Reducing Soil and Aquifer

Occurrence

The Reducing Soil Reducing Aquifer Environment occurs predominantly in lowland areas where there are fine textured silt or clay rich, imperfect to poorly drained soils and oxygen limited underlying aquifers. Where the soil is reducing, grey colours and distinctive rust-coloured spots are visible. Aquifers can be moderate to highly reducing depending on the carbon content of the underlying geology. Typical reducing geologies include mudstones, marine or estuarine sediments, lignite and coal measure sediments, and peats. This environment may also be associated with the fringe of wetlands.

Nationally 3.21% or 844,921 ha of New Zealand is classified as Reducing Soil and Aquifer Environment. Waikato, Manawatu, and Northland have the largest areal extent. Northland has the highest proportion of the region classified as Reducing Soil and Aquifer Environment.

Extent of the Reducing Soil and Aquifer Environment in New Zealand.
Region Area (ha) Regional (%) National (%)
Waikato 196,505 8.25 0.75
Manawatu 157,337 7.09 0.60
Northland 127,768 10.25 0.49
Southland 80,455 2.59 0.31
Canterbury 71,361 1.61 0.27
Otago 54,137 1.74 0.21
Auckland 41,506 8.47 0.16
Hawkes Bay 40,072 2.84 0.15
Gisborne 37,859 4.52 0.14
Bay of Plenty 25,667 2.13 0.10
Wellington 21,906 2.73 0.08
West Coast 10,965 0.47 0.04
Taranaki 10,574 1.46 0.04
Tasman 1,856 0.19 0.01
Nelson 317 0.75 0.00
Marlborough 305 0.03 0.00
slope map of new zealand with the parts classified as 'Reducing Soil over Reducing Aquifer' showing.

Water Source and Flow Pathway

In this environment there is a low potential for water dilution from Alpine or Bedrock Environments as local rainfall is the main source of water in this setting. Local rainfall infiltrates the soil and is modified by microbial activity under limited oxygen in the reducing soils and underlying aquifer. Aquifer contributions to stream tend to be reducing and of minor volume relative to the Oxidising Soil and Aquifer Environment.

The high water table results in drainage occurring as shallow lateral flow or as runoff over the land surface. In agricultural areas, artificial drainage 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 nitrogen and microbial losses from animal wastes, and both sediment, and phosphorus. This risk is reduced in areas with artificial drainage as water can drain deeper through the soil. However, as the soil is more oxygenated the ability to remove nitrate 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 will 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.

Sibling and Variations

The Reducing Soil and Aquifer Environment has four siblings:

  • High soil reduction occurs where soils are predominantly poorly drained.
    Extent: 297,992 ha (1.13% 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 soils during dry periods which increases the likely attenuation of phosphorus in the soil and any leached nitrogen is likely to be denitrified in the underlying aquifer.
    Extent: 344,744 ha (1.31% of New Zealand)

  • Over strong bedrock is a transitional class between the Reducing Soil and 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: 59,744 ha (0.23% 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. Weak bedrocks can be more susceptible to erosion than stronger bedrock settings.
    Extent: 142,442 ha (0.54% 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 they apply for your location MAPS โ€“ PHYSIOGRAPHIC ENVIRONMENTS.

  • Artificial drainage variant is common in areas of agricultural land and changes the predicted contaminant profile, especially over the wetter months. See Artificial Drainage 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 Reducing Soil and Aquifer Environment. If the landscape function is high it is good at reducing the risk to the receiving environment.
Reducing Soil & Aquifer Environment 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
High soil reduction Lateral drainage through the soil zone either to stream or a neighbouring environment. Recharge to the underlying aquifer is limited by the soil permeability. Overland flow is common due to seasonal wetness. Artificial drainage is common under agricultural land uses (see Artificial Drainage variant details if present). Low 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. Artificial drainage is likely under agricultural land uses (see Artificial Drainage variant details if present). Low Moderately low - Moderate Low - Moderate Moderately high - High Low โ€“ Moderately low 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 โ€“ 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 โ€“ 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. Low Moderate โ€“ Moderately high Moderate โ€“ Moderately high Low - Moderate Moderate โ€“ 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/Aยน 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 when soils are very dry with the highest risk occurring after prolonged dry periods. Low Low Low Low Low Concentration & load to groundwater

ยน Dilution potential is dependent on the recharge domain of the Physiographic Environment.

Contaminant Profile

Inherent susceptibility of the landscape for contaminant loss in the Reducing Soil and Aquifer Environment.
Nitrogen, phosphorus, and microbes require a source or input for losses to occur. Sediment risk is elevated if nutrient status is also elevated.
Reducing Soil & Aquifer Environment Sibling Nitrogen Phosphorus Sediment Microbes
Nitrate & Nitrite Ammonium & Ammonia Organic (Dissolved & Particulate) Particulate Dissolved Reactive Particulate Particulate
High soil reduction Low High High High Moderately high High High
Moderate soil reduction Low - Moderate Moderate โ€“ Moderately high Moderate โ€“ Moderately high Moderate โ€“ Moderately high Moderately low โ€“ Moderate Moderate โ€“ Moderately high Moderate โ€“ Moderately high
Over strong bedrock Low - Moderate Moderate โ€“ High Moderate โ€“ High Moderate โ€“ High Moderate โ€“ Moderately high Moderate โ€“ High Moderate โ€“ High
Over weak bedrock Low - Moderate Moderate โ€“ High Moderate โ€“ High Moderate โ€“ High Moderate โ€“ Moderately high Moderate โ€“ High Moderate โ€“ High
Hydrological variants ย  ย  ย  ย  ย  ย 
Artificial drainage Moderately high Moderately high Moderately high Moderate Moderately low Moderate High
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.