Hydrogeology of the Cromwell Terrace Aquifer, Central Otago
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
A hydrogeologic model, groundwater chemistry and stable isotopic analysis were used to establish recharge resources and outflows so a water balance could be developed for the Cromwell Terrace Aquifer (CTA) in Central Otago, New Zealand. Increased popularity of the Central Otago region for viticulture, orcharding and tourism, has resulted in an increased demand for water. Groundwater is a viable option to meet this demand for water. The CTA is a single unconfined aquifer contained within a thin veneer of permeable Quaternary glacial outwash gravels that range in thickness between 10 and 50m. These gravels rest unconformably on less permeable folded Tertiary sediments. The buried surface of the Tertiary sediments is irregular and provides the main hydrogeologic control in the CTA. Buried topographic highs in the Tertiary sediments impede groundwater flow, while the buried paleochannels at the southern end of the Cromwell Flat allow groundwater to flow unrestricted. The saturated thickness of the aquifer varies between 10 and 30 m. The direction of groundwater flow is in south easterly and south westerly directions toward both Lake Dunstan and the Kawarau Arm respectively. This indicates that recharge is from the Pisa Range. Annual fluctuations in groundwater levels show that there is a seasonal effect on the groundwater table. Annual fluctuations in groundwater level are in the range of 0.4 – 0.5 m, with lowest levels in winter and highest groundwater levels in late summer. The higher groundwater levels in summer correlate with when higher rainfall occurs, but could also be due to artificial recharge from irrigation during summer, and/or seepage from the Ripponvale Irrigation Scheme canals and storage ponds. Groundwater chemical analysis showed the dominant facies to be calcium bicarbonate waters. The source of the calcium bicarbonate is considered to be calcite in the Otago Schist, with concentrations of calcium bicarbonate being higher closer to the bedrock schist of the Pisa Range. Concentrations decreased toward Lake Dunstan, where calcium bicarbonate concentrations were lowest. The trend of calcium bicarbonate concentrations decreasing toward Lake Dunstan produces a similar pattern to the direction of groundwater flow. This would suggest that calcium bicarbonate concentrations are being diluted by rainwater infiltrating into the aquifer. However stable isotopic analysis showed that lake water infiltrates into the aquifer around the lake margin, and would also dilute calcium bicarbonate concentrations. Stable isotopic analysis found that groundwater was more depleted in both δ¹⁸O and δ²H than water from Lake Dunstan. The average δ¹⁸O for groundwater was -9.5‰, whereas the average δ¹⁸O for samples from Lake Dunstan was -8.1‰. The average δ¹⁸O value of Pisa Range snow, Pisa Range streams and Cromwell Flat precipitation gave values of -9.2‰ +/- 1.4‰, which is very similar to groundwater. This suggests recharge to the CTA is from a combination of snow melt and surface stream flow from the Pisa Range, and some direct rainfall infiltration on the Cromwell Flat. A water balance was calculated for the CTA groundwater system using the information from this study, and from a limited Otago Regional Council (O.R.C.) database. The main inputs to the CTA were found to be recharge precipitation and subsurface flows from the Pisa Range. The main outputs were identified as surface evaporation and discharge from the CTA to Lake Dunstan. The water balance showed that the total flow of water through the CTA is 93 Million cubic metres per year (Mm³/yr). At present the CTA has limited groundwater allocation measures in place. Using the information from the water balance, a volume of groundwater that could be abstracted sustainably was estimated. This volume was estimated using the O.R.C. method of allocating 50% of the mean annual precipitation that recharges the aquifer for groundwater abstraction. The total mean annual precipitation for the Cromwell Flat and Pisa Range is 20 Mm³/yr. Using the 50% of mean annual precipitation method, 10 Mm³/yr can be allocated for groundwater abstraction. The total volume of groundwater currently abstracted is 3 Mm³/yr, leaving 7 Mm³/yr of unallocated groundwater. Due to the small land area, types of land use, low population density of Cromwell Flat and availability of surface water (i.e. Lake Dunstan), it is unlikely that the total volume of 10 Mm³/yr will be fully allocated.