Science Buddies: "Ask an Expert"

Hi,
My son originally chose Underground water flow and Darcy's Law project for his science project because it looked interesting to him. He is mostly interested in comparing the rate of flow of water through different materials rather than the effects of pressure on flow. Can he do this experiment just changing the materials and measuring the flow rates? If so, his independent variables would be the size of the materials and his dependent variables would be flow rate. But then how would he graph this? He was thinking of particle size and using pea gravel, aquarium gravel, sand, clay soil, and a mixture of sand and clay. But how would he know the size of the sand and clay soil? We also thought originally that the K in Darcy's law had to do with porosity and were going to maybe graph that value, but reseach and other help on this site says that it really measure hydrolic conductivity-which might not relate directly to porosity or particle size.
Can you suggest how to tweak our problem and hypothesis so that we can make this into a legitimate project for the science fair?
Can we get particle size somewhere for sand and soil?
What do you think? Please help!

scifreak4fair wrote:Can we get particle size somewhere for sand and soil?

Not likely for all samples!
In constructing drainage fields for septic systems a perculation test is used to evaluate the suitability of a given soil and how big a drain field is required for a given average flow rate. In some areas that have some highly variable ground conditions, these tests must be done at multiple locations within the proposed field and and there will be local rules on how to combine the values.

The data you are asking for is something that has to be measured on the samples you are testing. The hydrostatic pressure and the flow rate in an experimental setup are used to calculate the hydrolic conductivity of the sample being tested.

You should be able to come up with an experiment to measure the hydrolic conductivity of your samples. Try filling something like filling a 2 foot section of 4" PVC pipe with your test material (after presoaking it in an excess of water), use a piece of drainage fabric (the kind used to prevent soil from entering a perforated drainage tile) to close the outlet end of the pipe and then use an elbo and another section of PVC pipe as an input stand pipe. Slope your test section at 1/8 inch per foot, fill the stand pipe with some pre-measured quantity of water, measure the initial standing height and time for the standing height to disappear, and measure the amount of water exiting the pipe at the time the standing height disappears, then measure how long it takes and how much water is recovered when the exit flow is down to a drop per minute. A couple of 5 gallon plastic buckets would be helpful as the catch vessel and to partially fill to dump into the stand pipe. Run the experiment at least three times with each test sample. The data you get combined with the geometry of your sample (aka inside of the pipe section that was filled with the sample) should allow you to calculate the hydrolic conductivity of your samples.

For gravel purchased from an engineered aggregate supplier, the supplier can supply you with a typical range of hydrolic conductivity, range of aggregate size, density, etc for the material because they have to run tests for their customers on those materials and guarnatee they meet the specifications required.

Before you do the experiment, you need to come up with a hypothesis statement and determine if this kind of an experimental proceedure can be used to prove or disprove your hypothesis or if this is just getting you one of the numbers you need for another experiment.