I've just grasped the distinctions between permeability and hydraulic conductivity, as well as the concepts of Darcy flux and groundwater velocity from the video. Thank you immensely for the clarification. Your video is impeccable in every aspect, from theory to slides and explanations. It's undoubtedly the top lecture on basic groundwater flow on TH-cam.
@@GeosearchInternational Thank you for responding sir. I have a question and I would be very grateful if you could enlighten me on this. May I ask where did you get the 100m/day K at 14:20? is it computed through equation q=Q/A? please answer. thanks
because of all the references i have read and videos i've watched, it is the only video/reference that made me understood the darcy's law the most. the only question in my mind right now is from where the K=100/day came from that was mentioned at 14:20 of the video?
Hey great work man … lucid and clear style of explaining the concepts… became a big fan of yours .. how about some videos on Hydrogeology of Permafrost soils … early awaiting… warm regards
Hi Sachin, Thanks for your kind words. I have done geophysical surveys in Greenland and Kazakhstan at temperatures below minus 20C, but I don't know much about permafrost hydrogeology. Interesting topic for sure! All the best, Andreas
You are most welcome Saheb Das. You can find more videos on hydrogeology on the playlist: th-cam.com/play/PLp1lK6n-xb5O8RnVhcfYvqy1kzU_5IfDF.html All the best, Andreas
At 14:00, why did you calculate Δh as h2-h1 not as h1-h2? and if we have to do it as h2-h1 to get a negative i to prove that water flows from high to low elevation, then what is the purpose of the negative sign in the original Darcy Eq Q=-KiA
Hi Emad, the reason is to make the volumetric flow rate, Q, positive in the direction of flow under a negative change in head (i.e., head decreases in the direction of flow). We don't want to end up with negative flow rates. See: books.gw-project.org/hydrogeologic-properties-of-earth-materials-and-principles-of-groundwater-flow/chapter/darcys-law/ All the best, Andreas
At 8:15 you say the groundwater velocity is calculated with "porosity", but isn't it the effective porosity? If it's not clarified, it could be mistaken for the total porosity Edit: Okay you clarify it at 15:50, that it is indeed the effective porosity.
In all my videos you can have a look in the description, where I often share files. For this video there is a Link to pdf notes: bit.ly/3e86loF all the best, Andreas
Hi sir, at 7:55 you said that if the pipe is filled with Water, speed of water is hydraulic conductivity. So does it mean hydraulic conductivity is not a property of aquifer material..? I understood the concept like this: 1. Permeability is property of aquifer material. 2. Hydrauic conductivity is property of both aquifer material and specific weight of water. Is above mentioned analogy is right or wrong. Please clarify this doubt.
Hi SaiPrasad, What I said is that if the pipe is filled with only water, then the speed of water flowing down the pipe would be the same as the hydraulic flux (darcy velocity). However, it is not as the pipe is filled with aquifer material, therefore groundwater velocity = hydraulic flux/aquifer porosity. ie this section is about groundwater velocity, not hydraulic conductivity which is a property of the aquifer assuming it is filled with fresh water. This is discussed later in the video after 08:31 minutes. Hope that makes sense. All the best, Andreas
The hydraulic conductivity value for different grain sizes would have been based on many laboratory measurements. e.g., www.e3s-conferences.org/articles/e3sconf/pdf/2019/11/e3sconf_enviro2018_06010.pdf All the best, Andreas
Another way of course is to estimate the Transmissivity of an aquifer from a pumping test and then divide by the aquifer thickness to get the hydraulic conductivity. T = Kb This will be more accurate than guessing a number from a book, but if you have no field data a book is the best place to start.
A very good question Lihle ! Darcy’s law applies to laminar flow in porous media where the Reynolds number is less than about 1 to 10. In a fractured aquifer we can have laminar flow inside the rock matrix (e.g., in weathered bedrock), but a substantial amount of groundwater flow will occur within the fractures, and this flow can be turbulent especially near a pumping well. That means that Darcy’s law will no longer be accurate, and you may need to use a more complex approach (e.g., dual porosity models). Having said all that, mostly we just don't have the amount of data needed in a groundwater investigation - for example for a rural water supply project - so I normally just use the Darcy equations, but always keep in the back of my mind that reality is more complicated. In general Darcy’s law will apply if you are looking at a large enough area of an aquifer, because the higher flow in individual fractures will be averaged out. If you are interested in a guide to regional groundwater flow in fractured aquifers have a look at this book by Peter Cook: library.dbca.wa.gov.au/static/FullTextFiles/070659.pdf All the best, Andreas
Hi Mausami, Darcy's law applies to all groundwater flow in porous media as long as the flow is laminar. If you have very high hydraulic gradients - like next to a pumping well - you will start to get turbulent flow, which results in much higher head losses for the same volume of flow. In pumping wells we can correct for this additional drawdown based on step test data, or even better - just use observation well data. So it really depends on what you mean by a high hydraulic gradient. All the best, Andreas
Hi Theresa, I have not yet made a video on contouring groundwater heads, but maybe I should. In GIS I tend to use a spline method to make it as smooth as possible, but it is best to do it by hand and develop a flow net at the same time. Groundwater contouring is actually an interpretation method, and you need to take the different boundary conditions into account. e.g. groundwater will flow parallel to a no-flow boundary like the edge of a valley with impervious basement rocks next to the aquifer. Contours calculated by computer methods are often misleading, unless there are many data points. Stick to the pencil & paper method first. All the best, Andreas
@@GeosearchInternational apologies if I wasn’t clear-I meant how to hand draw them from your own hand calculations. I know there are programs to do the math for you but I’d like to understand the hand calculations more.I’m in a hydrogeology course.
Hi Theresa, Have a look at the book: "Graphical construction of groundwater flow nets" by Eileen Poeter and Paul Hsieh, which you can download for free from The Groundwater Project at: gw-project.org/books/ It has some nice worked examples and links to TH-cam videos. All the best, Andreas
Hi JL Y, there are no stupid questions - only stupid answers! The only reason I use days is because I studied hydrogeology in the UK, and I find it easier to think of an aquifer Transmissivity of 200 m2/d than whatever it is in seconds. Just use whatever units make you comfortable. The Germans are always using seconds and we don't even talk about the Americans with their acre-feet per second... Just remember that the most common calculation errors are because of a mix up of the units. e.g., even a NASA satellite once missed the rendezvous with Mars because of a mix up in units... That's why I always stick to units of days and metres in hydrogeology. All the best, Andreas
@@GeosearchInternational thank you for the early response and thank you for this video it helps me a lot 😊 , more blessings to come to the people behind this channel tysm ❤
Dear Mohammad, some 30 years ago I did an MSc in hydrogeology and I will always remember my professor telling us that we should first work for about 10 years in groundwater before using groundwater models. I think he was right, and in most projects I have worked on modelling is always the last thing I do, and only if it is absolutely necessary. A model is just a tool, and it does not matter so much which one you use - the most important thing is to understand the basics of groundwater flow, and then you will also understand which is the best model for a particular project. Please have a look at my video on Groundwater flow around wells - Excel model: th-cam.com/video/PvUWXQCEjpc/w-d-xo.html and Steady state in pumping wells: th-cam.com/video/9Wmkugy3XdQ/w-d-xo.html All the best, Andreas
I'm sorry but you are mistaken. The smaller the grain size, the lower the permeability. The reason is that the smaller grain sizes result in more narrow paths for the groundwater to flow through, and they also have a larger surface area/volume which means more friction to groundwater flow. The only scenario where gravel will have a lower permeability than sand is if the pore spaces between the gravel are filled with an even smaller sediment - such as silt/clay. I hope that makes sense. All the best, Andreas
@@GeosearchInternational that is not the the scientific logic. The effective permeability is not directely propertional to the grain size. Meaning it is not the higher the grain size the higher the permeability. You mean do you think if you have bolder size your permeability is very high, that is not true. Please refer to any hydrogeological book about effective permeability of grains.
Please have a look at Table 2.2 Range of Values of Hydraulic Conductivity and Permeability in Freeze & Cherry. It is available online at: fc79.gw-project.org/english/chapter-2/#2.3 As I already mentioned above, the sorting of the sediments also plays an important role, so if your aquifer contains a lot of fine material, you will have a low permeability. All the best, Andreas
I was just chatting with a hydrogeologist from Afghanistan, and he has come up with an interesting question for you: "If sand is more permeable than gravel, why are we using gravel packs for water wells instead of filling them with sand?".
@@GeosearchInternational That is an excellent question. That is traditional wording and traditional size. But by experiment the sorted and coarse sand has the highest effective permeability. But not easily available and the sieve analysis to obtain coarse sand is more expensive than gravel grain size. But in the USA they use an excellent dominantely silica based coarse sand and the well efficiency is very high.
Hello I have a question hopefully you can answer: I have a freswhater well where i plan to dig some swales or ditches or whatever theyre called ,that would be covered in topsoil and some sort of gravel in layers and other filtration like charcoal,in order to help the aquifer recharge. Would i be contaminating the freshwater aquifer or is it just mimicking a natural process and i have nothing to worry about. Something similar to this th-cam.com/video/NliixNvK-vg/w-d-xo.html
Hi, There is no simple answer to your question as it depends on many factors related to the well design, the properties of the aquifer/unsaturated zone, how the aquifer is being naturally recharged at the moment and the water quality of the surface water. In the video you linked, they are basically just building an underground water storage pit filled with angular rocks. The only filtering is from the thin sand & charcoal layers and I don't think it will remove much more than the sediments from the surface water. Probably bacteria and other contaminants will still get through. As a rule of thumb for water supply wells we want to have at least 50 to 60 days of travel time between the time of recharge and when the groundwater is extracted from the well, so we are talking about 10s to 100s of meters of fine aquifer materials, not just a 20cm sand layer. Charcoal can be effective as a water filter, but it has a finite life and would need to be replaced frequently, so just burying it in the pit is not a long term solution. Artificial recharge or MAR as it is called nowadays is a complex subject, and you need to have a good understanding of the whole water cycle in your area to be able to make an effective intervention.
The clearest and most coherent explanation of Darcy's Law that I've come across to date. It really brought everything together for me.
I'm glad that you liked it Peter! All the best, Andreas
Agree 100%, keep up Andreas i really appreciate your efforts
Most welcome! All the best, Andreas
Totally. Beautify put together. makes it easy to follow
Thanks Noor! All the best, Andreas
Came to this video after not understanding the 120+ lecture slides assigned for comprehension, this video made the concept so clear to me
Glad you found it useful! All the best, Andreas
learnt a lot more in this video than i did in my hydrogeo class
Thanks for your kind words Grace. Keep studying! All the best, Andreas
wow! i couldn't have gotten a better introduction to hydrogeology than this. Great Video. Thanks for this
You're very welcome! Thanks for your kind words Ian! All the best, Andreas
I've just grasped the distinctions between permeability and hydraulic conductivity, as well as the concepts of Darcy flux and groundwater velocity from the video. Thank you immensely for the clarification. Your video is impeccable in every aspect, from theory to slides and explanations. It's undoubtedly the top lecture on basic groundwater flow on TH-cam.
Thanks so much for your kind words @hydromanor! Glad you found it useful. All the best, Andreas
I have really enjoyed this lesson. You're the best of all my lecturers at my university school..
Glad you liked the video! Which university are you attending? All the best, Andreas
much better than other videos i've watched. thank you so much! please do more videos. your an excellent teacher!
Thanks for your kind words. I will try to make more videos, though am very busy at the moment with projects. All the best, Andreas
@@GeosearchInternational Thank you for responding sir. I have a question and I would be very grateful if you could enlighten me on this. May I ask where did you get the 100m/day K at 14:20? is it computed through equation q=Q/A? please answer. thanks
because of all the references i have read and videos i've watched, it is the only video/reference that made me understood the darcy's law the most. the only question in my mind right now is from where the K=100/day came from that was mentioned at 14:20 of the video?
It is from Kruseman & deRidder. See page 21:
www.hydrology.nl/images/docs/dutch/key/Kruseman_and_De_Ridder_2000.pdf All the best, Andreas
I'm enjoying this series. You do a great job explaining it all.
Thank you so much for your kind words Professor! All the best, Andreas
Thank you, Sir. The illustration is so clear to understand. I will love more such videos. Keep up the brilliant job.
Thank you Andrew,
All the best, Andreas
Very subtle and informative. Your effort is underrated sir.
So nice of you Sina! All the best, Andreas
Oh my!, This lecture is above and beyond. Thanks
You are most welcome Jean Lee! All the best, Andreas
Excellent explanation and well understood. Thank You!
You are welcome Aung! All the best, Andreas
Thanks for this video sir🙏 now I'm clear on Darcys law 😊
Most welcome Puja! All the best, Andreas
Thank you for all these precious information.
You are welcome Sneha! All the best, Andreas
Great presentation
Thank you @mhick3333! All the best, Andreas
Again a great video. Also consider groundwater monitoring videos as this is very hot topic now.
Thanks! Will do! All the best, Andreas
Hey great work man … lucid and clear style of explaining the concepts… became a big fan of yours .. how about some videos on Hydrogeology of Permafrost soils … early awaiting… warm regards
Hi Sachin, Thanks for your kind words. I have done geophysical surveys in Greenland and Kazakhstan at temperatures below minus 20C, but I don't know much about permafrost hydrogeology. Interesting topic for sure! All the best, Andreas
ty professor great work that help a lot
You are welcome Omar! All the best, Andreas
Thanks Mr Andreas, the best way to learn.
You are welcome, Samir!
Thank you very much for this wonderful explanation. Waiting for more
You are most welcome Saheb Das. You can find more videos on hydrogeology on the playlist:
th-cam.com/play/PLp1lK6n-xb5O8RnVhcfYvqy1kzU_5IfDF.html
All the best, Andreas
Excellent video, very clearly explained! Thank you so much! :)
You're very welcome Rupesh! All the best, Andreas
Very beautiful lecture on the topic. Thanks. I strongly recommend to students.
Many thanks Puneet, glad you found it useful!
All the best, Andreas
Thanks you, very clear
You are welcome Algassimou! All the best, Andreas
Very well done! Fascinating stuff
Glad you enjoyed it!
Brilliant video. Thank you.
Glad you enjoyed it Emmanuel! All the best, Andreas
Great video as always, thank you for that ❤
You are welcome Qadr! All the best, Andreas
Thank you.
You're welcome Ken! All the best, Andreas
what a nice explanation :), thank u sir
Most welcome Hendry! All the best, Andreas
Wow this explanation is so informative. Keep reminding the basic principles is very useful for me working in underground mining! A subscribe from me💪🏻
Thanks Gilrandy! My first job as a geologist was gold exploration so mining is always interesting for me! All the best, Andreas
At 14:00, why did you calculate Δh as h2-h1 not as h1-h2?
and if we have to do it as h2-h1 to get a negative i to prove that water flows from high to low elevation, then what is the purpose of the negative sign in the original Darcy Eq Q=-KiA
Hi Emad, the reason is to make the volumetric flow rate, Q, positive in the direction of flow under a negative change in head (i.e., head decreases in the direction of flow). We don't want to end up with negative flow rates. See: books.gw-project.org/hydrogeologic-properties-of-earth-materials-and-principles-of-groundwater-flow/chapter/darcys-law/ All the best, Andreas
thank you very nice and interesting video
Glad you liked it! All the best, Andreas
thanks for this helpful presentation, if you don't mind I will consider it in my student lecture in Arabic language
You are very welcome Rani to use any of this material. Here is a link to the PDF notes for this lecture: bit.ly/3e86loF All the best, Andreas
@@GeosearchInternational many thanks for your help
Does Darcy's Law apply to both confined and unconfined aquifers?
Yes, Darcy's Law applies to all groundwater flow. All the best, Andreas
thanks for your videos.
You are welcome Osama! All the best, Andreas
At 8:15 you say the groundwater velocity is calculated with "porosity", but isn't it the effective porosity? If it's not clarified, it could be mistaken for the total porosity
Edit: Okay you clarify it at 15:50, that it is indeed the effective porosity.
Yes, you are right - it is the effective porosity. All the best, Andreas
Thank you, Sir.
You are very welcome Poltak! All the best, Andreas
Could you please provide best notes for hydrogeology sir
In all my videos you can have a look in the description, where I often share files. For this video there is a Link to pdf notes: bit.ly/3e86loF all the best, Andreas
Thank you for the educational video. How are we supposed to get the time that will groundwater will flow.
You are welcome Siyanda! Do you mean the speed? All the best, Andreas
Yes
Have a look at page 14 of the notes: bit.ly/3e86loF
All the best, Andreas
Thank you.
Welcome! All the best, Andreas
Hi sir, at 7:55 you said that if the pipe is filled with Water, speed of water is hydraulic conductivity. So does it mean hydraulic conductivity is not a property of aquifer material..?
I understood the concept like this:
1. Permeability is property of aquifer material.
2. Hydrauic conductivity is property of both aquifer material and specific weight of water.
Is above mentioned analogy is right or wrong. Please clarify this doubt.
Hi SaiPrasad, What I said is that if the pipe is filled with only water, then the speed of water flowing down the pipe would be the same as the hydraulic flux (darcy velocity). However, it is not as the pipe is filled with aquifer material, therefore groundwater velocity = hydraulic flux/aquifer porosity. ie this section is about groundwater velocity, not hydraulic conductivity which is a property of the aquifer assuming it is filled with fresh water. This is discussed later in the video after 08:31 minutes. Hope that makes sense. All the best, Andreas
thank you so much sir
Most welcome Hamadullah! All the best, Andreas
where did you get the 100m/day K at 14:20? is it computed through equation q=Q/A? please answer. thanks
100m/d is just a typical K for a gravel or sandy gravel
see: th-cam.com/video/O7K00PQaQIw/w-d-xo.html
All the best, Andreas
@@GeosearchInternational thanks for the response sir! God bless
But, how did they came up with that value? I mean, is there a formula to compute that? Thank you..
The hydraulic conductivity value for different grain sizes would have been based on many laboratory measurements. e.g., www.e3s-conferences.org/articles/e3sconf/pdf/2019/11/e3sconf_enviro2018_06010.pdf All the best, Andreas
Another way of course is to estimate the Transmissivity of an aquifer from a pumping test and then divide by the aquifer thickness to get the hydraulic conductivity. T = Kb
This will be more accurate than guessing a number from a book, but if you have no field data a book is the best place to start.
May I ask, why Darcy's law cannot be applied in fractured aquifers?
A very good question Lihle !
Darcy’s law applies to laminar flow in porous media where the Reynolds number is less than about 1 to 10. In a fractured aquifer we can have laminar flow inside the rock matrix (e.g., in weathered bedrock), but a substantial amount of groundwater flow will occur within the fractures, and this flow can be turbulent especially near a pumping well. That means that Darcy’s law will no longer be accurate, and you may need to use a more complex approach (e.g., dual porosity models).
Having said all that, mostly we just don't have the amount of data needed in a groundwater investigation - for example for a rural water supply project - so I normally just use the Darcy equations, but always keep in the back of my mind that reality is more complicated. In general Darcy’s law will apply if you are looking at a large enough area of an aquifer, because the higher flow in individual fractures will be averaged out.
If you are interested in a guide to regional groundwater flow in fractured aquifers have a look at this book by Peter Cook: library.dbca.wa.gov.au/static/FullTextFiles/070659.pdf
All the best, Andreas
thank you
You're welcome Lilian! All the best, Andreas
Can Darcy law be used to quantify groundwater induced in high hydraulic gradient??
Hi Mausami, Darcy's law applies to all groundwater flow in porous media as long as the flow is laminar. If you have very high hydraulic gradients - like next to a pumping well - you will start to get turbulent flow, which results in much higher head losses for the same volume of flow. In pumping wells we can correct for this additional drawdown based on step test data, or even better - just use observation well data. So it really depends on what you mean by a high hydraulic gradient. All the best, Andreas
thank you so much
You're welcome! All the best, Andreas
May I ask, do you have one of your well explained videos on how to calculate potentiometric lines?
Hi Theresa, I have not yet made a video on contouring groundwater heads, but maybe I should. In GIS I tend to use a spline method to make it as smooth as possible, but it is best to do it by hand and develop a flow net at the same time. Groundwater contouring is actually an interpretation method, and you need to take the different boundary conditions into account. e.g. groundwater will flow parallel to a no-flow boundary like the edge of a valley with impervious basement rocks next to the aquifer. Contours calculated by computer methods are often misleading, unless there are many data points. Stick to the pencil & paper method first. All the best, Andreas
@@GeosearchInternational apologies if I wasn’t clear-I meant how to hand draw them from your own hand calculations. I know there are programs to do the math for you but I’d like to understand the hand calculations more.I’m in a hydrogeology course.
Hi Theresa,
Have a look at the book: "Graphical construction of groundwater flow nets" by Eileen Poeter and Paul Hsieh, which you can download for free from The Groundwater Project at: gw-project.org/books/ It has some nice worked examples and links to TH-cam videos. All the best, Andreas
Sorry for being dumb, may I ask why the time is measured in days and not seconds?
Hi JL Y, there are no stupid questions - only stupid answers!
The only reason I use days is because I studied hydrogeology in the UK, and I find it easier to think of an aquifer Transmissivity of 200 m2/d than whatever it is in seconds. Just use whatever units make you comfortable. The Germans are always using seconds and we don't even talk about the Americans with their acre-feet per second... Just remember that the most common calculation errors are because of a mix up of the units. e.g., even a NASA satellite once missed the rendezvous with Mars because of a mix up in units... That's why I always stick to units of days and metres in hydrogeology.
All the best, Andreas
@@GeosearchInternational thank you for the early response and thank you for this video it helps me a lot 😊 , more blessings to come to the people behind this channel tysm ❤
You are welcome! All the best, Andreas
thanks sarry
From which software can I model groundwater?
Dear Mohammad, some 30 years ago I did an MSc in hydrogeology and I will always remember my professor telling us that we should first work for about 10 years in groundwater before using groundwater models. I think he was right, and in most projects I have worked on modelling is always the last thing I do, and only if it is absolutely necessary.
A model is just a tool, and it does not matter so much which one you use - the most important thing is to understand the basics of groundwater flow, and then you will also understand which is the best model for a particular project. Please have a look at my video on Groundwater flow around wells - Excel model: th-cam.com/video/PvUWXQCEjpc/w-d-xo.html and Steady state in pumping wells: th-cam.com/video/9Wmkugy3XdQ/w-d-xo.html
All the best, Andreas
Thaaaaaaaanks
Welcome Raghad! All the best, Andreas
danke
Bitteschön! Grüße, Andreas
The gravel does not have more permeability than sand. Sand has the highest permeability.
I'm sorry but you are mistaken. The smaller the grain size, the lower the permeability. The reason is that the smaller grain sizes result in more narrow paths for the groundwater to flow through, and they also have a larger surface area/volume which means more friction to groundwater flow. The only scenario where gravel will have a lower permeability than sand is if the pore spaces between the gravel are filled with an even smaller sediment - such as silt/clay. I hope that makes sense. All the best, Andreas
@@GeosearchInternational that is not the the scientific logic. The effective permeability is not directely propertional to the grain size. Meaning it is not the higher the grain size the higher the permeability. You mean do you think if you have bolder size your permeability is very high, that is not true. Please refer to any hydrogeological book about effective permeability of grains.
Please have a look at Table 2.2 Range of Values of Hydraulic Conductivity and Permeability in Freeze & Cherry. It is available online at:
fc79.gw-project.org/english/chapter-2/#2.3
As I already mentioned above, the sorting of the sediments also plays an important role, so if your aquifer contains a lot of fine material, you will have a low permeability. All the best, Andreas
I was just chatting with a hydrogeologist from Afghanistan, and he has come up with an interesting question for you: "If sand is more permeable than gravel, why are we using gravel packs for water wells instead of filling them with sand?".
@@GeosearchInternational That is an excellent question. That is traditional wording and traditional size. But by experiment the sorted and coarse sand has the highest effective permeability. But not easily available and the sieve analysis to obtain coarse sand is more expensive than gravel grain size. But in the USA they use an excellent dominantely silica based coarse sand and the well efficiency is very high.
Hello I have a question hopefully you can answer:
I have a freswhater well where i plan to dig some swales or ditches or whatever theyre called ,that would be covered in topsoil and some sort of gravel in layers and other filtration like charcoal,in order to help the aquifer recharge.
Would i be contaminating the freshwater aquifer or is it just mimicking a natural process and i have nothing to worry about.
Something similar to this th-cam.com/video/NliixNvK-vg/w-d-xo.html
Hi,
There is no simple answer to your question as it depends on many factors related to the well design, the properties of the aquifer/unsaturated zone, how the aquifer is being naturally recharged at the moment and the water quality of the surface water.
In the video you linked, they are basically just building an underground water storage pit filled with angular rocks. The only filtering is from the thin sand & charcoal layers and I don't think it will remove much more than the sediments from the surface water. Probably bacteria and other contaminants will still get through.
As a rule of thumb for water supply wells we want to have at least 50 to 60 days of travel time between the time of recharge and when the groundwater is extracted from the well, so we are talking about 10s to 100s of meters of fine aquifer materials, not just a 20cm sand layer. Charcoal can be effective as a water filter, but it has a finite life and would need to be replaced frequently, so just burying it in the pit is not a long term solution.
Artificial recharge or MAR as it is called nowadays is a complex subject, and you need to have a good understanding of the whole water cycle in your area to be able to make an effective intervention.