How do I disinfect my lawn from floodwaters?

May 17, 2006 7:57 AM

The Merrimack River in Massachusetts decided to become part of my backyard these past few days, but thankfully has receded. My question is, how do I disinfect my lawn from all the bacteria and what not that was in the river without actually killing the lawn?
posted by kraagenskul to Health & Fitness (15 answers total)

You don't worry about it. Here's a hint: the outdoors has bacteria in it.

Next time it rains it'll all get washed down into the soil, where it belongs. Besides, the river flooding delivered a *lot* of valuable silt and stuff that will make your lawn very green this year. Flooding used to be an essential part of the fertilization cycle for crops, and was absolutely necessary in areas that would otherwise be infertile, like egypt and mesopotamia.
posted by SpecialK at 8:07 AM on May 17, 2006

You can't, in a practical manner, and there is no reason to. The bacteria present in the water are either no different from the bacteria already present in the soil or are facultative or obligate anaerobes that will die when the water recedes and oxygen levels become too high for them. There are also bacteria that are specific to the aquatic environment that will also die when the water recedes and the environment becomes too dry for them.

Given the diversity of bacteria, I am sure that there are a few that don't meet the previous criteria for dying but they would already be present in the soil from the last time it was flooded.

Season flooding is a good thing that delivers nutrients and is what makes river valleys so fertile. Without seasonal flooding in the indus and nile river valley modern society would not exist.
posted by 517 at 8:14 AM on May 17, 2006

I'll second SpecialK's post. As long as you are not out kissing the grass and rolling in the lawn, you should have very little to worry about. Even if there were a few sewage treatment plants that overflowed into the river while it crested, the E-coli isn't going to sit and fester in your lawn forever.
posted by JJ86 at 8:17 AM on May 17, 2006

Flooding used to be an essential part of the fertilization cycle for crops, and was absolutely necessary in areas that would otherwise be infertile, like egypt and mesopotamia.

The Merrimack River, while cleaner now thanks to some environmental acts, used to be pretty much a toxic soup of well shit - industrial, sewer and human varieties. It still faces some challenges are far as pesticide runoff and some issues with raw sewerage overflow during heavy rains. (source) Hopefully, everyone is right where it'd seep back into the soil, but given the history of the Merrimack and the industrial nature of the area at one time, I understand why you'd be a bit concerned.

I'd presume in the next few days, kraagenskul, that more information about this will be in the media as far as treatment. You could probably speak to your local DPW or Health Department too and find out more.
posted by jerseygirl at 8:26 AM on May 17, 2006

I don't have an answer, but in several areas the water overwhelmed sewer systems causing sewage to flow into rivers and the regular water system. Kraagenskul could have more than just the Merrimack in his back yard.
posted by MasonDixon at 8:29 AM on May 17, 2006

Water from a nearby stream filled my backyard but luckily no flooding in my basement (unlike most of my neighbors).

From the Mass DEP site - Cleanup of External Areas section (near bottom of page):

"The majority of the microbial population from sewage flooding onto lawns, tarmac and paved areas will be inactivated within several days due to exposure to UV radiation from sunlight. A disinfectant can be used on tarmac and paved areas. Contamination on grass could be left to degrade naturally. Typically, bacterial numbers on turf are reduced to background levels expected in the environment within 13 days, but can extend to 20 days on soil and sand in the autumn and spring. Generally, the least absorbent or pervious surfaces absorb the least sewer and bacterial concentrations and return to background levels the quickest."

Fingers crossed for sunny weather the next few days!
posted by bCat at 9:04 AM on May 17, 2006

Yep, but sewage is still sewage, and unless you're kissing it or wading in it with lacerated feet, you're not going to get harmed by it that easily.

I think the more important part is that if you feel worried about it, after you walk outside in an unprotected manner for the next week, take a shower.
posted by SpecialK at 9:07 AM on May 17, 2006

If you're downstream from Lawrence (actually, Sutton St. in N. Andover), you might have some traces of sewage. Wait a week or two before you resume rolling in that hay. If you're upstream from Lowell's Duck Island STP, you can probably resume rolling much sooner. (I haven't heard that any NH treatment plants were purging into the Merrimack, has anybody else?)
posted by Kirth Gerson at 9:47 AM on May 17, 2006

Response by poster: Thanks for all the answers, especially the Mass DEP site and link; I couldn't find anything on the MEMA site, never occurred to me to look on the DEP site.

As for rolling and kissing in the grass, I won't be doing that, but I have 3 little ones that would if they could, hence the worry and the post. I basically wanted to know what I needed to do to let them start playing in the yard again, even if that is just simply waiting.

Thanks
posted by kraagenskul at 10:09 AM on May 17, 2006

Oxygen and sunlight are your friends here. Going over your yard with a rake or mowing it when it's dry to break up any clumps of stuff that got washed up might be a good idea. Otherwise, you want a nice healthy microbial load on your lawn.

And to put things in perspective, any surface that's not been fairly brutally sterilized and sealed will be colonized and blanketed by microbial organisms in a matter of days.
posted by KirkJobSluder at 11:30 AM on May 17, 2006

Off-topic, but who knew that "pervious" was a word?
posted by i_am_joe's_spleen at 1:19 PM on May 17, 2006

Not me. It sounds becilic.
posted by Kirth Gerson at 1:43 PM on May 17, 2006 [1 favorite]

If you're downstream from Dover, NH, look for some corn. We had Shepherd's Pie.

The parts per million in the part on your lawn after a couple days sunlight = not enough to worry about.
posted by yerfatma at 3:01 PM on May 17, 2006

Um, I wish river silt only had fertilizers in it. In the Merrimack, one might worry about perchlorate, metals including cadmium and chromium, polycyclic aromatic hydrocarbons (PAHs), VOCs like TCE, chlorinated solvents...

But it's not going to kill you or your kids in the amount you'll find there, especially since (presumably), everyone would just be exposed this one time. A guess from my layperson's perspective, what won't break down by sunlight (particularly the metals) might be heavy enough, or attached to heavy things like dirt, that it'd wash off. So if I were you, I might use the hose to squirt off any silt that's on top of the grass, and encourage the little ones to always wash their hands before eating. But it's probably much more important what you regularly eat.
posted by salvia at 11:30 PM on May 17, 2006

salvia, it depends on where kraagenskul lives. The Concord River (in your first link) enters the Merrimack near the downstream end of Lowell. If his house is upstream from there, it's not a worry for him.

The report in your second link says: "Sampling of Merrimack river surface water during the RI/FS [in 1993] detected no cyanide, VOCs or metals."

Your third link is a 1987 report on ocean fish. Unless kraagenskul's lawn is attached to a houseboat, I don't think it's relevant. It does say those hydrocarbons "are found in high concentrations in" the Merrimack, but it doesn't say where, and 19-year-old data is not really useful here.

Your fourth link is a report referencing (again) the Concord River, with no data more recent than 1989.

Your last link is a report on geophysical testing at the same site as your second link, "to confirm stratigraphic information collected during earlier investigations and to characterize zones of highly conductive groundwater that may indicate the
presence of a groundwater contaminant plume." It draws no conclusions about actual levels of contamination, either on the site or in the Merrimack.

FWIW, the Merrimack above Pawtucket Falls in Lowell has been certified safe for swimming for over ten years.
posted by Kirth Gerson at 8:21 AM on May 18, 2006