Is it safe to plant a vegetable garden after the area was covered by polluted flood waters for three days? Is it safe to eat existing vegetables (kale, garlic, etc) that survived? Our well tests positive for coliform bacteria and E. coli... two weeks later there is still floodwater remaining in our pasture. (This refers to the flooding of the Willamette river in Corvallis on April 10th 2019.) Should I wait a week, a month or a year to plant???
The April 2019 flooding event did cause two releases of untreated sewage from the Albany treatment plant into the Willamette River. There were likely other releases from septic systems along with contaminated runoff that distributed higher than normal levels of animal waste into flooded areas. Therefore, it is important that you protect yourself and your family from exposure to a number of different harmful hazards, including foodborne pathogens (e.g., Salmonella, E. coli), that could impact the safety of existing crops in your garden. Flood waters also carry chemical hazards (farm runoff, industrial pollutants, etc) that could be dangerous if consumed.
Do not consume any produce from your garden that was exposed to flood water (submerged or splashed). Your kale and garlic have likely been exposed to dangerous bacteria and chemicals that may not be completely removed by washing and/or cooking. Also, do not preserve any produce that was exposed to flood water. Never sell produce from a flood-damaged garden at a farm market or farm stand. The University of Wisconsin has a good 2-pager with more details on evaluating various edible plants in your garden.
You can safely grow a garden this year. With normal gardening activities (tilling, etc – as appropriate for the stage of your garden), the microbiology of your garden will correct itself in about a month. Flood water can be assumed to contain “non-composted manure” and we can apply the National Organic Program (NOP) standards that commercial farms use to manage risk. The NOP requires a 90-day period between non-composted manure application and harvest when the crop is not in direct contact with the contaminated soil (e.g., corn, peppers). A 120-day period is used if crops are in direct contact with contaminated soil (e.g., root crops, strawberries). Assuming flood waters receded in your garden on April 15, crops not contacting the soil could be safely harvested after July 14 and crops contacting the soil could be safely harvested after August 13.
It is not uncommon for groundwater sources (i.e., wells) to become contaminated with coliforms and E. coli from a flood event. It is also possible that your well may contain chemical contaminants from the flooding event. You may want to have your water tested for potability (microbial and chemical safety). There is a good explanation of water tests and testing resources on the OSU extension website.
If you are only concerned with microbial contaminants, you may boil your water (i.e. rolling boil for >1 min) and it will be safe to use. EPA has instructions for inspecting your well and considerations for treating your well. For a long term solution, your well will need to be chemically treated (i.e., “shocked”) or you may want to treat your water. OSU extension has a list of recommendations for considering water treatment strategies. Your local well drilling contractor can be a helpful resource for determining your best course of action.