Water Salinity: Pollution's Impact And The Future

Water salinity is a growing issue that poses a threat to food production, drinking water quality, and public health. Salinity in water is influenced by both natural and human factors. Natural salinization can occur in arid regions with low aquifer recharge rates or in basins with naturally saline rocks that erode over time. On the other hand, human activities such as agriculture, irrigation, and the use of de-icing salts on roads contribute significantly to the increase in water salinity. These activities introduce salts and chemicals into water bodies, leading to a range of environmental and economic consequences.

How pollution affects water salinity in coastal areas

Water salinity is a measure of the total dissolved salt content in water, mainly sodium chloride. It is typically measured in parts per thousand (ppt), with average river water containing 0.5 ppt or less, and ocean water containing 35 ppt. While salinity can occur naturally, human activities such as pollution have been identified as contributors to the salinization of water in coastal areas.

Coastal pollution is largely influenced by direct discharges from offshore oil activities and illegal discharges from ships. Operational discharges consist of production water and drilling cuttings, with cuttings accounting for 75% of the oil that enters the sea as a result of normal operations. The effects of this pollution on the marine environment have been extensively studied. In addition, accidental spills, though relatively small sources of oil pollution, directly affect birds and mammals and have devastating effects on local vulnerable economies.

The growing demand for freshwater in arid countries has led to a significant increase in the number of desalination plants, with over 80% of these plants located in coastal areas. The discharge of brine, a byproduct of the desalination process, into the coastal ocean has a strong impact on the benthic coastal ecosystem. Brine spread along the seabed not only increases salinity but also decreases oxygen levels and increases turbidity in the brine layer.

Agricultural pollution is another significant contributor to coastal pollution. Emissions and inputs from agriculture, such as the discharge of methane and ammonia, the use of insecticides and other pesticides, and nutrient runoff from fertilizers, have a substantial impact on coastal ecosystems. These pollutants enter coastal waters via rivers, direct discharges, diffuse sources, and deposition from the atmosphere. Eutrophication, an increase in nutritional resources in a water body, is a common result of agricultural pollution and can lead to harmful effects on ecosystems, such as toxic algae blooms and mass mortality of fish.

Human activities, such as the disposal of liquid or solid waste containing soluble salts, also contribute to the salinization of water. This waste can leach into underground water sources or be buried beneath the water table. Additionally, the extraction of groundwater can cause the intrusion of saline water as the extraction process disrupts the interface between saline and freshwater. The use of saline surface water for irrigation can further increase salinity levels if the water returns to underground aquifers as recharge.

The impact of pollution on water salinity in coastal areas has significant consequences. High salinity levels can affect the taste of drinking water and even have laxative effects on humans and grazing animals. It can also lead to the corrosion of infrastructure, such as roads, bridges, and sewage pipes, and cause damage to motor vehicles and building foundations. Salinity is a major threat to biodiversity, as it can lead to the loss of both aquatic and terrestrial habitats. Additionally, increased salinity can impact agricultural production by interfering with plant growth and reproduction, leading to reduced crop yields.

The impact of salinity on agricultural production

Salinity has a significant impact on agricultural production. It is one of the most detrimental environmental factors limiting the productivity of crop plants, as most crops are sensitive to salinity caused by high salt concentrations in the soil. Salinity affects almost all aspects of plant development, including germination, growth, and reproduction.

Salinity interferes with water absorption in plants through a process called osmosis, which is controlled by the level of salts in the soil water and in the water contained in the plant. If the level of salts in the soil water is too high, water may flow from the plant roots back into the soil, resulting in dehydration and yield decline or even death of the plant. Salinity also interferes with nitrogen uptake, reduces growth, and stops plant reproduction. Some ions, such as chloride, are toxic to plants and can lead to poisoning and death.

Agricultural losses caused by salinity are challenging to assess but are expected to be substantial and increase over time. It is estimated that about 20% of irrigated lands are salt-affected, with some estimates reaching as high as 50%. As the world's population continues to grow, meeting the need for increased food production will require higher yields per land area. Genetic engineering of crop plants for enhanced salt tolerance will play a crucial role in achieving this goal.

In arid regions where freshwater is scarce, irrigating moderately salt-tolerant crops with brackish water is a feasible option. Transgenic lines of some crop species have been developed to grow and develop at relatively high salinity levels in controlled environments. However, these transgenic crops must undergo rigorous testing for yield potential under field conditions.

To address the impact of salinity on agricultural production, various strategies can be employed:

• Using salt-tolerant crops: Developing and utilising crop varieties with salt tolerance can help mitigate the negative effects of salinity on plant growth and yield.
• Efficient resource management: Improving water application and monitoring systems, as well as adopting sustainable water management practices, can minimise the problems associated with salinity.
• Biological methods: Microorganisms, such as plant growth-promoting bacteria, can play a significant role in salinity stress management. These beneficial microbes can enhance plant growth and yield by improving nutrient uptake, producing plant growth hormones, and protecting against plant diseases.
• Changing farming systems: Adopting practices such as phase farming, alley farming, intercropping, or precision farming can help manage salinity by incorporating perennials in rotation with annual crops.
• Improving irrigation practices: Implementing better irrigation techniques, such as partial root zone drying, drip irrigation, or micro-jet irrigation, can optimise water use and reduce secondary salinisation.
• Soil and water management: While these practices have facilitated agricultural production on marginalised soils, achieving further gains from these approaches may be challenging.

By employing a combination of these strategies, it is possible to alleviate the impact of salinity on agricultural production and improve crop yields, even in saline environments.

Salinity's effect on water quality for drinking and irrigation

Salinity is a measure of the salt content in water or soil. It can be classified as primary or secondary salinity. Primary salinity is a natural feature of the landscape, formed by processes such as the weathering of rocks or the deposition of salt over thousands of years. Secondary salinity, on the other hand, is caused by human activities such as vegetation clearance, poor land management, irrigation, and industrial practices.

Drinking Water Quality

Increased salinity in drinking water sources poses a significant threat to public health. High levels of salinity in drinking water have been linked to various adverse health effects, including cardiovascular diseases, hypertension, preeclampsia, gestational hypertension, infant mortality, cholera outbreaks, skin diseases, and diarrheal diseases. For example, a study in coastal sub-districts of Bangladesh found a significant association between high water salinity and hospital visits for cardiovascular diseases, diarrhea, and abdominal pain.

The World Health Organization (WHO) has an aesthetic guideline value of 200 mg/L for salt in drinking water, but this may not adequately protect against health risks in areas with extremely high levels of salt in their water sources, such as coastal regions.

Irrigation Water Quality

Salinity also affects the quality of water used for irrigation, which can have economic, social, and environmental consequences for rural and urban communities. High salinity in irrigation water can impair the growth and health of salt-intolerant crops, reduce crop yields, and even lead to plant death. This is because high salt levels in the soil water can disrupt the osmosis process, causing water to flow out of plant roots, resulting in dehydration and reduced growth.

Additionally, certain ions, particularly chloride, are toxic to plants and can lead to plant poisoning and death as their concentrations increase.

Managing Salinity

The management of salinity in water sources is crucial to mitigate the potential negative impacts on drinking water quality, irrigation practices, and the wider economy. Various strategies have been proposed to address this issue, including:

• Establishing salt interception schemes to divert saline groundwater to evaporation basins.
• Maintaining the health of wetlands.
• Planting crops that reduce drainage.
• Restoring vegetation cover with native species to control the surfacing of salt.
• Employing efficient farming, irrigation, and drainage techniques.
• Redesigning the timing, volumes, and locations of irrigation.

The ecological health of streams

Impact on Biodiversity:

• Salinity changes in streams can have significant effects on the biodiversity of these ecosystems. Increased salinity levels, whether from natural or anthropogenic sources, can reduce species richness and alter community compositions. This can lead to a loss of sensitive species and changes in interactions between species.
• In dryland regions, where rivers and streams are often naturally saline, the introduction of additional salinity through human activities can exacerbate the situation, further threatening freshwater biodiversity.

Effects on Organic Matter Decomposition:

• Salinity has been consistently linked to reduced rates of leaf litter and organic matter decomposition in streams. This can lead to habitat modifications, increased downstream transport of organic matter, and indirect negative effects on secondary production.
• However, in arid regions with sparse vegetation, autotrophic organisms like algae are typically the dominant carbon source for animal production, so reduced decomposition may not directly impact secondary production.

Metabolism and Biomass Production:

• Salinity can influence the metabolism and biomass production of organisms in streams. While some studies show a negative correlation between salinity and metabolism, the relationship is complex and context-dependent.
• In dryland rivers, which are often net carbon exporters, changes in metabolism and primary production can have cascading effects on terrestrial biota, such as birds, which are valued by society.

Nutrient Cycling:

• Salinity can impact nutrient cycling processes in streams, such as nitrate uptake and denitrification. However, the available research is limited, and further studies are needed to understand the full extent of these effects.
• Nutrient cycling is crucial for self-purification capacity and preventing downstream eutrophication of coastal zones.

Soil and Vegetation:

• Increased salinity in streams can lead to salt accumulation in the surrounding soil, degrading its structure and integrity. This makes the area more susceptible to erosion and can result in the loss of productive farmland.
• Additionally, saline water can stunt plant growth by dehydration, prevent nitrogen uptake, and poison plants with chloride ions, further impacting the health of the surrounding ecosystem.

Economic Consequences:

• Salinity in streams and the resulting soil degradation can have significant economic impacts. The loss of irrigated farmland due to salt damage costs an estimated $27 billion globally in lost crops each year.
• The costs of addressing salinity, such as through desalination plants and water purification systems, can be substantial and are often passed on to businesses and residents.

Addressing the ecological health of streams requires a comprehensive understanding of the interconnectedness between human activities, water salinity, and the surrounding ecosystems. Implementing effective measures to reduce pollution and manage salinity is essential for maintaining the delicate balance of these freshwater environments.

Salinity's influence on the solubility of oxygen in water

Salinity influences the solubility of oxygen in water in a negative way. As salinity increases, the solubility of oxygen decreases. This is known as the "salting-out effect".

When an ionic salt like NaCl is added to water, the ions from the salt are attracted to the water molecules as they attempt to "solvate" the ions. This decreases the weak affinity of non-polar oxygen molecules to water, driving dissolved oxygen out of the polar water.

The solubility of a gas in a solvent is significantly affected by the presence of other solutes in the solution. The maximum amount of dissolved oxygen a body of water can hold (a saturated solution) depends on several factors: water temperature, atmospheric pressure, and salinity. Cold water can dissolve more oxygen than warm water. As the temperature increases, water releases oxygen into the air. Water also holds less dissolved oxygen at higher elevations due to decreased pressure.

The solubility of oxygen in water usually decreases with the addition of other solutes, particularly electrolytes. The extent of the "salting-out effect" varies with different salts, but the relative decrease in solubility remains almost the same for different gases, including dissolved oxygen.

The "salting-out effect" is important in coastal waterways. Salinity is a dynamic indicator of the nature of the exchange system and can inform us about the mixing of freshwater with seawater. It is also an important determinant of the mixing regime, as salinity stratification inhibits vertical mixing in estuaries, which can impact dissolved oxygen concentrations.

While salinity does influence the solubility of oxygen in water, the effect of temperature on dissolved oxygen is more significant. As water temperature rises, the amount of dissolved oxygen decreases, regardless of salinity. This is why fish often die from oxygen starvation during the summer months when water temperatures are higher.

To ensure the survival of aquatic life, it is crucial to maintain healthy levels of dissolved oxygen. According to the Environmental Protection Agency (EPA), dissolved oxygen levels below 3 mg/L are dangerous for common aquatic life, and levels below 1 mg/L cannot support any aquatic life. Therefore, it is essential to monitor and manage water quality parameters, including salinity and temperature, to protect aquatic ecosystems.

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