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One of the early events in my story involves a 2x2 mile section of a suburban town that is encased in invisible walls 100 feet (30m) high and perpetually subjected to heavy rain at a rate of about an inch an hour. It will keep filling up with water until it goes over the 100 foot (30m) walls roughly 50 days later, and from then on it will become a massive four-directional waterfall that, I suspect, will start carving massive rivers and radically altering the landscape. The specifics of what that would look like are a question for another day, however. Right now what I want to know is when, if ever, the water flowing out of that flooded suburb will be drinkable.

Given that nearly the entire planet has been subjected to frequently-dangerous supernatural ordeals like this one, modern society has entirely collapsed, and smaller communities need to figure out how to survive without things like advanced plumbing infrastructure. Living near such a huge source of water would be an incredibly valuable asset... if it were drinkable. But when I asked questions about how quickly said suburban deathtrap would start flooding, several people brought up something that had completely slipped my mind: the sheer amount of pollution that would get into that flood water due to the suburb it's built on and render it completely undrinkable.

If a 2x2 mile section of a modern American suburb were encased in 100ft (30m) high walls and flooded, how long would it take before the water flowing out over those walls would no longer be too contaminated and polluted to drink?

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    $\begingroup$ How long under which conditions? $\endgroup$
    – L.Dutch
    Apr 8 at 16:42
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    $\begingroup$ @L.Dutch-ReinstateMonica Do I not have enough details in the OP? What's missing? $\endgroup$ Apr 8 at 16:46
  • $\begingroup$ The time needed for the polluting load to be neutralized depends on the conditions in which the water is kept. On those conditions you are saying nothing $\endgroup$
    – L.Dutch
    Apr 8 at 16:49
  • $\begingroup$ You mean how clean the rain constantly filling it is? It so, completely clean. $\endgroup$ Apr 8 at 16:51
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    $\begingroup$ What's your standard for potability? There are very few existing natural sources of water that meet most (first world) government standards for potability. For that matter, some municipal water supplies don't meet some of the stricter standards for potability. Also, are we assuming the rain is pure H₂0, or...? $\endgroup$
    – Matthew
    Apr 8 at 18:02
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Not as much pollution as you think

The reservoir is filling so quickly, draining from the top, and the area exposed to the suburbs is so small relative to the volume that it just cannot contaminate it quickly enough to matter at the top of the water column. The "box" is acting like a giant settling tank, so very few particles will even make it to the top.

If you boil the water it will be completely drinkable. Even without boiling the water will be safe by most survival standards. There is just too much water compared to the amount of contaminants. Remember, we have reservoirs that literally contain flooded towns. Only in the very beginning when there is a lot of sediment and contaminants relative to the amount of water will it be dangerous.

You are looking at 300,000,000 cubic meters of water. That is more than most reservoirs hold. For scale, NY's largest reservoir, New Croton Reservoir, hold 70,000,000 cubic meters. All of the reservoirs in NY state's combined only hold 2,000,000,000 cubic meters of water. 4 square miles of suburbs just cannot supply enough contaminants in a few weeks of being submersed in essentially still water.

Keep in mind this is a survival situation. Minor impurities that may have long terms effects in 40 years are irrelevant. A suburb just cannot supply enough chemical contaminants in such a short time scale, so you only have to worry about biological contaminants, hence boiling. Even without boiling the risks involved are small, the influx of fresh water is just too fast. Would I want to drink it today? Not for long. Would I drink it in a post apocalyptic/survival setting? Absolutely.

Once you settle into rebuilding a civilization and years start going by, you may want to start actually purifying it, because things inside will start failing and breaking down, but even then the water coming out of your box will be cleaner than most reservoirs. The refresh rate on the water is outstanding, and the shape minimizes the migration of contaminants. Your water is clean by most pre-industrial standards and tolerable by modern ones.

Eventually the contaminants from the suburbs will be exhausted and in a few hundred years you have the cleanest water on the planet. Again many real reservoirs contains submerged towns. Monument City, Indiana was filled in 1965 submerging an entire town. NY contains dozens of flooded towns in its reservoirs, Ashokan Reservoir alone contains at least four towns flooded in their entirety. Keep in mind water in real reservoirs is flowing because it is coming in from rivers not magic rain so it is also far better at destroying and thus exposing stuff in the towns.

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    $\begingroup$ So, my city is wasting a lot of money into maintaining a whole set of purification plants while they could simply boil off the water they feed in the city pipes? $\endgroup$
    – L.Dutch
    Apr 8 at 17:02
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    $\begingroup$ @L.Dutch-ReinstateMonica If the city had a fresh water reservoir of this scale filled this quickly absolutely. Of course boiling that much water uses way more energy than normal purification which is Why cities don't use it. Keep in mind there is a big difference between legally potable and drinkable in a survival setting. $\endgroup$
    – John
    Apr 8 at 17:07
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    $\begingroup$ @john Your last number suggests you don't know that a cubic meter is 1000 liters... $\endgroup$
    – Zeiss Ikon
    Apr 8 at 17:12
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    $\begingroup$ @ZeissIkon thansk for catching that, got lazy and tried to use google $\endgroup$
    – John
    Apr 8 at 17:16
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    $\begingroup$ @L.Dutch-ReinstateMonica Note that boiling water takes a lot of energy. It might be that the purification plant they have is more energy efficient and hence (in the long term) cheaper than 'simply boiling'. $\endgroup$
    – quarague
    Apr 9 at 9:43
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That which can float, will soon float to the sides and eventually go over the waterfall.
(your barrier is spilling 20.43 cubic meters of water per hour, per meterlength of outside wall.)
And yes, even at a mere 2x2 miles it will be subject to a bit of tides, a lot of wind-driven waves, and will slosh over the sides instead of just gently dribbling.
Note that this will include most of the noxious chemicals we use, such as oil, gasoline, etc.

That which wants to sink, will sink and settle on the bottom.

The only problem will be anything that dissolves in the water, either once-off or via continuous contamination (like rotting organics)
And these will be continuously diluted by the influx of pure water.

I expect that the water will be fine as soon as all the organics have finished rotting, so give it maybe 6 months. This will also allow any rapid pollutant releases to deplete themselves. For example, gas tank of automobiles.

But for many, many years the water will have a slight sheen of oil on it, as the many machines and devices and storage tanks and containers rust through and gradually release their oily residue. The water will not be undrinkable, but it will not be a fresh mountain stream!


later comment: Remember that we are looking at a 100ft deep container, completely full, with rainwater falling on top at a rate that would have filled the container in 50 days. If using full mixing and only dilution to get rid of pollutants, we would only be halving the concentration every 25 days.
But because both the inlet (rainfall) and outlet(spillage over the edges) occurs at the very top of this reservoir, and there is no force at all to cause internal turnover currents, there will be very little mixing of the water. Mostly what will go over the edge is newly fallen rainwater, and anything that floats in the water.
Dissolved contamination, even without any source of replenishment, will dilute at a rate of something less (possibly much less) than halving every 25 days.

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    $\begingroup$ Rusting through a tank enough to produce a visible slick takes decades, starting from a serviceable container. Oil in crankcases and such will wash out early; they aren't sealed. And suburbs rarely have above-ground storage for large amounts of hydrocarbons -- underground storage will last longer than that, because it's required to be very resistant to corrosion. $\endgroup$
    – Zeiss Ikon
    Apr 8 at 17:16
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    $\begingroup$ @ZeissIkon a 2x2mile of suburbia will typically include 6 to 10 gas stations, each storing many thousands of gallons, plus approximately 12000 automobiles (if american suburbia. 1/3rd that in europe) , most of them with half-full fuel tanks. $\endgroup$
    – PcMan
    Apr 8 at 17:24
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    $\begingroup$ @ZeissIkon as for leaking. virtually NO hydrocarbon liquid storage is airtight. For the simple reason of thermal expansion, almost all containers and absolutely al large storage tanks include air venting. They rely on gravity to keep the liquids inside. Thus slow leakage is inevitable. $\endgroup$
    – PcMan
    Apr 8 at 17:28
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    $\begingroup$ I ran the tidal flux for the last question I got an amplitude of 1mm, so it does not matter much. $\endgroup$
    – John
    Apr 8 at 17:39
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    $\begingroup$ @ZeissIkon yep, hence "tend". I have actually seen a diesel tank uproot itself during flooding. But it was 90%+ empty, thus effectively an air bladder. I have also seen many tanks that just took in water, and ruined the contents. Most usually though, the tank stays where it is, and can be easily located in (still) water by the rainbow sheen on the water above it. $\endgroup$
    – PcMan
    Apr 8 at 17:58
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Some math: 25mm/m2/h and about 10km2 area leads to approximately 70 m3/s of water. Not even close to large rivers, but it is a decent stream. Approx 1/100 of Danube.

Population density varies, taking a reasonably high estimate of 1k/km2: 10k people, each with 100l of problems. 5k cars, each with 100l of problems. 5k "misc" (homes and whatnot), each with 100l of problems, and you end up with 20 m3 of issues.

Now, threshold for pollution greatly depends on source. I can't be bothered to find thresholds for all sources of problems, so I took the first number I found: 50 ppm = 50 mg/l threshold (EU threshold for nitrates).

This means water is "clean" as soon as outflow of problematic stuff is below 3.5 l/s. With 20 m3, even constant linear outflow of problems (the worst case estimate) would give you mere hours to have stuff below that threshold. Even estimating 10 times as many pollutants will have the water clean enough in less than a day.

So, we can ignore suburbs. Pollution is negligible with that amount of water flow.

EDIT: we can most likely also ignore rainwater problems. With that amount of rain, it will be essentially pure water falling down. You are still going to need approximately 30 tons/day of generic pollutants to make water problematic for drinking.

(so, to make water undrinkable you need specific pollutants that have way lower threshold of causing problems. Generic post-apocalyptic pollution will not do)

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    $\begingroup$ I'm not sure that the captured rainwater = filtration even holds here. The rain is going to be cleaning out the air, so the particulates in the air should go to 0. The question would be if bacteria in the water can outpace the overflow, but I am thinking maybe not considering that we don't usually have bacterial problems in tap water, etc. $\endgroup$ Apr 9 at 16:22
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    $\begingroup$ @user3067860 yeah, you are right. With that amount of rain you will flush all pollutants out of the air way before the day is over. $\endgroup$ Apr 9 at 19:09
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    $\begingroup$ @user3067860 You hve obviously never lived in Africa or Indonesia, where tapwater and cholera can go hand-in -hand. $\endgroup$
    – PcMan
    Apr 10 at 6:15
  • $\begingroup$ @ZizyArcher HOW is a 2x2 mile permanent rainstorm supposed to clear the air of an entire planet? Remember the scenario: each 2x2 mile region of earth has its own 10ft-tall enclosing walls, and its own purely local weather. $\endgroup$
    – PcMan
    Apr 10 at 6:16
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    $\begingroup$ @PcMan it won't clear the air of the planet, but it doesn't have to. It will clear the air in the region where it rains. Then, to have problematic levels of pollution to make water undrinkable, you require many tons of pollutants per day entering this local rainstorm thingy. This is hard to do even deliberately. $\endgroup$ Apr 10 at 18:44
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At the start, all the gasoline from underground tanks at filling stations, all the motor oil and similar substances from cars, all the contents of sewage treatment and septic systems, plus things like pesticides from garden stores and the like will wind up mixed into the water. It'll be a veritable stew of coliform bacteria and chemical pollutants (not to mention all the drowned residents and their pets adding to the mix).

By the time the water is twenty or so feet deep, however, there'll be enough dilution that chemical pollutants will become nearly negligible and, as noted in another answer, boiling will be sufficient. Assuming there's no fresh sewage or bodies going into the Fish Tank, it should be safe enough to drink without boiling in a matter of months -- the worst of the pollutants will have either washed out soon after the water starts to overflow the invisible wall, or settled to the bottom of the new "lake", leaving what's left almost completely rain water. Given the rate of rainfall, the water inside the Fish Tank will be purer than most surface water ca. 2021 within a year (likely less).

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You would have to include what kind of and level of toxins were frequent in the suburb, account for erosion, how fast is the water churning ? How is it leaking out ? Is the water remaining muddy ? Is new sediment being introduced at all (which it sounds like it is not). What kind of air pollution may be nearby (consider acid rain), and also what kind of potentially parasite microorganisms were present to begin with that might be reproducing from preexisting waterways or soil layers that are now thriving or being introduced by some other means.

The question is kind of complex, so the answer will spark more questions.

In brief it may be that the soil pollutants will settle in a layer of silt within a month or two, but if there are harmful critters in the water, or if there were high levels of pesticides etc that were water solubale, then you may be SOL.

In the wild we tend to filter and/or treat water for a reason, there are many parasites and pollutants that make us very sick. Near springs and head waters wit natural aquifers the water is less polluted, but the idea of a crystal lake is not realistic these days and what you are describing sounds sort of like a manmade lake in an area flooded, which contained ridiculous amounts of modern pesticides herbicides gasoline, and other pollutants and trash.

There might be a breakdown of garbage and trash by natural bacteria which help things alongs and some contagions might very well die in the water, which were aerobic.

Just some thoughts.

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    $\begingroup$ there is basically no churn, water only escapes by overflowing. Keep in mind there is also a big difference between modern legal safety levels, and save to drink in a post apocalypses setting. $\endgroup$
    – John
    Apr 8 at 17:41
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    $\begingroup$ yeah but you can still be poisoned if the water is 10% raid :P I'll think a bit more. $\endgroup$
    – John Sohn
    Apr 8 at 17:48
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    $\begingroup$ to be 10% raid there would have to be a raid factor with a giant fluid retaining tower in the suburbs. 300,000,000 cubic meters of water is a LOT of water. $\endgroup$
    – John
    Apr 8 at 23:50
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Water treatment plants daily process rain water coming from the surfaces of urban area

The goals of the treatment are to remove unwanted constituents in the water and to make it safe to drink or fit for a specific purpose in industry or medical applications. Widely varied techniques are available to remove contaminants like fine solids, micro-organisms and some dissolved inorganic and organic materials, or environmental persistent pharmaceutical pollutants. The choice of method will depend on the quality of the water being treated, the cost of the treatment process and the quality standards expected of the processed water.

The processes which are carried out are:

  1. Pretreatment
  2. Coagulation and flocculation
  3. Sedimentation
  4. Dissolved air flotation
  5. Filtration
  6. Removal of ions and other dissolved substances
  7. Disinfection

If the water is simply kept still, only step 3 would happen naturally at a significant rate, and the growth of bacteria and algae in the water would just make the problem worse.

It would take years at best to make the water drinkable without any intervention.

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    $\begingroup$ But the water is being continually added to (replacing outflow) at a terrific rate -- inch-an-hour rainfall, remember? $\endgroup$
    – Zeiss Ikon
    Apr 8 at 17:18
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    $\begingroup$ Keep in mind this is not runoff, this is a massive amount of water put on a very small area of suburban landscape, with no drainage, in a very short time. $\endgroup$
    – John
    Apr 8 at 17:20
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    $\begingroup$ Most of those are descriptions of how a filtering step is done, which isn't that relevant for a flood of magically induced torrential rainwater; you need a list of specific threats removed. $\endgroup$
    – Stilez
    Apr 8 at 17:28

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