A drain field, also called a leach field, is the final stage of a septic system, responsible for treating and dispersing liquid wastewater (effluent) into the surrounding soil. The soil above this system is unique because it houses a network of perforated pipes and gravel trenches, which alters the normal soil profile. When the grass covering this area begins to die, it indicates that the delicate balance of moisture, chemistry, or physical structure underground has been disrupted. Diagnosing the specific cause of the dying grass is the first step toward protecting the health of your entire septic system.
Causes Related to Excessive Moisture or Saturation
The primary reason grass dies over a drain field is often hydraulic overload, which leads to soil saturation and waterlogging. A properly functioning drain field allows effluent to slowly percolate through the soil, where microorganisms treat and disperse the wastewater. When the system receives more water than the soil can absorb, the ground becomes oversaturated, and the wastewater rises closer to the surface.
This constant saturation creates anaerobic conditions, depriving the soil of the oxygen grass roots require to function. The grass roots essentially drown in the waterlogged soil, leading to the visible browning and death of the turf. This issue is frequently compounded by the formation of a biological mat, or “biomat,” a dense, slimy layer of bacteria and organic solids that naturally forms around the drain field trenches. If the biomat grows too thick due to excessive water use or lack of maintenance, it severely restricts the soil’s permeability, causing effluent to back up and saturate the topsoil.
Excessive surface water, such as runoff from downspouts or improper grading, can also contribute to saturation. When the soil is already struggling to absorb the septic effluent, external water pushes the saturation level past the point where grass can survive. This waterlogging is a serious symptom, indicating the soil’s ability to treat and disperse wastewater is compromised, which can lead to system failure.
Chemical Imbalances and Toxicity
The death of grass over a drain field can also result from chemical imbalances or toxicity within the effluent. Wastewater contains high concentrations of nutrients, primarily nitrogen and phosphorus, which normally act as a fertilizer. However, an excessive, localized surge of these compounds, particularly nitrogen, can cause a “fertilizer burn” effect that damages and kills the grass roots and foliage.
This chemical burn is distinct from waterlogging and can occur even if the system is relatively functional but the effluent is highly concentrated. High levels of sodium or other salts, often found in water softener discharge, can also cause desiccation. These salts draw moisture out of the grass roots through osmosis, essentially dehydrating the plant and preventing it from taking up necessary water.
Harsh household chemicals dumped down the drain can have a direct toxic impact on the vegetation and beneficial soil microbes. Products like heavy-duty cleaners, bleach, paints, or solvents should never enter a septic system because they kill the microorganisms responsible for breaking down waste. When these toxins reach the drain field, they poison the grass directly, creating dead patches.
Physical Diagnostics and System Failure Indicators
Visible grass death can be a symptom of severe physical or structural problems requiring professional attention. Soil compaction from driving vehicles or heavy machinery over the drain field is a common culprit. This compaction crushes soil particles, reducing the pore space needed for oxygen and water movement, which suffocates grass roots and prevents proper effluent percolation.
Another major threat comes from tree and shrub root intrusion. Deep-rooted plants seek out the water and nutrients in the effluent, and their roots can infiltrate the perforated drain pipes, leading to blockages and damage. As the roots grow, they impede the flow of effluent, causing localized backups and saturated spots that kill the overlying grass.
The dying grass may also be one of several indicators of a full system failure. Signs like standing water or mushy ground, a persistent sewage odor, or noticeable depressions in the soil above the trenches all point to a serious system compromise. Sewage backing up into the house or drains running significantly slower are also urgent signs that the drain field has failed and can no longer absorb the wastewater, which requires immediate professional assessment and repair.
Restoring the Lawn and Preventative Measures
Restoring the lawn over a drain field begins with addressing the underlying problem, whether hydraulic overload, chemical imbalance, or physical damage. Once the system issue is resolved, focus should shift to choosing appropriate vegetation to maintain a healthy soil cover. Selecting shallow-rooted grasses, such as fescue or perennial ryegrass, is recommended because their roots are less likely to penetrate and damage the underground pipes.
Preventative maintenance is key to the longevity of the drain field and the health of the lawn. Homeowners should strictly avoid driving or parking any vehicles, including lawnmowers, over the drain field area to prevent soil compaction. It is also important to direct all surface water, such as rainwater from gutters and downspouts, away from the drain field to minimize saturation.
Never till or dig deeply into the soil above the system, as this can damage the pipes and gravel trenches. For long-term health, monitor water usage in the home to prevent hydraulic overload, and be mindful of what goes down the drain, avoiding harsh chemicals that can harm the septic system’s microbial balance. Regular septic tank pumping, typically every three to five years, also helps prevent excessive solids from entering and prematurely clogging the drain field.