SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 21 Overall, the testing indicates that there are some permeable soils in the near surface, but that most of the materials beneath the Willows Farm site have relatively low hydraulic conductivity. The main geological unit the access tunnel is to be driven through is the andesite, which has a geometric mean of 3.0 x 10-8 m/s. This is similar to that measured elsewhere in Waihi. Given the rockmass being dewatered is of low permeability, any associated dewatering effects are expected to be limited. 4.2 Conceptual Groundwater Model The conceptual geologic model for the site underpins the groundwater effects assessment in that it identifies risk pathways associated with dewatering. It also forms the basis for the numerical modelling undertaken to quantify the drainage risks. The conceptual model for the Willows Farm site is described as follows and is illustrated in Figure 18 and 19. • Rainfall that does not run-off infiltrates the soil profile • High permeability shallow soils store recharge water with some of this water moving downslope (interflow) • Water moving down the slopes and direct rainfall infiltration results in a perched water table locally in the regolith and terrace deposits • Interflow water continues to move down slope to the Stream • Some rainfall infiltration percolates down into the deeper rockmass with saturation below the perched water table in the regolith • Flow paths then result in deep groundwater discharge to the Stream as baseflow • Deep groundwater flow moves down gradient though the catchment • Fracture zones that are orthogonal to the flow direction intercept some of this groundwater • Higher permeabilities in the fracture zones results in preferential groundwater flow down the length of the zone resulting in high discharge zones in the Mataura Stream. Based on this conceptual model, the key risk to understand is how much stream flow will be intercepted as the access tunnel passes though the fracture zones prior to these zones being sealed off. The risk is higher at these locations due to the assumed higher permeability values and given the tunnel is still relatively shallow as it continues on a descent. However, intercepted water is to be diverted to the water treatment plant before being discharged to the Ohinemuri River. This water is not lost from the greater catchment.
RkJQdWJsaXNoZXIy MjE2NDg3