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Land-based activities are putting pressure on water quality

What we do on the land can affect our fresh water (Parliamentary Commissioner for the Environment, 2012). The clearing of native vegetation, New Zealand’s growing population, urbanisation, and farming/forestry, the drainage of wetlands, and the damming and modification of rivers and streams have all had significant effects on our land and placed increasing pressure on our water bodies and their ecosystems.

Land-based activities can discharge nutrients to surface water and groundwater (see The relationship between nutrients and algal growth in fresh water). The nutrients of most concern in New Zealand’s fresh water are nitrogen and phosphorus, as they cause excessive algal blooms and undesired plant growth (Davies-Colley, 2013). These nutrients occur naturally in rivers, streams, lakes, and groundwater and are essential for plants to grow. However, high nutrient concentrations can result in the excessive growth of freshwater plants and algae (periphyton in rivers or phytoplankton in lakes), which can reduce oxygen levels and prevent light from penetrating water, negatively affecting freshwater plant and animal species. Very high concentrations of nitrogen, either as nitrate-nitrogen or ammoniacal nitrogen, can be toxic to freshwater life; high nitrate-nitrogen concentrations can make water unsafe for humans to drink.

As of 2012, pastoral land covered about 40 percent of New Zealand (Ministry for the Environment and Statistics NZ, 2015). We assume pastoral land is used for livestock production; however, we recognise some areas are used for other purposes, such as turf production. In 2012, native forest land covered 24 percent, while exotic forest land covered 8 percent, cropping and horticulture land covered 2 percent, and urban land covered less than 1 percent of New Zealand. Even though agricultural land (pastoral, exotic forest, and cropping and horticulture) covers around 50 percent of New Zealand, 87 percent of New Zealanders live in urban areas so many people are potentially affected by degraded urban rivers.

The relationship between nutrients and algal growth in fresh water

Nitrogen and phosphorus come in many forms, which behave differently in water. This means they have different effects when they move through the environment. Nitrate-nitrogen is a soluble form of nitrogen and is highly mobile in water bodies. Once dissolved in water, nitrate-nitrogen can travel across the land surface or leach through soil into groundwater, ending up in rivers, lakes, and aquifers where it can affect water quality. Phosphorus tends to adhere to soil particles. When these soil particles are transported to rivers and lakes, sediment-bound phosphorus can accumulate on river and lake beds. Most phosphorus in water is bound to sediment, but under certain conditions, it is transported in dissolved form and can be taken up by plants and algae.

The concentrations and ratios of nitrogen and phosphorus in a water body are important. A supply of both nutrients is needed for excessive algal growth to occur (generally exhibited through periphyton in rivers and phytoplankton in lakes). An excess of nitrogen or phosphorus alone will not lead to excessive algal growth – both need to be present in the right ratio. If a river or lake has plenty of nitrogen to stimulate unwanted algal growth but not enough phosphorus, it is said to be phosphorus-limited. Conversely, water bodies with plenty of phosphorus but not enough nitrogen are nitrogen-limited. In theory, if a water body is phosphorus-limited, then there is less need to control inputs of nitrogen to prevent excessive algal growth. However, in many situations controlling one nutrient may be inadequate because the limiting nutrient in a water body can be different at different times, and it can be different in different parts of the same water body.

Periphyton is the algae found on the bed of streams and rivers. Healthy river ecosystems are characterised by the presence of periphyton (Biggs, 2000), but high periphyton abundance can reduce the diversity and productivity of invertebrates and fish, and erode recreational values such as swimming and fishing. Maximum periphyton abundance is mainly determined by the time available for biomass to accrue between floods (ie flow regime), and by nutrient (nitrogen and phosphorus) concentrations, temperature, and light.

Source: Parliamentary Commissioner for the Environment, 2012

For more detail see Environmental indicators Te taiao AotearoaLand cover [Stats NZ] and Land use [Stats NZ].

Agricultural practices have increased losses of contaminants to water bodies

New Zealand has one of the world’s highest rates of agricultural land intensification over recent decades (but our agricultural practices have yet to reach the intensity of other areas, such as the European Union) (OECD/Food and Agriculture Organization of the United Nations, 2015). Intensified agricultural practices put increasing pressure on our water bodies due to the increased use of fertiliser; urine and faecal matter deposited by livestock; the taking of fresh water for irrigation; accelerated erosion from forestry, livestock, and cultivated soils; and infrastructure and housing development (Davies-Colley, 2013).

Agricultural intensification has been underway in parts of New Zealand since the late 1970s, as indicated by increased stocking rates and yields; increased fertiliser, pesticides, and food stock inputs; and conversion to more intensive forms of agriculture, such as dairying (MacLeod & Moller, 2006). The rates of land-use change and the rates of intensification (both increasing and decreasing) vary in different parts of New Zealand (Anastasiadis & Kerr, 2013).

As dairy products have become more profitable over recent decades, many farmers have moved away from sheep farming to more intensive dairy farming (DairyNZ, 2013; see figure 2). In 2015, New Zealand farmed about 29 million sheep, 10 million cattle (6.5 million for dairy), and 900,000 deer. From 1994 to 2015, sheep numbers decreased 41 percent and dairy cattle increased 69 percent (Statistics NZ, 2015a). The largest numbers of dairy cattle are in Waikato (1.76 million), followed by Canterbury (1.25 million) and Southland (0.73 million). Canterbury and Southland also had the largest increases in dairy cattle numbers from 1994 to 2015 (excluding Nelson, see table 1). By percentage, Southland had the largest increase, with dairy cattle numbers rising by about 539 percent.

Figure 2

national livestock counts between 1972 and 2015
Click to enlarge view

This graph shows national livestock counts between 1972 and 2015. Visit the MfE data service for the full breakdown of the data.

Note: The current version of the Agricultural Production Survey started in 2002. Before 2002, the questionnaire design, coverage, and collection method varied, and the survey was conducted only in certain years. Numbers before 2002 are shown as a dotted line to indicate a change in livestock numbers; however, the data are not suitable for inclusion in time-series analysis. The current survey population (from 2002) includes businesses registered for goods and services tax.

Table 1: Dairy cattle numbers by region, 1994–2015

Region

Percent change, 1994–2015

Number, 2015

Northland

7

380,616

Auckland

–26

124,668

Waikato

23

1,761,949

Bay of Plenty

25

357,039

Gisborne

66

10,341

Hawke’s Bay

158

81,738

Taranaki

–10

541,931

Manawatu-Wanganui

47

451,421

Wellington

31

109,992

Tasman

59

77,863

Nelson

499

8,459

Marlborough

19

27,040

West Coast

130

182,298

Canterbury

490

1,253,993

Otago

368

384,979

Southland

539

731,209

Chatham Islands

–100

0

Total New Zealand

69

6,485,535

Note: The current version of the Agricultural Production Survey started in 2002. Before 2002, questionnaire design, coverage, and collection method varied, and the survey was conducted only in certain years. The current survey population (from 2002) includes businesses registered for goods and services tax.

Agricultural intensification can increase the discharge of nutrients into surface water and groundwater due to increased livestock urine and waste and the application of fertiliser containing nitrogen and phosphorus (Parfitt et al, 2008). To estimate agricultural nitrogen leaching (nitrogen lost from the root zone and potentially reaching groundwater), we used the Greenhouse Gas Inventory (a model that estimates livestock contributions to soil nitrogen). We assumed a 7 percent leaching rate of nitrogen applied to land – this value was adopted for New Zealand’s greenhouse gas reporting. In 2012, an estimated 137 million kilograms of nitrogen was leached from agricultural soils, an increase of 29 percent since 1990 (Ministry for the Environment, 2014; see figure 3).

Using the OVERSEER model (a model for estimating nutrient flows in a farm system), the main source of nitrogen leached from agricultural soils was livestock urine, accounting for an estimated 78 percent of nitrogen leached in 2012; the rest of the nitrogen leached was from fertiliser (Dymond et al, 2013). Waikato and Taranaki had the highest estimated agricultural nitrogen leaching rate per hectare (see figure 4 shows data only for agricultural soils).

Figure 3

modelled national nitrogen
Click to enlarge view

This graph shows modelled national nitrogen leached from agricultural soils between 1990 and 2012. Visit the MfE data service for the full breakdown of the data.

For more detail see Environmental indicators Te taiao Aotearoa: Livestock numbers [Stats NZ],Trends in nitrogen leaching from agriculture [Stats NZ] and Geographic pattern of agricultural nitrate leaching [Stats NZ].

Figure 4

modelled rates of agricultural nitrate-nitrogen
Click to enlarge view

This map shows modelled rates of agricultural nitrate-nitrogen leaching in 2011. Visit the MfE data service for the full breakdown of the data.

Urban run-off and modification of water bodies are degrading water quality

New Zealand’s population grew 17 percent from 1996 to 2013 (Statistics NZ, 2013) driving a 10 percent (20,922 hectares) increase in urban land area over a similar period from 1996 to 2012 (Ministry for the Environment and Statistics NZ, 2015).

Urban streams typically have flashy flows (rapid increases and decreases in peak flows), elevated concentrations of nutrients and contaminants, highly modified channels, and reduced biodiversity with more ‘tolerant’ species that can thrive in streams with poor water quality (Walsh et al, 2005). These streams typically have increased bank erosion and sediment transport, compared with native streams, which results in more sediments deposited in estuaries (NIWA, nd).

Urban areas have three types of water infrastructure – stormwater (rainwater plus any contaminants it picks up on surfaces and carriers through the system), wastewater (water used in houses, businesses, or industrial processes), and potable water (for drinking water supply). In this section we discuss the negative effects stormwater and wastewater can have on urban streams.

Stormwater run-off can contain elevated concentrations of heavy metals (Lewis et al, 2015). These metals come from sources such as vehicle wear (copper from brake pads and zinc from tyres), metal roofing, and industrial yards (Kennedy & Sutherland, 2008). Metals are transported by stormwater into urban streams (either by pipes or run-off from surfaces directly into streams), and can accumulate in sediment and plant and animal tissue to concentrations that are toxic to freshwater life. Nutrients can also enter urban streams through the stormwater system, from spills, fertiliser used on lawns, and golf courses (eg Webster & Timperley, 2004).

Urbanisation causes vegetation to be removed and soil to be covered by impervious surfaces such as roofing, asphalt and concrete that do not absorb water. This leads to increased stormwater volumes and peak flows (Suren & Elliot, 2004). The degree of contamination of surface water is linked to the amount of impervious area, as impervious surfaces and pipes sometimes direct stormwater run-off straight into urban streams (Lewis et al, 2015). Permeable areas in urban environments (eg lawns and gardens) allow run-off to soak into the ground rather than wash off hard, impervious areas, directly entering streams and stormwater systems. Currently, we do not have national information on the extent of impervious surfaces in New Zealand.

Wastewater networks carry waste from houses and businesses to treatment plants. Wastewater can contain many contaminants, in particular nutrients and faecal pathogens (indicated by E.coli).

Combined stormwater/wastewater networks, illegal connections to the network, and leaky pipes, pumps, and connections in urban environments have caused nutrients and pathogens to enter urban streams. Many combined stormwater/wastewater networks also have consented overflows for storm periods, meaning that wastewater is permitted to overflow into stormwater systems during storms (NIWA, nd). In some areas, we have moved away from historic combined stormwater and wastewater systems to networks that only convey and treat wastewater and minimise the risk of overflows.

Infrastructure not appropriately designed, maintained, and replaced when necessary can have negative impacts on urban streams. The replacement value of the entire national infrastructure network for wastewater and stormwater is $36.7 billion (Department of Internal Affairs analysis of local authority annual reports). This does not include any new infrastructure, only the replacement of existing infrastructure. From a national perspective, one-quarter of wastewater assets are more than 50 years old, with between 10 and 20 percent of the graded network requiring significant renewal or replacement (Local Government NZ, 2014). Some councils have made recent investments (such as Tauranga), whereas other councils have much older networks.