Oxidising Soil and Aquifer

Occurrence

The Oxidising Soil and Aquifer Environment occurs largely in lowland areas where there are moderately-well to well drained soils and oxygen-rich underlying aquifers. It is commonly associated with volcanic ash or wind and water deposited materials across New Zealand.

Nationally 21.4% or 5,636,510 ha of New Zealand is classified as Oxidising Soil and Aquifer Environment. Canterbury, Waikato, and Bay of Plenty have the largest areal extent. The Bay of Plenty has the highest proportion of the region classified as Oxidising Soil and Aquifer Environment.

Extent of the Oxidising Soil and Aquifer Environment in New Zealand.
Region Total Area (ha) Percent of Region (%) Percent of NZ (%)
Canterbury 1,111,424 25.06 4.22
Waikato 1,073,936 45.07 4.08
Bay of Plenty 755,516 62.82 2.87
Manawatu 548,805 24.73 2.08
West Coast 381,119 16.47 1.45
Hawkes Bay 377,189 26.72 1.43
Otago 349,656 11.23 1.33
Southland 301,571 9.69 1.14
Taranaki 232,199 32.03 0.88
Northland 88,167 7.07 0.33
Tasman 87,223 9.08 0.33
Gisborne 82,665 9.86 0.31
Wellington 70,057 8.74 0.27
Marlborough 66,213 6.33 0.25
Auckland 46,917 9.57 0.18
Nelson 1,492 3.55 0.01
slope map of new zealand with the parts classified as 'Oxidising Soil and Aquifer' showing.

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. Most rainfall infiltrates the well-drained soils and enters the underlying aquifer by deep drainage.

Landscape Characteristics

The Oxidising Environment has a high ability to filter and adsorb contaminants, reducing the risk of both particulate and dissolved phosphorus, sediment, and microbial losses when water can drain through the soil to depth. However, the risk of nitrate nitrogen leaching is elevated. As the environment has a low ability to remove nitrate naturally, greenhouse gas emissions are also inherently low.

Deep drainage and nitrate leaching are typically seasonal. Leaching to the underlying aquifer occurs when soil moisture is above field capacity or saturated, generally between late autumn and spring. However, deep drainage can occur at any time of the year in response to heavy or sustained rainfall. The deeper the unsaturated zone, the longer it takes for water and nitrate to be transported to the aquifer. This may be days, months to years depending on the water table depth and climate.

Under intensive land uses, nitrate can accumulate to high concentrations in the soil and underlying aquifer over time. This is particularly relevant in areas where there is a deep unsaturated zone or in older more weathered land surfaces.

Variation in the Oxidising Environment occurs where the land is sloping, where soil infiltration rates are low or impeded by slow permeability subsoils increasing the likelihood of runoff occurring. Artificial drainage may also be present in these areas to improve drainage. The environmental risk from all contaminants increases as the soil zone can be bypassed by drainage waters.

In the waterway, high nitrate 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 Variants

The Oxidising Soil and Aquifer Environment has four siblings:

  • High deep drainage occurs on flat topography where there are minimal impediments to water draining through the soil profile.
    Extent: 1,436,240 ha (5.45% of New Zealand)

  • Increased lateral and overland flow occurs on either undulating topographies or where soils are moderately well-drained resulting in increased lateral or overland flow.
    Extent: 1,527,673 ha (5.80% of New Zealand)

  • Over strong bedrock is a transitional class between the Oxidising 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: 1,520,765 ha (5.77% of New Zealand)

  • Over weak bedrock is a transitional class between the Oxidising Soil and 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: 1,151,833 ha (4.37% of New Zealand)

Variants are not as common in this environment. In MAPS – PHYSIOGRAPHIC ENVIRONMENTS check to see if any variants apply to your location. Artificial drainage may be present, most likely in the increased lateral and overland flow sibling. In MAPS - HYDROLOGY explore the Overland Flow map to see how runoff risk varies across this environment and the Water Table map for the predicted depth to groundwater. The depth to the water table is especially important in the ‘over bedrock’ siblings as shallow soils will become saturated faster.

The role of landscape in regulating contaminants in the Oxidising Soil and Aquifer Environment. If the landscape function is high it is good at reducing the risk to the receiving environment.
Oxidising Soil & Aquifer Environnent Sibling 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 deep drainage Deep drainage through the soil zone to an underlying aquifer. Low High High Low High Concentration & load to groundwater
Increased lateral and overland flow Deep drainage through the soil zone to an underlying aquifer with increased lateral and overland flow due to seasonal wetness. Low Moderately high - High Moderately high - High Low - Moderately low Moderately high - High Concentration & load to groundwater, minor surface water
Over strong bedrock Deep drainage until contact with bedrock which transitions to lateral flow. 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. Low Moderately high – High Moderate - High Low - Moderately high* Moderate* - High Concentration & load to surface water, minor groundwater contribution
Over weak bedrock Deep drainage until contact with bedrock which transitions to lateral flow. Slope and soil depth controls overland flow risk where the steeper the slope or shallower the soil the more likely runoff is to occur. Low Moderate - High Moderate - High Low - Moderately high¹ Moderate¹ - High 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 – 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 when soils are very dry with the highest risk occurring after prolonged dry periods. Low Low Low Low Low Concentration & load to groundwater

¹ If redox processes occur at the contact with bedrock.

² Dilution potential is dependent on the recharge domain of the Physiographic Environment.

Contaminant Profile

Inherent susceptibility of the landscape for contaminant loss in the Oxidising 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.
Oxidising Soil & Aquifer Environnent Sibling Nitrogen Phosphorus Sediment Microbes
Nitrate & Nitrite Ammonium & Ammonia Organic (Dissolved & Particulate) Particulate Dissolved Reactive Particulate Particulate
High deep drainage High Low Low Low Low Low Low
Increased lateral and overland flow Moderately high - High Low - Moderately low Low - Moderately low Low - Moderately low Low - Moderately low Low - Moderately low Low - Moderately low
Over strong bedrock Moderately low – Moderate* Low - Moderately high* Low - Moderately high* Low - Moderately low Low – Moderately high* Low - Moderately low Low - Moderate
Over weak bedrock Moderately low – Moderate* Low - Moderately high* Low - Moderately high* Low - Moderate Low – Moderately high* Low - Moderate Low - Moderate
Hydrological variants
Artificial drainage Moderately high Moderately high Moderately high Moderate Moderately low Moderate High
Overland flow High High High High High High High
Natural soil zone bypass High High Moderate Low Moderate Low High

* If redox processes occur at the contact with bedrock.

Key Actions

In this environment nitrate reductions should be the primary focus of mitigation activities. Limit the amount of surplus nitrogen in the soil especially prior to the wettest months. Consider deeper rooting crops or pastures to access deeper stores of nitrogen and limit leaching. Maintaining soil structure is also critical to prevent runoff from occurring. In areas that are prone to runoff, vegetated buffer areas, sediment traps, and riparian planting are all ways of intercepting runoff and minimising contaminant losses.