The hydrology and hydraulic characteristics of the unconfined aquifers of the Otaki-Te Horo area

Abstract

The aim of this thesis is to derive a better understanding of the hydrology and hydraulic characteristics of the unconfined aquifers of the Otaki - Te Horo area. The study focused on two aspects of the unconfined aquifer system - hydraulic characteristics of the aquifers, and the determination of a groundwater balance for these aquifers. The specific objectives of the study were to: improve the understanding of the shallow groundwater system within the study area; identify the important hydrologic components of the unconfined groundwater system and estimate a groundwater balance; describe groundwater flow characteristics and estimate the throughflow for the unconfined aquifer; relate hydraulic and hydrogeologic characteristics to pollution potential of the unconfined aquifer; construct a groundwater model which may be used as a management tool. Data were collected from December 1991 to June 1993 for the purposes of water balance calculation, and to provide input data for a groundwater model. This consisted of:- (i) Climatic parameters, including air temperature, rainfall, and pan evaporation; (ii) West - east spatial variation in rainfall over the study area; (iii) Spring discharge at three springs at the base of the 6,000 year sea cliff. (iv) Groundwater levels at representative wells throughout the study area. A further 60 wells were included for piezometric surveys conducted quarterly. To examine regional groundwater flow regimes analyses were carried out at three levels of detail. Salt water tracers were used at three locations to determine flow direction and velocity in specific materials. The tracer tests produced variable results, with the most successful indicating a seepage velocity of 14 metres/day in the Otaki unconfined aquifer. All tracer tests clearly indicated direction of groundwater flow, with the test in the Coastal sands indicating a seepage velocity of less than 1 metre per day. Tracer test results were in general agreement with hydraulic parameters calculated from regional hydraulic gradients and pumping tests. Groundwater and spring water quality analyses identified areas were landuse practice have caused a deterioration in water quality, and the relationship between the springs and the Hautere unconfined aquifer. Evidence of bacteriological contamination was observed at two springs. A DRASTIC pollution potential analysis of the study area showed that the Coastal and Otaki groundwater zones are more vulnerable to contamination from the surface than the Hautere zone, principally due to a shallow water table and higher permeability materials. A simple, steady state groundwater model was developed based on existing information, the results of piezometric contouring, salt water tracer tests, and water balance analysis. It provides a better understanding of the unconfined aquifers, and in particular the relationship between surface water and the unconfined aquifers.