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 are available as published maps of the Geological Survey of India, State Mines and Geology Departments, published/unpublished research papers/ reports, university theses etc.
Hydrogeological maps. These maps permit an understanding of the relationship between the groundwater and rock bodies. They comprise information on the different groundwater abstraction or monitoring structures, contours of the piezometric head/water level depth, direction of groundwater flow and variations in water quality. This information will enable a hydrogeologist to understand the extent of aquifers, together with geological, hydrogeological, meteorological and surface water features that are necessary for understanding the groundwater regime. Vertical
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 5 sections, such as borehole data, are incorporated in the hydrogeological maps to illustrate the relationship between aquifers and non-aquifers in relation to depth. Isopach maps reveal thickness of individual aquifers and give information on their regional extent.
Drilling Information. Drilling logs of all the exploratory/production wells and piezometers in and around the area of interest can assist in identifying different formations/aquifers occurring in the area. The drilled wells/piezometers should be located on a working base map, preferably a topographic map of 50,000 scale. Such a location map along with the drilling logs permit preparation of cross sections and three dimensional fence diagrams. These diagrams can assist a hydrogeologist in identifying various aquifers in the area and their domains in lateral and vertical dimensions. Superposition of the available water level/aquifer parameter data can lead to an understanding of the nature of the identified aquifers. Many of these wells need to be visited and information re-checked during the well inventory programme.
Geophysical exploration. Geophysical exploration permits an identification of various water bearing layers at a site and provides estimates of their thickness. The information gathered at a number of sites can lead to regional information on the extension of the aquifers, bedrock profile, nature of confining layers, etc. Electrical resistivity is the most popular geophysical exploration technique. While the resistivity survey work by itself is not complicated, the interpretation requires considerable training and experience.
2.2 Evaluation of the existing network
The scope of various water level monitoring networks established by the different agencies has largely been limited to gathering base line information on water levels and water quality. The information emanating from such networks has generally permitted conceptualization of the groundwater system and subsequently assessment of the resource.
Thus, prior to the proposed drilling of purpose built piezometers under the Hydrology Project, it is desirable to evaluate, aquifer wise, the existing networks. The evaluation should lead to identification of the data gaps (spatial and vertical).
The evaluation has to be based upon historical water level data available from these networks. The yardsticks for evaluation have to be designed keeping in mind the intended use of the basic water level data which, as already mentioned, could be one or more of the following:
• To estimate the average piezometric head (of a given aquifer) in a given area at a given time. • To estimate the spatial distribution of the piezometric head (of a given aquifer) in a given area at a given time. This shall essentially require multiple-interpolation.
The evaluation yardsticks in respect of these two possible data uses are discussed in the following paragraphs.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 6 2.2.1 Estimation of average piezometric head
The yardstick for evaluation shall be: How well does the data emanating from the existing network permit an estimation of the mean water elevation in a specified area? An existing network can be evaluated as per this yardstick by the Coefficient of variation method. This is a simple method and the necessary computations can be performed on a hand held calculator.
The method is based upon a study of the coefficient of variation of the water level, which is essentially a normalized spatial variation of the water level around its spatial mean. The expected error in the estimate of the mean water level is defined as a function of the coefficient of variation and the number of the gauge points. The step- wise procedure is as follows:
• Based upon the accuracy of other related data (e.g. pumpage, aquifer parameters, boundary conditions etc.) likely to be used along with the mean water level, select the permissible error in the estimate of the mean water level. (typical permissible error is 5 or 10%) • Retrieve the water level data observed at the gauging stations (falling in the specified area) at any discrete time (say pre-monsoon of the first year). Let the data be (hi, i =1,---N); N being the number of monitoring piezometers. • Compute the mean (µ) and standard deviation (σ)of the data as follows:
h µ = ∑ i N
1 ()h − µ 2 2 σ = ∑ i N −1
• The normalised spatial variation of the water level within the specified area and at the selected discrete time is indexed by the Coefficient of variation (CV ), and is computed as follows: σ CV = *100 µ • The percent expected error (p) in the estimate of the mean water level is computed as follows: CV p = N
• Repeat the above exercise for all discrete times at which data are available, and generate arrays of the coefficient of variation and the corresponding percent- expected error. • Study the generated array of the percent-expected error and evaluate the network.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 7 • If the errors are found to be excessive, pick up, from the generated arrays, the maximum encountered expected error and the corresponding coefficient of variation (CV). Stipulate an upper bound (P) on the percent expected error and compute the required number of the gauge points for restricting the percent expected error to P; in accordance with the following expression:
2 CV N = P 2.2.2 Interpolation
The yardstick for evaluation shall be: How well do the data emanating from the existing network permit interpolation of the water level at the ungauged points within a specified area?
Kriging can evaluate an existing network as per this yardstick. It is a stochastic interpolation technique based upon the theory of regionalized variables. Kriging, apart from yielding an interpolated value, also permits an estimation of its standard error. Thus, it can lead to delineation of the pockets (if any) within which the existing network does not ensure an accurate interpolation of the water level. The computations involved in kriging are enormous and can be performed only on a computer. The dedicated software for groundwater data processing to be developed and implemented under HP is expected to comprise the necessary software.
Kriging treats an attribute (water level in the present context) as a regionalized variable. A regionalized variable is a random function with a spatial continuity. The spatial continuity implies that the recorded values of the variable at close locations will be more similar than the values recorded at widely spaced locations. However, the continuity is considered to be too complex to be described by a deterministic function. The spatial continuity is expressed as a variogram. The same variogram is assumed to hold over the entire domain of the variable.
This uniqueness of the variogram is valid provided the variable is stationary over the entire domain. The stationality essentially implies that there is no well-defined trend in the spatial variation of the variable. Thus, the variable changes only locally without any regional trend.
Kriging at multiple points can provide the contours of the standard error. These contours permit an evaluation of the network of the observation points used for Kriging. Drilling of additional piezometers in the regions displaying high standard errors may be considered.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 8 2.3 Enhancement of the network
In case an existing network (in respect of a specific aquifer) is found to be inadequate, additional piezometers tapping that specific aquifer need to be provided. The first step towards planning of the enhancement shall comprise a macro-level planning, i.e., estimating the required number of additional piezometers and their location at a macro-level (say on a map of scale 50,000). The subsequent step shall involve pinpointing the sites for the additional piezometers on the ground, i.e., micro- level allocation.
2.3.1 Macro-level planning
Depending upon the intended use of the data from the network, the macro-level planning of the network enhancement can be accomplished in the following ways:
Coefficient of variation method: The method requires the user to specify the maximum permissible error in the estimate of the mean water level. Subsequently, based upon an analysis of the concurrent data from an existing network, the required number of the piezometers is computed. Thus, the additional number of the piezometers is computed.
The following procedure is adopted for locating the additional piezometers within the specified area.
• Employing the concurrent data from the existing network, draw contours of water level at a uniform interval. • Divide the entire area into zones, each zone representing an area falling between two successive contours. • Divide the required number of piezometers equally among all the zones. This will ensure a greater density of the piezometers in the regions of steeply sloping piezometric head and vice versa. • Count the number of existing piezometers in each zone and hence estimate zone-wise, the required number of additional piezometers. • Locate the additional piezometers in each zone in such a way that the piezometers (existing and additional) are uniformly distributed within the zone.
Kriging: As discussed earlier, kriging is a powerful tool for evaluating an existing network. It can also assist in the macro-level location of additional piezometers, in case the existing network is found to be inadequate. The steps involved shall be as follows:
1. Specify the level of permissible interpolation error. 2. Conduct kriging on the concurrent piezometric data from the existing network. This shall yield contours of piezometric head and of the interpolation error. 3. Study the error contours and hence identify the regions where the error is in excess of the specified permissible level. Additional piezometers are to be allocated to these regions. 4. Locate additional piezometers in the identified regions tentatively, generally ensuring that the increase in the network density is consistent with the error excess.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 9 5. Conduct kriging on the tentatively enhanced network and plot contours of the error. It may be noted that kriging permits generation of such contours even though the data from the newly introduced piezometers do not yet exist. 6. Study the modified error contours and check whether the error everywhere falls below the specified limit and the enhancement has not been over-done. An over enhanced network shall display interpolation errors far less than the prescribed limit. 7. Modify the network further, if necessary and go to step 5.
2.3.2 Micro-level planning
After having decided the location of the piezometer sites on the map, it is essential to pinpoint the site exactly on the ground. Certain micro-level deviations may be necessary to accommodate various hydrogeological and logistical considerations.
2.3.3 Hydrogeological considerations
These considerations originate from the primary expectation from a piezometer, i.e. it should record harmonized natural behaviour of ground water rather than local micro- trends. This can be ensured by keeping in mind the following:
• The site should show no impact of any external inputs such as from canal, tank, perennial river and irrigation return flows, except in special cases where the influence of these parameters on groundwater system is of interest. • The site should not fall within the radius of influence of a well, which is under pumping; but it should be capable of recording the effects of the pumping as a regional phenomenon. • The piezometric head/ water quality at the site should not be influenced by local recharge/pollutant sources.
2.3.4 Logistical considerations
There could be many general as well as area-specific logistical considerations such as:
• No other agency is considering constructing a piezometer tapping the same aquifer, in the vicinity. • The site is approachable by the rig and support vehicles. • Adequate space is available at the site for setting up drilling equipment, digging mud pit and draining the discharge, while the site should be clear of trees, overhead electric cables, under ground cables/ pipelines/ drainage lines etc. • The ownership of the site is clear and agreements have been made for drilling the piezometer and for continued monitoring. • The site should be safe from vandalism, as a costly DWLR will be installed. • The site should be neither too close nor too far off from the road.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 10 2.3.5 Documentation of the identified site
The finalised site needs to be documented as follows:
• Draw a rough sketch showing the identified site and important landmarks in the vicinity. The sketch should incorporate the north direction and the distance of the site from the landmarks. • Mark the site on the topo sheet. Record its longitude, latitude and the reduced level as read from the toposheet.
2.3.6 Geophysical exploration at the pinpointed site
Geophysical exploration at the pinpointed site can provide information on the nature of the subsurface lithological layers, and aquifer thickness at the chosen site. This information is useful in deciding upon the type of drilling rig to be employed, depth of drilling, casing depth, screen position, grout position, depth of surface casing etc.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 11 3 Piezometers in unconsolidated formations
Unconsolidated formations in peninsular India are largely localised to coastal tracts composed of beds of sand and clays, and sedimentary beds of Gondwana and Tertiary formations made of alternate layers of sandstone and shales. Piezometer construction in these areas is through rotary drilling.
3.1 Components of the piezometer
The main components of a piezometer in an unconsolidated formation are as described in the following paragraphs:
• Bore hole • Well assembly comprising blank casing pipe and screen • Gravel pack and cement seal • Sanitary seal comprising a surface concrete seal and a clay grout
Borehole. This is the primary component of a piezometer and acts as a host to the other components.
Well assembly. This is essentially the hardware of the piezometer and is accommodated in the borehole and also protrudes above the ground. Depending upon location of the aquifer in the vertical section, it may comprise one or more of the following parts:
Blank casing pipe: A blank casing pipe is provided to serve one or more of the following objectives:
• To prevent caving-in/sloughing of the drilled formation. • To prevent a hydraulic connection between the piezometer and the drilled formation other than the aquifer to be monitored. • A blank casing pipe connected to the lower end of the screen- to collect the fines entering into the screen. This part, known as bail plug/debris sump may be 1.5 m or longer, depending upon the expected entry of the fines. • A blank casing pipe protruding above the ground (say, by 0.6 m) and provided with a cover at the top- to permit an easy identification of the piezometer and to mark the measuring point. Screen: A screen provides a hydraulic connection between the piezometer and the aquifer to be monitored
• Gravel pack and seal. Gravel is provided in the annular space between the borehole and the well assembly around the screen and beyond, extending preferably over the entire thickness of the aquifer to be monitored. The gravel pack serves the following purposes:
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 12 • It inhibits the entry of the fines into the screen. • Due to its high hydraulic conductivity, it enhances the hydraulic connection between the piezometer and the aquifer (supplements the tapping provided by the screen) and thus, renders the water elevation in the piezometer more representative of the piezometric head of the aquifer (that is, creates a more efficient piezometer). The effective tapping provided by the combination of a short screen and a longer gravel pack shall nearly extend over the length of the gravel pack. Thus, a partially penetrating but fully gravel packed piezometer may behave like a fully penetrating piezometer.
A 50 cm thick cement seal is provided just above and just below the gravel pack to preempt any hydraulic connection between the piezometer and the overlying/ underlying formations, through the gravel pack and screen perforations.
Sanitary seal. A 50 cm thick concrete seal is provided at the ground surface to prevent the entry of surface water into the piezometer. The seal should be in the form of a cone around the casing to drain the water away from the well. The seal is underlain by a clay fill/packing for a more effective isolation of the aquifer to be monitored.
3.2 Pre-drilling design
The design of piezometers has essentially to be carried out in two stages, i.e. prior to the drilling and during drilling.
The features to be elaborated at the pre-drilling stage are design of well assembly and arrangement of materials at site. The design details are given in the following paragraphs.
Diameters. The water elevation in a piezometer is expected to fluctuate simultaneously with the vertically integrated piezometric head of the tapped layer at its interface with the piezometer. However, due to an inevitable storage effect, there is always a time lag between the fluctuation of the piezometric head and the response of the piezometer (that is, the corresponding fluctuation of the water elevation inside the piezometer). The lag is proportional to the square of the diameter. Thus, ideally a piezometer should have an infinitesimally small diameter. Further, drilling of a small diameter borehole is cheaper. However, the diameter must be large enough to satisfy the following logistical requirements:
• It must be large enough to permit monitoring of the water level inside the piezometer without any difficulty. • It must be large enough to accommodate the DWLR and also the pumping unit, if a water sample is to be pumped out for water quality monitoring. • It must permit a periodic removal of the accumulated sediment and trash.
A casing/screen diameter of 100 to 150 mm may be necessary to satisfy the above listed requirements. The drilled hole may have a diameter of 250 to 300 mm, to accommodate the recommended well assembly and the gravel pack. The thickness of the gravel pack must be at least 75 mm to ensure the expected performance.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 13 Screen length. The well screen should be long enough to ensure that the piezometer records the vertically integrated piezometric head of the investigated aquifer. Thus, there must be a perfect hydraulic connection between the piezometer and the aquifer over the entire aquifer thickness. Ideally, this requires a fully penetrating piezometer, that is, the screen provided over the entire thickness of the aquifer.
In case of thin aquifers, a fully penetrating piezometer may be provided. However, in case of thicker aquifers, a fully penetrating piezometer may not be economically feasible, and as such, a partially penetrating piezometer may have to be provided. But even a partially penetrating piezometer can provide an almost perfect hydraulic contact, if it is surrounded by a fully penetrating (that is, extending over the entire aquifer thickness) gravel pack of large enough thickness and hydraulic conductivity. The length of the screen, in such a case must be large enough to ensure a free inter- flow of water between the piezometer and the aquifer through the gravel pack. A screen length of two meters surrounded by a fully penetrating gravel pack may provide the necessary hydraulic contact and ensure the free inter-flow.
3.3 Installation
The installation involves several operations. Depending upon the drilling technique and the nature of the strata, some of these operations may be sequential and others may be overlapping. The operations may be classified as follows:
• Drilling of the borehole (including preparation of well log/ determination of the depth and thickness of the aquifer to be monitored) • Finalization of the design • Lowering of well assembly/gravel packing/sealing • Well development
3.3.1 Drilling of Borehole
In an unconsolidated formation, rotary drilling has to be adopted. Rotary drilling makes use of viscous bentonite mixed fluid as medium of drilling. The mud fluid acts as coolant to the rotating drilling bit as well as a medium for bringing out drill cuttings outside the borehole. Use of bentonite clay has been banned for water well drilling in many countries as they are not bio-degradable. Organic materials like guar gum are replacing bentonite clay as popular bio-degradable drilling fluid. The drilling fluid is circulated through the drill pipe, which is connected to the drill collar and to a square shaped kelly (tubular pipe). The drill bit is connected to the drill collar through a sub, which acts as a plumb to keep the borehole straight. The kelly fits into the rotary table groove through drive bushing, thus facilitating both rotary and up and down movements of the drill rod. The kelly in turn is connected to the swivel suspended by the travelling block on the rig mast and is linked to the mud pump for circulating mud inside the borehole.
3.3.2 Preparation of a well log
Samples of drill cuttings should be collected at intervals of 1.5 to 2 meters. Additional samples should be collected whenever there is a change in lithology. The change in
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 14 the lithology and hence the need for additional samples can be ascertained by monitoring the drilling speed.
In case of rotary drilling, there may be a considerable time lag between cutting of the formation and the appearance of the cuttings in the sump. Further, the time lag per unit depth of drilling may not remain constant during the drilling on account of inevitable variations in the mud density. This makes estimation of the time lag quite uncertain and it may not be possible to properly estimate the depth from which the sample cuttings were actually cut. To overcome this problem, the following sampling strategy is recommended. This strategy ensures collection of representative samples from the desired depths.
• Drill down to the sampling depth and halt further drilling. • Circulate the drilling fluid until all the transitional cuttings in the borehole are removed and cutting-free drilling fluid starts coming out from the borehole. • Drill through a depth (say about 25 centimeters) which may yield about one Kilogram of the cuttings. • Halt further drilling until all the material cut from this depth is removed from the borehole (by circulating fluid) and is collected in a sample catcher set below the delivery pipe of the surface casing. • Wash the sampled cuttings of the drilling mud and dry them. • Spread the cuttings over a piece of millimetre square graph paper and infer the grain size of the cuttings by a X10 magnifying lens. Also examine the cuttings for sorting, cementation, colour, composition etc. • Document the inferences • Store the cuttings in a duly labeled sample box. • Repeat the cycle for all sampling depths.
The above information shall permit the preparation of a litholog. The inferred litholog should be corroborated by geophysical well logging. The corroborated litholog shall provide the depth and thickness of the aquifer to be monitored.
3.3.3 Design of well assembly, gravel packing and sealing
Having identified the depth and thickness of the aquifer to be monitored, the necessary well assembly can be designed to satisfy the following requirements. These requirements, as described in the following paragraphs, depend upon the nature of the formation drilled and vertical location of the aquifer under investigation.
Unconfined aquifer. For monitoring the piezometric head of an unconfined aquifer, the piezometer essentially comprises a borehole drilled through the entire thickness of the aquifer, into the lower formation to accommodate a cement seal at its bottom. The well assembly, resting on the seal, comprises (starting from the bottom) a bail plug, a two meters long screen and finally a watertight casing pipe extending up to the ground surface and 0.6 m above.
The annular space between the bore hole and the well assembly is filled in as follows:
• A gravel pack from the bottom of the borehole till the highest position of the watertable
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 15 • A concrete seal at the top • A 2.5 meters long clay seal below the concrete seal
A typical well assembly is shown in Figure 1.
Figure 1 Piezometer for monitoring an unconfined aquifer
Confined/leaky-confined aquifer. For monitoring the piezometric head of a confined/leaky-confined aquifer, the piezometer essentially comprises a borehole drilled through the overlying formation and the entire thickness of the aquifer, into the lower formation to accommodate a cement seal at its bottom. The well assembly, resting on the seal, comprises (starting from the bottom) a bail plug, a two meters long screen and finally a watertight casing pipe extending up to the ground surface and a 0.6m above.
The annular space between the borehole and the well assembly is filled in as follows:
• A gravel pack from the bottom of the borehole till the top of the aquifer • A cement seal above the gravel pack • A concrete seal at the top • A 2.5 meters long clay seal below the concrete seal • Drill cuttings in the space between the clay seal and the cement seal
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 16 A typical well assembly is shown in Figure 2.
Sanitary seal Clay Clay seal Sand with shell
Laterite 100-150 mm dia well assembly
250-300mm dia Sand fine to medium borehole
Lignite
Back filling Sand fine to medium
Clay
Sand with shell
Clay
Cement seal Limestone
Sand with shell Gravel pack
Screen Coarse sand / gravel
Formation to be monitored Sand fine to medium Bail plug
Cement seal Clay
Figure 2 Piezometer for monitoring a confined/leaky unconfined unconsolidated formation
3.4 Design of gravel size and screen slot size
As already discussed, a screen and gravel pack have to be provided if the aquifer to be monitored is unconsolidated. The details of the sieve analysis are as follows.
Sieve analysis. For determining the screen size and nature of the gravel pack, it would be necessary to carry out a sieve analysis of the cuttings gathered from the aquifer. To carry out a sieve analysis, the washed and dried sample from the required depth is retrieved from the storage. The sample is mixed thoroughly and is heaped in a pile on a flat surface. The heap is flattened into a circular pile, which is divided in four quarters. A set of opposite quarters is removed and the remaining opposite quarters are thoroughly mixed and the resultant sample is used for performing the sieve analysis. The sample is weighed and passed through a standard set of IS sieves from No. 75 onwards. The coarsest sieve is placed at the
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 17 top and the finest at the bottom with a lid at the top and a closed pan at the bottom. The sample is placed on the top sieve and the lid is closed. The sieve system is shaken for a period of ten minutes. Weight of the material retained on each sieve and hence the percentage (by weight) of the sample passing through each sieve is determined. The percentage weight is plotted against the sieve size, on a logarithmic scale. The percentage weight represents the percentage of the material finer than the corresponding size. The size is usually represented as d with a suffix of the corresponding percent.
Design of slot size. The slot size of the screen is so designed that the aquifer material expected to be retained in the vicinity of the borehole, after the well development, does not enter into the piezometer. For example, assuming that only the fractions greater than or equal to the d60 of the aquifer material are to be retained, a slot size of d60 may be provided.
The expected efficiency of a well development in removing the fines is usually attributed to the uniformity coefficient (d60/d10) of the aquifer material; the higher the uniformity coefficient, the higher would the efficiency and vice versa. Thus, depending upon the uniformity coefficient and the extent of the expected well development, the usually recommended slot size for Natural Gravel Pack production wells (that is production wells without an artificial gravel pack) is d40 to d70 of the aquifer material. A higher slot size (d10 of the gravel) is usually permitted in case of the Artificial Gravel Pack (AGP) production wells.
The design criteria for piezometers (which are invariably AGP) should be more conservative since there is no real advantage in providing a larger slot size. (In case of production wells, a larger slot size may reduce the entry loss and hence may increase its yield.) Thus, the recommended slot size in case of piezometers is between d40 to d60 of the aquifer material; again with the same underlying principle, that is, a larger slot size within the range (say d60) in case the well development is known to be efficient and vice versa.
Design of gravel size. In case of piezometers, the main objective of providing a gravel pack is to improve upon the hydraulic connection between the aquifer and the piezometer. The other objective of inhibiting the entry of fines is relatively secondary due to two reasons. First, unlike production wells, large radial velocities responsible for the movement of the fines may not occur at all. Second, the slot size in the smaller range (see the preceding section) alone may be sufficient to inhibit the entry of the fines. Thus, the design criteria in case of piezometers need not be as rigorous as adopted for production wells.
The following criteria are recommended:
• The gravel should be as uniform as possible to avoid segregation during the placement. • The average size of the gravel should be 4 to 6 times the d50 size of the aquifer.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 18 3.5 Lowering of well assembly and gravel packing
After having designed the well assembly as indicated above, its sketch is prepared indicating the position of screen (against the identified zone) and the blank casing pipes. Arrange the assembly in the order of depth with the plugged debris sump at the bottom followed by the screen and blank pipes till the entire depth span is covered.
The borehole is sealed at its bottom by placing a 50 cm thick cement seal. The assembly is lowered after settling of the cement seal and the gravel is poured in the annular space between the borehole and the well assembly, till the required depth. The well is developed with a compressor so that the loosely placed gravel (if any) settles down. Thereafter, a sounding is conducted to determine the depth of the gravel. In case gravel has sunk below the required level, additional gravel is added to raise it to the design level. The gravel pack, so placed, is sealed at its top by a 0.5 meter thick cement seal. The annular space above the cement seal is back filled with drill cuttings followed by a 2.5 m long clay seal, and finally the top 0.5 m at the ground level and a sanitary seal is placed. The top of the casing is fitted with a threaded well cap to prevent any vandalism or filling.
3.6 Well development
The well development, in case of piezometers, is primarily aimed at ensuring an efficient hydraulic connection between the aquifer and the piezometer. Development of a piezometer drilled by rotary drilling is carried out after the completion of the drilling. The development in this case is very crucial since the drilling mud, which inevitably sticks to the borehole and invades into the aquifer may inhibit the necessary hydraulic connection between the aquifer and the borehole. The invasion of the drilling mud and thickness of the cake depends upon the hydraulic conductivity of the aquifer. The higher the hydraulic conductivity, the higher is the mud invasion and mud cake thickness. The development is thus aimed at removing the invaded /sticking mud and also the fines from the vicinity of the borehole. Under-developed piezometers will fail to provide the true information of the aquifer being monitored and the water level data emerging from such piezometers can lead to wrong conclusions.
As a part of the development, the mud cake around the screen should be dissolved using sodium tripolyphosphate. Sufficient volume of solution of sodium tripolyphosphate should be made and circulated to displace mud around the screen area as well as a portion of the casing for disaggregating the clays. The polyphosphate solution should be allowed to act for at least 24 to 36 hours. The solution should be circulated through the well screen that effectively acts on the mud cake. This should be followed by washing.
The development should be carried out through air compressor by alternatively surging and pumping with air. The air should be injected into the piezometer to lift the water. As the water level reaches the top of the casing, air supply should be shut off allowing the aerated water column to fall. Use of eductor lines is recommended when the static water level is deep.
High velocity jetting is another development technique that consists of a jetting tool fitted to the bottom of the drill string. The jetting tool should be lowered and washed
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 19 all along the screen length using fresh water. This should be followed by air lift. Careful jetting of the screened area is required. Jetting combined with air lift should be continued till pumped water is free from fine sand and bentonite, and the discharge from the piezometer stabilizes.
Development can also be done through back washing. In back washing, there is a reversal of flow through screen opening which agitates the sediments and leads to the removal of the finer fraction and rearrangement of the formation particles. As a part of back washing the water column should be alternatively lifted and allowed to fall back. The pump should initially be started at a reduced capacity and gradually increased to full capacity.
Mechanical surging needs to be carried out at times using surge blocks attached to drill rods. The surge block forces water into and out of the screen similar to a piston in a cylinder. The surging process at times forces fine material back into the screens and hence the fines should be removed before taking up surging.
Piezometers in consolidated formations
Consolidated formations occupy almost two – third of the country and a large part of peninsular India. The norms for design and drilling of piezometers in consolidated formations have to be different from the ones recommended for unconsolidated and semi-consolidated formations, due to vast differences in the modes of groundwater occurrence and flow and drilling technology. Most of the consolidated rocks have negligible primary porosity and it is only the secondary porosity, like fracturing and weathering, that provides the porosity and permeability necessary for the storage and flow of groundwater.
Groundwater yields are largely dependent upon the rock type. In granite, gneiss and khondalites highly productive groundwater zones are found in the vicinity of large lineaments, fractures and deep weathered areas. The lava flows are mostly horizontal and occasionally gently dipping and as such, groundwater occurrence is controlled by the water bearing properties of the flow units. In carbonate rocks like limestone, marble and dolomite, solution cavities serve as large repositories of groundwater. In all these rocks the drilling is usually carried out by the Down The Hole (DTH) drilling technique or a combination of DTH and rotory drilling.
3.7 Components of the piezometer
3.7.1 Unconfined aquifer
For monitoring the piezometric head of an unconfined aquifer, the piezometer essentially comprises a cased borehole of diameter 125 to 175 mm, drilled through the top collapsible/weathered rock zone, overlying the consolidated unconfined formation to be monitored. The casing protrudes above the ground by about 0.5 m. The cased borehole is underlain by an uncased borehole of smaller diameter (say 100 to 150 mm), drilled through the formation to be tapped. In case, this formation is underlain by a massive rock formation, a debris sump may be drilled into the massive rock. In case the formation to be tapped is underlain by another aquifer, the drilling should be discontinued at the bottom of the formation to be tapped and the borehole needs to be sealed with cement at its lower end. A sanitary seal, underlain by a clay seal is provided at the top. The
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 20 remaining annular space is filled in by drill cuttings. A typical well assembly is shown in Figure 3.
3.7.2 Confined/leaky-confined aquifer
For monitoring the piezometric head of a deep aquifer, the piezometer essentially comprises a cased borehole of diameter 125 to 175 mm, drilled through the top collapsible/weathered rock zone and other formation(s), overlying the consolidated deep formation to be monitored. Thus, for monitoring the piezometric head of a deep aquifer (or of a system of interconnected aquifers) lying just below the unconfined aquifer, the above drilling should be carried out till the bottom of the unconfined aquifer. The casing protrudes above the ground by about one meter. The cased borehole is underlain by an uncased borehole of smaller diameter (say 100 to 150 mm), drilled through the formation to be tapped. A concrete seal is provided at the top. A typical well assembly is shown in Figure 4.
Sanitary Seal
Top Soil GL
Highly Weathered Granite Cased Portion Drill cuttings Weathered Granite
Fractured & hard Granite
Formation to be monitored
Hard Massive Granite
Figure 3 Piezometer design in consolidated granitic formation for monitoring fractured unconfined or semi confined aquifers
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 21 Sanitary seal Top Soil
125 PVC casing Weathered Basalt
Drill cuttings
Weathered/fractured basalt
Hard massive belt
Cement Seal
Vesicular Basalt Formation to be monitored
Massive Basalt
Figure 4 Piezometer for monitoring a confined/leaky confined Consolidated formations
3.8 Installation
The installation involves drilling of the borehole, lowering of the blank casing pipe and well development.
3.8.1 Drilling of borehole
The consolidated formations are likely to be drilled by the DTH (down the hole) drilling technique. DTH drilling primarily involves passing compressed air through drill pipes connected to a hammer, which rapidly strikes the rock and gives crushing blows while rotating at a slow speed of 10 to 30 rpm. The hammer is made of Tungsten Carbide, a metal highly resistant to abrasion. DTH drilling technology has been used even in the remotest hard rock areas. DTH drilling is very fast and completion of one piezometer of 100 m depth takes only 12-18 hours. The drilling being very fast, supervision of DTH drilling becomes very important. The site hydrogeologist has to ensure that the compressor is in good condition to deliver the required air pressure and that the drill bit is of the required diameter.
The initial drilling should be carried out using a large diameter (say 125 mm) overburden bit. A casing, say of a diameter 112.5 mm, is lowered into the large- diameter borehole. The depth of drilling using the large diameter bit shall depend upon the position of the formation to be monitored. For monitoring the unconfined aquifer, the collapsible zone and weathered rock have to be drilled through. For monitoring a deeper aquifer, the initial drilling must continue till the top of the aquifer. Thus, for monitoring the piezometric head of a deep aquifer (or of a system of interconnected aquifers) lying just below the unconfined aquifer, the initial drilling should be carried out till the bottom of the unconfined aquifer.
On achieving the desired depth, the drilling should be stopped, the hole cleaned, the drill rods pulled out and the casing installed. The casing should pass through the entire drilled strata and be firmly positioned. The casing joints, if any, should be made leak proof, preferably by using threaded joints.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 22 After the installation of the casing, the drilling should continue using a small diameter (say 100 mm) drill bit. The drilling should extend over the entire thickness of the formation to be monitored. It should be ensured that the hole is constantly cleaned of the drill cuttings. During the change of the drill rod as well as when a water bearing zone is met, the well should be adequately developed and the discharge measured using a V notch. The drill cuttings should be collected and studied continuously. At the end of the drilling to the desired depth, the well should be cleaned for at least two hours. The cleaning should lead to de-clogging of all the fractures drilled through, and removal of all fines and cuttings.
3.8.2 Preparation of a litholog
A description of the lithology encountered at the drilling site is obtained by systematically collecting the drill cuttings during the drilling operations and recording the observations in the prescribed form. The cuttings should be sampled at one meter frequency, or whenever there is a change in lithology. An examination of the sampled cuttings can provide information on characteristics of the formations including rock type, color, texture and shape. The fact that the cuttings obtained are due to the action of the drill bit, should be kept in mind while examining the sampled cuttings. Further, the depths of the formations as revealed by the cuttings may not always be accurate - though they can be generally relied upon. The drill cuttings have to be classified on the basis of megascopic observations using hand lens, both for texture and mineral constituents. The description should identify the rock, colour, grain size, shape, fossils, trace minerals, etc. The drill cuttings should be dried, packed in polythene bags, marked with well number and depth interval, date and time. The samples should be stored in a box with numbered compartments. A correct procedure for collection and storage of drill cuttings ensures good correlation between the drillers log, VES interpretation, downhole logging and samples collected. The recorded drilling data should include the following:
• A drill log (time taken for drilling each meter of the drilled depth) • A description of drill action (such as nature of drilling noise and motion of the rig) • Depths at which moisture is struck • Depth at which water flows • Depths at which discharge increased • Colour, pH and EC of the water
3.8.3 Development of the well
Air drilling causes some plugging of fractures and crevices in hard rocks. Any material that clogs the openings in rock aquifer should be removed by development for realising the full information of the aquifer. Cleaning and development of the well in case of DTH drilling goes simultaneously with the drilling operations. At each stage or change of drill rod, cleaning and flushing of the well is carried out, and as a result the well is free of all finer chips and cuttings. At the end of the targeted drilling depth a final development should be done by running the compressor of the rig till the water is free of cuttings and the water is clear. The development can be carried out using eductor pipes if the well is very deep.
Development by air-lifting should be done at the change of each drilling rod, as well as after completion of the drilling. Jetting should be done where the depth of drilling is large, the discharge is low and the drilling speed is very high.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 23 4 Post-drilling activities
4.1 Recovery test
As indicated above, the development of the drilled piezometer shall involve a spell of pumping. The pumping shall inevitably be followed by a recovery of the water level. If technically feasible, a systematic recovery test should be performed. The test can permit an estimation of the transmissivity of the tapped aquifer. The test shall include measurement of the pumping discharge before closing the pumpage. The post- pumpage monitoring shall involve recording of the depth to water levels at pre- selected discrete times (after closure of the pumpage) till constant readings are obtained. The selected discrete times should be logarithmic (say, 1, 3, 5, 10, 30, 50, 100, 300 etc. minutes). Assuming as reference level the depth to static water level, the residual draw downs at the selected times can be computed. The evolved data of residual drawdown vs. time can be subjected to a standard recovery analysis to estimate the transmissivity.
4.2 Check for hydraulic connection
Following the well development, the adequacy of the hydraulic connection between the aquifer and the piezometer must be checked. The test involves filling up of the piezometer tube with water and recording the subsequent residual water levels at different discrete times. The residual water levels expected at the same discrete times under no-clogging conditions may be estimated using Theis equation, assigning the known orders of magnitude of the aquifer parameters. The respective theoretical and observed residual water levels are compared. In case the latter (observed residual water levels) are found to be significantly and consistently higher, an inadequate hydraulic connection may be inferred, and another spell of well development should be taken up.
After completion of the development of the well, it is important to record the depth to water level at rest, as the first initial reference water level of the piezometer.
4.3 Sampling for water quality
Water samples should be collected from the piezometer after completion of the drilling and development of the well, when the water is clear and free of sediments and drill cuttings. Proper sampling procedures should be followed. Samples should be collected in duplicate in 1 litre plastic containers, properly labelled and numbered. The relevant details like the date and time of collection, status of the well at the time of collection, method of sampling and the results of field measurements, if any, should be duly recorded. The time dependent parameters such as pH, EC, Redox potential, dissolved oxygen, carbon dioxide, soluble (ferrous) iron and sulphide will be altered if the sample is stored for long periods. Therefore, it is advisable to carry out these analyses in the field itself or immediately after bringing the samples to the laboratory. The laboratory must be intimated in advance of the proposed sampling from the piezometer, so that there is no inordinate time delay in analysis after bringing the sample to the laboratory.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 24 4.4 Survey
A measuring point is to be marked on the upper edge of the casing, after removing the cap. The reduced level (generally above MSL) of the measuring point should be determined by level survey work, commencing from a bench mark. Further, a detailed site plan should be prepared, say by plane-table survey. The site plan should show the piezometer, other prominent points/landmarks in the vicinity along with their distances from the piezometer, and the geographical directions. The site plan should be elaborate enough to permit an easy location of the piezometric site.
4.5 Aquifer tests
It is advisable to carry out a step-drawdown test and a pumping test after completion of the drilling and development.
4.5.1 Step-drawdown test
A step-drawdown test should be performed, primarily for ascertaining the hydraulic connection between the aquifer and the piezometer. The test involves monitoring of the steady (or near-steady) state drawdown in the piezometer, in response to a few successively increasing levels (steps) of the pumping discharge (Q). Thus, every pumping discharge level has to be sustained for a period long enough to reach steady (or near-steady) state, that is, till the drawdown in the well ceases to increase any further. The steady state drawdown(s) inside the piezometer can be expressed in terms of the following general expression.
s = B.Q + C.Q2
Where B.Q is approximately the formation loss, C.Q2 is approximately the loss of head as the water enters into the well, B is the formation loss coefficient and C is the entry loss parameter. The analysis of the discharge vs. drawdown data, say by Bierschenk’s method, permits estimation of B and C. The method essentially involves plotting s/Q against Q and passing the best straight line through the plotted points. The intercept and slope of the straight line provide estimates of B and C respectively. In case the estimate of C is less than 0.5 min2/m5, a good well development (and hence a good hydraulic connection) may be inferred.
4.5.2 Pumping test
A pumping test with a constant discharge may be performed for estimating the parameters of the tapped aquifer in the vicinity of the drilling site. The test involves monitoring of the time variation of drawdown in one or more observation wells in response to a pumping at a known discharge, from the piezometer. The discharge is also measured. The observation wells must be in the vicinity of the piezometer and must be tapping only the aquifer under investigation. If no such observation well is available, the drawdown may be monitored in the piezometer itself. This practice, however, is known to lead to an underestimation of transmissivity because of the non-linearity of the flow in the vicinity of the pumping well. The underestimation may also occur when the piezometer is partially penetrating. An analysis of the drawdown
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 25 data emanating from the test, as a function of time and distance, can provide estimates of the aquifer parameters.
4.6 Documentation of information
The whole history of the activities at the site along with the collected data should be documented in a single file. The file should include borehole diameter, depth, reduced level of the measuring point, lithology, complete details of the well assembly, complete details of the gravel pack (if provided), development discharge, static water level, step-drawdown test data/results, pumping test data/results, water quality results, details of the installation of Digital Water Level Recorder (if any), and grain size distributions of the cuttings from various depths. The documentation should be carried out in accordance with the format stipulated in the Data Entry Software provided under the Hydrology Project (refer Annexure I). Such documentation of information in a single file shall greatly facilitate computerization of the data. Copies of this file should be made available to the data center as well as to the libraries of the regional/central head quarter.
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 26 Annexure – 1 Drilling data collection formats
Part A Unconsolidated formations
District
Well No
Taluk
Block
Village
Hamlet
Latitude °"' Location Details
Longitude °"'
Toposheet No. ( 1:50,000 )
Survey No.
Owner
Well use
River basin
Sub-basin
Minor basin
Rock formation Regional Geomorphology
Select: Denudational hill / Pediment / Buried pediment / Valley / Laterite / Upland / Piedmont / Alluvial plain / Flood Plain / Deltaic plain Local morphology
Command area Yes / No Name of command
Surface water influence on local groundwater High / Marginal / Low / Nil
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 27 Drilling started on Drilling completed on
Drilling agency
Drilling rig type Select: Rotary / Reverse rotory / Diamond cord / Cable tool / DTH / Augur drilling / Hand drilling / Jetted / Calyx Drilling mud
Drilled depth . m
Well completed depth . m
Porous zones
From To Aquifer type
Pilot Hole / Dia From To Reamed Hole
Sieve Analysis Results
d10= d50= d60=
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 28 Casing details
Casing type Dia From To
Screen details
Screen type Dia Slot type Slot size Open area From To
Development method Select: Bailing method / Mechanical surging / Air lifting / Backwashing / High velocity jetting / Over-pumping / Chemicals / Explosives Disinfection / Block Surging Geophysical logging done Yes / No
Geophysical logging details
Water level details
Static water level Height of MP Reduced level
Gravel pack/Grout/Seal details
Gravel Pack / From To Remarks Grout / Seal
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 29 Part B Consolidated formations
District
Well No.
Taluk
Block
Village
Hamlet
Latitude °"' Location Details
Longitude °"'
Toposheet No. ( 1:50,000 )
Survey No.
Owner
Well use
River basin
Sub-basin
Minor basin
Rock formation Regional Geomorphology
Select: Denudational hill / Pediment / Buried pediment / Valley / Laterite Upland / Piedmont / Alluvial plain / Flood Plain / Deltaic plain Local morphology
Command area Yes / No Name of command
Surface water influence on local groundwater High / Marginal / Low / Nil
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 30 Drilling started on
Drilling completed on
Drilling agency
Drilling rig type Select: Rotary / Reverse rotary / Diamond Core / Cable tool / DTH / Augur Driling / Hand drilling / Jetted / Calyx Drilled depth . m Well completed depth . m Water bearing zones
From To Discharge
Casing details
Casing type Dia. From To
Development method
Select: Bailing method / Mechanical surging / Air lifting / Backwashing / High velocity jetting / Over pumping / Chemicals / Explosives disinfection / Block surging Geophysical logging done Yes / No
Geophysical logging details
Water level information
Static water level Height of MP Reduced level (MSL)
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 31 Lithological Descriptions
Depth to Rock type Colour Texture Shape Discharge
GUIDELINES FOR IMPLEMENTATION OF PIEZOMETERS 32