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Hurunui River

Dissolved Reactive Phosphorus Concentrations and Loads

A tool to manage periphyton growth ?

Review of Ecan & NIWA Datasetof DRP measurements at SH1

Samples taken between

2004 to June 2014

Purpose of Presentation

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• What do the water quality samples in the Hurunui River tell us about Phosphorus Concentrations and Loads?

• How relevant is an in-river load limit to achieving in-river water quality outcomes?

• How should P Loads be measured relative to the 2005-2010 baseline?

• Has the P Load Limit at SH1 really been exceeded?

• Should “landuse change” be a “permitted activity” or a “non-complying activity” under Rule 10.2(b) of the HWRRP?

Background: Periphyton Accumulation

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• NIWA study in April 2014;• Study to determine if adding N

and P to the Hurunui River results in additional growth of Periphyton;

• Results

• Downstream of Balmoral forest:• the growth of Periphyton is

limited by the low concentration of Phosphorus in the river;

• the concentration of nitrates in the river can be increased without increasing the growth Periphyton.

DRP - Dissolved Reactive Phosphorus

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12 Months of the Year

Ignoring season and river flow …

All year round DRP concentrations appear to be stable

DRP - Dissolved Reactive Phosphorus

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Good news: DRP concentrations during the Summer months at flows less than 60 cumecs have declined.

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Water Samples: DRP Concentration g/m3 (mg/L)

Summer Months at flows less then 60 cumecs

DRP (Concentration and Load) relationships with River Flow

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• There is a weak positive relationship between DRP Concentration and River Flow;

• There is a strong positive relationship between DRP Load and River Flow;

• High flow events that remove Periphyton also have high DRP loads;

More Good News: Hurunui River Flows since 2005 have been increasing

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Average River Flows for the period 2009-2013 are 14% higher than for the period 2005-2009.

But… Higher Flows = Higher DRP Loads

EXAMPLE - 2005 Low flows = Low DRP Load

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2005 DRP ConcentrationSH1 Flow 46.2 m3/sec Jan 0.0038 mg/l

Feb 0.0037 mg/lMar 0.0039 mg/l

DRP Load 6.1 Tonnes Full Year 0.0038 mg/l

0

50

100

150

200

250

300

350

400

450

500

1 31 61 91 121 151 181 211 241 271 301 331 361

SH1 Flow (m3/sec) by Day of Year

Low flows = limited flushing flows = lots of Periphyton accumulation = Low DRP Load Change of land use remains a permitted activity !!!

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High flows = flushing flows = Periphyton removal = DRP Load Limit exceeded Change of land use becomes a non-complying activity !!!

2013 DRP ConcentrationSH1 Flow 83.1 m3/sec Jan 0.0043 mg/l

Feb 0.0036 mg/lMar 0.0035 mg/l

DRP Load 15.5 Tonnes Full Year 0.0043 mg/l

0

50

100

150

200

250

300

350

400

450

500

1 31 61 91 121 151 181 211 241 271 301 331 361

SH1 Flow (m3/sec) by Day of Year

EXAMPLE – 2013 High flows = High DRP Load

DRP – Modelled* Annual Load and 6 year Rolling Average Load

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Using the Ecan method to calculate Load, the 6 year average DRP load is increasing. This is because river flows are increasing;

____

BUT.......

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DRP Concentrations appear to be in decline !!!

Has the Schedule 1 DRP load limit really been exceeded ???

To answer this question, we should apply the 2009-2014 DRP

concentrations to the 2005-2010 river flows to determine if the Schedule 1 DRP load (regulating water quality)

has been exceeded.

DRP – Average DRP Concentrations at River Flows between 20 to 90 cumecs

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• Taking River flow into account, DRP concentrations over the period 2009-2014 are around 15% less than concentrations over the period 2005-2010;

• After removing the impact river flows, we can conclude the 2009-2014 DRP load is around 15% less than the equivalent 2005-2010 DRP load;

25% less Statistically significant at 95% Confidence

DRP – Average DRP Concentrations : SUMMER River Flows (20 to 50 cumecs)

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Summer DRP concentrations are much lower than the 2005-2010 baseline

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

10 20 30 40 50 60

2005-20102010-2014

Load Limit - Conclusions

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• An in-river Load Limit is a very poor tool to manage in-river water quality and regulate landuse change;

• The HWRRP should replace “In-River Load Limits” with catchment wide:• On-Farm N Load limits (t/ha/yr); &• On-Farm Best Practice phosphorus

management requirements.

The 2009-2014 DRP load is around 15% less than the equivalent average annual 2005-2010 DRP load.

Landuse change under Rule 10.2(b) should remain a permitted activity.

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1

2

FARM

GROUNDWATER

MAINSTEM

SPRINGS/TRIBUTARY

InvertebrateToxicity &Periphyton

effects

InvertebrateToxicity,

Periphyton &toxic bloom

effects

Concentration Limits

Nitrate-NitrogenToxicity Limits &

Low-flow Phosphorus Limits

Periphyton Objectives Concentration Limits

Nitrate-Nitrogen Toxicity Limits &

Low-flow Phosphorus Limits

Periphyton Objectives

High Medium Low

High Medium LowLimit: Effectiveness

Farmer: Response and Incentive

Nitrate-Nitrogen & Phosphorus management

High

Groundwater Quality Limit

On-Farm Load Limit

N-Loss (kg/ha/yr) &P-Loss Management

Requirements;

Replace in-river Load limits with:• On-Farm (Catchment wide) N Load limit

(t/ha/yr); &• Best practice phosphorus management

requirements.

Water Quality Management

- Set Limits where they can be managed- On-Farm Cause-Effect-Consequence-

Reward

Human Health&

ConsequentialLowland Stream,

Tributary &Mainstem

effects

Medium

Low

What is Ngai Tahu Property doing over the next 6 months...

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• A more comprehensive NIWA study this summer in the Hurunui River to measure the accumulation of Periphyton biomass over time, and in response to additional N & P;

• A Lincoln Agritech research project on groundwater flows and in-river N & P concentrations impacting the Hurunui River;

• Preparation of a “Best Practice Phosphorus Manual” specific to the Hurunui Catchment;

• Based on the above information (April/May 2015), consider the implications for N & P management and the need for a Plan Change.