The Simplicity of Karst Aquifers
-Kevin Thuesen, Water Quality Protection Lands Program Manager
Karst Aquifers are pretty simple things, but as simple as they are, they are very difficult to regulate and some are difficult to protect.
Many people think “how could this be simple?” Karst aquifers are complex and diverse and big in scale, etc. So how are they simple? Well, they are simple in so far as what goes in is what comes out (whether it leaves the aquifer from a well or spring). That is, good quality water goes in – good quality water comes out. If dirty water goes in, then dirty water comes out.
So as long as we know where the water originates (contributing zone) and where it goes under the landscape to reach the aquifer (recharge zone) – we know what needs to be protected in terms of allowing water to continue to reach the aquifer. The difficulty then is how to protect it? There are many different jurisdictions over the contributing and recharge zones and each may have a range of protections that benefits their unique circumstance, many may have none. Obviously, in terms of protection, the complications can compound and multiply from here.
So let’s focus the simple part – what goes in is what comes out.
We have protected a lot of land over the Barton Springs recharge zone. How much? Well with land we own and conservation easements we have protected ~25% of the RZ. Not bad as a first major step protecting the land where water can enter the aquifer (good or bad water), but we only protect 7% of the contributing zone. We also manage the land to protect or provide for the continued recharge of high quality water, but the details of that are best left to another note.
Let’s focus instead on how best to get that water back underground – the RZ has many caves, sinkholes and other naturally created conduits that transmit rainfall and streamflow rapidly underground. It would be very simple and make a lot of sense to use such features that have evolved from the geologic history of this area. And so we do. The most effective features in terms of volume are caves located in the beds of streams that cross the RZ. Some of these features are obvious, but many are very small and very difficult to find. Make no doubt though – the water will find them even if we can’t.
When we find features that are humanly accessible – the definition of a cave requires human- sized access – we explore them and try to understand if there’s lots of water flowing through them which usually ties back to their origins and geomorphology.
Some features may have whirlpools associated with them so you might imagine them operating very similarly to a vacuum cleaner pulling in water instead of air, but similarly pulling in everything else in the water including sediment, rocks, leaves, sticks, turtles, you name it! Obviously it can pull in many pollutants as well. Over geologic time you could imagine such feature eventually pulling in enough debris that their capacity to recharge water is limited by all this debris blocking the passage that would otherwise transmit water just like pipeline. Big storm events could put a tremendous amount of sediment into such a feature in a single event. So some important, but simple things we can do is:
- Prevent more sediment from entering
- Remove any sediment (rocks, sticks) that is blocking the water function of such features.
We have come up with some very elementary strategies to accomplish this which have benefits to water quality as well. We use a simple grate that forms a solid super- structure and then use separate fine debris covers on top of these grates that are intended to clog up with debris and block much of the water from entering the feature. That is until we are ready to let the water in by manually clearing the debris form their structure. That allows us to let the normal big pulse of initial runoff - the first flush (or the most likely to be polluted runoff) to pass by and let in the much cleaner water that follows. Sometimes this is a very dramatic event!
So here we have discussed how we protect the features that allow water to get back into the aquifer and how we can improve the water quality going into the aquifer through such features, but we have not discussed where this water originates. That will have to be for another note.