Core Topic Briefs: Ground Water Model
Well we are going to introduce a groundwater and talk a little bit about how groundwater moves beneath the earth when it's out of sight and our groundwater model is here, we've got a valley with a river flowing through it or a stream, downstream is toward me and it drains through a tube into a basin on the floor, as you come across a valley you would naturally come to a mountain and we've eliminated the mountain to make the model reasonable in size but we've stored the water that's under the mountain in a jar and we're going to feed it through this feed tube on the side and allow the water to move from there across the valley through the aquifer and make its way to the stream. We can detect the movement of that water by injecting some dye. We add it here and it will show up as a spot at that point so this is just some green food coloring and I'm injecting it with a syringe.
So as you can see we've added dye here and the purpose of that is to allow us to visualize water movement through the model and so the water that comes from under the mountain and flows across a valley is carrying the dye and forms a streamline and you can see that it's exiting over here into the stream. At this point, I am going to create a spill by adding more green food coloring at this location and it's going to represent a point source of pollution.
Let me tell you a story as that is entering the the ground; I had a call from a family in Pennsylvania and they wanted to know how you get petroleum, gasoline, out of your drinking water and I was kind of curious on how you get it in your drinking water and the story was: they had been clearing snow from their driveway and ran out of fuel so they refueled the snow blower and set the can down on the snowbank and finished clearing the driveway, put the snow blower away and forgot about the gas can so the next day the sun came out, melted snow, and tipped a gas can over and it leaked and that was a source of petroleum in their water supply. So this point source that we've just created could be an accident, something like that.
What I want to do now also is trace where the water table is in this model. So we can locate that by using observation wells.
I'm going to add a little bit of red food coloring to one of the observation wells so to help us find the water table easily so I'll just add a drop and then I'll wash it down with some water and when it settles back down it will settle to the water table level and then I'm going to mark the some points on the water table. The first one is at the water level under the mountain which is that this water level right here; and we'll put a mark there and the second one is at this water level where we just added the red dye, we'll put a mark there, and third one is at this observation well with the green water and we'll mark this level right here and the last one is the water level in the stream and we'll mark that right here and now I'll put a smooth line through those and that will describe the water table.
And that shows us the location of the water table.
The water table, we can find a in reality by digging a hole, the hole will be empty until we reach the water table and if we dig further below the water table it will fill up with water that's what these observation wells are showing us.
We have a pumping well, drinking water well, located right here and I'm going to add a pump to that well, just a syringe. This represents a pump and we'll extract some water from that well and I usually like to ask the audience what color water they expect to find? and often they're divided. It will either be green or it'll be clear and so we will pull a little bit out and see what happens and here we're getting green water immediately so our point source has drifted down and across the aquifer and has reached the well and the well is heavily contaminated as we can see by the green color in the syringe.
I'm going to get rid of that water, that went into the basin which is collecting all of the outflow water from this well and I'm gonna install a pump again and I'm gonna pump one more time and this time I want you to observe these observations wells and see what happens to the water level in those as we extract water from this well and you should be able to see the observation water levels drop and that indicates that this water table changes shape and doesn't remain more or less horizontal. It forms a cone of depression around the well.
It drops toward the well and you may be able to see the the dye from the point source has dropped to the well and and sort outlines that cone of depression.
So we've shown that pollution moves or groundwater moves slowly, in fact let me describe how slowly it moves, when we first started this model it took about a half an hour to move the dye from here to the exit point which is a little bit more than foot long so if we make it a foot in half an hour that's two feet an hour that's about 50 feet a day and that is in the range that groundwater typically moves is 50 to 100 feet per day There is one or two more things that we can learn from this groundwater model. You can see that our streamline ends at the stream and this plume of contamination is moving toward the stream and will be intercepted and collected by the stream. So if we have a contamination event, usually it stays between the mountaintop and the stream.
Water flows from the mountain to the stream.
If we had a second mountain over here which we typically would, that water would be flowing from that mountain toward the stream and so a pollution event ends at the stream and stays since the water starts to flow toward a stream at the mountain top it stays between the mountain top and ridge and so one way to illustrate that is to ask a question: this is my well and it's been contaminated, you have a friend who has a well on this side of the stream over here and they are concerned about this contamination event that got my well. Will there's be contaminated?
And the best answer is: No, it will stop at the stream so unless something unusual happened such as a drought that dries up the stream this contamination will not be pulled to the other side.
Those are the things that we can really learn from this groundwater model.
This model is a physical representation of a three dimensional drawing made by EPA a few years ago and its parts are. The aquifer is down here at the bottom and it has a ridgeline right at this location here and the water table is represented by this blue covering and its slopes from the ridgeline to the stream which is right at this location where that what aquifer meets the wall the model and the ridge divides the water that flows the opposite way on the other side of the ridge. What this allows us to see is: we have a well, represented by this dowel rod and when we withdraw water from a well we know that water level drops around the well.
So if we push on the rod and sink it down, we can perform a cone of depression around this well and see that the water table is depressed in all directions but more so in the uphill direction and in fact if we traced the depressed area of the water table on top of this membrane, we would come up with an area that looks something like this in shape and this is the circular part the cone of depression and it extends all the way up the hill to the ridge line then and if we bring them down from where we drew it to the ground surface and centered around the well then this is the recharge area for the well and it also then represents the area that needs to be protected from anything that we don't want to drink so we would limit the application of fertilizers and pesticides and we'll try real hard not to have anything stored that might be spilled such as gasoline and it's probably not the best place to burn trash, that sort of thing.
So this is our area where we try very hard to protect what happens so that the water going through there will be as clean as possible and we get the highest quality water possible from this well as we pump water from the well.
Our purpose here is to show you how to put together a model, something like this which is a physical representation of a three dimensional drawing made by EPA to show how groundwater moves around in the neighborhood of a well and so we need a base to set it on and some side pieces to hold our groundwater model in. We need a groundwater base model which were going to cover with just a simple swimming cap.
We start with just a piece of Styrofoam® and end with something like this.
The other parts are the end walls and they are shaped a little bit to match the curve of the top and so they will go there and we need one more piece to support the Plexiglas® cover at the top and of course we need that Plexiglas® cover and this can be heat formed at 300 degrees (F) and then I simply put, before I cut this opening out. I just lay a piece of aluminum foil over the top and then lay the warm Plexiglas® over the top and held in place until cooled to get it shaped. You do have to be careful, I was testing and got distracted for about an hour and 300 degrees (F) at an hour gives you a piece of Plexiglas® that it's no longer clear so you do have to be careful of that. 300 degrees (F) for about 5 to 10 minutes is sufficient to get it warm enough to to form over a shape like that.
The starting point for this was a drawing of the shape of the aquifer and I was able to cut that shape with the with a bread knife and slice it. The interior part of this, you can make an opening and you can saw it out with a serrated knife or saw it out with a coping saw. Smoothing this so that you don't have little pieces of Styrofoam® flying around, I was able to do that with some coarse sandpaper and I was able to smooth out the round part with just a sanding drum so those were the tools. I guess one other thing the the Plexiglas® can be scored with a utility knife and snapped along the line to cut it to the size that you need the final thing would be the well, it's just a dowel rod and it should be rounded at the end so that it's blunt but has no sharp edges so that it doen't perforate the swimming cap. Those are the parts and it was all just put together with with screws. This Plexiglas® just drills with an ordinary drill bits for steel.
You do need to be careful and drill pilot holes and then enlarge them with with other drills and don't advance a drill very rapidly. Allow it to spin and build up some heat so that it sort of melts as well as cuts its way through the Plexiglas®.
If you drill to aggressively it can catch and then crack the Plexiglas® There are more details available along with a model that has all the dimensions on the website listed below: