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How we monitor

Where and how we collect the data

Monitoring sites

We monitor pesticides in groundwater in two networks:

  • Community supplies: 40 school sites. These monitoring wells were selected without any consideration of pesticide use.
  • Regional network: 40 regional sites. Chemical use was considered when selecting some of these monitoring wells, so pesticides are more likely to be detected in groundwater from these sites.

Monitoring frequency

The sites are monitored every four years. The regional network sites were initially sampled in 2003-2004 and the community supply sites in 2004-2005.  The indicator will next be updated in 2017.

Monitoring history

Data was collected in 2012/13.

Previous to this an investigation was undertaken of pesticide contamination of groundwater supplies looking at a likely worst case selection of sites where agrichemicals were known to have been used. This study is reported in an Waikato Regional Council technical report by JC Hadfield and D Smith.

Measurement technique

We collect groundwater samples from each of the monitoring wells, using the groundwater sampling protocol developed by the Institute of Geological and Nuclear Sciences. The samples are analysed in the laboratory for trace levels of organonitrates, organophosphates and organochlorines (OCNPT).

We also check for pH, conductivity, total dissolved solids, alkalinity, CO2, calcium, magnesium, nitrate, hardness, sodium, potassium, chloride, sulphate, boron, total iron, manganese copper and zinc.

Data storage

The data is stored in Waikato Regional Council’s Hydrol database.

How this indicator is compiled

The indicator has four categories for pesticides in groundwater. This indicator shows the proportion of monitoring wells in each category. The categories for pesticides in groundwater are:

Category Concentration (mg/L) Description
Excessive > MAV Occurrence of pesticides exceeds the maximum acceptable value (MAV) for drinking water
High 50-100 % MAV Safe for drinking when sampled, but pesticides detected at over half the MAV
Low < 50% MAV Pesticides detected, but at less than half the MAV
Non-detect < detection limit No pesticides were found at the time of sampling

 

Guidelines and standards

The Ministry of Health specifies the maximum acceptable values for pesticides in drinking water.

This indicator's data file provides pesticide analysis parameters and detection limits.

Limitations

This indicator is limited by the extent to which the selected sites represent the Waikato region’s very large and highly variable groundwater resource.

Care is taken to base this indicator on information from sites which are well documented (with log, construction and location information), distributed widely throughout the region and represent all major aquifer areas. The monitoring networks have been selected primarily on their hydrogeological setting rather than high chemical use.