Assessment of Shoreline Changes and the Groundwater Quality along the Coast of Kuakata, Patuakhali, Bangladesh

Shoreline changes and groundwater quality monitoring have become pressing issues for the coastal region of Bangladesh. This study investigated the shoreline changes from 2000 to 2020 and evaluated the groundwater quality, as well as SWI in the Kuakata coastal area. While analyzing satellite images, the temporal shoreline changes were assessed through the Digital Shoreline Analysis System (DSAS) in ArcGIS 10.4.1. Higher erosion rates (>2 m/ year) were found in the southernmost part and the SE part of the studied area. Twenty-five groundwater samples were collected, and the physicochemical parameters were measured to assess the groundwater quality. The geo-graphic information system (GIS) was used to assess the spatial variation of the EC, pH, and Cl⁻ contents through the inverse distance weighted (IDW) interpolation method. The EC, Cl⁻, and pH values of the studied groundwa ter ranged from (1.2 mS/cm to 19.5 mS/cm), (79.88 mg/L to 11241.67 mg/L), and (7.9 to 8.7), repectively. The analysis revealed that the majority of the groundwater samples were unsuitable for drinking purpose due to higher EC and Cl⁻ values. Saline water intrusion (SWI) was determined by using the alternative EC vs Cl⁻ method in the southern part of the Kuakata beach area, which was closest to the sea.


INTRODUCTION
The coastal region of Bangladesh covering almost 29,000 sq. km, is one of the vulnerable places on this planet due to sea-level rise, saline water intrusion (SWI), erosion, accretion, frequent occurrence of paleo-saline in aquifers, and periodic inundation of saline water by the Sidr, Aila, and Mahasen super cyclone It is essential to monitor the quality and sustainability of the groundwater resources of coastal aquifers as the decline of groundwater quality will have serious impacts on sustainable development (Elango et al., 2007;Singh et al., 2015;Rahman and Islam, 2019;Shammi et al., 2019). The intrusion of saline water in the coastal areas of Bangladesh is exaggerated by rapid erosion, accretion, sea-level rise, and other natural calamities that bring saline water to the inland area (Shibly and Takewaka, 2012;Brahmmar, 2014). Erosion and accretion denote the changes in offshore islands and shorelines which can be measured by analyzing time-series satellite images (Chand and Acharya, 2010;Patra et al., 2018). Erosion, and accretion are prominent phenomena in the southern part of the country (Ghosh et al., 2015;Salauddin et al., 2018). When erosion occurs, the shoreline shifts towards the mainland, and saline water inundate the area that was once dry land. The inundation situation becomes worse when there is a rapid sea-level rise. Due to sea-level rise, the southern part of the coastal districts of Bangladesh was found extremely fragile, while the coastal sections of the Sundarbans were found quite vulnerable (UNDP, 2019). Greater erosion activity with sea-level rise implies flooding of coastal lands and more saline water intrusion.
Very few works have been done highlighting the shoreline changes and saline water intrusion in the groundwater of the Kuakata beach area. The prime goals of this study were to explore the transient changes of shoreline over two decades and groundwater quality in the Kuakata coastal region. In addition, this study also focused on the potential of groundwater as a drinking water supply. For long-term groundwater utilization and management, an assessment of coastal erosion and its implications on groundwater quality is essential.

Study area
The study area lies in the Kuakata sea beach area, at Kalapara Upazila in the Patuakhali district. It covers from 21° 48' 33.23" N to 21° 52' 16.03" N latitude and 90° 6' 16.45" E to 90° 12' 48.46" E longitudes (Fig. 1). Kuakata is situated in Latachapli union Parishad, and the Kalapara Upazila town is located to the north, a panoramic sea beach is found to the south, eastern and western parts are bounded by the confl uence of two rivers namely Andharmanik in the east and the Galachipa river in the west. The Kuakata area accommodates 2,065 households with a 9,077 population (BBS, 2011). The study area lies within the moist tropical monsoon and experienced moderate rainfall. In addition, the area experiences the highest temperature in January (25.1°C) and 33.8°C in April (BMD, 2020), with a mean annual rainfall of 2580 mm/year. The period from June to September receives 90% of precipitation of the year (BBS, 2011). Tropical cyclone strikes the study area in the month from May to November causing tidal surges (Ahmed, 2006; Subhani and Ahmad, 2019).

Geological and hydrogeological settings
Patuakhali, one of the coastal districts in Bangladesh, is located in the Bengal Foredeep Basin of the late Holocene age which is infl uenced by The upper shallow aquifers of the coastal regions are usually recharged by the rainwater and fl owing surface water bodies (Bahar and Reza, 2010). Figure 2 shows the hydrogeological crosssection along N-S across Bangladesh. The direction of the regional fl ow pattern is mainly from north to south during the dry period since the aquifers are at or exposed above the ground surface (BGS and DPHE, 2001).

Satellite image analysis
Satellite images were used for investigating the temporal shoreline changes from 2000 to 2020 in ten years intervals (Table 1). A digital

Groundwater sample collection and analysis
Then fieldwork was carried out to assess the current scenario of groundwater quality. Electric conductivity (EC), temperature, and pH data were taken along a North-South and East-West transect from twenty-five tubewells using a multiparameter probe during the fieldwork, and the tubewells were chosen using a simple random sampling procedure. An alternative graphical method Cl⁻ versus EC was employed to study the SWI. The determination of the salt content of seawater is an important area of research because the salt content affects ocean currents and global climate. The chloride content of salt waters was measured using chloride titration. A precipitation reaction between the silver nitrate and sodium chloride was used to determine the chloride content in salt water.

Estimation of the shoreline changes
The shoreline change investigation in the study area identified frequent movement within the last 20 years from 2000 to 2020 (Fig. 3). The total length of the analyzed shoreline was 55.07 km in 2000 whereas it decreased in 2020 to 54.64 km. Not only erosion worked in this area, but it also experienced some accretionary processes. The total shoreline was categorized into six classes according to erosion and accretion rate. The southern, southwestern, and southeastern portions of the Kuakata beach area showed higher erosion rate (>2 m/year) and progression rate (>2 m/year) at the confluence of two rivers with the sea. This high erosion may be due to the newly exposed loose bank sediments, steep slopes, and high tidal pressure. On the other hand, a higher rate of accretion is found in the areas of gentle slopes because of the recent sediments deposited by rivers.
The total number of transects drawn is 1052 using Arc GIS 10.4 DSAS extension and among them, 786 of those are erosional transects, 263 of those show accretionary characteristics and four of those are stable. The eroding transects indicate the landward direction of shoreline movement and the average movement rate is 3.65 m/ year toward land.
The comparison of erosional patterns throughout the study area was illustrated in Figure 4. The polygons A, B, and C are located at different positions in the study area where sampling points meet the water body. The red and yellow lines indicate the shorelines of 2000 and 2020 respectively. In every aspect, there is an evident indication of an erosive shoreline. Because of the shoreline changes, this shifting is consistent with the   The spatial variation of EC EC varies with temperature and is the direct measurement of salinity providing the degree of SWI in aquifers (Hem, 1991;Sreekesh et al., 2018). The variation of conductivity gives important information on groundwater chemistry. It rises in response to the changes in temperature and total dissolved salts (Detay and Carpenter, 1997). The EC of groundwater ranges from 1.2 mS/cm to 19.5 mS/cm with a mean of 10.35 mS/cm. The feasibility of the collected samples based on EC is shown in Table 2. About 80% of the analyzed samples were unsuitable for drinking and about 12% are hazardous for use in this area.
The spatial distribution of EC was presented in Figure 5. The southeastern and northwestern parts show higher EC where the shoreline is protruding toward land. The southern and southeastern part is a zone of high erosion, and the rate of shoreline changes is higher. On the other hand, the northern and southwestern parts which is the middle zone of the two river confl uence, showed lower EC than other parts of the study area.
The changing shoreline due to erosion has adverse impact on coastal environments, and EC as well as the salinity in coastal aquifers (Sarwar and Figure 5. The spatial distribution of EC shows the higher values in the southern, western, and southeastern parts of the study area

The spatial distribution of pH
The pH regulates the chemical characteristics of groundwater as well as the mineral precipitation in it. It varies from 7.9 to 8.7 in the studied area, with a mean of 8.1 in GW, which is within the permissible limit for drinking water (WHO, 1993(WHO, , 2010. The pH fl uctuation in the studied area is shown in Figure 6. Lower pH or slightly acidic water encourage mineral precipitation and may be soft and corrosive, whereas higher pH values (pH>8.5) indicate hard water (Preda and Cox, 2000;WHO, 2010). The aquifer is alkaline, as the pH of most water samples ranges from 7.02 to 8.2 (Saxena et al., 2003;Luo et al., 2018).

The spatial distribution of Cl −
The spatial variation of Cl⁻ in the groundwater of the research area is depicted below in Figure 7.  An alternative graphical method (Cl⁻ vs. EC) was used to assess the SWI, which is shown in Figure 8

CONCLUSIONS
The study revealed that the shoreline is changing rapidly in the Kuakata beach area and the total length of the shoreline decreased from 55.07 km to 54.64 km between the years 2000 and 2020. It is moving towards land at about 3.65 m per year. The areas with higher erosion rates and shifting of shoreline have been observed due to steep slopes, newly exposed loose bank materials, and heavy tidal pressure. The EC of groundwater ranged from 1.2 mS/cm to 19.5 mS/cm with a mean of 10.35 mS/cm. The values of Cl⁻ in groundwater ranged from 79.88 mg/L to 11241.67 mg/L and almost all samples had the values greater than 300 mg/L indicating high salinity of the groundwater. The analyzed samples demonstrate that the majority of the groundwater is unsafe for drinking purposes. The higher concentrations of EC and Cl⁻ in most of the groundwater samples along the shore suggest that the studied aquifer was affected by saline water. The spatial distribution maps of physicochemical parameters reveal that the saline water-fresh water interface is migrating towards the north of the study area. Overall, the potential aquifer in the research area appears to be located in the northern section of the area. A well-structured monitoring system for assessing the shoreline changes and SWI in the coastal aquifers should be established for the sustainable groundwater development.