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Arsenic in Groundwater of the Waikato Region

Report: 2006/14
Author: Jeremy Piper and Nick Kim


Arsenic is a toxic but naturally occurring chemical element, present at low levels in all soil, water, plants, animals, and foods.  For members of the general population, most exposure to arsenic is through the small amounts of this element that are naturally present in food and drinking water.

Exposure to too much arsenic can result in a range of cancers, and a wide range of non-cancer effects (Table 1).  Population-wide studies overseas have shown that the most common cancers caused by too much arsenic in drinking water are those of the bladder and lung.  Susceptibility of an individual to such conditions is partly determined by genetic factors (Ghosh et al., 2005), and exposure while young appears to amplify risks of developing arsenic-related diseases later in life (Smith et al., 2006).  In most countries, the probability of such conditions occurring is kept to a practical minimum by setting upper limits for arsenic in food and drinking water.  In New Zealand, the tolerated maximum concentration of arsenic in drinking water is 10 µg/L (Ministry of Health, 2005).

In previous work on arsenic in the Waikato Region’s water supplies, the focus has been on surface waters, particularly the Waikato River.  To date, little has been documented about the prevalence and geographical distribution of arsenic in Waikato groundwater. However, such information is of public health significance, because groundwater comprises a major source of drinking water in the Waikato Region’s rural areas. The rural population of the Waikato Region is estimated to comprise about 75,000 people.    

In addition to implications for human exposure and health, there is a possibility that some agricultural practices may cause a gradual increase in concentrations of arsenic in regional groundwater over time.  If it were occurring, such an effect might require development of new management approaches to safeguard the groundwater resource.  However, in order to either confirm or discount the existence of a link between agricultural activities and arsenic in groundwater, there is a need for robust baseline data, against which future data can be reliably compared.

Over the last few years, arsenic analysis has been carried out on a total of 302 groundwater samples collected by Environment Waikato staff.   The aim of this report was to compile and assess these analytical results to present the first region-wide picture of arsenic concentrations in Waikato groundwater.  This information can then be used for a number of purposes, from protection of human health to gaining a better understanding of arsenic geochemistry in regional groundwater.  Features of this report include the following:

  • Estimates of rates of compliance with drinking water standards on both a regional basis and for sub-regional areas.
  • Identification of areas of the Waikato region with comparatively high arsenic concentrations in groundwater.
  • Identification of mechanisms that can result in high arsenic concentrations in Waikato groundwater.
  • Delineation of the chemical form (speciation) of arsenic in cases where drinking water standards are exceeded.
  • Assessment of the reliability of point-in-time measurements at specific locations, in relation to variation of arsenic concentrations in groundwater with time.
  • Discussion of possible links between concentrations of arsenic in Waikato groundwater and specific agricultural practices.

Arsenic in Groundwater of the Waikato Region
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Table of contents

1 Background 1
1.1 Rationale and scope 1
1.2 Technical background 2
1.2.1 Human exposure to arsenic in drinking water 2
1.2.2 Occurence of arsenic in groundwater 4
1.2.3 Other New Zealand studies 7
2 Methodology 10
2.1 Sample sites 10
2.2.1 General sampling and determinands 10
2.2.2 Sampling for speciation determination 10
2.2.3 Analysis 12
3 Results and discussion 14
3.1 Regional summary statistics 14
3.2 School supplies 14
3.3 Temporal variability 16
3.4 Reasons why arsenic becomes elevated in Waikato groundwater 19
3.4.1 General 19
3.4.2 Possible relationships with water-type 19
3.4.3 Correlations 22
3.4.4 Linear regression 25
3.4.5 Summary 25
3.5 Arsenic speciation in cases where arsenic consentrations in groundwater are high 26
4 Potential effects of resource use on arsenic in groundwater 31
4.1 General 31
4.2 Dissolved organic matter 31
4.3 Dissolved anions 32
5 Findings 34
5.1 Occurence of high arsenic in Waikato groundwater 34
5.2 Reasons why arsenic becomes elavated in Waikato groundwater 34
5.3 Arsenic speciation 35
5.4 Potential influence of resource use 35
6 Recommendations 36
6.1 Individual water supply bores, human exposure and risk 36
6.2 Regional trends and speciation 36
7 References 37