Southland Physiographic zones
Lignite/Marine Terraces Technical information
Physiographic zones are part of the Water and Land 2020 & Beyond project that aims to maintain and improve water quality in the Southland region, and to help us as a community achieve our goals for water.
Understanding our Physiographic zones water and water quality The Physiographics of Southland Physiographic zones represent areas What’s inside project was developed to better of the landscape that have a common understand the evolution of water influence over water quality. The Overview and map of the across Southland. By understanding proposed Southland Water and Land Lignite/Marine Terraces where water comes from and the Plan identifies nine physiographic zones zone and variants processes it undergoes as it moves in Southland that can be used to manage through drainage networks, we can land use effects on water quality. Variants and associations better understand the reasons for different water quality outcomes across Landscape characteristics the region. Physiographic zone technical information Surface zone The findings of the Physiographics of characteristics Southland project have been published The purpose of this booklet is to in two reports, which identify the key describe generalised water quality Soil zone characteristics drivers of hydrochemistry and water risks associated with individual quality variability in Southland (Rissmann physiographic zones. A companion Saturated zone et al., 2016), and describe a classification booklet titled Guide for using the characterisitcs system for managing land use effects on Southland physiographic zones water quality (Hughes et al., 2016). technical information explains the terms Water quality implications and source data used in this booklet. Lignite/Marine terraces physiographic zone
Lignite/Marine Terraces zone Variants Artificial Drainage Overland flow
Lignite/Marine Terraces Physiographic Zone
Approximate area 100,710 hectares
Geographic distribution Along the south coast between Bluecliffs and Awarau Bay, Fortrose and the Haldane Estuary. Inland areas of Eastern Southland where lignite measure sediments occur near the land surface. Go to our online BEACON mapping service to view Main surface waterways the physiographic zones of Te Tipua Stream, Hedgehope Southland in more detail. Stream
2 Overview
The Lignite/Marine Terraces physiographic zone occurs along the south coast and in areas of eastern and western Southland where organic-rich sediments occur at or near the land surface.
Key features Water quality • Located where the underlying implications geology is highly to moderately • When soils are wet, episodic reducing. contaminant loss occurs via • Mostly flat to gently undulating overland flow or artificial drainage terrain. in response to precipitation. • Soils are fine-textured with slow • The potential for elevated nitrate subsoil permeability. concentrations in groundwater can • The redox state of groundwater be limited by high denitrification varies according to the thickness potential. of alluvium overlying organic-rich sediments. • Surface water quality is influenced by groundwater quality (baseflow), overland flow and artificial drainage.
Water quality risk Lignite/Marine Lignite/Marine Lignite/Marine Terraces Terraces(a) Terraces(o)
Contaminant pathways Deep drainage Artificial drainage Overland flow
Dilution and attenuation Reducing aquifers Reducing aquifers Reducing aquifers processes
Primary receiving Aquifers Surface waterways Surface waterways environments
Water quality risk Low Nitrogen, phosphorus, Nitrogen, phosphorus, sediment, microbes sediment, microbes
3 Variants and associations
Variants Associations Gleyed The Lignite/Marine Terraces zone Variants identify areas within The Lignite/Marine Terraces occurs adjacent to the Gleyed zone in physiographic zones where there is physiographic zone is commonly the Hedgehope and Waimumu areas increased water quality risk when soils associated with three physiographic where Quaternary alluvium thins and are wet. Contaminant losses from zones in Southland (Figure 1). lignite sediments occur at or near the variants occur along alternate drainage land surface. pathways that have lower attenuation Bedrock/Hill Country potential. Streams originating from the Bedrock/ In the lower Aparima catchment, the Hill Country zone provide headwater Lignite/Marine Terraces zone occurs The Lignite/Marine Terraces discharge to surface waterways on remnant marine terraces, which physiographic zone has two identified draining the Lignite/Marine Terraces are located within the more extensive variants. zone along the south coast, east of alluvial deposits of the Gleyed zone. Riverton. Drainage from the Lignite/Marine Peat Wetlands Terraces zone contributes to baseflow Lignite/Marine of small streams that also drain the Terraces(a) – Artificial Along the south coast, east of Gleyed zone. Drainage variant Invercargill, the Lignite/Marine Terraces zone occurs on near-surface lignite Areas of undulating land that measure sediments adjacent to the have slow subsoil permeability Peat Wetlands zone. Discharge from the and may be seasonally wet. Lignite/Marine Terraces zone provides This results in an increased flow to wetland areas. potential for artificial drainage to maintain agricultural production.
Approximate area: 37,000 ha
Lignite/Marine Terraces(o) – Overland Flow variant
Areas of gently undulating to rolling land that have elevated potential for overland flow, due to a combination of steeper slopes and/or less well drained soils.
Approximate area: 58,730 ha
Figure 1: Landscape context image illustrating the relationship between the Lignite/Marine Terraces zone and neighbouring physiographic zones. The Lignite/Marine Terraces zone occurs in two physical settings: elevated terraces on the Southland Plains which are underlain at shallow depths by Tertiary (lignite measure) sediments and low-lying marine terraces along the south coast, which are often associated with the Peat Wetlands zone. The Lignite/Marine Terraces zone receives recharge from rainfall and often discharges to streams draining the Gleyed zone inland of the coastal margin, or to the coast.
4 Dominant characteristics that affect water quality
1 2 3 5 4
Landscape characteristics
Surface zone characteristics
Soil zone characteristics
Saturated zone characteristics
Water quality implications
5 1 landscape characteristics
The Lignite/Marine Terraces physiographic zone 1 occupies marine terraces along the south coast between Bluecliffs and Awarua Bay, and between Fortrose and the Haldane Estuary. It also extends across a large area between Grove Bush and Upper Charlton where Gore Lignite Measure sediments occur near the land surface. In the Ohai area, this zone occurs where Beaumont Coal Measure sediments are similarly exposed.
Topography Geology Climate Elevation This zone is associated with two distinct Average annual rainfall in this zone geological environments: varies spatially between 773 and 6,553 Most of this zone occurs at elevations less mm, with higher rainfall occurring on than 220 metres relative to sea level (m Marine terraces of late Quaternary age isolated marine terraces along the RSL). This reflects the spatial distribution (Q5 to Q7) Fiordland coast. of this zone across downlands in the lower Mataura catchment and alluvial • Located along the south coast terraces around the coastal margin. • Comprised of a mix of reworked alluvium and variable amounts of Slope fine-grained sediment and organic Most of this zone occurs on flat to materials undulating land (≤7o slope). Organic-rich Tertiary sediments of the East Southland Group, with surface or Steeper areas generally occur in the near surface exposure (<15 m below Overland Flow variant, which occupies ground level) predominately flat to rolling land (≤15o slope). • Located across a broad area extending along the southern Flatter areas with less well drained soils margin of the Hokonui Hills (from typically occur in the Artificial Drainage Grove Bush to Upper Charlton) and o variant (≤3 slope). in the vicinity of Ohai • The main sub-rock types are lignite and coal
6 Lignite/Marine Lignite/Marine Lignite/Marine Terraces Landscape Terraces(a) Landscape Terraces(o) Landscape zone characteristics zone characteristics zone characteristics
Elevation Elevation Elevation 0 – 220 m RSL 0 – 140 m RSL 0 – 320 m RSL Slope Slope Slope Flat to undulating Flat to gently undulating Flat to rolling Geology Geology Geology Hard rock and Quaternary Quaternary sediments Hard rock sediments Landform age Landform age Landform age Q2-Q4 and Pre-Quaternary Pre-Quaternary Pre-Quaternary Average annual rainfall Average annual rainfall Average annual rainfall 1,102 mm per year 1,506 mm per year 1,346 mm per year
7 2 Surface zone characteristics
Recharge to the Lignite/Marine Terraces zone predominantly occurs from local precipitation infiltrating through the soil matrix. Runoff from 2 adjacent higher elevation land provides minor recharge in some areas.
short, higher order streams (3rd order Dilution and above). These drain either to the Overland flow This zone is predominately recharged coast or to adjacent physiographic In steeper areas, or on rolling via land surface recharge (matrix flow). zones. topography where subsoil permeability There is less potential for dilution of is low, overland flow can be generated Temporal discharge is characterised contaminant concentrations with this during sustained wet periods. by frequent high flow events following recharge mechanism. precipitation, particularly when soils Soils in this zone have a high water are wet. holding capacity. Therefore, soil Surface waterways moisture may be maintained at or As soils dry, the water holding capacity around field capacity for extended This zone is predominately drained by and permeability of the soils mean periods, particularly in coastal areas a network of lower order streams. In increasingly large precipitation events with regular precipitation. This flatter areas that are prone to seasonal are required to initiate quickflow. increases the potential for saturation waterlogging, streams are augmented This results in extended periods of excess overland flow. by artificial drainage. stable baseflow when soil moisture is seasonally low. This network of lower order streams and artificial drainage discharges to
Lignite/Marine Lignite/Marine Lignite/Marine Terraces Surface Terraces(a) Surface Terraces(o) Surface zone characteristics zone characteristics zone characteristics
Dilution potential Dilution potential Dilution potential Low recharge flux Low recharge flux High to low recharge flux
Drainage density Drainage density Drainage density Moderate High Low
Stream Order Stream Order Stream Order Mixed Mixed Predominately small
Overland flow potential Overland flow potential Overland flow potential Moderate to very low Low to very low Moderate to moderately high
8 3 Soil zone characteristics
Soils in this physiographic zone are typically deep and fine-grained. These soils are generally formed in windblown loess on older alluvial terrace surfaces and 3 comprise moderately permeable topsoil overlying a compact, moderate to slowly permeable subsoil.
Soils Reduction potential Artificial drainage Brown soils occur extensively across Soils in this zone have moderate During sustained wet periods, areas with near surface exposures of Gore reduction potential due to their compact, slowly permeable subsoils Lignite Measure sediments in eastern moderate to poor internal drainage are prone to waterlogging. Where this Southland, with Gley soils more and slightly elevated organic land is developed, these soils often have commonly associated with marine carbon content. This means some artificial drainage to maintain agricultural terraces along the south coast. denitrification may occur within the soil productivity. Such areas are included in profile, reducing nitrate losses from the the Artificial Drainage variant. Soils in this zone typically have very land surface to underlying aquifers. low to moderate base saturation and moderate to high soil anion storage Lateral drainage capacity. In soil types that have restricted drainage, lateral flow may occur along slowly permeable layers within the soil profile.
Lignite/Marine Lignite/Marine Lignite/Marine Terraces Soil zone Terraces(a) Soil zone Terraces(o) Soil zone characteristics characteristics characteristics
Soil order Soil order Soil order Brown Brown Brown Profile drainage Profile drainage Profile drainage Poorly to moderately well drained Poorly to imperectly drained Moderately well to poorly drained Permeability Permeability Permeability Moderate over slow and No data** No data** moderate, slow Anion storage capacity Anion storage capacity Anion storage capacity No data** No data** Moderate Reduction potential Reduction potential Reduction potential Moderate High to moderate Moderate Artificial drainage density Artificial drainage density Artificial drainage density Moderate to high Low to none Low to moderate Lateral drainage potential Lateral drainage potential Lateral drainage potential Localised Localised Localised
**A majority (>90%) of the zone does not have available data
9 4 Saturated zone characteristics
Groundwater in the Lignite/Marine Terraces physiographic zone varies according to the underlying geology. 4
Groundwater Groundwater levels Deep drainage Groundwater levels are generally close Deep drainage to groundwater occurs Alluvial aquifers to the land surface (<2 metres) in lower at a slow rate through poorly to Due to their origin and age (typically lying areas adjacent to rivers and imperfectly drained soils and comprises Q5 and older), the surficial alluvium streams. a significant component of the water generally is comprised of quartz-rich However, levels may be up to 15 metres balance in this zone. gravels in a highly weathered silty clay below ground under elevated marine matrix. As a consequence, aquifers However, deep drainage is not a major terraces along the south coast. hosted in such sediments exhibit low to contaminant pathway due to filtration very low permeability. and sorption in the soil profile and Reduction potential denitrification associated with strongly The extent of the groundwater resource reducing conditions in the underlying varies according the thickness of Alluvial aquifers geology. alluvial deposits overlying lignite and Alluvial aquifers tend to exhibit a coal measure sediments. vertical redox gradient where alluvium Groundwater from these aquifers is sediments are thick. Groundwater primarily discharged as baseflow to is oxidised (oxic) when shallow, but surface waterways. becomes increasingly reduced with depth (due to increasing residence time Tertiary aquifers and disconnect from oxygen source). Tertiary sediments underlying inland areas of this zone host extensive Tertiary aquifers aquifers. These are located in coarser- Tertiary aquifers groundwater hosted grained sand and gravel zones, in Tertiary lignite measure sediments is which are interspersed between low generally anoxic. permeability mudstone and lignite sediments.
These aquifers form part of a regional- scale flow system which is slowly permeable and ultimately discharges at or near the coast.
Recharge to these deeper confined aquifers is likely to form a minor component of the overall water balance in this zone.
10 Lignite/Marine Lignite/Marine Lignite/Marine Terraces saturated Terraces(a) saturated Terraces(o) saturated zone characteristics zone characteristics zone characteristics
Water table depth Water table depth Water table depth Moderate to shallow Moderate to shallow Moderate*
Aquifer permeability Aquifer permeability Aquifer permeability Low Low Low
Active groundwater Active groundwater Active groundwater storage storage storage Moderate Moderate Minor
Reduction potential Reduction potential Reduction potential Low to high Low to high Low to high
Deep drainage potential Deep drainage potential Deep drainage potential Moderate Moderate Moderate
* A significant proportion (>50%) of the zone does not have available data
11 5 Water quality implications
The main water quality issues for this physiographic zone are nitrogen, phosphorus, sediment and microbial contaminant losses via artificial drainage 5 and overland flow to surface waterways.
Influencing factors Groundwater Denitrification within aquifers Factors that influence water quality in the containing elevated organic carbon Lignite/Marine Terraces zone include: moderates nitrate concentrations in Key hydrochemical • the presence of near-surface this zone. features carbon-rich (reducing) sediments Surface waters which have high reduction • Water types are potential Nitrogen concentrations are typically predominantly Na-Cl in • variable reduction potential in soils low in surface waters, particularly coastal areas (reflecting with moderate to poor drainage during baseflow. This reflects the marine aerosol deposition) influence of reducing groundwater to and Ca-HCO in inland • elevated potential for artificial 3 baseflow. Phosphorus concentrations drainage in flat-lying developed areas (reflecting elevated are also generally low due to retention areas with poorly drained soils alkalinity associated with of P in the soil zone. soil zone processes) • elevated potential for overland flow on sloping land with poorly drained Contaminant losses can become • Where waters are reducing, soils significantly elevated during high flow dissolved iron (Fe2+), events that occur in response to heavy manganese (Mn2+) and or sustained precipitation events. This ammoniacal nitrogen can reflects the rapid transport of water to be elevated while nitrate Water quality issues streams via overland flow and artificial (NO3) is low drainage. Nitrogen losses from this zone are moderated by soil and aquifer denitrification (Figure 3).
However, the actual extent of denitrification depends on nutrient load and residence time of water within the soil zone.
In overland flow (Figure 4) and artificial drainage (Figure 5), large amounts of nitrogen can be mobilised during heavy precipitation events due to the limited time available for denitrification to occur.
12 What affects water quality in the Lignite/Marine Terraces zone?
Key Contaminant Attenuation Water quality Where characteristics pathways processes risk
All areas Land surface Lateral drainage Attenuation Low water quality recharge through the soil zone of particulate risk and microbial Moderately well to contaminants well drained soils by filtration and adsorption. Variable denitrification potential in the soil zone
Deep drainage to Attenuation Low water quality groundwater when of particulate risk soils are wet and microbial contaminants by filtration and adsorption. Variable denitrification potential in the soil zone and elevated denitrification potential in the saturated zone
Artificial drainage Low subsoil Artificial drainage Some filtration, Nitrogen variant permeability in response to adsorption may Phosphorus sustained or heavy occur in water Microbes Flat topography precipitation events moving through the Sediment soil matrix. Limited denitrification may Contaminants also occur in water discharged rapidly to moving slowly surface water through soils with an elevated reduction potential
Overland flow Sloping topography Overland flow Limited attenuation Nitrogen variant in response to of contaminants Phosphorus Low subsoil sustained or heavy Microbes permeability precipitation events Sediment
Contaminants discharged rapidly to surface water
13 Contaminant pathway Mitigation objectives
Overland flow Protect soil structure, particularly in gullies and near stream areas
Manage critical source areas
Reduce phosphorus use or loss
Artificial drainage Protect soil structure, particularly in gullies and near stream areas
Reduce phosphorus use or loss
Reduce the accumulation of surplus nitrogen in the soil, particularly over autumn and winter
Avoid preferential flow of effluent through drains
Capture contaminants at drainage outflows
Figure 2: Main zone – Groundwater nitrate concentrations are strongly influenced by distribution of organic-rich lignite and marine sediments. Generally, aquifer denitrification potential aggrades from low to high within alluvial aquifers and is high throughout Tertiary aquifers. Phosphorus may be mobilised in reduced groundwater.
14 Figure 3: Overland flow variant – Overland flow provides a pathway for episodic losses of nitrogen (N), phosphorus (P), sediment (S) and microbes (M). Overland flow occurs because of seasonal water excess in moderately to slowly permeable soils.
Figure 4: Artificial drainage – Mole-pipe drainage provides a pathway for episodic losses of nitrogen (N), phosphorus (P),sediment (S) and microbial (M) contaminants to surface waters.
15 Environment Southland uses reasonable endeavours but does not warrant that this information is current, complete or accurate. Professional or specialist advice should be obtained before taking or refraining from taking any action on the basis of this information. To the extent permitted by law, Environment Southland will not be liable for any loss, liability or costs suffered or incurred as a result of any reliance placed on this information.