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Understanding groundwater quality – why it’s not easy Waiology Dec 11

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By Chris Daughney and Magali Moreau

Un-muddying the Waters : Waiology : Oct-Dec 2013Groundwater resources are very important for New Zealand. Groundwater supplies about a third of our abstractive water needs and an even greater proportion of the water required by the agricultural sector.

There is understandably much concern about groundwater quality, particularly in terms of nitrate. High concentrations of nitrate in groundwater that is used for drinking can impair oxygen transport through the bloodstream, particularly in infants. High concentrations can also pose risks to aquatic ecosystems where nitrate-rich groundwater discharges to rivers, lakes and estuaries.

So what is the state of health of New Zealand’s aquifers, as far as nitrate goes? There are a couple of reasons why this question is not easy to answer.

The first challenge is to determine the concentration of nitrate that we should expect in the absence of human influence. It is hard to define this “baseline” because many of our aquifers are already impacted to some degree by human influence, meaning that nitrate concentrations are not representative of natural conditions. One elegant way to estimate the baseline condition is to compare the age of groundwater to the concentration of nitrate that it contains. Application of this technique shows that nitrate concentrations in New Zealand’s groundwater were typically below 1 mg/L (as NO3-N) before the export meat industry started. A similar conclusion was reached in a separate study that used a statistical technique (without water dating) to show that nitrate concentrations above 1.6 mg/l are probably indicative of human influence, whereas nitrate concentrations above 3.5 mg/L are almost certainly caused by human activity.

Idealised shape of the capture zone for a well in a homogeneous isotropic unconfined aquifer. The regional groundwater flow direction is from right to left (modified from Ministry of the Environment, British Columbia 2004).

Idealised shape of the capture zone for a well in a homogeneous isotropic unconfined aquifer. The regional groundwater flow direction is from right to left (modified from Ministry of the Environment, British Columbia 2004).

The second difficulty is that capture zones have been mapped for very few wells in New Zealand. A capture zone is the area of land through which rain or river water enters the aquifer and is ultimately extracted from the well of interest. The land use activities within the well’s capture zone can alter the groundwater quality. So if the capture zone has not been mapped, if we find nitrate concentrations above the baseline, the source of the nitrate may not be identifiable. This leaves us asking “where did this nitrate come from?”

A third difficulty is that groundwater flow is relatively slow. This results in a time lag between nitrate infiltration into the aquifer in a well’s capture zone and detection of that nitrate at the well some distance away. The groundwater dating techniques mentioned above can assist us to understand these time lags, but there are still many locations around New Zealand at which groundwater ages remain unknown. This leaves us with the question “when did this nitrate enter the aquifer?”

Returning to the question posed above, what is the state of health of New Zealand’s aquifers, as far as nitrate goes?

A recent survey evaluated the concentrations of nitrate in groundwater from over 900 wells across the country, using measurements made between 1995 and 2008. Roughly 1/3 of the tested wells had median nitrate concentrations above 3.5 mg/L, which as mentioned above is the upper threshold for natural conditions. The wells with above-baseline nitrate concentrations were found across the country, indicating pervasive degradation of groundwater quality. At roughly 5% of the wells tested, the nitrate concentration was in excess of the drinking water standard that is set for protection of human health.

Although relationships between groundwater nitrate concentrations and well depth were observed, no relationships to land use or land cover around the wells were detected. This lack of relationship between groundwater quality and land use around the wells is in fact a common result that has been observed in several studies in New Zealand and overseas. It can be explained by the factors presented above: it is hard to understand relationships between groundwater quality and land use unless the age and source (capture zone) of the groundwater are known.

Given the importance of groundwater to this country, and given the evidence that groundwater quality is pervasively degraded relative to natural conditions, there is urgency for better understanding of our aquifers. Determining groundwater age and mapping capture zones must become priority activities. Without the ability to unequivocally relate cause (land use) to effect (increasing nitrate concentrations), we will keep telling each other “it wasn’t me!”


Dr. Chris Daughney is the Director of the National Isotope Centre, and Magali Moreau is a groundwater geochemist, both GNS Science.

References
Ministry of Environment (British Columbia). (2004) Well Protection Toolkit Step 2 (electronic resource). 24 p.; last accessed 30/01/2013.
Morgenstern, U.: Daughney, C.J. 2012. Groundwater age for identification of baseline groundwater quality and impacts of landuse intensification – The National Groundwater Monitoring Programme of New Zealand. J. Hydrol., Vol. 456–457: 79–93.
Daughney, C.J.; Raiber, M.; Moreau-Fournier, M.; Morgenstern, U.; van der Raaij, R.W. 2012. Use of hierarchical cluster analysis to assess the representativeness of a baseline groundwater quality monitoring network : comparison of New Zealand’s national and regional groundwater monitoring programs. Hydrogeology journal, 20(1): 185-200
Daughney, C.; Randall, M. 2009. National Groundwater Quality Indicators Update: State and Trends 1995-2008, GNS Science Consultancy Report 2009/145. 60p. Prepared for Ministry for the Environment, Wellington, New Zealand.

Nitrate in Canterbury groundwater Waiology Nov 27

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By Carl Hanson

Un-muddying the Waters : Waiology : Oct-Dec 2013Nitrate concentrations in Canterbury groundwater have been prominent in the media recently. Headlines have included phrases like “ticking time bomb”, “scaremongering” and “freaking out much of Canterbury”.

What I want to do in this article is to present the state of nitrate concentrations in Canterbury groundwater, and the trends we see in those concentrations, as objectively as I can, avoiding any emotive language.

First, the concentrations. Based on the data from our regional long-term monitoring programme, which includes approximately 300 wells distributed across the region, nitrate concentrations in Canterbury groundwater fall into two groups:

  • About 30% of the samples we collect have concentrations less than 1 mg/L (recorded as nitrate nitrogen).
  • The rest of the samples have concentrations distributed over a broader range, with an average of about 5 to 6 mg/L and the highest concentrations exceeding 20 mg/L.

Risk of finding groundwater in Canterbury with nitrate concentrations above the MAV (C. Hanson).

Risk of finding groundwater in Canterbury with nitrate concentrations above the MAV (C. Hanson).

Groundwater with low nitrate concentrations is found in the areas coloured green on the map to the right. This groundwater is derived mainly from alpine rivers, which have low nitrate concentrations themselves. In addition, the groundwater in some areas is anoxic as a result of organic material in the aquifer sediments. We find this particularly along the northern coast of the Canterbury Plains, where the land has historically been covered by swamps. Nitrate is generally not compatible with anoxic environments.

Groundwater with higher concentrations is found in the areas coloured yellow and red on the map in Figure 1. This covers most parts of the region where groundwater is used. The red colouring shows areas where we frequently measure concentrations greater than the New Zealand drinking-water standard (the Maximum Acceptable Value, or MAV, equal to 11.3 mg/L when reported as nitrate nitrogen).

In the yellow areas, concentrations are variable. They change from one location to another, they change with depth, and they change over time. Nitrate concentrations often display a seasonal pattern, with the highest concentrations occurring in the springtime after winter rainfall. Within these yellow-coloured areas, concentrations are generally below the MAV, but they may exceed the MAV in some years and/or at some locations.

There have been some questions in the media as to the source of the nitrate contamination. In my opinion, farming is the main source. Point sources like septic tanks and wastewater discharges contribute discrete, localised plumes of contamination, and in some cases the resulting concentrations are quite high. But we see elevated nitrate concentrations across the entire region. Nitrate contamination is ubiquitous. If you drill a well almost anywhere within the yellow and red areas on the map above, you will find groundwater with nitrate concentrations that are higher than they would be naturally. Contamination of this extent can only be explained by diffuse leaching from the farmland that covers the region. There are no other plausible sources.

Regarding trends, we can say comfortably that nitrate concentrations are increasing. Over the past ten years (2003-2012), we’ve noted increasing trends in about 30% of the wells in our regional-scale long-term monitoring programme. In contrast, we find decreasing trends in only 3% of the wells. Concentrations in the rest of the programme wells rise and fall over time but show no clear long-term trends, either upward or downward.

Trends in nitrate concentrations in Canterbury groundwater, 2003 to 2012 (C. Hanson).

Trends in nitrate concentrations in Canterbury groundwater, 2003 to 2012 (C. Hanson).

The results have been similar since our monitoring programme began in the 1980s. For any time period that we look at, we find that while most of the wells in the survey show no clear trend, the trends that do show up are mostly increasing. The increases are distributed across the region.

Concentrations have probably been increasing since farming began in Canterbury. As more land has been cultivated and farmed, as more fertiliser has been added to increase crop production, and as more stock have been grazed on the paddocks, the nitrogen balance in the soil has increased, and more nitrate has been available to leach into the groundwater.

Finding ways to minimise nitrate leaching from farming is a key part of Environment Canterbury’s work under the Canterbury Water Management Strategy.


Carl Hanson leads the groundwater quality team at Environment Canterbury.

Monitoring the diversity of NZ groundwater quality Waiology Nov 18

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By Magali Moreau, Chris Daughney and Zara Rawlinson

Un-muddying the Waters : Waiology : Oct-Dec 2013To date, more than 200 aquifers have been mapped across the country. These aquifers vary widely in their volumes, depths, host-rock lithologies, related geological structures, water circulation pathways and water age. Our knowledge of the individual characteristics of these aquifers grows as we gather more data through our active monitoring networks.

Active monitoring of groundwater is undertaken by the National Groundwater Monitoring Programme (NGMP) and, at the regional scale, by the State of the Environment (SOE) networks. Initiated in 1990, the NGMP achieved national coverage in 1998 (currently 108 sites) and is a collaboration between GNS Science and 15 regional authorities. Groundwater samples are collected quarterly at NGMP sites and tested for more than 25 parameters using a consistent sampling protocol and suite of analytical procedures. At the regional scale, SOE networks are operated by each regional authority, with groundwater samples collected at varying intervals (from monthly to annually). Nation-wide, the total number of monitored SOE sites exceeds 1,000. Monitored groundwater indicators vary between each region based on individually set regional monitoring objectives. The denser SOE network is accurately represented by the NGMP, which provides a perspective on groundwater quality throughout New Zealand.

Key groundwater chemical indicators used to assess the state and trends are:

  • Nitrogen species (nitrate-nitrogen and ammoniacal-nitrogen) for environmental and health reasons (blood circulation disorder). Both species need to be monitored because under low oxygen conditions, nitrate-nitrogen is converted to ammoniacal-nitrogen by natural processes. Groundwater is unsuitable for human consumption when its nitrate-nitrogen concentration exceed the Drinking Water Standard for New Zealand (DWSNZ) maximum admissible value (MAV) of 11.3 mg/L.
  • E. coli, indicative of faecal contamination. Groundwater used for drinking-water purposes should not contain any E. coli.
  • Iron and manganese, indicators of the oxidation state of groundwaters (high concentrations of these ions occur in anoxic conditions). The aesthetic guideline values for iron and manganese concentration are 0.2 mg/L and 0.04 mg/L, respectively (unpleasant taste and for manganese, laundry staining). Chronic health problems can also appear if groundwater containing manganese concentration above 0.4 mg/L is ingested. Some elements such as arsenic can also become mobile in groundwater under anoxic conditions.
  • Electrical conductivity is an indicator of total dissolved solids content. Infiltrated rain water conductivity will naturally increase as groundwater flows through the aquifer material, due to water-rock interaction. Electrical conductivity is a good indicator of saltwater intrusion in coastal aquifers.

New Zealand aquifers and groundwater types at NGMP sites (adapted from White et al., 2001 and Daughney and Reeves, 2005).

New Zealand aquifers and groundwater types at NGMP sites (adapted from White et al., 2001 and Daughney and Reeves, 2005).

Based on data from the NGMP, three categories of groundwater quality have been identified in New Zealand:

  1. “Natural fresh” groundwater (32% of NGMP sites, shown on the map in white) has little to no evidence of human impact and is chemically similar to clean river water. NGMP sites that have this type of groundwater are found throughout New Zealand with high proportions in central Otago and in the West Coast.
  2. “Natural evolved” groundwater (26% of NGMP sites, shown on the map in green) has chemistry that is indicative of natural water-rock interaction and may contain elevated concentrations of iron, manganese and/or arsenic. Most of the NGMP sites with this character are found in the North Island, for example Gisborne.
  3. “Impacted” groundwater (42% of NGMP sites, shown on the map in red) has nitrate concentration above natural levels due to some human or agricultural influences, and neither iron nor manganese are present. NGMP sites in this category are found across the country, but especially in regions of more intensive agriculture, such as Southland and Waikato. Only 11% of all NGMP sites have nitrate concentration above the guideline for protection of aquatic ecosystems, and only 5% of NGMP sites have nitrate concentration above the DWSNZ MAV.

Magali Moreau is a groundwater geochemist and manager of the National Groundwater Monitoring Programme, Dr. Chris Daughney is the Director of the National Isotope Centre, and
Zara Rawlinson is a geophysicist at GNS Science.

References:
Ministry of Health. (2005) Drinking-Water Standards for New Zealand 2005. Ministry of Health, Wellington, New Zealand.
Daughney, C. J.; Randall, M. (2009) National groundwater quality indicator update: state and trends 1995-2008. GNS Science Consultancy Report 2009/145. 62p.
Daughney, C.J.; Reeves, R.R. (2005) Definition of hydrochemical facies in the New Zealand National Groundwater Monitoring Programme. Journal of Hydrology, New Zealand, 44(2): 105-130.
Rosen, M. R.; White, P.A. (2001) Groundwaters of New Zealand. New Zealand Hydrological Society Publication. 498 p. ISBN 0-473-07816-3

Un-muddying the Waters: Series on NZ water quality Waiology Oct 14

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By Daniel Collins

Un-muddying the Waters : Waiology : Oct-Dec 2013The state of our waters is a hot button topic. Water quality has become an election issue from Southland to Northland, from towns to nation, and it is often in the news. Swimming water quality and implications of the proposed Ruataniwha water storage and irrigation scheme in Hawke’s Bay are cases in point.

 Sediment plume from Hinemaiaia Stream into Lake Taupo (D. Rowe).

Sediment plume from Hinemaiaia Stream into Lake Taupo (D. Rowe).

What is at stake varies from place to place, but in general it is our health, wealth and well-being, which include industrial activity and the environment’s natural character and ecosystem services.

These high stakes are reflected in public surveys. In a 2010 report from Lincoln University, water pollution and other freshwater issues were identified as the most important environmental issues facing New Zealand. While respondents generally thought our waterways were at least in adequate condition, rivers and lakes ranked the worst. And in terms of meeting our “clean, green” image, a 2013 report by Horizon Research found that addressing river and lake water quality topped the to-do list, followed by farm runoff and industrial discharges.

The ‘Dirty Dairying’ campaign mounted by Fish and Game NZ in 2002 no doubt help prime these opinions, while the catch phrase remains in use to this day. But land and freshwater practices are also changing. The campaign was followed a year later by the signing of the voluntary Dairying and Clean Streams Accord between Fonterra, regional councils, and the Ministries for the Environment and of Agriculture and Fisheries, which high-lighted a to-do list for farmers. The 2003 Accord was succeeded by the Sustainable Dairying Water Accord in August 2013, this time a mandatory agreement involving all dairy companies.

Given the nature of the debate, then, it is no surprise that the messages we receive via the media may be as muddied as the rivers or estuaries we’re trying to protect. Is the Manawatu River one of the dirtiest in the world? Are we “100% Pure”?

The importance of these questions and the complexity of the science behind them led the Parliamentary Commissioner for the Environment to produce a report in 2012 on the main concerns of water quality – pathogens, sediment and nutrients – and why they are concerns. It was designed as a guide to the complex science for the public and freshwater professionals alike. But so much of the important science and management frameworks are out of reach from the public, even from many of the freshwater professionals ourselves, either because they don’t know where to look or don’t have the background to fully make sense of it.

What would be valuable, and somewhat lacking in New Zealand when it comes to freshwater issues, is a communication middle ground – an open forum that fills the gulf between what we hear on the radio and what we hear at a conference. On the one hand this would benefit the public as more and more people engage in collaborative decision-making. On the other hand, freshwater professionals would also benefit as they make their research or policy planning more interdisciplinary.

So it is with this middle ground in mind that Waiology has convened the current series of articles on water quality. The series brings together over 25 experts from across scientific, educational, environmental, industrial, and regulatory organisations to answer important questions:

  • What is the state of our freshwaters?
  • How is it changing?
  • Why it is changing?
  • What are the implications?
  • And what could or should we do about it?

Articles will range from measurement to management, with two or three articles published each week from October to December. So stay tuned, subscribe, and take part in the discussion.


Dr Daniel Collins is a hydrologist and water resource scientist at NIWA.

Phreatogammarus fragilis: The fragile well shrimp Waiology Dec 06

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By Daniel Collins

Phreatogammarus fragilis is an endemic New Zealand crustacean that lives in aquifers. It is an amphipod (a relative of the sand hopper), and is one of the largest (commonly up to 25 mm excl. antennae) and strongest swimming of NZ’s stygofaunal* crustaceans. Because it is so rarely observed, it does not have a common name; the best translations are ‘fragile well shrimp’ or ‘fragile groundwater lobster’, ‘fragile’ probably because its appendages broke off when early specimens were being identified and preserved.

The individual below is a 12 mm-long female with a brood pouch beneath the abdomen. It is white and translucent because there is no point in investing in pigments if it’s too dark to see or if there’s no risk of sunburn. This individual was caught in a trap in a 6 m-deep well beside the Selwyn River in Canterbury by Nelson Bousted and identified by Graham Fenwick. It was photographed live in water in a custom-built aquarium with several off-camera flashes.

Photo credit: Nelson Boustead

Waiology will have some more in-depth science of stygofauna in a future post.

* Stygofauna: animals that live in groundwaters, named after the river in Greek mythology, the Styx, which separated the Earth from the Underworld.


Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.

Map: Projected effects of climate change on New Zealand freshwaters Waiology Nov 27

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By Daniel Collins

Maps are helpful tools in communicating and understanding the potential implications of climate change. We have national maps of projected changes in temperature that show faster warming in the north, and in precipitation that show more rain in the south and west and less in the north and east. We also have national maps of projected changes in drought, that show much of the country is likely to experience more severe droughts.

Now, I am able to give you a map of the potential freshwater changes across New Zealand. This includes changes in snow, ice, river flow, groundwater, aquatic ecology, geomorphology, and water use/management.

This is an important step in synthesising and understanding climate change impacts, drawn from existing case studies across the country. Projections are pin-pointed on the map below; in some cases they are more national in scope (e.g., salinisation of coastal groundwater).

This illustrates quite a complex picture. Retreating snow and ice. More flow in Alps-fed rivers, less flow in others. Higher lake levels and lower lake levels. More water demand from both agriculture and city. Higher erosion as well as channel aggradation. Higher lake nutrient levels and more frequent algal blooms.

There is a lot we know but also a lot we don’t know. As yet, we cannot provide a complete national assessment for river flows, nor for groundwater recharge. And very little research has connected the dots between climate change and aquatic ecology. But as new studies are carried out this map will be expanded and the gaps filled in.

In the near future I will describe the projected changes in more detail, so stay tuned.


Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.

Canterbury Lysimeter Network: Measuring the hydrologic inputs to aquifers Waiology Jan 30

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By MS Srinivasan

Irrigated agriculture is growing in Canterbury. This growth has resulted in a greater rush for accessing water resources — surface and ground waters – across the region. Since these water resources are finite, limits on their takes are imposed to conserve them and make them available for other uses. However, setting limits on groundwater has remained a challenge. There are more unknowns than knowns. Does groundwater recharge occur uniformly over a year? Does irrigation add to recharge? How much groundwater could be allocated for irrigation sustainably?

To answer these questions, we need to take a closer look at how much water is draining through the ground and recharging the aquifers, as Paul White discussed two weeks ago. In Canterbury, where there is substantial abstraction from groundwater and where groundwater-fed streams and rivers are vitally important, a new initiative has begun to provide some much-needed data.

The Canterbury Lysimeter Network (CLN) is a scientific endeavour to identify how much land-based recharge, from rainfall and irrigation, happens across the Canterbury Plains at daily, monthly, seasonal, and annual time scales.

The CLN was originally an Environment Canterbury initiative, with NIWA, Aqualinc Research and HydroServices brought in to make it happen. Currently, the CLN has four sites spread across the Canterbury Plains, representing a variety of conditions — foothill vs coastal, high vs low rainfall, well-draining vs poorly-draining soils. All sites are under irrigation.

Each site has a set of three drainage lysimeters, and each lysimeter is 700 mm deep and 500 mm in diameter. A drainage lysimeter is a large cylinder, buried vertically in the soil. These lysimeters measure how much water drains through a soil column. The volume of water draining through a lysimeter is a direct expression of recharge of groundwater. The lysimeters are buried with their top at the soil surface. Water that infiltrates through the lysimeters is collected and measured at the bottom.

CLNSites

CLNlysimeters

Using these continuously recording lysimeters, we can find out how much water drains towards aquifers and when. Monthly data from one of the lysimeter sites, below, shows that groundwater recharge is largely occurring in autumn and winter, outside the irrigation season, even if more water falls during the summer.

CLNmonthlydepths

If we look more closely, we can see that it’s the longer or more intense storms that tend to generate large groundwater recharge.

CLNrechargeevents

What’s next? Our data collection has just started. What we have shown here are preliminary results. We are looking to continue this network over the next few years to quantify precipitation and irrigation recharge over time. Data from this network will also be useful in verifying models of recharge and soil moisture developed by NIWA and others, and allowing better management and allocation of groundwater in Canterbury.

Kinky relationships among Canterbury’s springs Waiology Jan 23

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By Daniel Collins

As Ross mentioned some time ago, one of the frontiers of hydrological research at present is the interface between surface water and groundwater. On the one hand, we need to understand how aquifers are recharged from the surface; on the other, how aquifers in turn discharge water back to the surface. This is important to water resource managers so that they can determine how water use at one location may affect water availability and aquatic ecosystems elsewhere.

One question, particularly relevant to both Canterbury and Hawke’s Bay, is how fluctuations in groundwater levels affect spring flow near the coast. This is actually part of a larger research programme we have looking at the environmental effects of water use.

As part of this research I have been examining streamflow and groundwater data around Lake Ellesmere/Te Waihora, Canterbury. Environment Canterbury has an extensive network of groundwater monitoring wells and streamflow monitoring sites in the area. The wells measure what is known as the piezometric head of an aquifer, derived from the Greek piezein meaning ‘to press or squeeze’, and which refers to the height that water would rise to if subject to atmospheric pressure. For the top-most aquifer, this height basically means the water table (see this earlier post on groundwater anatomy for more).

IrwellSWGWBy comparing piezometric and streamflow data, we can develop a picture of how fluctuations in groundwater translate into fluctuations in streamflow. In the figure below I’ve chosen just one well (L36/0141) and one stream (Irwell River). The first thing you see is a roughly linear increase in flow with piezometric height, at least on the right. Linear relationships like this are typical of water flowing through a saturated porous medium (see Darcy’s law for more). But where the flow drops to around zero, at a piezometric height of about 68 m above mean sea level, there is a hint that this line bends — a kink in the groundwater-streamflow relationship. In fact, it has to bend because you can’t go lower than zero flow.

This kink indicates a threshold groundwater level that divides streamflow behaviour in two. Above the threshold, streamflow increases roughly in step with groundwater level; below the threshold, there is basically no flow at all, no matter how low the groundwater drops. So if the level in well L36/0141 drops below about 68 m AMSL, Irwell River is likely to go dry.

While in some instances this drying is entirely natural, changes in groundwater recharge (say, due to climate change) and abstraction for irrigation could shift the hydrological regime of the stream to become more ephemeral. It could also mean less water flows into Lake Ellesmere. This is all very important when allocating water while avoiding undue water resource or environmental impacts.

Threshold behaviour like this is actually quite common in hydrology, and can be traced back to the underlying physics of water movement. Having a physical explanation of the interaction between aquifers and streams allows us to make more robust predictions of spring flow, and of the ecological and water resource implications that follow.

Rainfall recharge to groundwater Waiology Jan 17

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Guest post by Paul White, Senior Groundwater Scientist at GNS Science.

Groundwaters are very important water resources in many New Zealand regions — important because they are used for water supplies (urban and rural) and because they supply flow to many springs, streams, rivers and wetlands. The two major inflows to groundwater are from rainfall and from surface water.

We need to know the rates of recharge to groundwater so we can manage groundwater use. For example, groundwater use must be significantly less than groundwater recharge to ensure that groundwater wells and springs do not go dry.

Groundwater recharge from rainfall is the subject of this post which will cover some concepts, how it is estimated, measured, uncertainty and some relevant New Zealand water management polices.

Groundwater recharge from rainfall occurs as rainfall trickles through the soil into aquifers. However, only a portion of all rainfall actually reaches aquifers as recharge. This is because some rainfall evaporates from the ground, and some rainfall is transpired by plants back into the atmosphere – processes termed evapotranspiration. Groundwater recharge first reaches a shallow aquifer (termed the water table or unconfined aquifer). Then the recharge may discharge from the unconfined aquifer to surface waters or to deeper, confined aquifers. Flow paths in aquifers are typically understood using models of groundwater flow systems including geological models and groundwater flow models.

It is common for rainfall recharge to be the largest source of groundwater recharge. In those circumstances sustainable groundwater allocation policies should ensure that allocation is less than recharge and that actual use is less than allocation, i.e.

R > A > U

where
R = rainfall recharge estimate provided by science, e.g. projects to characterise rainfall recharge and uncertainty undertaken in the Waterscape research programme;
A = allocation of groundwater, which is a policy decision by the groundwater management authority;
U = use of groundwater.

Estimates and measurements of rainfall recharge are very useful for the development of groundwater allocation policies. Regional councils are responsible for policy decisions on groundwater allocation. Central government also has an input to decision-making. For example a National Environmental Standard, proposed by Ministry for the Environment (2008), recommends a default (in lieu of regional policies) maximum groundwater allocation as 35% of groundwater recharge.

Estimates of groundwater recharge from rainfall are often made using computer models that typically consider rainfall, evapotranspiration and soil properties. Measurements of groundwater recharge can be made with lysimeters — this is typically a tube sunk into the ground that encases a soil column. Water flow from the base of the lysimeter column is measured over time. Models of rainfall recharge at the local scale are typically tested against measurements of groundwater recharge at the local scale. Quantifying rainfall recharge at the regional scale involves the use of models and up-scaled measurements from lysimeters.

Rainfall recharge measurements demonstrate significant inflows of rainfall to groundwater. For example, the Canterbury lysimeters measured groundwater recharge in the range 26% (Lincoln) to 37% (Winchmore) of rainfall in the period 1999 — 2000 (White et al. 2003). These results were used to estimate regional rainfall recharge to groundwater in the area between the Waimakariri River and Rakaia River in the range 19.2 to 23.9 m3/s providing a useful indication of groundwater sustainability in comparison with annualised groundwater allocation (approximately 42.6 m3/s) and estimated groundwater use (6.8 m3/s) in the period.

Environment Canterbury established groundwater allocation zones in 2004 and adopted allocation limits based on estimates of land surface recharge (rainfall plus irrigation). Zones in which total allocation exceeded those limits were designated as being fully allocated and referred to as ‘red zones’. The Rakaia-Selwyn groundwater allocation zone has been classified as a ‘red zone’ from the initial adoption of this management policy and Environment Canterbury has recommended decline of further groundwater allocation (e.g. in the Rakaia-Selwyn hearings), reviewed groundwater consents, and placed annual volume limits on groundwater pumping.

This post shows some of the applications rainfall recharge measurements to groundwater resource management and groundwater resource characterisation.

References:

Ministry for the Environment 2008. Proposed National Environmental Standard on Ecological Flows and Water Levels. Discussion Document. 61p.

White, P.A., Hong, Y-S., Murray, D., Scott, D.M. Thorpe, H.R. 2003. Evaluation of regional models of rainfall recharge to groundwater by comparison with lysimeter measurements, Canterbury, New Zealand. Journal of Hydrology (NZ) 42(1), 39-64.

Related Waiology posts:

The low down on groundwater
The importance of groundwater
Where does NZ take its water from?

Where does NZ take its water from? Waiology Nov 28

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By Daniel Collins

I mentioned previously how much water we are allowed to use in NZ. The amount varies markedly from region to region, and is growing over time, with Canterbury and Otago accounting for well over half of the consumptive takes (excluding the Manapouri hydro scheme). But seeing as Kiwis seem to know less about our aquifers than our rivers, I’d like to turn now to the issue of where we can take our water from — rivers, lakes, aquifers or reservoirs (or storage lakes).

SWGWAllocation

About two thirds of the water NZers are allowed to take is from surface water — rivers and lakes. This is for all non-hydro schemes and other non-consumptive uses; the data come from a 2010 report from Aqualinc Research for MfE. About a third comes from groundwater. Five percent comes from reservoirs, which are largely fed by rivers.

Looking across New Zealand, we see that the relative importance of surface water, groundwater and reservoirs differs among regions. In Otago, surface waters are relied upon for 84% of the allocated water supplies; in Auckland, it’s 4%. In Hawke’s Bay, groundwater accounts for 74%; in Otago and Taranaki it’s 7%. And in Auckland, reservoirs account for 74%; in Waikato and Manawatu-Wanganui it’s zero.

So why do these proportions vary so much?

There are three key factors behind this: whether there’s enough water in the rivers; whether there are accessible and productive aquifers; and how important reliability of supply is to the water user.

Otago, for example, only has a smattering of significant aquifers, so they get most of their water from rivers. Hawke’s Bay, Southland and Tasman each have productive aquifer systems available. In the Auckland region, where over a third of NZ’s population needs to be provided with a reliable supply of drinking water, they turn to reservoirs and more recently to a reliable supply from the Waikato River via a tunnel. On the Canterbury plains, water from rivers and aquifers are both readily accessible and both highly used, though it is typically easier and cheaper to take water from rivers, even if the reliability of supply isn’t as high. You can see maps of NZ’s key aquifer systems here.

So we can see, yet again, that the physical environment has a huge effect on the availability and reliability of water supply in New Zealand. In posts to come, we’ll see what exactly this water is used for.

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