Heretaunga Springs: Gains and losses of stream flow to groundwater on the Heretaunga Plains

Abstract or Summary

Executive summary Springs are the link between groundwater use and stream flows. Managing water use for the protection of stream ecosystems benefits from an understanding of spring inputs, because most water use on the Heretaunga Plains is from groundwater. This groundwater is used for irrigation, industry and drinking water. Springs of the Heretaunga Plains were investigated to help inform flow-ecology investigations and groundwater modelling. This report is primarily a technical reference, focussing on the location of flow gains and losses, as well as the methods used to quantify them. Readers looking for a general overview are directed to Section 1 (Introduction) and Section 4 (Synthesis). The results (Section 3) provide details for specific sub-catchments. The location of springs was investigated using a range of methods, starting with aerial photographs to narrow down the areas of interest. Spring locations and property access were then discussed with landowners before walking or kayaking the length of targeted streams. Measuring electrical conductance often revealed spring inputs that were sourced from larger catchments with less dissolved ions (e.g. Ngaruroro water). More quantitative surveys included flow gauging and longitudinal surveys of electrical conductance. Stable isotopes provided further insight into the source of spring water. Loss of flow to groundwater was estimated from the difference in gauged flow between sites. The investigation identified 64 springs throughout the Heretaunga Plains, which represents the largest number of springs documented to date. This report does not provide a complete list of springs, instead capturing the major gains and major losses for sub-catchments of the Heretaunga plains. Most of the major flow losses to groundwater have been described previously, including losses from the Ngaruroro River that continue through dry summers when the aquifer receives little recharge from local rainfall. During these times, flow losses from the Ngaruroro River are vital for sustaining spring flow to the Raupare, Tutaekuri-Waimate and Waitio streams. Likewise, flow losses from the Tutaekuri River probably sustain springs in Moteo Valley (Tutaekuri-Waimate headwaters). This investigation located large springs that contribute more than half of the low flow to the Karamu Stream. The Tukituki River was probably the main source of spring inflows direct to the Karamu Stream and to the Mangateretere Stream under low flow conditions on 3/3/2015. This changed in winter (23/8/2017), when isotope results indicated that nearly half of the groundwater originated from the Ngaruroro River. These investigations also revealed a tufa coating (calcite deposited from flowing water) on the bed of the Paritua Stream. The Paritua Stream has run dry in summers past. This stream loses flow to groundwater where it crosses unconfined alluvial gravels upstream of Bridge Pa. The tufa coating is important in extending the length of flowing stream because it probably reduces the rate of flow loss to groundwater. An extensive area of shallow Taupo pumice sands contributes groundwater to several streams, including the Karewarewa, Louisa and Awanui. Little is known of the groundwater in this pumice sand layer because it is not used for irrigation or domestic water supply. However, it may be an important source of flow and nutrients for these streams, and hence deserves further investigation. This report goes beyond describing where streams are, to also describe their location in the past. For example, the Hawke’s Bay earthquake of 1931 changed the drainage patterns of the Heretaunga Plains, including shifting the Paritua outflow from Irongate Stream to Karewarewa Stream. On alluvial plains such as the Heretaunga, floods and channel processes generate a naturally dynamic river network. In addition to these natural processes, people have made significant changes to the stream network to reduce flooding and drain soils for horticulture.