1. Introduction

The subject of Groundwater science is very fascinating, but once you go through the Well Hydraulics chapter of any Groundwater, Hydrogeology, or Hydrology books, you may get confused by coming across so much complicated mathematics. We know that the volume of water, expressed as a percentage of the total volume of the saturated aquifer, that can be drained by gravity is called the Specific yield. Hydraulic conductivity is defined as the volume of water that flows through a unit area of aquifer in a unit time under a unit hydraulic gradient., but these aquifer hydraulic constants-Specific yield (Sy) and Hydraulic conductivity (K) of unconfined granular aquifers(unconsolidated formations /alluvial /sand dunes), so far derived from different pumping test methods-are not reliable; also, it cannot be fairly estimated from particle size or from measurements of water retention, whereas laboratory or field tracer tests are relatively expensive and time-consuming compared to pumping methods. The aquifer parameters of porous media are particularly important for both qualitative and quantitative studies of groundwater resources, such as the transport of contaminants, flow modelling, estimation of groundwater resources, etc.

In order to determine the (Sy) and (K) values, many scientists* have been researching since the beginning of last century. They compromised with the help of type curves matching that involve personal judgement, making an analogy with heat flow pattern in metal, application of least squares method, non-least square method, numerical method, discrete kernel method, and error function methods, etc. Their mathematical models are based on numerous faulty assumptions. Their research papers contain long discussions, involved complicated mathematical applications without any concrete conclusion, and are meant for academic popularity only. The author believes that they adopted all kinds of indirect methods having absurd assumptions.

A new direct method has been discovered based on correct flow theory. It is not an alternative method; rather, it is one and only method to evaluate the hydraulic characteristics of unconfined aquifers. Based on this correct theory, two perfect model equations have been developed by the author using residual drawdown of water level data during recovery phase of the production well; the aquifer constants (Sy) and (K) could be evaluated correctly. So one can obtain the spot (in situ) value of (Sy) and (K) of the sub-surface aquifer wherever a single tube well exists by just stopping the running pump. Generally speaking, residual water level recovery data is more reliable than falling drawdown data because there is no pumping effect involved.The (K) value thus obtained completely satisfies Darcy's law and is free from Dupuit’s assumptions. Moreover the aerial extension of aquifer could be limited, inhomogeneous, and anisotropic.

This unique method of evaluation of the hydraulic properties not only saves your time in doing pumping tests but is also very economical, requires no observation well, and is without unnecessary waste of water. With little modification, the equations can be used for confined aquifers to derive the parameters, viz. Storativity (storage coefficient) and Transmissivity. Accordingly, a web-enabled tool has been hosted to determine (Sy) and (K) values correctly. The esteemed visitors are requested to share their comments and feedback after getting the aquifer constants.

Author :M.Iswar Prasad /2023

2. Aquifer Test Procedure

This is known as single well recovery test or source well recuperation test (SWRT) in Unconfined (Water table/Phreatic) aquifer which does not require any observation well. As shown below in the cross section diagram, a tube well (outer diameter: d) is penetrating an unconfined granular aquifer. At first measure the discharge of the well (Q) when the water level has ceased to decline (steady state). Note the drawdown imposed (Sw). Than stop the pump and note down the rising water levels reading (residual drawdown) during recovery phase at every minute. Accordingly keep the data set ready as per the format below.
The imposed drawdown (Sw) should not go below the screen bottom and should not be more than the vertical screen length (SL). The accuracy of the hydraulic constants i.e., Specific yield (Sy) and hydraulic conductivity (K) values thus determined depends upon the higher (stable) drawdown, discharge values, and narrower casing diameter. The depth of the tubewell (TD) should be measured from its original (static) water level (i.e. pre-pumping stage).

3. Input of Measuring Units

4. Cross Section

5. Data Requirements

The following data are required for this SWRT in unconfined aqufier and their input units and ranges are mentioned below.

Residual drawdown in metre VERSUS lapsed time at 1,2,3,4,5,6,7,8,9,10,20,30 minutes just after pump stopped.