Natural Soil Bypass Variant

Occurrence and Hydrological Flow Pathway

A significant variation from the predicted hydrological drainage pathway occurs in areas where there are clay rich soils that are prone to cracking when dry. Soils formed in calcium-rich parent materials and magnesium-rich mafic parent materials are most likely to have these shrink-swell clays. As the soil dries out, typically during summer to early autumn, cracks form leading to drainage water bypassing the soil matrix. Older soils with joints that form between poorly permeable areas are also important bypass pathways for water.

Landscape Characteristics

Cracks or joints are like highways for contaminant transport as they rapidly conduct water and limit the time water has in contact with the soil. This reduces the likelihood that particulates are filtered, and nutrients are adsorbed to the soil particles. Plants cannot readily exploit fast moving water in soils. Rather, this water and the nutrients within it, can be rapidly lost from the root zone. Bypass also limits chemical reactions, such as denitrification in reducing soils, from occurring.

Bypass risk is often significantly elevated where subsurface artificial drainage is present, as cracks can connect the surface with the subsurface drain resulting in rapid discharge to a waterway with minimal interaction with the soil. Irrigation of wastewater, such as farm dairy effluent, over areas with artificial drainage prone to bypass can result in discharge of the effluent to stream.

The risk of microbial contamination in shallow water supply bores across areas of cracking or jointed soils is also elevated. Cracks and joints can cause the bore casing to pull away from the soil matrix creating a pathway for drainage water to enter the underlying aquifer.

After prolonged rainfall, the clays rehydrate and swell, closing the bypass pathway. Drainage water may infiltrate the soil and percolate slowly to depth, or more likely, a significant component will be discharged as surface runoff or through an artificial drainage network.

The role of landscape in regulating contaminants when the natural soil zone bypass pathway is active. If the landscape function is high it is good at reducing the risk to the receiving environment.
Natural Soil Bypass Variant Contaminant pathway (dominant hydrological pathway) How the landscape regulates water quality contaminants Risk to receiving environment
Dilution Resistance to erosion Filtration and adsorption Attenuation: N-Reduction Attenuation: P-Reduction
Natural soil zone bypass Occurs when soils are cracked (under soil moisture deficit) or jointed. Pathway is active when soils are very dry with the highest risk occurring after prolonged dry periods. Low Low Low Low Low Concentration & load to groundwater

Contaminant Profile

Inherent susceptibility of the landscape for contaminant loss when the natural soil zone bypass pathway is active.
Nitrogen, phosphorus, and microbes require a source or input for losses to occur. Sediment risk is elevated if nutrient status is also elevated.
Natural Soil Bypass Variant Nitrogen Phosphorus Sediment Microbes
Nitrate & Nitrite Ammonium & Ammonia Organic (Dissolved & Particulate) Particulate Dissolved Reactive Particulate Particulate
Natural soil zone bypass High High Moderate Low Moderate Low High

Key Actions

In areas prone to natural soil bypass, avoid effluent irrigation over soils vulnerable to cracking as soil bypass minimises the capacity of the land to treat wastewater. Limit intensive land use practices on land surrounding water supply bores and increase protection around the well head. Use an alternative water supply if heavy rainfall occurs after prolonged dry periods.