5. Summary
This section provides a summary of the understanding developed of the aquifer functionality and surface water interaction for the various groundwater basins identified in the Waitaki Catchment.
5.1 Upper Waitaki
The upper Waitaki Catchment includes all groundwater basins above Kurow township. Aquifer basin functionality and surface water interaction for the upper Waitaki Catchment are discussed as follows.
Tekapo Basin
The Tekapo Basin aquifer extends from Grays Hills in the south to Lake Tekapo in the north. The aquifer is bounded by the Mary Range in the west and the Rollesby Ranges in the east.
Groundwater flow direction is from north to south toward the basin outlet at the Tekapo River at Grays Hills. The water table aquifer is of moderate permeability and saturated depth based on a small amount of data. Water table measurements at ECNZ piezometric sites in the north of the basin indicate that there is a degree of stratification of gravel layers in that area. Differences in water table measurements were observed at adjacent bores drilled to variable depths in that location. The connection between layers and their lateral extent is unknown.
The rainfall recharge component estimated for the aquifer is 25.8 Mm3/annum (817 l/s), coupled with a large estimated groundwater storage in the aquifer of 2,579 Mm3.
Stream flow leakage occurs in the basin from the Irishman Creek. However, limited flow measurements have been taken to fully understand stream/aquifer interaction. Piezometric contours and some gauging data show that the Mary Burn gains flow from groundwater in lower reaches. This is also true for the Grays and Tekapo Rivers. The water table is shallow in most reaches of the Mary Burn and the Greys River (Plate 7 - Figure 1).
Plate 7. Grays River, 7km upstream of Grays Hills
A previously indicated, groundwater in the aquifer basin ultimately provides some baseflow for the Greys River, Mary Burn and Tekapo River. Given the potentially large storage capacity and local recharge component, it is likely that suitably located and appropriately screened bores may abstract limited amounts of groundwater without directly affecting surface waters on a seasonal basis. Shallow bores located in this basin are however likely to intercept aquifer recharge, representing a proportion of longterm baseflow for streams.
Twizel Basin
The Twizel Basin aquifer extends from Lake Benmore in the south to Lake Pukaki in the north. The aquifer is bounded by the Ohau and Ben Ohau Ranges in the west, Benmore Range in the south, and the Mary Range in the east. Groundwater in the basin flows from north to south toward Lake Benmore. However, in the western extent of the aquifer, groundwater flows in an easterly direction from wetland areas adjacent to Lake Ohau and the Upper Twizel River, toward Lake Ruataniwha and the Ohau River bed. The aquifer is of moderate permeability and saturated thickness, based mostly on drill data around the Twizel township area.
The rainfall recharge component estimated for the aquifer is 27.4 Mm3/annum (868 l/s), and estimated groundwater storage in the aquifer is 2,740 Mm3.
Piezometric contours and limited gauging data indicates that the upper Twizel River gains in flow from groundwater contribution, whilst it is expected that the Twizel River would lose water to the aquifer in lower reaches. Below SH8, it is likely that significant flow gains from groundwater may occur in the reaches of the lower Ohau and Twizel Rivers before entering Lake Benmore.
The Tekapo River, which enters the Twizel Basin south of the Grays Hills, may also lose water to the aquifer as it flows toward Lake Benmore. Piezometric contours suggest that significant losses may occur in reaches of the Tekapo River below Grays Hills (Plate 8 - Figure 1). However, no concurrent river flow gaugings have been carried out to confirm this hypothesis.
Plate 8. Tekapo River below Grays Hills
The potentially large storage capacity and local recharge component for the aquifer (from rainfall and stream losses) indicates that suitably located deep bores may abstract limited amounts of groundwater without directly affecting surface waters on a seasonal basis. Bores located in the upper part of the Twizel Basin are however likely to intercept aquifer recharge and as such a proportion of long-term baseflow for streams in the area. Whilst bores located in the lower part of the basin may abstract quantities of groundwater without directly affecting surface water flows or long-term stream baseflows. This is most likely for bores located between the Tekapo River and Lake Benmore, as natural leakage to groundwater from surface waters in the area may be substantial. However, a long-term reduction in groundwater baseflow recharge to Lake Benmore may be measurable given the size of any abstraction, due to direct interception of river and rainfall based recharge to the aquifer, which ultimately discharges to Lake Benmore. Limited abstraction may not necessarily show any seasonal variation in measurable water table height and slope, and hence discharge to the lake. Although this remains unquantified at present.
Omarama (including upper Omarama) Basins
Rainfall recharge to the upper Omarama Basin provides baseflow to the Quail Burn and the Hen Burn streams, which ultimately flow to the Ahuriri River. There is limited information on groundwater in this basin, which is separated from the main Omarama Basin by the 'Clay Cliffs'. It is unlikely that large potential for groundwater exists in this basin, and abstraction of groundwater in the basin risks a reduction in baseflow to local streams.
The main Omarama Basin is within the Ahuriri River Catchment, which is bounded by the St Bathans Range to the south, and by a groundwater divide to the north, differentiating this basin from the Twizel Basin. The groundwater divide to the north is not very well known and the exact position may depend on seasonal variation in water table heights.
Groundwater flow in the Omarama Basin is mainly in an easterly direction, mostly toward and parallel with the Ahuriri River, ultimately draining to Lake Benmore. The storage in the aquifer is estimated at 235 Mm3. However, the Ahuriri and Omarama Rivers bisect the Omarama Basin and it is likely that the groundwater is mostly shallow, in conjunction with the major surface streams. Abstraction from the basin would intercept a proportion of baseflow contribution to surface waters. However, adequately sited bores within the basin, having regard for the size of the abstraction, may not necessarily impact on surface waters on a seasonal basis. That is, the depressurisation cone of influence from groundwater abstractions may not come into contact with surface waters in any irrigation season. This may be due to the size of aquifer storage, bore location and rate of take, and mitigating seasonal recharge effects.
Benmore Terrace
No information exists for the groundwater resources of the Benmore Terrace. There is limited storage in the aquifer, and rainfall recharge is not large. However, some gauging information suggests that natural leakage from the Otamatapaio River would provide a groundwater resource there. The aquifer ultimately discharges to Lake Benmore.
Aviemore and Waitaki Flats
Again, no groundwater information is available to assess the groundwater potential in these areas. The aquifer(s) ultimately discharge to Lakes Aviemore and Waitaki.
5.2 Lower Waitaki
The lower Waitaki Catchment includes all groundwater basins below Kurow township, and including the Hakataramea Valley. Aquifer basin functionality and surface water impacts for the lower Waitaki Catchment are discussed as follows.
Hakataramea Valley
Limited groundwater information exists for the Hakataramea Valley. A few drill holes were put down in recent times as part of exploration for a deep groundwater irrigation source. The results of the drilling indicate that the aquifer is of limited saturated thickness and extent, being locally recharged by rainfall infiltration.
Piezometric contours indicate that the direction of groundwater flow in the basin is generally toward the Hakataramea River.
The overall storage within the basin is quite large (907 Mm3). However, abstraction from the aquifer risks the interception of some baseflow to the Hakataramea River. Given the potential baseflow contribution from the aquifer to the river, groundwater abstraction from the basin may directly impact on river flows.
Lower Waitaki Valley - north bank
Piezometric contours indicate that the direction of groundwater flow is parallel to the Waitaki River. However, some aquifer recharge from foothills and streams traversing the area is expected to provide a hydraulic gradient toward the river.
Depth to the water table is small in the vicinity of the Wainui Station area, probably as result of the lower lying topography and the influence of the Waitaki River. Previous reporting (URS, 2003), suggested that some recycling of Waitaki River water through the Kurow Trench area was possible. The layout of the topography and the relationship of aquifer water levels to the Waitaki River tend to confirm this hypothesis. However, the Waitaki River throughflow may only occur on a seasonal basis, when groundwater recharge from other sources is low and Waitaki River flow is comparatively high.
Groundwater abstraction from the area is likely to induce throughflow from the Waitaki River. Also, abstraction of groundwater in the area is likely to have an impact on local spring flows to the Waitaki River.
Lower Waitaki Valley - south bank
The direction of groundwater flow in this area is generally toward the Waitaki River, with local rainfall and stream flow recharge providing an estimated 174 l/s input (average annual flow), to the river. Piezometric contours do suggest some recycling of Waitaki River water in the lower terrace areas; however this has not been quantified in any report.
Various streams flow on to the area (Kurow Creek, Oteike Stream, Otekaike and Maerewhenua Rivers), which in turn are expected to lose a proportion of (or all) flow to groundwater. The Duntroon Springs (adjacent to Duntroon township) are most likely to be sourced from the Maerewhenua River, and to a lesser degree, the Papakaio Formation, as some iron staining is evident from the spring waters. The springs then flow directly to the Waitaki River.
Maerewhenua Basin
The Maerewhenua Basin alluvial aquifer, which lies adjacent to the Maerewhenua River, generally has a thin saturated thickness, and water table levels are consistent with local river levels. Some local rainfall recharge provides a net gain to the Maerewhenua River, but it is only small at 38 l/s.
The storage volume in the aquifer coupled with proximity to the Maerewhenua River indicates that groundwater abstraction would have a direct impact on Maerewhenua River flows.
Natural flow losses occur in the Marerewhenua River in its mid reaches (Otago Regional Council, 2004). The flow losses are likely to sustain the Duntroon Springs located in the Waitaki Valley, and also recharge the Papakaio Formation (MWH, 2004), as discussed in the following section.
Papakaio Formation
The Papakaio Formation is a confined aquifer system extending in a spoon-shaped pattern over the Maerewhenua Basin (MWH, 2004). The aquifer receives outcrop rainfall recharge, Maerewhenua River leakage, and aerial distributed leakage from overlying water table aquifers. Outcrop rainfall recharge is defined as the amount of water percolation to the aquifer provided by rainfall to Papakaio Formation exposed at the surface in the Basin. Rainfall above and below Papakaio outcrop typically runs off to surface waterways, or is prevented from recharging the aquifer by overlying low permeability strata. The direction of groundwater flow is toward the northeast, to the Waitaki Valley, Awamoko Stream and Waiarekia Stream (part of the Kakanui River Catchment).
Abstraction of groundwater from this aquifer risks a reduction in baseflow contribution to the Awamoko and Waiareka Streams (Otago Regional Council, 2004). However, the contribution of groundwater to the Waitaki Valley, and the Awamoko and Waiareka Streams is of a diffuse nature, which is difficult to directly measure (Otago Regional Council, 2004).
Lower Waitaki - Otago
The Lower Waitaki alluvium has been studied in reasonable detail in the past. The basin extends from Black Point in the west, to the Pacific Ocean in the east. The northern and southern boundaries are the Waitaki River and the Tertiary sedimentary rocks of the Maerewhenua Hills respectively.
Groundwater flow in this aquifer is provided mainly by surface irrigation infiltration, including some rainfall recharge. The direction of groundwater flow is toward the Waitaki River and the Pacific Ocean. It is estimated that a 50/50 split of Lower Waitaki groundwater, discharges to the Waitaki River (including Welcome Creek), and to the Pacific Ocean.
The depth to groundwater in the Lower Waitaki is generally small (within 2 to 5 metres), and has a seasonal variation of up to 3.5 metres (SKM, 2000). The shallow water table in most areas reflects the seasonal influx of recharge from surface irrigation practices.
The major spring outflow (Welcome Creek) is sourced entirely from groundwater in the Waitaki Alluvium (Plate 9 - Figure 1), although it is unclear if there is Waitaki River water contributing in lower reaches.
Plate 9. Welcome Creek above Ferry Road Bridge
The Awamoko, and Waikoura-Henderson Streams traverse the basin. However, they are generally dry as flow from these streams leaks to the aquifer within 100 to 300 metres of where they flow on to the plain.
Groundwater abstraction from the Lower Waitaki in the vicinity of Welcome Creek may impact on stream flow. However, given the large storage in the aquifer, the high artificial recharge and high throughflow flux on an annual basis (groundwater retention times in the aquifer are approximately only one to two years), abstraction of groundwater from the basin overall is likely to have limited impact on the Waitaki River.
Lower Waitaki - Glenavy (Canterbury)
The Glenavy groundwater basin extends from Black Point (across river) in the west, to the Pacific Ocean in the east. The southern boundary of the aquifer is the Waitaki River, and the northern boundary is a combination of Tertiary sedimentary rocks forming the catchment boundary of the Waihao River, and a local groundwater divide in the alluvium between Grays Corner and Morven, based on piezometric groundwater flow patterns.
The Glenavy basin is similar to that of the lower Waitaki (Otago) basin, in that a 50/50 split of groundwater contribution to the Waitaki River and to the Pacific Ocean based on piezometric contours is likely to occur.
The aquifer has a high recharge flux from the Morven-Glenavy Irrigation Scheme in the area, which also culminates in seasonally high water tables in a large part of the basin. The spring fed Waikakiki Stream receives recharge from the aquifer, which in turn flows to the Waitaki River immediately upstream of the SH1 bridge at Glenavy.
Groundwater abstraction from the basin, given aquifer storage and recharge components, is not likely to have significant impact on the Waitaki River. However, again the recharge flux is a large component of storage, as such the annual flushing of the groundwater system is very high.
