>>In today's lecture we're going to talk about another type of water quality concern. Previously
we've talked about pathogens in the water that can cause cholera, typhoid fever, cryptosporidium,
and can lead to immediate deaths or deaths in a very in a timeframe of days to weeks.
But now we're going to talk about water quality concerns that come from groundwater. We have
surface water, rivers and lakes, and we have groundwater. We have water below the ground,
and a large percentage of the people in the world end up relying upon groundwater as their
drinking water supply if they're not next to a river. Or in some cases, for example,
Memphis, Tennessee is right on the Mississippi River, but that water has water quality concerns,
and so they actually use groundwater in Memphis even though they're located right on the Mississippi
River. But there can be issues with the groundwater as well. Groundwater as it flows through the
subsurface comes into contact with the geologic material, and there's certain things in that
geologic material that may dissolve into the water and may make that water unsafe for human
consumption.
And one example, an example that we'll talk about to begin with is arsenic. Arsenic is
naturally occurring in geologic materials, and in certain places they mine arsenic for
use in various products. And as a result as the groundwater flows through those formations,
some arsenic may dissolve into the water, and if people drink that water, they may have
health concerns that result. Now, arsenic can exist in the subsurface, in the groundwater
in several different forms. And not to go too much into the chemistry, but two forms
of special interest are arsenite and arsenate. And the reason I'm mentioning that is because
the arsenite form is more mobile in the subsurface. It'll move more readily, and thus it's more
likely to show up in the groundwater and and as you bring that groundwater up aboveground,
whereas the arsenate is less mobile unless there's certain geologic conditions that help
to mobilize that arsenate.
For example, right here in Norman, Oklahoma, University of Oklahoma we have in our geology,
we have naturally occurring arsenic. It turns out that those concentrations were below the
health concerns, the old standard of 50 parts per billion, and so Norman had a certain number
of wells that it was using. In 2006 the standard was lowered from 50 parts per billion down
to 10 parts per billion, and at that time about half of the wells the city of Norman
had were no longer in compliance. And so the city of Norman abandoned those wells and thought
it was cheaper to go find other water, looking at maybe bringing the water from 20-30 miles
away, rather than to remove the arsenic from those wells. So in the case of Norman, they've
come back now and realized that because of the cost of that other water, it's begun to
look at treating the water coming out of a well. And the material that they use to treat
that water is an iron-based material. And that as that water flows through the column
with this iron-based material inside it, the arsenic sticks to that material, and the water
coming out is relatively free of the arsenic. It turns out this is very similar to the process
that's happening in the subsurface. There's iron coatings on the geologic material belowground,
but the pH is high enough that the arsenate is liberated from those surfaces. As the groundwater
comes aboveground, the first steps the city of Norman does is to lower the pH and now
when it goes into the column, the arsenate will stick on to the material, will adsorb
onto the material because of a more neutral pH actually. So in the case of Norman, the
arsenic is in the groundwater because of the elevated pH, which liberates the arsenic from
the subsurface geologic materials. In other instances, the arsenic is in the groundwater
because of what we call reducing conditions. Reducing conditions just means the oxygen
has all been consumed from the water, and you're in an oxygen-depleted situation. And
we call that chemically, we call that a reducing condition. And under those conditions, iron
that's in the subsurface will actually dissolve, and you'll see this sometimes when you pump
a well. You'll see the water comes out has a rusty color to it, a red color to it, and
that's because of the dissolved iron. And as that iron dissolves, it liberates the arsenic
that was associated with the iron. So in those situations, what you do is you bring that
water aboveground, and you oxidize it. You add oxygen or even to make the process faster,
you add something, a chemical like potassium permanganate or hydrogen peroxide as an example
that we might know, an oxidant that we use at home, and that will help to oxidize that
arsenite to the arsenate and make it easier to remove on the iron-based removal material.
So that's arsenic.
Now, a second chemical of concern that can be in the groundwater is fluoride, and in
both of these cases, these are naturally occurring. This is not because of human activities. It's
just because of the geologic material releases these compounds into the water as the groundwater
flows past that geologic material. Fluoride is an interesting case because we actually
add fluoride to our water at low levels to improve our teeth. It was realized in former
times that people that drank groundwater that had low levels of natural fluoride in it had
fewer cavities, and so we started adding fluoride to our water at low levels. We added fluoride
into our toothpaste for similar reasons. But it turns out at higher concentrations, the
fluoride can have negative effects. And that can begin to damage your teeth, and you can
have something we call dental fluorosis, fluorosis of the teeth. And at even higher concentrations,
it can begin to damage the bones, and I've seen people in Ethiopia and India whose leg
bones are are permanently bowed, kind of like a bow and arrow bow because of the fluoride,
the high levels of fluoride in the water. So there's a little exercise I like to do,
and I first saw this in Ethiopia—I saw a medical doctor doing this out in the rural
areas and villages in Ethiopia, a little exercise to see if you might possibly have skeletal
fluorosis. I ask people to take their hands and to raise their hands and then to put their
put your hands behind your behind your neck, and if you can do that, then you don't have
skeletal fluorosis. People that have skeletal fluorosis may only be able to lift their hands,
say, this far, and that means they can't do the manual labor that they need to grow the
crops that they live on.
So now we've looked at arsenic and fluoride as two water quality issues in groundwater,
and let me share with you a case study that kind of illustrates the comparison of these
water quality issues versus the surface water quality issues. And that case study would
be the country of Bangladesh. Bangladesh has major rivers that flow through it, and so
there's not a lack of water. There's plenty of water, but it so happens because of animal
activities, and human activities, and the rain flushes those materials into the river,
the rivers are unsafe to drink. And so the idea was well, instead of having people get
their drinking water from the rivers, let's put in these wells, and the well water will
be safer than the river water. And that's in fact true. What they didn't realize when
they put in the wells was that those groundwater had naturally occurring arsenic in it. One
of the basic premises in doing work in any country and especially in developing countries
is do no harm. And unfortunately in the effort to prevent the people from being exposed to
the pathogens in the river water, ended up exposing these people to arsenic in the groundwater.
Now as it turns out, if you gave me a glass of water that had pathogens in it, cholera,
or if you gave me a glass of water that had low levels of arsenic in it, I'd drink the
arsenic water. However, I'd really ask for a third glass of water that had neither the
pathogens nor the arsenic in it, and that's what we should be attempting to do as we're
bringing water to those less fortunate. So this episode, this occurrence of putting in
these wells has been called by some people the the largest mass poisoning of a human
population in the history of the world. Unfortunate, unintended. Now we know that whenever you
put in well, you should test that well to make sure that that water doesn't have arsenic
or high levels of fluoride in it, and nonprofit organizations working in developing countries
do that now.
So we've talked now about arsenic and fluoride. We've talked about what that looks like in
the U.S., the city of Norman, and what that looks like in developing countries. And part
of the research that we're doing at the University of Oklahoma Water Center is to develop low-cost
materials that can help remove the arsenic or the fluoride from water in developing countries
where people are living on less than a dollar per day, and so the cost of what we implement
in the U.S. may be out of their reach. And so we're trying to find local materials, local
technologies that we can use to help make that water safe to drink. Well, in addition
to arsenic and fluoride—those are two of the major water quality issues naturally occurring
in groundwater, but there are other issues as well. For example, selenium can be naturally
occurring, uranium can be naturally occurring. There can be any of a number of compounds,
and so we need to evaluate our groundwater for these different compounds to make sure
that water is safe for human consumption. Now what we find is if we have just arsenic
or if we have just fluoride or if we have just selenium or just chromium, the certain
technologies may be most effective at removing those individual compounds. But sometimes
it turns out what's most effective for one compound isn't necessarily effective for removing
another compound. And so then we might find ourselves: do we put in multiple technologies
to target, be most efficient with each compound? Or do we put in a comp- a technology that's
maybe more robust, that's able to address multiple compounds but maybe not as efficient
with any one compound? For example, reverse osmosis can be very effective at removing
a wide range of compounds, but maybe not as cost-effective for any individual compound.
And so that's part of the challenge is deciding how to most effectively address groundwater
that has multiple compounds naturally occurring in it. So hopefully from this lesson we've
learned about the important groundwater issues, groundwater quality issues. And as we're considering
surface water versus groundwater, we need to be aware of and look for these constituents
to make sure the water we're supplying is safe for the consumer.