Different Rock Types, pls help!?

Question

How will the following affect conductivity of a stream and infiltration rates,

Aluvial/ glacial sediments
Old Red Sanstone
Carboniferous Limestone
Millstone Grit

Please help, im desperate! Can't find anything!

Any knowledge whatsoever appreciated, only one person answered so your likely to get best answer!

Answer 1

The rock types won't affect the conductivity of the stream itself, but will affect the infiltration rate of the water in the stream into the aquifer below the stream. Infiltration rate or flow is called Q in Darcy's law (Q=KAi) when K = Hydraulic Conductivity, A= Area and i = Hydraulic gradient. So you can already see that the gradient of water will affect the infiltration rate. Assuming the water level in the stream is say 5m above the water level in the rock you have then 5m of HEAD (pressure) pushing the water into the rock.

Hydraulic conductivity is the rate of flow of water through a unit cross sectional area under a unit hydraulic gradient.

However the flow is affected by a variety of things including the viscosity of the liquid, and the properties of the rock. The most important property that affects flow is primary porous (though the pores) flow or secondary fissure flow through the cracks in the rock. Generally, alluvial sediments will be soft and not cracked, so flow will be predominately through pores. Carboniferous limestone is more crystalline with little pore space if any, but lots of cracks. Millstone Grit and Old Red Sandstone tend to be made of little balls of sand, so have porous flow, but also have big cracks, so fissure flow as well.

The next factor that affects the flow is the sorting and packing of the sediments. A well sorted rock like Old Red Sandstone will have very well sorted and well rounded sand grains. That means when it is packed together, the water can still flow through it easily. The Millstone grit is less well sorted and has big and small grains. The small grains fit in the gaps between the big grains, so reduce the flow. With the alluvial deposits, the clay and silt can stuff up the pores, and significantly block the flow, so alluvial deposits often block the flow between the rock and the stream.

In addition, the direction of flow is important. The sandstone and the limestone may have good cracks horizontally along the bedding planes, but the water is trying to flow vertically not horizontally. If flow is the same in all directions it would be isotropic and homogenous. Normally flow is ten times better horizontally than vertically. It is the same for the alluvium, which is also deposited in layers but the difference between vertical and horizontal is up to 1000 times.

Consolidation is also important. A sand before it forms rock will allow water to flow through it far more easily than once it has been glued together reducing the paths through the rock. Often a rock has a weathered horizon, so the sandstone will have a sand layer between it and the alluvial deposits which act as a reservoir to aid infiltration.

Nikki got it wrong when she thought that the alluvial sediments will have a higher conductivity because they are unconsolidated. They will have a lower conductivity because all their pores spaces for flow are blocked up. (Good try though - what Uni are you at?)

Your homework is to go look up the typical hydraulic conductivity’s of your respective rock types now you know the principles.

Answer 2

Hydraulic conductivity(K) of a stream is described as the ease that water moves through pore spaces or fractures. It has nothing to do with electrical current.
Consolidated material has a lower (k) than unconsolidated material.
Alluvial deposits are unsorted therefore they have larger pore spaces so the hydraulic conductivity will be high.
Sandstone typically has a medium (k) unless it is severely fractured. Typically 10-5 (10 to the negative 5 cm per second) Limestone will have medium (k) on the rate of
10 -7(10 to the neg. 7) cm per second.
Grit is probably around the same as sand, 10-4(10 to the neg. 4 cm/sec)

Infiltration is the process by which water on the ground surface enters soil. It is mainly governed by gravity.
Soil texture, structure, water content play a part in infiltration.
Again, the material you have listed will have higher or lower infiltration rates depending how it is consolidated. Unconsolidated material such as alluvium will have higher infiltration rates than sandstone.

Answer 3

The question is essentially based on porosity. That would have to do mostly with grain size of the sediment/rock in this example I believe. Infiltration has to do with water leaving a stream and entering groundwater, so the rate at which infiltration occurs would have mostly to do with grain size/porosity as there are no other hydrogeological parameters given in the question.

Answer 4

Conductivity, to the best of my understanding, is usually related to either electricity or magnetism.

If you can't find any sites when you are using a search engine you need to broaden your parameters a bit.

Just to shoot some ideas at you....

Are you talking about natural filtration?
Natural chemical purification?

You mentioned the word "RATE."
A rate is a relationship between two things.
What two "things" are you talking about:
'purity of the water' and 'time',
'purity of the water' and 'distance',
'purity of the water' and '?',
biological contamination over time,
biological purification over time,
chemical contamination over distance,
chemical purification over distance?

RATE is a mathematical function such that your final answer is "x per y".
.

Answer 5

Try google, but use " inverted commas" to force it to give you the word string you want.
For example - "types of rock"