What about toxic chemicals in the water?

In the US, just know where you are hiking and the history of the water. For example there are old mining areas where the water isn't safe due to trailings. But overall the water is quite safe. Tap water isn't always safe either, just saying. You have to know the source. I had a boyfriend years ago who lived next to a superfund cleanup by a Navy base, his water reeked of jet fuel. On the other hand I have stood at the headwaters of the local river (it is on the PCT near Snoqualmie Pass) and not been afraid to drink it unfiltered…..

IMO you can only worry so much.

http://www.npr.org/2011/12/08/143386908/epa-connects-fracking-with-water-contamination should be epa-connects-fracking-with-water-contamination-by-violating-their-own-test-protocols-in-a-study-that-won't-survive-peer-review-but-got-the-desired-scary-outcome.

Sarbars right, you're worrying way too much.

Can I or anyone tell you exactly the risk of drinking any water? No… and that include municipal water supplies

Oh… by the way, municipal water supplies have no way of removing those same chemicals, and they are pumping million gallons every day. Here in LA much of the water we drink comes from open water sources. When I was up in the mountains I could see the water bubbling out of the ground before it flowed down the mountain. It is ridiculous to think there was considerable industrial waste in it, because there was no industry… there wasn't even any roads.

You talked about water 1 hour from a municipality. Where does that municipality get its water? My municipality gets its water from that same stream, but they wait until it flows past housing and industry and countless places to be polluted. Then they put it into an open aquaduct that flows for hundreds of miles.

It is a good idea to know where your water comes from… but you should also have the same concern about your tap water.

The PCT has at least one water source that's known to be contaminated with uranium. I still drink from it because the consequences of not drinking is higher than of drinking contaminated water.

Since you mentioned mercury, read about the Lewis & Clark expedition. Those guys put incredible amounts of mercury into their bodies. Some of those guys lived long lives. Yeah, it had some effects, but I wouldn't stress about consuming a relatively teenie tiny infinitesimal amount of mercury.

So what do I do? I know where my water sources are, try to know about the quality of water there, and plan accordingly. Sometimes I can and do carry enough water to skip an undesirable water source, but I doubt I'll ever choose to die of thirst rather than consume water with chemicals in it.

In any case, water goes downhill, hikers usually hike at higher elevations, and people usually live at lower elevations. With rare exception, water is going to pick up more and more chemicals as it goes downhill.

Wht water source on the PCT is contanminated by uranium. I need to know if I'm dying. Well, I guess I know I am, but anyway.

Most rivers in southern New England were terribly polluted with industrial chemicals and sewage until environmental laws started taking effect. I lived next to the Naugatuck River in Connecticut in the 1950s and I would never ever drink water from that river — it had an eerie beauty, with purples, blues and greens. This area was industrialized from the early 1800s. I imagine the damage from sewage is gone, but I'm sure there is still a witch's brew of toxic chemicals in the stream bottom. The same would be true of all medium size streams in CT, MA, RI and most in VT and NH. Possibly the large rivers (Conn. River, Hudson River, etc.) dilute it enough to be safe. The same would be true of NY, NJ, and PA, at least those that pass through areas that were industrial in the last 250 years. Some of the pollutants will take forever to wash out.

The Housatonic River next to the AT in CT and MA would be an example. The current concern is PCBs from a GE factory in, I think, Pittsfield, MA. There was iron mining and steel furnaces close to the AT in CT and MA, and at least one lead mine not too far (Litchfield, CT).

Smaller streams away from the valley bottoms are more likely to be safe, but this is not a certainty. Generally, I would drink from brooks or small streams that haven't gone through old industrial towns.

There are many bucolic towns in New England that were industrial many years ago, especially valley towns. Hill top towns in New England tended to be more farming than industry. Mining did occur in New England, generally on a small scale and I have no idea how that effects water safety here.

You would need to know the industrial history of an area to guess if a stream is actually safe.

It is actually a very interesting topic and discussion. Sometimes I get worried about these kind things. While i'm up in the wilderness I have to think we are drinking pretty good water and I don't really worry about it. In our day to day lives we breath and ingest a lot of nasty things. Just take a look at some of your favorite local diners and dive bars. Some of those beer lines from the keg to the tap probably have not been cleaned in years if not decades!! If I worried about all the potential risks of everything in my environment I would go crazy.

But I guess you never know what is up stream…

An "unknown level of risk" is CYA-speak for "We haven't tested it", rather than "it's #$%^ing close to chugging liquid mecury".

Eugene makes a good point about drinking from side brooks and streams, rather than valley bottoms. Oily stains in pools, or persistent surface foam below rapids are warning signs. Frogs are supposed to be sensitive to chemicals – if you can hear frogs, it is probably chemically safe (but doesn't mean it is bacterially safe).

Generally, it should be noticeable if you are hiking in an area where there are (former?) industrial sites. But those areas probably also have municipal supplies nearby.

Mining is the more problematic one, as you can get old mines in what are now 'wilderness' areas, and they aren't always obvious. Small scale and undocumented mines may well have used nasty chemicals (gold mines in particular can be an issue, as cyanide was (and still is) a key chemical for separating gold from associated ores).

It is worth reading trail notes and Park notes of areas you are visiting. Occasionally they will warn of particular known contaminated streams.

Open farm fields are probably much less of an issue (unless pesticide or fertilizer has very recently been applied), but you should avoid drinking immediately downstream of barns, sheds or stock yards, where concentrations can be much higher.

But in general, I think this issue is overblown for hikers. While drinking from a chemically contaminated source might be unsafe if you lived there (due to cumulative exposure), if you are drinking a liter or two and then moving on, the exposure and risk is extremely small.

If you want a backpacking filter which does remove chemicals, they are available:
http://www.drinksafe-systems.co.uk/

looks like they are trialling a product which removes radiation too.

I used to survey for frogs and other critters on an Air Force base that had ground and surface water contamination due to leaking rocket fuel from cold war area missile silos.
My advice, the presence of frogs most definitely does NOT mean the water is free from contamination.

-Steve