Justificación del problema

The Downlands zone is the area between the Broomfield Fault Southwards to about Sefton (Figure 4.6). This zone is has the highest median concentrations of Chloride, Sodium, Arsenic, Iron, Manganese, Magnesium and Reactive Silica. This zone has relatively high concentrations most of the other determinants except Nitrate Nitrogen in which it has overall lowest median concentration (Appendix 4c and Table 4.4). Almost all HCA subgroups are represented in this zone.

Figure 4.2 and Figure 4.6 show the spatial distribution of TDS and the HCA. Both figures suggest a complex pattern however generally TDS concentration in wells seems to be lower near ephemeral streams and both braches of the Kowai River. Water level data discussed in section 3.5.1 showed that shallow wells are being recharge by proximal ephemeral streams.

Examination of the limited bore logs from wells greater than 100m total depth show a variety of lithologies; gravel, sand, mud, clay, peat and silt. Aquidef when plotting all

the wells with bore logs show that the sediments are dominated by claybound gravels. There are no obvious descriptions referring to Tertiary sediments in any of the bore logs examined by the author.

Table 4.8 Average rainfall chemistry from long term (5 – 12 years) sampling from Nichol et al (1997). Kelburn is a coastal site located in Wellington, Lauder is located in central Otago and Puruki is a forested site located near Rotorua.

Kelburn (M8389) Kelburn (M9091) Kelburn (F9192) Lauder (M8391) Lauder (F9194) Puruki (M8791) mg/L mg/L mg/L mg/L mg/L mg/L SO4 1.89 1.41 1.43 0.18 0.12 0.74 Cl 9.00 8.38 7.45 0.62 0.21 1.11 NO3 0.03 0.02 0.04 0.02 0.02 0.04 Na 4.91 4.72 4.24 0.32 0.11 0.63 K 0.19 0.22 0.20 0.09 0.03 0.11 Ca 0.25 0.16 0.92 0.11 0.03 0.13 Mg 0.55 0.49 0.04 0.05 0.01 0.07

The Chloride median concentration for this zone is 63 mg/L, ranging up to 190 mg/L (shown in meq/L in Figure 4.8). Hem (1992) and Hounslow (1995) state that Chloride are typically sourced from seawater, rainwater (Table 4.8), connate seawater, or from soluble salts within porous sediments that have been submerged in seawater. Chloride bearing minerals in igneous rocks are a less common source. Moore (1995) suggested that high concentrations of Chloride detected at Amberley Beach golf course wells (N34/0062, Figure 4.1) was related to sea spray. Figure 4.1 shows that the concentration of the Chloride in the deepest of the wells increased until 2000 after which it has remained at similar concentrations suggesting a change in Chloride input. N34/0062 and nearby wells have been identified as being susceptible to saltwater intrusion due their to vicinity to the coastline (Aitchison-Earl et al 2003). Saltwater intrusion may well be contributing factor to the Chloride levels in wells within a couple of kilometres from the coast, however, wells further inland cannot be affected by this process. This interpretation is supported by the fact that the total depths of many of these wells are less than the depth to mean sea level. High concentrations of Chloride are detected in the Downlands and North Waipara zone but not Saltwater Creek and South Waipara zone. This suggests that sea spray or rainfall is not the dominant source of Chloride to the study area. Furthermore, loess of the Downlands does not have any obvious accumulation of salts in their profiles (per com. P Tonkin, 2009). Connate seawater is discounted as the dominate source of Chloride as flushing of the system should have removed it from the system. Given the arguments above it

is suggested that the majority of the Chloride in the study area is from soluble salts contained within the sediments.

The interpretation of the Chloride being sourced from the salts within the sediments is supported by geomorphic and geological observations (section 2.5.1) which suggest that the sea in recent geological history has transgressed into this area.

Figure 4.7 and Figure 4.9 indicate that the Downlands zone typically have excess Bicarbonate when compared to Calcium, and less Sulphate than the North Waipara zone. On both plots this zone has a large scatter which is reflected by the results from the HCA and in the Piper plot.

HCO3 vs. depth -250 -200 -150 -100 -50 0 0 50 100 150 200 250 300 350 400 450 500 HCO3 (mg/L) D e p th ( m b g l) Saltwater Downlands South North

Figure 4.11 Bicarbonate versus depth by Hydrochemical zone

Comparing the results from the HCA to the classification of redox state reveals an interesting pattern in this zone. Group 2 sites are have been typically classified as reduced or complex and group 3 as oxidised. Group 3 sites in this zone are usually shallow (median -9.0 mbgl, n=19, containing four sites >20 mbgl) and Nitrate Nitrogen concentrations >1.0 mg/L. Group 3 are deeper (median -35.5 mbgl, n=20), contain <1.0 mg/L Nitrate Nitrogen. Figure 4.11 and Figure 4.12 show a general increase in bicarbonate with depth and a general decrease in Sulphate concentration with depth. This pattern would be expected for reduced groundwater (Rosen, 2001).

Reduced groundwater is caused by the consumption of available oxygen in a closed system. PDP (2001) suggested that the sporadic occurrences of Arsenic (± Iron and Manganese) in the Saltwater Creek – Waikuku area are a result of groundwater interacting with silts, clays and peat. Similar conclusions were found by Nickson et al. (2000).

Sulphate concentrations vs. depth

-250.0 -200.0 -150.0 -100.0 -50.0 0.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 SO4 (mg/L) D e p th ( m b g l) Saltwater Downlands South North

Figure 4.12 Sulphate versus depth by Hydrochemical zone.