2. Methodology
Groundwater contamination in the catchment was quantified by monitoring six wells at monthly intervals during the irrigation season. The wells were selected based on the depth of well screen, adequate well-head protection, and the absence of other contamination sources (e.g. septic tanks). Data collection in this the second year of the study consisted of approximately monthly groundwater for three of the wells with more intensive (twice weekly) sampling undertaken on two wells (J41-0026 & J41-0008) through the February-March irrigation period. Farm maps were obtained where possible and the groundwater flow direction was estimated based on local topography. Information was collected for the four fields up-gradient of the well that were considered to be most likely to impact the well. Location maps for each well site are given in Appendix 2.
Irrigation and stocking data including numbers and dates of stocking for each paddock and irrigation dates was obtained from landholders for paddocks up-gradient of the wells. There were occasional time periods where the stocking and irrigation data was not recorded. For these periods landholders have indicated that irrigation occurred approximately every 14 days and stocking occurred approximately every 21 days. This is consistent with the recorded data.
Unfortunately continued sampling from well J40/0131 was not possible during the second year due to the landholders closure of the well.
Well samples were analysed for E. coli, total coliforms, and Campylobacter. E. coli and total coliforms were analysed quantitatively using Colilert. Campylobacter was quantitatively analysed using a MPN/PCR format for C. jejuni and C. coli, these are the two major pathogenic species. A three tube MPN format was used, with volumes of 1000 ml, 500 ml, and 100 ml. This gave a quantification range of < 0.6 to >3.1 MPN per L. It is acknowledged that there is significant uncertainty with using a three tube MPN but its use was due to budget constraints.
A section of the project, which involved the study of Campylobacter within cow faecal samples, was undertaken in the first year of the study. The study comprised the collection of samples of cow faecal material from three sites on each sampling occasion, with the other three sites being sampled on the following sampling occasion (each sample was a composite of 5 cowpats). The samples were analysed for the presence of Campylobacter and indicated that all but two samples gave a positive result for C.jejuni and therefore this part of the study was discontinued in year two.
Figure 1: Location of monitoring wells in the Waikakahi catchment
A specific capacity test was undertaken at one of the six wells (well J41-0008) and recorded a value of 2.4 l/s/m. This was much lower than an earlier recorded value of 56 l/s/m for this well but there were no details as to when or how this value was measured, and it was significantly higher than other specific capacities from surrounding wells. A transmissivity of between 700 and 2200 m2/day was estimated from this present test using two different techniques, which translates to a hydraulic conductivity value of between 29 and 88 m/day, assuming an aquifer thickness of 25 m. Taking into account the groundwater gradient, the groundwater flow rates are generally likely to range from approximately 0.6 m/day to 2 m/day.
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