Characterisation and numerical simulation of the Lower Manawatu Catchment hydrogeological system : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University, Manawatu, New Zealand

Abstract

The Lower Manawatu Catchment (LMC) hydrogeological system presents an example of extensive stratified heterogeneous aquifers. A conceptual model was developed for the system through systematic characterisation of its geology, soil, climate, hydrology, hydrogeology and hydrochemistry. Numerical groundwater flow modelling provided an effective integrated framework for the analysis. The developed knowledge and models are useful for the identification and comparison of land and water resources management options in the catchment. The research involved the development of a soil moisture balance modelling code to evaluate recharge. Stratigraphical modelling has been possible through incorporating imaginary lithological well logs and stratigraphical cross-sections in data-sparse areas. Geological material heterogeneity was represented in the groundwater flow model through hybridising zonal and pilot point calibration techniques. The developed soil moisture, geology and heterogeneity modelling techniques have universal applications. The study indicates that the LMC hydrogeological system is more suitably represented as a continuous groundwater flow system rather than a sequence of discrete aquifers and aquitards. Average groundwater recharge is estimated at about 25% of average rainfall. Average baseflow is estimated at about 10% of the average runoff, the equivalent to more than half the estimated average recharge. The LMC groundwater resource is mainly tapped at shallow depths, the locus where most of the interaction with surface water occurs. Catchment-scale steady-state numerical groundwater flow modelling suggests that in average groundwater abstraction may have been depleting overall surface water flow by about 5%. Groundwater levels in the LMC were found to be generally stable, implying sustainable resource development. Rising groundwater levels noted in the eastern and southern outskirts of Palmerston North may be related to prolonged practice of irrigation. No evidence of land use impacts on groundwater quality was found in the catchment. Nitrate concentrations are believed to have been kept at acceptable levels in groundwater due to denitrification stimulated by extensive organic content in some geological units. This thesis represents a one stop shop for information on groundwater in the LMC. The knowledge and tools developed through this research have immediate use in the LMC and elsewhere, and they provide solid basis for further work.