delivering contamination after the first clear. Retest on the schedule county health gives you — often again in a few weeks, then later in the season — then return to at least annual bacteria and nitrate.
What to do tonight
- If the wellhead was underwater or floodwater reached the casing, stop using that water for drinking and cooking. Call county environmental health and a licensed well / pump contractor. Read EPA’s flood response steps.
- Schedule disinfection, then put sample day on a Monday–Wednesday 7–10 days later. Ask the lab their bacteria hold time and Friday rule before you collect.
- Order bacteria + nitrate at minimum. Add VOCs if fuel or chemical storage flooded nearby. County first; otherwise a certified drop-off or a named mail-in kit (Tap Score / NTL).
- Do not collect Friday. Do not use a screening strip as the all-clear. Do not invite a free in-home “test” that exists to sell a softener.
Not this if…
- Someone offered a free post-flood “test” that ends in a treatment pitch. That is lead-gen. See the Home Depot free-kit comparison. We refuse door-to-door, free-kit, and Culligan-style sales.
- You only bought a strip from the aisle. Screening-only. It will not stand in for certified bacteria after a flood.
- You sampled the day you shock-chlorinated. Wait for the chlorine to clear (7–10 days per EPA/CDC). Early samples lie.
- You shipped bacteria Friday and hoped. Hope is not a hold time.
- You needed a lender chain-of-custody file. That is a different errand — see the home purchase / lender page. A post-flood health PDF and a closing file are not always the same SKU.
Related: Florida · How to read a report · PFAS 533 / 537.1 · How it works · Tap Score vs NTL vs county cost table · Home purchase / lender CoC · Home Depot free kit vs a lab · Michigan · Minnesota · Pennsylvania · Texas · back to the directory.
Sources checked September 2, 2026: EPA Protect Your Home’s Water; EPA What to Do After the Flood PDF; CDC well disinfection after an emergency; CDC well-testing guidelines; Wisconsin DNR sample hold-time sheet (coliform 30 hours / nitrate 48 hours). Hold times and Friday rules are lab-specific — confirm before you collect. Affiliate disclosure: links to Tap Score / SimpleLab may earn WellTestList a commission (rel="sponsored nofollow"). National Testing Labs links are not affiliate links; we earn nothing from them. That does not change who we refuse to list.