Sediment in Water

[zabanda]

One of my sons ideas about a science fair project has to do with preventing erosion. He wants to try different ways to prevent sediment from ending up in our waterways, like grass, gravel, and two more ways he hasn't come up with yet. He then wants to test how much soil ends up in the water and had hoped to also test for some sort of pollution in the water. Do you have any suggestions as to what he could test for? We thought about Nitrates since we had done several projects on that subject and understand it, but I am not sure that the nitrates would show different levels in the water based on the amount of sediment. Just wondering if you have any suggestions, as we have been researching this and haven't come up with anything.

[sunmoonstars]

Hi,

The nitrates is a good idea because there are known ways to test for it, it is known to increase when fertilizer waste is running off into the water, and has a good connection to real-world problems. If you are using soil that has fertilizers in it, I think the amount of nitrate you measure should be proportional to the amount of sediment washing into the water. You may also decide to test the size of the sedimented particles, not just the volume of the total sediment.

Does that help with your question? Let me know if some additional questions come up.

Tonya

[zabanda]

Yes, this helps with our question. I am a little confused about how to test for particle size. I am assuming the sediment would be a thick mud and so I am not sure how I would measure that. I hope that doesn't sound ridiculous, but I am not a science person. Also, do you think this is an appropriate project for a sixth grader? Thanks again for your help. I really don't know what those of us who are scientifically challenged would do without this service!

[deleted-141593]

I agree that testing particle size would be challenging. In terms of experimental design there are two hypotheses here. 1st is that treatment X prevents erosion/soil accumulation in water. Measure soil by volume or weight here. Depending on the experimental setup you can either measure erosion by the amount of soil lost from the source or by the amount of soil that ends up in the water runoff. A coffee filter could be a good way to drain the water from the soil in the runoff sample to do the measurement. Another hypothesis is that nitrate content of the water runoff correlates with soil content. The most straight-forward way to do this would be to add different amounts of soil directly to the water, then test for nitrates. The reason for this is the different interventions to prevent erosion could have independent effects on nitrates, for example these other interventions could themselves add nitrates directly or remove them irrespective of soil runoff. I think it is best to test the hypotheses separately. You could then always include the nitrate data from the erosion tests as well with the disclaimer that that nitrate data is correlative and not well-controlled. How has your son decided to prevent erosion? From where will you collect water? Do you need help designing apparatuses to do this experiment?

Cheers,
Colin

[zabanda]

Thanks for the great info.

He has decided to test gravel, sand(in bags) with vegetation on top, plain soil and he hasn't found his 4th type. He was going to test the amount of sediment that ended up in the water. His initial set up was that he was going to use topsoil from a store to control the type and make-up of dirt. He was then going to add some fertilizer to it and run regular water from a hose through it to see how much dirt ran into the water. Then he would test for nitrates but now I do see what you are saying about the levels of nitrates may not be solely connected to the soil runoff but what was used to stop erosion. I think he will use the disclaimer idea. For the apparatus, we were thinking somehow doing it in pieces of gutters. We have done other similar types of experiments and have yet to find a great way to control all of the water flow. Do we sound like we are on the right track? His sandbag idea came from a website he found. Thanks again for all the help!

[deleted-141593]

Sounds like he's on the right track. Let me know if he needs any more help.

Cheers,
Colin

[zabanda]

Just a few questions.

First, my son is using a gutter at an angle to simulate a hill. At the top of the gutter he has placed plain dirt. Next he place plain dirt with the natural filter on top (gravel, sandbag, grass, etc.). The end of the gutter is in a collection container. He then ran water from the top to see how much sediment ended up at the bottom. My question is whether or not the natural filter should be on dirt or by itself. Our thinking was that if, lets say, gravel is being used in the real world, they would place the gravel on the existing dirt. Just wondering what you thought.

Second question is whether or not this same setup could be used to see how much cow manure would end up in the water. Since we live by a river that has a lot of cows on it, he was wondering if he could replace the plain dirt at the top with cow manure. I wasn't sure if the cow manure would run down the gutter as well as the dirt and didn't want to go purchase a bag if you didn't think it would work. He read that even though the benefits of natural fertilizers are great, they can be negated by the cow manure actually getting into the water and doing harm. Also, is there a way to determine what is cow manure or what is dirt in the runoff? Any thoughts would be appreciated. thanks again for all of the help!

[zabanda]

Forgot to add one last question from my post before. Should we dry the sediment out once we run it through a coffee filter, before we weigh it? Thanks so much for your help!

[deleted-141593]

Hi Zabanda!

Sorry I missed this. See my responses below.

"First, my son is using a gutter at an angle to simulate a hill. At the top of the gutter he has placed plain dirt. Next he place plain dirt with the natural filter on top (gravel, sandbag, grass, etc.). The end of the gutter is in a collection container. He then ran water from the top to see how much sediment ended up at the bottom. My question is whether or not the natural filter should be on dirt or by itself. Our thinking was that if, lets say, gravel is being used in the real world, they would place the gravel on the existing dirt. Just wondering what you thought."

A: I think the filter should be by itself. If you have a substrate of dirt it can runoff and skew your results. You are specifically testing how well the filter prevents sediment from getting from the top of the filter to the bottom, correct? If so you place dirt on top and see how much gets trhough. If you are testing how weel gravel works to prevent erosion of soil underneath it then you put the gravel on top and soil at the bottom. Does that make sense to you?

"Second question is whether or not this same setup could be used to see how much cow manure would end up in the water. Since we live by a river that has a lot of cows on it, he was wondering if he could replace the plain dirt at the top with cow manure. I wasn't sure if the cow manure would run down the gutter as well as the dirt and didn't want to go purchase a bag if you didn't think it would work. He read that even though the benefits of natural fertilizers are great, they can be negated by the cow manure actually getting into the water and doing harm. Also, is there a way to determine what is cow manure or what is dirt in the runoff? Any thoughts would be appreciated. thanks again for all of the help!"

A: Not sure how you could differentiate between manure and soil in the runoff. However, as I suggested above, I think you want to place soil, manure etc on the the and the filter on the bottom and see how much sediment makes it through the filter. I would not mix soil and manure for the reasons you already stated. Also not sure if manure placed on top of the filter will make it to the bottom, but that's part of the point of the experiment, right?

"Forgot to add one last question from my post before. Should we dry the sediment out once we run it through a coffee filter, before we weigh it? Thanks so much for your help!"

A: Good question. Probably makes sense to dry it out, you might need to carefully dry the sample in a low temp oven.

Here is a link to a process for drying sediment samples I just found: http://www.tdi-bi.com/analytical_servic ... Weight.pdf

Cheers,
Colin

[zabanda]

Thanks so much Colin!

My son did a test run just so we could see what problems we would encounter. We watered his pan for 1 min but ended up getting a large amount of runoff to strain the sediment from (about 2200 ml). We determined the amount of time and strength of water flow by how much sediment was in runoff, to ensure we had enough sediment to be measured with a regular kitchen scale. Our problem was how to strain. We tried the coffee filter, and it took long, which isn't a problem but when we called the coffee filter people they said that the filters were only really good for one cup and eventually would stop filtering in that amount of water. We did find a paint strainer net but it isn't as fine as a coffee filter. Does it matter as long as the same method is being used for each soil sample? My concern lies with the fact that after we run the experiment with the four samples, it takes quite awhile to filter through each soil pan, about 1 hour. Does that bring in any variable we haven't thought of? Should we run one soil pan test and then measure that right away so that no pans are sitting for a couple of hours before being filtered? And finally, can we dry the filtered sediment naturally, for a specified amount of time?

Again, thank you so much for the help. Sorry for all the questions but my son is really enjoying this and he really wants to do this so it is following the "scientific method" as much as possible.

[deleted-141593]

Hi there,

Maybe you need to try several measurements and see which ones are reproducible and/or feasible. If you think the amount of water that comes through the system is reproducible you can weigh the entire water/sediment mix and subtract from this the weight of several trials of water alone. For example if the average sample weight with sediment is 5lbs and without is 4 then you can subtract and get sediment weight. You can also record volume if possible. I would make these two measurements as soon as each run is finished. Once you get these measurements you can also allow the samples to dry. As long as you prevent sediment from blowing away this might work but it might take a long time. It would be best to completely dry the samples (in an oven like in the link I sent) but this may not be feasible (particularly with the manure samples!). I'm not sure how to best speed up the process. In any case once you have all the data in hand your son can present all the measurements in which you have confidence. It is common in science to make measurements of the same using different instruments or techniques in order to increase confidence that the observed phenomenon is real. If any of the methods of measurement turns out to have lots of variability or other technical problems you need not present the data, but to have different kinds of measurement of the same thing is great!

A reminder that it is always good to do multiple trials of each condition, usually at least 3, and then take the averages.