Hydrogeological Evaluation of Proposed Landfill Sites in the Greater Auckland Area (N.Z.): Mt Wellington and Peach Hill Valley
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Expanding landfill sites without careful consideration of their hydrogeological suitability can present a serious threat to the environment. The groundwater system is the common pathway for leakage of contaminants from these sites. Hence, the hydrogeological setting is the dominant consideration in the evaluation of any proposed disposal site. This study identifies some of the major hydrogeological controls on the migration of contaminants in a heterogeneous and fractured aquifer. Two proposed landfill sites in fractured media at the Mt Wellington quarry and Peach Hill valley were studied. The combination of methods used to evaluate these sites can be applied to other studies, especially those involving fractured and multiple aquifers within complex groundwater systems. The first objective of the study was to define the physical framework and nature of the groundwater flow system at each site. The second objective was to develop three-dimensional groundwater flow and transport models and to demonstrate their application to complex field problems. The unconfined basalt aquifer and a confined/unconfined aquifer within the Waitemata sediments are the main water-bearing zones at the Mt Wellington site. Tracer tests indicate that the velocity of the groundwater movement in the basalt aquifer varies with direction due to different preferential flow paths. However, circular resistivity soundings (CES) and detailed mapping of joints in the quarry suggest random anisotropy. The study showed that the CES method is effective in a medium with multiple joint orientations and relatively regular joint spacing for identifying the dominant orientation of near surface joints. The second proposed landfill site in the Peach Hill valley is underlain by four major lithological units, greywacke, coal measures, volcanic rocks and fill materials. The valley is cut by two major fault zones, the Hunua and Drury Faults. Three gravity profiles across the approximate trace of the Hunua Fault, together with other geological data, indicate that the fault is located next to the northern boundary of the Peach Hill quarry and does not occur beneath the fill as previously assumed. A multi-well tracer method with an instantaneous injection of non-reactive rhodamine WT revealed that the movement of groundwater in the quarry is toward the Hunua Fault. The fault, however, acts as a groundwater barrier, the fracture zones associated with the fault facilitate the movement of the groundwater toward low hydraulic pressure areas. The bulk of the local groundwater within the valley moves upward behind the Hunua and Drury Faults and forms the base flow component of the Peach Hill stream. Three dimensional numerical models were developed using the MODFLOW programme for groundwater flow simulations and a random walk code (INTERTRANS) for the transport simulations. Two other codes based on different numerical techniques (TOUGH2 and MT3D) were used for comparison and verification. Sensitivity analysis shows that the hydrological parameters used in the calibrated models are appropriate. The flow simulations for the Mt Wellington site indicate that a dewatering system can eliminate the risk of contamination downstream from the site; however, the failure of such a system would constitute a potential risk of contamination throughout the valley. The transport simulation of selected contaminants (COD, Cl, Cr, Zn, Cd and TCE) showed that attenuation of contaminants would not be reached until a significant portion of the basin has been contaminated. The transport simulations for the Peach Hill valley showed that the zones with high hydraulic conductivities adjacent to the Hunua Fault provide a pathway for the movement of contaminants downstream from the site. The results, however, demonstrate that with a conservative compliance distance of 200m, the concentration of the contaminants will generally be within the required safety levels.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1996