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PUBLICATIONS Water Resources Research RESEARCH ARTICLE 10.1002/2016WR020344 Key Points: Evaporation increased supratidal pore water salinity to higher than 85 g/L within a shallow layer of the beach Evaporation as well as precipitation significantly altered pore water flow in beach supratidal zone Evaporation and rainfall generated spatially different driving force for pore water flow in the beach supratidal zone Supporting Information: Supporting Information S1

Correspondence to: M. C. Boufadel, [email protected] Citation: Geng, X., and M. C. Boufadel (2017), The influence of evaporation and rainfall on supratidal groundwater dynamics and salinity structure in a sandy beach, Water Resour. Res., 53, 6218–6238, doi:10.1002/ 2016WR020344.

The influence of evaporation and rainfall on supratidal groundwater dynamics and salinity structure in a sandy beach Xiaolong Geng1

and Michel C. Boufadel1

1 Center for Natural Resources Development and Protection, Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, New Jersey, USA

Abstract Evaporation has been recognized as a major driving force affecting coastal aquifer systems. However, its effects on subsurface flow and salinity structure have not been investigated in sufficient detail. This paper presents field measurements and numerical simulations of pore water flow and subsurface salinity structure in the supratidal zone of a sandy beach subjected to evaporation as well as rainfall. It was found that evaporation significantly increased pore water salinity, up to 85 g/L, within a shallow layer, approximately 10 cm below the beach surface. The induced density gradient generated salt fingers near the beach surface, which caused local groundwater circulation (i.e., fingering flow). However, unlike inland aquifers, the salt fingering was significantly diminished by tidal action that prompted the horizontal mixing of salt in the beach. The subsequent precipitation (e.g., rainfall) diluted the evaporation-induced high saline plume near the beach surface and drove the plume to migrate downward; the plume gradually dispersed and was diluted along the groundwater pathways. The simulation results indicated that evaporation as well as precipitation at the beach surface induced complex driving mechanisms for supratidal groundwater flow. Depending on the intensity at the beach surface, evaporation and rainfall significantly altered the pore water flow and associate solute transport processes in the supratidal zone of the beach.

1. Introduction Received 1 JAN 2017 Accepted 4 JUL 2017 Accepted article online 10 JUL 2017 Published online 28 JUL 2017

Coastal beaches subjected to oceanic forcing (e.g., tides and waves) involve very complex driving mechanisms for subsurface pore water flow and solute transport processes [Abdollahi-Nasab et al., 2010; Geng et al., 2015; Heiss, 2011; Michael et al., 2005; Moore, 1999; Robinson et al., 2006; Xin et al., 2010]. Tidal action induces significant saltwater-freshwater mixing and recirculation and forms two distinct saline plumes in beaches: the classical saltwater wedge and an upper saline plume that overlays a ‘‘tube,’’ whereby fresh groundwater discharges near the low tide mark [Boufadel, 2000; Brovelli et al., 2007; Heiss and Michael, 2014; Robinson et al., 1998]. Waves superimposing on tides enhance the mixing and recirculation between seawater and groundwater and induce a convoluted driving force for groundwater flow and solute transport due to the nonlinear interactions between tides and waves [Baldock and Hughes, 2006; Geng and Boufadel, 2015c; Heiss et al., 2014; Sous et al., 2013; Xin et al., 2015]. These effects could even extend to the beach supratidal zone due to intensified waves, storms, and hurricanes [Personna et al., 2015; Robinson et al., 2014; Xin et al., 2014]. As a fundamental component of the hydrologic cycle, evaporation has been recognized as an important driver for subsurface water flow and associated solute transport [Fujimaki et al., 2006; Geng and Boufadel, 2015a; Gran et al., 2011a; Mahfouf and Noilhan, 1991; Wooding et al., 1997a]. Previous studies revealed that evaporation from bare soils in absence of salt drives pore water to flow upward [Geng and Boufadel, 2015b; Gran et al., 2011b; Zhang et al., 2014]. The magnitude of the driving force is dependent on meteorological condition and subsurface water content [Mahrt, 1996; Saito et al., 2006; Smits et al., 2011; Zhang et al., 2015].

C 2017. American Geophysical Union. V

All Rights Reserved.

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The influences of evaporation on subsurface flow are more complex when evaporation occurs from saline soil; pore water flow is driven initially upward due to the evaporation at the soil surface. As salt accumulates near the soil surface, high saline plume forms near the soil surface and migrates downward with a ‘‘finger’’ shape [Boufadel et al., 1999b; Wooding et al., 1997b; Zimmerman et al., 2006]. The subsequent density gradient drives pore water to circulate around these fingers [Duffy and Al-Hassan, 1988; Geng and Boufadel, 2015a; Wooding et al., 1997a]. However, these studies are mostly for inland aquifer systems. In coastal

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beaches with shallow water table, the near-surface region usually has a relatively high water content (due to capillary forces), which facilitates pore water evaporation from there; meanwhile, precipitation (e.g., rainfall) causes freshwater infiltration into the beach, which alters pore water flow and subsurface salt distribution. Therefore, evaporation interacting with tides and waves as well as precipitation might induce very complex driving mechanisms affecting subsurface flow and fate and transport of solutes/contaminants. This interaction is expected to be more obvious in the supratidal zone of beaches, due to relatively longer exposure time to the air compared to the intertidal zone. However, the contribution of evaporation and precipitation to coastal groundwater dynamics and their interaction with oceanic forcing (e.g., tides) have not been investigated, especially in supratidal zone of beaches. These water and salt dynamics are critical for understanding aspects of beach aquifer systems such as subsurface geochemistry, solute fate, and ecosystem function. To explore evaporation and rainfall affecting coastal groundwater system, here we present field measurements complemented with numerical simulations in the supratidal zone of a beach located in Gulf of Mexico. Based on field and modeling results, we quantified the influence of evaporation as well as rainfall on subsurface pore water flow and salinity structure within the supratidal zone of this tidally influenced beach.

2. Field Work The study was conducted at a beach located in Grand Isle State Park in Grand Isle, Louisiana (29816.1800 N, 89857.3130 W). As one of many elongated barrier islands fringing the Gulf of Mexico, Grand Isle is an island immediately off the coast of Louisiana, approximately 1.2 km wide and 12 km long (Figure 1a). The shoreline consists of a smooth straight beach that is approximately 5 km along shore, and the remainder consists of irregular features [Collins and Easley, 1999]. Tides in Grand Isle are primarily diurnal (one high tide and one low tide each day). The tidal range varies from 0.37 m at neap tide to 1.8 m at spring tide. There is a large shallow tidal lagoon known as Barataria Bay, lying behind the island (Figure 1a). Therefore, the fluctuation of groundwater in Grand Isle is impacted by dual tide from Gulf of Mexico and Barataria Bay. Two transects were installed perpendicular to the shoreline and were 40 m long extending from the intertidal zone to the supratidal zone of the beach (Figure 1b). The alongshore distance between the transects was 2 m. One transect consisted of four stainless steel multiport sampling wells (MP wells) that were used to collect pore water samples at several discrete depths below the beach surface for the measurement of salinity (among others), and the other consisted of four galvanized steel piezometer wells (PW wells) that were used to measure the groundwater table. The piezometer wells were made of 3.8 cm 3 91.4 cm galvanized drive point (J48–12-HomeDepot). The drive points were perforated to allow water passing through them. A self-logging pressure transducer (Cera-Diver, Schlumberger) was placed at the bottom of each piezometer well to record the water pressure and temperature at every 10 min. The multiport sampling wells were made of stainless steel and contained ports at various levels. Pore water samples were collected from the sampling ports of MP wells on 28 January 2011 (3:00 P.M.; high tide). The pore water salinity for each of the sample was measured using a digital refractometer (300035, SPER SCIENTIFIC). The groundwater table was monitored from 3 through 28 January 2011. The approach followed our previous work for different beaches for various research purposes, including oil spill investigation [Boufadel et al., 2011; Geng et al., 2013, 2015; Li and Boufadel, 2010]. The local meteorological conditions including air temperature, relative humidity, and rainfall were obtained from National Solar Radiation Database (NSRDB, Station ID: 837406) (https://data.noaa.gov/dataset/nationalsolar-radiation-database-nsrdb station-data-output-for-1991-to-2010) and are reported in Figure 1c. Sediment samples were collected using a stainless steel core sampler that was driven into the sediment by a sliding hammer at different locations of the beach. The collection is for identifying physical properties of sediments such as grain size distribution (sediment size D10 ranges from 80 to 100 mm), porosity, saturated hydraulic conductivity, and capillary parameters. Soil moisture was measured at different depths at different locations by using a soil moisture meter (Lincoln Irrigation, Nebraska) on 28 January 2011. In the laboratory at NJIT, capillary-retention experiments were conducted using some of the collected sediment samples. The measured capillary-retention data were fitted by the van Genuchten model [van Genuchten, 1980] using the least square method to estimate capillary parameters, such as the van Genuchten parameters a and ‘‘n.’’ The approach followed that of Boufadel et al. [1998].

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Figure 1. (a) Location of the studied beach on Grand Isle State Park, Louisiana; (b) cross-sectional view of studied site showing beach topography and locations of four piezometer wells (crosses) and four multiport sampling wells (squares); (c) observed data of relative humidity, temperature, rainfall, and wind speed during the simulated period between 4 and 28 January 2011 (total 600 h). All measuring wells were installed in the beach supratidal zone, except for PW4 which was installed in the intertidal region to monitor tidal fluctuation. The 30 min meteorological data were obtained from National Solar Radiation Database (Station ID: 837406), which is approximately 2.4 km southeast of the studies site (https://maps.nrel.gov/ nsrdb-viewer). The insets in Figure 1b show the simulated domains (Domains 1 and 2) located at landward and seaward sides of the supratidal zone, respectively. The salt fingering shown in Domains 1 and 2 are from the simulation results.

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3. Numerical Modeling 3.1. T-MARUN Model The model MARUN was expanded to incorporate heat and vapor transport, named T-MARUN, as explained herein. New formulae (equations (16), (30), and (31)) were adopted in the model to calculate the relatively humidity at the soil surface, taking into account the effects of temperature, matric potential, osmotic potential, and salt crust on the evaporation from the saline soil, which was not considered in Geng and Boufadel [2015a, 2015b, 2016a]. The equation for the conservation of the fluid mass is expressed as follows: @ðb/SÞ @ðbdqLx Þ @ðbdqLz Þ @ðqVz Þ 52 2 2 @t @x @z @z

(1)

where b is the density ratio defined as q=q0 , and d is the viscosity dynamic viscosity ratio defined as l=l0 ; q and q0 are salt-dependent water density and freshwater density (kg m23), respectively; l and l0 are salt-dependent water dynamic viscosity and freshwater dynamic viscosity (kg m21 s21), respectively; / is the porosity of the porous medium (m3 m23); qLx and qLz denote liquid flow in horizontal and vertical directions (m s21), respectively; qVz denotes vertical vapor flow (m s21); and S denotes soil moisture ratio. The liquid and vapor flows used in equation (1) are expressed as follows: @w @T 2KLT @x @x @w @T 11 2KLT qLz 52KLh @z @z qLx 52KLh

qVz 52KVh

@w @T 2KVT @z @z

(2) (3)

(4)

where w denotes pressure head (m); T denotes temperature (K); KLh and KLT denote the isothermal and thermal hydraulic conductivities of the liquid phase (m s21), respectively; and KVh and KVT denote the isothermal and thermal hydraulic conductivities of the vapor phase (m2 K21 s21), respectively. The two terms on the right side of equations (2)–(4) represent the liquid/vapor flow due to pressure head gradient (i.e., Darcy’s Law) and temperature gradient, respectively. The soil moisture ratio and freshwater hydraulic conductivity are correlated by the van Genuchten [1980] model: For w 0; S51:0; KLh 5KLh0

(5)

where KLh0 is the saturated hydraulic conductivity (m s21). For w < 0, the effective saturation ratio, Se , is given by m S2Sr 1 Se 5 5 12Sr 11ðajwjÞn

(6)

and KLh is given by KLh 5KLh0 Se ð1=2Þ ½12ð12Se 1=m Þm 2

(7)

where m512ð1=nÞ, Sr is the residual saturation ratio, and jwj is the absolute value of w. The thermal hydraulic conductivity function, KLT , is defined as follows [e.g., Noborio et al., 1996]: 1 dc KLT 5KLh : wGwT (8) c0 dT The parameter GwT is the gain factor (unitless), which quantifies the temperature dependence of the soil water retention curve [Nimmo and Miller, 1986], c is the surface tension of soil water (g s22), and c0 is the surface tension of water at 258C, which is equal to 71.89 g s22. The expression for c as function of temperature is given by

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c575:620:1425T22:3831024 T 2

(9)

where T represents temperature in 8C. Thus, as the temperature decrease, the surface tension decreases. The isothermal vapor hydraulic conductivity KVh (m s21), and the thermal vapor hydraulic conductivity, KVT (m2 K21 s21), are described as [e.g., Fayer, 2000; Nassar and Horton, 1989; Noborio et al., 1996] KVh 5

DV Mg Hr qSV RT q

(10)

DV dq gHr SV q dT

(11)

KVT 5

where DV is the vapor diffusivity in soil (m2 s21), qSV is the saturated vapor density (kg m23), M is the molecular weight of water (kg mol21), g is the gravitational acceleration (m s22), R is the universal gas constant (J mol21 K21), and g is the enhancement factor (unitless). The vapor diffusivity, DV , of the soil is defined as DV 5sha Da

(12)

where ha is the air-filled porosity (m3 m23) and s is the tortuosity factor as defined by Millington [1959]: s5

ha 7=3 hs 2

(13)

The parameter hs denotes saturated volumetric water content (m3 m23) and Da is the diffusivity of water vapor in air (m2 s21) at temperature T: 2 T Da 52:1231025 (14) 273:15 When liquid and vapor phases of water in soil pores are in equilibrium, the vapor density of the soil can be expressed as the product of the saturated vapor density and the relative humidity (i.e., qV 5qSV Hr ). The saturated vapor density qSV (kg m23) as a function of temperature is expressed as qSV 51023

expð31:37162 6014:79 27:9249531023 TÞ T T

(15)

The relative humidity Hr can be calculated from the pressure head, w, using a thermodynamic relationship between liquid water and water vapor in soil pores as [Philip and De Vries, 1957] ww Mg Hr 5exp (16) RT where ww is the water potential (m), which can be calculated according to ww 5w1wo

(17)

where wo is the osmotic potential (m), dependent on the solute concentration [e.g., Campbell and Norman, 2012]: wo 52xmvCRT

(18)

21

where x is a unit-conversion factor (m kg J ); m is the number of ions per molecule (2 for salt taken as sodium chloride); v is the osmotic coefficient (mol J21); and C is the molar concentration of the solute (mol kg21). The osmotic coefficient, v, can be calculated as follows: v511ax c1bx cpx

(19)

where c is the concentration of the solute (g L21); ax , bx , and px are fitting parameters. For salt, they are equal to 0.0026, 0.0026, and 0.053 [Fujimaki et al., 2006]. The parameter g is called enhancement factor, which is used to describe the increase in thermal vapor flux as a result of liquid trapped within the pore space and increased temperature gradients in the air phase [Philip and De Vries, 1957]. An equation for the enhancement factor g was derived by Cass et al. [1984] and is expressed as

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h 2:6 h 4 28:5 exp 2 11 pffiffiffiffi hs fc hs

(20)

where h is the total volumetric water content (m3 m23) and fc is the mass fraction of clay in the soil (unitless). The solute transport equation (advection-dispersion equation) is written as uS

@c 5brðuS~ DrcÞ2q~L rc @t

(21)

The term ~ D represents the liquid water physical dispersion tensor given by /SDXX 5aL

ðqLx Þ2 ðqLz Þ2 1aT 1/Ssc Df kqL k kqL k

(22)

/SDZZ 5aT

ðqLx Þ2 ðqLz Þ2 1aL 1/Ssc Df kqL k kqL k

(23)

/SDXZ 5ðaL 2aT Þ

qLx qLz kqL k

(24)

where aL and aT are the longitudinal and transverse dispersivities (m), respectively, Df is the diffusion coefficient (m2 s21), sc is the domain tortuosity, and kqL k5ðqLx 2 1qLz 2 Þ1=2 . The dispersion tensor accounts for mechanical dispersion and molecular diffusion. The cross dispersion terms account for the situations where velocity does not coincide with the major axes. The equation for heat transport within the porous medium is written as @Sh @ 5rðkT rT2CL q~L TÞ2 ðL0 qV 1CV qV TÞ @t @z

(25)

Where L0 (J m23) is the latent heat of vaporization of liquid water (L0 5 2.501 3 106 – 2369.2T), and Sh is the storage of heat in the soil (J m23), which is expressed as Sh 5ðCS 1CL hÞT

(26)

where CS , CL , and CV denote the volumetric heat capacity of dry soil particles, liquid, and vapor (J m23 K21), respectively. The bulk thermal conductivity kT (J m21 s21 K21) is described as follows [Chung and Horton, 1987]: pffiffiffi kT 5b1 1b2 h1b3 h

(27)

where b1 , b2 , and b3 are empirical regression parameters (W m21 K21) and equal to 0.228, 22.406, and 4.909, respectively. 3.2. Numerical Implementation Figure 1b shows the cross-shore view of the studied transect in Grand Isle. A very fine mesh (1.0 cm) and a small time step (usually less than a few seconds) were needed for simulating the near-surface processes of evaporation, groundwater flow and associated solute transport. For these reasons, two segments (e.g., Domains 1 and 2) at the landward and seaward sides of the supratidal zone were selected and simulated, respectively. Each domain was approximately 2 m long and 2 m deep with a mesh resolution of 1 cm in both horizontal and vertical directions. The simulated period was for 25 days starting from 4 to 28 January 2011. To obtain the boundary conditions for the two simulated domains (Domains 1 and 2), large-scale simulations for the whole transect were first conducted using a coarse mesh (spatial resolution of 0.1 m 3 0.1 m) without considering the evaporation, in which observed water table, salinity (equal to 20 g/L in groundwater and 35.5 g/L in seawater), and tide level were used as boundary conditions. The observed water table at four piezometer wells (PW1–PW4) along with the model prediction was shown in supporting information Figure S1. Note that the water table measured at PW1 and PW4 was used as boundary conditions for the large-scale simulations. The simulated water table and salinity were then output at the boundaries of Domains 1 and 2 at each time step (10 s) for the small-scale simulations to take into account

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evaporation as well as rainfall at the beach surface. To consider evaporation effects, Neumann boundary condition calculated by the aerodynamic model was used to represent the evaporation flux E (m/s) at the exposed portion of the beach surface using the meteorological conditions shown in Figure 1c, which is described as follows [Fujimaki et al., 2006]: E5

Hr 2Ha ra 1rsc

(28)

where Ha is the relative humidity in the air and ra is the aerodynamic resistance (s m21) expressed as [Liu et al., 2006] ra 594:909UZ 20:9036

(29)

where Uz is wind speed at the atmospheric reference level (2 m above the soil surface). The parameter rsc (s m21) denotes the additional resistance due to formation of salt crust near the surface, which is described as [Fujimaki et al., 2006] rsc 50; C < exp ð2br =ar Þ

(30)

rsc 5100½ar ln ðCÞ1br ; C exp ð2br =ar Þ

(31)

where C (mg cm22) denotes the accumulated mass of salt above a depth of 0.25 cm and ar and br are the fitting parameters equal to 0.69 and 21.04, respectively. A Cauchy boundary condition was used to simulate the evaporation-induced salt accumulation below the beach surface [Geng and Boufadel, 2015a]. The large-scale simulations provided a uniform salinity of 20 g/L in Domains 1 and 2, which was because neither evaporation nor rainfall was considered. Due to this fact, 20 g/L was assigned as the initial condition for the small-scale simulations. The salinity of 20 g/L is similar to the measured value (14–24 g/L) in Barataria Bay lying behind Grand Isle, reported in United States Geological Survey (USGS). Rainfall was observed during the studied period, and thereby was considered in the simulations of Domains 1 and 2 as influx with zero salinity at the beach surface. Due to the lack of the information (temperature near the beach surface was not measured), heat transfer and vapor transport were not considered in our current study. The time step for the simulations was selected at 1.0 s, resulting in a grid Courant number less than 0.5 (a value less than 1.0 is required). The simulations were started from Cases 1 and 2 for the landward and seaward domains (i.e., Domains 1 and 2), respectively. The parameter values used in the simulations are based on measurements, shown in Table 1. Eight simulations (Cases 3– 10) were performed to examine effects of rainfall intensity on supratidal flow and salt transport. Of the base case rainfall intensity, 1%, 80%, 120%, and 200% were used in Cases 3–6 and 7–10 for the inland and seaward domains, respectively. The simulations were also conducted to test effects of osmotic potential on the supratidal flow and salt transport. The results indicate negligible effects of osmotic potential on the studied beach system, which is not shown in the paper for brevity. This is probably due to the minor effects of osmotic potential on the relative humidity near the beach surface (the osmotic potential of 50 m only causes 0.3% relative humidity difference, equation (16)).

4. Results and Discussion 4.1. Comparison With the Studies of Fujimaki et al. (2006) and Gran et al. (2011a, 2011b) The model T-MARUN was first run and tested by comparing to experimental data obtained from previous studies. The first experiment compared to was an isothermal soil column study conducted by Fujimaki et al. [2006] using two types of sand (Masa loamy and Toyoura). The diameter and height of the soil column were 3.8 and 5.2 cm, respectively. A porous plate (5.0 mm thick) was installed at the bottom of the column, and hydraulically connected to a Mariotte flask to supply a saline solution while maintaining a select pressure head at the bottom. The soil samples were initially saturated from the bottom with 3.0 g/L NaCl solution. After the moisture reached near saturation, the pressure head at the bottom was lowered and kept at either 275 or 240 cm by adjusting the level of the Mariotte flask. Evaporation was applied on the uncovered soil surface under constant meteorological conditions, except for radiation, which was regulated using a thermostat to maintain the soil temperature at 258C. Figures 2a–2e show measured evaporation rate and the profiles of salinity and moisture ratio in the soil column along with the simulation results obtained by the

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EVAPORATION AFFECTS GROUNDWATER DYNAMICS ð12-Þ

ak ak 1 11bk S1bk

Isothermal Isothermal 3.0 300.0 25.0 25.0 0.4

Isothermal 3.0 300.0 25.0 25.0 0.25

x 5 0.75, a1 5 0.026 cm21, n1 5 33.6, a2 5 0.021 cm21, n2 5 1.84

K5K0

½11ð2a2 wm Þn2 121=n2

Isothermal

Sr 5 0.07, a 5 0.042 cm21, n 5 1.37, m 5 0.32, w0 5 2107 cm, w 5 7.2

K5K0 Sw

1

½11ð2a1 wm Þn1 121=n1

x

0.44 1027 (0.23, 0.23)

0.43 1027 (0.14, 0.14)

S5

72.0

12Sr ½11ð2awm Þn m ( ) ln ð2wm 11Þ 2 1Sr 12 ln ð2w0 11Þ S5

Case of Toyoura Sanda

1.9

Case of Masa Sanda

12bSr

1

24.0 24.0 0.5

7.0 300.0

750

8.75 days24

Sr 5 0.08, a 5 0.04 cm21, n 5 10.5, w0 5 26.7 3 106 cm

ln ð2w0 Þ2ln ð2wm Þ ln ð2w0 Þ 1=2 S2Sr K5K0 12Sr n n21 S2Sr ðn21Þ ð n Þ 2 12 12 12Sr

bSr ;

½11ð2a1 wm Þn 121=n

b5

S5

0.4 1027 (0.1, 0.1)

100.8

Case of Silica Sanda

The parameter values used for cases of Masa sand, Toyoura sand, and Silica sand are obtained from Fujimaki et al. [2006] and Gran et al. [2011a, 2011b], respectively.

a

Thermodynamic Properties Cs, J mg21 8C21, specific heat capacity Rn, W m2, net radiation Solute Properties C0, g L21, initial concentration Solubility, g L21 Aerodynamic Conditions Air temperature, 8C Soil temperature, 8C Air relative humidity

Parameter values

Hydraulic conductivity equation

Hydraulic Properties K0, cm/h saturated freshwater hydraulic conductivity (see supporting information Table S1) U, porosity S0, cm21, specific storage (aL, aT), cm, longitudinal and transverse dispersivity Vadose Zone Properties Water retention curve equation

Symbol, Units, Definition

Table 1. Parameter Values Used in the T-MARUN Model for Each Simulated Case

12Sr

1Sr

300.0

Isothermal

Isothermal

Sr 5 0.01, a 5 0.013 cm21, n1 5 7.9

K5K0

½11ð2a1 wm Þn 121=n

1=2 S2Sr 12Sr n n21 S2Sr ðn21Þ ð n Þ 2 12 12 12Sr

S5

0.37 1027 (10.0, 1.0)

36

Case of Grand Isle

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Figure 2. Simulated and measured results of (a, b) evaporation rate, (c, d) salinity, and (e) moisture ratio for cases of Masa sand and Toyoura sand, respectively. The measurements were obtained from Fujimaki et al. [2006].

model T-MARUN. The T-MARUN model was run by specifying the initial and boundary conditions, and by using a spatial increment of 0.01 cm and a time step of 0.1 s. It is shown in Figure 2 that the model underestimated the evaporation rate for the first 48 h, and therefore slightly overestimated the moisture content near the soil surface. This is probably due to the uncertainty of the parameter estimate. Nevertheless, the T-MARUN model generally reproduced the evaporation rate occurring at the soil surface (Figures 2a and 2b), and the associated impacts on subsurface salinity (Figures 2c and 2d) and moisture distribution (Figure 2e). Another evaporation experiment used for further validating the T-MARUN model was the nonisothermal soil column study conducted by Gran et al. [2011a]. The soil column was 24 cm long and 14.4 cm in

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(b)

0.0

Moisture ratio 0.2

0.4

0.6

0.8

1.0

(c) 20

0

0

5

5

5

10 Simulation at 74% sat.

15

Simulation at 50% sat.

10 15

Depth, cm

0

Depth, cm

Depth, cm

(a)0

10.1002/2016WR020344

Temperature, oC 25

30

35

40

45

50

10

15

Simulation at 32% sat.

20

Measurements at 74% sat.


20

20

25

25


25

Figure 3. Simulated and measured results of (a) salinity, (b) moisture ratio, and (c) temperature for Silica sand case. The measurements were obtained from Gran et al. [2011a, 2011b]. The curves denote simulation results and symbols denote measurements. Note that the condensation effects were not considered in our current modeling work.

diameter enveloped in a thermal insulator. The column was initially saturated by saline water at the concentration of 7 g/L. Evaporation was imposed on the soil surface by an infrared lamp. The experiment continued until the overall saturation fell to 0.32. Figures 3a–3c show the measured vertical profiles of salinity, moisture ratio, and temperature at the soil column saturation of 74%, 50%, and 32% along with the modeling results from the model T-MARUN, which was used with a spatial increment of 0.1 cm and a time step of 0.1 s. The T-MARUN model was able to predict reasonably well the three dependent variables: salinity, moisture, and temperature at various depths in the domain. The model slightly overestimated the moisture ratio at the depth of 15–22 cm (Figure 3b) and the temperature at shallow depth (Figure 3c), but the overall comparison is good, and lends strong credibility in TMARUN. Note that vapor condensation was not considered in the model, which might cause the mismatch between the simulations and observations. 4.2. The Supratidal Salt Structure in the Gulf of Mexico Beach Figure 4 reports the results of three capillary-retention experiments using sediments from the Grand Isle beach (Figure 1a) at three depths (surface, 0.70 m, and 1.0 m deep) at a location between PW2 and PW3 (Figure 1). It is also shown the best fit to the data using the least square method as conducted in Boufadel et al. [1998]. We have adopted the inverse of the van Genuchten parameter a as an approximation of the height of the capillary fringe, and in this case, one notes that the capillary fringe was around 60 cm.

Figure 4. Measured capillary-retention curve obtained based on the sand collected at the studied site in Grand Isle. The sediment samples were collected at different depths between well PW2 and PW3. The van Genuchten model was fitted to the data using the least square method. Capillary fringe estimated using the approach of Boufadel et al. [1998] is reported as a horizontal dashed line.

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High moisture content and high salinity were observed near the beach surface. Figures 5a and 5b show measured moisture and salinity profiles at well MP1–MP4 at high tide (28 January 2011 3:00 P.M., t 5 591 h in the simulation) along with the simulated results. Observed and simulated groundwater tables at well MP1 and MP2 are shown in supporting information Figure S1. Measured moisture profiles at different well locations, shown


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in Figure 5a, indicated almost water-saturated conditions in the beach unsaturated zone (i.e., the beach region above the groundwater table), which is due to a relatively shallow groundwater table (approximately 50 cm below the surface) and the fine-texture of the sediment, which resulted in a high capillary fringe, estimated at approximately 60 cm based on soil water retention experiments (Figure 4). Measurements of the pore water salinity, reported in Figure 5b, indicate that the salinity within Figure 5. Measurements of (a) moisture ratio and (b) salinity at MP1–MP4 along with the simulation results for MP1 and MP4. The profiles of moisture ratio and salinity were measured on the top 10 cm layer of the 28 January 2011 3:00 P.M. (t 5 591 h in the simulation). beach reached 85 g/L, while seawater salinity was 35.5 g/L. This could only occur due to evaporation. The results demonstrate that the T-MARUN simulations closely matched the observations of groundwater table, moisture, and salinity at the sampling locations. The model slightly overestimated the moisture content close to the surface, which is probably due to the underestimation of the evaporation at the beach surface. Evaporation and rainfall greatly affected the supratidal pore water salinity. The temporal evolution of pore water salinity simulated at different depths in Domain 1 (landward side of the supratidal zone, Figure 1) and associated observed tide fluctuation and rainfall intensity are shown in Figures 6a and 6b, respectively. The pore water salinity at the beach surface increased rapidly due to evaporation and suddenly decreased to almost zero when rainfall occurred. The sudden drop due to rainfall was apparent down to a depth of

Figure 6. (a) Simulated temporal variation of pore water salinity at different depths in the landward domain (i.e., Domain 1 in Figure 1) along with the simulated evaporation rate at the surface. The salinity value shown in the figure represents the horizontal average at each depth. (b) Observed tide fluctuations and rainfall during the simulated period. (c) Simulated salinity profiles at different time points (marked in Figure 6a by circle symbols). Note that the evaporation rate was assumed to be zero during rainfall periods.

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Figure 7. (a) Temporal variation of pore water salinity at different depths in the seaward domain (Domain 2, Figure 1) along with the simulated evaporation rate at the surface. (b) Temporal variation of moisture ratio at different depths along with the tide level. (c) Simulated salinity profiles at different time points. The time points selected for Figure 7c are marked in Figures 7a and 7b by circle symbols, which delineate the fluctuations of moisture and salinity near the beach surface during a tidal cycle. The salinity value shown in the figure is the horizontal average of each depth. Note that the evaporation rate is assumed to be zero during the rainfall periods.

10 cm (no change was noted at 50 cm depth). However, the increase due to evaporation became weaker with depth, also down to 10 cm deep. The salinity at the surface increased to 150 g/L at t 5 135 h when high evaporation rate occurred (corresponded to observed low air relative humidity on 9 January, Figure 1c). The high salinity dropped to zero at the beach surface on 10 January 2011, which is due to the intense rainfall at rate of approximately 1.4 cm/h. Figure 6a shows that the salinity started to decrease several hours before the rainfall took place. This is most likely due to relatively high air relative humidity during that period, which significantly lowered the evaporation rate at the beach surface. Moreover, subsequent tidal actions elevated the moisture content near the beach surface, and therefore resulted in a decrease in salinity (Figure 6b). The increase in salinity due to evaporation and the ‘‘quenching’’ effects due to rainfall as well as tides, persisted at the shallow portion of the beach (i.e., 10 cm or shallower) through the measurement period. The salinity at 50 cm deep almost remained a constant value (20 g/L), indicating negligible effects of meteorological change there, which is probably due to the presence of the water table at that depth, which provided sufficient supplement of horizontal flow with salinity around 20 g/L to Domain 1. The salinity profiles selected at different time points (following an evaporation-precipitation cycle) are shown in Figure 6c, and they illustrate the drastic effects of precipitation on pore water salinity. At t 5 200 h, due to the evaporation, the pore water salinity at the beach surface was 40 g/L, and gradually decreased as moving downward until reaching a depth of 5.0 cm, and then increased back to 20 g/L. As the evaporation continuously extracted the pore water from the beach, at t 5 315 h the salinity at the beach surface reached 120 g/L, and the associated high salinity profile expanded further downward. The increase in subsurface pore water salinity was apparent only within a 30 cm depth. The precipitation effects on the supratidal salt structure were observed at t 5 357 h, during which the salinity at the surface decreased to zero, and consequently formed a vertical profile where the salinity increased from the value of zero at the surface to a maximum value of 35 g/L at 15 cm deep, and then decreased downward to 60 cm deep, below which the salinity remained a constant value (e.g., 20 g/L). Note that the salinity was around 25 g/L at 15 cm deep prior to rainfall, and thus the increase of salinity was due to the advection of high-salinity water from the surface into the water column. Therefore, in spite of the low permeability of the sediments in the beach, and the associated high dispersion (i.e., dilution), advection continues to play a major role. The results indicate that the supratidal pore water salinity near

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Figure 8. (a–f) Simulated salinity contour at different time points for the landward domain (i.e., Domain 1, Figure 1). The white dashed lines represent the groundwater table. The black arrows represent the velocity vectors. Note that the increase in pore water salinity near the beach surface (shown in Figures 8a–8d and 8f) is due to evaporation, while the sudden decrease (shown in Figure 8e) is due to the rainfall.

the beach surface (within 60 cm deep) could be significantly modified by evaporation as well as rainfall, and demonstrate periodic change in response to rainfall-evaporation cycles. A similar variation in pore water salinity was observed at the sea side of the supratidal zone (Domain 2), which is shown in Figure 7a. The mean water table was at a depth of 53 cm. The pore water salinity near the beach surface gradually increased during the evaporation period and suddenly dropped when the rainfall occurred. Compared to Domain 1, the pore water salinity near the beach surface demonstrated periodic fluctuations during the spring tide cycles (e.g., between t 5 240 h and t 5 340 h). In a spring tide cycle, the pore water salinity near the beach surface increased during the low tide, and decreased at subsequent high tide. This periodic fluctuation is most likely due to the moisture variation near the beach surface caused by the relatively large tidal range, which is shown in Figure 7b. In particular, at the lowest low tide (t 5 270 h), as the surface moisture ratio dropped to 0.6, the pore water salinity at the beach surface increased to 140 g/L, while at the subsequent high tide, as the beach surface became saturated, the salinity decreased to 100 g/L. The periodic evolution of the vertical salinity profile at different tidal levels was further illustrated in Figure 7c. It is observed that the pore water salinity close to the beach surface significantly increased during the falling tide, and decreased at subsequent high tide; the fluctuation of the pore water salinity due to tides seemed to only appear within the top 10 cm beach layer, which is to some extent consistent with the field measurements that are shown in Figure 5. During the spring tidal cycles, the moisture at the beach surface dropped during falling tides and was saturated at subsequent rising tides. The results indicate a strong hydraulic connection between this near-surface process and deep groundwater flow. Therefore, our results indicate that besides meteorological change that extracted pore water from the beach by evaporation or infiltrated freshwater into the beach by precipitation, replenishment of pore water from the deep location of the beach by rising tides and capillary forces played an important role in altering near-surface salt structure in tidally influenced beaches.

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Figure 9. (a–e) Simulated salinity contour at the time points selected within a tidal cycle for the seaward domain (i.e., Domain 2). (f) Fluctuations of tide and evaporation rate during the selected tidal cycle period. The elevation of the ground surface is approximately 2.3 m. The time points shown in Figure 9a–9e are marked in Figure 9f by circle symbols. The white dashed lines represent groundwater table. The black arrows represent the velocity vectors. Note that the formation of salt fingers and associated pore water circulation near the beach surface is due to evaporation; the fingering was significantly diminished at time t 5 312 h.

Note that the moisture content in Domain 1 remained saturated over the whole simulation period, which is not shown in the paper for brevity. The extremely high moisture content is due to relatively low surface elevation (z 5 1.85 m) in comparison to Domain 2 where the surface elevation is equal to 2.1 m. The lower surface elevation made the groundwater table and associated capillary fringe closer to the beach surface, thereby resulting in nearly saturated condition at the surface of Domain 1 even during the spring low tides. Evaporation caused salt fingering near the supratidal surface, while subsequent rainfall and tidal forcing diminished the salt fingers. Figures 8a–8f show the simulated salinity profiles along with pore water velocity vectors at the land side of the supratidal zone (Domain 1). Surface evaporation initially drove the pore water to flow upward and caused salt accumulation near the beach surface (t 5 203 h, Figure 8a). The induced density gradient generated salt fingers near the beach surface, which caused pore water circulation around these fingers (t 5 263 h, Figure 8b). After several tide cycles, the salt fingering was diminished by forming the large high saline plumes near the beach surface which gradually expanded downward (t 5 312 h and 333 h, Figures 8c and 8d). The evaporation-induced saline plumes were diluted and driven downward by the rainfall (t 5 357 h, Figure 8e). This resulted in a distinct salt structure formed in the subsurface: a layer of freshwater retained on the top and evaporation-induced high saline layer buried below. After the rainfall, evaporation started to raise the pore water salinity near the beach surface again, and the subsurface high salinity plume formed by the previous evaporation event was gradually diluted by surrounding groundwater, and migrated seaward along the large-scale groundwater discharge pathway. The simulation results highlight the effects of evaporation, rainfall, and tidal action on the supratidal salt structure. Unlike inland aquifers where formed salt fingers have sufficient time to migrate downward, coastal systems are rapidly flushed by tidal action, which causes significant horizontal mixing. Therefore, the formation of large salt

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Figure 10. Temporal movement of neutral particles in (a, b) the horizontal and (c, d) vertical directions in Domain 1 (Figure 1) without and with evaporation and rainfall, respectively. The particles are released at the locations of x 5 3.5 m and z 5 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, and 1.6 m, indicated by parentheses in the legend. The particle movement was computed based on the velocity field simulated at each time step (0.01 s).

fingers in coastal systems appears to be less prominent than that in inland aquifers, as reported in various works [Boufadel et al., 1999a; Geng and Boufadel, 2015b; Wooding et al., 1997a]. The evolution of pore water salinity in the seaward domain (Domain 2), shown in supporting information Figure S2, was similar to that in Domain 1. However, the effects of tide-caused horizontal mixing on supratidal salt structure were more obvious in Domain 2. The temporal variation of pore water salinity within a tidal cycle period in Domains 1 and 2 was shown in supporting information Figure S3 and Figure 9, respectively. Salt fingers due to evaporation developed at t 5 300 h in both domains. However, as the groundwater table got closer to the beach surface between t 5 300 and t 5 312 h, the salt fingers were significantly reduced in size and smeared by the horizontal groundwater flow driven by tides. The smearing is more evident at t 5 312 h in Domain 2, where the lower edge of the high salinity plume was almost straight. This significant mitigation of the fingering could be also due to relatively low evaporation rate on the beach surface, and thereby tidal action became the dominating factor affecting subsurface salt distribution. After the groundwater table dropped 60 cm below the beach surface (t 5 318 and 324 h), the salt fingers reappeared, indicating that evaporation-induced density gradient dominated the salt transport near the beach surface. 4.3. Pathways of Supratidal Groundwater Flow Particle tracking was conducted to further evaluate the influence of evaporation as well as rainfall on supratidal groundwater flow pathways and associated solute transport. The neutrally buoyant particles were released in the simulated domains. The simulations were run using the particle tracking code NEMO3D [Geng et al., 2016b,], and by using the simulated transient velocity field. For comparison, the particle tracking was performed for both cases with and without evaporation and rainfall effects. Particle tracking was also performed for all the rainfall cases, which is discussed in the next section.

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Figure 11. The temporal movement of the particles at (a, b) horizontal and (c, d) vertical directions in the seaward domain (i.e., Domain 2) without and with effects of evaporation and rainfall, respectively. The particles are released at the locations x 5 13.5 m and z 5 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, and 1.8 m. The particle movements were computed based on the velocity field simulated at each time step (0.01 s).

Evaporation and rainfall significantly affected subsurface flow pathways. Figures 10a and 10b show the horizontal movement of particles released in the landward domain (Domain 1) at the location of x 5 3.5 m with various depths for the cases without and with evaporation and rainfall, respectively. Generally, the particles moved seaward at the low tide and receded landward at the subsequent high tide. However, comparing the two cases, a clear difference in the particle movement is observed at the later simulated period, between t 5 400 and 600 h. During that period, a relatively low precipitation was noted. Therefore, evaporation-induced density gradient significantly altered groundwater flow near the beach surface. In general, surface evaporation as well as precipitation accelerated the seaward movement of the particles. In particular, when the evaporation was present on the beach surface, the particle released at the elevation of 1.6 m transported 0.35 m seaward after 600 h in comparison to 0.15 m for the no-evaporation case. This is probably due to the fact that as evaporation-induced high saline plume migrated downward, the subsequent density gradient prompted the seaward movement of the particles. While the evaporation played an important role in altering subsurface flow path, tidal effects could be observed by periodically oscillating the horizontal movement of the particles. In contrast, tidal effects were not apparent in the vertical direction; as shown in Figure 10c, with tides and without evaporation and precipitation, the particles moved almost horizontally to the seaward. This is as expected since the domains investigated were located in the supratidal zone of the beach, at which the vertical variation due to the tide was negligible. However, the horizontal contribution of the tide extended much farther inland. Significant effects of evaporation and rainfall on subsurface flow is observed in the vertical direction, which is shown in Figure 10d. Apparently, when the evaporation and rainfall were present, the particles transported seaward with remarkable oscillations in the vertical direction. It is apparently shown that precipitation drove the particles to migrate downward at t 5 130 h. In contrast, the evaporation effects on groundwater dynamics were more complex. During the periods of the evaporation, the particles released near the beach surface migrated upward (e.g., between t 5 150 and 300 h) and downward (e.g., between t 5 300 and 350 h). This is because on the one hand, evaporation

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extracted water from the beach surface and induced the pore water to flow upward; on the other hand, evaporation caused salt accumulation near the beach surface, which generated densitydriven flow circulating around the salt fingers. Therefore, the driving force induced by evaporation for groundwater flow spatially and temporally varied at different beach location, depending on local density gradient and evaporation intensity at the beach surface. Evaporation as well as rainfall greatly affected trajectories of the particles released in the seaward domain (Domain 2). Figures 11a and 11b show the horizontal movement of the particles for the cases without and with evaporation, respectively. Comparing the two cases, at the shallow locations (within 50 cm deep) evaporation at the beach surface caused the particles to migrate landward, while as moving Figure 12. Temporal variation of pore water salinity at the beach surface for (a) landward deeper (70 cm below the beach cases (Cases 1 and 3–6) and (b) seaward cases (Cases 2 and 7–10) along with observed tide level. surface) evaporation induced the particles to transport slightly more seaward. The results demonstrate complex driving forces induced by evaporation in the subsurface: by generating different local density gradient, evaporation could either prompt or inhibit groundwater discharge along its pathways. The significant effects of evaporation on groundwater flow are also observed in the vertical direction, which is shown in Figures 11c and 11d for the cases without and with evaporation and rainfall effects, respectively. In the presence of evaporation, the particles flowed gradually upward. In particular, at the elevation of 1.8 m, due to the evaporation, the particle gradually migrated upward and reached the beach surface after 600 h. Compared to the landward domain, the upward movement of particles induced by evaporation was more consistent in the seaward domain (i.e., Domain 2), which is most likely due to the greater horizontal mixing, which smeared the salt fingers, and thereby inhibited downward fingering flow. The results further indicate that evaporation as well as precipitation generated important driving forces in the supratidal zone of the beach, which superimposed on the tidal forcing and perturbed groundwater flow. The sequence of mechanisms is (1) when evaporation initially occurred at the beach surface, the pore water flow was driven upward; (2) as the evaporation kept accumulating salt near the beach surface, salt fingers were generated and subsequently drove the pore water to circulate around each finger; and (3) subsequent precipitation (i.e., infiltration) and/or rising groundwater table diluted the near-surface pore water salinity and to some extent drove and/or expanded the high saline plume downward. Therefore, the evaporation drove the near-surface pore water upward again, and the resulting high saline plume generated density gradient at deeper location and perturb groundwater flow along its pathways. 4.4. Effects of Rainfall Intensity Rainfall intensity remarkably affected pore water salinity near the beach surface. Figures 12a and 12b show the temporal evolution of the pore water salinity at the beach surface for inland and seaward cases, respectively. The pore water salinity demonstrated the same change in the first 40 h for all the cases, gradually

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rising to 40 g/L due to the evaporation and abruptly dropping to zero at t 5 40 h due to the rainfall. However, after the rainfall ceased (t 5 42 h), the subsequent evaporation caused a less increase in salinity for the cases with higher rainfall intensities (e.g., Cases 6 and 10). This is as expected because a higher rate of rainfall caused more freshwater infiltrating into the beach. It diluted the pore water near the beach surface, which attenuated the subsequent evaporation-induced increase in salinity. With a higher intensity, rainfall demonstrated deeper impacts on the pore water salinity in the beach. Figures 13a and 13b show the simulated salinity profiles at t 5 80 h for the landward and seaward cases, respectively. Obviously, for the cases with higher rainfall intensities, the pore water salinity near the beach surface was significantly lower, and the low-salinity zone (less than 20 g/L) induced by the rainfall extended deeper in the beach. This is due to the fact that a higher rainfall rate generated a higher infiltration rate in the beach, which caused the dilution of the pore water to occur at deeper location of the beach. The results indicate that the rainfall intensity had significant impacts on the subsequent temporal and spatial evolution of pore water salinity near the beach surface induced by evaporation. Rainfall intensity also affected trajectories and transit time of the particles released near the beach surface. Particle tracking was conducted for all the cases (Cases 1–10) to further evaluate the rainfall effects on the flow pathways and associated transit time in the supratidal zone of the beach. Figures 14a and 14b show the horizontal displacement of the particles released at the locations x 5 3.5 m and z 5 1.2, and x 5 13.5 m and z 5 1.2 m for the landward and seaward cases, respectively. It is observed that the particles moved Figure 13. Simulated salinity profiles at t 5 80 h for (a) landward cases (Cases 1 and 3–6) and (b) seaward cases (Cases 2 slightly more landward at the high tide for the cases and 7–10) along with observed tide level. with higher rainfall intensities. An apparent difference was observed at the highest high tide (t 5 150 h): for Cases 6 and 10 with the highest rainfall intensity, the particles moved 1 and 3 cm more landward, respectively, in comparison to Cases 3 and 7 subjected to the least rainfall. The difference is probably due to the fact that rainfall-induced freshwater infiltration altered the density gradient near the beach surface. Higher-intensity rainfall events greatly lowered the pore water salinity in the shallow zone of the beach, and resulted in a larger density gradient generated during the high tide. Therefore, the particles demonstrated slightly more landward movements for the high rainfall intensity cases. Rainfall intensity had significant impacts on vertical movement of the particles released near the beach surface. Figures 14c and 14d show the vertical movement of the released particles for the landward and seaward cases, respectively. As expected, due to more infiltration, higher rainfall intensity caused the particle to move more downward. In particular, for Cases 6 and 10, significant downward movement is observed at t 5 150 h when large rainfall occurred on the beach surface. In contrast, negligible downward movement was observed for Cases 3 and 7 with the least rainfall on the beach surface. The results indicate that rainfall provided an important force driving the pore water to flow downward. The magnitude of the downward flow was dependent on the rainfall intensity. In addition, rainfall drove freshwater into the beach systems, which altered the density gradient generated in tidally influenced beach systems.

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Figure 14. The temporal movement of the particles at (a, b) horizontal and (c, d) vertical directions for landward cases (Cases 1 and 3–6) and seaward cases (Cases 2 and 7–10), respectively. The particles were released at the locations x 5 3.5 m and z 5 1.2 m, and x 5 13.5 m and z 5 1.2 m for landward and seaward cases, respectively.

5. Conclusion In this paper, field measurements, combined with numerical simulations, demonstrated the influences of evaporation as well as rainfall on subsurface salinity distribution and groundwater dynamics in the supratidal zone of a beach located in Gulf of Mexico. It was revealed that evaporation significantly increased the pore water salinity, up to 85 g/L, within a shallow layer, approximately 10 cm below the beach surface. However, unlike inland aquifers, the salt fingering was significantly diminished by tidal actions which prompted the horizontal mixing of salt in the beach. The subsequent precipitation (e.g., rainfall) diluted the evaporation-induced high saline plume near the beach surface, and drove the plume to migrate further downward. The plume gradually dispersed and was diluted by surrounding groundwater along its pathway. The results indicated that evaporation could induce distinct geochemical condition near the beach surface, which might affect ecological function and associated biological zonation there. The subsequent precipitation and/or rising water table due to tides could build the hydraulic connection between near-surface layer and deep groundwater system, which might induce intensive mixing and exchange of pore water and chemical species between the shallow and deep layers of the beach, and thereby increase the complexity of subsurface physical and geochemical processes in the beach. The simulation results revealed that evaporation as well as precipitation at the beach surface significantly altered the supratidal groundwater flow pathways. Evaporation initially induced the pore water to flow upward; however, as salt accumulated near the beach surface, density gradient generated fingering flow around the high saline plume. Subsequent precipitation diluted the high saline plume and drove it further downward, and thereby resulted in the perturbation of the groundwater flow at deeper location; after the precipitation, due to the dilution, upward flow reappeared near the beach surface when evaporation was present. Our results indicated that in the supratidal zone of the beach, evaporation and rainfall provided spatially and temporally different driving forces for the pore water flow. Depending on the intensity at the

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beach surface and the local density gradient, evaporation could drive the pore water flow either upward (evaporation dominated) or downward (density dominated). Similarly, in the horizontal direction, evaporation, superimposing on tides, could either facilitate or inhibit groundwater flowing along its discharge pathways. Rainfall induced freshwater infiltration into the beach, which generated downward flow, and meanwhile altered the density gradient in the beach. It significantly affected supratidal flow and salt transport. Therefore, our combined field and modeling study indicated important forcing mechanisms for groundwater flow and solute transport in coastal beaches subjected to evaporation and rainfall. It could interact with oceanic oscillations (e.g., tides and waves), and further impact coastal groundwater systems. In the current study, heat transfer and vapor transport were not considered because the temperature was not measured in the field. This assumption is reasonable because the moisture content measured near the beach surface was extremely high (almost saturated), which could attenuate effects of heat transfer and vapor transport on near-surface evaporation processes. Furthermore, our simulations generally matched the salinity observation, which to some extent, suggests a minor role of hear transfer and vapor transport in this beach. However, it might still affect the water flow and salt distribution in the supratidal zone of the beach. Vapor condensation could be another factor affecting supratidal salt structure that is not considered in the paper. Therefore, our results demonstrated a very dynamic response of the supratidal pore water flow and salinity to combined effects of tides, evaporation as well as precipitation. It highlighted the need to consider a field study with the high-resolution and high-frequency pore water sampling, taking into account heat transfer and vapor transport processes. Acknowledgments This research paper was made possible in part by a grant from The Gulf of Mexico Research Initiative to the Consortium CARTHE II, and subsequently to the New Jersey Institute of Technology. Funding has also come from the National Oceanic and Atmospheric Administration and the Exxon Valdez Trustee Council through contract 11100836. However, no official endorsement should be implied from any of the funding entities. The data for this paper are available upon e-mail request ([email protected]) at Center for Natural Resources Development and Protection.

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