Salinity is an ambiguous term which is commonly referred to as a high concentration of dissolved salts in a liquid media (e.g. groundwater). Salinity contamination of groundwater is generally defined by high concentrations of chemical ions in water that is measured by electrical conductivity (EC).<sup>5</sup> The EC is a measure of salt in the water, usually expressed in micro-Siemens per centimeter (μS/cm) or deci-Siemens per meter (dS/m).<sup>6</sup> Salts or other chemicals that dissolve in water can be broken down into positively and negatively charged ions. These free ions in the water conduct electric current. The concentration of dissolved ions may affect the taste of water and is measured by EC. The major cations influencing water salinity are sodium, potassium, calcium, and magnesium (Hem, 1985) – all are essential macro-minerals for humans needed in bulk daily amounts (Cook, 2013). According to the Environmental Protection Agency (EPA) some dissolved mineral matter is desirable. High drinking water salinity (i.e., elevated EC) contains higher concentrations of these minerals in drinking water (Hoque and Butler, 2015; Alfarrah and Walraevens, 2018; Mahlknecht et. al. 2017; Park et. al. 2012).

Typically, sea-water has an average EC value of ~50,000 μS/cm. EC for fresh groundwater varies from very low values (<50 μS/cm) to high values (2,000 μS/cm). These values are generally considered unsuitable for drinking and irrigation. Salinity level in coastal Bangladesh groundwater varies greatly with an average EC value of 5,500 μS/cm. The level of salinity measured by electricity is classified as follows (Zahid et al., 2013): fresh, 0-1,000 μS/cm; brackish, 1,000-2,000 μS/cm; slightly saline, 2,000-6,000 μS/cm; moderately saline, 6,000-20,000 μS/cm; and extremely saline, 20,000-50,000 μS/cm.

Infants and children need a relatively higher volume of water and a high concentration of minerals considering body weight (Sievers, 2005). About fifty percent of very low birth weight infants who weigh 1 kg at birth and who live for over 28 days will develop bronchopulmonary dysplasia (Caddell, 1996; Seelig, 1980). According to the World Health Organization (WHO), they are especially vulnerable and hence drinking-water mineral contents need to be stabilized by adding calcium and magnesium, and other minerals based on regional dietary composition (WHO, 2005). Use of freshwater or low mineral water for drinking, cooking infant food, and preparing formula-milk can compromise the nutrient intake in infants (Sievers, 2005; WHO, 2005).

This study attempts to fill the research gap by exploiting and combining data that cover a large sample population from the entire nation with a long temporal span and spatially-matched features. These

> 5 EC is a measurement of dissolved material in a fluid media (e.g., groundwater) that relates to the ability of the material to conduct electrical current through that media. EC is measured in units called Siemens (S) per unit area (e.g. micro-Siemens per centimeter or μS/cm in short form). Water’s capacity to conduct electricity is directly related to the concentration of ions in the water. The conductive ions come from dissolved salts and many inorganic chemical materials (e.g. chlorides, sulfides and carbonate compounds). Compounds that dissolve into ions are also known as electrolytes. Higher the ions present, higher the conductivity of water. Likewise, the presence of fewer ions makes the water less conductive.

> 6 1 deci-siemens per meter = 1,000 micro-siemens per centimeter, 1 ds/m = 1,000 μS/cm.

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