Modelling lower Taieri Plain groundwater :

Edge, Brendon (Brendon Roberts)

The ground water system of the Lower Taieri Plain is an important resource supplying water to Dunedin, Mosgiel, dairy farms, and market gardens. The plain is the largest area of flat land near Dunedin and as the city expands, landuse conflicts are occurring more regularly. To preserve the groundwater resource a thorough understanding of the hydrogeological processes is required for sustainable management. Approximately 140 drill logs exist f~r the 200 km2 plain. Three drill logs deeper than 100 m deep do not reach basement schist, although two may have Tertiary units at lower levels. The 154 m deep Victoria University drill log in the southwest of the plain does not appear to have Tertiary units and -~ is used as the basis for interpretation of the marine layer. Correlation is poor between drill holes for lithologies other than those associated with a Holocene marine transgression, and hydraulic connection across the basin is inadequately quantified. The Lower Taieri Plain is the surface expresswn of a SW-NE trending down-dropped basin that has accumulated a complex array of sands, silts, clays and gravels due to a long-lived association with fluvial processes. A Holocene marine transgression deposited a fine-grained sand and silt layer over two thirds of the plain, thickest in the southwest (25 m thick), pinching out west of Mosgiel, and this layer is the likely cause of confined ground water west of the Taieri River. The East Taieri cause of confinement is less obvious and may be due to the effect of discontinuous multiple finegrained layers in the top 10-20 m. Recharge is interpreted to be from flow loss of rivers proximal to where they enter the plain, runoff from adjacent foothills into alluvial fans, and infiltration from rainfall. Discharge is poorly 11 understood, with previous interpretations suggesting losses to the drainage system, and also to the tidal parts of Taieri River and subsequently to the sea. Several years of data for river and lake levels, river flows, rainfall, potential evaporation and groundwater levels at five automatic recorder sites are used to interpret spatial hydrological relationships. Rainfall recharge at the surface is calculated as part of the water balance. A regional model of the ground water system was developed using G MS (Groundwater Modeling System) and MODFLOW. A two layer, 500 m by 500 m grid is used to produce a model with several hundred active cells per layer. Measured values for transmissivity range from 40- 1000 m2day-1 • Groundwater levels and results from flow gauging are used as targets in model calibration. When the flow loss target for the Upper Silverstream is met the piezometric head is well replicated throughout most of the plain with hydraulic conductivity at 12 m day- 1, except for west ofthe lakes where the heads appear to be controlled by the lake levels given in the model. Several parameters remain unknown, and the model does not replicate the system in some areas, for example a very high flow gain to the Lower Sl.lverstream was modelled, artesian conditions are not replicated, and the inferred recharge area at · Outram becomes a discharge zone. These issues are not amended for the purpose of this dissertation, but remain to be addressed when sufficient field data are available. The model indicates the deficiencies in our understanding of the ground water system. Useful field measurements would include further flow gauging of rivers and drains, particularly the Silverstream and Taieri Rivers, and the West Taieri drainage system. Chemical isotope analyses and further research into available chemical data could help to ascertain source waters. Further quantification of aquifer hydraulic parameters is required, especially in the West Taieri where the greatest landuse issues are currently occurring.

xi, 119 leaves : ill. (some col.), maps (some col., some folded) ; 30 cm.

2000Edge