1 Introduction

Over 412 million hectors of coastal lands in about 100 nations worldwide are affected by soil salinization, a global concern [1]. Salinization is the most important land degradation process along Bangladesh's whole coast, particularly the southern shore. In Bangladesh's coastal areas, salinity has risen by approximately twenty-six percent in the previous 35 years [2]. Soluble salts accumulate in the soil in certain places, leading in a steady decrease in agricultural output. Bangladesh as a deltaic country with low-lying territory, is particularly vulnerable to climate change and its consequences [3]. With their virtually flat topography and "funnel" tip of the Bay of Bengal, Bangladesh's coastal lands are vulnerable to a variety of natural hazards, including cyclones and storm surges, saline intrusion, bank erosion, and coastal recession. Despite the fact that all threats are harmful to agriculture, sea level rise is likely to pose the greatest threat of land submersion and saltwater intrusion. Soil salinization negatively impacts agricultural and aquaculture productivity [4, 5]. Plant toxicity from excess ions and altered nutritional balance are two negative effects of excessive salt accumulation in the soil root zone on crop yield [5]. The Noakhali district's accreted land has more soil salinity than non-accreted land, and that there is a positive correlation between soil salinity and depth [6]. The Hatiya sub-district, located in the southwest coastal region, is highly susceptible to salinity intrusion and widespread poverty, making it one of the most vulnerable areas.

The world's highest-value renewable resource is groundwater [7]. The increasing use of groundwater for human needs such as drinking, irrigation, and industry is forcing demand for this natural resource to grow at an arbitrary rate [8]. Because of saltwater incursion, storm surges, and too much groundwater withdrawal for diverse uses, groundwater in Bangladesh's coastal regions is contaminated by salinity and other metallic ions [9]. In the southern region of Bangladesh, the Noakhali District faces significant challenges related to ground water salinity, which can lead to serious health issues such as pre-eclampsia and gestational hypertension [10]. The WHO and Bangladeshi standards state that this region's groundwater is unsuitable for human consumption [11].

According to recent research, Bangladesh's groundwater water systems are increasingly susceptible to heavy metal and microbial contamination, particularly from arsenic (As) [12]. The annual groundwater extraction is found to be larger than the natural replenishment. The ability to identify the acceptability of water for residential and irrigation uses necessitates the assessment of groundwater quality. Geochemical analyses of groundwater aid in the detection of potential changes in quality [13]. In the Hatiya Island, Bangladesh groundwater is an essential supply of fresh water for agriculture and drinking. About 60% of Hatiya Island's groundwater (60–90 m deep) is unsafe to drink [14]. A serious threat to Bangladesh's clean water supply is the geogenic enrichment of arsenic (As) in groundwater and the intrusion of saltwater [14].

However, there has yet to be published a complete geochemical assessment of the entire coastal region of Nijhum Dwip. The nature of coastal aquifers in relation to saline intrusion and other anthropogenic consequences will be highlighted in this study. The purpose of this study is to use geochemical analysis to analyze the soil salinity and groundwater quality in Nijhum Island southern coastal region of Bangladesh. The objective of the research is to assess depth wise soil salinity distribution pattern and ground water quality by measuring physico-chemical parameters in the agricultural land of Nijhum Island, Noakhali.

2 Materials and methods

2.1 Overview of the study area

This study was conducted on the agricultural land of Nijhum Island, Noakhali. The island emerged from a group of islands, including Ballar Char, Kamlar Char, Char Osman, and Char Muri, formed in the 1950s as sediment deposited in the shallow estuary of the Bay of Bengal, located to the south of Noakhali [15].

The location is situated within the coordinates of 22°1´ and 22°6´N latitude and 90°58´ and 91°3´E longitude, specifically in the northern region of the Bay of Bengal. Located in the western region of Char Damar, to the east of Manpura, to the south of Hatiya Mainland, and to the north of the Bay of Bengal [16], it falls within the Hatiya Upazila of the Noakhali District (Fig. 1).

Fig. 1
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Study area map

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The islanders' livelihood is based on primary economic activities like agriculture, fishing, and cattle farming. The islanders' primary source of income is fishing. The island has a network of canals through which tidal saline water sweeps the island, so the transportation system is primarily water-based [17]. A main road runs along the eastern side of the island, linking all of the important destinations such as Bondortila Bazar and Namar Bazar. On the island, there are a few brick and mud roadways. Nijhum Island was chosen for this study because of its abundant water resources and tourism potential. The research area is depicted in Fig. 2.

Fig. 2
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Sampling station map

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2.2 Sampling design

2.2.1 Soil sample

Forty-two soil samples were gathered from fourteen stations at three distinct depths: 1–5 cm, 10–15 cm, and 20–25 cm, utilising a hand auger (Fig. 2). The stations were evenly distributed across 12 segments, with each segment spanning approximately 6 kms from north to south. Each station possesses distinct longitude and latitude coordinates, which are determined using a GPS metre as one moves from station to station.

2.2.2 Measuring soil salinity

Soil EC measures its ability to conduct electricity. Most importantly to fertility, EC indicates soil nutrient availability. Higher EC means more negatively charged sites (clay and organic particles) in the soil, which holds more cations (positive charges). Cations that benefit plants include sodium (Na+), ammonium (NH4+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), hydrogen (H+), iron (Fe2+), aluminium (Al3+), copper (Cu2+), zinc (Zn2+), and manganese (Mn2+). Like most things in the soil, the EC should not be too high because too many nutrients, especially Na and Mg, can harm soil health. Thus, soil EC should be 110–570 mS/m. Too low EC levels indicate low nutrients, and too high indicate excess nutrients. Sandy soils with low organic matter have low ECs, while clay soils have high ECs.

Conducting soil salinity measurements with a high sampling density is a significantly expensive and time-consuming process. Fortunately, there is a convenient method called in-situ EC that can be used to assess soil salinity. The correlation between EC and soil salinity is intricate due to the potential influence of additional factors such as soil texture, water content, and bulk density on EC measurements [18]. In this study, laboratory measurements of EC were conducted to determine the salinity levels of the chosen soil samples.

The soil samples were collected (3 replicates per site) and initially dried in the open air for 72 h, exposed to sunlight, and subsequently thoroughly crushed. The laboratory measured soil salinity using EC of a soil and distilled water suspension, using a 1:5 weight-to-volume ratio. The EC was measured 3 h after mixing, using the DPIRD method [6]. The salinity classes for loam texture soil, as defined by the Department of Primary Industries and Regional Development (DPIRD) of the government of Western Australia, are provided in Table 1. Due to the proportional relationship between the EC reading and temperature, where the EC reading increases by approximately 2% for each degree Celsius increase in temperature from 25 ̊C, the reading was adjusted to account for the temperature of 25  ̊C.

Table 1 Salinity Class
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Two different classifications of salt-affected soils were used to evaluate the soil salinization of the study area: The initial measurement was conducted using EC units expressed in decisiemens per metre (dS/m). This measurement was subsequently compared to the salinity classification system established by the Department of Primary Industries and Regional Development (DPIRD), a government organisation in Western Australia.

2.2.3 Groundwater sample

Groundwater samples were collected from 18 open dug wells of various locations as same as soil sampling stations, which are extensively used for drinking, domestic and irrigation purposes.

From in situ measurement to laboratory analysis, tools and materials that are used to collect soil samples and water samples including Multiprobe GPS Meter, pH, EC Meter, Hammer Scraper, Zipper Bag, Pen, Notebook etc. Auger Data collected by using mentioned instruments and samples are taken by using appropriate chemicals in plastics bottle and zipper bag.

2.2.4 Measuring groundwater quality

A total of eighteen water samples were collected from predetermined sampling locations. The tube-well water in the study area was primarily utilised for drinking. The majority of the wells that were tested were equipped with a typical Bangladesh number-6 hand pump.

Groundwater samples were collected using 500-ml polyethylene bottles that were clean and dry. Prior to collecting the groundwater samples, tube-wells were pumped for 5 min to flush out the stagnant water within the tube-wells and obtain fresh groundwater. Following the process of rinsing with clean water, the water sample was transferred into containers. Subsequently, the sample bottles were labelled and tightly packed. The pH, oxidation–reduction potential (ORP) in millivolts (mv), EC in microsiemens (µs), salinity in parts per million (ppm), and temperature of the water were measured on-site using a YSI Multiparameter Probe metre.

3 Results and discussion

3.1 Soil salinity spatial distribution (Class—DPIRD)

In ArcGIS 10.8, by using interpolation for spatial analysis of soil salinity EC has shown in this map at three different classes according to the salinity class of the DPIRD, Govt. of Western Australia [6]. The soil salinity in the accreted region of Noakhali exhibits a clear seasonal cycle that impacts farming techniques [19]. In a study spanning nine years on soil salinity, Dasgupta et al. (2015) discovered that the maximum seasonal salinity in both the accreted and non-accreted portions of the Noakhali district declines by around 1.5 dS/m from the dry season (Rabi) to the rainy season (Kharif) [20].

The salinity distribution pattern of three different depths of soil is presented in Fig. 3. Depth A between 1 and5 cm depth indicates that most of the soil in the area of agricultural land is in severe state of salinity means EC range in between 1.46 and 2.90 ds/m. And the rest of the land EC shows its extreme state of salinity. Extreme salinity covers the south-eastern part of the Nijhum Island where 3 sampling had done for soil salinity assessment.

Fig. 3
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Salinity distribution pattern for the depth 1–5 cm, 10–15 cm, 20–25 cm

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Depth B between 10 and15 cm depth indicates that most of the soil in the area of agricultural land is in severe state of salinity means EC range in between 1.64 and 3.94 ds/m. And the rest of the land EC shows its extreme state of salinity. Extreme salinity covers the south-eastern part (same as depth A) of the Nijhum Island where 3 sampling had done for soil salinity assessment.

Depth C between 20 and 25 cm depth indicates that almost 99% of the soil in the area of agricultural land is in severe state of salinity means EC range in between 1.66 and 3.11 ds/m.

According to Corwin and Yemoto (2019), the accumulation of salt in the root area of soil has a negative impact on crop yield by changing the nutritional balance of plants and making them toxic due to an excess of ions [21]. The soil salinity in the Noakhali region is much higher (p < 0.01) at 45–60 cm depth than it is at 1–2 cm and 15–20 cm depth and Comparably, a significant difference (p < 0.01) was found in the impact of accreted and non-accreted land [22].

3.2 Soil salinity spatial distribution

In addition, this is another soil spatial variation based on my soil EC value except following the salinity class. By using interpolation of spatial analysis tool in ArcMap, the data has shown into 4 classes. It indicates that the agricultural land of Nijhum Island has largely affected by salinity (Fig. 4).

Fig. 4
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Proposed salinity distribution pattern for the depth 1–5 cm, 10–15 cm, 20–25 cm

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3.3 Pie chart—soil electrical conductivity

Salt (or specifically the dissolved ions in solution) is the primary component of the soil matrix that conducts electricity. While EC is highly dependent on the level of soil salinity, it will also rise and fall with soil moisture. In this pie chart, the circle represents a whole class of EC of soil. Figure 5. Shows the pie visualization for EC of soil salinity ranges between 1.64 ds/m to 4.36 ds/m.

Fig. 5
figure 5

Pie Chart of Soil EC at three different depth

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Salinization is a widespread and significant issue for crop production and ecological restoration in coastal regions. Most plants are usually not stressed by salinities of 0.7 to 1 dS/m (EC value) or less. Das et al., 2020 stated that several crops are seriously stressed by the salinity of the recently accreted land in the Noakhali district and the soil in accreted areas was found to be slightly to severely saline at the surface (1–2 cm) and mid-layer (15–20 cm), and high to extremely saline at a depth of 45–60 cm [6].

3.4 Pie Chart—soil salinity: severe Vs extreme (Station-wise)

In this pie chart, the circle represents a whole class of EC of soil. Figure 6 showed pie chart of the total station count for soil salinity according to severe and extreme class at three different depths. The findings were represent in Table 2.

Fig. 6
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Pie chart soil salinity (Severe vs Extreme)

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Table 2 Salinity class
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In Noakhali region, soil salinity is significantly higher (p < 0.01) at 45–60 cm depth, that ranges from 0.28 to 4.70 dS/m, than at 1–2 cm (which ranged from 0.14 to 2.39 dS/m) and 15–20 cm (which ranged from 0.18 to 2.37 dS/m) [6].

Rainfall and irrigation-induced saline water leaching can have significant impacts on the salinity of the soil by moving salt from the upper to lower layers [23]. An association between drinking water salinity and hypertensive diseases was reported by Khan et al. 2014 [10].

3.5 Groundwater salinity spatial variation

The groundwater of the Noakhali district is moderate to highly saline [24, 25]. Therefore, the use of saline groundwater for irrigation purposes can be responsible for the secondary soil salinization in Noakhali [6]. The EC values vary from 685.8 to 1041 µs/cm with an average value of 851 µs/cm in terms of avg deep tube well depth 800ft (Fig. 7). The salinity value in ppm vary from 439 ppm to 666.2 ppm with an average value of 544.7 ppm in terms of avg deep tube well depth 800ft.

Fig. 7
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Spatial variation of groundwater salinity (ppm)

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Analysis of groundwater salinity spatial variation of has found that in the average depth of 800ft for deep tube wells, salinity was like fresh water cause each station contains saline water less than 1000 ppm.

According to Prosun et al. (2018), all of the locations had very little to slight salinity, which is not within the acceptable limits in the Noakhali region. The greatest salinity value was 4%, and the minimum value was 1%, which was not in accordance with the suggested limit. All of the dissolved salts in water are indicated by salinity [26].

3.6 Groundwater pH & ORP variation

pH and ORP are distinct measurements of water quality that exhibit a somewhat inverse relationship in terms of their high and low values on the scales. They are loosely connected due to the influence of chemicals and water composition on both parameters. For instance, the presence of chlorine will elevate the ORP levels. However, if the pH is excessively high (alkaline), it will counteract the effectiveness of the active chlorine, resulting in a decrease in ORP. Chlorine plays a significant role in increasing the ORP. Therefore, the decrease in chlorine levels will have a substantial impact on reducing ORP levels in swimming pools and wastewater treatment systems, as chlorine is the primary factor contributing to high ORP and sanitation [27].

As per Sarker et al. (2020), the maximum tube-well water contains a large amount of TDS (6040 mg/l), EC (1786 µs/cm), salinity (6.8%), and hardness (1050 mg/l), that is not safe for human health. However, pH, sulfate, and potassium were found to be within the acceptable limit in the Noakhali region according to WHO and Bangladesh standards [11].

Conversely, the addition of different acids, such as muriatic acid, to water will result in a decrease in pH (as acids have a pH below 7). However, the ORP will increase because muriatic acid acts as a potent oxidising agent, which promotes sanitation and leads to higher ORP levels [28]. Prosun et al. (2018) found that drinking water in the Noakhali region had a maximum pH of 8.4 and a minimum of 7.1, both within the acceptable range. pH in groundwater usually ranges from 6 to 8.5 [26]. Optimal water quality is achieved by maintaining a neutral pH, which ensures appropriate ORP levels for effective sanitation and the promotion of high water quality. (Fig. 8).

Fig. 8
figure 8

Comparison of PH and ORP (mv) value for salinity of groundwater

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4 Conclusions

The research conducted on soil salinity and groundwater quality in the agricultural land of Nijhum Island, Noakhali, Bangladesh, reveals significant findings regarding the extent and impact of salinity on the region's environment and agricultural productivity. The study highlights the severe and extreme levels of soil salinity across different depths, indicating substantial challenges for crop cultivation. Moreover, the analysis of groundwater quality underscores the moderate to high salinity levels, posing risks to human health and further exacerbating soil salinization. The findings emphasize the urgent need for comprehensive measures to address soil and groundwater salinity, including improved irrigation practices, sustainable land management strategies, and the adoption of alternative cropping systems resilient to salinity. Additionally, the research underscores the importance of continued monitoring and assessment of soil and groundwater quality to inform targeted interventions and mitigate the adverse impacts of salinization on agriculture, public health, and environmental sustainability in coastal regions like Noakhali, Bangladesh. Overall, this study provides valuable insights for policymakers, researchers, and stakeholders to develop effective strategies for mitigating soil salinity and ensuring the sustainable management of coastal resources.