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Factsheet: Calculating groundwater quality trends

What do we mean by ‘trend’?

Groundwater quality is always changing. It may show short-term changes, like responses to rainfall events, chemical spills or groundwater pumping. It may show seasonal cycles, and it may respond differently to periods of wet or dry years. On top of these shorter-term variations, groundwater quality may also show gradual changes over periods of many years or decades.

It is these longer-term changes that we refer to as ‘trends’. The trend is the overall direction of change over the selected time period. LAWA presents trends calculated for the last 10 and 15 years.

 

LAWA trend categories for groundwater quality

LAWA categorises trends into five classes.  These are: very likely degrading, likely degrading, indeterminate, likely improving, and very likely improving. 

 

Improving Trends          

Very likely improving trend Likely improving trend
Very likely improving trend Likely improving trend

 

 

 

 

 

 

The improving trend symbols are used for sites that show an improving trend in water quality and are either classified as 'very likely improving' or 'likely improving'.  For the five groundwater quality indicators on LAWA, an improvement is a decrease in values over time.

A 'very likely improving' trend is given when there is 90 - 100% likelihood of an improving trend.  A 'likely improving' trend is given when there is 67 - 90% likelihood of an improving trend.  The lower certainty reflects the fact that while there is an indication of an improving trend, there is less statistical support for it. 

With enough data, the trend evaluation method can detect even a very subtle trend.  In these cases the trend can be classified as improving even if there is only a small change in water quality over time.

 

Indeterminate Trend

Indeterminate trend_Sept2018
Indeterminate trend

This classification is given to sites where there is insufficient evidence to confidently determine if water quality is showing an improving or degrading trend.  An indeterminate trend means that the data do not show an upward or downward trend direction with sufficient statistical certainty. 

 

Degrading Trends  

Very likely degrading trend Likely degrading trend
Very likely degrading trend Likely degrading trend

The degrading trend symbols are used for sites that show a degrading trend in water quality and are either classified as 'very likely degrading' or 'likely degrading'. For the five groundwater quality indicators on LAWA, a degrading trend is an increase in values over time.

A 'very likely degrading' trend is given when there is 90 - 100% likelihood of a degrading trend.  A 'likely degrading' trend is given when there is 67 - 90% likelihood of a degrading trend.  The lower certainty reflects the fact that while these is an indication of a degrading trend, there is less statistical support for it.

With enough data, the trend evaluation method can detect even a very subtle trend.  In these cases, the trend can be classified as degrading even if there is only a small change in water quality over time.

 

Trend Not Assessed 

 

      Not Assessed

Sites are not assessed for trends when they do not meet the criteria to be included in the trend analysis (e.g. there were not enough data/samples over the period, not enough variability in the data to assess a trend, too many below-detection-limit measures in the data, or long runs of the same value).  

 

How do we calculate water quality trends? 

Data requirements

Groundwater quality trends were calculated for the last 10 and 15 years. Data were evaluated to determine whether water quality is showing improving, degrading, or indeterminate trends.

The data used to calculate groundwater quality trends is generally collected quarterly. Median values were used when multiple samples are collected within a quarter. LAWA does not calculate trends for wells that are only sampled annually. A few wells are sampled monthly and have trends analysed on a monthly basis instead of quarterly. If these wells do not meet the monthly data requirements, the monthly data are reduced to quarterly values by taking a median value for each quarter and analysed on a quarterly basis.

Monitoring wells were excluded from the analysis if they had less than 75% of the data expected, and data from less than nine (or thirteen) out of the last ten (or fifteen) years.  They were also excluded if they had less than 75% of the most recent year’s (2019) data (3 of 4 quarters or 9 of 12 months).

Censored values (data that are less than or greater than laboratory detection limits) were included in the trend analysis as representative low or high values.  However, the trend evaluation method used can become unreliable if too many of the data are censored. We chose not to calculate trends where there were fewer than five non-censored observations, or where there were less than three unique values among the non-censored results.  Sites which did not meet these requirements for trend analysis were labelled as 'Not assessed'.  This label is also used when sites did not have enough measurements available, such as only six years of measurements available for ten-year trends. 

The data requirements are summarised here:

  • At least 75% of the expected data in the trend period
  • At least 3 of 4 quarters or 9 of 12 months in the final year (2019) analysed
  • At least 9 years must be represented in the data for 10-year trends and at least 13 years represented for 15-year trends.
  • At least 5 non-censored values
  • At least 3 unique non-censored values

 

The data requirements for trend calculation are quite strict, to ensure that the results are statistically robust. However, this also means that many wells do not have enough data for trend calculation. Several councils, such as Auckland, Bay of Plenty, Manawatu and Otago, have only established their quarterly monitoring programmes within the past ten years, so they do not yet have ten years of quarterly data. Some councils, such as Canterbury, Hawke’s Bay, Northland, Waikato and West Coast, have many wells in their monitoring programmes that are sampled only annually or bi-annually. The wells may have been monitored for many years, but they do not have quarterly data, so they do not meet the requirements for trend analysis.

The trend methodology

To determine whether groundwater quality at a monitoring well is showing an improving, degrading or indeterminate trend, LAWA follows the methodology implemented in R functions (R Core Team) provided by LandWaterPeople (Snelder & Fraser, 2018). 

Trends were calculated on data for one well and one indicator at a time. 

The data for each well/indicator combination were tested for seasonal effects, and then analysed with either a seasonal or non-seasonal version of the non-parametric Mann-Kendall Slope Test.  This test evaluates all pairwise combinations of the data, evaluating whether the later observation is higher or lower than the earlier observation, for each pair. The magnitude of the difference has no importance, only the sign.  The seasonal version of this test compares the water quality data of each season separately which means any changes present are not hidden by seasonal patterns.  

Probability values from this test were interpreted in a Bayesian context as the probability of a mis-classification.  Using this probability, each trend is classified into one of five categories: very likely improving, likely improving, indeterminate, likely degrading, and very likely degrading.

What do groundwater quality trends show us?

We use trend calculations to help us identify changes in groundwater quality caused by human activities. By setting our minimum calculation period to 10 years, we disregard many of the shorter-term changes described above that might be caused by weather patterns or isolated contamination events. Even 10-year trends may be influenced by longer-term weather patterns, but they are more likely to indicate regional-scale changes in land use or land management practices.

The trends calculated on LAWA help us determine where changes in management have been effective and where further work may be needed, keeping in mind that changes in water quality often take years to be seen, and it may be decades for any restoration actions to show effect in water quality trend results.

Where do I find more information?

Environment Aotearoa 2019. A joint report by the Ministry for the Environment and StatsNZ.

Snelder T & Fraser C. 2018. The LWP-Trends Library; December 2018. LWP Ltd Report. https://landwaterpeople.co.nz/