Clean Water: Which Method Makes Which Water Safe

Boil it: one minute at a rolling boil, three minutes above 6,500 feet. That kills everything alive in it. If you cannot boil, plain unscented bleach at 8 drops per gallon (16 if the water is cloudy), wait 30 minutes; or calcium hypochlorite, which stores for a decade as a powder and treats thousands of gallons; or chlorine dioxide tablets. Let cloudy water settle first and pour off the clear top, or run it through a cloth, because dirt shields germs from the chemical.

No. Chlorine kills bacteria and viruses and, given time, Giardia. It does not reliably kill Cryptosporidium, a protozoan common in surface water that causes weeks of diarrhea. It also does nothing about chemicals, fuel or metals. Where crypto is a risk, which is any creek, pond or flood water, boil, use chlorine dioxide tablets (4 hours for crypto), a UV pen, or a filter rated at 1 micron absolute or finer.

It depends on the pore size. A 0.1 or 0.2 micron hollow fiber or ceramic filter stops bacteria and protozoa, including Giardia and crypto, and lets viruses through. A 0.01 to 0.02 micron ultrafilter, or a purifier bottle like the Grayl, also stops viruses. Activated carbon takes out chlorine taste, many pesticides and some metals. No ordinary filter removes dissolved salt, nitrates or fluoride; that takes reverse osmosis or distillation.

First fill the bathtub and every pot and jug from whatever pressure is left. Then use what is already in the house: the water heater holds 30 to 50 gallons (power or gas off first, inlet closed, a hot tap opened upstairs, drain from the bottom valve), the pipes hold a few gallons (open the highest tap to let air in, drain from the lowest), the toilet tank holds a couple of gallons if no cleaner has ever been in it, and the ice maker is a reserve. Treat anything you are not sure about. If a boil notice comes with the outage, boil or treat everything from the tap when it returns until the notice lifts.

One gallon per person per day, half for drinking and half for cooking and washing, and more in heat, for nursing mothers and for the sick. Three days is the bare minimum; two weeks is the target. For a family of four that is 56 gallons: a few cases of bottles you can carry, plus stackable 5 to 7 gallon jugs, plus a 30 or 55 gallon drum if you have the space. Rotate it every 6 to 12 months or treat it and forget it.

Test it before you decide. A deep, properly sealed well is usually clean, but private wells are not regulated by anyone but you, and they pick up nitrates from farms, arsenic and radon from rock, and bacteria from a cracked casing or a flood. Test for bacteria and nitrates every year and after any flooding or work on the well; test once for arsenic, lead, radon and hardness. If bacteria show up, shock chlorinate the well and retest.

Three levels. Strip kits from the hardware store give a rough read on pH, hardness, nitrates, chlorine and sometimes bacteria for a few dollars. A digital meter gives accurate pH and total dissolved solids. A mail-in laboratory test is the only one that reliably finds bacteria, arsenic, lead, radon and pesticides; your county health department or state lab usually offers one for less than a hundred dollars. Do the lab test when you buy the property and every few years after.

Yes, and it is the biggest single reserve in most houses. Turn off the power at the breaker or the gas at the valve, close the cold water inlet on top of the tank, open a hot tap somewhere upstairs so air can get in, and drain from the valve at the bottom into clean containers. Let the first quart run off if it is rusty. Do not turn the heater back on until the tank is full again. Treat it if the city water was already under a boil notice.

Only distillation or reverse osmosis takes salt out; no filter or chemical does. A solar still or a pot with a lid and a catch cup over a fire will produce a slow trickle of pure water. Flood water is a chemical problem as well as a biological one, with fuel, sewage and farm runoff in it, so filter through carbon and then boil, and use it for washing rather than drinking if there is any other source. The how water becomes unsafe guide covers what floods carry.