.

.

.

.

.

Putika Ashfar .K August 2016

Identify Saltwater Intrusion in Coastal Aquifer

Introduction

Almost all cases of seawater intrusion are recorded were caused by human activities because of intensive water pumping in coastal area. The overexploitation of groundwater results in the quantitative and qualitative degradation of the water reservoirs (Papazotoz et al., 2016). The transitional zone between the fresh and salt water in coastal groundwater aquifer can be modeled based on evolution of water quality distribution influenced by a range of effect like tidal, morphology of shore, and groundwater reservoir, geological type (Vandenbohede and Lebbe, 2006). Since this transition zone movement is very dynamic as the result of aquifer stresses changes (Young Kim et al., 2009), this movement depends on factors such as aquifer withdrawal, hydraulic properties, and confining unit of coastal area.

The increase of population, the extensive water use, the interbasin water transfer, and the climate change effect have and will impact the groundwater level change. It has been showed that groundwater overexploitation leads to salt water intrusion in coastal area (Pousa et al., 2007). Long-term groundwater abstraction pushes land subsidence occurrence Furthermore, land subsidence may contributes to sea level rise that will lead to increase concentration of chloride as salinity plume forward to the inland. Based on this background, it is important to identify the extend of seawater intrusion and its impact for the environment.

Materials and Method

The presence of salinity in coastal aquifers can be detected by:

1. Geophysical method

Geophysical methods are indirect methods to know saline water intrusion. The geophysical survey are conducted to know subsurface condition on the formations

In geophysical method, it is important to know :

1. The geological units Explain the conditions of geological conditions on the study area. Land use and soil conditions around its study area. Past salt intrusion observation can also be used to as a prediction for forecasting.

2. Type and properties of aquifer The formation of aquifer controls the groundwater flow, and its necessary to know the aquifer properties. Define aquifer properties such as hydraulic conductivity, storage coefficient, porosity, aquifer thickness. The average weighted of hydraulic conductivity, porosity, specific yield and transmissivity were accounted as point value of these aquifer properties in each borehole data.

3. Meteorological data Consists of climate condition in study area, rainfall , precipitation and evaportranspiration conditions, average monthly / annual temperature, etc. (Fig. 1)

Fig. 1 Temperature – Precipitation diagram from Marathon station (in period 1986-1997)

The EFDC Model

Salinity intrusion range in the Geum River were obserbved using EFDC model (Jeong et al., 2010). In this study, a three-dimensional numerical model, Environmental Fluid Dynamics Code (EFDC) was used in the analysis of the salinity intrusion characteristics in the downstream of Geum River. The numerical simulation was performed to investigate the influence range for salinity intrusion when the gates were fully opened.

In order to attain accurate numerical analysis, the horizontal grid was resoluted to identify the main channel and the floodplain sections of the study reach. The vertical grid resolution was created following the flow direction in Fig. 2

Fig. 2 Horizontal curvelinear-orthogonal grid in the Geum River in EFDC model (Jeong et al., 2010)

The EFDC model is a general purpose modeling package for simulating three-dimensional flow, transport, and biogeochemical processes in surface water systems including rivers, lakes, estuaries, reservoirs, wetlands, and coastal region. EFDC solves the vertically hydrostatic, free-surface, turbulent-averaged equations of motions for a variable-density fluid. Dynamically coupled transport equations for kinematics energy, turbulent length scale, salinity and temperature are also solved.

MODFLOW Model

Numerical model combined to geological interpretation and boreholes measurement are commonly used by hydrologist to evaluate sea water intrusion development in coastal aquifer. MODFLOW is numerical based tools for solving sea water intrusion (Vandenbohede and Lebbe, 2006). The aquifer is considered as an unconfined aquifer and was set for groundwater modeling condition in Coastal Lowlands of Semarang City (Rahmawati et al., 2013) Steady state simulation was performed for the model. The model discretized uses 56 rows time 154 columns. Fix head boundaries were used as boundary condition for three major rivers in this area and Java Sea. These three rivers were Beringin, Garang, and Babon Rivers.

Due to the lack of data set, recent measurement of the last 10 years water level data from Ministry of Public Works were used to define average water level in each river and set as initial hydraulic heads.Ten years sea water level was used from the Department of Ocean and Fishery to input the initial hydraulic head for Java Sea in the northern part. No no-flow boundaries was defined for fault lines ocalized in this area based on Geological map scale 1:100,000 from Department of Energy and Mineral Resources (see Fig. 3). In addition, no-flow boundary was set for topography boundary in the southern part where hill topography is observed as boundary for Coastal and Lowland of Semarang City.

Fig. 3 Illustration of the boundaries conditions used in the numerical model of the groundwater of Coastal Lowlands of Semarang City (Rahmawati et al,. 2013)

2. Gecochemical investigation

Geochemical investigation are direct measuremets of chemical analysis of groundwater samples and/ or soil samples obtained from well or monitoring boreholes. It can also use isotope studies (age of water to identify the source of salinity). Time to take groundwater samples depend on the purpose of the study and climate condition.

Papazotoz (2016) identify seawater intrusion and nitrate pollution in coastal aquifer of Marathon Basin, use 2 samples for each wells. One samples preserved for laboratory analysis and the others for in-situ measurements of pH, EC and temperature. The samples were taken in the last month of dry season, as the natural recharge is decreased and potable water uses is intense. So the water table is expected to be in lower position.

Assessment of seawater intrusion in aquifers of Thi Vai river in Southern Vietnam by (Hieu et al ., 2015) using automatic groundwater monitoring was analyzed salinity influenced by tidal fluctuation. This direct measurements using automatic monitoring device with frequency is 10 times/month for 1 year.

Fig. 4 Some of monitoring well and monitoring devices (Hieu et al ., 2015)

Results

1. Results From Geophysical Method

1. Groundwater level result

Groundwater level as the result of tidal influence through automatic monitoring are presented below. There is a little difference between regular monitoring and automatic monitoring when track the fluctuations into details. It help to predict the influence of water level to salt concentration levels

Fig. 5 Automatic monitoring in 2014 (left) versus regular monitoring in 2007-2008 (right)

(Hieu et al., 2015) 2. Groundwater Flow

Geological approach of coastal areas of Semarang City was conducted by (Rahmawati et al., 2013).Groundwater condition in study area can be explained by groundwater flow model in MODFLOW figure selut in Fig. 6. The coastal and lowland areas of Semarang City consist of alluvium deposit which created the delta. The material is mostly sand, clay and sea shell (carbonate material).Formation that consists of tuffaceous sandstone, conglomerate and volcanic breccias. River and lake also contributed for material deposit in this study area included sand and silt. As a result, the area presents higher aquifer productivity than another area surrounding Semarang Coastal Area. This area included the groundwater basin of Semarang-Demak. In this area, groundwater flows from Southern to Northern part leading to Java Sea.

Result from MODFLOW which are converted to GIS for groundwater flow modeling in Semarang

City (Rahmawati et al .,2013) 3. Salinity intrusion range

Salinity intrusion range in the estuary was performed due to drought flow, low flow, normal flow and flood flow . The salinity concentration decreases as the distance from the estuary barrage gets farther. The maximum influence rate for the salinity intrusion during different flow regimes: drought flow, low flow, normal flow, and flood flow. Salinity intrusion range in Geum River during drought flow has shown in Fig. 7

Fig 7. Example of saline intrusion range in Geum River with EFDC model (Jeong et al., 2010)

Comparing with (Jong et al, 2010), (Hieu et al, 2015) using the automatic groundwater monitoring in the land to identify salinity intrusion in each observation wells. The result of this study showed decreasing saltwater intrusion into groundwater in the study area.

4. Spatial map for saltwater intrusion Interpolation results from ArcGis for salt concentration in Semarang City developed particularly in Coastal and Lowland Areas. There was a significant change of saline groundwater development during the period 1995–2000 (see Fig. 8).

Fig. 8 Saline groundwater map in Semarang City of year 1994 and 2000 (Rahmawati et al .,2013)

2. Results From Geochemical Method

1. Statistics of sampling

BEX is proposed by Stuyfzand (1986, 2008) and is given by the formula:

BEX = (Na++K++Mg2+) - 1.0716 x Cl- (all in meq L-1)

When the index value is positive, it indicates recharge process and when it is negative, it indicates salinization conditions. Almost all values are negative (Koumantakis et al., 1993). Calculation of BEX describe saline water concentration from ddescriptive samplings of several parameters in Table 1.

Table 1 Example of descriptive statistics sampling in coastal aquifer of Marathon Basin ( Papatoz et al ., 2016)

2. Spatial Distribution The spatial distribution of some parameters which are presented in Table 1 show a specific point source of groundwater contaminant source (see Fig. 9)

Fig. 9 Spatial distribution maps example of EC, CL, Na adn NO3 with ArcgGis and Google Earth

(Papazotoz et al., 2016)

Validation of Results Validation use to compare whether the obsevation results are correlating with computing results. The EFDC model was validated by computing the RMSE (Root Mean Square Error) and NRMSE (Normal Root Mean Square Error). The RSME and NRSME were uded to compare between the obserbved and stimulated data. Jeong et al (2010) use it to stimulate water surface elevation in Geum River near estuary to validate data. Equations (1) and (2) give the formula of RSME and NRMSE respectively,

𝑅𝑀𝑆𝐸 = 1𝑛∑ (ℎ𝑜𝑏𝑠,𝑖 − ℎ𝑠𝑖𝑚,𝑖)2𝑛𝑖−1 (1)

𝑁𝑅𝑀𝑆𝐸 =𝑅𝑆𝑀𝐸

(ℎ𝑜𝑏𝑠,𝑖 − ℎ𝑠𝑖𝑚,𝑖) (2)

References Craig P M, 2005. EFDC-DS/EFDC-Explorer User Manual: 3D Hydrodynamic and Water Quality Modeling

System. Dynamic Solutions International, LLC, Knoxville. TN. 1–10. Hieu, Nguyen Duy., Thang, Mai Toan., Quan, Nguyen Hong., Dung, Lam Quang., Patsch, Matthias. 2015.

Assessment of saline intrusion in aquifers of Thi Vai river using automatic groundwater monitoring. International Workshop on Enviroment and Climate Change Challenge, Response and Lesson Learnt. 103-107

Jeong, Sangman., Yeon, Kyusung., Hur, Youngteck., Oh, Kukryul. 2010. Journal of Environmental Sciences. Salinity intrusion characteristics analysis using EFDC model in the downstream of Geum River. , 22(6). 934–939

Papazotos P., Koumantakis I. and Vasileiou E.2016. Proceedings of the 14th International Conference, . Seawater intrusion and nitrate pollution in coastal aquifer of Marathon basin. Thessaloniki.

Pousa, Jorge, Tosi, Luigi, Kruse, Eduardo, Guaraglia, Dardo, Bonardi, Maurizio,Mazzoldi, Andrea, Rizzetto, Federica, Schnack, Enrique, 2007.Coastal processes and environmental hazards: the Buenos Aires (Argentina) and Venetian (Italy)Littorals. Environ. Geol. 51, 1307–1316

Rahmawati, Novi., Vuillaume, Jean-Francois., Purnama, Ignasius Loyola Setyawan. 2013. Journal of Hydrology. Salt intrusion in Coastal and Lowland areas of Semarang City. 494. 146-159

Vandenbohede, A., Lebbe, L., 2006. Occurrence of salt water above fresh water in dynamic equilibrium in coastal groundwater flow system near De Panne, Belgium. Hydrogeol. J. 14. 462–472.

Young Kim, Kue, Suk Park, Yoon, Pyo Kim, Gee, 2009. Dynamic freshwater–saline water interaction in the coastal zone of Jeju Island, South Korea. Hydrogeol. J. 17. 617–629