Greta Jimenez
Drainage pipes are used to remove excess water from poorly drained lands. While they are essential for agricultural productivity, they can also be a source of soil pollution. This occurs when the water in the pipes contains contaminants that are not filtered out before reaching the soil. The type of soil and its absorption rate play a crucial role in determining whether drainage pipes will pollute the soil. If the soil cannot adequately filter the water, pollutants can seep into the groundwater, leading to environmental harm and even violating drinking water standards.
| Characteristics | Values |
|---|---|
| Soil conditions that make drainage pipes a necessity | Slow water permeability, dense soil layers that restrict water movement, flat or depressional topography, high salt levels at the soil surface |
| Soil pipe materials | Clay, concrete, corrugated plastic |
| Soil pipe termination | Underground in a dry well, above ground with a "pop-up emitter" |
| Soil pipe pollution prevention | Controlling drainage levels, recycling drainage water, creating new wetlands alongside crop fields, adjusting water levels with in-line control structures |
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What You'll Learn
Poorly drained soils
To address this issue, farmers often install subsurface drainage pipes, typically placed at depths of 3 to 6 feet in the ground. These pipes are perforated and allow excess water to enter and flow away from the field. This type of drainage system lowers the water table, improves soil aeration, and enables faster drying and warming of the soil in the spring.
However, it is important to assess the suitability of the soil before installing drainage pipes. A percolation test, or "perc test", is used to determine the rate of water absorption into the ground. This test measures how long it takes for a measured amount of water to drain away from a saturated hole dug into the ground. The results indicate the permeability of the soil and ensure that the ground has sufficient drainage capacity to handle the water flow.
If the soil's percolation rate is too slow, effluent can drain away too quickly and potentially pollute the groundwater. On the other hand, if the percolation rate is too fast, the untreated effluent may not have enough time to be treated by the soil, again leading to potential groundwater contamination. Therefore, it is crucial to determine the appropriate drainage field size and ensure that the soil can effectively remove contaminants before they reach the water table.
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Nitrate loads and nutrient loss
Nitrogen is a crucial nutrient for crops, but it can become a contaminant when it enters water systems. This is a particular concern in agricultural contexts, where nitrogen-based fertilisers are often used to improve crop yields. When excess nitrogen is applied to fields, it can be washed into drainage ditches and pipes, leading to high levels of nitrate in water systems. This process is known as nitrate-nitrogen (NO3-N) loss or leaching.
Agricultural drainage systems are designed to remove excess water from fields, preventing waterlogging and promoting crop growth. However, when drainage pipes carry water contaminated with nitrogen away from fields, they can pollute the surrounding soil and water sources. This is a particular problem in areas with poor natural internal drainage, such as the Upper Midwest, where artificial drainage systems are widely used.
Nitrate-nitrogen loss through drainage pipes can have significant environmental impacts. Excess nitrogen in water bodies can contribute to the occurrence of seasonal hypoxia, as seen in the northern Gulf of Mexico. It can also lead to the contamination of groundwater sources, making it unsafe for human consumption.
To mitigate these issues, farmers can implement nutrient management plans. These plans involve applying nitrogen fertiliser at lower rates and using cover crops, such as soybean, to reduce the amount of residual nitrate-nitrogen in the soil. Controlled drainage (CD) is another structural conservation practice that can help. By managing the drainage outlet elevation, CD reduces drain flow volume and nutrient loads, including nitrate, discharged into water bodies.
While these practices can help reduce nitrate loads and nutrient loss, it is challenging to implement site-specific practices that meet crop needs while also reducing environmental impacts. Estimating crop nitrogen needs is difficult due to variable factors such as weather conditions and nitrogen cycle interactions. However, by synthesising data from multiple studies, researchers can gain a better understanding of how nutrient management affects nitrogen levels in drainage and develop more effective conservation practices.
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Soil infiltration
Agricultural practices can impact soil infiltration rates. For instance, farming activities that leave the soil bare, such as incorporating, burning, or harvesting crop residues, can make the soil susceptible to erosion, reducing infiltration. Tillage methods and soil disturbance activities that disrupt surface-connected pores and prevent the accumulation of soil organic matter can also negatively affect infiltration.
Additionally, soil infiltration is crucial for crop growth and management. In regions with poor natural internal drainage, such as the Upper Midwest, artificial drainage systems are essential for agriculture. Subsurface drainage pipes are installed at depths of 3 to 6 feet to remove excess water, lower the water table, and improve soil aeration, promoting better root growth and plant health.
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Contaminants and impurities
Drainage pipes can introduce contaminants and impurities into the soil in several ways. Firstly, perforated pipes used in subsurface drainage systems can allow untreated effluent to enter the soil, potentially leading to soil pollution. This is particularly true if the absorption rate of the soil is too high, allowing water to drain away too rapidly, or too low, causing water to pool or saturate the ground.
To prevent this, it is essential to perform a percolation test before installing a drainage system. This test measures the rate at which water is absorbed into the ground and helps determine the suitability of the soil for a drainage field. By assessing the absorption rate, it is possible to ensure that contaminants are removed from the water before it reaches the water table.
Another way drainage pipes can introduce contaminants is through the release of gases. Soil pipes are designed to release gases produced from bodily waste, such as methane. If these gases are not properly vented, they can build up and cause blockages or leak into the surrounding soil, contributing to soil pollution.
Additionally, drainage pipes can become clogged or backed up over time due to sediment buildup or changes in the environment, such as increased rainfall or runoff. This can cause water to pool in the pipes and potentially leak into the surrounding soil, carrying with it any contaminants or impurities that were in the water.
Furthermore, drainage pipes have been implicated in nutrient loss and environmental harm, particularly in the case of Midwestern drainage pipes contributing to an annual hypoxic area in the Gulf of Mexico. The nitrate from these farm drainage systems enters the Gulf via the Mississippi and Atchafalaya Rivers, promoting excessive algae growth. As the algae decompose, they consume oxygen, leading to environmental damage and violations of drinking water standards.
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Water absorption rates
The water absorption capacity of soil is crucial for agriculture and plant growth. Soils with higher water retention can provide an ongoing supply of water to plants, promoting their growth and survival during dry periods. In regions like temperate Victoria, Australia, soil water retention in wet winters enables the survival of perennial plants during dry summers. Additionally, water absorption rates impact the thermal properties of soil, influencing temperature-related biological triggers such as seed germination and flowering.
Artificial drainage methods, such as subsurface drainage pipes, are often employed to manage excess water in poorly drained soils. These pipes are typically placed at depths of 3 to 6 feet and are used to remove standing water, lower the water table, and improve soil aeration. However, the installation of drainage pipes can also impact the natural water absorption rates of the soil. By facilitating the removal of excess water, drainage pipes can affect the natural water balance in the soil, potentially altering the moisture content and, consequently, the thermal properties and biological triggers associated with soil moisture.
It is important to note that the water absorption capacity of the soil is not static and can be influenced by various factors. For example, the incorporation of plant residues, such as leaves and roots, can impact water absorption rates and influence microbial activity. Additionally, soil texture, pore size distribution, and topographic features all play a role in determining water absorption rates and the overall water balance in the soil.
Understanding the water absorption rates of different soils is essential for various applications, including agriculture and environmental science. By studying the percolation rates and water retention capacities of different soil types, we can make informed decisions about soil usage, plant selection, and water management practices. Optimizing water absorption rates can lead to improved crop yields, enhanced soil sustainability, and a better understanding of the hydrological cycle.
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Frequently asked questions
Drainage pipes are used to remove standing or excess water from poorly drained lands. They are placed at a depth of 3 to 6 feet below the soil surface and are perforated to allow water to enter the pipe and flow away from the field.
Drainage pipes can pollute the soil if they are not properly maintained or if they are not installed correctly. If a drainage pipe becomes clogged or backed up, it can cause water to pool or saturate the ground, leading to soil erosion and contamination.
Drainage pipes have been linked to environmental issues such as nutrient loss, drinking water contamination, and harm to aquatic life. For example, in the Midwest, drainage pipes have been blamed for contributing to an annual hypoxic area in the Gulf of Mexico where there is almost no dissolved oxygen to support aquatic life.
