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Wairarapa Valley Groundwater Resource Investigation Upper Valley Catchment Hydrogeology and Modelling

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Wairarapa Valley Groundwater Resource Investigation Upper Valley Catchment Hydrogeology and Modelling Wairarapa Valley groundwater resource investigation Upper Valley catchment hydrogeology and modelling Wairarapa Valley groundwater resource investigation Upper Valley catchment hydrogeology and modelling M. C. Gyopari (Phreatos Limited) D. McAlister (Greater Wellington) For more information, contact Greater Wellington: Wellington Masterton GW/EMI-T-10/74 PO Box 11646 PO Box 41 November 2010 T 04 384 5708 T 06 378 2484 F 04 385 6960 F 06 378 2146 www.gw.govt.nz www.gw.govt.nz www.gw.govt.nz [email protected] 1 DISCLAIMER This report has been prepared by Environmental Monitoring and Investigations staff of Greater Wellington Regional Council and as such does not constitute Council’s policy. In preparing this report, the authors have used the best currently available data and have exercised all reasonable skill and care in presenting and interpreting these data. Nevertheless, Council does not accept any liability, whether direct, indirect, or consequential, arising out of the provision of the data and associated information within this report. Furthermore, as Council endeavours to continuously improve data quality, amendments to data included in, or used in the preparation of, this report may occur without notice at any time. Council requests that if excerpts or inferences are drawn from this report for further use, due care should be taken to ensure the appropriate context is preserved and is accurately reflected and referenced in subsequent written or verbal communications. Any use of the data and information enclosed in this report, for example, by inclusion in a subsequent report or media release, should be accompanied by an acknowledgement of the source. The report may be cited as: Gyopari M. and McAlister D. 2010. Wairarapa Valley groundwater resource investigation: Upper Valley catchment hydrogeology and modelling. Greater Wellington Regional Council, Technical Publication No. GW/EMI-T-10/74. Executive summary Greater Wellington Regional Council (Greater Wellington) has undertaken a comprehensive investigation of groundwater in the Wairarapa Valley to re-assess the sustainable yields of aquifers in the valley. Phase 2 of the investigation, reported here, provides a technical analysis of the groundwater environments of the Wairarapa Valley and presents three sub-regional numerical groundwater flow models. These models will be used in the third phase of the investigation to evaluate aquifer sustainable yields and assist in a review of Greater Wellington’s existing groundwater allocation policy for the Wairarapa Valley. This report documents the results and outcomes of the Phase 2 hydrogeological and groundwater modelling investigation for one of three sub-regions of the Wairarapa Valley – the Upper Valley catchment. This catchment encompasses an area of 160 km2 largely to the northeast of the Waingawa River and centred on the town of Masterton. The smaller Waipoua, Kopuaranga and Whangaehu tributaries of the Ruamahanga River also occur in the catchment along with numerous small streams and spring systems on the lowland fans and plains. Agriculture is the dominant landuse in the catchment, particularly sheep and beef farming. The Te Ore Ore plain is an intensively farmed area within the catchment where a significant amount of groundwater abstraction occurs for irrigation purposes. The Phase 2 investigation entailed the development of a geological framework followed by a hydrogeological analysis from which conceptual and numerical groundwater models were formulated. A field investigation programme, designed to address critical information gaps, included drilling of monitoring bores, river and spring flow gaugings, a water metering study, piezometric surveying and hydrochemical sampling. Core research themes of the investigation were: • Geological and structural characterisation of the catchment • Analysis of temporal and spatial groundwater levels and regional flow patterns • Rainfall recharge quantification • Groundwater–surface water interaction characterisation • Groundwater abstraction analysis and modelling, and • Hydrochemical investigations and statistical modelling. Formulation of a three-dimensional geological framework assisted in the characterisation of the Upper Valley groundwater environment. A heterogeneous succession of late Quaternary and Holocene unconsolidated sediments comprise the dynamic groundwater environment of the catchment. Variable degrees of sediment sorting, reworking, compaction and deformation by faulting and folding have resulted in a complex aquifer system. Major structural features, such as the Masterton and Mokonui faults, have dislocated and folded the sediment sequence and created the Te Ore Ore basin. Four broad hydrostratigraphic units were identified – the most important, in terms of groundwater resource potential, are recent (Holocene age) alluvium connected to major river systems, and older Tararua-sourced alluvium in the Te Ore Ore basin forming semi-confined aquifers. The groundwater head distribution shows that the Upper Valley groundwater environment behaves as a hydraulic continuum with variable degrees of impedance across major structural features (such as the Masterton Fault) where groundwater is forced to discharge. The groundwater flow pattern reflects a system in which rivers interact closely with adjacent shallow aquifers – groundwater and surface water are indistinguishable in such areas. Rainfall recharge was modelled using a soil moisture balance technique on a 500 m2 grid incorporating detailed spatial climate modelling and soil property mapping. The modelling showed that about 30-40% of rainfall goes to recharging groundwater over the western and northern areas of the catchment, while less than 10% of rainfall reaches the water table over the drier eastern part. The average recharge, for the 16-year period 6 3 3 ending in 2008, was 48 x 10 m /year, or 130,000 m /day. Temporal changes in the dynamics of the groundwater system are attributable to a combination of natural climatic variability and abstraction stresses. Inter-seasonal variability in recharge rate reflects temporal rainfall patterns driven by the El Nino Southern Oscillation (ENSO). Areas such as the Te Ore Ore basin also show clear evidence of abstraction-related seasonal declines in groundwater level. Fluxes between shallow groundwater and surface water dominate the groundwater balance for the Upper Valley catchment. Natural groundwater discharges occur as river base flow, spring flow and diffuse seepage into wetlands. Some reaches of the main river channels recharge groundwater by losing part, or sometimes all, of their flow into underlying aquifers. Concurrent river gauging surveys show that the three principal river systems – the Ruamahanga, Waipoua and Waingawa rivers – exhibit complex patterns of flow gain and loss with respect to underlying shallow aquifers. Groundwater abstraction in the catchment has more than doubled over the past 10 years primarily due to demand for seasonal pasture irrigation. At the time of initial groundwater model development in 2008, the total consented groundwater allocation was about 46,000 m3/day and 8.25 x 106 m3/year. About 85% of the total abstraction occurs on the Te Ore Ore plain and from bores near the Ruamahanga River upgradient of Lansdowne Hill. Annual meter readings show that water users do not normally exceed 50% of their annual allocation (10-30% being the norm). A metering study showed that resource consent holders tend to abstract about 60-70% of their consented daily rate. During the 2007/08 irrigation season the metered abstraction was about 27,000 m3/day – 60% of the allocated daily volume. Historical groundwater abstraction rates for the catchment were modelled using soil moisture deficits to estimate demand periods in conjunction with available annual meter records. Multivariate statistical analysis of groundwater and surface water data, in conjunction with mean residence time and stable isotope data, supported the conceptual hydrogeological model development. In particular, water chemistry helped with stratigraphic correlation work and the identification of aquifer flow paths. Conceptually, the Upper Valley groundwater catchment is characterised as a ‘closed’ groundwater basin in which the dominant water balance components are rainfall recharge and fluxes between surface water and groundwater. Rainfall infiltration and river bed leakage are both important for recharge. The most important hydrogeological characteristic of the catchment is the strong interdependence of surface water and groundwater. Groundwater abstraction constitutes more than 10% of the catchment water balance during the summer months. The effects of abstraction on shallow, unconfined, dynamic aquifers are of particular significance since these aquifers are freely connected to the surface water environment (rivers, streams, springs and wetlands). The conceptual hydrogeological model for the Upper Valley catchment was verified and transformed into a numerical transient flow model using FEFLOW finite element code. The model was qualitatively and quantitatively calibrated to field measurements of groundwater level and fluxes to and from surface water environments. The calibration process followed procedures that minimise non-uniqueness and predictive uncertainty. Calibration robustness was achieved for the principal aquifers – the shallow unconfined Holocene aquifer of the Waingawa, Waipoua and Ruamahanga floodplains, and the semi-confined aquifers in the Te Ore Ore basin. Simulated
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