A lot of people depend on ground water, aquifers, for drinking and irrigation. Some aquifers recharge on their own, and do it pretty quickly, such as the Edwards Aquifer in central Texas, or the Sandhills portion of the Ogallala Aquifer. Others either recharge very, very slowly, or not at all. Those are the ones that tend to get lots and lots of attention, unless Central Texas is dry, and Austonio begins talking about sending a pipeline up to the Panhandle to tap the Ogallala.
A quick note to clarify here, before I go any farther. I’m talking about aquifers in sediment like sand and gravel, not groundwater in bedrock, as is found in New England, Canada, and a few other places. That is a different formation, with different flow patterns, and I know next to nothing about how those “work” other than general theory. If you are in New Hampshire and you have a well drilled into bedrock, please contact a local expert.
How do aquifers recharge? It depends on the material above and below the porous layer. That’s what most aquifers are – a layer of sand and gravel that at one time was exposed to rain and snow, or was a river bed (large swaths of the Ogallala and Equus Beds). Under that layer is a watertight layer, usually a shale or something. Over time, that sand and gravel got buried by other things and now lies below the land surface. A few, like the Edwards in central Texas, have access today through caves and sinkholes, where rain can fall right in, or have a very porous layer above that lets rain and snow melt trickle down pretty quickly. The Nebraska Sandhills are pure sand, and water that falls there soaks in, recharging the Ogallala below. Unless there is an extended drought, recharge is not as much of a concern (over-pumping that draws down the water too fast is a different matter.) Other aquifers, like those in Arizona, coastal Georgia, and most of the Ogallala, would take hundreds to thousands to regain their water, if they can at all. When the aquifer is buried hundreds of feet below the surface and topped with firmly-packed dirt, caliche, and so on, water has a harder time soaking in. These are “fossil” waters, and you just assume they won’t recharge without help. How to help without destroying the formation, is another problem.
First, there has to be water to go back in. Without that, it’s pretty moot. Also, the material in the aquifer layer has to still be loosely-packed enough to accept water. If you draw enough out, the layer compresses, and that’s that. No recharge ever, unless all the surface material erodes away and rain falls directly on the sand and gravel.
Ideas for recharging aquifers all involve “putting the water back in down there,” or at least, giving the water an assist. Drilling a well and pouring water back in . . . has a lot of technical difficulties, including the fear of contaminating the rest of the aquifer if some chemical or biological contaminant seeps in – think fecal coliform, or avian cholera, or . . . So the water would have to be filtered, and dust kept out, and the water released high enough that the layers between the end of the well and the aquifer would filter some of the stuff. Oh, and you have to hope that on the way down, the water won’t pick up salt, gypsum, or dig a hole that causes a sink hole.
Around here, attempts were made to deepen the natural rainwater lakes, punching through the clay layer at the bottom of the shallow depression to allow more water to seep in. It started well, but the clay swells, and sediment filled in the holes, closing them. Also the rate or recharge did not justify the cost of the work, which has to be maintained. And depends on moisture. In a year like 1940-41, when the area got 40″ of rain or more, no problem! In a decade like the 1950s, or 2010-2014? Rain? What rain?
Most aquifers were “laid down” when the local/regional climate was much wetter. The Ogallala was sediment dumped from the Rockies by huge, enormous, massive, gargantuan rivers that wandered back and forth over the region for millions of years. Then things changed. In the case of the Ogallala, the goal in 90% of the region is to balance draw-down over time, so that X% of the current depth will remain in Y years. Some places are changing types of crops, other areas revert to range land, and irrigation is much, much more efficient than it used to be. The down side to better irrigation is that less excess water seeps back in to return to the aquifer.
Eventually, a way might be found to return water to places like the Ogallala, Equus beds, coastal aquifer, and so on. If the stuff has not compacted, and if there is sufficient rain and snow to permit that. And if people are willing to spend the money and time needed to do it.
Groundwater flow in rock is much easier to reckon. You can essentially ignore compaction, and that’s a beast of a formula.
In general, try not to think about groundwater formulas too much. They’re full of black box variables in impossible units. (Because “That’s how the math works “.) I’m pretty sure I hurt my brain in that class.
Interesting. Thank you. I live in an area that is semi arid so the aquifers here are very important. No pun intended (well not much anyway) but I think I will dig into the subject a little more.
I have a textbook _Groundwater in the Environment_ by Paul L. Younger that is quite good. It’s a general introduction, written for people who have a basic understanding of the water cycle, but not about groundwater in detail. The author is English, so some of his examples are a bit obscure for American readers.
_Ogallala Blue_ is an excellent, well-written book about the Ogallala Aquifer’s different sections and problems.
Thank you for the recommendation. I do have a book put out by the Bonneville Power Administration on local aquifers but it is very, very general in nature. It also covers some of the local geology so is a bit of a 100,000 ft. overview.
Lots of ifs there… And not enough $$$ or commitment, as far as I can see.
The Ozarks area is karst topography. Drawdown/recharge is, at worst, a local problem. (Someone wanted to start an ethanol plant. That was a major objection.)
See a depression in the middle of a field? A sinkhole.(a collapsed cave) at the surface. Water drains into the local cave system, like water from your bathtub. Unfortunately, some people are careless enough to dump trash into sinkholes.
When someone drills a new well (or rehabilitates an old one) the well is cased for most of its length, to reduce the chance of contaminated surface water seeping in.
My understanding of the Ogallala aquifer is that it was charged mainly with meltwater from the last big glaciation. Recharge is almost nil because, well, there ain’t no melting ice sheets n’more. That’s based on a college basic geology class from [mumble] years ago, though. Is that still the conventional wisdom on the Ogallala?
More or less. The layer and initial water was laid down during the Rocky Mountain Orogony, long before the glaciation. Imagine rivers twice as large as the Amazon, eroding the fast-rising Rockies. That’s where the sand and gravel came from. Then the other layers came later. The ice ages . . . didn’t contribute as much water, in part because the area might not have been all that wetter. The geologists are arguing over that bit. We still get some rainfall and snowmelt recharge, but again, it really depends on where you are – Nebraska’s different from southern Nebraska’s different from Kansas from the northern Texas Panhandle from the southern Texas plains (where the formation peters out.)