Archive for January, 2010

Where does Tucson get its water?

January 20, 2010

Hi all.  As I was riding my bike to school avoiding all the puddles, I was thinking about where the water we drink in Tucson comes from.  If you think about it, we don’t have any flowing rivers (for all of you not from Tucson, we have “washes,” which are dry river beds that flood every so often during a large rain event – image to the right), but when we turn on our taps, water flows out.  So where do we get our water?

Rillito River, Tucson, AZ. Courtesy of http://www.tucsonazrealestateblog.com/tucson-real-estate/.

Historically, Tucson has pumped water from the regional aquifer (sediments below the surface that are saturated with water – like a sponge); while our population was small, this was a renewable source of water via rainfall percolating down to the aquifer.  As Tucson’s population began to rise, with it the need for water, we started to overpump, or mine, our aquifer.  This lead to lowered water tables, higher costs for water, and a bleak outlook for the future of Tucson and its water needs.

In 2001, Tucson began to mix Colorado River water brought down south via the CAP (Central Arizona Project – which is a canal that brings Colorado River water from northern Arizona to southern Arizona).  Mixing the water with groundwater allowed the water utility to recharge some of the aquifer with CAP water.  Adding CAP water to the mix has allowed the water utility to shut down some pumping wells and as a result the water table has risen in some places.  If you’re interested in learning more about what Tucson’s water plan, please follow this link for more information: http://www.ci.tucson.az.us/water/waterplan.htm.

Central Arizona Project (CAP): Brings billions of gallons of Colorado River water to southern Arizona each year.

However, the population of Tucson has continued to increase and we only have access to a set amount of CAP water, 40 billion gallons of water each year to be exact.  We only use about 22 billion gallons per year currently, but with people moving to Tucson for the sun and the golf, we will exceed our allocation of CAP water in the near future. This means that we’ll have a “water problem” again with no clear solution in sight.

So what can we do?  Well there are a few things we can do to help our water situation.

(1) Water Conservation: The less water we use the less water Tucson needs to use as a city.  We can put in native vegetation in our yards that need minimal watering or no watering  (just imagine never having the mow your lawn again…).  Shut off the tap when we brush our teeth and shave, take showers rather than baths, only wash our cars once in a while rather than every weekend.  Making simple changes will help conserve millions of gallons of water each year, and lower your water bill at the same time.

(2) Reuse Water: Reusing water that we’ve used for clothes washing may seem a little weird, but water your garden with washing machine water (sometimes called “gray water”) can save a lot of water in the long run.  Besides, your plants don’t care what kind of water they get; it’s pretty much all the same to them.  Here’s an example of gray water reuse: http://www.builditsolar.com/Projects/Water/Water.htm.

(3) Rainwater Harvesting: Now this sounds a little hippyish, but rainwater harvesting is big business in Tucson.  Saving rainwater in cisterns to water your yard is easy and cheap (i.e. free, if your subtract the cost of the cistern).  Go to this link for more info: http://www.builditsolar.com/Projects/Water/Water.htm, http://www.harvestingrainwater.com/rainwater-harvesting-inforesources/.

Water is a precious and limited resource, if we all make small changes in our lives, we will have a large impact on conserving this vital resource for the future.

Groundwater

January 11, 2010

I know, I know, It’s been a while since my last posting, but I really have an excuse – I’ve switched graduate programs and am fixing to be a environmental engineering graduate student come January 13th.  I’m looking forward to a change in pace, a different project, and the math!!  I’ve always been attracted to applied science (i.e. how can science help everyday people, such as biomedical engineering, waste water treatment design, etc.) and I think this will be a better fit for me.

Well enough about me, I wanted to talk about ground water today.  Now most people that I talk to have heard of the term groundwater, they associate it with a well and water that’s clean (with a bit of a funny aftertaste is some cases).  I remember once that co-worker of mine thought that groundwater is just underground rivers flowing in caves.  There are caves that have underground rivers flowing, but this is by far the least common form of ground water.

So let’s take a scientific look at what groundwater is.

What do we know?  First, we know that when it rains, not all water evaporates or flows over the land into rivers, some does, but not all of it.  Second, we know that springs are found in peculiar spots.  Third, we know that we can get water if we drill a well.

So where does the water go if it doesn’t all evaporate or flow into rivers?  – If you take a look at dirt, you notice that it’s composed of small grains of clay, silt and sand. If you pour water on the ground, it wicks away into the ground like water into a sponge.  The spaces in between the clay, silt and sand particles create voids, and these voids go all the way down into the ground until they hit bedrock.  These voids are called porosity, or the volume that can be occupied by either air or water.  I know that it doesn’t look like it, but a soil’s porosity can be a lot.  For example, a gravel soil may have up 25% porosity, or 25% of the total volume is empty, a sandy soil can be 35% porosity, and a clay soil can be up to 70% porosity!!  Now you may notice that if you try to squeeze a handful of wet sand, water come out, but when you squeeze a handful of clay mud, it just remains sticky.  This brings me to another term – permeability.  permeability is the connectedness of each pore to each other.  Think of a Styrofoam cup, if you cut it open and look closely at it, you’ll notice that it’s composed of lots of small air pockets.  A Styrofoam cup may have 95% porosity, but because those pores aren’t connected, they have zero permeability.  That’s why your hot coffee doesn’t seep through the cup and burn your hand.

So what’s a spring?  Well, as water percolates through the soil, it’ll eventually reach an area of the soil where it’s saturated, like a saturated sponge.  Here it forms a boundary between saturated and unsaturated soil – this is called a water table, and everything below is called an aquifer.  The water table moves up and down with the seasons and from pumping the aquifer.  The water table generally follows the landscape topography, so when we get a steep hill, and the water table hits the surface, we get a spring.

So what are pumping from a well?  A well is just a straw that we drill into the ground to access the groundwater.   You can imagine that a well is made up of a long tube that has a lot of tiny slits cut into it where we find good water called a screen.  It’s from this screened area that we pump water.  In developing countries, they usually dig the well by hand and line it with stones, then use well bags to draw the water up.  So why do some wells dry up?  If the water table drops below the screened area, then all we’re pumping is air in the pore space.  This can happen due to drought or more commonly due to over pumping the aquifer.


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