Irrigation's Water Pollution: Causes And Effects

Irrigation is a vital practice in agriculture, but it can also contribute to water pollution. This occurs through various direct and indirect means, including the overuse of fertilisers, biocides, and pesticides, which can contaminate both groundwater and surface water sources. Poor drainage and soil management can also lead to water pollution, as can the unsafe use of non-conventional water sources, such as wastewater. These issues are exacerbated by the expansion and intensification of agriculture, which accounts for 70% of total water consumption worldwide. The consequences of irrigation-induced water pollution are severe, impacting human health, the environment, and socio-economic factors.

Over-irrigation and poor distribution uniformity

Irrigation systems are designed to provide uniform water application to ensure even crop growth, nutrient uptake, and fertigation. However, poor distribution uniformity can lead to over-irrigation, causing environmental and economic issues.

Distribution uniformity (DU) is a measurement of how evenly water is applied to a landscape. It is expressed as a percentage, with higher percentages indicating better distribution uniformity. DU is critical for plant health, efficient water use, and the longevity of the irrigation system. Poor DU can lead to over-irrigation, resulting in the waste of water, chemicals, and increased costs associated with excess water usage and runoff.

Over-irrigation can cause deep drainage from rising water tables, leading to problems of irrigation salinity. This requires watertable control through subsurface land drainage or leaching to remove toxic salts from the soil surface. Poor DU can also lead to under-irrigation, resulting in poor soil salinity control and increased soil salinity. Irrigation with saline or high-sodium water can damage soil structure by forming alkaline soil.

The causes of poor DU can be attributed to system design, maintenance, and operation. Inadequate system design, such as incorrect emitter sizes or poor pressure regulation, can lead to uneven water distribution. Poor maintenance, including worn parts, leaks, and clogged sprinklers, can also impact DU. Operating irrigation systems at pressures different from their designed pump pressure can cause problems with uniformity.

To improve DU, it is essential to regularly inspect and maintain irrigation systems. Smart irrigation technology can also help achieve high DU by monitoring and adjusting watering schedules based on real-time data. Field evaluations of water application uniformity, such as catch can tests, can help identify problems with DU and guide necessary adjustments or repairs.

Agricultural chemicals and biocides

Agriculture accounts for 70% of total water consumption worldwide and is the largest contributor to non-point-source pollution to surface water and groundwater. The use of agricultural chemicals and biocides in irrigation water can lead to water pollution in several ways. Firstly, the overuse or misuse of fertilizers in irrigation water can increase soil erosion, salinity, and sediment loads in water bodies. As drainage water moves through the soil, it can dissolve fertilizer-based nutrients like nitrates, leading to a buildup of these nutrients in groundwater aquifers. High nitrate levels in drinking water are harmful to humans, particularly infants, who are at risk of developing "blue-baby syndrome" or methemoglobinemia.

Agricultural chemicals, such as biocides, can also directly pollute water sources. Biocides are used to kill or control harmful organisms and include pesticides and herbicides. When these chemicals are applied to crops or fields, they can be washed into nearby water bodies through runoff or leach into groundwater. This can result in increased nutrient levels in irrigation and drainage water, leading to algal blooms, aquatic weed proliferation, and eutrophication in irrigation canals and downstream waterways. The excessive growth of aquatic weeds can obstruct waterways and have ecological, navigational, and health consequences.

Additionally, the unsafe use of non-conventional water sources, such as wastewater, in agriculture can further contribute to water pollution. Wastewater can contain high levels of chemical and microbiological pollutants, which, if not adequately treated before agricultural use, can accumulate in crops, livestock products, soil, and water resources. This can ultimately lead to severe health impacts among exposed food consumers and farm workers and exacerbate antimicrobial resistance.

The impact of agricultural chemicals and biocides on water pollution is not limited to the immediate vicinity of agricultural lands. Polluted river water can enter the sea, adversely affecting the ecology along the seashore. For example, the detention of sediments behind dams can disrupt the natural cycle of sediment dispersion, which is critical for replenishing nutrients in the soil and supporting the livelihood of plants and animals that rely on downstream sediments.

To mitigate the pollution caused by agricultural chemicals and biocides, proper management and control measures are necessary. This includes the safe treatment and application of wastewater, as well as the careful use and disposal of fertilizers, pesticides, and other biocides to minimize their impact on water sources. By addressing these issues, we can help reduce the negative effects of agricultural practices on water quality and the environment.

Soil salinity and toxic salt buildup

Soil salinity and the buildup of toxic salts on the soil surface are significant issues in irrigation, particularly in arid and semi-arid regions. Salinity is a critical factor in limiting crop productivity, as most crop plants are sensitive to high salt concentrations in the soil.

Irrigation water is often the primary source of salt in the soil. When precipitation is insufficient to leach ions from the soil, salts accumulate, leading to soil salinity. This can be exacerbated by poor irrigation management, resulting in secondary salinization. Inadequate irrigation practices, such as over-irrigation or under-irrigation, can further contribute to the problem. Over-irrigation can cause deep drainage issues, leading to rising water tables and requiring subsurface land drainage. On the other hand, under-irrigation results in poor soil salinity control, allowing salt buildup on the soil surface, especially in areas with high evaporation rates.

The salts commonly found in irrigation water include sodium chloride (common salt), calcium and magnesium bicarbonates, chlorides, and sulphates. These salts can affect plant growth in two ways: the salinity effect and the toxicity effect. With the salinity effect, plants struggle to absorb water from the soil due to the higher concentration of salts outside their roots, leading to reduced growth and yields. The toxicity effect occurs when salt concentrations reach toxic levels, causing leaf scorching, wilting, and eventually plant death.

To address soil salinity and toxic salt buildup, several strategies can be employed. These include leaching to remove excess salts, implementing improved irrigation practices such as partial root zone drying and drip irrigation, and adopting salt-tolerant plant species. Additionally, changing farm management practices, such as reducing water application rates and incorporating deep-rooted perennial plants, can help prevent the movement of salt to the soil surface.

By understanding the impact of soil salinity and toxic salt buildup, farmers can make informed decisions to optimize their irrigation practices and minimize negative environmental impacts.

Eutrophication and algal blooms

Irrigation has a range of environmental impacts, including water pollution. One of the ways in which irrigation pollutes water is through eutrophication and harmful algal blooms (HABs). Eutrophication is characterized by excess algal or plant overgrowth, often due to the introduction of nitrogen and phosphorus into water bodies. This process disrupts the natural balance of aquatic ecosystems by increasing turbidity levels, reducing light penetration, and lowering dissolved oxygen, which in turn compromises water quality and biodiversity.

Agricultural, urban, and industrial activities have been identified as major contributors to aquatic nitrogen and phosphorus pollution (eutrophication). These activities introduce nutrients into water bodies, creating an environment conducive to HABs. HAB-contaminated water can have adverse effects on water quality, aquatic ecosystems, and both animal and human health. For example, HABs can produce toxins that affect the development of plant structures and inhibit essential physiological processes. Additionally, HABs can create taste and odor problems, further compromising the safety of drinking water and aquatic food supplies.

The relationship between eutrophication and HABs is complex and influenced by various factors, including shifts in food webs, habitat changes, and climate change. The composition of the nutrient pool, rather than just the total quantity, impacts HABs. High biomass blooms, for instance, require exogenous nutrients to sustain themselves. Furthermore, both chronic and episodic nutrient delivery can promote the development of HABs.

While challenges remain in fully understanding and managing eutrophication, advancements in technology have been made. For instance, new tools and techniques have been developed to improve the detection of HABs, and emerging technologies are now being used to predict HABs and their toxins. Experimental studies are also being conducted to enhance our understanding of the role of nutrients in HAB expression, which will aid in prediction and mitigation efforts.

Microbiological and chemical pollutants

Irrigation can introduce microbiological and chemical pollutants to water sources in several ways, leading to water pollution and adverse health effects.

Microbiological Pollutants

Pathogenic microorganisms, including bacteria and zoonotic agents, can contaminate irrigation sources, posing risks to human health. For instance, Escherichia coli O157:H7 has been found on plants affected by contaminated irrigation water. Additionally, wastewater-irrigated lettuce in Egypt and vegetables in Morocco have been associated with microbial contamination. The use of wastewater in informal irrigation practices, such as in urban and peri-urban areas of Kumasi, Ghana, further highlights the potential for microbiological hazards in water sources.

Chemical Pollutants

Agricultural operations, including irrigation, contribute chemical pollutants to water sources. Fertilizers, pesticides, and livestock manure contain nutrients and chemicals that can dissolve and accumulate in groundwater, leading to increased nitrate levels. High nitrate concentrations in drinking water pose health risks, particularly to infants under six months old, who may develop "blue-baby syndrome" (Methemoglobinemia).

Additionally, chemical pollutants from irrigation can result in the salinization of water sources. Over-irrigation or poor soil salinity control can lead to the buildup of toxic salts on the soil surface, which then leaches into the groundwater. This process can alter the soil structure, forming alkaline soil, and negatively impact water quality.

Furthermore, increased levels of nitrogen and phosphorus from fertilizer and manure runoff can stimulate algal blooms in lakes and rivers. These blooms contribute to the development of hypoxic conditions, depleting oxygen levels and causing harm to aquatic life.

Frequently asked questions