Tue, Oct. 22

Verde Valley Aquifers

There are lots of different kinds of aquifers, some large and productive, and others that can be depleted in no time. In the Verde Valley, most of the aquifers occur in porous limestone and/or sandstone that can hold and transmit water. To visualize a typical Verde Valley aquifer, think more of a sponge than an open cavern full of water. The rocks containing the water are full of tiny holes and cracks that are full of water. Usually this water flows slowly in the same direction as the streams and rivers on the surface.

The tops of our aquifers are generally less than 800 feet deep, but there are some deeper exceptions. Nearly all of the aquifers around here are connected to each other to some extent. This called a hydrological connection, and it generally means that if you take water from one place, you'll affect the others.

Sometimes an exceptional aquifer is discovered that yields more water than average, and this is usually due to cracks and shifts (faults) in the earth that "funnel" water toward a particularly wet spot underground. Wells in the upper Verde Valley can normally produce 30-150 gallons of water per minute, but the best might yield up to 1.000 gallons in a minute.

There are also "under-performing" aquifers. These can be "perched" or "confined" aquifers that are not connected to the others, and which receive their water from a very small area of rainfall.

The branch of geology that studies aquifers is called hydrology, and the scientists who study hydrology are hydrologists. Their concerns, especially in the desert Southwest, center around two dynamics, which are: how much water is leaving the aquifers by natural or man-made means, and how much is refilling, or recharging, them.

Of course, the best situation is to have the amount used equal to or less than the amount recharged, and this has historically been the case around here. Some local hydrologists believe that recently the Verde Valley has started "over-drafting" its aquifers. They believe we may already be using more than is recharged.

To decide whether we are using more than we are receiving in rainfall, they calculate the average total rainfall on the watershed that refills our aquifers, and they know about how much of that rain runs off into the river and how much evaporates and is used by plants. The rest, they assume, ends up in some sort of groundwater.

Next, they make an estimate of how much water is being used out of the ground by calculating the average water use per household, the amounts needed for agriculture and industry, etc., and they add it all up. If the usage exceeds the recharge, the area is in overdraft.

You can overdraft any aquifer for a while, hoping for more rainfall in coming years, but you can't do it forever. Sooner or later an over-drafted aquifer will simply run dry. Sometimes a drained aquifer will collapse and then it can never be recharged.

There is another "aquifer" that has been discussed a lot lately, and that is called the "saturated alluvium," or "Holocene alluvium" of the Verde River. This water is generally within about 50 feet of the river level, and is really the same water that is flowing in the river, it's just contained in the ground adjacent to the river.

Recently, the Arizona Supreme Court decided that this water, although it is underground, is not groundwater, but surface water. This is important because surface water is allocated to (that is, owned by) specific downstream users, such as ditch associations, mining companies, and the Salt River Project.

Their claim on the surface water depends on when they made their claim, the most senior users having precedence over late comers. The upshot of the court's decision is that 3,500 or more well owners in the Verde Valley may actually be drawing their water from the surface water owned by SRP. What SRP can and will do about this is to be decided.

The bottom line is that our aquifers are a precious resource, just like our streams and rivers, forests and our air. We need to take care of them and use them wisely if we want to have an uninterrupted supply for our use, to fill the streams and rivers, and for continued economic development.

For more information, see;

Doug Von Gausig is a Clarkdale resident and current mayoral candidate for the town.

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