Field experiment of artificial recharge through a well with ... - CiteSeerX

Engineering Geology 50 (1998) 187–201

Field experiment of artificial recharge through a well with reference to land subsidence control N. Phien-wej, P.H. Giao *, P. Nutalaya School of Civil Engineering, Asian Institute of Technology, P.O. Box 4, Khlong Luang, Pathum Thani 12120, Thailand Received 10 August 1997; received in revised form 13 January 1998; accepted 13 January 1998

Abstract Land subsidence due to groundwater overdraft is one of the most urgent environmental dilemmas facing Bangkok Metropolis and its vicinities. Prolonged flooding, salt water encroachment, groundwater quality deterioration, damage to building foundations, roads, bridges etc. are some of the adverse consequences of the groundwater over-exploitation and the resulting land subsidence. Because the control of groundwater pumping within a safe limit is not likely to be achieved in the near future due to the poor town planning and rapid economic growth of the city, artificial recharge has been proposed as one supplemental means to help restore the declined piezometric levels and mitigate land subsidence. As the first step of recharging the Bangkok aquifer system, a field experiment aimed at investigating particular features of recharge into its uppermost aquifer, that is, the First Sand Layer, has been recently performed. The recharge operation was successfully carried out from October 1993 to June 1994, and some rebound in the recharged aquifer was recorded. The experiment provided useful guidance with reference to setting up of a field test, data monitoring and analysis which can be applied to other sites with similar conditions. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Artificial recharge; Field experiment; Groundwater; Land subsidence

1. Introduction Bangkok is situated ca 25 km from the Gulf of Thailand ( Fig. 1), on the Chao Phraya River flood plain, which is a part of the Lower Central Plain. The Bangkok Metropolitan area is ca 1500 km2 and has a population of 8 million. The area is underlain by a nine-aquifer system extending to depths of >500 m (Fig. 2). Groundwater extraction for domestic and industrial uses at a total rate of >1 million m3 day−1, has caused a drastic * Corresponding author. Fax: 66 2 524 5544; e-mail: [email protected]

decline in the piezometric levels in the exploited aquifers, that is, Phra Pradaeng, Nakhon Luang and Nonthaburi, as well as in the adjacent aquitards and aquifers. Consequently, thousands of square kilometers are affected by land subsidence. For the last 20 years the problem of land subsidence in Bangkok has been continuously investigated and artificial recharge has been proposed as one supplemental means for its mitigation (AIT, 1981). Well recharge has long been practised for water supply, waste water disposal etc. But mitigation of land subsidence, generally speaking, still remains more of a theoretical concept than a practical

0013-7952/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 9 8 ) 0 0 01 6 - 7

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Fig. 1. Location and land subsidence of Bangkok and its vicinities.

Fig. 2. Sketch of the Bangkok multiaquifer system.

application to be adopted on large scale. This is due to the high cost and technical nature of the recharge operation as well as still-to-be elucidated recharge hydraulics. A number of laboratory and field experiments have been carried out in many countries to investigate artificial recharge through wells, some of which are briefly mentioned here. Laboratory experimental results by Rahman et al.

(1969) obtained by Sakthivadivel and Enstein (1970) and Bichara (1986) have been important contributions to the study of spatial and temporal development of the progressive clogging process around a recharge well due to suspended solids. The field experiment results by Rehbun and Schwarz (1968) are corroborating evidence of clogging by suspended solids. Vechioli (1970) investigated bacterial clogging and proposed remedial measures. The first recharge experiment in Bangkok was conducted in 1972 (Ramnarong, 1989) and involved injecting storm water into the Bangkok aquifer. In Ramnarong’s experiment, since the maximum injection pressure was not taken into account in the well design, during one of the recharge tests some cracks formed ca 10 m away from the recharge well and through which water was pushed upward. The work by Monkhouse and Philips (1978) is the first to provide guidance on design and operation of a recharge well, in which clogging is incorporated as a design parameter. Huisman and Olsthoorn (1983) gave a concise presentation on recharge well hydraulics, emphasizing the different types of clogging and with attention paid to cracking due to injection pressure. Despite such practical and comprehensive guidance on the setup, performance and data analysis for a field recharge test as well as a quantitative assessment of clogging around a recharge well based on field test data remain important topics to be resolved. Bangkok is, perhaps, one of very few cities in the world for which artificial recharge is seriously considered as a means of controlling land subsidence. This is due to the fact that abandoning groundwater usage is not envisaged in the near future since the inadequate surface water supply has not caught up with the rapid economic growth of the city. As a first and indispensable step, a field recharge experiment was conducted by the Asian Institute of Technology (AIT ) during 1993–1995.

2. Experimental artificial recharge through a well The recharge experiment conducted at AIT was aimed at investigating particular aspects of artificial recharge through a well into the uppermost aquifer of the Bangkok multiaquifer system, that is, the

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N. Phien-wej et al. / Engineering Geology 50 (1998) 187–201 Table 1 Characteristics of water from the recharge reservoir and those of groundwater from the First Sand aquifer Parameter

Recharge reservoir

First Sand aquifer

Physical Turbidity (NTU ) Specific conductivity (mhos cm−1) pH Total solids (mg l−1) Suspended solids (mg l−1)

1.2–3.5 2409–2471 7.6–8.1 1570 1–6

9.2 11 190 7.3 6370 56–86

Chemical DO (mg l−1) Alkalinity (mg l−1 as CaCO ) 3 Total hardness (mg l−1 as CaCO ) 3 Calcium hardness (mg l−1 as CaCO ) 3 Chloride (mg l−1 as Cl ) Sulphate (mg l−1 as SO ) 4 Silica (mg l−1 as SiO ) 2 Total iron (mg l−1)

7.6–8.6 50–64 308–326 47–61 465–474 — — —

— 1000 1380 420 2650 748 31 1.0

Bacteriological Total coliform (MPN per 100 ml ) Faecal coliform (MPN per 100 ml )

49–510 2

— —

Upper Bangkok aquifer (the First Sand aquifer). Another goal was to generalize these results in deep well recharging for larger applications. 2.1. Recharge site The experimental site is located on the AIT campus, 42 km north of Bangkok. The test location and the experimental layout are shown in Fig. 4. The recharge site is located ca 100 m away from a large man-made reservoir with a volume of ca 0.8 million m3 which was used as the source of recharge water. The reservoir is ca 12 m deep and has a catchment area of ca 10 ha, connected with three inlets which allow surface run-off to flow in during the rainy season. These inlets are connected to the a tributary of the Chao Phraya River during the high flood period but not during the dry season. The physical, chemical and bacteriological characteristics of the reservoir can be seen in Table 1. The Bangkok area is covered by a marine clay of the Holocen age, deposited on an erosion surface of an older stiff clay; altogether they constitute the so-called Bangkok clay, which is underlain by a complex sequence of Quarternary and Tertiary deltaic sediments. The First Sand aquifer is immedi-

ately below the Bangkok clay; at the recharge site it is located at depth intervals of 16–28 m. The upper portion of the aquifer is fine sand and from 19 m downwards it is essentially gravely sand with sizes ranging from 2.5 to 5 mm. The aquifer is a confined type, with a piezometric head ca 6 m below the ground surface. Groundwater from this aquifer is not exploited due to its high salt content with very high concentrations in dissolved solids. The chemical characteristics of the groundwater from the First Sand layer are presented in Table 1. Inside the AIT campus there are several deep groundwater wells drilled into the Nakhon Luang aquifer up to 200 m deep. Their interaction with the Upper Bangkok aquifer is not understood, but the subsidence caused by deep well pumping may contribute to the compression of the surficial Bangkok clay and differential settlements of buildings as evidenced in many parts of the AIT campus (see Fig. 3). 2.2. Experimental setup A well recharge experiment has five main components: a recharge well; recharge-discharge pipe lines; piezometers to monitor static pore pressure profile versus depth; piezometers to monitor head

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Fig. 3. Land subsidence induced differential settlements of buildings on the AIT campus.

changes in the recharged aquifer; and compression indicators to monitor ground movement ( Fig. 4). 2.2.1. Recharge well A recharge well is essentially an abstraction well in reverse. However, besides the basic design of a

water well one has to take into account the specific features of a recharge well such as boiling, clogging due to quality of recharge water and the method of injection, that is, free flow or under pressure. The design of a recharge well, therefore, may be considered as consisting of three steps:

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Fig. 4. Setup of AIT recharge experiment (not to scale).

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(1) processing pre-design information and modelling possible responses of the injected aquifer to recharging; (2) basic water well design; and (3) specific recharge well design. Using the steady solutions of recharge well the flow rates and permeability were calculated (Fig. 5). Based on the initially estimated permeability of the First Sand aquifer, that is, k=10 m day−1, and its piezometric head ca 6 m below the ground surface, a value of 250 m3 day−1 was suggested as the optimal recharge rate for the free flowing injection [see Fig. 5(c)]. The back-pumping rate for well cleaning is usually double the recharge rate. The structure of the AIT recharge well is shown in Fig. 6. 2.2.2. Other Components A recharge pipe line 115 m long was installed to take water from the reservoir to the well with the intake point being located 30 m off the lake shore

Fig. 5. Steady state responses of a homogeneous aquifer with reference to a recharge well design for the Upper Bangkok aquifer.

and 2 m below the lake surface level. A discharge pipe line of 8 m length was installed to discharge water from the well into the AIT drainage canal. The recharge pump, installed outside the recharge well, was a centrifugal type whereas the discharge pump installed inside the recharge well was a vertical turbine type. Two groups of standpipe piezometers were installed to monitor the piezometric pressure profile down to 40 m, that is, the UG4 group next to the well and the UG3 group far from the well ( Fig. 4). The installation is depicted in Fig. 7(a). Head changes in the recharged aquifer during a test were monitored by means of a line of standpipe piezometers in the N–S direction [Figs. 4 and 7(b)]. For recording the ground movement, a group of compression indicators were installed 5 m from the recharge well ( Fig. 8). Three types of compression indicators: surface; auger; and cone were installed at depths of 0.3, 2.0, 6.0, 11.0 and 19.5 m in soft clay, stiff clay and the First Sand layers. 2.2.3. Performance There were three main stages in the experiment: (1) the preparatory stage from February to September 1993 for the experimental setup; (2) the initial testing stage from October through December 1993, during which the initial pumping and recharging tests at different rates were carried out to check the experimental setup, the recharge well efficiency, manifestations of clogging, to investigate the optimal recharging rate and to establish a suitable field work schedule etc. Based on these results, a subsequent program of recharge tests was begun with longer recharging times and larger volumes of water being recharged; (3) second testing stage from March through June 1994, during which recharging testing was carried, mostly at a rate of 250 m3 day−1. A notable feature of this stage was the extensive program of water sample collection and analysis at different time intervals during the recharging process to investigate possible clogging effects. Soil stratigraphy and properties at the recharge site were determined from a borehole drilled at 2.5 m from the well to a depth of 22 m. The samples were tested to determine the following soil parameters: unit weight (c); water content (w);

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Fig. 6. AIT recharge well structure.

plastic and liquid limit (PL and LL); undrained shear strength (S ); effective and maximum past u pressure; and compression ratio (CR). The soil profile and the corresponding geotechnical properties are shown in Fig. 9. The steps of the recharge experiment were as follows. 2.2.3.1. Pumping and recharging tests. A pumping or discharge test consists of pumping water from an aquifer through a well. In a recharging test, instead of being pumped out of the well, water from an exterior source is injected into the aquifer through the well. Discharge and recharge are not reversible processes. Various clogging

effects give rise to this difference. More than 50 pumping and recharge tests were performed. Some of the pumping and recharging test curves are shown in Fig. 10 and depict the differences between a pumping test and a recharging test as well as the effects of clogging on the recharge well operation. The well curves in the recharge test have a specific feature of continuous head increase while in a pumping test this levels off after a while. 2.2.3.2. Piezometric pore pressure monitoring. Piezometric pressure profiles were monitored from August 1993 until the end of December 1994. These are shown in Fig. 11 compared with a pore

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Fig. 7. Installation of monitoring piezometers and soil profile at the test site. (a) UG3 and UG4 piezometers groups to record pore pressure profile versus depth. (b) Line of piezometers to monitor head changes in radial distance from the recharge well.

pressure profile measured in 1975 (Premchit, 1978). Some important observations can be drawn from these results: (1) the water table at the test site fluctuates from 1.0 m below ground surface in the rainy season to 1.5 m in the dry season; (2) there was almost no pore pressure deficit in the soft clay layer. The decline in pore pressure commenced, first, in a thin sand layer at 9.0–10.0 m, continued in the stiff clay from 10.0–16.0 m and was clearly manifested in the First Sand layer with a constant decline of 50 kPa or 5.0 m of piezometric head. The deficit of pore pressure in the AIT soil profile increased at an annual rate of 2 kPa or 0.2 m of piezometric head in the First Sand layer. This

Fig. 8. Leveling network and installation of compression indicators. (a) Leveling network plan view, not to scale. (b) Section of compression indicators.

decline in pore pressure justifies the injection into the Upper Bangkok aquifer. 2.2.3.3. Monitoring of ground response. Ground movements in response to the pumping and recharging were monitored. During the test period daily readings were made with reference to a benchmark located 160 m from the recharge well (see Fig. 4). Weekly readings were taken from the benchmark to a deep well (drilled into Nakhon Luang aquifer) 700 m from the recharge well. The monitoring continued until the end of December 1994. Ground movement of each separate soil layer, that is, weathered clay, soft clay, stiff clay and the recharged aquifer was calculated and presented in Fig. 12 from where it can be seen that during the first and second stages of testing the elastic rebound of the recharged aquifer are clearly manifested. However, the rebounds of the overlaying clay layers were less evident because either the

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Fig. 9. Geotechnical properties of the subsoil at the AIT campus.

Fig. 10. Pumping and recharge test curves.

pore pressure change there has yet to be induced or the injected water volume was still not high enough. The scattering of ground movement data

is likely to be due to the alternative testing of discharge and recharge as well the seasonal change in the piezometric heads of the soil layers.

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Fig. 11. Pore pressure profile up to 40 m depth, AIT recharge site.

curves that can be analysed based on commonly used techniques such as Theis’s, Jacob’s, Thiem’s or Hantush–Bierschen’s to determine the aquifer hydraulic parameters. In traditional pumping test analyses, only the head curves monitored some distance from the well are used, while the well curves are not often analysed. But, in the case of a recharge experiment, it is recommended to make use of well curves as their analysis could give useful information on the properties of the clogging zone around the well as well as the specific capacity of the recharge well ( Fig. 13). In the actual experiment, >150 analyses were made for curves monitored at the well, 2.5 and 10 m away from the well. It was found out that for the zone furthest from the well the permeability and storage coefficient of the First Sand aquifer are ca 30 m day−1 and 10−3, respectively. For the zone nearest the well the permeability was lower and gradually reduced to a value k=6 m day−1 at the end of the experiment. 3.2. Clogging analysis

2.2.3.4. Water sampling and chemical analysis. Water samples from the reservoir and aquifer were collected prior to, during and after the recharge test. In the Second Testing Stage, 134 water samples were collected during recharging periods at different distances from the well and at different time intervals after recharging. They were then analysed to determine the pH, conductivity, total hardness as CaCO , turbidity, total sus3 pended solids ( TSS), total dissolved solids ( TDS), total solids ( TS), ionic concentrations in Na, Ca, Mg, K. The main purpose of the water sampling and analysis is to investigate the possible chemical causes of clogging of the recharge well.

3. Analyses of test data 3.1. Aquifer hydraulic parameter analysis Recharging was carried out alternatively by pumping (Fig. 13). This was done for developing the well after drilling, for well cleaning after a recharge run etc. as can be seen in the experiment schedule. Thus, there are always some pumping

This type of analysis is specific for recharge testing. Clogging analysis consisted of quantitatively identifying and evaluating each clogging type based on the field recharge test data. 3.2.1. Clogging due to air entrainment This type of clogging can be easily recognized by an abrupt increase of head rise in the first 5–10 min after the start of recharging [Fig. 14(a)]. 3.2.2. Clogging due to chemical incompatibility between the recharge water and the native groundwater This type of clogging can be asserted through two parameters, namely, the sodium adsorption ratio (SAR) and the sodium percentage (SP), which have been proposed from irrigation practice ( Krone, 1970) for characterizing the clogging potential of a water mixture. Problems usually arise when the Na concentration is high. For recharged water if SAR>3 and SP>50% there is clogging potential. In the actual recharge experiment, the SAR curves usually showed values >5, and all the SP curves showed values >65%

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Fig. 12. Ground movement of individual soil layers.

[Fig. 14(c)], indicating that clogging had occurred during the recharging period when the lake water was mixed with the native groundwater of the First Sand aquifer.

(3) a high concentration of suspended solids of water samples collected near the well by backpumping immediately after the recharging has stopped [Fig. 14(d )].

3.2.3. Clogging due to suspended solids This is the most common and one of the most dangerous clogging types (Bichara, 1986), it can be recognized by: (1) a continuous head rise in the well; (2) a decrease in the specific recharge rate with time or volume of injected water;

3.2.4. Quantification of well clogging effects 3.2.4.1. Recharge clogging factor (RCF). To estimate the permeability of the clogging zone around the well a new parameter called recharge clogging factor (RCF ) is introduced. The concept of RCF is explained in Fig. 15. RCF can be used

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to detect the time-dependent reduction of permeability of the clogging zone and to simulate recharge well curves. The difficulties in simulating a recharge well curve lie in the fact that this curve is continuously increased inside the well due to a continuously decreasing permeability of the clogged zone around the well. An empirical relationship of RCF deduced from the test data as depicted in Fig. 15 can represent the time-dependent change of permeability of the clogged zone and by incoporating it into a FEM program the particular feature of continuously increasing head of a well curve could be successfully simulated (Giao, 1997).

3.2.4.2. Clogging development radius (CDR). To investigate clogging development around the recharge well quantitatively, back-pumped water after recharging was sampled for chemical analysis. The sampling time, the pumped water volume and the radial distance from which the water is mobilized can be correlated in the following manner: Fig. 13. Reduction in specific pumping rate of the recharge well.

V=Qt=pCDR2L n scr

(1)

Fig. 14. Identification of clogging types in AIT recharge experiment. (a) Clogging by air entrainment, recharge test on 20 December 1993. (b) Identification of clogging by bacteria. (c) Identification of clogging by geochemical incompatibility, recharge test on 3 June 1994. (d ) Clogging by TS and TSS, recharge test on 3 June 1993.

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Fig. 15. RCF concept, field observations and derivation. (a) Two test curves used for determining RCF. (b) Derivation of RCF parameter.

where V (m3) is the volume of pumped out water when cleaning; Q (m3 min−1) is the back-pumping or cleaning rate; t (min) is the pumping time; CDR (m) is clogging development radius; L (m) is the scr length of well screen; n is the porosity. A CDR chart (Fig. 16) was constructed based on Eq. (1) for the Upper Bangkok aquifer with

Q=500 m3 day−1 (0.347 m3 min−1), porosity n= 0.4, corresponding to a void ratio of e=0.7 and L =10 m. Such a chart can be used in assessing scr the cleaning operation time. Given the values of Q and t, one can roughly estimate how far from the well the clogging zone is developed. The value of t here is understood as the time at which the

Fig. 16. CDR chart for the First Sand aquifer.

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water becomes clean due to back-pumping and is observed by the field engineer. The application of the CDR chart can be illustrated by the following example, namely, with back-pumping being carried after a recharge test to clean the well, the water being pumped out would look very turbid in the first few minutes but would gradually clear. Suppose that after 5 min the water looks clean, with a pumping rate of 500 m3 day−1 (which is double of the recharge rate) based on the chart in Fig. 16 a CDR value of ca 0.35 m can be deduced which provides a preliminary estimation on the distance of the clogged zone radius. At a higher value of CDR more clogging and more well maintenance should be carried out. For each recharge site, charts of this type can be used with respect to different values of Q, t, L and n. scr 4. Results and discussions (1) The AIT experiment is one of very few experiments of well recharge with reference to land subsidence mitigation. Although the volume of recharged water was not large, a rebound of 3 mm in the recharged aquifer was recorded after two stages of intermittent recharging, ca 10 h daily at a recharge rate of ca 250 m3 day−1. No rebound was observed in the overlying clay layers. (2) Guidance for the design of a recharge well into the First Sand layer was obtained. Some important design features were: (1) the geotechnical properties of the soil column overlying the recharged aquifer are needed to calculate the maximum allowable injection pressure, which should not exceed the horizontal effective stress at the level on top of the gravel pack; (2) as a rule of thumb, the optimal recharge rate is one-third or half of the maximum discharge rate; (3) any simple software for two-dimensional horizontal flow is sufficient for design calculations; (4) for well construction two pumps, one for recharge and another for discharge, are a must; (5) the intermittent mode of recharging is preferable to continuous recharging as it allows clean-

ing of the recharge well when the clogged zone has not yet extended too far from the well. Daily recharging with 15 min cleaning by pumping out at the end of each recharging period was considered a good idea. (6) Successful use of lake water as the recharge water source suggested the possibility of using water from other retention basins for recharging. It was estimated that there are ca 450 retention basins in Bangkok and its vicinity. If the Chao Phraya River and its tributaries as well as the irrigation channels networks are included, the total area of surface water is >100 km2 (Giao, 1997). Consequently, artificial recharging can be used to mitigate not only subsidence but also flooding. (7) Water sampling and analysis of the collected water samples proved to be a necessary and indispensable aspect of the artificial recharge experiment. The physical properties (pH, TSS, TS, TDS, conductivity, turbidity) and the chemical properties such as ionic concentrations in Ca, Mg, Na, K, Fe can be used for an investigation on clogging around a recharge well. Analyses of the physical properties of water are much easier and cheaper than those of chemical properties, therefore the former are strongly recommended. Two parameters, SAR and SP, can be used in clogging analysis. For recharged water, if SAR is 5 recharging is not recommended ( Krone, 1970). For native groundwater, when SAR