Water and nutrient flow pathways in the Papanui Catchment : source, mean residence time, and connection between ground- and surface-water

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Morgenstern, U.; Gordon, D. 2017 Water and nutrient flow pathways in the Papanui Catchment : source, mean residence time, and connection between ground- and surface-water. Lower Hutt, N.Z.: GNS Science. GNS Science report 2017/08 27 p.; doi: 10.21420/G2KG6G

 

Abstract: Hawke’s Bay Regional Council aims to reduce algal growths in surface waters through improved management of nutrients in the Papanui Catchment. In this study, age, gas, isotope, and chemistry tracers were analysed in groundwater and surface water to understand the source, pathways, and lag times of the water and the nutrients that travel with the water from land to the waterways. All investigated groundwater wells contained anoxic water, and four of the six wells contained highly anoxic water to the stage of methane fermentation. The groundwaters appear not to have been recharged from surface waters, including Waipawa River, but from local rain. Most of the wells contain very old groundwater (> 100 years), even though wells were relatively shallow, indicating very little connection between these groundwaters and the surface waters. Only two of the shallower wells contained younger water, with mean residence times (MRTs) of 1.5 and 28 years, indicating active recharge from the surface. The old groundwaters contained very little nitrate, but the younger groundwaters contained up to around 2 mg/L, indicating anthropogenic sources of nitrate. Older groundwaters had elevated phosphate (P) concentrations, indicating that the dissolved P in the groundwater originated from leaching from the aquifer material, rather than P from land-use activities. The large rivers (Tukituki and Waipawa) discharge relatively young water (c. 2 years), reflecting the basement rock conditions in their upper catchments. The stream waters in the Papanui Catchment are significantly older, with MRTs of up to 18 years, reflecting favourable conditions for rain water to infiltrate into the deeper groundwater system in the limestone, sandstone, and mudstone areas. The rivers appear to have highly conductive gravel beds. For the Papanui Stream at Walkers Road the stable isotope composition indicates that about half of the water originates from the Waipawa River, and the other half from local rain recharge, which contributes significant nitrate to the stream (1.7 mg/L). At Camp David, younger water in the stream and a significant increase in flow indicate that Papanui Stream just before its confluence with Tukituki River already contains a large fraction of water from the Tukituki River. The highest P concentrations are also observed in the oldest surface waters, indicating that the soluble P in the surface water at baseflow originates mainly from leaching of P from the geologic formations. Elevated P in stream waters with a limestone signature indicates P leaching from the limestone as one of the main sources of dissolved P in the streams during baseflow conditions. (auth)