Is city rainwater "dirtier" than country rainwater?
November 27, 2010 5:41 PM   Subscribe

Is city rainwater "dirtier" than country rainwater?

Is rainwater that falls in the city "dirtier" (eg from material it collects as it falls, or from other pollutants that might evaporate with the water around a city) than rainwater in the country?

And if so, would it make a significant difference, for example for the purposes of laundry, bathing, or drinking?

If it makes any difference, assume the city is by the ocean.
posted by UbuRoivas to Science & Nature (12 answers total) 3 users marked this as a favorite
Ubu, story might answer your question.

More broadly - and I can't find the link but have read about this as it pertains to Sydney - yes, the water is dirtier than in the country, mainly from traffic pollution. But no, it's not bad enough to seriously affect your health.

The worst pollution comes from the run-off once the water has hit the ground and soaked up the contaminants from tyres, cigarette butts etc. The amount it gets from the sky here is comparitively minor.
posted by smoke at 5:53 PM on November 27, 2010

depending on the wind rain blowing off a large body of saltwater the rain could be salty. Rain water starts as pure water so it's clean to start with.
posted by patnok at 6:08 PM on November 27, 2010

But no, it's not bad enough to seriously affect your health.

Uhh, depends greatly on your region. Collecting rainwater for household use is pretty common in many areas of Australia and New Zealand, so smoke's story is a little skewed.

For example: Cubatao is a city by the ocean, in Brazil, but had had horrendous amounts of air pollution in the 80s. Here's an article from the NYTimes about it, where it says "The factories on this swampland have turned the nearby town into a place of superlatives: in Cubatao, pollutants in the rain have reached some of the highest levels known in the world..."

So clearly, yes, it can make a difference. This is an extreme case, however. Again, depends on the city, depends on the location, depends on the weather, etc. etc. etc.
posted by suedehead at 6:25 PM on November 27, 2010 [1 favorite]

Short answer is yes to the question of whether, all other things being equal, rain falling through more polluted air will be dirtier than rain falling through less polluted air. Both clouds and falling rain will pick up chemicals and particulates in the air. Whether this matters is going to depend on the use of the collected rain, and can probably only be properly determined by testing the water, which is a very accessible thing for anyone wanting to use collected rainwater to do.
posted by nanojath at 6:42 PM on November 27, 2010

(Assuming the use of collected rainwater is the basis of this question...)
posted by nanojath at 6:43 PM on November 27, 2010

Rain collecting particulates in the air is an issue, but another major one is the trip across your roof which, in areas that have long periods without rain, collects crap on it, then washes into your tank as the rain starts. You can get systems that "divert" the first x litres of water to the drain before they start to fill up the tank in order to improve this situation.
posted by Jimbob at 8:17 PM on November 27, 2010 [1 favorite]

To point out the (hopefully) obvious, have someone who knows how to deal with filters and pollutants give you good local advice before you heavily and regularly use collected rainwater for bathing. And please don't even think about drinking collected rainwater without expert input including appropriate filters, sterilization and precautions. Be aware that even with those measures plumbing rainwater to any household fixtures is illegal in many jurisdictions.

The danger is more with the stagnant collected rainwater and pollutants that have run-off the surfaces the rainwater lands on as it is with anything picked up while falling through the air.
posted by meinvt at 8:24 PM on November 27, 2010

San Francisco has much less air pollution than towns in California's Central Valley, because the prevailing winds blow all of our gunk east. So it would depend on your (micro)climates, I'd imagine.
posted by rtha at 8:26 PM on November 27, 2010

I should add that this applies for the majority of the year when the winds are onshore (West to East); when the winds are offshore, our air quality is terrible. But that mostly only happens during our dry season (roughly May-Oct), when it doesn't rain, and only lasts for a few days at a time.
posted by rtha at 8:38 PM on November 27, 2010

The answer is - it depends. Do you want to drink this stuff or just water the garden? If it's for the garden, you really don't need to worry. If you want to drink it, read on. (On preview - sorry for the length!)

Are you collecting run-off from a roof? If so, you will certainly end up with particular matter as well as the usual bird-crap and dust - simply from the tyres and exhausts of vehicles.

Do you live near a dirt road, beach or industrial area? All these things will contribute to the amount of crap in your water.

Do you have animals in your roof? Birds, rodents and possums need to be taken into consideration.

What size tank are you collecting into? All tanks should be fitted with netted intakes and out-flows to prevent anything climbing or flying into the tank. Mostly mosquito's but there are some suicidal possums out there and nothing will kill you faster than water polluted by even a tiny corpse.

The size of your tank will also determine how much time the water gets to settle. Most particulate matter will eventually accumulate on the bottom of the tank. A large tank won't be emptied and filled as much and the bottom sediment will stay undisturbed for longer.

I personally think water filters are overkill, but the idea of a flow diverter that lets the first few liters wash off the roof is a good one.

Disclaimer: I lived for 20-something years on tank water alone. This was the country so low traffic but high dust and plenty of animals. We had multiple tanks and ran everything off them - toilets, showers, washing machiens and all drinking water. From a good size house roof and a shed roof we were able to harvest enough water that we never EVER ran dry even through the worst of the droughts. Australia doesn't have a rain problem - it has a storage problem. The only problem I've encountered with rain water is that it doesn't have fluoride added to it so I have a few cavities that might have been preventable.

And if you think city water is that much cleaner, you really need to 1) have a good look at the catchment area and get some idea of all the things going on it in (including water sports, farms and so on), 2) have a look at the pipes coming out of whatever treatment plant you've got (and the sludge that grows inside the older ones is outstanding). It can be interesting.
posted by ninazer0 at 9:51 PM on November 27, 2010 [1 favorite]

Thanks for the answers, everybody. This actually came out of a discussion (not exactly an argument; more of a "huh, interesting that you see it that way" conversation) about whether or not you should wash your laundry again if it gets rained on.

I've always assumed that rainwater is about as pure H2O as you're ever likely to get, so as long as the clothes don't get so wet that they take days to dry, it's like you just got an extra awesome rinse. Plus, you can give the clothes another spin in the machine if you really want to get rid of the excess water. The city water v country water angle was an attempt to set a benchmark for what the purest rainwater might be.

For what it's worth, as much as I might feel like installing a rainwater tank, it's just not cost-effective now, when my quarterly per-volume costs for tap water are around $35. It'd take more than 10 years to recoup the expense.
posted by UbuRoivas at 3:24 PM on November 28, 2010

The only problem I've encountered with rain water is that it doesn't have fluoride added to it so I have a few cavities that might have been preventable.

We gave our children flouride tabs for a while, but were then informed that the water had enough flouride from being stored in a concrete tank. Their teeth were/are fine, FWTW.
posted by GeeEmm at 3:10 PM on November 29, 2010

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