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“Trigger” values for New Zealand rivers

Executive Summary

Trigger values for ecosystem protection are needed in the new ANZECC guidelines that are currently being drafted. If median water quality at monitoring sites exceeds these trigger values, then it is intended to “trigger” a management response. Ministry for Environment has asked NIWA to estimate such trigger values for New Zealand rivers using percentiles taken from the National Rivers Water Quality Network (NRWQN) datasets. Of the 77 sites in the NRWQN, 32 are considered baseline or pseudo-baseline (minimally or lightly impacted) and 11 of these sites are lake-fed or affected by alpine headwaters. The remaining 21 sites were used for estimation of trigger values (as 80 or 20 percentiles) after being further categorised into 3 lowland sites (elevation at monitoring site < 150 m) and 18 upland sites. Variables for which trigger values were estimated were: total phosphorus, dissolved reactive phosphorus, nitrogen, oxidised nitrogen (nitrate + nitrite-N), ammoniacal nitrogen (ammonia + ammonium-N), dissolved oxygen as % saturation, pH, black disc visibility, turbidity, and temperature. In order to obtain a meaningful trigger value for temperature, data for February alone (high insolation, low flow) were used. The percentiles obtained for pH and especially for %DO, may require professional judgement in order to derive trigger values because these variables are subject to diurnal fluctuation. The percentiles are presented and discussed with reference to trigger values for SE Australian rivers.

Introduction

Trigger values for ecosystem protection are needed for the new ANZECC guidelines that are currently being drafted. These trigger values indicate marginal water quality for supporting ecosystem health (Hart et al. 1999). Running medians of water quality data measured in monitoring programmes may be compared with these trigger values. If the median value of a water quality variable for a particular site exceeds the trigger value, then it is intended to “trigger” a response on the part of water managers, which might be to initiate special sampling or carry out an investigation of reasons for the degraded water quality.

Several ways of specifying trigger values are envisaged by Hart et al. (1999), but in most cases, and for most water quality “issues” (and therefore variables), the use of statistics from datasets for “reference” sites with minor or minimal impact is recommended. The 80 percentile has been (arbitrarily) chosen where high concentrations or values indicate degraded water quality, and the 20 percentile where a low benchmark is appropriate.

The Ministry for Environment (MfE, Mr Bob Zuur) has asked NIWA to estimate trigger values for New Zealand rivers using percentiles calculated for the National Rivers Water Quality Network (NRWQN) dataset. The NRWQN involves sampling of 77 river sites on 49 different rivers at monthly intervals for a variety of water quality variables (Smith & McBride 1990). This report describes the approach taken for site selection and categorisation, presents the relevant percentiles for selected sites in the NRWQN, and gives a brief commentary on the percentiles, including a comparison with SE Australian trigger values.

Description of the Approach

Ten years of data from the NRWQN have been used to calculate percentiles in order to estimate “trigger” values for the new edition of the ANZECC guidelines. Both 80 and 20 percentiles were calculated: the upper percentiles provide an estimation of the trigger value for most variables, but lower percentiles are needed for some variables (e.g. visual clarity) and both for others (dissolved oxygen, pH).

Of the 77 river sites in the NRWQN, 32 sites are regarded as “baseline” (essentially un-impacted) or “pseudo-baseline” (lightly impacted) (Smith and McBride 1990). Data from these sites were used for the calculation of percentiles.

Temperature is one variable that is strongly seasonal in its distribution, so data for February visits (late summer) at which time a combination of comparatively low flows and still high insolation gives seasonal temperature maxima, were used to derive percentiles for this variable. Note that February temperatures are typically higher than January temperatures in the NRWQN dataset.

The minimally or slightly impacted sites were categorised as “upland” (site elevation > 150 m) and “lowland” (< 150 m) following the approach taken in Australia. Unfortunately, most un-impacted to slightly impacted sites are at relatively high elevation, and only four suitable lowland sites are available in the NRWQN: The Waipapa River (Northland), the Upper Waipa and Ohinemuri Rivers (Waikato Region) and the Haast River (Westland). The last river is alpine in its headwaters, and so the percentiles were re-calculated with Haast data removed.

There are fully 28 upland un-impacted to slightly impacted sites in the NRWQN. However, 10 of these are either lake fed (e.g., Tarawera @ lake outlet, Waikato @ Reids farm) or have alpine headwaters with glaciation (e.g., Waimakariri @ the Gorge, Shotover @ Bowens) or both (Waitaki @ Kurow, Clutha @ Luggate), and so the analysis was repeated with these sites removed, leaving only 18 sites.

Appendices 1 and 2 list the baseline and pseudo-baseline sites, give their elevations and other data, and record notes about their catchment characteristics.

Site categorisation and the estimation of percentiles was done in DataDesk which apparently uses a slightly different algorithm for percentile interpolation from MS EXCEL. Appendix 3 summarises the DataDesk output.

Comments on the New Zealand data

Table 1 summarises the percentiles calculated for a range of variables for the different categories of baseline or pseudo-baseline sites. Percentiles for visual clarity (black disc range, m), turbidity, and temperature are included, as well as for the variables (five nutrient measures, dissolved oxygen, pH) covered for various regions of Australia (trigger values for SE Australia, taken from the draft ANZECC guidelines, are given for comparison in Table 1).

The dissolved oxygen (DO) percentiles seem unlikely to be very useful for estimating trigger values, since they are for daytime sampling whereas diurnal minima usually occur near dawn. For a similar reason pH minima may not be very useful. The temperature percentiles for February alone (e.g., 80 percentile temperature for upland sites = 18°C, and 21.5°C for lowland rivers) are probably more useful as trigger values than year-round values. For all three diurnally (and seasonally) varying variables (temperature, pH and DO), professional judgement might need to be used for deriving trigger values.

Unfortunately, the NRWQN does not include data for chlorophyll a in periphyton, although observations of periphyton as % cover are routinely made. Nor are water column chlorophyll a analyses made, so no trigger values can be given for Chla in the water or on the bed of New Zealand rivers.

Note that turbidity and visual water clarity are closely, inversely, related, and the 80 percentile for turbidity is broadly consistent with the 20 percentile for visibility and vice versa.

Comparison with SE Australian Values

A comparison was made with trigger values for SE Australia (including Tasmania) a region that may be more comparable climatically with New Zealand than the other (tropical and or arid) regions of Australia. This comparison can only be made on “face value”, because analytical methods may differ.

Trigger values for SE Australia, for both total phosphorus (TP) and total nitrogen (TN), are similar to the 80 percentiles from the NRWQN, which is “comforting”. Percentiles for dissolved reactive phosphorus (“DRP” equivalent to FRP) and ammoniacal-N (“NH4”) are also similar to, but somewhat lower than, for SE Australia.

Of the nutrients, the greatest difference between NZ and SE Australia is for oxidised-N (“NO3”). The 80 percentiles for oxidised-N in Table 1 are appreciably higher than for SE Australia, particularly for lowland sites. This raises questions about the designation of these three lowland sites as “baseline” or “pseudo-baseline”. Nitrate-N is a good indicator of land use and it seems likely that the Ohinemuri River in particular (designated as “pseudo-baseline”) is actually appreciably impacted by agricultural runoff. I notice that the trigger value (190 ppb) for upland rivers in Tasmania, which may be climatically more comparable with NZ, is much higher than for upland SE Australia generally (15 ppb).

The 20 and 80 percentile values for % saturation DO are very close, suggesting a very "tight" distribution of the data around 100% saturation. The 80 percentile pH for upland NZ rivers is 8.00 pH units, appreciably above that (7.5) for SE Australia. Other trigger values for pH and DO are well within SE Australian values. As noted above, the DO and pH percentiles may not be very useful as trigger values because of diurnal and seasonal variation.

The 80 percentiles for turbidity in NZ are low compared to the wide range given for SE Australia. No comparative visibility data are available for SE Australia.

Acknowledgements

Thanks to Mark O’Donohue, Monash University, Victoria, for explaining the concepts behind trigger values, and to Bob Zuur, Ministry for Environment, for providing liason. Graham Bryers provided data from the NRWQN and carried out preliminary site sorting and data processing. Graham and Bryce Cooper reviewed the report.

References

Hart, B.T.; Maher, B.; Lawrence, I. (1999). New generation water quality guidelines for ecosystem protection. Freshwater biology 41: 347-359.

Smith, D.G.; McBride, G.B. (1990). New Zealand’s National Water Quality Monitoring Network- design and first year’s operation. Water resources bulletin 26: 767-775.

Table 1: Trigger Values for New Zealand rivers

Data from the National Rivers Water Quality Network (NRWQN). Percentiles compiled 18/5/2000 by R. J. Davies-Colley

Baseline or Pseudo-baseline Sites, JANUARY 1989 TO DECEMBER 1998 (10 YEARS)

 

 

 

TP

DRP

TN

NO3

NH4

 

DO%

pH

 

Clar

Turb

 

Temp

Temp

 

 

 

ppb P

ppb P

ppb N

ppb N

ppb N

 

%

units

 

m

FTU

 

Deg C

Feb. only

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Deg C

Upland sites, elevation > 150 m ( = 28 SITES)

 

 

 

 

 

 

 

 

 

 

 

 

NO. OBSERVATIONS

3388

3394

3032

3396

3047

 

3365

3360

 

3382

3399

 

3400

278

80th PERCENTILE

 

24

6.0

230

108

9

 

103.7

8.01

 

 

5.00

 

14.4

18.1

20th PERCENTILE

 

 

 

 

 

 

 

99.1

7.42

 

0.65

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Upland sites, elevation > 150 m, glacial and lake-fed sites removed  ( = 18 SITES)

 

 

 

 

 

 

 

 

NO. OBSERVATIONS

2194

2199

1960

2200

1973

 

2176

2179

 

2188

2202

 

2202

179

80th PERCENTILE

 

26

9.0

295

167

10

 

102.8

8.00

 

 

4.10

 

13.9

17.9

20th PERCENTILE

 

 

 

 

 

 

 

98.9

7.3

 

0.8

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SE Australia (upland)

20

15

250

15

13

 

110

7.5

 

 

2 to 25

 

 

 

 

 

 

 

 

 

 

 

 

90

6.5

 

 

 

 

 

 

Lowland sites, elevation < 150 m ( = 4 SITES)

 

 

 

 

 

 

 

 

 

 

 

 

NO. OBSERVATIONS

473

473

422

474

426

 

474

470

 

473

473

 

474

38

80th PERCENTILE

 

28

9.0

555

391

15

 

103.1

7.77

 

 

4.89

 

16.9

20.3

20th PERCENTILE

 

 

 

 

 

 

 

97.4

7.2

 

0.75

 

 

 

 

Lowland sites, elevation < 150 m, Haast River (alpine headwaters) removed ( = 3 SITES)

 

 

 

 

 

 

 

NO. OBSERVATIONS

353

353

315

354

318

 

354

351

 

353

353

 

354

30

80th PERCENTILE

 

33

10.0

614

444

21

 

105

7.87

 

 

5.58

 

16.1

21.5

20th PERCENTILE

 

 

 

 

 

 

 

98

7.15

 

0.56

 

 

 

 

SE Australia (lowland)

50

20

500

40

20

 

110

8

 

 

6 to 50

 

 

 

 

 

 

 

 

 

 

 

 

85

6.5

 

 

 

 

 

 

Appendix. 1.  NRWQN baseline or pseudobaseline site data

Region

Site Code

Place

Map Series

MapID

x

y

Tideda #

Catch. Area km2

Highest Catch. Elev m

Site Elev m

Whangarei

WH1

Waipapa @ Forest Ranger

NZMS260

P05

734

584

47804

122

237

30

Hamilton

HM1

Waipa @ Otewa

NZMS260

S16

156

234

43481

304

413

80

Hamilton

HM6

Ohinemuri @ Karangahake

NZMS260

T13

506

172

9213

305

248

10

Rotorua

RO1

Tarawera @ Lake outlet

NZMS260

V16

169

297

15341

414

455

320

Rotorua

RO4

Whirinaki @ Galatea

NZMS260

V17

368

953

15410

509

528

205

Rotorua

RO6

Waikato @ Reids Farm

NZMS260

U18

778

763

1143444

3305

655

349

Turangi

TU2

Tongariro @ Turangi

NZMS260

T19

537

417

1043459

786

1005

363

Wanganui

WA2

Manganui @ SH3

NZMS260

Q20

189

130

39508

19

725

320

Wanganui

WA5

Rangitikei @ Mangaweka

NZMS260

T22

504

512

32702

2689

864

518

Wanganui

WA7

Manawatu @ Weber Rd

NZMS260

U23

747

26

32503

716

371

152

Gisborne

GS2

Waikohu @ No. 1 Br.

NZMS260

X17

83

997

19734

30.5

722

457

Havelock Nth

HV1

Makaroro @ Burnt Br.

NZMS260

U22

928

489

23218

121

863

329

Havelock Nth

HV4

Ngaruroro @ Kuripapango

NZMS260

U20

969

974

23104

384

1108

488

Havelock Nth

HV6

Mohaka @ Glenfalls

NZMS260

V20

240

188

21803

1040

820

320

Wellington

WN2

Hutt @ Kaitoke

NZMS260

S26

942

150

29808

87

576

200

Wellington

WN5

Ruramahanga @ SH2

NZMS260

S25

299

461

29254

78

829

268

Nelson

NN2

Motueka @ Gorge

NZMS260

N28

28

526

57008

166

1049

376

Nelson

NN3

Wairau @ Dip Flat

NZMS260

N29

36

234

60114

521

1422

655

Nelson

NN5

Buller @ Longford

NZMS260

M29

590

378

93202

1404

888

183

Greymouth

GY3

Grey @ Waipuna

NZMS260

K31

100

718

91404

642

774

171

Greymouth

GY4

Haast @ Roaring Billy

NZMS260

G37

129

895

86802

1027

1009

53

Christchurch

CH1

Hurunui @ Mandamus

NZMS260

M33

725

240

65104

1060

976

442

Christchurch

CH3

Waimakariri @ Gorge

NZMS260

L35

331

605

66402

2387

1034

244

Tekapo

TK3

Opuha @ Skipton Br.

NZMS260

J38

482

790

69614

458

1020

238

Tekapo

TK4

Waitaki @ Kurow

NZMS260

I40

80

88

71104

9741

1047

250

Alexandra

AX1

Clutha @ Luggate Br.

NZMS1

S12

4 78

85

75241

4453

973

305

Alexandra

AX2

Kawarau @ Chards

NZMS260

F41

844

698

75262

4302

1043

305

Alexandra

AX3

Shotover @ Bowens Peak

NZMS260

F41

722

710

75276

1079

1200

320

Dunedin

DN2

Sutton @ SH87

NZMS260

H43

832

84

74338

151

672

220

Dunedin

DN6

Mataura @ Parawa

NZMS260

E43

635

73

77505

787

787

259

Dunedin

DN7

Oreti @ Lumsden

NZMS260

E44

541

892

78607

1139

694

220

Dunedin

DN10

Monowai below Gates

NZMS260

C44

853

751

79712

258

701

213

Notes - Sites in bold have alpine headwaters or are lake-fed

Appendix 2. NRWQN site description

Site Code

Type

Brief Description & Reasons for Inclusion

WH1 Baseline Largely native bush and small amount exotic.
HM1 Baseline Catchment largely native bush; unlikely to change. Little pasture.
HM6 Baseline* *Pseudobaseline. Major tributary of the Waihou River. Contains or will contain discharges from several large gold mining operations. Some pasture
RO1 Baseline Catchment includes several Rotorua lakes; mainly bush but some pasture/exotics. Some natural geothermal inputs.
RO4 Baseline With major forestry. Major tributary of Rangitikei. Useful paired with RO3 - geology upstream very different (RO3 -Taupo pumice, flat; RO4 - Urewera grewwacke, steep).
RO6 Baseline* *Psuedobaseline for Waikato region. Major recreation river. Downstream 0f Lake Taupo (area 623 km2). See TU2 and Waikato region.
TU2 Baseline For Waikato system although some catchment development for pasture/exotics. Major international trout fishery. See RO6 and Waikato egion for downstream stations.
WA2 Baseline With small amount of pasture development. Representative of radial streams from Mt Taranaki. Local trout fishing.
WA5 Baseline* *Pseudo baseline. Largely undeveloped bush/tussock catchment; some pasture. Nationally important for recreation (rafting, fishing).
WA7 Baseline* *Pseudo baseline. Some upstream pasture development.
GS2 Baseline Baseline for Waipaoa and, because of similar geology to Motu River catchment, can also be used as Motu baseline.
HV1 Baseline Baseline for Tukituki (HV2). Some pasture development but mainly bush.
HV4 Baseline Bush/tussock.
HV6 Baseline* *Pseudo baseline. Mainly bush catchment but some pasture and exotic development.
WN2 Baseline Bush catchment
WN5 Baseline Bush catchment with a small amount of exotics and pasture.
NN2 Baseline  Native bush catchment.
NN3 Baseline  Native bush catchment.
NN5 Baseline* *Pseudo baseline. Beech forest catchment; some pasture development. (Impact site is GY1).
GY3 Baseline Beech forest catchment. Some minor upstream development for holiday homes.
GY4 Baseline Largest alpine-fed Westland river; representative in terms of catchment geology and lack of development.
CH1 Baseline Typical high alpine catchment, tussock, beech.
CH3 Baseline A major (and representative) braided river of the Canterbury Plains. Major fishery and recreation value. Alpine fed.
TK3 Baseline* Pseudo baseline, for Opihi system. Some development.
TK4 Baseline For river at end of hydroelectric lakes system. Main river in region. Little upstream development.
AX1 Baseline Takes L. Wanaka and L. Hawea waters. Some catchment devpt. Starting point of the country's major river upstream of hydro electric development. Major tourist attraction.
AX2 Baseline* *Pseudo baseline; Little catchment development. Major tourist vlues (and pressures) being downstream of Queenstown and L. Wakatipu. The major tributary of the Clutha River.
AX3 Baseline* *Pseudo baseline; little catchmentdevelopment. Major tributary of Kawarau R. Subject to tourist pressure, past and current alluvial gold mining.
DN2 Baseline Tussock catchment typical of region. Largely undeveloped. Typical input to taieri.
DN6 Baseline Tussock with beech forst patches. Small amount of pasture development.
DN7 Baseline* *Pseudo baseline. Improved pasture on plains. Extensively grazed tussock elsewhere. Regional fishery.
DN10 Baseline (Baseline (although from man-enhance lake) for Waiau system. Wholly beech forest. National Park status.

Notes - Sites in bold have alpine headwaters or are lake-fed

Appendix 3: DataDesk summaries of percentiles for baseline or pseudo-baseline NRWQN sites

1. Lowland sites (< 150 m elevation) a. Four baseline lowland sites: WH1 (Waipapa), HM1 (Waipa @ Otewa), HM6 (Ohinemuri), GY4 (Haast).

Summaries No
Selector
Percentile 20

Variable Count Mean Median Lower ith %tile Upper ith %tile
TP  473  32.3087  15   28
DRP 473  6.12262 9
TN  422 318.749 212.5 70 555
NO3  474  199.586  78.5   22  390.7
NH4  426 13.5845   7 15
DO% 474  100.349 99  97.4 103.1
pH  470   7.55787 7.56 7.2  7.775
Clar  473  2.15696  1.9  0.75  3.119
Turb  473 8.04655   1.8  0.991   4.89
Temp  474 12.8091 12.5      16.9
BOD5  473 0.45803  0.4  0.2  0.65

b. Three baseline lowland sites (Haast removed being alpine affected)

Summaries cases selected according to

Select NOT Haast

Percentile 20

Variable Count Mean Median Lower ith %tile Upper ith %tile
TP  353 39.6941 18 8 33
DRP 353 6.65609 6 1.4 10
TN  315 390.251 325 74 614
NO3  354 258.895 195 36 444.1
NH4  318 16.5031 8 4 21
DO% 354 101.448 99.7 98 104.94
pH  351 7.55564 7.53 7.15 7.87
Clar  353 2.12649 1.8 0.561 2.998
Turb  353 9.81598 1.8 0.991 5.58
Temp  354 12.2342 11.5 8.4 16.14
BOD5  353 0.49957 0.4 0.2 0.7

c. Temp summary is not very useful because of seasonality, so we select just February data

Summary of Temp cases selected according to Select Feb 474 total cases of which 436 are missing Percentile 20

Count 38

Mean 17.8842

Median 18.75

Lower ith %tile 14.67

Upper ith %tile 20.28

Summary of Temp cases selected according to Feb, NOT Haast 474 total cases of which 444 are missing Percentile 20

Count 30

Mean 19.4633

Median 19.25

Lower ith %tile 17.75

Upper ith %tile 21.45

2. Upland sites (> 150 m elevation)

d. 28 Upland baseline sites

Summaries

No Selector

Percentile 20

Variable Count Mean Median Lower ith %tile Upper ith %tile
TP 3388 28.9153 9 4 24
DRP 3394 4.26827 2.7 1 6
TN 3032 159.67 91.5 54.9 230
NO3 3396 76.6623 31 9 108
NH4 3047 6.32622 5 3 9
DO% 3365 101.415 101 99.1 103.7
pH 3360 7.71887 7.73 7.42 8.01
Clar 3382 3.12273 2.4 0.65 5.2
Turb 3399 8.7449 1.2 0.5 5
Temp 3400 10.6379 10.35 6.95 14.4
BOD5 3390 0.35792 0.3 0.1 0.5

e. 18 Upland baseline sites (alpine and/or lake-fed sites removed)

Summaries cases selected according to Select NOT glacial, lake Percentile 20

Variable Count Mean Median Lower ith %tile Upper ith %tile
TP 2194 28.737 11 5 26
DRP 2199 5.96016 4 2 9
TN 1960 205.702 140 63 295
NO3 2200 106.626 49.5 15 167
NH4 1973 6.99088 6 3 10
DO% 2176 100.945 100.7 98.87 102.8
pH 2179 7.65826 7.62 7.3 8
Clar 2188 3.03631 2.385 0.8 4.718
Turb 2202 8.43552 1.3 0.55 4.1
Temp 2202 10.1807 9.75 6.6 13.9
BOD5 2197 0.40318 0.3 0.15 0.6

f. Temp summary is not very useful because of seasonality, so we select just February data

Summary of Temp cases selected according to Select Feb 3404 total cases of which 3126 are missing Percentile 20

Count 278

Mean 15.9004

Median 16

Lower ith %tile 13.6

Upper ith %tile 18.09

Summary of Temp cases selected according to Feb, NOT glacial, lake 3404 total cases of which 3226 are missing Percentile 20

Count 178

Mean 15.4309

Median 15.5

Lower ith %tile 13.2

Upper ith %tile 17.9