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A steady state groundwater flow model was built for Ruataniwha basin. The results of this model
will be used to build the transient state model. The transient model will couple groundwater flow
model (MODFLOW) with the surface water flow model (MIKE 11) to simulate the groundwatersurface
water interaction.
Ruataniwha basin is a closed basin; there...
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Citations
... The process takes into account provision of additional irrigation water from the RWSS and the resulting land intensification. The groundwater flow modelling on which this work is based has been described in detail (Baalousha, 2009Baalousha, , 2010). These reports provide full details regarding model conceptualisation and parameterisation. ...
... Calibrated hydraulic properties were unchanged from the flow model, including calibrated hydraulic conductivity, storativity and streambed conductance (Baalousha 2009Baalousha , 2010). The solute transport model also requires specification of effective porosity and dispersivity. ...
... Contaminant transport model output includes estimates of the 3D spatial distribution of nitrate concentrations. The model consists of three layers (see Baalousha 2009 Baalousha , 2010) representing the top Young Gravel, the second thin layer of intermittent aquitard and the lower layer, which represents the Old Gravel Formation.Figure 4 shows the nitrate concentration predicted at the end of 2052 in the top layer (Layer 1). It predicts that the maximum permissible limit of 11.3 mg/L has been exceeded at four reasonably isolated locations, with an absolute maximum predicted nitrate concentration of 14.7 mg/L. ...
The proposed Ruataniwha Water Storage scheme will create the potential to irrigate approximately 25,000 hectares of the Ruataniwha Plains. This is almost four times that of the 6,000 hectares of land currently irrigated. Increased irrigation generally favours intensification of farming practices, which increases the potential for nitrogen loss to groundwater through leaching. As a consequence, adverse impacts on groundwater quality may be anticipated, in particular from increasing nitrate concentrations.
Contaminant transport modelling was used to explore the fate of nitrogen in groundwater and help identify areas where greatest risk of nitrate contamination is likely. The modelling used a three-dimensional groundwater flow model (Baalousha, 2009, 2010) to provide input to a solute transport model (Zheng, C. and Wang, 1999), which was used to simulate advection (horizontal and vertical movement) and dispersion (dilution) within the aquifer.
The concentrations of nitrate in the aquifer recharge (a key transport model input) were prepared by NIWA (Rutherford 2013), from leaching losses predicted for future land use intensification (Wheeler, 2013). Groundwater flow and contaminant transport models were run for a 35 year period starting in 2017, the date proposed for commencement of the irrigation scheme.
The contaminant transport model predicts the likely concentration of a solute (in this case nitrate), at any point within the model domain and at any time within the model timeframe. Three groundwater layers were defined in the model in terms of depth.
The modelling process predicted:
Concentrations were predicted to increase in all three groundwater layers over time.
Highest nitrate concentrations were predicted in layer 1 (shallow groundwater).
In general, the predicted nitrate concentrations are below the MAV.
Nitrate concentrations were likely to exceed the Maximum Acceptable Value (MAV) of the New Zealand Drinking Water Guidelines (11.3 mg/L as nitrate-N) at three locations.
The maximum nitrate concentration predicted is 14.7 mg/L, likely to occur 35 years from the start of the simulation.
It must be noted that groundwater attenuation has not been considered in this modelling, which is conservative approach (i.e. predicts a worst-case scenario for nitrogen reduction).
Although this approach is suitable to identify areas where there is a risk of exceeding the NZ Drinking Water Guidelines (in certain isolated areas where slightly elevated nitrate concentrations are predicted), there are significant uncertainties involved in this study, as in all solute modelling approaches. Therefore, the model-derived estimates of likely future nitrate concentrations should be used qualitatively to guide monitoring and management of future land use intensification and related groundwater nitrogen concentrations. The monitoring activities will provide data to validate the model and will confirm areas where nitrate leaching is occurring and the extent and magnitude of any groundwater contamination. If necessary, mitigation strategies may be developed in response to land use change and groundwater contamination.
... A water budget developed for the area identified groundwater pumping as being only a small proportion of the total budget. Baalousha (2008aBaalousha ( , 2008bBaalousha ( , 2010 developed steady and transient groundwater-surface water flow models for the Ruataniwha Basin. The finite difference-based Modflow model was used for that study (Harbaugh et al., 2000). ...
