Nitrate Transport Characteristics in the Soil and Groundwater

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 157 (2016) 246 – 254

IX International Conference on Computational Heat and Mass Transfer, ICCHMT2016

Nitrate Transport Characteristics in the Soil and Groundwater Jinqiao Wu, Jing Ding*, Jianfeng Lu *School of Engineering, Sun Yat-Sen University, Guangzhou, 510006, China

Abstract Nitrate as heat transfer and storage media can be widely used in solar thermal power, nuclear power, and other industrial engineering. If the leaked nitrate cannot be timely recovered, it will probably dissolve in the water, and then transport to the soil and groundwater. In this paper, a numerical model was built to simulate large-scale migration of nitrate in the soil and groundwater, and then the unsteady diffusion performance of nitrate in the groundwater was studied to prevent and reduce the environment pollution of leaked nitrate. The soil system was constructed by the loam and sand, and different groundwater levels and crossing flow rates were also considered. The results showed that the vertical nitrate diffusion range was mainly impacted by the groundwater depth and annual precipitation. As the groundwater depth was deeper or the local annual precipitation was larger, the nitrate pollution can permeate deeper. As the crossing velocity increased, the horizontal pollution range remarkably increased and the vertical pollution range decreased, and the saturated soil layer was more obviously influenced by the crossing flow. © 2016 2016The TheAuthors. Authors.Published Published Elsevier © by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of ICCHMT2016. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICCHMT2016 Keywords: nitrate; soil and groundwater; permeation; crossing flow; numerical model

1. Introduction As heat transfer fluid and storage medium, molten salt can be applied in many thermal energy storage systems [1]. In China, the present energy situation makes that the majority of application is in the solar power technology [2]. According to chemical composition, molten salt has many types as nitrate molten salt, carbonate molten salt and

* Corresponding author. Tel.: +86 020 39332320; fax: +86 020 39332321 . E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICCHMT2016

doi:10.1016/j.proeng.2016.08.363

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fluoride molten salt, etc. Because of favorable stability at high temperature and low vapor pressure, the nitrate is widely used in realistic utilization [3]. Ternary nitrate (40%NaNO2-53%KNO3-7%NaNO3) was adopted in the concentrated solar system of Themis plant, and its operating temperature range is 150oC-550oC [4]. By using nitrate (60%NaNO3-40%KNO3) as the solar receiver and storage fluid, the MSEE generated 4700 kg/h steam at 504 oC in the salt-to-water stream generator [5]. Many other concentrated solar plants also used the multiple nitrates as working medium to transfer and store energy [3, 6]. The fluid using popularity of multiple salts [7] is in order to decrease the fluid melting point temperature for more steady running conditions. Although the nitrate is widely used in many thermal transmission and storage device, in case of exceeding the service life of the equipment, aging fluid tank and tubes or other accidental situations, the salt would leaked from heat transfer system. Thus if there is no unexpectedness, the leaked nitrate would timely recovered, but if the uncontrollable emergencies was encountered, such as earthquake, mountain torrents and rainstorm, etc., leaking nitrate couldn’t be handled immediately. The contamination would flow to the ground and then migrate in the groundwater. Furthermore, the solute transport research about inorganic salt corroborated that nitrification was not supposed to happen on the fully saturated soil samples [8]. Hence, diffusion range and concentration of the pollution at specific location should be confirmed. Then reasonable and effective environment protection range will be provided by this investigation about nitrate diffusion in the groundwater. As Simulation model, HYDRUS-1D model have been used to study the leaching in the soil [9-11]. Wang et al. [12] detected the leaching of accumulated N under heavy rainfall, high irrigation rate in growing season and with different amounts of initial accumulated N. Ramos et al. [13] successfully simulated water and solute transport in two multicultural experiments, in which water with different salinity and nitrogen concentrations was used. The model of transient water flow and nitrogen transport were established by Kurtzman et al. [14] to calibrate the data of two vadose zone profiles in the sandy-loam soils. In this paper, the large-scale migration of nitrate in the soil and groundwater was simulated by HYDRUS-1D model, and then the unsteady nitrate diffusion in the groundwater and the scope of nitrate contaminant were evaluated and investigated. Nomenclature θ K S

volumetric water content (m3/m3) unsaturated hydraulic conductivity function (m/s) sink term (1/s) C solute concentration in the liquid (kg/m3) s solute concentration in the solid(kg/m3) u pore-water velocity (m/s) D dispersion coefficient tensor (m2/s) W liquid volume of somewhat introduced source sink term (m3/t) ρ soil bulk density (kg/m3) ГD, ГN Dirchilet, and Neuman type boundary segments, respectively Гg gradient type boundary segment q outward fluid flux (kg/m2s) C0 outward fluid flux (kg/m3) h0 initial pressure head (m) 2. Modelling and analysis 2.1. System description In this paper, a 2D system (10u10 m˅was built to simulate large-scale migration of nitrate in the soil and groundwater (Figure 1). The upper and lower layers of the soil system were constructed by the loam and sand respectively, and the material characteristics were shown in Table 1. The groundwater depth is the vertical distance between the highest position of groundwater and the ground surface. The local precipitation flows from the top to

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bottom, and the soil was infiltrated by the precipitation. The upper boundary was open to atmosphere and the water head of lower boundary retains 900 cm. In some situation, the underground water in the soil derives from the river nearby, so the crossing flow from left to right was also considered.

Table 1. The characteristics of two soil layers Mat

Sand

Silt

Clay

Bulk Density

Porosity

%

%

%

g/cm3

%

1

82.95

14.44

2.61

1.475

44.54

2

94.21

5.36

0.43

1.402

46.79

Z x

Fig. 1. Nitrate permeation system in the groundwater

2.2. Physical model The numerical simulation package HYDRUS [15] was used to simulate the process of water flow, solute transport, heat transport and root water uptake. For solute transport, the advective-dispersive transport in the liquid phase is regarded as gaseous phase. The governing equation of water flow can be calculated by Richard’s Equation [16]: ·º w ª § wh wT (1)  Kiz ¸ »  S = « K ¨¨ Kij ¸» wx «¬ © wx j wt ¹¼ where θ is the volumetric water content, h is the pressure head, t is time, K is the unsaturated hydraulic conductivity function, S sink term, which represents the volume of water removed per unit time from a unit volume of soil due to plant water uptake [17]. Van Genuchten [18] used the statistical pore-size distribution model of Mualem [19] to obtain a predictive equation for the unsaturated hydraulic conductivity function in terms of soil water retention parameters. The equation of van Genuchten is:

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T  h

Ts  Tr ­ T  ° n m ° r ª 1 D h º ® ¬ ¼ ° T ° ¯ s

h0 h!0

m K S Sel ª«1  1  Se1/ m º» ¬ ¼

K  h

(2)

2

(3)

1 m 1 , n ! 1 n

(4)

where θr and θs are the saturation water content and residual water content, α, n, Ks and l are parameters predicted by HYDRUS. The pore-connectivity parameter l in the hydraulic conductivity function was estimated to be about 0.5. The classical solute transport equation is derived by the mass conservation equation, and the non-equilibrium equation is taken as w ˄T C +Ub s) ’(T D ’C )  ’(T uC )  WCw  I wt

(5) where C and s are solute concentration in the liquid and solid phases respectively, D is the dispersion coefficient tensor for the liquid phase, and ρ is the soil bulk density, u is pore-water velocity, W is liquid volume of somewhat introduced source sink term, Cw is fluid concentration of the introduced source sink term, I is other source sink terms. In order to solve equation (5), it is necessary to know the water content θ, and the other variables can be obtained from Richard’s equation. The water content can be calculated by: TR

wC ˄+Ub s ) wt

w wxi

§ wC ¨¨ T D xj w ©

· wqC  WC  I ¸¸  ¹ wxi

(6) where q represents the outward fluid flux. Three types of conditions was implemented to describe system-independent interactions along the boundary conditions of the form x, y, z *D h  x, y, z, t \  x, y, z, t for  (7) specified flux (Neumann type)boundary conditions given by ª § ·º wh  « K ¨ Kij  Kiz ¸» ni ¨ ¸» «¬ © wx j ¹¼

V1  x, y, z, t

for 

x, y, z * N

(8)

and specified gradient boundary condition § · wh  ¨ Kij  Kiz ¸ ni ¨ ¸ wx j © ¹

V 2  x, y , z , t

 x, y , z  *

g for (9) where D, N, g indicate Dirchilet, Neuman and gradient type boundary segments, respectively; <, V1 and V1 are prescribed functions of x, z and t; and ni are the components of the outward unit vector normal to boundary N or g . Whereas third-type (Cauchy type) boundary condition is used to prescribe the concentration flux along a boundary segment c as follows:

T Dij

wC n  qnC wx j

qnC0

x, y, z *C for  (10) in which q represents the outward fluid flux, n is the outward unit normal vector and C0 is the concentration of the incoming fluid.

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2.3. Calculation conditions The pressure head at the top and bottom can be described as:

h  x,0, t 1mH2 O

h  x, 10, t 9mH2 O

for t 0

(11a)

for t

(11b)

0

where h0, Ci are initial pressure head and initial solute concentration. Beijing Inner Mongolia Sinkiang

Precipitation (mm)

140 120 100 80 60 40 20 0

1

2

3

4

5

6

7

8

9

10

11

12

Time (month)

Fig.2. The monthly precipitation of three typical plants site in a typical year

The flow velocity rate at the top and left side can be described as: v  x,0, t v0

for 2.5m d x d 2.5m

u  5, z, t u0

(12a) (12b)

where u0 is the crossing flow rate, and v0 has direct ratio with the precipitation. The initial pressure heat can be described as: h  x, z, t h0  x, z

for t 0 where h0 is calculated from the steady parameter of the system with boundary condition of Eqs. (11-12). The initial solute concentration can be described as: C  x,0,0 C0

for 2.5m d x d 2.5m

C  x, z, t 0

(13)

(14a)

for z  0m (14b) In the article, the local precipitation data of solar plant in Beijing, Inner Mongolia and Sinkiang (Figure 2) was used in Eq. (12a). In the present article, D=10.1 cm2/d, C0=0.651 g/cm3 (saturated nitrate concentration). The maximum allowable nitrate concentration is 0.0113 mg/cm3 by the WHO (World Health Organization). From the Chinese hygienic standard for drinking water (GB5749-2006), the permissible concentration must not exceed 0.01 mg/cm3, and this value was used to judge whether the target zone was contaminated in this simulation.

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3. Results and discussion 3.1. Basic nitrate transport characteristics

(1) 0 day

(2) 180 days

(3) 365 days

Fig.3. The nitrate concentration change under Beijing precipitation condition

0

Depth (m)

-2

-4

-6

1 year 5 year 10 year

-8

-10

0

2

4

6

8

10

3

Concentration (mg/cm )

Fig.4. The nitrate concentration change with different annual precipitation

After the nitrate diffused in the groundwater, the concentration range fell by the time duration. The transport characteristics of nitrate also were influenced by the other boundary or configurable conditions. Figure 3 presents the basic nitrate concentration distribution under Beijing precipitation condition during one year. As the total precipitation increases with time going, the nitrate diffusion range in the vertical direction moved down, while the nitrate range in the horizontal direction changed very little. Figure 4 presented the concentration distribution under Beijing precipitation condition after the precipitation of 1 year, 5 years and 10 years. From Figure 4, the range of nitrate diffusion peak remarkably enlarged as moving down, while its concentration value decreased. 3.2. Precipitation effect

(1)Sinkiang

(2) Inner Mongolia

(3) Beijing

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Jinqiao Wu et al. / Procedia Engineering 157 (2016) 246 – 254 Fig.5. The nitrate concentration change with different annual precipitation

Figure 5 presents he nitrate concentration change with different annual precipitation. When the annual rainfall reduced, the accessible depth decreased, so the nitrate permeated into the deepest level for high annual precipitation in Beijing. As the annual precipitation is larger, the contamination range is bigger and deeper. So, solar power plant what used nitrate as working fluid was better to be constructed in the rainless area for environment problem. As Figure 5 was demonstrated, the depth Sinkiang could reach was the shallowest. Therefore if the nitrate is migrated into that groundwater, it will be necessary to dig out the soil near surface and refill it after leaching the nitrate. Once the contaminant isn’t handled timely, it would exist for a long time, and affect further environment.

3.3. Groundwater depth effect

(1) 1 m

(2) 2 m

(3) 3 m

Fig.6. The concentration change with different groundwater depth

The soil characteristics and the groundwater depth changed in different regions. From an example, the groundwater depth is 2.0-6.0 m in Beijing, 0.6-2.4 m in Inner Mongolia and 4.5-8.7 m in Sinkiang, respectively. Figure 6 presented the nitrate concentration change, where the groundwater depth was set as 1m, 2m and 3m. As the groundwater depth is larger, the nitrate can permeate deeper and the concentration was lower. Under the level with groundwater depth, the soil is turned from unsaturated porous media to the saturated one. Because the nitrate was leached faster in the unsaturated soil rather than saturated soil, the nitrate with groundwater depth of 3m diffused the fastest. 3.4. Crossing flow effect There are always accompanied adjacent rivers or horizontal flows disturbing the subsurface leaching function. In this simulation, the crossing flow which from left to right and perpendicular to the direction of rainfall was designed to research the impact of velocity change. From Fig.7, the crossing flow velocity would decrease the vertical nitrate permeation range, but increase the horizontal range. It provided demonstration that the solution range should be considered larger under the certain flowing situation.

(1) no flux

(2) 0.2 cm/day

(3)1 cm/day

(4)5 cm/day

Jinqiao Wu et al. / Procedia Engineering 157 (2016) 246 – 254 Fig.7 The nitrate diffusion contours of different crossing flow velocity at the 250th day

Fig.8. described the trace of the maximum concentration particle at the horizontal direction with different crossing flow velocity. The position migrated towards upper right at varying degree, the more quick the flowing, and the more obvious migration was. It is also certified that the velocity of crossing flow would influence the horizontal and vertical pollution range. In addition, the position where the traces started to deviation was close to the groundwater depth. This is translated to that the crossing flow affected the saturated layer more than the unsaturated layer. And the other reason of this trend is that the precipitation affected saturated layer less than the unsaturated one. 12 10

Month

8 6 4 No flux 0.2 cm/day 1 cm/day 5 cm/day

2 0

0

1

2

3

4

5

X (m)

Fig.8 The trace of the maximum concentration particle at the horizontal direction

4. Conclusions The nitrate transport with different conditions was simulated and analyzed by HYDRUS model. By inputting the local precipitation data of solar plant in Beijing, Inner Mongolia and Sinkiang in the simulation, the results suggest that the contamination range is bigger and deeper for the larger annual precipitation. Because the nitrate leaching was more actively in the unsaturated layer rather than saturated layer, the whole diffusion region would move down as the groundwater depth was deeper. The nitrate concentration removal is remarkably dependent on the water content, but it was less influenced by soil material. Crossing flow also affects the nitrate transport process in the actual situation. As the crossing velocity increases, the horizontal pollution range remarkably increased, while the vertical pollution range decreased. In addition, the migration trace of maximum concentration particle showed that the crossing flow affected the saturated layer more than the unsaturated layer, and the unsaturated region was mainly controlled by precipitation. Acknowledgements This paper is supported by National Natural Science Foundation of China (No. 51476190, 51436009), National High Technology Research and Development Program of China (2012AA050604), National Key Technology Support Program (2014BAA01B01) and the Fundamental Research Funds for the Central Universities. References [1] E. Rivas, E. Rojas, Heat transfer correlation between Molten Salts and helical-coil tube bundle Steam Generator, J. International Journal of Heat and Mass Transfer, 2016, 93, pp. 500-512. [2] Z. Zhu, D. Zhang, P. Mischke, X. Zhang, Electricity generation costs of concentrated solar power technologies in China based on operational plants, J. Energy, 2015, 89, pp. 65-74. [3] Soleimani Dorcheh, M.C. Galetz, Slurry aluminizing: A solution for molten nitrate salt corrosion in concentrated solar power plants, J. Solar Energy Materials & Solar Cells, 2016, 146, pp. 8-15. [4] V Mascle, F Pharabod, B Rivoire, Themis Plant Operation Progress Report, J. Springer Netherlands, 1985, 2, pp. 62-72. [5] J.T. Holmes, The solar molten salt electric experiment, J. Intersociety energy conversion engineering conference, 1984, San Francisco, CA, USA.

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