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ISSN : 1225-6692(Print)
ISSN : 2287-4518(Online)
Journal of the Korean earth science society Vol.42 No.4 pp.425-444
DOI : https://doi.org/10.5467/JKESS.2021.42.4.425

Hydrogeological Characteristics of a Riverine Wetland in the Nakdong River Delta, Korea

Hang-Tak Jeon1, Eun-Ji Cha2, Woo-Ri Lim1, Sul-Min Yoon1, Se-Yeong Hamm1*
1Department of Geological Sciences, Pusan National University, Busan 46241, Korea
2Korea Environment Institute, Sejong 30147, Korea
*Corresponding author: hsy@pusan.ac.kr Tel: +82-51-510-2252
August 2, 2021 August 15, 2021 August 15, 2021

Abstract


Investigating the physical and chemical properties of riverine wetlands is necessary to understand their distribution characteristics and depositional environment. This study investigated the physical (particle size, color, and type) and chemical properties (organic, inorganic, and moisture contents) of sediments in Samrak wetland, located in the Nakdong River estuary area in Busan, South Korea. The particle size analysis indicated that the hydraulic conductivity values for the coarse grain and the mixture of coarse and fine grains ranged from 2.03 to 3.49×10−1 cm s−1 and 7.18×10−3 to 1.24×10−7 cm s−1 , respectively. In-situ water quality and laboratory-based chemical analyses and radon-222 measurement were performed on groundwater and surface water in the wetland and water from the nearby Nakdong River. The physical and chemical properties of Samrak wetland was characterized by the sediments in the vertical and lateral direction. The concentrations of chemical components in the wetland groundwater were distinctly higher than those in the Nakdong River water though the wetland groundwater and Nakdong River water equally belonged to the Ca-HCO3 type.



초록


    Introduction

    The formation, persistence, size, and function of wetlands primarily depend on hydrologic processes (Carter, 1996). Distribution, types, vegetation, and soils of wetlands are mainly determined by topographic, geologic, and climate factors. Ramsar (2007) defines wetlands as marshes, peatlands, floodplains, rivers and lakes, salt marshes, mangrove swamps, seagrass areas, coral reefs, intertidal zones of 6 m or less deep, and artificial wetlands (wastewater treatment ponds and reservoirs). On the other side, the US Environmental Protection Agency (https://www.epa.gov/wetlands/ what-wetland) defines wetlands where water covers or is located near the surface of the soil layer throughout the year or during crop seasons. Wetlands are places where water is stagnant or flowing slowly and plays the role of water resource storage, flood control, and groundwater recharge and maintenance. Wetlands are habitats for various species since they have abundant nutrients high productivity. When wetland water flows out, it becomes a nutrient source for organisms downstream and in coastal areas. In addition, wetlands maintain the stability of lakeside, protect erosion, purify sediment and water quality, circulate nutrients, and secure a stable climatic environment of the target area. It also brings economic benefits to fishing, agriculture, biological resources, entertainment and tourism. According to article 8 of the Wetland Conservation Act in Korea, as of December 2020, 27 wetlands are registered by the Ministry of Environment, 12 intertidal wetlands (tidal flats) by the Ministry of Oceans and Fisheries, and 7 wetlands by city mayors and provincial governors, totaling 47 wetlands. On the other hand, according to article 9 of the Wetlands Conservation Act, there are 23 Ramsar wetlands nationwide, including Daewangsan Yongneup wetland. Moon (2005) defined wetlands as a transition zone between the terrestrial and hydrosphere, including both terrestrial and hydrosphere characteristics. He divided wetlands into major, intermediate, and sub-groups. The major group is divided into mountain wetlands and riverine wetlands based on geographic location. Mountain wetlands are divided into slope wetlands and basin wetlands. On the other hand, riverine wetlands are divided into river wetlands, lake wetlands, and fen wetlands. Finally, sub-group wetlands are classified according to inflow and outflow conditions of wetland water.

    Ryu et al. (1997, 1998), Lee et al. (2004), Lee and Cho (2005), Kang et al. (2007) studied the sedimentary environment and distribution characteristics of wetlands. Chi et al. (2018) evaluated the ecological health of estuary wetlands in China, based on the characteristics and spatial heterogeneity of the ecosystem of the Yellow River estuary. Chen et al. (2014) evaluated the nitrogen removal effect of plants in wetlands by batch experiment, using stable isotopes and mass balance. Rho (2007) presented spatial distribution characteristics and temporal changes of Han River estuary wetlands. Eser and Rosen (1999) revealed groundwater flow and hydrogeochemical characteristics of the Stump Bay wetland adjacent to Lake Taupo in New Zealand. Bragg (2002) characterized the hydraulic properties of peat beds in the wetlands in Scotland. Cha et al. (2010a, 2010b) reported the hydraulic and physical properties of sedimentary layers in mountain wetlands. Baek and Chun (2004) studied seasonal changes of sedimentary structure in Duuri wetland in Yeongwang-gun, Jeonnam province. Garcia et al. (2007) recognized the emission of greenhouse gas (GHG) by underground flow in artificial wetlands and the biodegradability of anaerobic environments. Picek et al. (2007) studied Greenhouse gas (GHG) emission from artificial wetlands, and Phillips and Beeri (2008) suggested a hydro soil role in GHG emission from agricultural wetlands in Prairie Pothole Region. Keller (2011) reviewed the role of wetlands in the global carbon cycle and predicted that the CH4 release of wetlands due to anthropogenic effects would cause global climate change. Ilyas et al. (2021) predicted the removal efficiency of emerging organic pollutants (pharmaceuticals, personal care products, steroids, etc.), using artificial wetlands. Zhao et al. (2021) reviewed the methods for removing polycyclic aromatic hydrocarbons (PAHs) using artificial wetlands. Im et al. (2015) analyzed the topographic changes of the river-side areas and downstream and midstream river islands in the Nakdong River basin after the completion of the Four Major Rivers Project (FMRP) and reported limited ecological roles of created riverine wetlands along the Nakdong River by the FMRP.

    This study intended to characterize the hydrogeological characteristics of Samrak wetland located at the estuary of the Nakdong River in 2010, before the end of the FMRP in 2012.

    Study Area

    Samrak wetland, the study area, is a riverine wetland located in Samrak-dong, Busan city, and currently becomes Samrak Ecological Park (Fig. 1), within 35 ° 10'24.2''N-35 ° 11'14.3''N to 128 ° 57'56.0''E- 128 ° 58'47.0''E. The wetland consists of Quaternary alluvium of sand, clay, and gravel. The bedrock of the wetland is biotite granite and andesitic breccia belonging to the Cretaceous period (Chang et al., 1983). The wetland has a surface elevation of ~0-3 m, an area of ~941,978 m 2 . Samrak wetland is a riverine wetland (Ramsar type code: permanent stream, M) and is formed along the main Nakdong River (Nakdong River Basin Environment Agency, 2008). 27 shallow sediment samples shallower than 1-meter depth and 107 deep sediment samples deeper than 1- meter depth at three locations (SN1, SN2, and SN3) were collected at relatively equal intervals in the wetland to reveal the vertical and lateral characteristics of sediments in Samrak wetland (Fig. 2). Wetland groundwater, wetland surface water, and the surrounding Nakdong River water were collected during the wet season (August to October) and dry season (October to November) for recognizing chemical properties.

    Two piezometers (SNM1 and SNM2) were installed in the northern and southern parts of the wetland, respectively, to understand the physical and chemical characteristics of groundwater in Samrak wetland in July 2010 (Table 1, Fig. 2). The piezometers have 1 m in length and BX size in diameter and are made of HDPE slotted pipe (Eijkelkamp Agrisearch Equipment). The depths of the SNM 1 screen and SNM 2 screen are 83 and 41 cm below the ground surface, respectively. Soon after the installation, due to the water level decrease of SNM 2, seasonal groundwater level and temperature changes were only observed on SNM1, using a pressure transducer (Diver from Eijkelkamp, measurement range 10 m).

    Methods

    Sampling of sediments

    Sediments in riverine wetlands give very important information on the mechanism for wetland formation and the change of wetlands. Shallow sediments reflect the current environment of the wetland and deeper sediments indicate changes in the sedimentary environment from the past. In order to estimate organic and inorganic matters, moisture content, and particle size, shallow sediments were sampled at three points in the north of the wetland (SNS13, SNS14, SNS15) and seven points in the south (SNS1, SNS2, SNS3, SNS4, SNS5, SNS6, SNS7), five points (SNS8, SNS9, SNS10, SNS11, SNS12) in the middle part, were collected on July 22 and 23, 2010, using a gouge hand auger suitable for collecting undisturbed samples (Fig. 2). The samples were collected at a depth of about 1 m (Table 2).

    However, the shallow sediments are fragile to be disturbed by artificial factors (field farming, agricultural land, reclaimed land, etc.) and river flooding. Such that, deeper sediments of the riverine wetland should be collected using drilling equipment (Geoprobe, model no. SD30). Geoprobe can excavate a maximum of 30 m depth, with the advantage of obtaining undisturbed samples in the PVC pipes during excavation. At three locations (SN1, SN2, SN3), A total of 107 sediment samples were collected at three depths (shallow, medium, and deep depths) on October 12 and 13, 2010 (Table 2, Fig. 2).

    Measurement of water content and total organic matter of sediments

    Shallow (SNS1-SNS15) and deep (SN1-SN3) sediment samples were heated at 105±5 ° C for 24 hours in a furnace. After that, the water content (w) was measured by comparing the weight of the wet natural sample and the dried sample and is expressed by the equation:

    w = w f w d w d w c × 100 ( % )
    (1)

    Here, wt is the weight of the wet natural sample and the container (g), wd is the weight of the dried sample and the container (g), and wc is the weight of the container (g).

    The total content of organic matter in sediments was measured by loss-on- ignition (LOI) method (Oliver et al. 2001;Jung et al., 2014). When about 1 gram of the sample is heated at 450 to 550 ° C for ~3 hours, total organic matter content by the LOI method is the percentage of the reduced weight by heating to the dried sample weight.

    Particle size measurement and estimation of hydraulic conductivity

    Particle size analysis is performed to determine particle size distribution, classification, and hydraulic conductivity of the sediment. The particle size distribution provides information on the characteristics, transport and deposition mechanism of the sediment in a certain sedimentary environment. Also, the particle size is related to the properties of the sediment, such as hydraulic conductivity, porosity, and rheology, and these properties are also predicted by the particle size distribution (Chough et al., 1995). The particle size is determined by the wet sieve method for coarse particles or hydrometer analysis for fine particles (finer than 0.074 mm). The wet sieve method stacks sieves in the order of finer size below coarser size then adds distilled water to the collected samples and loosens the samples not to become agglomerate. The weight of the particles was measured and the average particle size of the particles was calculated by weight ratio. The particle size of the fine particles passing through the No. 200 (0.074 mm) sieve is determined by hydrometer analysis. Based on Stokes' law, fine particles in suspension settle down over time in order from largest to smallest. The maximum diameter (D, mm) of the particles in the suspension is

    D = 30 η 980 ( G s G w ) × L t
    (2)

    where η is viscosity depending on water temperature (mm poise), Gs is the specific gravity of the sediment, Gw is the specific gravity of the water depending on temperature, t is sedimentation time (min), and L is effective depth (settled distance of particles in a certain time, cm).

    Hydraulic conductivity (K, LT−1 ) relies on porosity, structure, particle shape, effective particle size, and fluid viscosity of sediments and is estimated by various empirical equations (Kasenow, 2002):

    K = ( g / ν ) × ( β ) × ( ν ( n ) ) × ( d e )
    (3)

    where g is the gravitational acceleration (MT−2 ), ν is kinematic viscosity coefficient (L 2 T−1 ), β is dimensionless coefficient according to the characteristics of the porous medium (texture, particle shape, particle composition, anisotropy, etc.), v(n) is porosity function, n is porosity (dimensionless), and de is effective particle size (L). In this study, Hazen (1892), Beyer (1964), Kozeny (1927), Carman (1956), Sauerbrei (Kasenow, 2002), and Slichter (1899) formulas were used for estimating hydraulic conductivity of the sediments.

    Chemical analyses of organic and inorganic constituents and oxides of sediments

    Organic matters are formed by combining nitrogen (N), oxygen (O), sulfur (S), hydrogen (H), phosphorus (P), etc. around carbon (C) and are essential elements for constituting an organism. The organic matter (C, H, O, N, and S) content in the sediment was analyzed for the nine samples identical to the particle size analysis. The analysis was performed with an Elemental Analyzer (Elementar Analysensysteme GmbH, Germany) at the Busan Center of the Korea Basic Science Institute. For a sample of up to 2.0 mg, the analysis condition was an operating temperature of 1,150 ° C with the standard reagent of sulfanilic acid (8.1% of N, 41.6% of C, 18.5% of S, 4.1% of H, and 26.2% of O).

    Through pre-treatment of the sediment samples, cations (Al 3+ , As 3+ , Ca 2+ , Cr 3+ , Fe 2+ , K + , Mg 2+ , Mn 2+ , Na + , and Zn 2+ ) were analyzed by ICP Optical Emission Spectrometer (ICP-OES) of PerkinElmer Inc. and anions (F - , Cl, NO3, PO43−, and SO42−) were analyzed using ion chromatography (ICS-1500) of Dionex at the Busan Center of the Korea Basic Science Research Institute.

    The oxides (SiO2, Al2O3, Fe2O3, MnO, CaO, MgO, K2O, Na2O, P2O5, and TiO2) of the nine sediment samples as those for organic analysis were analyzed by X-ray Fluorescence Spectrometer (Philips PW2400) at the Busan Center of the Korea Basic Science Institute. Before oxides analysis, sediment samples that contain organic matter were pre-treated by the LOI method to decompose organic matter before the analysis.

    Water quality analysis

    In-situ water quality measurement of wetland groundwater, surface water, and the Nakdong River water was done for temperature, pH, oxidationreduction potential (Eh), electric conductivity (EC), salinity, total dissolved solids (TDS), dissolved oxygen (DO), alkalinity, and turbidity. Water temperature was measured using digital thermometer (model SM- 1250MC of Sato), pH and Eh were measured by using portable pH meter (model 250A of Orion), EC, salinity, and TDS were measured by Orion 115 EC meter, DO was measured by Orion 810 DO meter, and alkalinity was measured using Lovibond's Photometer System Multidirect.

    Water samples for laboratory analysis were taken at the same locations as the in-situ water quality measurement. Water samples were collected after the stabilization of water quality that was recognized by no change in water temperature in time. After sample collection, suspended material was filtered using 45 μm filter paper, and then the samples for cation analysis were acid-treated with 0.05 N nitric acid to prevent adsorption of ions to the sample bottle. The filtered sample was placed in sterile sample bottle of 100 mL and transported at the state of 4 ° C or less. Eleven cations (Al 3+ , As 3+ , Ca 2+ , Cr 3+ , Fe 2+ , K + , Mg 2+ , Mn 2+ , Na + , Zn 2+ , and Si 4+ ) were analyzed by ICP Optical Emission Spectrometer (Perkin Elmer ICPOES) and five anions (F, Cl, NO3, PO43−, SO42−) were analyzed using ion chromatography (Dionex ICS-1500) at the Busan Center of the Korea Basic Science Research Institute.

    Measurement of Radon-222 concentration

    The samples for radon-222 (Rn-222) analysis of water were collected at the same point as the in-situ water quality analysis, were collected in 500 mL brown glass bottles without filtration and were sealed with a rubber stopper to prevent gas escape and then transported at the condition of 4 ° C or less. The Rn- 222 concentration was measured in the laboratory using Durridge RAD-7, with the measurement error of ±5% within 3.8 days (the half-life of Rn-222) after collection. Durridge RAD-7 consists of alpha-detector and desiccant.

    The actual Rn-222 amount (Cwater) in the collected water sample can be expressed as the sum of the radon present in the sample and radon present in the air of the sample bottle (Weigel, 1978):

    C w a t e r V w a t e r = C a i r V a i r + k C a i r V w a t e r
    (4)

    k = 0.105 + 0.405 e 0.0502 T
    (5)

    Here, Cwater is radon concentration in the sample, Cair is radon concentration in the air of the sample bottle, and Vwater and Vair are the volumes of water and air in the sample bottle, respectively. Radon solubility in water, k, is a function of temperature (T):

    Results

    Water content of the sediments

    The water content in the shallow sediments (about 1 m in depth) of Samrak wetland decreases as depth increases from the surface, but the water content increases from about 40 cm below the surface (BS), with showing 13.27-35.07% as well as a similar trend at all locations (Fig. 3). The highest value (35.07%) was shown in SNS3-1-1 (0-19.5 cm depth) at the SNS3 point, and the lowest value (13.27%) in SNS9- 2-1 (23-39 cm deep) at the SNS9 point (Table 3). The lowest water content of the SNS9 point seems due to that the elevation of the SNS9 point is higher than the water level of the surrounding water puddle. On the other hand, the higher water content of the SNS4 point may be because the SNS4 point maybe located below surface water.

    The water contents of the deep sediments (SN1, SN2, and SN3) were between 12.17 and 41.43%, with an average water content of 28.35% which is higher than that of shallow sediments. The higher water content is due to the saturated zone. The water content increased from the surface to depth of 17 m (Fig. 4).

    Organic matter contents in the shallow and deep sediments

    The organic matter content in wetlands is high due to the continuous inflow of organic matter and the production of organic matter by plants in the wetland (Shin et al., 2005). The organic matter content of the shallow sediments obtained by the LOI method showed a high value at a depth of 0 to 40 cm, showing a tendency to decrease as depth increased (Fig. 5). This indicates that organic matter is actively accumulated near the surface of the wetland (around 40 cm depth). The organic matter content in the shallow sediment ranged from 0 to 7.73% (Table 4). The depth of 0-10 cm (SNS6-1-1) at SNS6 point in the southern part, demonstrated the highest organic matter content (7.76%) whereas the depth of 56-64 cm (SNS11-4) at SNS11 point in the north showed 0% organic matter content.

    Based on organic elements analysis for the deep sediments at SN1, SN2, and SN3 points in the wetland, oxygen (O) accounts for the largest amount with an average of 3.94%. Carbon (C) accounts for the second-highest with an average of 0.66%, showing the highest value (0.84%) at the SN3 point in the southern part. Concerning organic matter content according to the depth, SN1 displayed an increasing tendency from shallower zone to the deeper zone, while SN2 and SN3 showed the highest value at the deeper zone and the second highest value at the shallower zone and the lowest value in the intermediate zone (Table 5).

    In comparison, the sediments in Mt. Geumjeong wetland presented higher C contents (2.33-7.61%) than those in this study area (Cha et al., 2010a). The mountain wetland exhibited a relatively high C content near the surface while the riverine wetland displayed a relatively high value in the deep zone. This is caused by a higher accumulation of organic matters content near the surface zone in the mountain wetland, while the accumulation of organic matters is comparatively small near the surface zone in riverine wetlands due to river flooding or rainfall during the wet season. Hence, it is judged that the distribution of organic matter content in the riverine wetlands reflects depositional environment.

    Inorganic components and oxides contents

    As a result of inorganic analysis for the nine samples as the organic analysis, the mean concentration of K + was as high as 21,121 mg/kg, the mean Al 3+ concentration was 19,116 mg/kg, and the mean Fe 2+ concentration was 13,211 mg/kg. Among the anions, the mean Cl concentration as high as 165.7 mg/kg (Table 6).

    As a result of oxides analysis on the nine samples of the inorganic analysis point, the concentrations of SiO2 and K2O decreased from the surface to the depth, whereas the concentrations of the remaining oxides mostly increased as the depth increases (Table 7).

    Particle size distribution and hydraulic conductivity of the deep sediments

    Particle size analysis was performed using samples of SN1-2-1 (shallow depth, 1.5 m BS), SN1-7-3 (medium depth: 1.5-7 m BS), and SN1-14-3 (7-14 m BS) at SN1 point; SN2-2-1 (1.5 m BS), SN2-6-3 (1.5- 6 m BS), and SN2-17-3 (6-17 m BS) at SN2 point; and SN3-2-1 (1-2.85 m BS), SN3-8-3 (2.85-8 m BS), and SN3-16- 3 (8-16 m BS) at SN3 point. As a result of hydrometer analysis for the sediment that passed through no. 200, the SN1-7-3 zone showed the size particles of 0.0124 to 0.0002 mm and the SN1-14-3 zone showed the size particles of 0.0106 to 0.0002 mm. Therefore, the deep depth indicated fine grain, the intermediate depth indicated a mixture of coarse and fine grain, and the shallow zone indicates coarse grain. The degree of sorting becomes better from the deep part (SN1-14-3) to the shallow part (SN1-2-1) (Table 8).

    As a result of hydrometer analysis for the sediment that passed through no. 200, the SN2-2-1 zone displayed the size particles of 0.0114 to 0.0002 mm and the SN2-17-3 zone showed the size particles of 0.0144 to 0.0003 mm. The deeper zone of the SN2 point consists of finer grains, the intermediate zone indicates coarse grains, and the shallow zone indicates the mixture of coarse- and fine-grained sediments. The degree of sorting becomes better from the deeper part (SN2-17-3), the shallower part (SN2-2-1), and to the intermediate part (SN2-6-3) (Table 8).

    As a result of hydrometer analysis of the SN3-2-1 zone, particle sizes ranged from 0.0095 to 0.0002 mm and the SN3-16-3 zone showed the particle range of 0.0126-0.0006 mm. At the SN3 point, the deeper part was fine, the intermediate depth was coarse, and the near-surface part was a mixture of coarse and fine particles. Sorting at the SN3 point in the intermediate part (SN3-8-3), the deeper part (SN3-16-3), and the shallower part (SN3-2-1) became poorer in order (Table 8).

    Based on the empirical formula using grain size distribution, the hydraulic conductivity values at the SN1 point were obtained as 2.68×10−1 cm/sec in SN1- 2-1 (the shallower part of SN1) and 1.56×10−5 cm/sec in SN1-7-3 (the intermediate part of SN1), indicating higher hydraulic conductivity in the shallower zone of coarse grains than in the intermediate part of the mixture of coarse and fine grains. At the SN2 point, the hydraulic conductivity value (7.18×10−3 cm/sec) was smaller at the shallower SN2-2-1 of a mixture of coarse and fine grains while the hydraulic conductivity value (2.03×10−1 cm/sec) was greater at the mid-depth SN2-6-3 of coarse grains. At the SN3 point, the hydraulic conductivity values were estimated to be 1.24×10−7 cm/sec at the shallower SN3-2-1 of a mixture of coarse and fine grains and 3.49×10−1 cm/ sec at the mid-depth SN3-8-3 of coarse-grained material. It is inferred that at the SN2 and SN3, the surface infiltration rate is low due to a lower hydraulic conductivity and groundwater flow rate will be faster in the middle depth. It was impossible to calculate the hydraulic conductivities by the empirical formula since the deeper part was composed of fine grains.

    According to the USDA (1993) soil classification, the sediments of the SN1, SN2, and SN3 points in the triangular diagram mostly belonged to clay, clay loam, and sandy clay loam (Fig. 6).

    Groundwater level in the wetland

    A pressure transducer (Eijkelkamp CTD-Diver, measuring range 10 m) was installed in the SNM 1 piezometer in the northern part of Samrak wetland, and the groundwater level and temperature were measured at 10-minute intervals from July 25 to August 9, 2010 (Fig. 7). During the observation period, the groundwater level changed according to the amount of precipitation and showed a decrease of about 40 cm from the beginning to the end of the observation period. The relationship between the change rate of the groundwater level and the amount of precipitation indicated the negative change of the groundwater level, that is, groundwater level rising as the amount of precipitation increased (Fig. 8). On the other hand, groundwater temperatures ranged from 21.4 to 23.0 ° C and showed an increasing tendency with atmospheric temperature (Fig. 9). However, the relationship between evaporation and groundwater level did not show any tendency (Fig. 10), indicating that the groundwater level change due to evaporation of groundwater was insignificant.

    Chemical characteristics of the wetland groundwater, surface water within the wetland, and the Nakdong River water

    In-situ physicochemical properties (pH, Eh, EC, salinity, TDS, DO, and alkalinity) of groundwater (SNM1), surface water in the wetland (SNW1- SNW5), and river water (SNR1) were measured on August 9, October 13, and November 17, 2010. (Table 9). The temperature of the SNM1 ranged from 16.1 to 24.1 with a mean temperature of 20.2 ° C while the temperature of the Nakdong River water was between 9.8 and 30.1 ° C with mean temperature of 20.9 ° C. Groundwater exhibited a smaller variation than river water. In contrast, surface water (SNW1-SNW5) in the wetland displayed similar temperatures (8.65- 10.6 ° C, average 9.1 ° C) with the river water.

    The pH values of the groundwater were in the range of 7.480-8.073 (a mean of 7.771) but the pH values of the river water (7.440-8.460, a mean of 8.185) were slightly higher than the groundwater. On the other side, the surface water in the wetland exhibited pH values of 7.272-8.568 (a mean of 7.268) with a wider range of values than the groundwater and river water.

    The EC of the wetland groundwater ranged from 777 to 827 μS/cm with an average value of 805 μS/ cm; the EC of the Nakdong River water ranged from 189 to 457 μS/cm with an average value of 301 μS/ cm; and the EC of the surface water in the wetland is in the range of 239 to 603 μS/cm with an average value of 426 μS/cm. A higher EC of the wetland groundwater than both the surface water in the wetland and the river water may be derived from chemical interaction between groundwater and the wetland sediments, in the condition of very slow groundwater flow. The Eh values were 91.9-123.5 mV (a mean value of 104.7 mV) for the wetland groundwater, 113.4-169.5 mV (a mean value of 147.4 mV) for the surface water in the wetland, and 65.3- 152.9 mV (a mean value of 109.3 mV) for the nearby Nakdong River (SNR1), showing lower Eh of groundwater than that of the surface water in the wetland and the Nakdong River water due to the reduction environment of NO3 to N2, Mn 4+ to Mn 2+ , and Fe 3+ to Fe 2+ , etc. (Pezeshki and DeLaune, 2012).

    Among the cation components of the shallow wetland groundwater (SNM1), the mean concentration of Ca 2+ (69.11mg/L) was the highest with a decreasing order of the Na + concentration (39.70 mg/L), Mg 2+ concentration (18.31 mg/L), K + concentration (2.86 mg/L), SiO2 concentration (22.89 mg/L), and Mn 2+ concentration (0.82 mg/L) (Table 10). Among the anion components of the shallow wetland groundwater (SNM1), the mean concentration of HCO3 (180.19 mg/L) was the highest, followed by SO42− (57.22 mg/ L), Cl (45.48 mg/L), F - concentration (0.39 mg/L), PO43− concentration (0.05mg/L), and NO3 concentration (0.22 mg/L).

    Among the cation components of the nearby Nakdong River water (SNR1), the mean concentration of Ca 2+ was the highest at 19.88 mg/L, followed by Na + (16.65 mg/L), Mg 2+ (4.94 mg/L), K + (5.12 mg/L), SiO2 (3.42 mg/L), and Mn 2+ (0.03 mg/L). On the other hand, in anions, HCO3- concentration was the highest at 113.0 mg/L, SO42- concentration (29.36 mg/L), Cl - concentration (18.20 mg/L), F - concentration (0.16 mg/ L), PO43− concentration (0.09mg/L), and NO3 concentration (6.70 mg/L). The wetland groundwater and river water belonged to Ca-HCO3 type in the Piper diagram (Fig. 11) which is typical in shallow groundwater (Yun et al., 1998). Hence, the concentrations of constituents in the wetland groundwater were markedly higher than those in the Nakdong River water even though both the wetland groundwater and Nakdong River water belonged to the Ca-HCO3 type.

    Rn-222 concentration of the wetland groundwater, surface water within the wetland, and the Nakdong River water

    The Rn-222 concentration of the wetland shallow groundwater (SNM1), the wetland surface water (SNW1-SNW5), and the Nakdong River water (SNR1) were 0.07-0.99, 0.11-0.45, and 0.10-2.86 Bq/ L, respectively (Table 11). Rn-222 concentrations in September were largely higher than those in Novermber. In general, it is known that the concentration of Rn- 222 in groundwater is 100 to 1000 times higher than that of most surface waters (Dulaiova et al., 2010). Ok et al. (2011) reported that the Rn-222 concentration of groundwater in the coastal area of Busan city was 0.17-43.39 Bq/L (a mean value of 18.22 Bq/L) and the Rn-222 concentration in the Ilgwang Stream water was 0.79-9.25 Bq/L (a mean value of 2.22 Bq/L). Meanwhile, the concentration of Rn-222 in seawater in Busan city ranged from 0.009 to 0.949 Bq/L (a mean value of 0.047 Bq/L). In contrast, the concentrations of Rn-222 in the study area were highest in the Nakdong River water, followed by the wetland shallow groundwater and the wetland surface water.

    Conclusions

    This study revealed the physical and chemical properties of sediments within Samrak riverine wetland in vertical and lateral direction at the estuary of the Nakdong River. According to particle size distribution of the SN1, SN2, and SN3 locations, the SN1 location indicated fine particles in deeper part, a mixture of coarse and fine particles in the middle part, and coarse particles in the shallower part. The degree of sorting become better from the deeper part to the shallow part. The SN2 point consists of finer grains in the deeper part, coarse grains in the middle part, and the mixture of coarse- and fine-grains in the shallow part. The degree of sorting become higher from the deeper part, the shallower part, and to the middle part. The location of the SN3 presented fine particles in the deeper part, coarse particles in the middle part, and the mixture of coarse and fine particles in the shallower part. Sorting at the SN3 point became poorer from the middle part, the deeper part, and to the shallower part.

    The water content in the shallow sediments ranged from 13.27 to 35.07% and increased as the depth from the surface and was higher in deeper shallow sediments due to the saturation zone. The mean organic matter content of the shallow sediments was 2.77%. Among the organic components (C, N, S, H, and O) of the deep sediments at the SN1, SN2, and SN3 points, O was the most abundant with a mean of 3.94% and C was the second most abundant with a mean of 0.66%. The contents of the organic components increased with depth. In comparison, the C content in the sediments in the riverine wetland showed 0.04-1.52% that was relatively lower than that of Mt. Geumjeong wetland (2.33-7.61%). Besides, the deep sediments in the riverine wetland displayed a relatively low C content near the surface while the mountain wetland showed a reverse trend. This may be due to the continuous increase of organic matters in the mountain wetland with the accumulation of plants near the surface, but there was not much opportunity of continuous accumulation of organic matter near the surface in the riverine wetland because of river flooding or rainfall during the flood season. The concentrations of SiO2 and K2O decreased with depth, but the concentrations of the remaining oxides mostly increased with increasing depth.

    From July 25 to August 9, 2010, in the summer season, the wetland groundwater level changed according to the amount of precipitation and decreased by about 40 cm during the observation period. The wetland groundwater temperature changed between 21.4 and 23.0 ° C and tended to rise with the increase in atmospheric temperature. In contrast, the wetland groundwater level irrelatively changed with the evaporation rate. The hydraulic conductivity values of the wetland sediments by empirical formula based on the particle size distribution were obtained as 2.68× 10−1 cm/sec in the shallower part and 1.56×10−5 cm/ sec in the middle part at the SN1, with coarse grains SN1 point and the mixture of coarse and fine grains in the middle part. At the SN2 point, the hydraulic conductivity values were 7.18×10−3 cm/sec of a mixture of coarse and fine grains at the shallower part and 2.03×10−1 cm/sec of coarse grains in the middle part. At the SN3 point, the hydraulic conductivity values were estimated to be 1.24×10−7 cm/sec of a mixture of coarse and fine grains in the shallower part and 3.49×10−1 cm/sec of coarse grains at the middle part. Hence, it is interpreted that at the SN2 and SN3, the surface infiltration rate is low due to a lower hydraulic conductivity but in the middle part, groundwater flow rate will be faster.

    The wetland groundwater showed a mean temperature of 24.1 ° C, pH of 7.48, Eh of 123.5 mV, EC of 811 μS/cm, TDS of 388 mg/L, DO of 5.22 mg/L, and the salinity of 0.4‰. The wetland groundwater exhibited mean Ca 2+ concentration of 69.11 mg/L, mean Na + concentration of 39.70 mg/L, and mean Mg 2+ concentration of 18.31 mg/L. On the other hand, the Nakdong River water displayed mean Ca 2+ concentration of 19.88 mg/ L, mean Na + concentration of 16.65 mg/L, and mean Mg 2+ concentration of 4.94 mg/L. The concentrations of constituents in the wetland groundwater were clearly higher than those in the Nakdong River water though the wetland groundwater and the Nakdong River water belonged to the same water type of Ca- HCO3. The Rn-222 concentration of the wetland shallow groundwater, the wetland surface water, and the Nakdong River water were in the range of 0.07- 0.99, 0.11-0.45, and 0.10-2.86 Bq/L, respectively.

    This study will be helpful to evaluate changes in the hydrogeological characteristics of the riverine wetlands before and after the creation of artificial wetlands in the Nakdong River estuary region by the FMRP as well as hydrological changes of the Nakdong River estuary due to opening the Nakdong River estuary bank carried out by the Korean government since 2019.

    Acknowledgments

    This work was supported by Pusan National University Research Grant, 2021.

    Figure

    JKESS-42-4-425_F1.gif

    Location map of the study area (Samrak wetland).

    JKESS-42-4-425_F2.gif

    Sampling location of sediments in Samrak wetland.

    JKESS-42-4-425_F3.gif

    Natural water content of shallow sediments with depth.

    JKESS-42-4-425_F4.gif

    Natural water content of deep sediments (SN1, SN2, and SN3) with depth.

    JKESS-42-4-425_F5.gif

    Organic matter content of shallow sediments with depth.

    JKESS-42-4-425_F6.gif

    Soil classification of the sediments in the study area based on USDA (1993). Red, blue, and black colors correspond to shallower sediment, middle sediment, and deeper sediment, respectively.

    JKESS-42-4-425_F7.gif

    Groundwater level (a) and temperature (b) of the piezometer SNM1.

    JKESS-42-4-425_F8.gif

    Precipitation vs. daily groundwater change.

    JKESS-42-4-425_F9.gif

    Groundwater temperature vs. atmospheric temperature.

    JKESS-42-4-425_F10.gif

    Evaporation vs. daily groundwater change.

    JKESS-42-4-425_F11.gif

    Water types of the wetland groundwater (SNM1) and the nearby river water (SNR1).

    Table

    Specification of piezometers in Samrak wetland

    Sampling locations of shallow and deep sediments in Samrak wetland

    Water contents of sediment in Samrak wetland in shallow and deep sediments

    Total organic matters of the shallow sediments in Samrak wetland

    Weights of N, C, S, H, O in the deep sediments (SN1, SN2, and SN 3)

    Concentrations of inorganic constituents in the deep sediments at different depths

    Concentrations of oxides in the deep sediments at different depths

    Mean size and degree of sorting of particles in the sediment at SN1, SN2, and SN3

    In-situ physicochemical property of groundwater (SNM1), surface water in the wetland (SNW1-SNW5), and river water (SNR1)

    Concentrations of inorganic constituents of groundwater (SNM1) and the Nakdong River water (SNR1)

    Radon concentrations in shallow groundwater of the wetland, surface water in the wetland, and the Nakdong River water

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