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Guide Lines for Drill Site Supervision

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Guide Lines for Drill Site Supervision Guidelines for implementation of piezometers July 1998 TABLE OF CONTENTS Preface ........................................................................................................................2 1 Introduction .....................................................................................................3 1.1 Use of production well as piezometer ...............................................................3 1.2 Use of piezometer as production well ...............................................................3 1.3 Data emanating.................................................................................................4 2 Network design................................................................................................5 2.1 Identification of the aquifer system....................................................................5 2.2 Evaluation of the existing network.....................................................................6 2.3 Enhancement of the network.............................................................................9 3 Piezometers in unconsolidated formations................................................12 3.1 Components of the piezometer .......................................................................12 3.2 Pre-drilling design............................................................................................13 3.3 Installation .......................................................................................................14 3.4 Design of gravel size and screen slot size ......................................................17 3.5 Lowering of well assembly and gravel packing ...............................................19 3.6 Well development............................................................................................19 4 Piezometers in consolidated formations ....................................................20 4.1 Components of the piezometer .......................................................................20 4.2 Installation .......................................................................................................22 5 Post-drilling activities...................................................................................24 5.1 Recovery test ..................................................................................................24 5.2 Check for hydraulic connection .......................................................................24 5.3 Sampling for water quality...............................................................................24 5.4 Survey .............................................................................................................25 5.5 Aquifer tests ....................................................................................................25 5.6 Documentation of information .........................................................................26 Annexure – 1 Drilling data collection formats......................................................27 Part A Unconsolidated rocks......................................................................................27 Part B Consolidated rocks.........................................................................................30 GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 1 Preface One of the aims of the Hydrology Project is to implement a large number of scientifically designed and correctly installed piezometers to monitor the piezometric head of top unconfined and deeper confined aquifers. The practising groundwater professionals, responsible for the implementation are mostly very well familiar with the design criteria and installation operations. This familiarity, however, emanates generally from their experience in drilling of production wells and not from the drilling of piezometers. While the design criteria and field operations remain by and large the same, there are important differences, which need to be understood and assimilated into the practice of implementing a piezometer. These guidelines highlight these differences, and are expected to assist the professionals in realizing the necessary re-orientation of their drilling expertise and experience. GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 2 1 Introduction A piezometer is a purpose-built, non-pumping observation well that facilitates measurement of vertically averaged piezometric head of a single layer. The selected layer is an aquifer, though for some specific studies it may be thinner than an aquifer (for example for monitoring the vertical variation of the head in an aquifer). A piezometer taps only the selected layer, blinds the rest of drilled strata and finally isolates the selected layer from the rest of the drilled strata and ground by properly placed seals. The tapping is accomplished by a screen and a surrounding gravel pack in an unconsolidated formation, while the piezometer is left as a naked hole in consolidated formations. The water elevation in the piezometer represents the vertically averaged piezometric head of the tapped layer. Piezometers are also used for sampling groundwater from the tapped aquifer for water quality monitoring. One can immediately see the underlying difference between the criteria of efficiency/success in respect of a piezometer and a production well. The efficiency of a piezometer shall depend upon how closely its water level matches with the vertically averaged piezometric head of the aquifer under investigation. On the other hand, a production well yielding high discharge of good quality water at a technically/economically feasible drawdown is deemed to be successful, irrespective of what its water level represents. 1.1 Use of production well as piezometer An existing production well tapping the specified layer/aquifer may not be a good piezometer since it may also be tapping other water bearing layers/aquifers and hence its water elevation may not represent the desired piezometric head. Further, even if it taps only the specified layer/aquifer, it may not serve as a good piezometer due to the following reasons: • The tapped layer/aquifer may not be adequately isolated. • The recorded water elevation in the well may also comprise the residual drawdown from a preceding pumping spell. • Due to pumping, the water elevation in the well may not be representative of the regional piezometric elevation in its vicinity. 1.2 Use of piezometer as production well Although it may be tempting to use a piezometer to pump water, the practice has to be avoided as it may lead to corruption of the data emanating, due to the residual drawdown. GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 3 1.3 Data emanating A periodic monitoring of the water elevation in a piezometer provides discrete points on the hydrograph of the piezometric head of the tapped layer/aquifer. The high frequency monitoring permitted by the DWLRs shall lead to almost a continuous hydrograph. The concurrent water elevation data from a number of piezometers tapping the same layer/aquifer permits contouring of the spatial distribution of the piezometric head at discrete times. The hydrographs along with the contours shall permit a variety of computations to meet the following objectives: • Understanding of the groundwater system • Understanding of the groundwater regime of the aquifer under investigation • Estimation of groundwater resource including the rejected recharge • Estimation of aquifer parameters GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 4 2 Network design A piezometric network is required for estimating the average piezometric head in an area at discrete times. Such an averaging is required for estimating the groundwater storage. The network is also necessary to arrive at spatial distribution of the piezometric head at discrete times, for understanding the groundwater regime and also for the aquifer response modeling. The design of a piezometric network is accomplished in the following sequential steps: • Identification of the aquifer system • Evaluation of the existing network • Enhancement of the network comprising Macro-level planning and Micro-level planning 2.1 Identification of the aquifer system The first step towards design of the piezometric network for a specific area involves the identification of its aquifer system, comprising the following components: • Identification of the number of aquifers in the vertical section and the extent of hydraulic connections among the adjacent ones. • Identification of the lateral extension (that is, domain) of each aquifer • Variation of thickness of each aquifer in its identified domain The identification can be accomplished by studying the following data/documents as detailed in the following paragraphs: • Geological maps • Hydrogeological maps • Drilling information • Geophysical exploration Geological maps. The geological map of the area permits an understanding of the rock type, the nature of the consolidated or unconsolidated material and the distribution of structures The interpretation provided in the geological maps by lithostratigraphical units should lead to a broad understanding of the aquifer systems, groundwater flow systems and types and conditions of the boundaries as controlled by geology and structure. Geological maps should be read with a three-dimensional perception for inferring the geometry of the aquifers and boundary compositions of the aquifer system. Based upon such an understanding, a hydrogeologist can identify the aquifers and aquitards. Geological maps
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