Groundwater in the Upper Waikato River Catchment is an important resource for economic commercial and domestic water supply, and the source of direct inflow to streams, lakes, rivers, and geothermal systems. Groundwater - surface water (GW-SW) interaction occurs throughout the catchment which can influence surface water flow rates and water quality. Increases in groundwater use have occurred throughout the Waikato region, and are predominantly driven by land use intensification. Consequences of intensification include increased groundwater abstraction, which potentially reduces the availably of water to recharge streams, rivers, and lakes (and therefore can effect hydropower generation); and an increase in discharge of nutrients to groundwater and surface water, leading to potential water quality degradation in lakes, waterways, and groundwater.

The aim of this project was to use novel techniques to improve the understanding of GW-SW interaction in the Upper Waikato River Catchment, and involved collaboration between Waikato Regional Council and the GNS Science-led Smart Aquifer Characterisation (SAC) research programme. Three case study sites were selected based on pre-existing knowledge that GW-SW interaction processes occur in the local hydrological system, and to represent a range of scales and settings (e.g., lake, river, and stream).

The primary aim of this project was to apply novel temperature sensing techniques from the SAC program, which include satellite thermal infra-red (satellite TIR), airborne thermal infra-red (airborne TIR), fibre optic distributed temperature sensing (FODTS), to test if the methods complimented each other for improved characterisation of GW-SW interaction in the Upper Waikato River Catchment. The specific objectives of the project were to: identify whether satellite TIR can be used in combination with airborne TIR to identify locations of GW-SW interaction to inform placement of FODTS equipment; and identify whether FODTS is a suitable method to determine GW-SW interaction within the large scale hydrological system of the Upper Waikato River Catchment. In addition, water sampling for hydrochemistry and age dating was completed at two sites (Waikato River reach and Torepatutahi Stream), to identify whether hydro-chemical parameters can be used as an age proxy in these systems.

Hydrogeological setting at Whakaipo and Whangamata Bays, Lake Taupo

Whakaipo Bay is a south-westerly facing bay, located along the northern shore of Lake Taupo. The bay is approximately 1.3 km long, and its surface water catchment extends approximately 10 km from the lake shore to the north. While the topographic map indicates up to five streams discharge into the bay, only the Mapara Stream located in the southeast corner flows continuously. The other streams are ephemeral following rainfall events. Whangamata Bay is located to the northwest of Whakaipo Bay, and is approximately 1.8 km long. The township of Kinloch and Kinloch marina are situated along the shore of Whangamata Bay. Previous studies at Whangamata and Whakaipo Bays identified groundwater flow into the lake through diffuse seepage zones located between 2 m and 6.5 m of water depth (Gibbs et al., 2005). These zones were identified to be deep groundwater inflow zones, and different to inflow that also occurs along the lake shore. Gibbs et al. (2005) identified that groundwater flowed in a thin layer down the lakebed slope, or pooled in depressions due to density differences between the warmer lake water (20°C) and the cooler groundwater (c. 12°C). The Gibbs et al. (2005) study used divers who could feel by hand differences in the water temperature between the inflowing groundwater and the warmer lake water.

FODTS temperature profile for the period 22:30 – 23:29 on 11.03.2014 during Horizontal 2 deployment, Whakaipo Bay, and locations of vertical deployments (I – III)

Figure 5.13 (from GNS Science Report 2014/64): FODTS temperature profile for the period 22:30 – 23:29 on 11.03.2014 during Horizontal 2 deployment, Whakaipo Bay, and locations of vertical deployments (I – III).

Plot of corrected temperature over time for Location II vertical FODTS deployment, Whakaipo Bay

Figure 5.15 (from GNS Science Report 2014/64): Plot of corrected temperature over time for Location II vertical FODTS deployment, Whakaipo Bay. Note: Temperature at t=55 min is due to a calibration change and is not an observable change.

Hydrogeological setting at Torepatutahi Stream, Broadlands

Torepatutahi Stream originates along the Kaingaroa Fault scarp, and flows in a westward direction, following the topographic gradient, towards the Waikato River. A conceptual model proposed by Piper (2005) describes the Torepatutahi Stream catchment to have an upper zone (where there is minimal surface flows and where groundwater recharge occurs), and a lower zone where groundwater discharges from springs at the fault zone. Many of the upper reaches of the stream have been reported to be dry, and it is unknown if these streambeds have ephemeral flow. The Torepatutahi Stream has several large springs at the fault zone, and downstream reaches are reported to both gain and lose groundwater. It has been estimated that approximately 74% of the total flow in Torepatutahi Stream comes from springs in the upper catchment, and the remainder is from groundwater inflow between the springs and Waikato River inflow (Piper, 2005). A single water dating sample was collected from a main spring (named ‘Hillside Spring’) of Torepatutahi Stream in 2001, and was reported to indicate a mean residence time (MRT) of 130 years (Piper, 2005; WRC, 2014). Based on local MRT values for groundwater sampled in up-gradient wells, it was interpreted that the source of groundwater for the spring was likely to be up-gradient (e.g., from the Kaingaroa fault scarp). At the springs site, the Torepatutahi Stream is joined by a tributary that is fed by several springs including the RDC water supply spring, and a spring discharging from a fracture in the hillside. The water age sample described in the previous section was collected from the hillside spring. A second spring-fed tributary joins Torepatutahi Stream approximately 250 m downstream of the RDC site. RDC have recently installed a continuous stream flow gauging site at the Torepatutahi Stream footbridge (Woods, 2014).

Schematic location of FODTS deployment at Torepatutahi Stream

Figure 4.6 (from GNS Science Report 2014/64): Schematic location of FODTS deployment at Torepatutahi Stream, RDC Deep Creek Supply. The unnamed tributary (stream No.2) is predominantly spring-fed from the valley (c. 300 m upstream). Inset shows location of Torepatutahi Stream in the Upper Waikato region.

False colour TIR image of Torepatutahi Stream at the RDC supply

Figure 5.30 (from GNS Science Report 2014/64): False colour TIR image of Torepatutahi Stream at the RDC supply, and modified aerial image from Google Earth (2014). The TIR image shows the Torepatutahi Stream, a spring fed tributary, and the confluence of these streams. Orientation and spatial scales are approximately the same between images. TIR colour scale is refined from the RAW value of 0 – 60,000 (see GNS Science Report 2014/64, Section 4.1.2.1).

Hydrogeological setting at Nga Awa Purua Rapids to River Road Reach, Waikato River

The Waikato River case study site included the reach between Nga Awa Purua Rapids and River Road. This reach was selected as it was ideal for acquisition of TIR imagery due to the river channel being orientated in a relatively straight direction towards the east. This maximised the reach of the river that could be surveyed using the airborne TIR method, which required a single straight line flight path. The Nga Awa Purua to River Road reach of the Waikato River lies within the Rotokawa Geothermal Field. Perennial and ephemeral streams flow into the Waikato River along this reach. The main stream that flows into the Waikato River along this reach is the Parariki Stream, which originates at the outlet of Lake Rotokawa and is strongly geothermally influenced.

Temperature plot of the Waikato River reach 3 FODTS deployment

Figure 5.26 (from GNS Science Report 2014/64): Temperature plot of the Waikato River reach 3 FODTS deployment, from 17:00 on 06.08.2014 to 12:00 on 07.08.2014.

Hydrochemical proxies

Of the eleven hydrochemistry sampling sites in the Upper Waikato study area, three sites from the Waikato River reach (S1 – S3) were influenced by geothermal processes and were excluded from hydrochemistry proxy studies involving cold groundwaters. For the nine cold groundwater samples, comparisons for proxy development were made between: silica and MRT, chloride and sodium, silica and sodium, sodium and MRT, fluoride and MRT, and nitrate-nitrogen and MRT. Chloride showed a slight negative relationship with MRT, and supports the idea that differences in chemistry are due to differences in recharge sources and land-use. Fluoride showed an increasing trend with increasing MRT for sites from Whakaipo Bay and the Waikato River, but not at the Torepatutahi Stream site. Analytes commonly associated with land-use changes and intensification, such as nitrate, often show negative relationships with MRT, however it is difficult to use such relationships as an age proxy as the variability in concentrations for young groundwaters is primarily a function of land-use not MRT. No apparent relationship was observed to exist for the other proxy comparisons. It is likely that hydrochemistry-age proxies area-specific, and to verify this, more data needs to be collected for each area independently.

Table 5.3 (from GNS Science Report 2014/64): Summary of tritium and radon measurements for the Waikato River sampling sites including Tritium Ratio (TR), significance (Sig.), Mean Residence Time (MRT), and radon. (*MRT values are calculated based on 80% Exponential Mixing)

Site Tritium (TR) Sig MRT* Radon (Bq L-1) Sig
Waikato River, S1 (Gas seep spring) 0.034 0.013 247 0.7 0.2
Waikato River, S2 (Hot spring) 0.048 0.013 240 2.5 0.4
Waikato River, S3 (Bank seep) 0.015 0.013 260 6 0.5
Waikato River S4 1.308 0.031 8 1.8 0.2
Waikato River S5 1.134 0.025 13 0.3 0.1

General findings of the project include:

Satellite TIR was not able to be used at the upper Waikato River scale to detect areas of GW-SW interaction due to lack of detail in the TIR caused by the large pixel size. Airborne TIR had much finer pixel resolution (0.7 m) and provided very good information on location of groundwater inflow to the river, particularly geothermal influenced warm discharges into the river. The river stage influenced the usefulness of the technique in some reaches where springs were submerged and not detectable at medium to high flows. It is recommended that any future airborne TIR surveys be undertaken during low flow conditions to maximise interpretation. The airborne TIR was very useful for informing the placement of FODTS and has considerable merit and potential in this regard. FODTS was used to identify the location of both warm and cold seeps/springs of variable flow, and identify considerably more of these features than was able to be achieved with TIR and visual observation. A limitation of the FODTS technique in a river of this size with dozens of closely spaced small springs is that it was unable to be reliably used for groundwater flux estimation.

  • Overall, satellite and airborne TIR were both useful for detecting general locations of groundwater inflow into river, lake and stream systems, and to inform placement of more detailed FODTS deployments.
  • Airborne TIR was very useful for identifying groundwater discharge along the Waikato River, particularly in areas that had strong thermal contrasts such as warm geothermal influenced springs. A clear contrast between groundwater and surface water systems at some locations in Lake Taupo was observed, and requires further investigation.
  • FODTS performed well and provided specific information on groundwater seepage locations at all sites.
  • FODTS was successfully used to determine flux at Lake Taupo sites using both the horizontal and vertical methods.
  • The usefulness of airborne TIR and FODTS was somewhat dependent on Waikato River flow rate (and river level) where springs discharge close to elevation of the river. Best results were obtained during low flow conditions.
  • There is the potential for hydrochemical age proxies for the Upper Waikato River Catchment groundwater system to exist. However, there was a high variation in sample hydrochemistry and too few samples in this project to confirm any age proxies. Proxy development therefore requires additional work

References

Lovett, A.; Cameron, S.; Reeves, R.; Meijer, E.; Verhagen, F.; van der Raaij, R.; Westerhoff, R.; Moridnejad, M.; Morgenstern, U. 2015. Characterisation of groundwater - surface water interaction at three case study sites within the Upper Waikato River Catchment, using temperature sensing and hydrochemistry techniques, GNS Science Report 2014/64. 83 p.

Waikato Regional Council. 2014. Lake Taupo website http://www.waikatoregion.govt.nz/ Environment/Natural-resources/Water/Lakes/Lake-Taupo/. Accessed July 10, 2014.

Woods, Brian. 2014. Utilities Engineer, Rotorua District Council. Personal communication, 2.10.14.

Gibbs, M., Clayton, J., and Wells, R. 2005. Further Investigation of Direct Groundwater Seepage to Lake Taupo. Hamilton: Environment Waikato.

Piper, J. 2005. Water Resources of the Reporoa Basin. Environment Waikato Technical Report, 2005/57. Environment Waikato, Hamilton, p.55.