Open Journal of Marine Science, 2012, 2, 70-83 http://dx.doi.org/10.4236/ojms.2012.22010 Published Online April 2012 (http://www.SciRP.org/journal/ojms) Model for Predicting the Transport and Dispersal of Contaminants Incoming with Submarine Groundwater: Case Study for the Southwestern Taiwan Coastal Zone Konstantin A. Korotenko1, Peter O. Zavialov1, Ruey-Chy Kao2, Chung-Feng Ding2 1Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia 2Tainan Hydraulics Laboratory, National Cheng-Kung University, Tainan, Chinese Taipei Email: kkoroten@yahoo.com Received February 29, 2012; revised March 19, 2012; accepted March 26, 2012 ABSTRACT As was recognized recently, the submarine groundwater transports a significant amount of various contaminants into the coastal ocean. An assessment of the impact of intruded pollutants in the coastal ecosystems requires understanding fate of the pollutants and processes of their dispersal in ambient waters. In this paper, we proposed a 3-D coupled ocean circulation/particle-tracking model for predicting the transport and dispersal of pollution-containing groundwater dis- charged into a coastal environment of the southwestern Taiwan. The particle-tracking model takes currents and turbu- lent diffusivities predetermined by the ocean circulation model and uses the Lagrangian approach to predict the motion of individual droplets, the sum of which constitutes a contaminant plume in result of discharge of contaminant-rich submarine groundwater. The ocean circulation model was forced by tides and seasonal favorable winds for the south- western coast of Taiwan. The initialization of the coupled model was set using field data obtained in 2009 on the Ping- tung shelf where shallow aquifer seepages were discovered. Several types of numerical experiment scenarios were set up to elucidate the transport and dispersal of conservative and nonconservative (nitrate) contaminants in the shallow coastal zone. The comparison of obtained numerical results with observations performed by other researches was dis- cussed. Keywords: Submarine Groundwater; Coastal Zone; Nitrate; Eulerian-Lagrangian Model; Taiwan 1. Introduction Submarine groundwater discharge (hereinafter SGD) has been recognized as the potentially significant contribu- tion to the coastal ocean [1]. Although not as obvious as river discharge, groundwaters (hereinafter SGs) can also discharge directly into the coastal ocean. Like surface water, groundwater flows downgradient. Therefore, ground- water flows directly into the coastal ocean wherever a coastal aquifer is connected to the sea. Furthermore, arte- sian aquifers can extend for considerable distances from shore, underneath the continental shelf with discharge to the coastal ocean at their points of outcrop. In some cases, these deeper aquifers may have fractures or other breaches in the overlying confining layers, allowing groundwater to flow into the sea Figure 1(b) schematically illustrates shallow and deep aquifers and processes associates with SGD. Very importantly, it is now recognized that groundwa- ter discharge may be an important pathway for diffuse pollution to enter the coastal zone where coastal aquifers have become contaminated by septic systems or other pollution sources [2]. Despite SGD contribution in the ocean’s water budget is generally small, it represents an important source of nutrients and other contaminants that affect the ecology of the estuaries and coastal fresh water bodies [1,3-7]. There are reports that SGD may also be an important source of alkalinity and carbon to shelf wa- ters [8]. Submarine groundwater coming from coastal aquifers into the coastal ocean carries dissolved contaminants, concentration and type of which considerably varied from region to region. Multiple studies indicate that the con- centration of groundwater contaminants increases with increasing housing density and agriculture activity. Ex- panding residential and commercial near-shore develop- ment is leading to increased nutrient inputs to groundwa- ter that eventually migrate into to coastal waters. Several- decades long research shows that nitrogen inputs via C opyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 71 Figure 1. Map of the Pingtung coastal zone showing the bathy metry off southwestern Taiw an and stations of hydrographical observations (marked by solid circles) in the area of the shallow SG well (marked by the solid square). The lower inset (2b) epicts the schematic (no scale) of processes associated with SGD (after [1]). d Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 72 non-point sources over large coastline areas cause de- cline of ecological health and may support harmful algal blooms [9]. In Taiwan, investigations of the offshore discharge of groundwater, for many years, were largely motivated by water resource related issues. As was noted by [10], there were at least two reasons why scientific studies have de- veloped so slowly in this field. First, the SGD process is inherently very difficult to measure and monitor, which tended to discourage serious investigations. Nearshore seepages around Taiwan typically have very diffuse and highly variable unit fluxes although the cumulative dis- charge can be very significant when it occurs over a wide area. Second, SGD is a process that occurs across a land- sea interface that spans different scientific disciplines as well as environments. Unfortunately, there are distinct cultural and structural differences that separate terrestrial and marine scientists. Literally, hydrologists and coastal oceanographers are looking at the same problem from different ends. For the last decade, intensive field investigations of coastal environment pollution associated with SGD are undertaken in coastal zone of Taiwan including its south- western part (Figure 1). In [11], it was found out that aquifers are significant sources of trace elements and other chemical constituents to the southwestern coastal waters of Taiwan where onland chemicals as NH4, SO4, NO3, PO4 and SiO2, hydrocarbons, trace metals (Ni, Cu, Cd), NaOH, fertilizers, and sewage were. Reference [11] also reported the exploratory discovery of SGD by using salinity in Taiwan for the first time in the coastal area and point out that SGD may occur near the Kaoping River estuary. Recently [12,13] have shown that subma- rine groundwaters are also clearly indicated by tracers of oxygen, strontium isotopes and barium content. They noted that the concentration of many contaminants in groundwater is typically several times higher than in seawater. Usually with distance from seepage the con- centration of contaminants decreased rapidly. As obser- vations of nitrate (NO3) plumes near shallow seepages by [7,14] shows, the flux of nitrate towards the marine en- vironment was highly variable even over a short distance from the coast. In addition, greater dispersion near the seepage face due to movement of water masses in re- sponse of tides and to seasonal cycles of recharge to the fresh aquifer. Outside an aquifer, nitrate diminished sub- stantially in concentration so that horizontal and vertical sizes of a detectable inclined nitrate plume did not ex- ceed 6 and 4 m, respectively [7]. In the coastal ocean, processes of spreading and fate of contaminants coming from SG aquifers are governed by ocean dynamics and depend on many factors as seepage zone, discharge rate and type of contaminants. Mathe- matically, predicting such processes is very a compli- cated task that requires developing numerical models for reproducing/predicting the ocean circulation and trans- port of contaminants. Until now, numerical modeling directed to the solution of problems associated with pol- lution inputs from SGD is mostly focused on modeling the transport and biogeochemical processes [9,15,16] inside aquifers. Some works have been dedicated to ana- lysis and modeling an influence of tides on oscillation of groundwater discharge rate [2,17,18]. By theoretical mo- del for SGD and the associated chemical transfer to the ocean, [2] have demonstrated that wave setup and tidal pumping may be the processes largely responsible for the high rate of SDG and, in turn, a considerable intensifica- tion of the transport of chemicals to the ocean. Despite wide spectra of models and approaches that were built for the solution of various problems associated with con- tamination of coastal zone due to SDGs, yet, no attempt has been made on the development of a complex ap- proach for predicting the regional circulation and its ef- fect on the transport and mixing of pollutants coming into seawater from submarine seepages. Therefore the objective of this work is to fill this gap and develop a pollution transport model coupled with an ocean circula- tion model. In the work, we will focus on the shelf zone of the southwestern Taiwan, however, based on the gen- eralized approach, the model can be adapted easily for any region of interest. The paper is structured as follows: In Section 2, a short description of region of interest, hydrology and charac- teristics of SG based on field observations are given. In Section 3, the coupled circulation/particle-tracking model is introduced. In Section 4, results numerical experiments with conservative and nonconservative (nitrate) tracers under wind and tidal forcings as well as discussion are presented. Summary is given in Section 5. 2. Study Region Figure 1 shows the coastal zone of the southwestern Taiwan. The marine environment in this area is rather unique in terms of geomorphology. A major feature the latter is Gaoping (submarine) Canyon, belonging to the river extension type canyon, and Siaoliuciou Island. The canyon is well aligned with the Gaoping River on land, runs across the continental shelf, continues its course southwestward onto the continental slope, and terminates at a depth of about 3000 m. The head of the canyon, cut- ting the continental shelf, is characterized by high and steep walls, and the cross-sectional morphology of the canyon varies from V-shaped to broadly U-shaped [19]. As field observations exhibited [19,20], such peculiari- ties in geomorphology have a great effect on coastal cir- culation and mixing processes. Complex hydrographic study of submarine groundwa- Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 73 ter discharge in the Pingtung coastal zone [12,13] al- lowed finding seepage locations by tracing oxygen, stron- tium isotopes. Areas with notable SGD1 were found to locate a long distance from the shoreline (~25 km) and likely to be widespread off southwestern Taiwan. To detect shallow SGDs, quantify their rate and influ- ence on the local hydrology as well as define the content of contaminants, the joint oceanographic observations were conducted by Taiwanese and the Russian scientific groups in February and October 2009 on the Pingtung shelf [21]. Figure 1 schematically depicts the transec- tions of the polygon where hydrophysical and hydro- chemical observations have been conducted. The solid circles depicted along the transections denote location of hydrographical stations conducted during both expedi- tions. Distinct manifestations of submarine groundwater dis- charge were revealed near-shore, west of the Siaoliuciou Island at stations 11 and 16 (Figure 1). In the vicinity of the SG seepage (marked by solid squared in Figure 1), CTD measurements revealed salinity inversions on pro- files in the 2 m bottom layer (Figure 2(b)), where the magnitude of the salinity drop reached 0.06 psu. For such salinity drop, the SGD rate, as was estimated by [21], should be in range between 0.1 and 1 gm–2·s–1. The esti- mation was based on the advection-diffusion balance of salinity in the water column affected groundwater inflow, dd z wSK Sz, where S is salinity, z is vertical coordi- nate, w is mean vertical velocity of groundwater seepage and Kz is the eddy diffusivity. Assuming that salinity difference, , in the bottom inverse layer, S w , significantly less than mean salinity in the bottom ambient water, S0, i.e., 0, we ob- tain the velocity and SGD rate per unit area, SS 1 1 0z wKSS H Q , where is mean density. Taking average values S0 = 30 psu, Kz = 10–4 m 2·s–1 and = 1025 kg·m–3, for = 0.06 and S = 2 m, we obtain w = 0.864 cm·day–1 and Q = 1.025 g·m–2·s–1. The deviation of vertical velocity and magnitude of the SGD rate, obtained by [21], is associated with variations of and S estimated from field experiments. Be- sides uncertainties in estimation of and S the rate, Q, depends on vertical diffusivity, Kz, which was chosen from general assumptions. Actually, Kz can significantly vary in the bottom boundary layer and thus the estimates of w and Q can be considered as an order of magnitudes. Nevertheless they agree with the characteristic values reported earlier for well-developed SGD at other loca- tions in the ocean [22]. The field observations also revealed that the salinity inversions coincided with maxima of dissolved oxygen, organic matter content, iron and phosphorus concentra- tions, and the minimum of turbidity. Note that, in non- impacted by SGD areas, ambient waters were character- ized by regular salinity profiles with the maximum of salinity at the bottom (Figure 2(a)). Many studies show that SGD makes a significant con- tribution to nutrient budget of coastal water that causes significant research interest in this phenomenon [6,12,18]. We made chemical analyses of water samples collected from the bottom layer of the ocean, as well as the near- shore land groundwater well (marked by solid square in Figure 1) and Kaoping River in February 2009. The re- sults for nitrate, NO3, which we focus on, are depicted in Table 1. As seen, measurements conducted at stations 1, 6, 11, 16 and near the SG well revealed that nitrate concentra- tions were higher than those at other stations not affected by SGD. This difference in NO3 concentrations appears to be consistent with the figures reported by [8] for the same area. Note that the increasing concentrations of NO3 observed at stations #11 and #16 cannot be ex- plained by the river inputs, because the nitrate content in Kaoping River water was considerably lower. Above the SG well, the nitrate concentration reached 0.48 mg·L–1 that significantly exceeded that in seawater. Current velocity measurements conducted near by the SG well (Figure 1) indicated that along- and cross-shore components of the velocity exhibited a superposition of about 0.1 m·s–1 amplitude of the tidal and background northwestward coastal current of about 0.25 m·s–1. 3. Coupled Model Description Modeling the transport and dispersal of contaminant- containing fresh groundwater from a coastal aquifer into (a) (b) Figure 2. Typical temperature, T, and salinity, S, profiles obtained in the 5-m layer (a) far from the seepage (at station 15) and (b) those obtained near the seepage (after [21]). 1Note that SGD can be submerged springs or point sources and often are diffuse across large areas in the ocean. Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 74 Table 1. Chemical indicators of the water samples collected from the bottom layer of the ocean, groundwater well, and Kaoping River in February, 2009. Bold values indicate SGD impacted seawater (after [21]). Station # Distance, km Nitrate, mg·L–1 1 0.5 0.04 2 1.7 0.03 5 1.9 0.04 6 0.4 0.04 7 6.8 0.02 8 5.0 0.02 9 2.9 0.02 10 1.2 0.02 11 0.3 0.03 12 6.9 0.02 13 5.0 - 14 3.0 0.01 15 1,3 0.01 16 0.4 0.02 Average N/A 0.02 Average sites N/A 0.03 GW Well N/A 0.48 Kaoping River N/A 0.06 the adjacent shallow marine environment presents a real challenge to oceanographers. Difficulties are associated with many factors such as the hydrostatic instability of bottom fresh water discharging into the saline environ- ment, uncertainties of the SGD rate estimate, seepage location(s) as well as with difficulties of predicting coa- stal currents induced by variable forcing (wind, tides, etc.). All these factors are necessary to take into account in deciding on the choice of a particular numerical model and approach. Our approach to the solution of the trans- port and dispersal of contaminant-containing fresh ground- water is based on the Lagrangian particle tracking me- thod (LPTM) that is significantly more effective than approaches based on finite-difference models since LPTM describes the advective transport with a high accuracy. This is a very important for realistic reproducing the movement a plume of contaminants. In addition, the La- grangian method is better suited to modeling the fate and dispersal of contaminants, when the attribution of fate properties to a particular contaminant is required. As any other similar coupled models, ours consists of a hydrodynamic model embedded into a particle trans- port model (PTM). PTM takes currents and turbulent diffusivities predetermined by the hydrodynamic model and uses LPTM to predict the motion of individual parti- cles, the sum of which constitutes a contaminant plume. The basic concept of the proposed model is similar to that presented in [23-30]. Therefore, herein, we shortly discussed only those extensions/improvements of the model that are made for the present specific problem. Generally, the procedure of predicting behavior and fate of a contaminant plume is divided into two parts: 1) predetermination of currents, turbulent diffusivities with the hydrodynamic model; and 2) applying the predeter- mined three-dimensional motions to individual particles, the aggregate of which constitutes the plume. The model thus simulates the advection and turbulent diffusion of particles. The physicochemical decay and processes mo- difying the structure and properties of a contaminant plume are simulated due to specific algorithms. 3.1. The Hydrodynamic Module To predict circulation, we chose the sigma-coordinate Princeton ocean model (POM) [31] and adapted it for the southeastern part of Taiwanese coastal zone marked in Figure 1(a) by the square. The POM has a resolution of 1/60 Deg along longitude (Δx) and latitude (Δy). The calculation domain, 39 × 72 cells, covers the region of interest from 22.00˚N to 23.05˚N and from 120.05˚E to 120.70˚E. Over the vertical, the model has 31 uneven σ-levels, which are distributed so that to provide the maximal resolutions near the surface and the bottom. The calculation time steps for the external, DTE, and for the internal, DTI, modes were chosen from the CFL criterion of stability [31] and equal to 6 s and 120 s, respectively. For the momentum fluxes in the near-bottom layer (1 + σkb–1)H/z0, we used the quadratic dependence on the velocity: 1/2 22 12 iz i uDCuu u for 1, where H(x, y) is depth, D = H + η, η(x, y) is the elevation of the free surface, KM, z0 is the rough- ness parameter is the coefficient of turbulent viscosity, ui (i = 1, 2) are the horizontal components of the mean cur- rent velocity, and drag coefficient, Cz is defined as 2 2 10 max ,0.0025 ln 1 z kb C Hz , where k = 0.4 is the Karman constant. 3.1.1. Open Boun dary Conditions At the open (western and southern) boundaries of the calculation domain, two types of boundary conditions for the temperature and salinity were used: the condition for the inflow and that for the outflow. In the case of the outflow outside the calculation domain, the values for T Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 75 and S at the corresponding open boundary were setup. In the case of the inflow into the calculation domain, the radiation equation ,, n TSt uTSn was solved [25]. The index n, here, represents the coordinate normal to the open boundary. The components of the mean cur- rent and the turbulent diffusivities were output to the particle transport model at every time step during the simulations. 3.1.2. For cing Two types of forcing were applied for modeling: winds and tides. For the wind forcing, we used observations by [32] presented favorable seasonal winds. For the tidal forcing, we referred to [19,20] who analyzed the ratio between amplitude of tidal oscillations of coastal currents along the southwestern Taiwan induced by the astro- nomical semidiurnal, M2, S2 and diurnal, K1, and O1 con- stituents. They showed that the semidiurnal, M2, con- stituent was the major one in energy spectra of tidal os- cillation. Thus, for the first order approximation, we can simplify tidal forcing by a use of the dominant constitu- ent M2 only and, for tuning the circulation module, we utilized current velocity measurements conducted by [21] near by the SGD well. We applied a radiation condition based on the long gravity-wave speed, to determine the boundary elevation and a gravity-wave radiation condi- tion to specify the normal component of the depth-aver- aged current at the model open boundaries. 3.1.3. POM Initialization The hydrodynamic model is spun up from rest and with temperature and salinity from the ocean climatological database [33] with adjusting T, S-profiles, in the water column over the SG well, to those shown in Figure 2(a). 3.2. Particle Tracking Model In the Lagrangian particle tracking method, an algorithm for updating particle coordinates (Equation (1)) is the following: at every time step, particles are moved in the 3-dimensional Cartesian reference frame by an advective displacement added to a diffusive jump [23]. , ,, 13;1,2, ,;1,2, , iiji jk jk tp xut ij Nk N (1) The displacements, , i k x, are defined as sum of a deterministic displacement caused by mean velocities, ,ij u, and a random displacement, , i k , due to fluctua- tions of velocity. Here Nt is the number of time steps, t is the time step and Np is the total of particles released during a numerical experiment. The advective movement within a grid cell is deter- mined by linear interpolation of the velocity values from the 8 nodes of the grid cell, and computation of the dis- placement vector is a product of the interpolated velocity vector and the time step Δt. Diffusive jumps of particles (random displacement due to sub-grid fluctuations of velocity) along horizontal (i = 1, 2) and vertical (i = 3) axes are determined differently. For the horizontal axes, we used so-called “naïve random walk” (NRW) scheme, i.e., 1/2 , 2 ii ij t to simulate diffusive jumps. The random vector, i , normally distributed with an aver- aged value of zero and unit standard deviation is con- verted later to yield the Gaussian distribution with zero mean and unit standard deviation. Coefficients ,ij rep- resent time dependent diffusivities along the i-axis. Such simple scheme for the lateral transport and dispersal of particles is feasible due to a weak variation of horizontal diffusivities, j,i , along the correspondent axes. Unlike ,ij , profiles of the vertical diffusivity, 3, , usually exhibits significant variations in coastal waters where current and density structures are formed under tidal and wind-driven circulation and, often, under strong influence of freshwater input [29]. Such combined forc- ing leads to the formation of non-uniform vertical diffu- sivity profiles that, in case of the use of the NRW scheme, can form artificial particle accumulation zones in layers with weak vertical mixing. To avoid this effect we em- ployed so-called a “consistent random walk” (CRW) approach [29,34,35] in order to obtain vertical particle displacements correctly. For this, we applied the formula 1/2 * 33 33 2 ztKz t , adopted from [35]. As seen, this formula includes deterministic and diffusive (or random) components. The deterministic component causes a net displacement of the center of mass of the neutrally buoyant particles toward increasing diffusivity at a rate 3 (a local gradient of K3 in the vertical direc- tion), thus allowing avoidance of the artificial particle accumulation within layers of low vertical diffusivity. The diffusion coefficient K in the CRW model is esti- mated from the diffusivity profile at a vertical coordinate z* shifted from the particle coordinate z by a small dis- tance 3 0.5 t . Note that the CRW model should be used for simulating horizontal displacements too. How- ever, according to our assessment, the largest horizontal diffusivity gradient, in the coastal zone studied, is the order of 10–3 m·s–1 (cf. m·s–1) so that the effect of 1,2 1 3max 10K on horizontal distribution of particles is negligible for a given model grid spacing and time step (see below). As was mentioned about, the horizontal and vertical diffusion coefficients, 1,2 K and 3V K as well as the mean current velocity components, ,,,, ij uxyzt , and density, ,,, yz t, are provided by the POM. The horizontal diffusivities are computed with a use of Sma- gorinsky formula and the vertical diffusivity is obtained Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 76 from the level-2.5 turbulence closure scheme [36]. Thus, 3-dimensional parameters available from the POM are used as forcing in the particle tracking model, which es- timates particle coordinates at every time step Δt that can be equal to (or longer than) the internal time step, DTI, of POM. The time steps is chosen to be 360 s that is long enough but prevents particle jumping more that one grid cell, and, thus, guarantees an accurate estimate of particle displacement in each of 3 directions. It should be noted that, based on the random walk concept, particle tracking models coupled with the hy- drodynamic models have certain structural features asso- ciated with the ratio between model time step, DTI (Δt), and the Lagrangian timescale, ag . For the vertical mo- tion, the external timescale for smallscale turbulence in the ocean, turb, is of the order 1 - 10 sec, so that T T t turb . It means that vertical velocity fluctuations and displacements of a Lagrangian particle at the time lag are not correlated we can use Equation (1) for predicting vertical (i = 3) particle coordinates. In contrast, for horizontal currents in the ocean, the typical value of the Lagrangian timescale DTI T t ag T is 1 - 3 days, so that lag and the random horizontal velocities and dis- placements of a Lagrangian particle within the time lag are correlated. For this reason, the horizontal motion (i = 1, 2) of the particle is described by equations tT t ,1, 1 ijijlaga i uu tTA t, (2) ,1,,,1 , , ijijiij iijij2 xuxztuut , (3) where a is the root mean square random acceleration, i is the normally distributed random vector value with zero mean and unit variance, ,ij and ,1ij are the horizontal radius-vector of the particle at moments t and , respectively. Note that (2) is a finite difference analogue of the Langevin equation. Note that, from POM, we have explicitly only one parameter to describe hori- zontal random motion of a Lagrangian particle, namely the horizontal diffusivity, tt , while Equations (2) and (3) include two parameters, ag T and a . However, taking into account that POM uses the Smagorinsky formula for the horizontal diffusivity, : 2 11 1/2 22 112 22 2 0.5 HH KCxyux ux uxux where C is a constant, we can infer the Lagrangian timescale as ag H TxyK and the random accelera- tion as 1/2 3 2 aH tx y [26] and thus close the system of Equations (2) and (3). 3.2.1. Definitio n of the Source A very important step for the model setup is to set cor- rectly the near-seepage zone processes and the discharge rate of contaminant-rich SG. Submarine groundwater coming into the seawater from coastal aquifers carries dissolved contaminants, type and properties of which depend on regional factors. Contaminants may be con- sidered as passive and either conservative or nonconser- vative tracers but, being dissolved in fresh groundwater, such tracers would have vertical velocity due to the buoyancy force. The magnitude of the rise velocity de- pends on difference between densities of fresh SG and adjacent saline water. In the particle tracking approach applied, we presume that submarine groundwater discharging from an aquifer breaks up into fresh water droplets with diameter, d, de- fined by empirical formula [27] 1/3 2/3 2/3 0 9.52 1dg , where 0 is density of SG droplets, and ν is density and viscosity of ambient water, respectively and g is the acceleration of gravity. The rise velocity of the freshwa- ter droplet can be estimated as 1/ 2 0 81 3wgd . Thus, the magnitude of rise velocity grows with the droplet diameter and the ratio 0 . For d = 1 mm, the rise velocity is about 0.25 m·s–1 and thus in shallow wa- ters (~30 m), SG droplets may appear at the sea surface in a several minutes after their release. In the work, we applied a “stationary” source of parti- cles, i.e., a certain number of “particles-in-SGW” (clus- ter), N, was released every seconds at the position (X0, Y0) of seepage at the bottom. Amount of particles in a cluster t N is set to prescribe the initial concentration V 0 CN0j0 Two types of particles were used in numerical experi- ments: 1) Conservative particles that mimicked “long-live” tracers or contaminants (as 222Rn, 226Ra, 3H, 4He) without significant attenuation in their concentration due to physico-chemical reactions; and 2) Nonconservative par- ticles that emulate “short-live” contaminants experienc- ing a relatively rapid decay in result of their own proper- ties and reactions. As an example of a nonconservative contaminant we will take nitrate (NO3) that, as recog- nized now, is the major contaminant of SG. Recalling measurements by [21] discussed above, we can set the initial concentration of nitrate as C0 = 0.48 mg·L–1), The reduction of nitrate due to denitrification and other reac- tions can be described by a first-order degradation proc- ess with an exponential decrease in concentration in the cell-volume . Vxyz 0expln 2CC , where is “half-life” parameter. In the particle tracking approach, effects of degradation can be treated in a similar way as radioactive decay using e-folding times 1 e T [23,25] for the probability of removal of particle in a time step. According to research by [37] the process of denitrification in seawater spans Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. Copyright © 2012 SciRes. OJMS 77 about 100 hours and thus, in the model, we can set e to be 30 h and use procedure of randomize half-life pa- rameter as was described in [25]. T 3.2.2. Particle Re l e a se To simulate a continuous source, particles are launched every DTI (180 s) time step. Coordinates of the source were chosen at the site of SG well (Figure 1) where depth was about 32 m. In numerical experiments with conservative particles, we launched total 105 particles every t = DTI and obtained concentration in conven- tional units. In experiments with nitrate, every time step, we launched 15,480 particles to fit to the initial concen- tration of 0.48 mg·L–1 as was obtain in the observations [21]. We limited our simulation by short-term effects of simulated currents on the development of particle plumes. Therefore in numerical experiments with wind forcing, duration of simulations was chosen to be 24 hours while simulations with tidal forcing last up to 72 hours to study behavior of nitrate plume in tidal current and the effect of nitrate degradation. 4. Results and Discussion Dynamics of shallow water of the region of interest is very complicated and exhibits nonlinear effects of tidal and wind-induced currents [19,20,32]. In order to illus- trate the ability of the proposed model, we simplified our simulation considering the effect of winds and tides separately. We run the model with two types of particles to mimicking conservative and nonconservative con- taminants as was discussed above. In the first case, we simulate a wind-induced circulation forced by two fa- vorable for autumn and summer winds. In the second one, we simulate a tidal circulation to scrutinize its effect on the nitrate plume. 4.1. Transport of Contaminants under Wind-Induced Circulations 4.1.1. N NE Wind To scrutinize the transport and dispersal of passive con- servative tracers discharged by a SG well, we forced the hydrodynamic model with the north-northeasterly (NNE) wind favorable for autumn [32]. For simplicity, we set constant wind speed with magnitude of 8 m·s–1. Figure 3 shows the computed near-shore circulation at the sea surface and depth of 30 m. As seen, the NNE wind in- duced jet-like currents along the coast and eddy struc- tures over the Gaoping Canyon. The wind-induced SSE current impinging on the Siaoliuciou Island created the circulation around the Island that is clearly seen at 30 m. (a) (b) Figure 3. Examples of computed velocity at (a) the sea surface and at (b) 30 m under predominant NNE wind of 8 m·s–1.
K. A. KOROTENKO ET AL. 78 At the surface, current velocity magnitude reached 0.3 m·s –1 while at 30 m the magnitude was about 0.1 m·s–1. At 30 m, the circulation revealed distinct onshore flow at the easternmost end of the Gaoping Canyon. Velocity profiles (not shown here) indicated a veering of velocity throughout the water column near the site of interest. Figure 4 presents a planar plot of the integral2 con- centration, C(x, y)H, of particles in 24 hours after their release. Under NNE wind, the particles moved to south- east along the coast, although some particles, as seen, moved side- and backward due to the inhomogeneous structure of the current velocity. Length and width of the integral plume were about 15 and 7 km, respectively. There were two maxima of concentration indicated the slope of the particle plume that is clearly seen in Figure 5. Presented in Figure 5, the 2-D cumulative X-Z and Y-Z transections of the concentration give characteristic details of vertical structure of the particle plume formed in 24 h after their release. Here, “cumulative” means a sum of all particles swept by the X-Z (Y-Z) area moving along the Y (X) axis. The asymmetric vertical structure of the plume was associated with shear effects of wind-induced currents on the vertical transport and dispersal of rising particles. 4.1.2. SS E Wind The second numerical experiment was performed under summer favorable wind, which, according to [32], blows from south-southeast (SSE). Similarly to the previous experiment, wind speed was chosen to be of 8 m/s. As seen in Figure 6, the SSE wind induced strong jet-like currents at the sea surface (cf. Figure 3) and partly at 30 m. A weak distortion of the surface currents occurred over the Gaoping Canyon while the strong cha- otic circulation was at depth of 30 m indicated a signifi- cant influence of the steep structure of the Canyon on the current Figure 7 shows the integral particle concentration dis- tribution formed under the SSE wind of 8 m·s–1 in 24 h after their release. As seen, the area covered by particles was larger than that formed under the NNE wind; the surface plume reached the Gaoping River estuary. Length and width of the integral plume were about 17 and 8 km, respectively. The maximum of particle concentration, as seen, was near the source location. Figure 8 presents the X-Z and Y-Z transections of particle concentration. As in the experiment with SSE wind, the concentration maximum was found to be at 3 - 4 m above the bottom indicating that the particle accu- mulation increased when the rise of particles was re- tarded. In contrast to the plume formed under NNE wind, Figure 4. Integral concentrations of particles formed under NNE wind of 8 m·s–1 after 24 h of discharge. (a) (b) Figure 5. The X-Z (a) and Y-Z (b) transections through a particle plume relatively source coordinates (X0, Y0); the experiment with NNE wind of 8 m·s–1. in this case, the plume asymmetry indicated that margins with lower gradients of particle concentrations are ex- tended to the west (negative X) and to the north (positive Y) from the source location. 4.2. Transport and Dispersal of Nitrate in Tidal Current The groundwater discharging into the sea often contains nitrate with high concentrations. However, denitrification 2C(x, y)H = (N/ΔV), where N-number of particles, ΔV = Δx·Δy·Hand (x, y) is depth. Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 79 (a) (b) Figure 6. Examples of computed velocity at (a) the sea surface and (b) 30 m under predominant SSE wind of 8 m·s–1. Figure 7. Integral concentration of particles formed under SSE wind of 8 m·s–1 after 24 h of discharge. (a) (b) Figure 8. The X-Z (a) and Y-Z (b) transections through a particle plume relatively source coordinates (X0, Y0); the experiment with SSE wind of 8 m·s –1. and dilution of nitrate apparently reduces its concentra- tion. As measurements indicated [37], the attenuation of nitrate concentration in the seawater proceeds quite rap- idly. Of interest here is to estimate scales of the nitrate contamination of adjacent seawater nearby the SG well Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 80 as well study an influence of tidal circulation on the dis- tribution of nitrate in the impacted zone. For this, we forced the hydrodynamic model with dominant constitu- ent M2 and tune the model to adjust computed current velocity to that obtained in the field observations with ADCP [38]. Figure 9 presents the time series of horizontal velocity components, U and V, and horizontal, KH, and vertical diffusivities, KV, at 15 m above the bottom. As seen, KH and KV exhibit significant variation during the tidal cycle with KH ranges from about 4 to 20 m2·s–1 and KV ranges from about 10–4 to 10–2 m 2·s–1 that is very close to KV derived from direct measurements of turbulence obtained with a high-frequency ADCP in tidal bottom layer [38]. Minima of the viscosities lag the minima of the current velocity; the time lag is about 1.5 h. Figure 10 illustrates successive phases of the nitrate plume evolution in the tidal current. For the parameters of the model initialization, nitrate reached the sea surface in about 3 min after its release. Tidal oscillations cause to periodic variations of the plume position relatively the vertical axis. An asymmetry of current velocity and tur- bulent diffusivities echoes in the nitrate plume structure and variability in the water column. As seen in Figure 10, the curvature of the plume is more pronounced when the tidal current directed to the southeast. The equalization of nitrate concentration level comes after 24 h. Before, we see the vertical extension of the high concentration core (~10–1 mg·L–1) of the plume. Figure 11 shows the integral nitrate concentration formed under tidal circulation in 24 h after nitrate release. As seen, the area occupied by particles is noticeably less than those formed in the experiments with NNE and SSE winds, and particles are mostly dispersed around the source point, through the northwestward drift is clearly seen. 5. Summary Although submarine springs and seeps around Taiwan have been known for many years, these features have traditionally been perceived as hydrologic “curiosities” rather than objects for serious scientific investigation. Despite low volume (relatively the riverine input) of SGD inputs into coastal waters of the southwestern Tai- wan, SGDs are recognized as potentially significant sources of various pollutants. An assessment of SGD-originated contamination im- pact on ecosystems requires a complex approach to this problem including experimental study and numerical modeling. Until now, numerical modeling focused on the solution of problems associated with contaminant distri- bution inside aquifers and only a few works focused on the processes near-seepage zones. In the paper, we pre- sented the 3-D coupled circulation/particle transport model for predicting coastal circulation and its effect on the transport and distribution of particles mimicking conser- vative and nonconservative (nitrate) contaminants dis- charging in seawater in result of seepage of submarine groundwater. To our knowledge, this couple model is the first one that was developed for such specific problems. Numeri- cal experiments were performed with favorable for au- tumn and summer winds and under simplified tidal cir- culation forced only by M2 constituent. However, the model gave reasonable results for scales and distributions of discharged particles/contaminants in shallow zone under the applied forcing. It is worthwhile to note that Figure 9. Tidal cycle of computed velocity components, U and V (both in m·s–1), vertical = and horizontal diffu- sivities = K H/40 (both in m2·s–1) at 15 m above the bottom. * V K50 * V K * H K Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 81 Figure 10. Successive phases of the nitrate plume (the X-Z transection) development in tidal current. Figures in upper part of graphics indicate time after release. Figure 11. An integral concentration of nitrate formed by tidal circulation after 24 h of discharge. despite the model was designed for Taiwanese coastal waters, but being built on general principles, it can be applied, with an appropriate adaptation, to any other re- gions of the ocean. As any other model, ours requires further improve- ment that we associate with realistic forcing and initiali- zation of the model as well as the consideration of real contaminants with their specific properties and reactions in seawater. 6. Acknowledgements This work was supported by the bilateral Taiwanese- Russian Project “Monitoring, Assessment, and Manage- ment Implications of Submarine Groundwater Discharge in Taiwan” between P. P. Shirshov Institute of Oceanol- ogy, Russian Academy of Sciences, Russia, and Tainan Hydraulics Laboratory, National Cheng Kung University, Taiwan. REFERENCES [1] E. A. Kontar and I. S. Zektser, “Submarine Discharge and Its Effect on Oceanic Processes in the Coastal Zone,” Water Resources, Vol. 26, 1999, p. 512. [2] L. Li, D. A. Barry, F. Stagnitti and J.-Y. Parlange, “Sub- mariane Groundwater Discharge and Associated Chemi- cal Input to a Coastal Sea,” Water Resource Research, Vol. 35, No. 11, 1999, pp. 3253-3259. doi:10.1029/1999WR900189 [3] P. K. Weiskel and B. L. Howes, “Differential Transport of Sewage-Derived Nitrogen and Phosphorus through a Coastal Watershed,” Environmental Science & Technol- ogy, Vol. 26, No. 4, 1992, pp. 352-360. doi:10.1021/es00026a017 [4] J. W. Portonoy, B. L. Nowicki, C. T. Roman and D. W. Urish, “The Discharge of Nitrate Contaminated Ground- water from Developed Shoreline to Marsh-Fringed Estu- ary,” Water Resources Research, Vol. 34, No. 11, 1998, Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. 82 pp. 3095-3104. doi:10.1029/98WR02167 [5] B. L. Nowicki, E. Requintina, D. Vankeuren and J. Port- noy, “The Role of Sediment Denitrification in Reducing Groundwater-Derived Nitrate Inputs to Nauset Marsh Estuary, Cape Cod, Massachusetts,” Estuaries and Coasts, Vol. 22, No. 2, 1999, pp. 245-259. doi:10.2307/1352981 [6] L. F. H. Niencheski, H. L. Windom, W. S. Moore and R. A. Jahnke, “Submarine Groundwater Discharge of Nutri- ents to the Ocean along a Coastal Lagoon Barrier, South- ern Brazil,” Marine Chemistry, Vol. 106, No. 3-4, 2007, pp. 546-561. doi.10.1016/j.marchem.2007.06.004 [7] K. D. Kroeger and M. A. Charette, “Nitrogen Biogeo- chemistry of Submarine Groundwater Discharge,” Lim- nology Oceanography, Vol. 53, No. 3, 2008, pp. 1025- 1039. [8] C.-T. A. Chen and S.-L. Wang, “Carbon, Alkalinity and Nutrient Budget on the East China Sea Continental Shelf,” Journal of Geophysical Research, Vol. 104, No. C9, 1999, pp. 20675-20869. [9] C. P. Slomp and P. Van Cappellen, “Nutrient Inputs to the Coastal Ocean through Submarine Groundwater Dis- charge: Controls and Potential Impact,” Journal of Hy- drology, Vol. 295, No. 1-4, 2004, pp. 64-86. doi:10.1016/j.jhydrol.2004.02.018 [10] E. A. Kontar and W. C. Burnett, “Study of Groundwater Discharge to the Coastal Zone and Evaluation of Potential Earthquakes,” Proceedings of International Conference on Marine Environment, the Past, Present and Future, National Sun Yat-sen University, Kaohsiung, 26-28 Janu- ary 1999. [11] R.-M. Wang, C.-F. You, H.-Y. Chu and J.-J. Hung, “Sea- sonal Variability of Dissolved Major and Trace Elements in the Gaoping (Kaoping) River Estuary, Southwestern Taiwan,” Journal Marine Systyem, Vol. 76, No. 4, 2009, pp. 444-456. doi:10.1016/j.jmarsys.2007.11.012 [12] I.-T. Lin, C.-H. Wang, C.-F. You, S. Lin, K.-F. Huang and Y.-G. Chen, “Deep Submarine Groundwater Dis- charge Indicated by Tracers of Oxygen, Strontium Iso- topes and Barium Content in the Pingtung Coastal Zone, Southern Taiwan,” Marine Chemistry, Vol. 122, No. 1-4, 2010, pp. 51-58. doi:10.1016/j.marchem.2010.08.007 [13] I.-T. Lin, C.-H. Wang, S. Lin and Y.-G. Chen, “Ground- water-Seawater Interactions off the Coast of Southern Taiwan: Evidence from Environmental Isotopes,” Journal of Asian Earth Sciences, Vol. 41, 2011, pp. 250-262. doi:10.1016/j.jseaes.2011.03.001 [14] M. S. Andersen, L. Baron, J. Gudbjerg, J. Gregersen, D. Chapellier, R. Jakobsen and D. Postma, “Discharge of Nitrate-Containing Groundwater into a Coastal Marine Environment,” Journal of Hydrology, Vol. 336, No. 1-2, 2007, pp. 98-114. doi:10.1016/j.jhydrol.2006.12.023 [15] Y. Uchiyama, K. Nadaoka, R. P. Peter, K. Adachi and H. Yagi, “Submarine Groundwater Discharge into the Sea and Associated Nutrient Transport in a Sandy Beach,” Water Resource Research, Vol. 36, No. 6, 2000, pp. 1467-1479. doi:10/1029/2000WR900029 [16] C. Spiteri, C. P. Slomp, K. Tuncay and C. Meile, “Mod- eling Biogeochemical Processes in Subterranean Estuar- ies: Effect of Flow Dynamics and Redox Conditions on Submarine Groundwater Discharge of Nutrients,” Water Resource Research, Vol. 44, 2008, Article ID: W04701. doi:10.1029/2007WR006071 [17] C. Leote, J. S. Ibanhez and C. Rocha, “Submarine Ground- water Discharge as a Nitrogen Source to the Ria Formosa Studied with Seepage Meters,” Biogeochemistry, Vol. 88, No. 2, 2008, pp. 185-194. doi:10.1007/s10533-008-9204-9 [18] R. Santos, W. C. Burnett, T. Dittmar, I. G. N. A. Surya- putra and J. Chanton, “Tidal Pumping Drives Nutrient and Dissolved Organic Matter Dynamics in a Gulf of Mexico Subterranean Estuary,” Geochimica et Cosmo- chimica Acta, Vol. 73, No. 5, 2009, pp. 1325-1339. doi:10.1016/j.gca.2008.11.029 [19] I.-H. Lee, R.-C. Lien, J. T. Liu, W.-S. Chuang and J. Xu, “Turbulent Mixing and Internal Tides in Gaoping (Kaop- ing) Submarine Canyon,” Journal of Marine Systems, Vol. 76, 2009, pp. 383-396. doi:10.1016/j.jmarsys.2007.12.011 [20] Y. H. Wang, I. H. Lee and J. T. Liu, “Observation of Internal Tidal Currents in the Kaoping Canyon off South- western Taiwan. Estuarine,” Estuarine, Coastal and Shelf Science, Vol. 80, 2008, pp. 153-160. doi:10.1016/j.ecss.2008.07.016 [21] P. O. Zavialov, R.-C. Kao, V. V. Kremenetskiy, V. I. Peresypkin, C.-F. Ding, R.-T. Hsu, O. V. Kopelevich, K. A. Korotenko, Y.-S. Wu and P. Chen, “Evidence for Submarine Groundwater Discharge on the Southwestern Shelf of Taiwan,” Coastal Shelf Research, Vol. 4, 2012, pp. 18-25. doi: 10.1016/j.csr.2011.11.010 [22] Intergovernmental Oceanographic Commission, “Subma- rine Groundwater Discharge: Management Implications, Measurements, and Effects,” IOC Manuals and Guides, Vol. 44, 2004. [23] K. A. Korotenko, R. M. Mamedov and C. N. K. Mooers, “Prediction of the Dispersal of Oil Transport in the Cas- pian Sea Resulting from a Continuous Release,” Spill Sci- ence and Technology Bulletin, Vol. 5-6, 2001, pp. 323- 339. doi:10.1016/S1353-2561 (01)00050-0 [24] K. A. Korotenko, “Warfare Chemicals Dumped in the Baltic Sea: Modeling Transport Processes of Pollution Resulting from Possible Leakages,” Oceanology, Vol. 43, No. 1, 2003, pp. 11-23. [25] K. A. Korotenko, R. M. Mamedov, A. E. Kontar and L. A. Korotenko, “Particle Tracking Method in the Approach for Prediction of Oil Slick Transport in the Sea: Model- ling oil Pollution Resulting from River Input,” Journal of Marine Systems, Vol. 48, No. 1-4, 2004, pp. 159-170. doi:10.1016/j.jmarsys.2003.11.023 [26] K. A. Korotenko and A. V. Sentchev, “Effects of Particle Migration on the Features of Their Transport by Tidal Currents in a Region of Freshwater Influence,” Oceanol- ogy, Vol. 48, No. 5, 2008, pp. 622-633. doi:10.1134/S0001437008050020 [27] K. A. Korotenko, M. J. Bowman and D. E. Dietrich, “High-Resolution Model for Predicting the Transport and Dispersal of Oil Plumes Resulting from Accidental Dis- charges in the Black Sea,” Recent Advances in Numerical Copyright © 2012 SciRes. OJMS
K. A. KOROTENKO ET AL. Copyright © 2012 SciRes. OJMS 83 Ocean Modeling and Prediction, Terrestrial Atmospheric and Oceanic Sciences, Vol. 21, 2010, pp. 123-136. doi:10.3319/TAO.2009.04.24.01(IWNOP) [28] V. Sentchev and K. A. Korotenko, “Stratification and Tidal Current Effects on Larval Transport in the Eastern English Channel: Observations and 3D Modeling,” Envi- ronmental Fluid Mechanics, Vol. 4, No. 3, 2004, pp. 305-331. doi:10.1023/B:EFMC.0000024246.39646.1d [29] V. Sentchev and K. A. Korotenko, “Dispersion Processes and Transport Pattern in the ROFI System of the Eastern English Channel Derived from a Particle-Tracking Model,” Continental Shelf Research, Vol. 25, No. 16, 2005, pp. 2294-2308. doi:10.1016/j.csr.2005.09.003 [30] V. Sentchev and K. A. Korotenko, “Modeling Distribu- tion of Flounder Larvae in the Eastern English Channel: Sensitivity to Physical Forcing and Biological Behavior,” Marine Ecology Progress Series, Vol. 347, 2007, pp. 233-245. doi:10.3354/meps06981 [31] F. Blumberg and G. L. Mellor, “A Description of a Three-Dimensional Hydrodynamic Model of New York Harbor Region,” Journal Hydraulic Engineering, Vol. 125, 1987, pp. 799-816. [32] B. W. J. Kern, Y.-L. Chen and M.-Y. Chang, “The Diur- nal Cycle of Winds, Rain, and Clouds over Taiwan during the Mey-Yu, Summer, and Autumn Rainfall Regimes," Monthly Weather Review, Vol. 138, No. 2, 2010, pp. 497-516. doi:10.1175/2009MWR3031.1 [33] S. Levitus, “World Ocean Atlas//NOAA Atlas,” US De- partment of Commerce. NOAA/NODC, 2009. [34] J. Hunter, C. Craig and H. Phillips, “On the Use of Ran- dom-Walk Models with Spatially-Variable Diffusivity,” Journal of Computational Physics, Vol. 106, No. 2, 1993, pp. 366-376. doi:10.1016/S0021- 9991(83)71114-9 [35] A. Visser, “Using Random Walk Models to Simulate the Vertical Distribution of Particles in a Turbulent Water Column,” Marine Ecology Progress Series, Vol. 158, 1997, pp. 275-281. doi:10.3354/meps15827 [36] G. L Mellor and T. Yamada, “Development of a Turbu- lence Closure Model for Geophysical Fluid Problems,” Review of Geophysics Space Physics, Vol. 20, No. 4, 1982, pp. 851-875. doi:10.1029/ RG020i004p00851 [37] J. J. Coering and J. D. Cline, “A Note on Denitrification in Seawater,” Limnology and Oceanography, Vol. 15, 1970, pp. 306-309. [38] K. A. Korotenko and A. V. Sentchev, “Turbulence Inves- tigation in a Tidal Coastal Region,” Oceanology, Vol. 51, No. 3, 2011, pp. 394-406. doi:10.1134/S000143701103012X.
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