 
SOIL: THE NATURE OF DRAINAGE
Why water and soil don't mix! |
When you were a kid, ever feel like you wanted to really get into playing with some dirt like the grown-ups could? Well, let's do it now.
To better understand the transfer of moisture through soil, we are going to mentally walk through a process of progressively visualizing the various states of solid material from rock to fine soil and how water flows through them. Then we will combine them and throw in a few variables to refine the recipe, make some real muck, and see what happens.
Imagine taking a five gallon bucket and punching a fist sized hole in the bottom so water can run into and out of the bottom of the bucket readily, and fastening a coarse screen on the bottom of the bucket to hold in small pieces of stuff. Just for reference, we also get a second container that holds water, has a spout and a handle or two.
Imagine cutting a solid rock so that it fits perfectly and tightly into the bucket like a big solid plug. To keep things simple lets say the rock is a good hard sandstone like the material that is readily available all around the world; not granite, not limestone, just plain common sandstone. Visualize pouring water into the top of the bucket; very little if any of the water would make it around or through the rock to exit at the bottom of the bucket. In fact the water would just run out over the top of the bucket. Next, lets get a big strong fellow to smack the top of the rock, while still in the bucket, hard enough to fracture the rock into many small pieces, yet still all fitted together like a three dimensional puzzle. Again pour water into the top of the bucket. This time, a little more water makes it to the bottom, but it takes a long time --maybe hours for even a small amount to get through. All those tiny fissures are still very small and tight.
Dump the fractured rock out, and beat it up some more so that what is left is a pile of gravel sized stones (a size somewhere between a marble and a golf ball). Put as much of the stone back into the bucket as will fit. Notice that there are a lot of pieces of stone left over after the bucket is filled. That's because now there is a lot of air space between the stones. The left over stone which could not be put back in the bucket roughly represents the amount of air in pockets or voids between the stones in the bucket. The water that is dumped into the bucket now flows almost freely through the stone, and out of the bottom of the bucket. This stone is similar to the stone placed into trenches to make a French type drain. It is very porous and allows water to readily pass through.
This is also the type of stone that is normally placed in French type drains surrounding a drain pipe, to accept, accumulate and channel errant water in a controlled fashion through a manmade trench.
Dump the stone in the bucket out again back on top of the left over pile of stone and continue to beat all of the stone into smaller sizes until the entire pile is reduced to pieces about the size of a pea (whole or split pea will do). Most people have heard the term "pea-gravel". Well we have just made "pea-stone".
After pouring the pea sized stone back in the bucket, we now find that the left over pile is much smaller. The smaller the pieces, the smaller the air gaps between the pieces. Also, when we pour water into the top of the bucket, the outflowing water takes a little longer, and comes out of the bottom a little slower. If the bucket is set up on a crate or a couple of concrete or wooden blocks so that the hole in the bottom of the bucket can be seen, we find that water will continue to drip out of the pea stone for a long time after the main body of water has run through. Thus, we see new phenomenon begin to show up; this is surface adhesion between the water to the now large exposed surface of the pea stone (the smaller the stone the greater the exposed surface area on the stone particles). And, this same increased surface area, in concert with greater adhesion, also provides more friction to the movement of the water also slowing it down. The finer the material the water has to pass through, the slower it moves through, with some water lingering behind, clinging to the surface of the stone. At this point, yet another phenomenon may also begin to manifest: some of the water may penetrate the surface of the stone, and be held inside the stone itself (this is absorption). Surface adhesion, friction, absorption, yes, it gets complex. There is more: an immense number of obstructions are provided by the matrix of almost endless passages and obstructions, because the water has to negotiate (or maneuver through) these passages, each drop of water now has to travel farther to get through the stone. But, the concept is simply that the finer particles of stone, the slower water moves through, with some being held and temporarily stored before eventually continuing on.
At this point a few other phenomenon that should be mentioned are also beginning to exhibit greater influence on the moisture transfer dynamics. These are capillary action, gravity and vapor pressure.
Capillary action is the natural travel of moisture over the surface of or through a material. This is exhibited in how water will travel through cloth. For example, if you put half the length of a shoe string or a small rag in a glass of half filled water, the water will climb through the rag, and follow the cloth (or shoe string) to whatever it is touching, delivering a considerable amount of water to that point, until the water in the glass drops sufficiently to stop the flow. Capillary action is related to surface tension, adhesion and absorption, but it tends to make water travel as opposed to holding it in place. Capillary action moves water in a slow subtle way, (sometimes against gravity) away from a body of liquid water.
Capillary action relates to soil moisture transfer because it tends to move moisture through soil. This promotes a more even overall level of soil moisture, and plays a big role in dispersing small pockets of liquid water. If some liquid water becomes trapped in a pocket in the ground, and can no longer run down hill, it is capillary action that rescues it by moving it up (or down) through, or outward away from that pocket. This moistens the surrounding soil, disperses and keeps the water moving, and eliminates standing bodies of water (if no regular additional inputs maintain a standing pool).
Gravity seems much simpler because its actions are more obvious; it simply keeps liquid water moving downward, when no overriding obstructions exists. But it also creates hydrostatic pressure (hydro=water, static=not moving & constant) --a constant pressure of water against the sides or walls of its container --even if the container is only earth. It can also cause liquid water to seem to move upward with force if there is a body of water higher which is pressuring, through a channel, under lower water so as to push it along or upward. This is the usual cause of a true spring. Gravity always moves liquid water from higher to lower.
Vapor pressure is just a bit more conceptually elusive than capillary action, but is another force that moves water independent of gravity. In semi-technical terms, it is molecules of water bouncing around that leave the body of water when they bounce in the direction away from the main body of water (however small that body of water is) and then keep moving away, thereby escaping to bounce around on their own, helter-skelter, as nature allows or provides a path. This happens at a higher rate as the temperature of water rises. It is this phenomenon that causes evaporation and humidity. Evaporation is, of course, the opposite of condensation. Condensation occurs when water molecules bounce into each other (there are lots of them around) and stick together. Water molecules stick together because they have a certain amount of affinity for each other, or in other words they have a weak attraction for each other. Water molecules stick to each other and stay stuck easier with lower temperatures. Vapor pressure is always present, but is higher with higher temperatures, and lower with lower temperatures. Vapor pressure is another way water moves away from water thereby dispersing itself.
Capillary action, vapor pressure and evaporation work together to disperse water.For the liquid water that is left in the bucket clinging to the stones, or absorbed into the stones or still slowly traveling through the stones, it is capillary action, gravity an and vapor pressure which will eventually clear the remainder of the water from the stones, thereby countermanding adhesion, absorption and friction. With water, there is always balance.
To continue our exploration we again dump the pea-stone back onto the small left over pile that did not fit back into the bucket and further crush it until all of the original large plug shaped rock becomes uniform, coarse sand-sized particles, about the size of a pinhead or large bread crumb, but not as fine as beach or river sand. And back into the bucket it goes. More of the crushed rock now fits back into the bucket as the spaces between the particle has become smaller yet. Still there is some left over which does not fit evenly into the bucket.
This time when we pour water into the top of this bucket of coarse sand size stone particles, we must pour more slowly or the water backs up and overflows the edge of the bucket; in fact it may take several minutes to slowly pour all the water into the bucket. The water moves through yet slower, takes longer to start to run through the bottom, and even longer to finally clear the bucket. In fact it may take hours before the water stops dripping out of the bottom. And, we have a noticeable amount less water than went in, since the finer particles are now holding onto more of the water. In fact it may take hours, or many hours for all of the water to clear. A small amount may never come out, or only come out eventually through capillary action and evaporation.
As we work through this, we are seeing a picture developing of water having a more and more difficult time moving through what is becoming more and more like soil. It also becomes apparent the more soil-like our medium of crushed rock becomes , the greater is its ability to store moisture. Also, we see water moving in ways besides just down.
We again dump the stone out of the bucket on the left over pile, allow it to dry out until it is manageable, and continue to crush and grind the fine particles of stone until is the exactly the equivalent of regular beach or river sand. Back into the bucket goes our sand. This time pouring the water through the sand is a slow process, it takes a long time to slowly pour it in without water running out over the top of the bucket. Eventually all of the water is poured, the sand is saturated with water, and water is running out the bottom of the bucket. Eventually, many hours later, we find that the receiving container under the bucket has a measurable less amount of water than when we started, and the sand is holding and storing a good bit of water. Days, and even maybe weeks, later if we come back and sink our hand deeply into the sand, it will still be damp. If we come back when the sand has become well drained and thoroughly wet, and try to pour an additional tank of water through it, we find that the second time around with the sand already wet, it is an even slower process to pour water through the sand. The sand is somewhat more resistant to water passage. When the sand is a great body of mass, not just a little bucket, and when the water travel is horizontal instead of a direct vertical fall, this damp-sand resistance is much greater. So we see that sand substantially resists the movement of water through it, tends to store a significant percentage of water, and when already wet, provides somewhat more resistance to the transfer of water.
Sand also makes a good filter material, trapping and stopping silt, and organic material. This can be plainly seen in the use of sand in swimming pool filters. Many people who have had sizeable swimming pools will remember the sand filters, a large barrel shaped container filled with sand. Over time the sand becomes clogged with the fine, mostly organic, material it screens out of the water, slowly allowing less and less water to pass through until, eventually, almost no water at all will pass.
A similar thing happens in nature when water continuously moves through a naturally sandy soil. Fine material, both organic and inorganic, is brought in through the flow, or down from above. Eventually, the passage of water is stymied, the material settles and decays further, and what is formed is a very thick, almost solid, muck. If and when the muck dries, it sits ready and waiting for the next input of water to quickly swell the small spaces between the particles of sand, again, absorbing, holding and resisting the flow of water. A similar event occurs when the sand bed in a septic system drain field becomes overloaded and clogged with waste and stops receiving sewage. The sewage then is forced to the surface (usually the path of least resistance) and the septic system is rendered useless.
For the final crush, we reduce the sand to the consistency of powdered sugar, or wheat flour. This done, we should have the closest thing to real nonorganic soil. What do you think is going to happen when this "soil" goes back into the bucket and we pour water on it. Most of the "soil" goes back into the bucket. You will find that pouring water into the soil is a major project, that water is very reluctant to travel through and only drips slowly out of the bottom of the bucket. At some point the soil may not take any additional water at all, and you will have to wait for the soil moisture to stabilize and some water to clear the bottom of the bucket. Depending on the actual makeup of the rock you started out with, at some point the fine particles may swell, and totally stop the passage of water; that is, until the soil dries out some, and can again accept more water. This soil will store some degree of moisture indefinitely, and left alone, the moisture level in the soil in the bucket will become uniform. Ultimately the only way the soil moisture will reduce is through evaporation from the exposed surfaces. Of course, if you plant grass in the top of the bucket it will also wick out some of the moisture more quickly.
As a last consideration about soil and water, lets take our bucket of soil and hammer on the sides and tamp the top until the soil is packed as tight as it would get if it were laid down one fraction of an inch per year, wetted continuously (by rain), allowed to congeal between wettings and tampings & shakings (as is done by cycles of the moon, wind, floodwater pressure and the longer term effects of gravity, and heat & cold, expansion & contraction. What do you now think we have? Try sinking your fingers into that soil, or for that matter even a shovel--yes, there would be a lot of resistance. It would be something similar to virgin, untouched, undug soil; it would be so tight as to be almost impervious to water penetration. Now that's the difference between soil around a house in the back-fill area and soil that has never been disturbed or dug. Rain water runs across natural soil and sinks into the disturbed soil --although slowly.
Another approach can be taken to prove or verify this "virgin soil" concept. Find a fairly flat area of normal soil that you are sure has not been dug or disturbed. Draw a real or imaginary 2 foot diameter circle and pour a large bucket of water (4-5 gallons) onto it and watch what happens; the water just spreads out and lays there. Now, in that same 2 foot circle dig a hole 2 feet deep. Then, after chopping up and loosening the soil, shovel it back in, tamping it and packing it down as best you can. You will surely have some dirt left over when you are done no matter how much you tamp and pat. Leave a cone shaped mound on top of the hole you just dug and refilled, and set the rest of the soil neatly aside. Leave it alone for a day or a week or a month, or even a year or more. Come back, level the soil directly over the top of the hole if necessary, and then pour that same amount of water over the top of the dug area. As a rule, the water will settle in quite readily. Another test is, without adding any water, start digging up the soil again. Notice how much softer and easier it is to dig. This is the difference between undisturbed soil and disturbed soil. One is very solid, the other is not.
This little exercise may be continued and expanded to find out what happens when organic material is added to this inorganic soil, or what happens when some stone and/or rock is placed into the mix. But the bottom line is that liquid water does not move easily through soil or even sand. If there is a true water table present in soil, it probably won't move very fast; the soil has characteristic qualities that tend to control and balance any movement.
And so the character of soil and land is to divide and control water, just like it is supposed to do.
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