Cromwell Terrace aquifer study

Abstract

Background: The Otago Regional Council (ORC) is responsible for managing Otago’s groundwater and surface water resources. Before this study ORC hadn’t investigated how much groundwater resource there was under the terraces surrounding Cromwell. This investigation assesses how much of the groundwater can be assigned for irrigation or other large uses of water that require consents. Assigning volumes of water in consents is termed ‘allocation’. Why is allocation of groundwater necessary? Groundwater is less transient and more vulnerable to over-exploitation than surface water. It may takes years or decades to find out if the underground water resource is being over-used. The setting of a limit on the amount of groundwater which can be taken from an aquifer helps prevent over-exploitation from being experienced in the first place. What this study found: The aquifer was found to be replenished by rainfall and irrigation returns percolating through the ground, and the seepage of water from Lake Dunstan into the aquifer. The aquifer was seen to respond immediately and completely to the filling of Lake Dunstan in the early 1990s, and from this we know that they are strongly connected by exchanges of water. We were able to account for the water exchanges that the aquifer makes with its surroundings, including the net rainfall that soaks into the ground; the irrigation from the lake and pumped from the aquifer that also soaks in; and the inflows and outflows with the lake. A computer model of the aquifer was also used to firm up the specifics of these exchanges, especially those that could not be measured directly. In this manner, we found that the average yearly contribution from the surface and lake was 2.4 million cubic metres per year (or approximately 1000 Olympic sized swimming pools every year). Of this volume, the Water Plan says that we can allocate 50%, or 1.2 million cubic metres per year, until a tailored volume is set. It should be noted that the groundwater allocated in existing resource consents is already 1.7 million cubic metres per year. We think that the default allocation of 1.2 million cubic metres per year is overly restrictive and that the ability of the lake to compensate for the extraction of groundwater should be recognized in a tailored allocation volume for the aquifer at Cromwell. What should be done next? The recommendations from this report should be discussed with the local community and other stakeholders. A new allocation regime would then be determined and be included in a future Water Plan change.