Extreme ranges of groundwater level

Oral presentation at New Zealand Hydrological Society & Meteorological Society of New Zealand Joint Annual Conference, 18-20 November 2008, Shantytown/Greymouth, New Zealand.

Large natural variations in groundwater levels can be an issue for selecting the depth of new water well installations or for evaluation of health risk from land treatment of waste, especially when these variations occur over periods of several years. Some parts of the Canterbury Plains show recorded variations in groundwater level that have a range of more than 30 m. In this presentation, the factors contributing to this range of groundwater level are examined and quantified. The time series of groundwater levels at selected monitoring wells in the alluvial plains area of Central Canterbury were used to calibrate a physically-based analytical groundwater model. This model is homogeneous and one-dimensional in the groundwater flow direction across the plains, and is represented for computational purposes as the eigenmodel solution to the boundary conditions of no-flow at the foothills and fixed-head at the coastal discharge zone. The results show that groundwater recharge from soil-water drainage is the dominant cause of natural groundwater level variation, even in the presence of a large component of recharge from the rivers that transect the plains. Soil-water drainage is determined by the partitioning effect of soil-plant-atmosphere processes on the rainfall climate (and, increasingly, irrigation). The time scale of dynamic response of groundwater levels and groundwater discharge to these climate-driven processes can be quantified by means of a single parameter that encapsulates the horizontal scale of the groundwater system and bulk values of the storativity and transmissivity of the aquifer. The magnitude of response is determined by the location of the monitoring well and the bulk transmissivity of the aquifer system. The geology, climate, and extent of the Canterbury Plains produce a combination of factors that result in ranges of natural dynamic groundwater level variation that are unusually large for aquifers in New Zealand.