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. The risk to the receiving environment is defined as concentration and/or load to surface water, groundwater, or both.
Natural Soil Bypass Variant	Contaminant pathway (dominant hydrological pathway)	How the landscape regulates water quality contaminants					Risk to receiving environment
Natural Soil Bypass Variant	Contaminant pathway (dominant hydrological pathway)	Dilution	Resistance to erosion	Filtration and adsorption	Attenuation: N-Reduction	Attenuation: P-Reduction	Risk to receiving environment
Natural soil zone bypass	Occurs when soils are cracked (under soil moisture deficit) or jointed. Pathway is active following extended periods of soil moisture deficit.	N/A¹	Moderate	Low	Low	Low	Concentration & load to groundwater

¹ Dilution potential is assessed by the Physiographic Environment recharge domain which is indicative of water source and relative volume. This does not change with the hydrological variant.

Contaminant Profile

Inherent susceptibility of the landscape for contaminant loss when the natural soil zone bypass pathway is active. When the pathways is active, the variant risk supersedes the risk for the environment. A high susceptibility equals a high risk of loss from agricultural, horticultural, forestry and urban land uses and assumes a source or input of nitrogen, phosphorus, and microbes for losses to occur. Sediment risk is elevated if nutrient status is also elevated. The contaminants have been colour coded red, orange, and yellow for high, moderately high, and moderate risk, respectively.
Natural Soil Bypass Variant	Nitrogen			Phosphorus		Sediment	Microbes
Natural Soil Bypass Variant	Nitrate & Nitrite	Ammonical	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.