Sampling and Analytical Methods

3.3.3..1 Suction Tubes

Suction tubes were prepared for sampling by first removing any residual water, then a 50 kPa tension was applied by removing a set volume of air (350 mL) with a 60 mL plastic syringe (Terumo®, Japan). Samples were collected the following day using a 60 mL plastic syringe. A 50 mL sample was taken for determination of NO3

-

-N concentrations using the standard methods (APHA 4500-NO3 I) of

automated cadmium reduction and Azo dye colourimetry and a flow injection analyser (Lachat Instruments, WI, USA) (APHA, 2005).

Another 50 mL sample was taken for NO3 -

isotopic analysis, with δ15N-NO3 -

and δ18O-NO3 -

values determined using the bacterial methods of Sigman et al. (2001) and Casciotti et al. (2002) at UC Davis, California, USA. All samples (n = 123) were stored frozen, shipped cool and filtered to 0.45 µm before analysis. Although the methodology of Sigman et al. (2001) states that samples with NO3-

concentrations as low as 0.014 mg N L-1 could be analysed, initial analysis indicated that a minimum of 0.03 mg N L-1 yielded more reliable results. As a consequence, subsequent samples with NO3

-

concentrations <0.03 mg N L-1 were not submitted for analysis.

Suction tubes were sampled 2 – 5 times per year from June 2008 to Sep 2010 during high water table periods so that all samples (n = 130) were taken during saturated conditions.

3.3.3..2 Well Sampling

Standard procedure was to remove a water volume equivalent to 3 times the standing water column (Daughney et al., 2006) and this was done on the same day as sampling at the Waihora site.

However, some wells in the Toenepi catchment recharged so slowly that wells were purged the day before sampling, and removal of more than one standing water column was unfeasible. In this instance, particular attention was given to the stability of the online readings before samples were taken on the following day. Low flow sampling methods (Daughney et al., 2006) were used to minimise water level changes, using a variable speed pump and flow controller, limiting flow rates to 0.1 – 0.15 L min-1. A flow cell was used with probes to measure DO, pH and electrical conductivity (EC), connected to a data logger (TPS 90-FLMV Field Lab) and logged every 30 sec. Samples were taken after readings had stabilised to the following criteria: DO ± 0.3 mg L-1, pH ± 0.1 pH units, EC ± 3% of reading.

The Toenepi wells were sampled to up to 3 times per year (during winter and spring) between 2008 and 2011, resulting in a total number of samples of 64. The Waihora wells were sampled twice in 2008 (September and November), in March 2009 and November 2011 (n = 72).

3.3.3..3 Well Sample Analysis

Well water samples (250 mL) were taken and analysed for NO3 - -N, NO2 - -N, ammoniacal N (NH4 + -N) and dissolved reactive phosphorus (DRP) according to standard methods (APHA, 2005). Then 50 mL samples were taken for isotopic analysis of NO3

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. Most samples (n = 100) were again analysed using the bacterial methods of Sigman et al. (2001) and Casciotti et al. (2002) at the Stable Isotope Facility at UC Davis, California, USA. However, some samples (n = 26) were analysed by the Stable Isotope

Centre at GNS Lower Hutt, NZ using the chemical method of McIlvin and Altabet (2005). Samples for isotopic analysis were stored frozen, shipped cool and filtered to 0.45 µm before analysis.

At selected sampling dates, the Toenepi wells were also sampled for silica (SiO2) analysis. A 50 mL

sample was field-filtered using 0.45 µm syringe tip filters and analysed using ICP-OES according to

standard methods (APHA 3120-B) (APHA, 2005). 3.3.3..4 Age dating

The concentration of SiO2 was used as a proxy for tritium age dating, since a good relationship was

observed between these two variables in samples taken in 2011 (Figure 3.8). The longer the groundwater is in contact with the aquifer, the higher the SiO2 concentration and the lower the TU

value. Due to the different geology and presence of the aquitard at 2 m depth, the Topehaehae (To) SiO2 data cannot be grouped with the Kereone (Ke) and Morrinsville (Mv) results and forms its own

dataset. The three To data points located close to 0 TU are from the To-390 well, below the aquitard, and are older (estimated at > 200 years). Other To data points, from above the aquitard, are younger (<3 years), despite relatively similar SiO2 concentrations (80 – 110 mg L

-1

). All the Ke data is classified as young (being recharged within the last 5 years) while only the uppermost layer of groundwater at the Mv site (2.6 m below ground surface) is classified as young. The groundwater from the 4.1 m depth is relatively old, with an estimated age of 50 years, while water at 4.7 m depth is older, at around 150 years.

Taking into account the relationships observed in Figure 3.8, samples were classified as being young or old. The Ke and Mv samples with SiO2 concentrations > 70 mg L-1 were classed as ‘old’, while To

data was handled differently; all samples from the To-390 well were classified as ‘old’ and all those taken from above the 2 m aquitard were denoted as ‘young’.

Figure 3.8: Linear relationships between SiO2 concentration (mg L-1) and tritium units (TU) for Toenepi groundwater samples. The solid green trendline is for the Kereone (Ke) and Morrinsville (Mv) data only, the dotted blue line is for the Topehaehae (To) results from wells shallower than 2 m depth. The three To data points close to zero TU are not included in either dataset. The

groundwater was classified as either ‘young’ or ‘old’ based on this data.

3.3.3..5 Statistical Analysis

All statistical analyses were conducted using Microsoft Excel 2007. Means, standard deviation and standard error were calculated for different parameters, and where appropriate, significant differences between means were determined using Student’s t-test.