Wednesday, May 2, 2012

Water Witching

We frequently get questions about “dowsing”, or “water witching”; a Dowser is sometimes also called a “water witch”. The word refers to people who use copper wire or split willow sticks and walk across the surface of the Earth, saying that they can with these means select a place to sink a well that will produce abundant water. The first question below is unusual in that the person was actually more concerned about “stray voltage and ground currents” than water. The second question is more typical.

    We are farmers in Central Minnesota, USA. We have a Dairy herd of about 50 cows and 60 heifers.  We are having trouble with stray voltage and ground currents. We have found these paths and other stray voltage problems by dowsing.
–Joe R.

Does dowsing really work?
– Name Missing

There is water everywhere. In the sense that anyone, waving an empty cardboard box over the surface of the Earth, is always standing over at least some groundwater, then the answer would technically be yes.

There is water beneath you no matter where you are - I even detected it while crossing the Empty Quarter of the Arabian Peninsula. I was crossing the driest desert in the world (where it sprinkles a little rain once every 10 to 100 years, typically).  In a sand-dune area where the humidity was only 2%, my VLF-EM (very low frequency electromagnetic) unit could detect a conductor - ground water - less than 60 meters (~200 feet) below my feet. For reference, the dry southwestern US typically has a humidity of around 20%, so there was NO water as deep as we could dig with a shovel. I tested this on the edge of an ancient dry lake bed: bone dry, dusty. People had wondered where the water comes from that feeds springs near the Arabian Gulf – and now we know. In part, it comes from the mountains of Yemen and Oman, and passes out of sight, beneath the sands, until it reaches the Gulf.

My back yard is part way down a slope, and is almost always mucky and damp. Ground water is pretty obvious there, but not so obvious in my front yard because the water lies deeper below the surface. But it’s the same aquifer, the same water. A radioactive isotope spilled in my front yard will eventually show up in my back swamp.

Some analogies may help here.

People commonly think of an aquifer, an interconnected zone of groundwater, as a great pool of water hidden under ground. In fact, a better description would be porous rock that is saturated with water. Think of a sponge. Some sponges have more “air space” than others, and they will therefore hold more water, so that would make it a better “aquifer”. Now think of a rubber sheet - water doesn’t cross this boundary, so it would be an “aquiclude” - a barrier to water movement. It excludes water. Sandstone and gravels tend to have significant porosity - they are full of “space” that water or air can occupy, and tend to make great aquifers. Granites, shales, and salt-diapirs (salt domes) have far less porosity - they tend to act as barriers to the free movement of groundwater. And oil, for that matter.

As a teenager I helped an old man dig a hole for a septic tank for his cabin in the southern Sierra Nevada mountains of California. The cabin was built on solid granite. I didn’t know much at the time, except that it was very, very hard work with pick and sledge hammer getting down through that salt-and-pepper colored granite. Later the old man (a retired policeman) couldn’t understand why the hole wouldn’t “perk” - i.e., why water poured into it didn’t sink down, but instead pooled in the middle.

Bad place to put a septic tank.

One of the biggest aquifers - continuous zones of water-loaded sediment and sedimentary rock - is the Ogallala Aquifer. This is a vast stretch of water-saturated porous rock that stretches from northwestern Texas to southern South Dakota. It’s continuous, in that if water is drawn from a well in South Dakota it could theoretically be pulling water along northward from Texas:

Most aquifers are smaller than this, however, and often comprise all or part of a sedimentary basin. If you have a big sedimentary basin, it superficially acts - sort of - like a bowl filled with sand. With a few exceptions, water drains from these basins via a river on the lowest end. In the middle where it is deepest there can typically be lots of water, but when you get to the edges, and an aquiclude like granite rock crops out, you find yourself out of the aquifer. 

That makes a basin sound very simple - but basins are rarely simple. I published a US Geological Survey Professional Paper ( ) where I used an airborne EM unit – think of a giant, aircraft-sized metal-detector - to measure conductivity beneath the ground. I went a lot deeper than where you find coins. I was able to measure electrical conductivity down to 400 meters (1300 feet) deep beneath the San Pedro Basin of southeastern Arizona and northern Sonora, Mexico. When I plotted out the conductor - the water - it turned out to be surprisingly three-dimensional. It also turned out to change with time. Hydrologists already knew that the water table would go up during the August Arizona “monsoon” season, and go back down during the dry spring and summer months. I found - using the magnetic part of the survey – how far down the granite basement (the underlying horizontal aquiclude) under the basin was. I also noticed in the magnetic data that a lava flow had long ago coursed down a steep ancient canyon now covered with the basin sediments. When that lava cooled, it acted like a barrier to the ground water flow - like a rubber sheet blocking it, a vertical aquiclude.      

Why would anyone care about this?

The San Pedro Basin and its water regime are obviously important to all of the people who live there. However, the San Pedro River also supports one of four major North American migratory bird flyways. If the river disappears because too much groundwater is taken out to run the town of Sierra Vista, the nearby Fort Huachuca Army base, AND all the possible agriculture in the valley – then the surface water could potentially dry up. You see, surface water and groundwater are always connected, in some way.

That problem was solved by wise land-use planning and water prioritization worked out years ago. But that only worked for the English-speaking,. American side of the international frontier that cuts the basin in half. On the other side of the Basin lies on Sonora, Mexico, and pumps there are drawing down the aquifer to provide water for farms and for the huge Cananea copper mine on its southwestern edge. In the late 1990's,  Grupo Mexico (representing the owners of the mine) announced it was going to dramatically ramp up production of copper. Alarm bells went off at the local Ejidos, the communal farms set up after the 1910 Mexican revolution. The people managing the San Pedro Riparian Area – a Congressionally-mandated effort to preserve the flyway on the American side of the basin – also got very worried. The US Army hired us to tell them what was going on with the water in the basin. Our study showed that there were barriers (vertical aquicludes) in the Mexican side of the basin that protected the Ejidos and the San Pedro river on the American side of the basin aquifer from water loss.

Science provided the information that allowed everyone to relax on that one.

Well, so far I’ve talked about the sides and edges, and the granite basement beneath a basin. But granite is at least a little porous, especially if fractured, so... how far down can water really get?

If you went down into one of the deepest mines on Earth - a diamond mine in South Africa can reach depths of 12,000 feet - you would notice two things. First, and most immediately, you would notice that the temperature was very hot: up to 60 degrees Centigrade, or 140 F.  Miners can work in these conditions only if they have been temperature acclimated AND if refrigerated air is pumped down into the mine. The other thing you would notice is that when you get away from the vertical shafts that move miners and ore into and out of the mine... that it is very dry.

Water is hosted in porous rock, but the more you compress the rock, the lower the porosity becomes: you squeeze out the empty space that water can occupy. Referring back to our previous analogy, you put your hand on the soaked sponge and lean on it. Sandstone that may have a porosity of up to 15% at the Earth's surface (that is, 15% total space between the grains is open) will have less and less porosity as it is buried deeper and deeper in the earth. The overlying crush of the rock and sediment above it - this is called “lithostatic pressure” - eventually squeezes out any water that might potentially be present. The overlying earth has squeezed the sponge.

While I could “see” where the water WAS by using geophysical methods, it took a hydrologist doing pump and isotope tests to figure out where the water was GOING - nor how fast it was going. If you start pumping and the flow rate stays the same - then you are in a large aquifer. If you start pumping and the water coming out falls off steadily, then your dowsing wasn’t so good after all. There is water everywhere beneath your feet – but in some cases there isn't much to start with in the first place. This could be due to relatively low porosity (shales instead of sandstones and conglomerates, for instance). It could also mean that the hydraulic conductivity is low. When you apply hydraulic pressure, the water you get is controlled by the hydraulic permittivity of the rock (how interconnected the pores are) times the distance the water has to move through it to get to your well. It’s like talking to someone through a sheet vs talking to someone through a pillow.

Back to Ground Zero: my backyard. I know where the water IS, because it’s muddy there, and it bothers me (it’s hard to push a lawn mower through mud). I am planning on digging a “French Drain” - a trench with a slotted plastic pipe, surrounded by gravel - to catch that excess water and divert it through the high-permittivity pipe - to the side of my yard where there is already a natural drainage leading down into the Greenway below my house. The rabbits will be happier then, because the blackberry bushes will grow thicker, and be less friendly to the coyote family that roams through my backyard every couple of nights.

So for me it is now real. I understand all this more or less: where the water is coming from (not the paved street, but from rain falling for six straight months on my front yard). I know where it is going (following gravity around and beneath my house to the back yard). I also have a rough idea of how much water is involved – and since it rains half of every year here in the Pacific Northwest, that is a LOT of water. But my back yard is about a millionth the size of the 1,000-square-kilometer San Pedro Basin, so I wouldn’t call myself a hydrologist here – just a backyard engineer.

But I’m an even lousier biologist. I can’t figure out how to get rid of the Mole from Hell that keeps drilling tunnels through my lawn - and pushing the muddy tailings up on TOP of my lawn. In most senses, doing geophysics to map ground water across an international frontier is the easier task.


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