MANAGING OUR ENVIRONMENT Nitrogen and Phosphorus Loads to Rivers in the Manawatu-Wanganui Region : An Analysis of Low Flow State : Technical Report to Support Policy Development Nitrogen and Phosphorus Loads to Rivers in the Manawatu-Wanganui Region: An Analysis of Low Flow State Technical Report to Support Policy Development August 2007 Authors Kate McArthur and Maree Clark Acknowledgements to Jon Roygard, Marianne Watson and the Environmental Compliance Team Internal Review Greg Bevin – Senior Environmental Compliance Officer Jeff Watson – Manager Resource Data External Review Bob Wilcock and Neale Hudson, NIWA Graham Sevicke-Jones, Hawkes Bay Regional Council Kirsten Meijer, Environment Southland August 2007 ISBN: 1-877413-81-X Report No: 2007/EXT/793 CONTACT 24hr Freephone 0508 446 749 [email protected] www.horizons.govt.nz Kairanga Palmerston North Dannevirke Cnr Rongotea & 11-15 Victoria Avenue Weber Road, P O Box 201 Kairanga-Bunnythorpe Rds Private Bag 11 025 Dannevirke 4942 Palmerston North Manawatu Mail Centre Palmerston North 4442 Levin 11 Bruce Road, P O Box 680 Marton T 06 952 2800 Levin 5540 Hammond Street F 06 952 2929 SERVICE REGIONAL P O Box 289 DEPOTS Pahiatua CENTRES Marton 4741 HOUSES Cnr Huxley & Queen Streets Wanganui P O Box 44 181 Guyton Street Pahiatua 4941 Taumarunui P O Box 515 34 Maata Street Wanganui Mail Centre Taihape P O Box 194 Wanganui 4540 Torere Road, Ohotu Taumarunui 3943 F 06 345 3076 P O Box 156 Taihape 4742 EXECUTIVE SUMMARY The growth of nuisance periphyton in many rivers and streams of the Manawatu- Wanganui Region is a significant environmental issue, requiring the reduction of nitrogen and phosphorus loads to surface water. Timeframes and implementation of reductions in point source dissolved reactive phosphorus, attempted through the Manawatu Catchment Water Quality Regional Plan, have not achieved the environmental outcomes sought. New water quality standards in the Proposed One Plan (Proposed Regional Policy Statement and Regional Plan) for dissolved reactive phosphorus, soluble inorganic nitrogen and other contaminants have been applied to all waterways in the Region at flows less than three times the median (Ausseil & Clark, 2007a). In most water management zones the Proposed One Plan dissolved reactive phosphorus standard is more stringent than the Manawatu Catchment Water Quality Regional Plan. Ensuring discharges to water comply with the proposed standards will be pivotal to achieving the desired water quality outcomes. Existing State of the Environment and compliance water quality monitoring data was used, together with continuous hydrological data to calculate loads across a number of catchments to assess the relative contributions of point and non-point source nutrient loads. Emphasis was given to low flow conditions; specifically mean annual low flow and half median flow. At low flows, the proposed nutrient standards were exceeded at a number of locations. Although nitrogen and phosphorus loads were estimated from a variety of data sources and loadings calculations of this nature can have considerable biases, the results were indicative of ongoing nutrient enrichment issues resulting from both point source and non-point sources at low flows. Phosphorus loads were attributed to point sources whereas in many catchments, nitrogen appeared to be non-point sourced. This report recommends a number of areas for further monitoring to better validate these results, and suggests the following changes to compliance and State of the Environment monitoring: 1. State of the Environment and compliance data should be collected on the same day, at similar flows within water management zones. 2. The effect of mixing zones and inflowing tributaries or confluences in relation to discharge sampling location needs examination. For significant discharges, mixing zones should be determined by mixing trials. 3. Flow should be gauged at the same time and location of water sample collection to establish flow relationships to the nearest continuous flow recorder. 4. The volumes of effluent discharged should be measured continuously, with results telemetered to the Regional Council for significant discharges, whereas smaller discharges should provide daily discharge volumes regularly. 5. Data derived from compliance and State of the Environment monitoring should be publicly available. Consideration should be given to making this data available via the internet to show compliance with receiving water standards and consent conditions. Nitrogen and Phosphorus Loads to Rivers in the Manawatu-Wanganui Region: An Analysis of Low Flow State i ii Summary table of compliance with proposed nitrogen and phosphorus standards during low flows at 17 State of the Environment (SOE) monitoring sites in the Manawatu-Wanganui Region between 1989 and 2006. Executive Summary SIN DRP meets SIN meets SIN meets DRP meets DRP SOE Monitoring Site predominant SIN predominant DRP standard at standard at ½ standard at standard at ½ ∗ source ∗ source MALF median MALF median Manawatu at Weber Road 8 8 non-point 8 9 geology/unknown Manawatu at Hopelands 8 8 non-point 8 8 point source Mangatainoka at SH2 8 8 non-point 8 8 point source Manawatu at Teachers College 8 9 non-point 9 9 - Manawatu at Opiki Bridge 8 9 point source 8 8 point source Nitrogen andPhosphorus Loadsto Rivers Oroua at Awahuri Bridge 8 8 point source 8 8 point source Rangitikei at Pukeokahu* 9 9 - 8 8 natural geology Hautapu at NIWA station Taihape 9 9 - 8 9 geology/unknown Hautapu upstream Rangitikei 8 9 point source 8 8 point source Rangitikei at Mangaweka 9 9 - 9 9 - Rangitikei at Vinegar Hill 9 9 - 9 9 - Whanganui at Cherry Grove* 8 8 non-point 9 9 - Whanganui at Te Maire* 8 9 non-point 8 9 point source Whanganui at Pipiriki 9 9 - 9 9 - Whangaehu at Kauangaroa* 9/ 8 9 non-point 8 9 point source Mangawhero at DoC Headquarters 9 9 - 8 8 natural geology in theManawatu-Wanganui Region: Owahanga at Branscombe Bridge 9 9 - 8 9 point source Key: 8: Does not meet the standard; 9: Meets the DRP standard; 9: Meets the SIN standard An Analysis of Low Flow State An Analysis ofLow ∗ or 75th percentile flow at sites affected by the Tongariro Power Development (TPD) CONTENTS Executive Summary i Contents iii 1. Introduction and Scope 1 1.1 Introduction 1 1.2 Project Scope 1 1.2.1 Project Aims 2 1.2.2 Planning Context 2 1.2.3 Peer Review 3 1.3 Background 3 1.3.1 The Impacts of Nitrogen and Phosphorus 3 1.3.2 The Importance of Understanding Nutrient Loads at Low Flows 4 2. Data Sources 6 2.1 Water Quality Monitoring 6 2.1.1 State of the Environment Data 6 2.1.2 Compliance Monitoring 6 2.1.3 Non-Point Source Nutrient Loads 7 2.1.4 Flow Data 7 3. Methods for Calculating Nutrient Loads 8 3.1 Flow-Stratified Load Calculation 8 3.2 Estimated Daily Load at Low Flows 9 3.2.1 Point Source Nutrient Loads 9 3.2.2 SOE Nutrient Loads 10 3.2.3 Proposed Nutrient Loads 10 3.3 Assumptions and Limitations 10 3.3.1 Limitations of Load Calculations 10 3.3.2 Assumptions Underlying the Interpretation of Results 10 4. Manawatu River Catchment Nutrient Status 12 4.1 What does our SOE monitoring tell us about the nutrient status of the Manawatu River at low flows? 15 4.1.1 SOE Nutrient Concentrations 15 4.1.2 SOE Nutrient Loads 17 4.2 What are the sources of nutrient enrichment in the Manawatu catchment? 18 4.2.1 Dairy Discharges 18 4.2.2 Point Source Nutrient Loads 21 4.3 Upper Manawatu catchment 23 4.3.1 Point Source vs. SOE Nutrient Loads 23 4.4 Manawatu catchment – upstream of Hopelands 26 4.4.1 Sources of Nitrogen Upstream of Hopelands 26 4.4.2 Sources of Phosphorus Upstream of Hopelands 27 4.5 Mangatainoka River Catchment Nutrient Status 29 Nitrogen and Phosphorus Loads to Rivers in the Manawatu-Wanganui Region: An Analysis of Low Flow State iii Contents 4.5.1 Sources of Nitrogen in the Mangatainoka River 30 4.5.2 Sources of Phosphorus in the Mangatainoka River 31 4.6 Lower Manawatu Catchment – downstream of Teachers College 32 4.6.1 Point Source vs. SOE Nutrient Loads 33 4.6.2 Sources of Nitrogen in the Oroua River 36 4.6.3 Sources of Phosphorus in the Oroua River 37 4.7 Conclusions and Recommendations - Manawatu Catchment 38 4.7.1 Recommendations for further investigation – Upper Manawatu 39 4.7.2 Recommendations for further investigation – Lower Manawatu 39 5. Rangitikei River Catchment Nutrient Status 41 5.1 What does our SOE monitoring tell us about the nutrient status of the Rangitikei River at low flows? 44 5.1.1 SOE Nutrient Concentrations 44 5.1.2 SOE Nutrient Loads 45 5.1.3 Rangitikei Catchment Point Source vs. SOE Nutrient Loads 47 5.1.4 Hautapu River Nutrient Loads 49 5.2 Conclusions and Recommendations – Rangitikei River Catchment 51 5.2.1 Recommendations for further investigation – Rangitikei 52 6. Whanganui River Nutrient Status 53 6.1 What does our SOE monitoring tell us about the nutrient status of the Whanganui River at low flows? 55 6.1.1 SOE Nutrient Concentrations in the Whanganui River 55 6.1.2 Point Source vs. SOE Nutrient Loads in the Whanganui River catchment 57 6.2 Conclusions and Recommendations – Whanganui River Catchment 59 6.2.1 Recommendations for further investigation – Whanganui River 60 7. Whangaehu River Catchment Nutrient Status 61 7.1 What does our SOE monitoring tell us about the nutrient status of the Whangaehu, Mangawhero and Makotuku Rivers at low flows? 64 7.1.1 Point Source vs. SOE Nutrient Loads in the Whangaehu River catchment 64 7.2 Conclusions and Recommendations – Whangaehu, Mangawhero and Makotuku River Catchments 67 7.2.1 Recommendations for further investigation – Whangaehu, Mangawhero and Makotuku River catchments 67 8.