Reducing Soil Oxidising Aquifer

Occurrence

The Reducing Soil Oxidising Aquifer Environment occurs predominantly in lowland areas where there are fine textured silt or clay rich, imperfect to poorly drained soils and oxygen rich underlying aquifers. It is commonly associated with wind and water deposited materials across New Zealand.

Nationally 8.64% or 2,274,664 ha of New Zealand is classified as Reducing Soil Oxidising Aquifer Environment. Canterbury, Southland, and Otago have the largest areal extent. Southland also has the highest proportion of the region classified as Reducing Soil Oxidising Aquifer Environment.

slope map of new zealand with the parts classified as 'Reducing Soil over Oxidising Aquifer' showing.

Extent of the Reducing Soil Oxidising Aquifer Environment in New Zealand.
Region	Area (ha)	Regional (%)	National (%)
Canterbury	534,862	12.06	2.03
Southland	467,624	15.03	1.78
Otago	336,433	10.81	1.28
West Coast	182,104	7.87	0.69
Manawatu	161,452	7.27	0.61
Hawkes Bay	146,093	10.35	0.55
Waikato	103,628	4.35	0.39
Gisborne	94,096	11.23	0.36
Wellington	90,834	11.33	0.34
Northland	66,829	5.36	0.25
Marlborough	53,786	5.14	0.20
Taranaki	34,390	4.74	0.13
Tasman	28,595	2.98	0.11
Auckland	18,872	3.85	0.07
Bay of Plenty	16,460	1.37	0.06
Nelson	859	2.04	0.00

Water Source and Flow Pathway

In this environment there is a low potential for water dilution from Alpine or Hill Country Environments as local rainfall is the main source of water in this setting. Local rainfall infiltrates the soils where the limited oxygen and microbial activity result in reducing soils. Where the soil is reducing, grey colours and distinctive rust-coloured spots are visible. The underlying aquifer in this setting is oxygen rich.

The high water table results in drainage typically occurring as shallow lateral flow or as runoff over the land surface. In agricultural areas, artificial drainage (a variant) is often used to lower the water table and improve soil drainage. This allows more oxygen into the soil and minimises the occurrence of runoff but creates a pathway for water to transport contaminants through.

Landscape Characteristics

The ability to filter and adsorb contaminants in the soil zone is largely dependent on the drainage pathway. Runoff has the highest risk of loss for ammoniacal and organic nitrogen, microbial losses, and both sediment, and particulate phosphorus. This risk is reduced in areas with artificial drainage as water can drain deeper through the soil. However, as the soil becomes more oxygenated the ability to remove nitrate nitrogen naturally through denitrification is lower, with nitrate nitrogen lost to surface waterways through the artificial drainage network. Any denitrification occurring in the soil zone or underlying aquifer can produce both the harmless dinitrogen gas, which makes up the majority of our atmosphere, and nitrous oxide, a harmful greenhouse gas.

In the waterway, sediment can lower water clarity and smother the stream bed, particularly after rainfall events. High nutrient concentrations can result in excessive algae and plant growth and can be toxic to aquatic organisms. This also reduces the ability to use the waterway for gathering mahinga kai and recreation.

Siblings and Variants

The Reducing Soil Oxidising Aquifer Environment has four siblings:

High soil reduction occurs where soils are predominantly poorly drained.
Extent: 698,116 ha (2.65% of New Zealand)
Moderate soil reduction occurs where soils are imperfectly drained. Soils with imperfect drainage are likely to take on similar characteristics to the Oxidising Soil and Aquifer Environment during dry periods.
Extent: 897,764 ha (3.41% of New Zealand)
Over strong bedrock is a transitional class between the Reducing Soil Oxidising Aquifer Environment and the Strong Bedrock Environment. This sibling identifies where soils occur over strong bedrock which affects how deep water can drain and limits the aquifer potential of the environment. Strong bedrock is more likely to host fractured rock aquifers than weak bedrock.
Extent: 358,075 ha (1.36% of New Zealand)
Over weak bedrock is a transitional class between the Reducing Soil Oxidising Aquifer Environment and Weak Bedrock Environment. It identifies where soils occur over weak bedrock which affects how deep water can drain and limits the aquifer potential of the environment.
Extent: 320,688 ha (1.22% of New Zealand)

Variants have a large role in changing the predicted hydrological response and contaminant risk profile in this environment and are highly seasonal. Check if variants apply for your location MAPS – PHYSIOGRAPHIC ENVIRONMENTS.

Artificial drainage variant is common in areas of agricultural land and changes the predicted contaminant profile, especially over the wetter months. See the Artificial Drainage Variant for more details.
Overland flow risk is elevated in areas of agricultural land and changes the predicted contaminant profile, especially over the wetter months when the water table is high and soils can become saturated easily. See Overland Flow Variant for more details.
Natural soil bypass is common in this environment where soils are derived from calcareous (calcium rich) or mafic (magnesium rich) parent materials. These soils are prone to cracking when dry (under soil moisture deficit) or are naturally jointed creating a natural bypass channel for water to drain bypassing the soil matrix. See Natural Soil Bypass Variant for more details.

The role of landscape in regulating contaminants in the Strong Bedrock Environment. 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.
Reducing Soil Oxidising Aquifer Environment Sibling	Contaminant pathway (dominant hydrological pathway)	How the landscape regulates water quality contaminants					Risk to receiving environment
Reducing Soil Oxidising Aquifer Environment Sibling	Contaminant pathway (dominant hydrological pathway)	Dilution	Resistance to erosion	Filtration and adsorption	Attenuation: N-Reduction	Attenuation: P-Reduction	Risk to receiving environment
High soil reduction	Lateral drainage through the soil zone either to stream or a neighbouring environment. Recharge to the underlying water table aquifer is limited by the soil permeability. Overland flow is common due to seasonal wetness (see Overland flow variant when pathway is active). Artificial drainage is common under agricultural land uses (see Artificial Drainage variant details if present).	Low	Moderate - Moderately low	Low	High	Low	Concentration & load to surface water, minor groundwater contribution
Moderate soil reduction	Lateral drainage through the soil zone either to stream or a neighbouring environment. Lateral drainage is likely to become more vertical (deep) during the drier months. Recharge to the underlying aquifer is limited by the soil permeability (likely higher than high soil reduction sibling as soils are imperfectly drained). Overland flow occurs due to seasonal wetness (see Overland flow variant when pathway is active). Artificial drainage is likely under agricultural land uses (see Artificial Drainage variant details if present).	Low	Moderate	Moderate	Moderately low - Moderately high¹	Moderately low - Moderately high¹	Concentration & load to groundwater and surface water
Over strong bedrock	Lateral drainage along contact with bedrock. Slope and depth to bedrock controls overland flow risk where the steeper the slope or shallower the bedrock the more likely it is to occur. Limited aquifer potential. Artificial drainage may be present under agricultural land uses (see Artificial Drainage variant details if present).	Low	Moderate – Low	Low - Moderate	Moderate - High	Low - Moderate	Concentration & load to surface water, minor groundwater contribution
Over weak bedrock	Lateral drainage along contact with bedrock. Slope and depth to bedrock controls overland flow risk where the steeper the slope or shallower the bedrock the more likely it is to occur. Limited aquifer potential. Artificial drainage may be present under agricultural land uses (see Artificial Drainage variant details if present).	Low	Moderate – Low	Low - Moderate	Moderate - High	Low - Moderate	Concentration & load to surface water, minor groundwater contribution
Hydrological Variants	Occurrence (See MAP VARIANTS to check if they apply at your location)
Artificial drainage	Likely where agricultural soils have impeded drainage or a shallow water table. Pathway is most active during the wetter months.	N/A²	Moderate – Moderately high	Moderate – Moderately high	Low - Moderate	Moderate – Moderately high	Concentration & load to surface water
Overland flow	Occurs when soils are saturated and/or infiltration is limited. Pathway is active after prolonged or intense rainfall.	N/A²	Low	Low	Low	Low	Concentration & load to surface water
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/A²	Moderate	Low	Low	Low	Concentration & load to groundwater

¹Dependent on how active the flood plain is, how well sorted the soil and unsaturated zone materials, and the depth to water table. Where the water table is shallow there is a high connectivity for N loss and risk of overland flow is elevated. P loss can be elevated where soils are dominated by large cobbles with little matrix and the water table is shallow.
² 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

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. Hydrological variants modify the ability of the landscape to regulate contaminants. When these pathways are active, the variant risk supersedes the risk for the environment. The contaminants have been colour coded red, orange, and yellow for high, moderately high, and moderate risk, respectively. Where the risk is provided as a range, the highest risk is used for the colour.
Reducing Soil Oxidising Aquifer Environment Sibling	Nitrogen			Phosphorus		Sediment	Microbes
Reducing Soil Oxidising Aquifer Environment Sibling	Nitrate & Nitrite	Ammoniacal	Organic (Dissolved & Particulate)	Particulate	Dissolved Reactive	Particulate	Particulate
High soil reduction	Low	High	High	High	Moderately high	High	High
Moderate soil reduction	Low - Moderately low	Moderate – Moderately high	Moderate – Moderately high	Moderate – Moderately high	Moderately low – Moderate	Moderate – Moderately high	Moderate – Moderately high
Over strong bedrock	Low - Moderately low	High	High	High	Moderately high	High	High
Over weak bedrock	Low - Moderately low	High	High	High	Moderately high	High	High
Hydrological variants
Artificial drainage	Moderately low - Moderately high	Moderately low - Moderate	Moderately low - Moderate	Moderate	Moderately low	Moderate	Moderate
Overland flow	Low	High	High	High	Low	High	High
Natural soil zone bypass	High	High	Moderate	Low	Moderate	Low	High

Key Actions

In this environment, mitigation activities should focus on preventing bypass pathways. In areas that are prone to runoff, vegetated buffer areas, sediment traps, and riparian planting are all ways of intercepting runoff and minimising contaminant loss. Maintaining good soil structure is critical to prevent runoff from occurring. In areas with artificial drainage, minimise surplus nitrogen in the soil prior to the wetter season to reduce losses through the artificial drainage network.