Bouwer-Rice(
https://doi.org/10.1111/j.1745-6584.1989.tb00453.x) have described in an article how we perform slug test analysis in an unconfined aquifer which can be extended to confined aquifers also. In the same article they mentioned that if we use the falling water table test in a well where the initial groundwater table is below the filter top then 1) the head will drop faster in the well and 2) it will overestimate the hydraulic conductivity. Neither does it mention anywhere why the head will drop faster and in comparison to which case will the head drop will be faster nor has it mentioned that accodring to which formula will the hydraulic conductivity will be overestimated.
I am not a geologist, and have limited experience with this approach. In unsaturated substrate below well casing, a portion of the water is used to fill voids to saturate substrate and help raise water table, and a portion is moved through hydraulic conductivity. Hydraulic conductivity is typically measured in saturated substrates. I do not know if you were to run the slug test long enough, if the substrate would eventually saturate and stabilize, so the hydraulic conductivity portion could be approximated.
The speed of the hydraulic head drop is both controled by the hydraulic conductivity. and the hydraulic gradient (vertical gradient in the case of a slug test). Since the hydraulic conductivity is lower in the unsaturated zone than in the saturated zone, you have to consider the hydraulic gradient to compare both situations and try to explain why you observe a faster drop for an injection of water that is above the water table.
Thanks William F. Hansen for the reply. I advise to the young hydrogeologists to do the same. Fill in water above the filter top and let it drain out. Repeat it two to three times and then do the slug test. How will I do an analysis of such a slug test. My intution is that I take the complete filter length in this case as the saturated thickness in order to estimate the hydraulic conductivity.
Thanks Romain for the reply. Intutively Ksat x bsat+ Kunsat x bunsat is the trasmissivity value instead of just Ksat x bsat. Thats the reason head drop rate should be higher doesnt matter how small unsaturated hydraulic conductivity is. However the article does not reveal the value of effective length of filter while giving the statement ' overestimation of hydraulic conductivity.
Well,
When water is poured into the well, the water column will also include the unsaturated part represented by the filter portion of the unsaturated sediments, because the water level is below the upper end of the filter. Here, part of the water will invade the top unsaturated screen part, causing a rapid drop in water levels.
Sincerely
Thanks Bayan for the reply! That one I understood. But why does it overestimate the hydraulic conductivity is something I dont get.
Which formula have they used to overestimate the hydraulic conductivity.
Well,
there are many disadvantages in sludge test application. you can consider that after reading the attached chapter, and it overestimate the hydraulic conductivity , because the value of hydraulic conductivity become for saturated and unsaturated zone, and not represent H.C of aquifer.
Sincerely
I suspect Quinn's reference to Butler had got it right. For the portion of the filter above the water table, the initial response physics of the unsaturared sediments near the piezometer is in part like that of the initial response of top surface unsaturated soil to a wetting/ponding intense rainfall event--you may have suction heads! However, your situation is more complex, with gravity direction normal to the initial wetting front, which evolves into a curved wetting front surface, and with radial symmetry--I would second Quinn's reference to Butler.
It seems to me Bayan Hussien is right: the filter (gravel pack) will introduce an additionnal volume influenced by the slug test.
Slug test has been modelled by Hvorslev (1952) whose equation for a partially penetrating well or piezometer in a “no-limit” aquifer includes the square of the casing radius (see for example http://www.aqtesolv.com/hvorslev.htm).
One of the important specificities of the well you are testing is that the water level is located at the screen level. It seems to me you could still use Hvorslev model, but instead of the square of the casing radius, you should use something equivalent to effective storage capacity in the well itself (e.g. square of borehole radius – % of annular space occupied by the filter - gravel pack).
- Badges
- Science topic
- Similar topics
- Computer Communications (Networks)
- Simulators