5 Comparison to previous studies
This section provides a brief comparison with previous work as a framework for assessing the results of this investigation. Emphasis is placed on the parameters of greatest concern: NO3-N, Fe, Mn, salinity and microbiological indicators (cf. section 2.2). Note that comparisons to previous studies must be made with caution, due to possible biases resulting from differences in monitoring objectives, sampling and analytical methods, data interpretation techniques, and so on.
5.1 Regional investigations
Many regional investigations of groundwater quality have been conducted in New Zealand, in most cases by regional councils. In some cases these investigations have focused on the same SOE monitoring bores considered in this study, whereas in other cases additional monitoring data are included. It is important to note that regional studies can usually account more specifically for location-specific details related to aquifer characteristics and groundwater quality than is possible in a national overview.
It is beyond the scope of this report to provide an exhaustive summary of previous regional results, but certain generalisations can be made. For NO3-N, regional council surveys report median concentrations of about 2 mg/L, with typically 5-10% of sites above the MAV (Close et al, 2001). For microbiological parameters, regional surveys report results above the MAV at 9-60% of sites (Sinton, 2001). There are few specific regional surveys of Fe, Mn or salinity.
5.2 National assessments of groundwater quality
There are relatively few previous national assessments of groundwater quality available for comparison with the results from this investigation. An early survey reported that about 30% of wells tested in Taranaki, Wairau, Waimea, Waikato and mid-Canterbury (n = 497) had an NO3-N concentration above 10 mg/L (Burden, 1982). Bright et al (1998) used a larger data set to demonstrate that a significant proportion of wells tested had NO3-N concentrations above 5 mg/L (half the MAV). Unfortunately, the survey by Bright et al (1998) did not include statistics that are directly comparable to the results in this report. Davies et al (2001) reported that 86% of drinking-water supplies that were found to have NO3-N above half the MAV were derived from groundwater sources. They also found that groundwater-derived water supplies had elevated Mn relative to supplies sourced from surface water.
In a national survey of Fe and Mn in New Zealand’s groundwater, Daughney (2003) reported median concentrations of 0.08 and 0.01 mg/L, respectively (n = 9,784). This same study reported that 36% and 39% of the samples tested had median Fe and Mn above the relevant aesthetic GVs for drinking-water, respectively. The NGMP data set has median concentrations of NO3-N, Fe and Mn of 0.77, 0.03 and 0.002 mg/L, respectively (Daughney and Reeves, 2003, 2005). Temporal trends observed in the NGMP (Daughney and Reeves, 2006) are very similar to those reported in this study. We are not aware of any other national assessments of trends in groundwater quality. Overall, the results of this study appear to be generally similar to previous investigations.
5.3 National assessments of river water quality
The National River Water Quality Network (NRWQN) includes 77 sites that are monitored monthly by NIWA for various parameters. A recent summary of the NRWQN data collected over the period 1989-2005 revealed national medians for conductivity of 95 µS/cm, NOx-N of 0.116 mg/L, and E. coli of 49 cfu/100 ml (Scarsbrook, 2006). As expected, New Zealand groundwaters have higher median conductivity and NO3-N but lower microbial counts (see Table 7). Thus it is possible that the discharge of nitrate-rich groundwater into a stream or river may pose a significant threat to its ecosystem, but the number of locations in New Zealand where this is presently occurring cannot be determined with the data used in this study. Similarly, the recharge of groundwater from a river may introduce microbial pathogens into the aquifer, but again the number of locations where this is a potential concern is not known. Median relative rates of change in the NRWQN were 0.15% and 0.47% per year for conductivity and nitrate, respectively (Scarsbrook, 2006) and were comparable to median rates for groundwater (see Table 12). In the future it would be useful to perform a more quantitative comparison of results from the NRWQN and groundwater monitoring programmes such as the NGMP.
5.4 International studies
Griffioen et al (2005) summarise groundwater quality at the global scale through the use of 11 status-based indicators. Of the 231 countries considered (including administrative units), 65 were identified to have a significant fraction of “high-NO3” groundwater, usually associated with intense agricultural activities, about 20 countries were identified as having high concentrations of Fe and Mn of natural origin, and “many” countries were identified as having problems with salinity, usually associated with salt-water intrusion.
Several investigations have discussed groundwater quality at the national scale. For example, groundwater in Finland is characterised by similar ranges of Fe and Mn as found in New Zealand, but concentrations of NO3-N tend to be lower, presumably due to less intense agricultural activity (Lahermo et al, 1999). In Canada, about 14% of wells tested have NO3-N concentrations above guideline values, and 20% have coliform counts above guideline values (Lesage, 2005). Broers and van der Grift (2004) reported increasing trends in NO3-N at 8% of monitoring sites in a region of intensive livestock farming in the Netherlands. These examples illustrate that, generally speaking, groundwater quality in New Zealand is similar to that in many other countries.
