Re:[gwmodel] Minimum permissible distance between bore holes in hard rock aquifers
- To Dr. A.K. Ghosh
1) Dr. BBS Singhal has used the ref of UNESCO 1972 for rad of Influence in fissured rocks.
2) In double porosity model, best match with type curve has been achieved with porosity=10 %. Double porosity model is more representative of uniformly fractured aquifer.
3) Equations in connection with pumping Test are developed based on the assumptions of homogeneous & isotropic aquifers with laminar flow during pumping...well is 100% efficient and effective diameter is the same as the diameter of the well. But in reality aquifers are heterogeneous, anisotropic with non-laminar flow during pumping...pumping well is not 100% efficient and effective dia is usually more than dia. of screen mainly, in case of unconsolidated sediments. Considering the above reality, pumping well & observation wells are installed into closely spaced & inter-connected fractures and also, nested observation wells at different depth horizon in different direction are installed to get reading of water level at different depth & in different direction and an av. T value is computed. With a view to get as much as radial,horizontal & laminar flow pattern, pumping well should be put in lower 1/2 or 1/3 rd of aquifer thickness and Observation
wells should be located at dist. 1.5 times the saturated thickness of interconnected fractured aquifer. Considering the above reality, Huntley used the Specific Capacity(Q/Sw) as (Q/Sw)1.18and he modifed Thiem's Eq for fractured aquifer as T=A(Q/Sw)1.18 using the coefficient A=(2.3*log R/rw)/2π (as in Thiem's Eq.) Thus Huntley's Eq holds good for i) small dia 100% efficient pumping well and ii) nested observation wells in different & different direction installed in closely spaced interconnected fracture zone. Therefore, I think earlier steps may be quite O.K. for finding out Rad of Influence (R).
4) Dr. BBS Singhal has not included the study of Anisotropic Aquifer Heterogeneous Aquifer with partial laminar flow and partial turbulent flow in his book.
Subrata HalderExecutive Engineer (Agri-Irrigation)
State Water Investigation Directorate
Department of Water Resources Investigation & Development
Govt. of West Bengal
--- On Mon, 12/11/12, asokg@... <asokg@...> wrote:
From: asokg@... <asokg@...>
Subject: Re:[gwmodel] Minimum permissible distance between bore holes in hard rock aquifers
To: "subrata halder" <shalder_2007@...>
Date: Monday, 12 November, 2012, 9:54 AM
Thank you for clarifying Hard Rock aquifer system so clearly. Do you
have the details of reference UNESCO 1972. Was ir developed through
observation or through some theoretical analysis?. If the original paper
is available in soft I will like to have a copy of the same.Most of the
equations in Step 1 are developed for Homogeneous isotropic aquifer. For
double porosity I will like to know for what value(range) of porosity
these equations are valid and what is the range of Reynold number for
these porosity values. Huntleys equation is an Empirical Equation based
on observation from field data. However, Huntley takes care of the
turbulent flow part by introducing Specific Capacity ( S = AQ + BQ^n).
Again in Step 3 we are using an Equation which is developed for Laminar
Flow. Dr. Singhal has written a book of Hydrogeology of Fractured Rocks .
I have not seen that book. I am sure you have gone through the book. I
will like to know whether Dr. Singhal has included study of Anisotropic
Aquifer Heterogeneous Aquifer with partial laminar flow and partial
turbulent flow .