Performance evaluation of fresh groundwater skimming wells in the Indus basin irrigation system of Pakistan: A selective review

Transcription

1 Mem. Fac. Agr. Kindai Univ. 50:5 ~ 23 (2016) 5 Performance evaluation of fresh groundwater skimming wells in the Indus basin irrigation system of Pakistan: A selective review Muhammad Aslam*, Yutaka Matsuno** and Nobumasa Hatcho** * International Water Management Institute (IWMI), 12 Km, Multan Road, Chowk Thokar Niaz Baig, Lahore-5400, Pakistan ** Department of Environmental Management, Faculty of Agriculture, Kindai University Nakamachi, Nara , Japan Synopsis This review paper intends to portray performance scenario of skimming wells being practiced for abstracting shallow fresh groundwater from the fresh-saline groundwater unconfined aquifers in the Indus Basin Irrigation System of Pakistan. The review discusses some of the published works done through modeling and field studies on performance evaluation of skimming wells. Current review findings conclusively revealed that (i) for 20-30m thick freshwater layer in aquifer, single-borehole wells with 30-60% penetration depth, 4-28 l/s discharge rates and 4-8 hours per day operational hours can provide good quality groundwater (salinity less than 1000 mg/l) on sustainable basis, (ii) a multi-strainer skimming well with 3-6 strainers, well penetration depth of 60%, 10 l/s discharge and 4 to 6 hours per day of pumping can provide better quality pumped water. More than 6 strainers will not increase well discharge significantly but they will increase cost of skimming wells without any additional benefit, (iii) continuous pumping deteriorates the pumped water quality. Intermittent pumping for 4 to 6 hours per day can control the deterioration of groundwater quality, (iv) the scavenger wells could provide better quality pumped water and serve the dual purpose of drainage and irrigation based on proper and effective design, operational strategy, maintenance and arrangement for disposal of saline water, (v) radial skimming wells are not easily adoptable by the farming community because of being expensive and difficult to construct, (vi) dug wells are also not commonly used by the farmers because of their limited skimming capacities, (vii) farmers use both the single strainer 30 m deep wells with 28 to 42 l/s discharge and the 15 m deep multi-strainer wells with 4, 6, 7, 10 and 16 strainers and l/s well discharge, (viii) main problems being faced by the farmers in adoption and operation of skimming wells include: well depth, number of strainers, deterioration of pumped water quality and reduction in well discharge, and (ix) depth-wise groundwater quality monitoring through a test hole, a pumping test and intermittent operation of wells could resolve the issue of well depth, number of strainers, quality deterioration and reduction in well discharge. Keywords: fresh-saline groundwater unconfined aquifers, Indus Basin Irrigation System of Pakistan, performance evaluation, saltwater upconing, shallow fresh groundwater, skimming wells 1. Introduction Rationale of the Present Study The unconfined alluvial groundwater aquifer of the Indus Basin Irrigation System (IBIS) of Pakistan has deep native saline groundwater due to geological formation of marine origin overlain by fresh groundwater because of seepage from the river and canal system. An estimate reveals that approximately 200 billion cubic meters (BCM) of freshwater (salinity less than 1000 mg/l) in the form of shallow thin water bodies are present over the deep saline groundwater (salinity greater than 1000 mg/l) in the IBIS of Pakistan (Sufi et al. 1992). The thickness of the freshwater layer varies from 120 to 150 m within the 24 to 48 m wide belt in the river flood plains and along the irrigation canals (Kori et al. 2013). Within doabs (land lying between two rivers), the thickness of freshwater

2 6 Muhammad ASLAM, Yutaka Matsuno and Nobumasa HATCHO layer varies from about 30 m in the central and lower parts to about 60 m along the periphery of the doabs. In IBIS of Pakistan, about 6.42 million hectares (Mha) have freshwater layer of variable thickness (Saeed et al., 2003b). Under the conditions of aquifers having freshwater layer overlying saline water layer, high capacity and deep wells pump poor quality groundwater (salinity more than 1000 mg/l) due to saline water intrusion and saltwater upconing below the pumping wells (Asghar et al., 2002; Aliew et al., 2003; Saravanan et al., 2014). Saltwater upconing is the vertical upward movement of saltwater in the form of a cone or mound from the saline water zone in response to pumping from the aquifer (Reilly and Goodman, 1987, See Fig.1). Under such conditions, properly designed and operated skimming wells help managing the freshwater withdrawals by controlling the saltwater upconing. A skimming well is a partially penetrating well in shallow fresh groundwater layer having pumping capacity generally less than 28 l/s. Skimming wells pump water from thin fresh groundwater layer without or with minimum disturbance to the underlying saline groundwater (Ahmad, 1979; Saeed et al., 2003a; Zardari et al., 2015). Fig. 1. Saltwater upconing beneath a pumping well Several researchers and investigators in Pakistan have done a lot of work to evaluate the performance of skimming wells. The present study reviews the work done on performance evaluation of skimming wells, and draws conclusions and makes recommendations for use of skimming well technology to obtain good quality pumped groundwater (salinity less than 1000 mg/l) on long-term sustainable basis in the IBIS of Pakistan. The main objectives of the present study were: To collect and review the literature (papers & reports) of studies and investigations on performance evaluation of skimming wells in the IBIS of Pakistan; To synthesize key findings on the performance of skimming wells; and To derive key conclusions and make recommendations for use of skimming well technology to obtain good quality groundwater on long-term sustainable basis in the IBIS of Pakistan. Indus Basin Irrigation System of Pakistan The Indus river system consisting of Indus, Jhelum, Chenab, Ravi and Sutlej rivers (Fig. 2) constitutes the Indus Basin Irrigation System (IBIS) of Pakistan. The mean annual river inflow is about 172 BCM (Aslam and Prathapar, 2006). The IBIS contains three main reservoirs; Tarbela, Mangla and Chasma with present live storage capacities of 10.33, 5.54 and 0.60 BCM, respectively. There are 15 barrages and 45 main canals withdrawing on average 130 BCM of river water per annum (Aslam and Prathapar, 2006). Also, there are 15 link canals, which were built after the 1960 Water Treaty with India to transfer water from western rivers to eastern rivers. Irrigated canal command area of the IBIS is 16 Mha (Aslam et al., 2014). The entire area of IBIS is underlain by a vast unconfined alluvial groundwater aquifer. In the upper Indus Basin (central and southern parts of Punjab), the Indus river tributaries; Jhelum, Chenab, Ravi and Sutlej Rivers divide the land into four doabs (land lying between two rivers), namely; Thal, Chaj, Rechna, and Bari doabs (Fig 2). The lower Indus Basin (Sindh) has only Indus River (Fig 2). In the IBIS of Pakistan, agriculture being one of the biggest economic sectors of the country contributes about 21% to the national Gross Domestic Product (GDP). It supports 60% of the population providing self-employment to 40% of

3 Performance Evaluation of Groundwater Use for Irrigation in the Indus Basin of Pakistan 7 Fig. 2. The Indus Basin Irrigation System of Pakistan rural families (Haq et al., 2008). Major irrigated agricultural cropping systems include rice and wheat, cotton and wheat, wheat and maize, sugarcane and wheat and mixed cropping based on various combinations of wheat, rice, cotton and sugarcane. The surface water through canal irrigation network provides about 50% of crop water requirement. Inadequate and unreliable canal water supplies and change in cropping patterns from low-delta to high-delta crops have made farmers depend on marginal-quality groundwater for irrigation. Depending on the circumstances, groundwater meets 10 to 90 percent (on average 50 percent) of the irrigation requirements (Aslam et al., 2014). Groundwater Irrigation in the IBIS of Pakistan In the IBIS of Pakistan, conjunctive use of surface and groundwater plays an important role for agricultural productivity by contributing more than 50 % of total irrigation supplies. In the fresh groundwater areas, about 10 BCM of public tubewell water and about 45 BCM of private tubewell water are being used directly or in conjunctive (blending/cyclic) form for irrigation by Pakistani farmers (Aslam and Prathapar, 2006). Presently, more than one million private tube wells have been installed by the farmers for conjunctive groundwater use (FODP WSTF, 2012). Consequently, cropping intensity has increased from 80 to more than 150 percent. Crop yields are % higher compared to farmers without tube wells. About 2.5 million farmers use groundwater for irrigation of croplands (Qureshi et al, 2008). In IBIS of Pakistan, declining groundwater tables due to over-exploitation of groundwater are causing intrusion (horizontal movement) of saline water from saline groundwater areas into the over pumped fresh groundwater areas thereby causing deterioration in groundwater quality in fresh groundwater areas. In the close vicinity of rivers and irrigation canals, aquifers have also shallow freshwater body overlying saline groundwater at deeper depths. Pumping with high capacity (more than 28 l/s discharge) and deep wells from these kinds of aquifers results in abstraction of saline groundwater. Under these conditions, skimming wells (partially penetrating wells in fresh groundwater layer overlying saline groundwater layer in the aquifer) play a key role in abstracting good quality groundwater (Aslam et al. 2014). In the IBIS of Pakistan, more than 30% of the irrigated area has potential for the installation of skimming wells to abstract fresh groundwater resource (Zuberi and McWorter, 1973). Skimming Wells Adopted in the IBIS of Pakistan In the IBIS of Pakistan, skimming wells which are partially penetrating wells in freshwater layer overlying saline water layer play an important role in minimizing the mixing of fresh and salt waters in the aquifers. The increased pumping rates of skimming wells cause abstraction of saline groundwater (Saravanan et al., 2014). Improved well design and better operating strategies are vital to obtain good quality groundwater (Kori et al. 2013). In the IBIS of Pakistan, majority of skimming wells are not properly designed and operated (Saeed et al. 2002a, b). Consequently, they pump poor quality groundwater. Skimming wells used in the IBIS of Pakistan to skim fresh groundwater can mainly be categorized into single-bore, multi-bore, scavenger, radial collector and dug-well skimming wells (Saeed et al., 2003a). Among these wells, single-bore and multi-bore wells are the most acceptable and popular among farming community for irrigation purposes due to their affordable economics, technical simplicity and the availability of local

4 8 Muhammad ASLAM, Yutaka Matsuno and Nobumasa HATCHO material and skilled labor (Ashraf et al., 2001; Saeed and Ashraf, 2005). A brief description of the skimming wells being adopted in the IBIS of Pakistan is portrayed below. Single-Borehole Skimming Well A single-borehole well with a discharge rate in the range of 28 to 42 l/s consists of a single borehole having a diameter in the range of 20 to 25 cm and it partially penetrates the upper freshwater layer overlying the saline groundwater layer (Fig 3). Multi-borehole Skimming Well A multi-borehole well with a discharge rate in the range of 14 to 28 l/s consisting of a number of vertical boreholes of small diameters ranging from cm is generally m deep. These boreholes are drilled in an arbitrary fashion, mostly around a circle and are joined together to be pumped by a centrifugal pump (Fig. 4). Farmers use multi-borehole wells in the areas having freshwater layer thickness less than 20 m (Saeed et al. 2003b). The multi-borehole wells have the advantages of shallower depth of penetration and small vertical head gradient than Fig. 3. The schematic diagram of a single-borehole skimming well (Source: Saeed et al, 2003b) A centrifugal pump powered with diesel engine or tractor is used to pump water (Ashraf and Idrees, 2000; Saeed et al., 2001; Saeed et al., 2002a). The depth of these wells depends upon the thickness of the freshwater layer. In the areas along the riverain belts and active flood plains, tubewells are more than 30 m deep and are 20 to 30 m deep in the centers of doabs ((Ashraf and Idrees, 2000; Saeed et al., 2001; Saeed et al., 2002a; Saeed et al. 2003b). In the IBIS of Pakistan more than one million single- bore wells (private tubewells) have been installed by the farmers to pump fresh groundwater for irrigation due to simple technology, locally available construction material, and skilled labor for installation and maintenance. However, improper design and operation of these wells cause pumping of saline groundwater. Fig. 4. The schematic diagram of a multi-bore skimming well (Source: Saeed et al, 2003b) a single-borehole well at the same discharge. Thus multi-borehole wells cause less saltwater upconing and consequently pump better quality water compared to the single-borehole wells (Sufi, 1999). The number and spacing of boreholes are constrained by operating efficiency of the pumping system and the cost of installation (Sufi, 1999). Ashraf and Idrees, (2000), Saeed et al. (2001) and Saeed et al. (2002a) reported that in the Chaj Doab (land between Jhelum and Chenab Rivers, see Fig. 2) of the IBIS of Pakistan, multi-borehole skimming wells having 4, 6, 7, 10 and 16 bores and 12 to 15 m deep are in common use by the farmers. Generally, these wells have 9 m long strainers of 5 cm diameter and 6 m long blind pipes of 5 cm size. The strainers are generally penetrated to the same depth of 15 m at variable horizontal distances of 1.5, 2.5, 3 and 3.5 m from the centrifugal pumps.

5 Performance Evaluation of Groundwater Use for Irrigation in the Indus Basin of Pakistan 9 Scavenger Skimming Well with Single Borehole A scavenger skimming well with single borehole pumps fresh and saline waters from the same site simultaneously without mixing, through two separate discharge outlets installed in the same single borehole as shown in Fig 5 (Milne et al., 1973; Saravanan et al., 2014). This well consists of a single borehole with two pumps each in fresh and saline water layers (Fig 5). The shallow borehole part is screened in the upper portion of freshwater layer to pump freshwater whereas the deeper borehole part is screened at lower portion of the freshwater layer to pump saline water. Fig. 6. The schematic diagram of a scavenger well with double boreholes (Source: Saeed et al, 2003b) Fig. 5. The schematic diagram of a scavenger well with single borehole (Source: Saeed et al, 2003b) Radial Collector Skimming Well A radial collector well consists of horizontal well screens (radial collectors) which open into a sump and drain water into it (Fig. 7). Radial collector wells help skimming of very thin freshwater lens (< 5 m in shallow water table areas) under gravity without pumping from groundwater aquifer without any disturbance in the underlying saline water layer. Radial collector wells provide large discharge per unit drawdown, a desirable feature of skimming wells (Saeed et al., 2003b). Scavenger Skimming Well With Double Boreholes The scavenger skimming well with double boreholes consists of double boreholes very close to each other, drilled in fresh and saline water layers with separate pumps and motors (Fig.6). The shallow borehole is screened in the upper portion of freshwater layer to pump freshwater whereas the deeper borehole is screened at lower portion of the freshwater layer to pump saline water. About 378 scavenger wells have been installed in the Lower Indus Basin (Sindh province) of Pakistan to manage agriculture drainage and supplement irrigation without mixing of freshwater with saline water (Zardari et al., 2015). Fig. 7. The schematic diagram of a radial collector well (Source: Saeed et al, 2003b) The subsurface horizontal tile drainage systems

6 10 Muhammad ASLAM, Yutaka Matsuno and Nobumasa HATCHO installed in the Indus Basin of Pakistan are the good examples of radial collector wells. Radial well performs better (pumps good quality water) compared to that of a scavenger well (Sufi, 1998a, b). Dug-Well Skimming Well A dug-well skimming well provides a simple, cost effective and traditionally familiar option for skimming freshwater (Zuberi and McWhorter 1973). A dugwell consists of a pit dug to the aquifer or to the material where permeability is reasonable. The pit is often lined with masonry or pre-cast concrete rings to support excavation (Zuberi and Mc Whorter 1973). The dug wells provide storage for water, as well as a source (NAS, 1974). They with the water lifting devices are used for extracting freshwater from shallow unconfined aquifers for domestic and irrigation purposes. The well penetration and well diameter are the main design parameters that influence the performance of dug wells. The rate of pumping is much less responsive to changes in well diameter as compared to the changes in well penetration depth. Thus, the well penetration depth in relation to the aquifer characteristics is one of the important design parameter to achieve higher discharge rates. The dug wells may be used in the areas where the freshwater thickness varies from 5-15 m (Akbar et al., 2001). The dug wells can skim fresh groundwater overlying the saltwater by keeping the dug well penetration depth less than the thickness of the freshwater layer in the aquifer (Akbar et al., 2001). The depth of a dug well (open well) ranges from 5 to 10 m below the static water level. The open excavation is usually circular in shape, the diameter varying from 1.5 to 4.5 m. The large diameters permit the storage of large quantities of water in the dug wells (Akbar et al., 2001). Skimming Wells Performance Concept The performance of a skimming well here refers to the quality of groundwater a skimming well abstracts or can abstract under certain well design and operation strategy for the hydrogeological conditions of the groundwater aquifer of the IBIS of Pakistan. The quality here refers to the salinity of pumped water by a skimming well. As in the IBIS of Pakistan, skimming wells are mostly installed and operated by the farming community for irrigation purposes, the preferred quality (salinity) of pumped groundwater ranges from mg/l (<1000 mg/l) for irrigation purposes. This is the useable water quality limit for irrigation that will not cause any harm to soils and crops (Saeed and Ashraf, 2005). The quality of groundwater pumped by skimming wells mainly depends upon the hydro-geological conditions of the aquifer (that include thickness of freshwater, sources of freshwater recharge, hydraulic parameters of aquifers and existence of natural barrier to prevent saline-upcoming), design and operation of wells (Chandio and Larock, 1984; Saeed et al., 2003a). The well design and operational parameters that affect performance (quality) of a skimming well include well penetration depth with respect to thickness of freshwater layer in the groundwater aquifer, well screen (perforated pipe; filter/strainer) diameter & length, number of boreholes & inter-boreholes spacing (well spacing) in case of a multi-borehole skimming well, well discharge rate and pumping duration (operational hours per day). The sensitivity analysis has shown that inter-boreholes spacing has negligible effect on salinity of pumped water (Saeed and Bruen, 2004). Chandio and Larock (1984) reported that with the same well discharge rate, with time well would pump more saline water in case of a thin freshwater layer as compared to a thick freshwater layer, a linear relationship exists between pumping rate and pumped water salinity, and also between increase in salinity of pumped water and elapsed pumping time. Table 1 provides an overview of certain definitions used in the description of performance of skimming wells in the present study. Study Area 2. Materials and Methods The present study was conducted in the IBIS of Pakistan the entire area of which is underlain by a vast unconfined alluvial groundwater aquifer having shallow freshwater layer overlying deep saline water layer. In order to skim fresh

7 Performance Evaluation of Groundwater Use for Irrigation in the Indus Basin of Pakistan 11 Table 1 Definitions used in the description of skimming wells performance Skimming well performance criteria: GW quality: Fresh GW: Saline GW: Good quality GW (for irrigation purpose)*: Reasonable/marginal quality GW (for irrigation purpose)*: Poor/hazardous quality GW (for irrigation purpose)*: Quality of the groundwater (GW) abstracted by a skimming well designed and operated for the GW aquifer conditions (physical, hydraulic and chemical characteristics) prevalent in the IBIS of Pakistan Salinity of GW expressed in milligrams per liter (mg/l) GW having salinity less than 1000 mg/l GW having salinity equal to or more than 1000 mg/l GW having salinity less than 1000 mg/l GW with salinity in the range of mg/l GW having salinity greater than 1700 mg/l Well design parameters: Well operation parameters: - Well penetration depth which is the well depth with respect to thickness of freshwater layer in the aquifer and it is generally expressed in percentage of the thickness of the freshwater layer overlying the saline water in the given GW aquifer [i.e. (depth to which well penetrates in the freshwater layer below water table/thickness of the freshwater layer) x100]. - Well discharge which is the pumping rate at which GW is abstracted by the well from the GW aquifer. It is expressed in liters per second (l/s). - Number of strainers/boreholes in case of a multi-strainer skimming well. - Well spacing which is the spatial distance between well strainers/screens/boreholes in case of a multi-borehole skimming well. Number of hours a skimming well is operated to pump GW in a day (daily operational hours; pumping hours per day). * Water quality standards for irrigation water (Sufi, 1999) groundwater overlying saline groundwater, various types of skimming wells are being practiced in the IBIS of Pakistan. The present study intended to review the performance of these wells. Study Approach The groundwater research and investigation community in Pakistan has continuously evaluated the performance of the skimming wells in terms of quantity and quality of the pumped groundwater under different design and operation parameters. This has created a good wealth of knowledge on the performance of skimming wells. In the present study, various research and technical reports and papers related to skimming wells performance studies and published by different academic, research and consulting institutions were collected and comprehensively reviewed to derive and synthesize key findings on performance of skimming wells. Key conclusions

8 12 Muhammad ASLAM, Yutaka Matsuno and Nobumasa HATCHO and recommendations were also derived from the review. 3. Results and Discussion In the IBIS of Pakistan, single-borehole, multi-borehole and scavenger wells are more commonly used to skim freshwater from the fresh-saline water aquifer. Among these wells, single-bore and multi-bore wells are the most acceptable and popular among farming community for irrigation purposes due to their affordable economics, technical simplicity and the availability of local material and skilled labor (Ashraf et al., 2001; Saeed and Ashraf, 2005). Huge work has been done on performance evaluation of single-borehole, multi-borehole and scavenger wells. The present review also focuses on performance evaluation of only these skimming wells. Performance Appraisal of Skimming Wells Single-borehole skimming wells performance scenario In the IBIS of Pakistan, presently, more than one million single-borehole wells (having diameter in the range of 20 to 25 cm) locally called as private tubewells have been installed by the farmers for irrigation purposes (FODP WSTF, 2012) due to simple technology, locally available construction material, and skilled labor for installation and maintenance (Saeed and Ashraf, 2005). However, due to improper design and operation, these high capacity (discharge rate more than 28 l/s; generally discharge rate ranges from l/s) and deep wells pump poor quality groundwater (salinity more than 1000 mg/l) due to saline water intrusion and saltwater upconing below the pumping wells (Asghar et al., 2002; Aliew et al., 2003; Saravanan et al., 2014). About 70% of private tubewells pump saline water use of which for irrigation has caused secondary salinization in about 2.3 Mha irrigated land (Qureshi and Barrett-Lennard, 1998). In the IBIS of Pakistan, majority of single-borehole skimming wells (private tubewells) are not properly designed and operated (Saeed et al. 2002a, b). Consequently, they pump poor quality groundwater. The depth of these wells is more than 30 m in areas along the riverain belts and active flood plains and it ranges from 20 to 30 m in the centers of doabs ((Ashraf and Idrees, 2000; Saeed et al., 2001; Saeed et al., 2002a; Saeed et al. 2003b). The depth of a skimming well is not determined on the basis of thickness of the freshwater. Farmers increase the well depth just to obtain more water without considering thickness of freshwater layer of the aquifer. Generally farmers want to obtain discharge rates more than 28 l/s (Saeed and Ashraf, 2005). This decision is not based on techno-economics rather it is a psychological decision in the fresh-saline groundwater environment of the Indus basin. Thickness of freshwater is a site-specific characteristic while the depth of well is a general trend (Saeed and Ashraf, 2005). Also, private tubewells are operated indiscriminately by farmers in the IBIS of Pakistan. Improved well design and better operating strategies are vital to obtain good quality groundwater by skimming wells (Kori et al. 2013). Several studies have evaluated the performance of the single-borehole skimming wells in the IBIS of Pakistan to provide guidelines for improved well design and operation strategies to skim good quality (salinity less than 1000 mg/l) groundwater (see Table 2). These studies employed the physical and numerical modeling approaches to assess the performance of the single-borehole skimming wells. These studies evaluated the performance of the wells in terms of quality (salinity) of the pumped water on the basis of well design parameters including mainly the thickness of freshwater layer in the groundwater aquifer, well penetration depth and well discharge (pumping rate), and well operation time (pumping time in hours per day). Key findings of various studies mentioned in Table 2, indicate that for 12-40m thick freshwater layer in the aquifer, single-borehole wells with 30-60% penetration depth, 4-28 l/s discharge rates and 4-8 h/d pumping duration can skim good quality (salinity less than 1000 mg/l) ground water on long-term sustainable basis. In the IBIS of Pakistan, farmers generally install single-borehole wells having discharge rates of l/s in m thick freshwater layer. Performance evaluation studies (Table2) suggest that for m thick freshwater layer,

9 Performance Evaluation of Groundwater Use for Irrigation in the Indus Basin of Pakistan 13 single-borehole wells with 30-60% penetration depth, 4-28 l/s discharge rates and 4-8 h/d operational hours per day can provide good quality (salinity less than 1000 mg/l) ground water on long-term sustainable basis. Multi-borehole skimming wells performance scenario The idea of multi-bore well is based on the concept that these wells have advantage to spatially distribute the pumping stress. The drawdown (which is the change in hydraulic head observed at a well in an aquifer due to pumping a well) is more in case of a single-bore well and this decreases with number of boreholes. The saline water in fresh-saline aquifers move upwards to maintain the equilibrium disturbed by the drawdown propagation around the well. The spatial distribution of drawdown helps to minimize the chance of saline water upconing provided that the well is designed and operated considering the local hydro-geological conditions (Saeed et al. 2003a). For the hydrogeological environment of the Indus basin, 2- and 4- borehole wells suppress the upward movement of saline water by more than 50% as compared to single-borehole well for the same total amount of withdrawal and the same aquifer and well parameters (Chandio and Larock, 1984). In the IBIS of Pakistan, farmers have installed the multi-borehole wells with discharge rates in the range of 14 to 28 l/s and consisting of a number of vertical boreholes of small diameters ranging from cm. Multi-bore skimming wells having 4, 6, 7, 10 and 16 strainers and 12 to 15 m deep are in common use by the farmers. The strainers are generally penetrated to the same depth of 15 m at variable horizontal distances of 1.5, 2.5, 3 and 3.5 m from the centrifugal pumps (Ashraf and Idrees, 2000); Saeed et al., 2001; Saeed et al., 2002a). The multi-borehole wells have the advantages of shallower depth of penetration and small vertical head gradient than a single-borehole well at the same discharge. Thus multi-borehole wells cause less saltwater upconing and consequently pump better quality water compared to the single-borehole wells (Sufi, 1999; Saeed et al. 2003a). The number and spacing of boreholes are constrained by operating efficiency of the pumping system and the cost of installatio n (Sufi, ). Farmers use multi-boreholes wells in the areas having freshwater layer thickness less than 20 m (Saeed et al. 2003b). Farmers prefer multi-borehole wells to single-borehole wells because of better performance of multi-borehole wells compared to single-borehole wells (Ashraf and Idrees, 2000). In the upper Indus Basin of Pakistan (Chaj Doab, Fig. 1), the number of multi-borehole skimming wells installed by farmers has increased tremendously during the drought period of 2000 to About 36% of these wells pump marginal quality groundwater having salinity of 1420 mg/l (Ashraf et al. 2011) There is a wide variation in various well design parameters. Farmers increase the depth of multi-borehole wells to obtain more water without considering thickness of freshwater layer of the aquifer. Decision regarding well depth by drillers or farmers is not based on the thickness of the freshwater layer. Thickness of freshwater is a site-specific characteristic while the depth of well is a general trend. The number of boreholes varies from 2 to 26. Decisions regarding number of boreholes in skimming wells are apparently arbitrary. Farmers do not have any idea about the optimal number of boreholes. In most of the cases farmers depend on local drillers to decide the number of boreholes. Majority of the farmers and drillers have a misconception that discharge of a well increases with the number of boreholes. Theoretically it is true up to certain limit. Generally farmers want to obtain discharge rates that exceed 28 l/s. This decision is not based on techno-economics rather it is a psychological decision in the fresh-saline groundwater environment in the Indus basin. Sometimes farmers follow the neighboring well design taking into account the discharge rate and salinity of the pumped water from that well (Saeed and Ashraf, 2005). Most of the time farmers operate the wells for 4-6 h/d and occasionally operate well more than 12 h/d during rice crop season. Farmers operate well almost daily or on alternate days during the crop growing season (Saeed and Ashraf, 2005). The pumped water quality in skimming wells

10 14 Muhammad ASLAM, Yutaka Matsuno and Nobumasa HATCHO Table 2 Single-borehole Skimming Wells Performance Evaluation Studies with Key Findings FW Well design and LT operation parameters Key Findings (m) considered in the Study Study Approach Sufi et al. (1998a, b) Ullah (1999) Asghar et al. (2002) Saeed and Ashraf (2005) Alam and Olsthoorn (2013) Aslam (2014) Aslam (2015) Numerical GW VDGWTRN Numerical GW MODFLOW-MT3D Numerical GW MODFLOW-MT3D Numerical GW MODFLOW-MT3D Numerical GW modeling study on performance evaluation of farmers skimming wells practices Numerical GW MODFLOW-MT3D Numerical GW MODFLOW-MT3D P w (%) studies Q w (l/s) T w (h/d) The single-bore wells with 45% penetration and 14 l/s well discharge rate skims good quality GW (salinity of 596 mg/l) on long-term sustainable basis. Well can pump good quality GW on long-term sustainable basis. Well can skim good quality GW on long-term sustainable basis. Well can provide good quality pumped GW. Well could provide pumped water of good quality on long-term sustainable basis. Farmers skimming wells with discharge rates between and l/s cannot provide good quality pumped water on long-term sustainable basis with their current operational practices. Well discharge reduced to l/s could provide good quality pumped groundwater on long-term sustainable basis. A one-borehole well can skim groundwater of salinity less than 1500 ppm on long-term sustainable basis. Well can provide pumped groundwater of salinity less than 1000 ppm. GW, groundwater; FWLT, freshwater layer total thickness in meters (m); P w, well penetration depth in percentage of thickness of freshwater layer [i.e. (well depth in freshwater layer/thickness of freshwater layer) x 100]; Q w, well discharge in liters per second (l/s); T w well operation time in hours per day (h/d) mainly depends upon the hydro-geological conditions of the aquifer (thickness of freshwater, sources of freshwater recharge, hydraulic & chemical properties) and design & operation of

11 Performance Evaluation of Groundwater Use for Irrigation in the Indus Basin of Pakistan 15 wells (Chandio and Larock, 1984; Saeed et al., 2003a; Chandio and Lee, 2012). The design and operational strategy development for skimming wells should be based on hydro-salinity in the aquifer, useable limit of pumped water quality and economics of operation (Saeed et al., 2003a). In the IBIS of Pakistan, improper design and operation of multi-borehole wells result in pumping of poor quality groundwater. Major design and operation problems being faced by farmers include well depth, number of strainers, horizontal distance of strainers from pump, deterioration in quality of pumped water and reduction in well discharge with time (Saeed et al. 2002b). Field experiments and numerical modeling studies have evaluated the performance of the multi-borehole skimming wells in the IBIS of Pakistan to provide guidelines for improved well design and operation strategies to skim good quality (salinity less than 1000 mg/l) groundwater (see Tables 3 & 4). The parameters considered in the numerical modeling studies included well penetration depth with respect to depth of the freshwater layer in the aquifer, number of boreholes and spatial distance between boreholes, well discharge rate and daily operational hours. Key findings of various studies mentioned in Tables 3 & 4, indicate that: The multi-borehole (2-, 3-, 4-, 6-, 8- and 16- boreholes) skimming wells perform better compared to single-borehole wells because multi-borehole wells suppress more saltwater upconing due to less vertical hydraulic gradient around the wells compared to single-borehole wells. Consequently, multi-borehole wells pump better quality groundwater compared to single-borehole wells. Multi-borehole wells with large number of boreholes do not result in any significant improvement in the quality of pumped water rather they increase installation and operational cost. The multi-borehole wells with a small number of boreholes perform better by delivering good quality groundwater compared to wells with excessive number of boreholes. They are economical also to install & operate. Wells with 4-8 boreholes are more cost effective in pumping good quality groundwater. The continuous operation of multi-borehole wells allowing no recovery period deteriorates the quality of pumped water significantly. The intermittent pumping (a couple of hours pumping per day and then allowing recovery for remaining hours of the day i.e. 4 h/d intermittent pumping means 4 h pumping and 20 h of recovery) results in significant improvement in the quality of pumped water. Four to 8 hours/day of intermittent pumping can help control groundwater quality deterioration. A regular schedule of intermittent pumping should be followed to skim good quality water and also to save energy cost. Pumped water quality in fresh-saline aquifer is hugely affected by pumping duration (daily operational hours). Salinity of pumped water increases with increase in operational hours for a given freshwater thickness, well penetration depth and well- discharge rate. For multi-borehole wells, the daily operation of 4-8 hour being cost-effective pumping operation can skim groundwater of salinity less than 1000 mg/l on long-term sustainable basis. The 4- to 8- borehole wells installed in m thick freshwater layer with 30-60% pumping can skim good quality penetration depth, 9-28 l/s discharge rate and 4-8 hours per day of intermittent groundwater on long term sustainable basis in the IBIS of Pakistan. Scavenger skimming wells performance scenario In the IBIS of Pakistan, about 378 scavenger wells have been installed to pump fresh and saline water separately for managing agricultural drainage and supplementing irrigation (Zardari et al., 2015). Various studies have evaluated the performance of these wells (see Table 5). These studies reveal that scavenger wells do not perform satisfactorily because they pump marginal to hazardous quality water through freshwater discharge outlets due to mainly improper operational strategies. Studies suggest that scavenger wells can perform better by delivering good quality water through freshwater discharge outlet if they are properly designed and operated.

12 16 Muhammad ASLAM, Yutaka Matsuno and Nobumasa HATCHO Scavenger wells can provide good quality pumped water through freshwater discharge outlet with operation time of h/d and recovery ratio (ratio of freshwater to saline water withdrawals) of 0.5:0.5. The increased pumping of freshwater and decreased withdrawal of saline water can be achieved with recovery ratio of freshwater to saline water withdrawals of 0.7:0.3 with the design operation time of 15 hours per day. 4. Conclusions and Ecommendations Single-borehole Skimming Wells Improved well design and better operating strategies are vital to abstract good quality (salinity less than 1000 mg/l) groundwater by skimming wells from the fresh-saline water aquifers. In the IBIS of Pakistan, about 70% of the single-borehole wells (more than one million) installed by the farmers for irrigation purposes pump saline water (salinity more than 1000 mg/l) due to improper design and operation. For m thick freshwater layer in aquifer, single-borehole wells with 30-60% penetration depth, 4-28 l/s discharge rates and 4-8 h/d operational hours per day can provide good quality (salinity less than 1000 mg/l) groundwater on long-term sustainable basis in the IBIS of Pakistan. Multi-borehole Skimming Wells In the Indus Basin of Pakistan, about 36% of multi-borehole skimming wells pump marginal quality groundwater having salinity of 1420 mg/l due to improper design and operation. The multi-borehole skimming wells perform better compared to single-borehole wells because multi-borehole wells suppress more saltwater upconing due to less vertical hydraulic gradient around the wells compared to single-borehole wells. Consequently, multi-borehole wells pump better quality groundwater compared to single-borehole wells. Multi-borehole wells with large number of boreholes do not result in any significant improvement in the quality of pumped water rather they increase installation and operational cost. The multi-borehole wells with a small number of boreholes perform better by delivering good quality groundwater compared to wells with excessive number of boreholes. They are economical also to install & operate. Wells with 4-8 boreholes are more cost effective in pumping good quality groundwater. The continuous operation of multi-borehole wells allowing no recovery period deteriorates the quality of pumped water significantly. The intermittent pumping (a couple of hours pumping per day and then allowing recovery for remaining hours of the day i.e. 4 h/d intermittent pumping means 4 h pumping and 20 h of recovery) results in significant improvement in the quality of pumped water. Four to 8 hours/day of intermittent pumping can help control groundwater quality deterioration. A regular schedule of intermittent pumping should be followed to skim good quality water and also to save energy cost. Pumped water quality in fresh-saline aquifer is hugely affected by pumping duration (daily operational hours). Salinity of pumped water increases with increase in operational hours for a given freshwater thickness, well penetration depth and well discharge rate. For multi-borehole wells, the daily operation of 4-8 hour being cost-effective pumping operation can skim groundwater of salinity less than 1000 mg/l on long-term sustainable basis. The 4- to 8- borehole wells installed in m thick freshwater layer with 30-60% penetration depth, 9-28 l/s discharge rate and 4-8 hours per day of intermittent pumping can skim good quality groundwater on long term sustainable basis in the IBIS of Pakistan.

13 Performance Evaluation of Groundwater Use for Irrigation in the Indus Basin of Pakistan 17 Table 3 Multi-borehole Skimming Wells Performance Evaluation Numerical Modeling Studies Study Sufi et al. (1998a, b) (Numerical GW VDGWTRN) Asghar et al. (2004) (Numerical GW MODFLOW-MT 3D) Saeed and Bruen (2004) (Numerical GW modelling) Saeed and Ashraf (2005) (Numerical GW MODFLOW-MT 3D) Chandio and Lee (2012) (Numerical GW FEMGWST) Aslam et al. (2014) (Numerical GW MODFLOW-MT 3D) Aslam (2015) (Numerical GW MODFLOW-MT 3D) Key Findings A 2-borehole skimming well installed in 12 m thick freshwater layer with 45% penetration depth pumps good quality water (586 mg/l salinity) compared to a 1-borehole skimming well with the same well discharge of 14 l/s. A multi-borehole well with 3 and 4 boreholes did not show any significant improvement in pumped water quality compared to a 2-borehole well. It reflects that multi-borehole wells with small number of boreholes can perform satisfactorily by delivering good quality GW. They will be economical also to install and operate. A 2-borehole well is recommended compared to a 3- or 4- borehole well as their performance is the same but installation and operation of a 2-borehole will be more economical compared to a 3- or 4- borehole well. Pumped water salinity of 887 mg/l after one week of continuous pumping and pumped water salinity of 735 mg/l after one week of intermittent pumping at 12 h/d shows significant improvement in the quality of pumped water. A regular schedule of intermittent pumping should be followed to skim good quality water and also to save energy cost. The farmers 16-borehole wells with l/s pumping rates cause high salinity in the pumped water (poor quality pumped water). A 6-borehole (maximum no. of boreholes) well with discharge ranging from 4-8 l/s and operation time ranging from of 2-4 h/d could provide good quality pumped water on long term basis. Pumped water quality changes when number of boreholes in skimming wells increases from one to four but it does not change significantly with further increase in number of boreholes. Four to 6 boreholes of 7.5cm diameter or 6-8 boreholes of 5 cm can give a total discharge of 28 l/s under the present skimming well practices. The 4- to 8- borehole wells spaced 3m apart from each other installed in m thick freshwater layer with 30% penetration depth, 9-26 l/s discharge rate and 4-8 hours per day pumping duration can skim good quality groundwater on long term sustainable basis. The multi-borehole wells can pump good quality groundwater following intermittent pumping (well operational) strategy. A 4-borehole well installed in 30m thick freshwater layer with 30-40% penetration depth, 6-14 l/s discharge rate and 4-8 hours per day operational duration can skim reasonable quality groundwater (salinity in the range of mg/l) on long-term sustainable basis. A 4- or 8-borehole well installed in 23m thick freshwater layer with 30-60% penetration depth, 9-14 l/s discharge rate and 4-8 hours per day pumping duration can provide pumped groundwater of salinity less than 1000 ppm. A 4-borehole well with 30% penetration, 9-14 l/s discharge and 4-8 h/d operation was recommended to skim groundwater of salinity less than 1000 ppm.

14 18 Muhammad ASLAM, Yutaka Matsuno and Nobumasa HATCHO Table 4 Multi-borehole Skimming Wells Performance Evaluation Field Studies with Key Findings Study Ahmad (1979) Kemper (1976) Hefeez et al. (1986) Mirbahar et al. (1997) Ashraf et al. (2001) Saeed et al. (2003a) Alam et al. (2004) Ashraf et al. (2011) Key Findings Multi-borehole wells connected to the same centrifugal pump can skim good quality GW successfully from the shallow freshwater layer without disturbing deep saline water. A 2-borehole well with the same discharge of 14 l/s as of a 1-borehole well pumps better quality water compared to a 1-borehole well reflecting multi-borehole wells (wells with more than 1 borehole) can pump water of better quality compared to single-borehole wells. Multi-borehole skimming wells with a small number of boreholes perform better compared to wells with excessive number of boreholes. They are economical also to install & operate. The continuous operation of a 3-borehole well for 15 and 32 days at a rate of 14 l/s increased the pumped water salinity from 994 to 1399 mg/l. This finding confirmed that ccontinuous pumping causes deterioration in quality of pumped groundwater. Multi-boreholes skimming wells with 2-3 boreholes can provide good quality pumped water. Intermittent pumping (pumping water from shallow aquifer for a couple of hours and then allowing it to recover to its original level) of 5 h/d (5-h intermittent pumping with 5 h pumping and 19 h of recovery) for 71 days of a 3-strainer well with 17 l/s discharge rate improved the quality of pumped water significantly (1221 to 1029 mg/l) because the well was fully recovered within 5 to 10 hours after stopping pumping. Intermittent pumping of 6 h/d (6 h pumping and 18 h of recovery) for 41 days of a 16-strainer well with discharge rate of 28 l/s also improved pumped water quality (852 to 639 mg/l). Pumped water quality in fresh-saline aquifer is hugely affected by pumping duration (daily operational hours). For 16-borehole well with 28 l/s discharge arte, an increase in daily operation from 2 to 12 h/d caused three-fold deterioration in the pumped water quality (salinity of pumped groundwater increased from 426 mg/l to 1278 mg/l). For 16-borehole well, the daily operation of 4-6 hour being cost-effective pumping operation can skim groundwater of salinity less than 1000 mg/l on long-term sustainable basis. The 4- and 6-borehole wells operated at 4 hours per day pumped better quality water compared to a single-borehole well. Multi-borehole (3-, 4-, 6- and 16-boreholes) skimming wells can provide good quality pumped water compared to a single-borehole well. Wells with 3-6 boreholes and 1.5 m horizontal distance of boreholes from pumping unit are more cost effective in pumping good quality groundwater. Continuous pumping deteriorates the pumped water quality. Four to 6 hours/day of intermittent pumping can help control GW quality deterioration. A well with 3-6 boreholes, 60% penetration depth, 10 l/s discharge rate and 4 to 6 hours per day of intermittent pumping can skim good quality GW. The 6-borehole wells of 7.5 cm diameter installed within a depth of 20 m having discharge rate of about 28 l/s could be operated at 4-12 hours/day to obtain good quality pumped water on long-term sustainable basis.

15 Performance Evaluation of Groundwater Use for Irrigation in the Indus Basin of Pakistan 19 Table 5 Scavenger Skimming Wells Performance Evaluation Studies with Key Findings Study Study Approach Key Findings Kemper (1976) Hefeez et al. (1986) Hasnain et al. (1997) Sufi (1999) Ali et al. (2004) Kori et al. (2013) Alam and Olsthoorn (2014) Zardari et al., (2015) Field experimentation Field study Physical & Numerical GW VDGWTRN Numerical GW MODFLOW-MT 3D Numerical GW MODFLOW-MT 3D Numerical GW SEAWAT Field study Scavenger wells by pumping fresh water with discharge rate of 14 l/s and saline water with discharge rate of 5 l/s (fresh to saline water recovery ratio of 0.74:0.26) separately provide good quality groundwater. Scavenger wells perform poorly by pumping marginal to hazardous quality water through freshwater discharge outlet due to improper operational strategies. Scavenger wells with 1:1 fresh to saline water recovery ratio can pump water of acceptable quality (salinity of 1229 mg/l). Pumped water salinity of 1090 mg/l after one week of continuous pumping and pumped water salinity of 1040 mg/l after one week of intermittent pumping at 12 h/d show that intermittent pumping has low effect on the performance of scavenger wells. The increased pumping of freshwater and decreased withdrawal of saline water can be achieved by changing the original (design) recovery ratios of freshwater to saline water withdrawals (of the two wells studied) from 0.5:0.5 (original) to 0.7: 0.3 (for the first study well) and from 0.3:0.7 (original) to 0.62:0.38 ((for the second study well) with the same design operation time of 15 hours per day. Scavenger wells can provide good quality pumped water with operation time of 12 or 13 h/d with recovery ratio (ratio of freshwater to saline water withdrawals) of 0.5:0.5. Scavenger wells prevent salinization in the pumped water due to saltwater upconing thereby they can pump good quality groundwater on long-term sustainable basis. Operation of the scavenger well at the design operational time of 14.4 hours/day provides a good quality water pumped through the freshwater pump installed in fresh water layer. Quality of water pumped through the freshwater pump deteriorates if the scavenger well is operated beyond the design operational time of 14.4 hours/day. Scavenger Skimming Wells Scavenger wells do not perform satisfactorily because they pump marginal to hazardous quality water through freshwater discharge outlets due to mainly improper operational strategies. Scavenger wells can perform better by delivering good quality water through freshwater discharge outlet if they are properly designed and operated. Scavenger wells can provide good quality pumped water through freshwater discharge outlet with operation time of h/d and recovery ratio (ratio of freshwater to saline water withdrawals) of 0.5:0.5. The increased pumping of freshwater and decreased withdrawal of saline water can be achieved with recovery ratio of freshwater to saline water withdrawals of 0.7:0.3 with the design operational time of 15 hours per day.