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Groundwater flow dynamics beneath a wastewater irrigation site, Hautapu, Cambridge

In this study, a qualitative groundwater hydrological model is developed to assist in evaluating the long-term environmental sustainability of a wastewater irrigation system at Hautapu, Cambridge. The model relies on the integration of site information, numerical groundwater modelling, geochemical data, and surface geophysical data. The characteristics of the site relating to the groundwater system are examined including climate, topography, natural and artificial recharge, hydrogeology, hydraulic characteristics, seasonal variations, and abstraction activities. Integration of this information indicates groundwater movement is predominantly horizontal and northwestward, with highly restricted vertical movement downwards due to the presence of thick sequences of silt-based materials beginning at an average depth of ~10m: A numerical groundwater model was developed incorporating the site information to estimate groundwater flow patterns within the study area. The model suggests that lateral flow northwestward within the unconfined aquifer predominates in the study area and that there is probably minimal vertical movement from the upper aquifer zones to the deep aquifer. An analysis of recent geochemical data is undertaken in this study to (i) determine the horizontal and vertical extent of modified groundwater movement; and (ii) provide ground truth for the electrical resistivity technique. The results of the analysis reveal elevated levels of sodium in shallow groundwater in a zone centred on the wastewater irrigation farm, with some localised lateral migration to the South, West and Northwest. Movement of modified groundwater vertically has occurred to a maximum depth of ~16 m on-site. The surface geophysical technique, electrical resistivity was employed at the site to: (i) provide input about flow dynamics beneath the site, through the delineation of the low resistivity modified groundwater plume; and (ii) evaluate the success of the technique in detecting spatial variations in groundwater quality. A comprehensive survey was undertaken involving vertical soundings and horizontal profiling measurements. In general, a modified groundwater plume consistent with that defined by geochemical data was identified thereby validating the application of the technique and confirming recognised movement patterns. However, the application of the technique is limited to areas where high conductivity modified groundwater controls the resistivity signal. Beyond these areas it is proven that the resistivity data may be an artefact of lithology. The development of the qualitative groundwater hydrological model was achieved through the integration of the flow information obtained via the outlined investigative methods. The main outputs of the groundwater model are that modified groundwater movement is: (i) Constrained within the unconfined aquifer; and (ii) Predominantly horizontal, therefore 2-dimensional. It is concluded that the current wastewater disposal activity appears to be performing sustainably in terms of nitrogen disposal, however consistent increases in sodium levels over the wastewater disposal site to the boundaries raises the possibility of future increases in sodium in groundwater just beyond the boundaries in some locations.
Type of thesis
Whatnall, N. (1999). Groundwater flow dynamics beneath a wastewater irrigation site, Hautapu, Cambridge (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13683
The University of Waikato
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