Current state and trends

Surface water quality

The section begins by assessing the water quality of surface waters (rivers, streams, and lakes) by considering their nutrient and bacteria levels, visual clarity, and their biochemical oxygen demand. The water quality of groundwater is then assessed, primarily by considering nitrate and bacteria levels, then water demand is assessed by considering levels of allocation.

Waikato River downstream of Lake Taupō – one of the rivers monitored as part of the national monitoring network.

 

Source: Ministry for the Environment.

Nutrients in rivers and streams

Nutrient levels increasing (nitrogen and phosphorus)

Figure 10.3 shows changes over time in the concentration of major nutrients in rivers within the national monitoring network (see box ‘Monitoring river water quality’). The dark blue line in each graph represents the rivers’ median nutrient concentration. The orange and light blue lines show trends for the rivers that have the highest and lowest concentrations of nutrients, respectively; that is, the rivers with nutrient concentrations in the top and bottom 5 per cent of the range of monitored sites.

The median levels of nitrogen and phosphorus have increased in rivers within the national monitoring network over the past two decades. More specifically, over 1989–2003, there was an average annual increase in levels of total nitrogen and dissolved reactive phosphorus of 0.5 per cent to 1 per cent (Ministry for the Environment, 2006d). While this increase may seem small, and is difficult to detect from the slope of the median (dark blue) lines in Figure 10.3, it signals a long-term trend towards nutrient-enriched conditions that are likely to trigger undesirable changes to river ecosystems.

Furthermore, New Zealand rivers with relatively high levels of nitrogen are deteriorating – becoming more enriched – more rapidly than rivers with low levels of nitrogen. This is illustrated most clearly in Figure 10.3 by the strength of the trends for nitrate nitrogen and total nitrogen (that is, the relatively steep orange lines).

Comparison with nutrient levels in other countries

The trend of increasing nitrogen in New Zealand rivers is consistent with patterns observed in other countries around the world. Globally, 30 per cent of 82 major river basins have higher nitrogen concentrations now than they had in the late 1970s (United Nations Educational, Scientific and Cultural Organization, 2006).

Nutrient levels in New Zealand rivers are still low by international standards. Our most nutrient-enriched rivers have about half the average nutrient levels of rivers in Europe, North America, and Asia that have been reported by the Organisation for Economic Co-operation and Development (Organisation for Economic Co-operation and Development, 2006).

Note that it is reasonable to compare only our most nutrient-enriched rivers with rivers reported by the Organisation for Economic Co-operation and Development (OECD). This is because, in general, OECD measurements are taken at the mouths of rivers flowing from large catchments. As a result, the OECD data represents relatively highly nutrient-enriched river systems that do not compare readily with New Zealand’s less nutrient-enriched and, in many cases, smaller river systems.

Rivers with high levels of nutrients

The most nutrient-enriched rivers (represented by the orange lines in Figure 10.3) are located throughout the country and include the Mataura (Southland), Waingongoro (Taranaki), Waihou (Waikato), and the lower Manawatū. While these rivers are in lowland areas and are surrounded by predominantly pastoral farmland, factors unrelated to the predominant landscape may be contributing to their poor water quality. For example, in the past, a large point-source discharged effluent from a meatworks on the mid-reaches of the Waingongoro River (Taranaki Regional Council, 2006), although it now discharges to land during periods of low river flow.

Some improvement in nutrient levels

On average, levels of dissolved reactive phosphorus have increased in rivers of the national monitoring network (described earlier). However, there has been a steady decrease in phosphorus in rivers with high levels of this nutrient since a peak in the mid-1990s (indicated by the orange line in the graph for dissolved reactive phosphorus in Figure 10.3). This may signal improved pasture management in intensively farmed areas (for example, through reduced erosion and better fertiliser application practices), which may have led to reductions in the amount of phosphorus run-off to waterways.

Effluent, particularly from humans and farmed animals, such as sheep and cows, is the primary source of ammoniacal nitrogen. In contrast with other nutrients, levels of ammoniacal nitrogen have decreased in most of New Zealand’s rivers over the past two decades. This improvement is consistent with reductions in point-source pollution, particularly the trend in recent decades towards applying ammonia-rich stock effluent to land, rather than discharging it into waterways (see Figure 10.2).

Land use and nutrient enrichment in rivers

The level of nutrients in our rivers is influenced by natural factors such as rainfall and river flow patterns. For example, rivers in areas with relatively low rainfall have higher median nutrient (and bacteria) levels than rivers in wetter areas (Ministry for the Environment, 2005a). This is because contaminants are able to accumulate in stagnant or slow-flowing waters rather than being flushed downstream. However, the largest impact on nutrient levels in our rivers comes from land use.

Figure 10.4 compares the median nutrient levels in rivers and streams in unmodified catchments with the levels in rivers and streams in pastoral and urban catchments.

Algal bloom in a freshwater lake.

 

Source: Nature's Pic Images.

Figure 10.4 shows urban streams are the most nutrient-enriched waterways in New Zealand, followed by rivers and streams in predominantly pastoral catchments. The median nutrient concentrations in both urban and pastoral waterways breach the Australia and New Zealand Environment Committee Council guidelines (see the notes to Figure 10.4) for ecosystem protection (Australia and New Zealand Environment Committee Council, 2000). Rivers and streams in unmodified catchments, such as those that are covered in native bush or alpine tussock, have the lowest levels of nutrients measured in New Zealand waterways.

Nutrients in urban waterways

The main source of nutrients in urban waterways is human wastewater (sewage). Wastewater may leak from broken sewer pipes or be discharged into stormwater systems through faulty pipe connections and sewer overflows. Some nutrients may also come from run-off from suburban lawns and gardens that have had fertilisers applied.

Urban streams with poor water quality can also have downstream impacts. Because most large towns and cities in New Zealand are situated on the coast, urban streams commonly discharge into harbours and estuaries (see chapter 11, ‘Oceans’).

Impact of agricultural activity

In rural environments, agricultural fertilisers and stock manure and urine are the major non-point-sources of nitrogen and phosphorus. These nutrients can enter water bodies relatively quickly if they are carried across the land surface by rainfall run-off, particularly if there are drains such as the mole and tile drains that are common on farmed pasture in Ōtago and Southland. (Mole and tile drains are subsurface channels constructed to drain surplus water.)

There is strong evidence at both the regional level (Environment Waikato, 2004; Hamill and McBride, 2003) and nationally that the levels of nutrients in rivers increase in proportion to the levels of agricultural activity in river catchments. The amount of nutrients going into the land from fertiliser application and livestock continues to increase in New Zealand as farming becomes more intensive (see box ‘More about intensive farming and land use’ in chapter 9, ‘Land’).

Nutrients in lakes

Monitored lakes make up only a small proportion (4 per cent) of all lakes in New Zealand, and many of the lake monitoring programmes focus on lakes that have poor water quality or are at risk of water quality being impaired by land use in their catchment (see box ‘Monitoring lake water quality’). This means care should be taken when interpreting the results of the monitoring of water quality of New Zealand lakes.

A more balanced picture of lake water quality across the country can be obtained by classifying all unmonitored lakes according to the environmental factors that drive water quality (such as climate, lake depth, and the size and types of land cover in the lake catchment).

Key findings for both monitored and unmonitored lakes are presented in the following section.

Nutrient levels in monitored lakes

Seventy-five of the 134 lakes in New Zealand for which nutrient data are available have high to very high levels of nutrients (see Figure 10.5). Thirteen per cent of these lakes are known as ‘hypertrophic’, meaning they are ‘saturated’ with nutrients and their water quality is extremely degraded. In such lakes, algal blooms are common and the health of aquatic animals is often at risk. While some recreation may take place on the surface of these degraded lakes (such as sailing), activities such as swimming are restricted because of the lakes’ prolific weed growth and poor water clarity.

Deep lakes hold more water than shallow ones and have a greater capacity to absorb incoming nutrients before showing definite signs of deterioration in water quality. In addition, the nutrient status of lakes is strongly related to their depth and the type of land use and human activities in the catchment.

All of the monitored lakes that have high levels of nutrients are shallow. They include lakes surrounded by farmland in the Waikato (for example, Lake Hakanoa and Lake Mangakawhere), several of the dune lakes in Northland (Lakes Ōmāpere, Kapoai, Rotokawau, and Waiporohita), and two coastal lagoons in Canterbury (Lake Ellesmere/Te Waihora and Lake Forsyth/Te Wairewa).

The monitored lakes with the lowest levels of nutrients are nearly all deep lakes in mountain country in the South Island (for example, Lakes Coleridge, Pūkaki, Wānaka, and Tekapō) and do not have particularly intensive farming or urban activity in their catchments.

Lake Tekapō.

 

Source: iStockphoto.

An estimate of water quality in unmonitored lakes

A large majority of the 3,820 lakes greater than 1 hectare in area in New Zealand are not monitored. By extrapolating the results for monitored lakes, it is estimated that the majority (about two-thirds) of all lakes are likely to have relatively low concentrations of nutrients and good to excellent water quality because they lie in natural, or only partially developed, catchments (Ministry for the Environment, in press c). The remaining third of lakes are likely to have high levels of nutrients and poor water quality.

Measured trends in lake water quality

Trends in water quality have been assessed for 49 lakes. Figure 10.5 shows that the levels of nutrients in most of these lakes have shown no signs of change since 1990. Ten of the 49 lakes show possible or definite signs of deterioration (that is, an increase in nutrient or algae levels or a decrease in visual clarity), and six show signs of improvement. Many of the lakes showing signs of deterioration are already moderately nutrient-enriched (meso-eutrophic) and lie in largely developed catchments (for example, Waikere in Northland, and Waikare and Rotomanuka in the Waikato).

Land use and lake water quality

Figure 10.6 compares the water quality of monitored lakes in predominantly pastoral catchments with lakes in catchments with predominantly natural land cover. Levels of nutrients (nitrogen and phosphorus) and algae are between two and six times higher in lakes in pastoral catchments than in lakes that are in natural catchments.

Because algal concentrations affect water clarity, the lakes in natural catchments have water that is, on average, five times clearer than water in lakes in pastoral catchments. For example, lakes in the mountainous terrain of the South Island commonly have underwater visibility for more than 10 metres (Ministry for the Environment, 2006b), although this is lowered naturally in some cases by tannins leaching from beech forests or by fine glacial sediment.

The photo below shows an example of a degraded lowland lake that is surrounded by farmland – Lake Spectacle in the Auckland region. Many lakes that lie in intensively used catchments are the subject of management programmes that aim to stem the inflow of pollutants from the surrounding land (see box ‘Local action to protect water quality in Lake Taupō and the Rotorua Lakes’).

Lake Spectacle in the Auckland region (with Lake Tomorata in the background)

 

Source: Courtesy of Auckland Regional Council.

Other factors affecting lake water quality

Natural factors such as air temperature and wind are also important determinants of water quality in lakes. Algal blooms are more likely to occur in lakes in warmer climates (those at lower elevations and in the north) and in the summer. Wind can create waves and currents, particularly in shallow lakes, and lift sediments from the lake bed into the water. As well as reducing the clarity of a lake’s water, this can cause the amount of nutrients that are available for algal growth to increase. Clarity and the appearance of lake water may be affected by soil type. For example, lakes surrounded by peaty soil, such as those commonly found in Westland and Waikato, have water that is naturally brown-stained or ‘dirty’ looking.

Bacterial (faecal) pollution in rivers and lakes

Water quality at freshwater swimming spots

Figure 10.7 shows that over the 2006–2007 summer, 60 per cent of the 230 monitored freshwater swimming spots had water quality that met New Zealand guidelines for water-based (contact) recreation almost all of the time (that is, at least 95 per cent of the samples taken at these sites had concentrations of E. coli that were within acceptable levels). Ten per cent of the sites breached the guidelines regularly (that is, more than 25 per cent of the samples taken from these sites were non-compliant), indicating that these sites often have poor water quality and are not suitable for swimming.

The number of freshwater sites meeting the New Zealand guidelines in 2006–2007 is higher than in previous years for which data has been reported (2003–2004 and 2005–2006). (See the graph in Figure 10.7.) While this increase is encouraging, the period of monitoring is not yet long enough to be able to determine whether there is a trend of improving recreational water quality over time.

Several natural and human factors may cause variations in water quality between seasons. For example, during a wet summer (with frequent rain), more faecal matter is carried from the land into rivers and lakes. Therefore, bacteria levels in the water during wet summers are often high compared with dry summers. Also, sediment mixing as a result of wind and wave action can elevate bacteria levels.

Freshwater swimming spots generally have higher background levels of bacteria and longer-lasting contamination events than coastal beaches. This is largely because faecal matter is more rapidly diluted and dispersed by currents and the large volumes of water at the coast. The difference between bacteria levels at freshwater and coastal swimming spots is illustrated by comparing monitoring results for the 2006–2007 summer; the water quality of 80 per cent of monitored coastal beaches met the guidelines for swimming almost all of the time (compared with 60 per cent of freshwater sites), while only 1 per cent of the coastal beaches breached the guidelines regularly (compared with 10 per cent of freshwater sites) (see chapter 11, ‘Oceans’).

Water-skiing on Lake Taupō.

 

Source: Ministry for the Environment.

Land use and bacterial (faecal) pollution in rivers

Rivers and streams with the highest average levels of faecal pollution are those in towns and cities (see the comparison of median levels in Figure 10.8). Faecal matter from birds, cats, and dogs may be carried by stormwater into urban waterways, although there is little evidence that this source on its own results in infectious levels of bacteria (Ministry for the Environment, 2002). A significant amount of faecal material comes from human waste leaking from sewerage systems.

Like those in urban areas, rivers and streams in pastoral areas also have high levels of bacteria (relative to waterways in natural catchments). While the levels of bacteria in pastoral waterways are lower than in urban sites, the worst pastoral sites that are monitored have significantly higher levels of bacteria than the worst urban sites that are monitored (as indicated by the relatively high 95th percentile for pastoral sites in Figure 10.8).

It is known that farm stock with access to river and stream beds can contribute high amounts of faecal matter directly to the water. One study has shown that if cows cross a stream on their way to and from milking, they are 50 times more likely to defecate in the water than on adjacent raceways (Davies-Colley et al, 2004).

Many sites in predominantly natural catchments, where land-use pressures are considered to be lowest, also have high levels of bacteria. These high levels could be caused by faecal matter from birds and other wild animals, such as possums, deer, and goats. Predominantly natural catchments may also have small pockets of urban or pastoral land use that deliver significant amounts of faecal material to the water.

Figure 10.8: Concentrations of Escherichia coli (E. coli) bacteria in rivers and streams in catchments with different land uses, 1997–2002

 

Notes:

(1) Statistics in the graph are derived from 95th percentile data.

(2) These results include winter sampling results, when faecal-bacteria loads are typically relatively high. This is because there is increased run-off from higher winter rainfall and bacteria tend to live longer in cooler temperatures. However, in general, recreational activities such as swimming do not take place in winter.

(2) mL = millilitres (of water).

Data source: Ministry for the Environment, in press a.

Text description of figure

Box and whisker plot comparing bacteria (E.coli) concentrations in waterways in natural, pastoral and urban catchments. 

 

Dominant land cover (number of sites in brackets)	95th percentiles (Average 1997-2002) (E.coli/100 mL)
	Median	5th	95th
Natural (122)	209	7	5406
Pasture (338)	1179	105	13996
Urban (26)	2558	920	7714

Visual clarity

Between 1989 and 2003, the median visual clarity improved in rivers within the national monitoring network. Almost half of the 77 monitored sites showed increases in visual clarity of more than 1 per cent per year over this time. Since 2000, the median visual clarity for monitored New Zealand rivers ranged between 1.0 and 1.9 metres (Ministry for the Environment, 2006d). This meets the requirements for ecosystem protection (Australia and New Zealand Environment Committee Council guidelines state that clarity of less than 0.7 metres, averaged for upland and lowland rivers, is unacceptable) but has, at times, been below the recommended minimum for human recreation (1.6 metres) (Australia and New Zealand Environment Committee Council, 2000).

Water clarity varies widely between rivers around the country. Visual clarity of only 10 to 40 centimetres is common in rivers with very high levels of sediment. These rivers include the lower Manawatū, the Waitara in Taranaki, and the Waipaoa in the Gisborne district.

Soil erosion is a common cause of low levels of clarity in New Zealand rivers and streams. This may be a consequence of poorly managed farmland (for example, the collapse of unprotected stream banks and sediment run-off from paddocks). Urban development and harvesting of plantation forestry can also produce high volumes of sediment run-off.

Natural factors can also determine clarity. For example, the low level of clarity in the Waipaoa River is caused by the geology of the catchment. Sandstones, mudstones, and gravels are easily eroded, which leads to high suspended-sediment loads (Gisborne District Council, no date). See also box ‘More about hill-country erosion and our geological history’ in chapter 9, ‘Land’.

Visibility of more than 10 metres is common in the country’s clearest rivers (the upper Motueka, Clutha, and Monowai in the South Island high country). The upper catchment of the Motueka River is almost entirely native bush or bare mountain rock and, as a result, the amount of sediment-laden run-off that enters the upper reaches of the river after rainfall is minimal. The photo on the opposite page shows an example of a river with high visual clarity – the Waiohine River in the Wairarapa.

Waiohine River Gorge in the Wairarapa, a river with very high visual clarity.

 

Source: Courtesy of Greater Wellington Regional Council.

It is not yet possible to identify why the water clarity in rivers has improved. However, the improvement may be related to a reduction in sediment in the water as a result of better forestry and farm management (for example, fencing to prevent stock trampling river and stream banks).

Water temperature and dissolved oxygen

Water temperatures and levels of dissolved oxygen remained stable in rivers in the national monitoring network over 1989–2005 (see Figure 10.9). This is encouraging because trends towards higher temperatures and lower levels of dissolved oxygen would indicate water quality was declining.

Large rivers are less susceptible to significant temperature changes than streams. There is some evidence from regional council reporting (for example, Tasman District Council, 2005) that streams in developed catchments regularly experience water temperatures that are high enough to threaten their ecology. These streams are typically unshaded because the riparian vegetation has been cleared.

Macroinvertebrate richness (%EPT)

High macroinvertabrate richness (%EPT) indicates good water quality. Monitoring of rivers in the national network over 1989–2005 showed that the relative abundance of macroinvertebrates that were sensitive to pollution (as measured by %EPT) increased in rivers that had low or median numbers of macroinvertebrates to start with (see Figure 10.10). These increases are consistent with decreases in ammoniacal nitrogen and biochemical oxygen demand (reported in earlier sections of this chapter), and further indicate that levels of organic pollution have decreased.

Biochemical oxygen demand

Biochemical oxygen demand (see box ‘What is biochemical oxygen demand?’) decreased steadily in rivers across the country in 1989–2002, indicating an improvement in water quality. This is illustrated in Figure 10.11, which shows that about half of the 77 sites monitored have had significant decreases in organic pollution and none of them has worsened over 1989–2002. This improved water quality is probably the result of better management of point-sources of pollution, such as dairyshed and factory wastewater discharges.

The average levels of biochemical oxygen demand in New Zealand’s most polluted rivers are three times lower than in large rivers in other OECD countries (Organisation for Economic Co-operation and Development, 2006).

The Mataura River in Southland is an example of a major New Zealand waterway in which water quality has improved since point-source discharges of organic waste were reduced and/or received improved treatment before discharge. In 1975, 15.5 tonnes of organic waste were discharged into the river each day. By 2000, because of improvements to effluent treatment at a large meatworks alongside the river, the organic waste discharged had decreased to just over 3 tonnes a day. Similar reductions in the amount of suspended-solid material were achieved over the same period.

While the Mataura River still has elevated nutrient and bacteria levels from non-point-sources, marked improvements in the appearance of the river (less surface scum and foam) have been attributed to the reduction in organic matter entering it (Environment Southland, 2000).

Groundwater quality

The most extensive aquifers in New Zealand are shallow, unconfined sand and gravel sediments. These aquifers contain relatively young, well-oxygenated, and fast-flowing groundwater. While the mineral content of this groundwater is typically low, it is vulnerable to pollution from human activities on the land. These pollutants, particularly nutrients and faecal material, can quickly reach the water table and, once there, disperse over wide areas.

Nutrients (nitrate) in groundwater

Nitrogen is found in groundwater in the form of nitrate, and is monitored for health and environmental reasons. Excessive levels of nitrate in drinking water have been linked with blood disease in infants (commonly known as ‘blue baby syndrome’) (Davies, 2001).

From an environmental perspective, elevated levels of nitrate often indicate the potential presence of other pollutants from human activities, such as faecal pathogens and pesticides (that is, nitrate can be a good indicator of general groundwater degradation). In addition, groundwater that is rich in nitrate has the potential to elevate nutrient levels in the surface water it drains into.

Current nitrate levels

More than one-third (39 per cent) of groundwater monitoring sites in New Zealand have levels of nitrate that are elevated above natural background levels, probably as the result of human activities (Ministry for the Environment, in press b), such as the leaching of fertiliser and stock effluent. The median nitrate level in New Zealand groundwater that is monitored is 1.3 milligrams per litre. Nitrate levels exceed the 2005 New Zealand drinking water standard of 11.3 milligrams per litre at almost 5 per cent of monitoring sites (see Figure 10.12). However, the proportion of bores at these sites used to supply drinking water for people is not known.

In New Zealand, nitrate concentrations are highest in shallow, well-oxygenated groundwater in unconfined aquifers. The median nitrate concentration in this type of groundwater is 2.8 milligrams per litre, which is more than twice the average of all monitored groundwater (1.3 milligrams per litre).

Monitored groundwater with nitrate concentrations that breach health standards are found in most regions, but are most common in the Waikato and Manawatū regions (Ministry for the Environment, in press b). In the Waikato, elevated nitrate concentrations have been attributed to intensive land uses such as dairying and market gardening in areas where free-draining soils overlie a shallow water table (Environment Waikato, 1998).

As well as affecting drinking water quality, excessive levels of nitrate in groundwater can lead to nutrient enrichment of downstream surface water. This has important implications for regional freshwater management, particularly because there can be a lengthy time lag between groundwater being polluted and its emergence at a downstream water body. For example, nitrogen levels in a lake may increase long after the activity responsible for delivering nitrogen into a catchment (such as intensive farming) has declined or ceased. This is because the groundwater entering the lake is still polluted from historical farming practices.

Trends in nitrate

There has been no clear nationwide trend in nitrate concentrations over recent years. Approximately equal numbers of monitored groundwater sites have shown increasing concentrations of nitrate (13 per cent) and decreasing concentrations of nitrate (11 per cent) over 1995–2006 (Ministry for the Environment, 2007b).

At a regional scale, increasing trends of nitrate are more widespread in some areas than others. Increasing nitrate concentrations have been reported in rural parts of Canterbury, probably due to the increasing intensity of human activities in the region, such as dairy farming and wastewater disposal (Environment Canterbury, 2002). Increasing concentrations of nitrate have also been recorded at some Waikato sites for which records are available from the 1950s (Environment Waikato, no date).

Faecal pollution (bacteria) in groundwater

There are 520 groundwater monitoring sites that have sufficient data on bacteria levels to derive medians. Eighty per cent of these sites comply with New Zealand guidelines for drinking water quality of 1 bacteria coliform unit per 100 millilitres of water sampled (Ministry for the Environment, in press b). However, it is not known how many of these monitored groundwaters are used to supply human drinking water. (Of the drinking water supplies that are registered with the Ministry of Health, about 2 per cent exceeded the guidelines for E. coli between 2002 and 2004 (Ministry of Health, 2006).

The drinking water standard is breached most commonly in Northland, Southland, and Canterbury (Ministry for the Environment, in press b).

Only 2 per cent of the monitored sites exceed the guidelines for stock drinking water quality (100 coliform units per 100 millilitres of water sampled).

Like nitrate, bacteria concentrations are highest in shallow, unconfined groundwater (Ministry for the Environment, in press b); faecal bacteria generally do not survive the long travel times needed to reach deeper groundwater. However, bacteria can be widely dispersed within shallow groundwater systems because these aquifers typically have relatively fast-flowing water and porous sediments.

Elevated concentrations of bacteria in groundwater are commonly attributable to faecal matter leaching from stock dung on the land surface, or from human waste disposal facilities such as septic tanks. However, high bacteria counts do not always represent general groundwater degradation. For example, poor groundwater bore design may allow faecal material from a localised source at the land surface (such as a farm animal defecating near the well head) to leak directly down the shaft.

Ninety per cent of the monitored groundwaters showed no change in concentrations of bacteria over 1995–2006.

Other information on groundwater quality

Regional councils also assess the quality of our groundwater in other ways. In particular, minerals, chemicals, and metals, such as sodium, chloride, sulphate, dissolved iron arsenic, dissolved manganese, and total dissolved solids are measured. Table 10.1 summarises the medians and ranges for these elements in monitored groundwater.

Table 10.1: National medians and ranges for minerals, chemicals, and metals in monitored groundwater, 1995–2006

View national medians and ranges for minerals, chemicals, and metals in monitored groundwater, 1995–2006 (large table).

Salinity

Concentrations of chloride, sodium, sulphate, and total dissolved solids are indicators of salinity. These exceed New Zealand drinking water standards for aesthetic quality at between 0 per cent and 5 per cent of the sites for which data is available (see Table 10.1).

High salinity, which may produce a salty taste, usually indicates that the groundwater is old, but it may also be associated with saltwater intrusion (the movement of salty water into fresh groundwaters) in low-lying coastal aquifers. Saltwater intrusion is a natural occurrence near the sea, but may worsen if too much water is withdrawn from coastal aquifers.

Although it is not widespread in New Zealand, saltwater intrusion is a serious resource management issue for local authorities in some coastal areas of Northland, Waikato (particularly in the Coromandel Peninsula), Manawatū, Tasman, and Canterbury (Ministry for the Environment, 2007b).

Metals: iron and manganese

Dissolved iron and manganese concentrations exceed the New Zealand drinking water standards for aesthetic quality in 27 per cent and 33 per cent, respectively, of groundwater sites for which data is available (see Table 10.1).

Elevated concentrations of iron and/or manganese in groundwater are commonly caused by natural microbial processes (such as respiration by bacteria) or interaction with iron-rich sediments (such as peat), and can produce an unpleasant taste and stain and clog water supply pipes.

If ingested in high enough concentrations, manganese can also harm humans by damaging the respiratory tract and nervous system. Manganese concentrations exceed health-related guidelines at 15 per cent of monitored groundwater sites for which data is available (see Table 10.1).

Metals: arsenic

Arsenic concentrations exceed the drinking water standards of 0.01 milligrams per litre at 10 per cent of sites for which data is available (see Table 10.1). Adverse health effects such as skin cancers and lesions have been observed in populations that drink arsenic-contaminated water in countries such as Bangladesh and India. However, concentrations in these countries greatly exceed the concentrations encountered in New Zealand – sometimes being up to 350 times our national standard (Davies, 2001).

Arsenic in groundwater may originate from both human and natural sources. It is used in a variety of industries, including timber treatment (tanalising), agriculture (herbicides and insecticides), mining, smelting, and pulp and paper production.

Groundwater that spends a long time interacting with arsenic-rich rock, particularly in geothermal areas, may have high arsenic levels. The extent to which natural processes or human activities cause elevated concentrations of arsenic in New Zealand groundwater is not clear.

Other metals

Other trace metals (such as cadmium, chromium, copper, nickel, lead, and zinc) are generally present in low concentrations in New Zealand groundwater and do not pose a risk to human health (Ministry for the Environment, in press b).

Pesticides

Pesticides, including herbicides, insecticides, and fungicides, are commonly used in agricultural activities. Inappropriate use of these agrichemicals may result in groundwater contamination. Drinking water that is contaminated with pesticides can be harmful to both people and stock.

Five national surveys of pesticides in groundwater have been conducted since 1990, the most recent of them in 2006. Note that the limitations of groundwater monitoring are particularly relevant for pesticide surveys. These are described in the box ‘Monitoring groundwater’ at the beginning of ‘Groundwater quality’. These surveys have focused on groundwater that is vulnerable to pesticide contamination; that is, shallow, unconfined groundwater in areas of known pesticide storage and application.

Of 163 groundwater bodies sampled in 2006, 19 per cent had detectable traces of pesticides. This proportion is comparable with earlier surveys, if differences between the survey methods are taken into account (Close et al, 2007).

The pesticide concentrations in all monitored groundwater in the 2006 survey, except one, complied with New Zealand drinking water standards and were similar to pesticide concentrations found during similar recent groundwater surveys in the United States (Close et al, 2007).

Groundwater in which pesticides have been detected usually also have elevated concentrations of nitrates. This further demonstrates the vulnerability of New Zealand’s shallow unconfined aquifers to pollutants leaching from land that is intensively used.

Freshwater demand (allocation)

In this section, current consumptive allocation is compared with historical allocation, the overall size of New Zealand’s renewable freshwater resource, and the amount of water flowing in our rivers.

Consumptive allocation does not include water used for electricity generation, because this water is normally returned directly to the water body from which it was taken; that is, it has a non-consumptive use.

Amount and use of allocated water

If all the water consents (see box ‘More about allocation’) are added together, the total allocation of water in New Zealand (in 2006) is 676 cubic metres every second (Ministry for the Environment, 2006c). The total allocation is equivalent to twice the average flow rate of the Waikato River. The Canterbury and Ōtago regions account for almost three-quarters of the total allocation, with 55 per cent and 18 per cent, respectively (Ministry for the Environment, 2006c).

On a per capita basis, it is estimated that the demand for water is two to three times higher in New Zealand than in most other OECD countries (Organisation for Economic Co-operation and Development, 2006). (This estimate is based on figures for total water use (including water used for economic development as well as domestic purposes) and is indicative only, because the methodologies for estimating water use differ between countries.)

Source of freshwater supply

Almost 20,000 resource consents are in place for taking water, 66 per cent of which are for groundwater takes (Ministry for the Environment, 2006c).

Figure 10.13 (left) reflects the even distribution of the river and stream network across the country and the relative ease with which water can be taken from surface water systems.

In contrast, consents for groundwater takes tend to be grouped together in areas where the water table is shallow and/or aquifers yield relatively high volumes of water (such as the gravel aquifers on the Canterbury Plains or the Auckland volcanic aquifers); that is, where it is most cost-effective to take groundwater (see Figure 10.13, right).

Although the majority of consents are for water from groundwater sources, the volume of water taken from surface water sources is higher. Sixty per cent of the total volume of water allocated comes from surface water sources, 34 per cent from groundwater, and 6 per cent from storage sources such as lakes and dam reservoirs.

There is considerable variation between regions in the proportions allocated from surface, ground, and storage sources. While storage sources contribute a relatively low proportion of the total national water allocation, in some regions (such as Auckland and Gisborne) reservoirs are the major source of supply.

Use of freshwater

As shown in Figure 10.14, on a national basis, 77 per cent of the total weekly allocation is used for irrigation, which is slightly higher than the global average of 70 per cent (Organisation for Economic Co-operation and Development, 2006). The remainder is shared among public water supply, manufacturing and industry, and stock watering. The use of water by manufacturing and industry and for public supply is generally low in New Zealand compared with more populous countries in Europe and North America, and represents a relatively small part of the overall demand in New Zealand.

Figure 10.14: Use of allocated water in New Zealand, 2006

 

Data source: Ministry for the Environment, 2006c.

Text description of figure

Pie chart showing what allocated water is used for

• Stock watering 3%
• Public water supply 9%
• Manufacturing processes 11%
• Irrigation 77%

Regional variations in the use of water are shown in Figure 10.15. In Canterbury, Marlborough, and Tasman, irrigation accounts for more than 80 per cent of water allocations. Water taken for industrial uses makes up a relatively large proportion of the total allocation in Auckland, Waikato, Taranaki, the West Coast, and Southland. Allocations for stock drinking water are underestimated in Figure 10.15, because this is generally a permitted activity under the Resource Management Act 1991 (that is, it does not require a resource consent), so full figures for stock drinking water allocations are not available.

In most cases, consent holders do not use the full volume of water they are allowed under the consent. The proportion of actual water used is highly variable. Regional consents indicate that actual use typically ranges from 20 per cent to 80 per cent of the allocated volumes (Ministry for the Environment, 2006c). Demand for water varies according to factors such as the time of year, the crop type, and the growth stage of the crop. Use of allocated water often declines in the margins of the irrigation season (that is, early and late in the growing season).

Allocation compared with renewable freshwater resource

Compared internationally, New Zealand has an abundance of freshwater. It is ranked 12th out of 193 countries, on a per capita basis, for the size of its renewable freshwater resource (United Nations Educational, Scientific and Cultural Organization, 2006). Within New Zealand, allocated water comprises less than 5 per cent of its renewable freshwater resource (Ministry for the Environment, 2006c).

However, not all of the renewable resource is actually available to be used – much of it needs to be retained in the rivers, lakes, and aquifers to maintain the various values of these water bodies (such as ecological, recreational, and cultural values). Furthermore, water is not always in the right place at the right time for users. A large proportion of New Zealand’s annual rainfall occurs in winter, when demand is relatively low.

Figure 10.16 shows the water allocated from surface water sources relative to the mean (average) annual low flow of rivers. The figure highlights the difference in water availability and demand between the western and eastern regions of New Zealand.

Several eastern regions (Hawke’s Bay, Wairarapa, Marlborough, Tasman, Canterbury, and Ōtago) have surface water catchments that are highly allocated (that is, 20 per cent to 50 per cent of the river flow during low flow periods is allocated to users). Therefore, rivers in these catchments are likely to be under pressure during the drier parts of the year. In these regions, closer regard to managing water resources is required to ensure water takes do not adversely affect aquatic ecosystems or other water users. For example, resource consents may have conditions that restrict water take when river flows are low.

Trends in allocation and irrigated area

Total water allocation in New Zealand increased by 50 per cent between 1999 and 2006. Over this period, allocation increased by almost 80 per cent and 40 per cent from groundwater and surface water sources, respectively, while allocation from storage sources tripled (Ministry for the Environment, 2006c). Figure 10.17 shows that allocation increased in all regions except Northland. This is likely to reflect changes in the way resource consent information is held, rather than an actual reduction in demand for water in the Northland region.

The increase in total water allocation in New Zealand between 1999 and 2006 can largely be explained by the increase in demand for irrigation. The amount of consented irrigated land in New Zealand increased by 52 per cent over this period, which was an annual rate of increase of 7 per cent.

Figure 10.17 shows that in terms of total land area, the biggest increases in irrigation occurred in Canterbury, Ōtago, Hawke’s Bay, and Marlborough. However, relative to the land irrigated in 1999, the regions with the biggest rate of growth are Southland, Wellington, Bay of Plenty, and the Waikato; irrigated land areas have at least doubled between 1999 and 2006 in these regions.

In 2006, the area of total consented irrigated land in New Zealand was just over 970,000 hectares, the majority of it in Canterbury (66 per cent) and the second largest amount in Ōtago (14 per cent).