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ISSN : 1225-6692(Print)
ISSN : 2287-4518(Online)
Journal of the Korean earth science society Vol.41 No.4 pp.381-390
DOI : https://doi.org/10.5467/JKESS.2020.41.4.381

Groundwater Productivity and Rehabilitation of Radial Collector Wells for Agriculture near Okseong Underground Dam

Hang-Tak Jeon1, Se-Yeong Hamm1*, Soun-Ouk Hong2, Sang Yong Lee3, Hyoung-Soo Kim4
1Department of Geological Sciences, Busan National University, Busan 46241, Korea
2Jeongbuk Regional Headquarter, Korea Rural Community Corporation, Jeonju 54969, Korea
3Impulse Tech Co., Ltd., Busan 46531, Korea
4Department of Renewable Energy, Jungwon University, Goesan 28024, Korea
*Corresponding author: hsy@pusan.ac.kr Tel: +82-51-510-2252
August 14, 2020 August 26, 2020 August 26, 2020

Abstract


When a radial collector well is installed and operated for agricultural purposes, negative impacts may be observed over time due to the clogging of horizontal arms, such as reduced groundwater discharge and water quality deterioration. When radial collector well No. 2 was rehabilitated using the high-pressure impulse generation technique, the specific capacity and transmissivity were increased by 43.1 and 100.6%, respectively. In contrast, according to air surging, the specific capacity and transmissivity increased by 33.8 and 85.8%, respectively, compared to the initial rate before rehabilitation. During the operation of radial collector wells since construction, the time of well rehabilitation can be effectively determined by continuously monitoring the groundwater level and pumping rate of the radial collector wells, thereby preventing a decrease in productivity.



초록


    National Research Foundation of Korea
    NRF-2020R1A2B5B02002198

    Introduction

    In the world, the first horizontal wells were developed in Texas in the 1920s, and then radial wells were first developed in 1927 in Malta, Ohio, for oil development. Currently, radial collector wells are installed at the sites of large-scale riverbank filtration water intake. The radial collector wells consist of a caisson and horizontal arms and groundwater is pumped through the screens of the horizontal arms. In general, compared to vertical wells, radial collector wells are widely contacted with aquifers and can intake a large amount of groundwater, with an advantage of small drawdown due to spatially dispersed effect of water intake (Lee et al., 2010;Bakker et al., 2005). However, when radial collector wells after development are operated for a certain period of time, phenomena such as reduction of the pumping rate and deterioration of water quality take place due to the clogging of horizontal arms screen. In general, well clogging is largely divided into physical and chemical/biological clogging (Van Beek et al., 2009;Van Beek, 2012). Well clogging results in a decrease in the pumping rate of radial collector wells, deterioration in water quality, and a decrease in well efficiency and eventually causes the well closure with a huge economic damage. The accumulation of particles around horizontal arms screens concentrically expands over time and produces a gradual decrease of permeability of the aquifer around horizontal arms (Houben and Treskatis, 2007). Therefore, appropriate rehabilitation should be performed at the time when the specific capacity has been decreased by 25% comparing to the specific capacity of the initial development of the well. The specific capacity below 50% indicates the large accumulation of particles around horizontal arms screens and greatly increased cost of rehabilitation of the well with a decreased well efficiency.

    Well rehabilitation increases the low efficiency of wells caused by aging or clogging of the wells and is largely classified into physical methods and biological/ chemical methods. Physical methods include brushing, surge block swabbing, high-pressure fluid jet, explosive technique, high pressured gas, high-pressure pulse sparking, ultrasonic jet, heating, etc. Biological/chemical methods include acid treatment, sequestering using organic solvents, and antibacterial agent treatment. According to various conditions in the field, several methods mentioned above may be applied together. Radial collector wells are mainly rehabilitated by using high-pressure fluid jet method which injects fresh water and air into horizontal arms to remove clogging materials and is effectively applied with a large amount of clogging materials in the horizontal arms. The high-pressure fluid jet method should be carefully applied because high pressure can damage the screens of the horizontal arms while low pressure cannot easily remove the clogging materials with a longer time of removing the materials.

    Domestic researches on the clogging of wells are such as ‘Study on proper maintaining and management for wells (Ministry of Agriculture and Forestry, 1997)’, ‘21st Century Frontier Project (Ministry of Education, Science and Technology, Korea Water Resources Corporation, 2011)’, ‘GAIA project (Ministry of Environment and Pusan National University, 2012)’. Jeon et al. (2019) evaluated the rehabilitation efficiency of vertical wells by using the high-pressure air impulse generation method at a riverbank filtration site in Daesan-myeon, Changwon City. In Korea, the clogging of vertical wells has been identified and rehabilitation equipments of the vertical wells have been developed. However, in Korea, the technology for the maintenance and treatment of the radial collector wells and rehabilitation of horizontal arms is not much progressed, with many tasks to be solved for the treatment of the clogged horizontal arms.

    In Korea, a total of 98 agricultural radial collector wells have a history of over 35 years. In 1983, the Korea Rural Community Corporation first installed radial collector wells in Ian-myeon, Gyeongsangbukdo province in a series of the underground dam project confronting drought in 1981. Most of the radial collector wells were installed between 1980s and 1990s (Hong et al., 2016). These radial collector wells are maintained and managed through repairing facilities and well rehabilitation. Changwon City in the early 2000s and Gimhae City in 2013 installed and have been operating the radial collector wells for producing municipal water through riverbank filtration. In addition, the radial collector wells near the Taehwa River in Ulsan City in 2009 and the Hongje Stream in Seoul in 2005 have also been installed and utilized to stably maintain streamflow rate of the drying streams. In contrast, a few researches have been conducted on radial collector wells (Chung et al., 2004;Lee et al., 2010;Kim et al., 2014;Park et al., 2015). Lee et al. (2010) evaluated the groundwater productivity of radial collector wells planned in Jeungsan-ri area, Changnyeonggun, by numerical simulation. Kim et al. (2014) studied the relationship between the pumping rate of the radial collector wells and the hydraulic characteristics of the aquifer in the riverbank filtration area in Daesan-myeon, Changwon City. Also, Park et al. (2015) evaluated the performance of the radial collector wells according to the arrangement of horizontal arms through groundwater modeling. Collins and Houben (2020) compared and reviewed hydraulic properties near radial collector wells by using several analytical and numerical models.

    To date, in Korea, no case has been reported that analyzed the effect of the rehabilitation of radial collector wells in the field. In this study, we evaluated the productivity of No. 2 and 3 radial collector wells among the four wells located upstream of the Okseong underground dam in Wooseong-myeon, Gongju City (Fig. 1).

    Study Area

    Geological and geographical setting

    The study area is surrounded by Mt. Chaejuk (170 m) to the east, Mt. Yak (277.9 m) to the north, and Mt. Mukbang (370.2 m) to the west. The Yugu Stream, a tributary of the Geum River, flow in the south of the study site from west to east. The Yugu Stream flows south near the Okseong underground dam and finally joins the Geum River. The topographic altitude is low around the streams, but the other parts of the study area are mountainous. The topographic elevation generally increases from east to west.

    According to Kim et al. (1976), the geology of the study area consists of Precambrian banded gneiss in the north with inclusion of garnet at different locations (Fig. 2). On the other hand, the southeastern part is composed of an age-unknown porphyroclastic gneissose granite, andesite, and Cretaceous conglomerate. Breccia, calcareous shale, and siltstone are intercalated within the conglomerate. The bedrock of the Precambrian banded gneiss is intruded by the porphyroclastic gneissose granite, conglomerate, and andesite in order. On the surface, the Quaternary alluvial bed covers other rocks with unconformity. The geology of the area where the radial collector wells is located, consists of the Quaternary alluvial bed (soil, sand, and sand/gravel layers). According to the geological section of the Okseong underground dam, the thicknesses of the soil layer, sand layer, and sand/gravel layer, respectively, are 0-10, 0-7, and 1-7 m, with spatial variation (Fig. 3). Weathered zone generally appears from 2.0 m below mean sea level under the alluvial bed.

    Configuration of the radial collector wells

    The four radial collector wells (No. 1-4) in Okseongri, Wooseong-myeon, Gongju City consist of a 3.5-m inner diameter caisson and 15-cm diameter horizontal arms, with the installation depth of mostly 10-12 m (Fig. 4). The radial collector wells are located at the upstream of the Okseong underground dam in Wooseongmyeon, Gongju City, which have been developed for more than 30 years ago (Fig. 1). In the radial collector well, one or two submersible pumps or ground pumps of 30 HP are usually installed depending on well productivity. A PVC screen with a diameter of 65 mm is usually installed in the horizontal arm of the length of 30 m. On the screen, a total of 12 slots are opened at intervals of 30°, with the length of ~100 mm and the opening of 2 mm. The spacing between slots is ~40 mm, and the opening rate is less than 8% when considering the connection part (socket) of the PVC screen.

    Due to long-term operation for over 30 years, a significant part of the horizontal arms has been deformed or damaged, and the clogging has progressed considerably to the surrounding alluvial bed including the screen wall and slot of the horizontal arms, with the deterioration of the water quality. Hong et al. (2016) reported that Fe(OH)3 acts as a major clogging material in this area. Fe and Mn concentrations in groundwater at the radial collector well No. 2, 13.88 and 2.566 mg/L, were 13.88 and 2.566mg/L, respectively that were significantly higher than groundwater quality standard (Ministry of Food, Agriculture, Forestry and Fisheries and Korea Rural Community Corporation, 2015). Choo et al. (2012) also mentioned ironhydroxides as typical clogging material in water wells in Mt. Geumjeong. Besides, the horizontal arms were deformed due to the aging and clogged by gravel and sand around the slots of the screen (Ministry of Food, Agriculture, Forestry and Fisheries and Korea Rural Community Corporation, 2015).

    Results

    Procedure of high-pressure impulse technique

    The PVC horizontal arms of the radial collector well have been deteriorated after more than 30 years, so a significant part of the horizontal arms has been deformed and damaged, with the deposition of sand and gravel inside the horizontal arms. Therefore, the withdrawal rate of groundwater at No. 2 radial collector well decreased sharply comparing to the initial state of the development. As consequence, the high-pressure impulse generation technique and air surging for rehabilitation of the radial collector well No. 2 were carried out in the following process.

    • 1) Pumping test was conducted to evaluate the hydraulic characteristics around the radial collector well and to estimate the application pressure of the high-pressure impulse generation technique for each horizontal arm.

    • 2) The water inside the horizontal arms was completely removed by using the intake pump and discharge pump in the radial collector well.

    • 3) The application pressure of the high-pressure impulse generation technique was determined based on the pumping test and the specific capacity of each horizontal arm.

    • 4) Image well logging was performed to investigate the clogging of the screen of each horizontal arm. The clogging materials including gravels can damage the horizontal arms when rehabilitating the radial collector wells using air surging or high-pressure impulse generation technique.

    • 5) A stainless steel wound wire screen of diameter 50-60mm with a semi-permanent lifespan and durability was inserted into horizontal arms for protecting the material (Fig. 5). The wound wire screen has 10 times more strength and 5 times more opening rate than the existing PVC screen of diameter 65 mm. The PVC screen has a tensile strength of 40.7 N/mm2 and an opening rate of less than 8%. By contrast, the stainless steel wound wire screen of the material of STS304 has a tensile strength of 517.0 N/mm2 and an opening rate of 40%, with the slot opening of 1 mm, the outer diameter of 53 mm, and the length of 2 m. In the radial collector well No. 2, a total of 20 wound wire screens were assembled with male and female screws at both ends of the screen.

    In order to maximize well rehabilitation, a 4-m wound wire screen was inserted into the lower horizontal arms of nos. 5, 8, 10, 12, 13, and 19, and a 16-m wound wire screen was inserted into the lower No. 15 horizontal arm (Fig. 5) A 4-m wound wire screen was inserted into other upper and lower horizontal arms. The wound wire screens could not be applied in the case of deformed or damaged horizontal arms.

    • 6) Before performing high-pressure impulse generation technique, water jetting using a high pressure selfrotating water jetting machine of 60-mm diameter was used to remove clogging matters to the surface of the PVC screen, with maximum rotation speed of 2000 rpm and maximum injection pressure of 400 kgf/cm2 .

    • 7) Using compressed air, a high-pressure impulse generator of 40 mm diameter with a filling volume of 300 cm3 and maximum impact pressure of 330 kgf/cm2 rehabilitated the radial collector well as removing clogging materials from the horizontal arm screen and surrounding aquifer (Fig. 6). Discharge pump withdrew the clogging materials that were removed by the highpressure impulse generating technique and was operated until fresh water was discharged.

    • 8) Image well logging was performed to confirm the removal of the clogging materials from the horizontal arms screen as comparing before and after the well rehabilitation.

    • 9) Pumping test was conducted for comparing hydraulic parameters of the aquifer and the productivity of the radial collector wells before and after the well rehabilitation.

    Hydraulic property of the radial collector well

    Pumping test was conducted to evaluate the groundwater abstraction before and after air surging for the radial collector well No. 2. Air surging blew clogging materials by spraying the mixture of water and compressed air with low pressure through a plastic pipe inside the horizontal arms. By operating two discharge pumps, the pumping test before air surging was conducted with a pumping rate of 3,120 m3 /day and the pumping time of 45 minutes, and resulted in the maximum drawdown of 2.54m (Table 1).

    Pumping analysis was executed by using AQTESOLV for windows 4.5 and interpreted by the methods of unconfined model for the well No. 2 and leaky confined model for the well No. 3. The different models for the wells No. 2 and 3 may be due to heterogeneous and anisotropic conditions of the aquifer in the study area as shown in Fig. 3. This kind of heterogeneity and anisotropy was also reported in alluvial aquifer near the Nakdong River (Hamm et al., 2002). By the pumping test analysis for the radial collector well No. 2 by using Theis solution for unconfined aquifer (Theis, 1935), the transmissivity (T) was 295.1 m2 /day and the storage coefficient (S) was 0.2930 (Fig. 7, Table 1). The pumping test after air surging was conducted for 70 minutes with a pumping rate of 1,512 m3 /day and the maximum drawdown of 0.92m. By the analysis of Theis solution for unconfined aquifer (Theis, 1935), the T value was 548.2 m2 /day, and the S value was 0.2063 (Fig. 8, Table 1).

    The pumping test after applying the high-pressure impulse generation technique subsequently air surging, was performed for 53 minutes, with a pumping rate of 2,688 m3 /day and the maximum drawdown of 1.53 m. By analyzing the pumping test with Theis solution for unconfined aquifer (Theis, 1935), the T value was 592.0 m2 /day, and the S value was 0.1277 (Fig. 9, Table 1).

    Pumping tests were conducted before and after air surging for the radial collector well No. 3 to evaluate the well productivity and hydraulic parameters. The pumping test before air surging was performed for 100 minutes with a pumping rate of 3,624 m3 /day and the maximum drawdown of 3.29 m (Table 1). Using Moench (1985) solution of leaky confined aquifer model, the T and S values were 74.4 and 0.2633 m2 / day, respectively (Fig. 10, Table 1). After the air surging, the pumping test was carried out for 106 minutes with a pumping rate of 6,216 m3 /day and the maximum drawdown of 3.34 m. Using Moench (1985) solution of leaky confined aquifer model, the T and S values were 507.8 and 0.2007 m2 /day, respectively (Fig. 11, Table 1).

    Meanwhile, the T value increased by the highpressure impact generation technique, but the S value decreased (Table 1). The decrease of the S value is inferred by the removal of the clogging matters accumulated around the horizontal arms. Jeon et al. (2019) reported that the S value decreased at a greater numbers among the rehabilitated wells while the T values increased at all the rehabilitated wells.

    Assessment of well productivity by rehabilitation

    The specific capacity (Q/s), T, and S values were compared for the radial collector wells before and after the rehabilitation (air surging and high-pressure impulse generation technique) (Fig. 12). By applying the high-pressure impulse generation technique to the radial collector well No. 2, the T value and Q/s, respectively, increased by 100.6 and 43.1% more than the initial state before air surging while by the air surging, the T and Q/s values, respectively, increased by 85.8 and 33.8% comparing to the initial state. Therefore, it was confirmed that the T and Q/s values were increased more by the high-pressure impulse generating technique than by air surging. Therefore, by conducting high-pressure impulse generation to only 8% (40 m) of the total length of the horizontal arms (520 m with upper 6 arms and lower 20 arms) could increase the T value by 100.6%.

    After air surging at the radial collector well No. 3, the T value increased by 582% and the Q/s value by 68.9% more than the initial state before air surging. In contrast, at the riverbank filtration site in Daesanmyeong, Changwon City, the abstraction rate increased by an average of 14.8% using brushing and air surging, and by 217.5% using high-pressure impulse generation technique (Ministry of Environment, Pusan National University, 2012). Besides, it was confirmed that the Q/s value increased at the radial collector well No. 2 by the high-pressure impulse generation technique and air surging and increased at the radial collector well No. 3, comparing to the initial state of air surging (Fig. 13). In other nations, it was reported that the drawdown was reduced from 1.42 to 0.46 m through air surging with an increase of pumping rate from 577 to 1,809 m3 /day (Subsurface Tech, 2008) and the drawdown was decreased from 5.34 to 2.49 m with an increase of pumping rate from 2,290 to 4,766 m3 /day (WellJet, 2011).

    Discussion and Conclusions

    This study evaluated the well productivity of the radial collector wells No. 2 and No. 3 located at the upstream of the Okseong underground dam in Wooseongmyeon, Gongju City. In this study, the high-pressure impulse generation technique which is one of well rehabilitation methods, was able to effectively remove clogging materials attached to horizontal arms. Before applying high-pressure impulse generation technique, stainless steel wound wire screen was installed into the horizontal arms to prevent damage of the aged PVC horizontal arms. For the radial collector well No. 2, the T value and Q/s increased by 100.6 and 43.1%, respectively, by the high-pressure impulse generation technique and increased by 85.8 and 33.8%, by the air surging, comparing to the initial state. On the other hand, by air surging at the radial collector well No. 3, the T and Q/s values increased by 582 and 68.9%, respectively, more than the initial state before air surging. Resultantly, the high-pressure impulse generating technique more effectively increased the T and Q/s values than the air surging did. Besides, the S values decreased by the high-pressure impulse generation technique and air surging. The reason of decrease in the S values may be due to the removal of the clogging matters accumulated around the horizontal arms.

    For proper maintenance and management of the radial collector wells, after the completion of the wells, well development should be conducted for ensuring flow path through the filter pack surrounding screen of the horizontal arms. It is also important to continuously monitor the horizontal arms. The specific capacity of the radial collector wells should be periodically checked in order to estimate the clogging materials around the horizontal arm screen. By the monitoring, when the horizontal arms are deteriorated or the pumping rate decreases due to clogging materials adhering to the screen, well rehabilitation should be implemented. In addition, when the radial collector wells are deteriorated, repairing and replacement of auxiliary facilities such as pipelines and valves inside and outside the wells and additional installation of the horizontal arms are performed.

    Acknowledgments

    This research was funded by by National Research Foundation of Korea (NRF) under the Ministry of Science and ICT (No. NRF-2020R1A2B5B02002198).

    Figure

    JKESS-41-4-381_F1.gif

    Study area.

    JKESS-41-4-381_F2.gif

    Geological map in study area (Kim et al., 1976).

    JKESS-41-4-381_F3.gif

    Cross section (A-A') in Fig. 1.

    JKESS-41-4-381_F4.gif

    Radial collector well No. 2 showing caisson with horizontal arms of upper layer (blue color) and lower layer (black color) (Ministry of Food, Agriculture, Forestry and Fisheries and Korea Rural Community Corporation, 2015).

    JKESS-41-4-381_F5.gif

    Scheme of wound wire screen into the horizontal arms at the radial collector well No. 2 (Ministry of Food, Agriculture, Forestry and Fisheries and Korea Rural Community Corporation, 2015).

    JKESS-41-4-381_F6.gif

    Installation scheme of impulse generation equipment (Ministry of Food, Agriculture, Forestry and Fisheries and Korea Rural Community Corporation, 2015)

    JKESS-41-4-381_F7.gif

    Drawdown vs. elapsed time by the pumping test before air surging at the radial collector well No. 2.

    JKESS-41-4-381_F8.gif

    Drawdown vs. elapsed time by the pumping test after air surging at the radial collector well No. 2.

    JKESS-41-4-381_F9.gif

    Drawdown vs. elapsed time by the pumping test after high-pressure impulse generation technique at the radial collector well No. 2.

    JKESS-41-4-381_F10.gif

    Drawdown vs. elapsed time by the pumping test before air surging at the radial collector well No. 3.

    JKESS-41-4-381_F11.gif

    Drawdown vs. elapsed time by the pumping test after air surging at the radial collector well No. 3.

    JKESS-41-4-381_F12.gif

    Comparison of specific capacity (Q/s), transmissivity (T), and storage coefficient (S) before and after rehabilitations (high-pressure impulse generation and air surging).

    JKESS-41-4-381_F13.gif

    Relationship of Q/s vs. S and Q/s vs. T for the radial collector wells No. 2 and No. 3.

    Table

    Result of pumping test analyses before and after air surging and high-pressure impulse generation technique at the radial collector wells No. 2 and No. 3

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