Stitt River Mixing Zone Assessment - EPA Websiteepa.tas.gov.au/documents/taswater, rosebery wwtp - appendix o2... · Stitt River Mixing Zone Assessment Final 1. Introduction TasWater

Stitt River Mixing Zone Assessment

TASWATER

For Rosebery STP DPEMP

Final

3 March 2014


PAGE i

Stitt River Mixing Zone

Project no: VT30538

Document title: Stitt River Mixing Zone Assessment

Document no: Final

Revision: R01

Date: 03 March 2014

Client name: TasWater

Client no: PO323635

Project manager: Elspeth Moroni

Author: Dr Bonnie Bonneville

File name: C:\Users\bbonneville\Documents\Organice\bbonneville\http\dmca-

apac.skmconsulting.com\80\sites\vt30538\DmcaConsult\Stitt River Mixing Zone

Assessment.docx

Sinclair Knight Merz

ABN 37 001 024 095

Floor 11, 452 Flinders Street

Melbourne VIC 3000

PO Box 312, Flinders Lane

T +61 3 8668 3000

F +61 3 8668 3001

www.globalskm.com

COPYRIGHT: The concepts and information contained in this document are the property of Sinclair Knight Merz Pty Ltd (SKM). Use or copying of

this document in whole or in part without the written permission of SKM constitutes an infringement of copyright.

Document history and status

Revision Date Description By Review Approved

R01 04/02/14 Review of draft S Treadwell 04/02/14 B Bonneville


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Contents

1. Introduction ................................................................................................................................................................................... 1

1.1 Mixing zones ................................................................................................................................................................................... 1

2. Discharge quality and quantity.................................................................................................................................................... 2

3. Water quality objectives for Stitt River ....................................................................................................................................... 3

3.1 EPA water quality objectives........................................................................................................................................................... 3

3.2 Ambient water quality data.............................................................................................................................................................. 3

3.3 Australian Water Quality Guidelines ............................................................................................................................................... 4

4. Stitt River flows ............................................................................................................................................................................. 5

4.1 7Q10 Flow in Stitt River .................................................................................................................................................................. 5

5. Mixing zone assessment .............................................................................................................................................................. 6

5.1 Dilution of effluent ........................................................................................................................................................................... 6

5.2 Dilutions of pollutant concentrations ............................................................................................................................................... 6

6. Conclusion .................................................................................................................................................................................... 8

7. References..................................................................................................................................................................................... 9


Final

1. Introduction

TasWater proposes to position an outfall for the new Rosebery Sewage Treatment Plant (STP) on the Stitt River

downstream of Rosebery. The new Rosebery STP will receive and treat sewage from the Rosebery Township to

Accepted Modern Technology (AMT) standard and discharge treated effluent to Stitt River via the outfall. The

treated effluent will have elevated nutrient concentrations and will form an area in the river where water quality

conditions are outside the natural background variability. This area is defined as the mixing zone. The objective

of this study is to determine the size of the mixing zone in Stitt River from the Rosebery STP outfall.

1.1 Mixing zones

If the water quality of the effluent does not comply with the water quality objectives (WQOs) for the receiving

waters, then a mixing zone may be designated around the point of discharge (Figure 1-1). The water quality

within the mixing zone is permitted to be outside of the normal range for the receiving environment, provided it

does not detract from the values or uses of the waters. The size of the mixing zone is calculated under the

7Q10 dry condition flows (i.e. lowest 7 day flow rate which can be expected to occur in a 10 year period) using

the 90th percentile effluent discharge pollutant concentrations (EPA Tas 2013). The result is a conservative

estimate of the mixing zone.

Figure 1-1 Mixing zone illustration

Mixing zones will not be designated in areas that (State Policy on Water Quality Management 1997; Clause

20.3 (d)):

receive significant and regular use for primary contact recreation;

are recognised areas of significant value as spawning or nursery areas;

are close to areas used for aquaculture;

are close to potable water supply intakes;

are of outstanding ecological or scientific importance; or

for which the protected environmental values include protection of pristine aquatic ecosystems.


Final PAGE 2

2. Discharge quality and quantity

The proposed outfall location (55G 377875E, 5373831N) is on the Stitt River downstream of the Stitt River falls,

about 2 km upstream of the confluence with Lake Pieman. The location is downstream of the township of

Rosebery and the Rosebery Cabin and Tourist Park. It is also downstream of the MMG Rosebery mine. The

proposed outfall location and the Stitt River downstream are situated in a forested gully, which is not easily

accessed by the general public.

The quality and quantity of the Rosebery STP effluent has not yet been finalised. However, the quality will meet

the AMT standard as described by the Emission Limit Guidelines for Sewage Treatment Plants that discharge

pollutants into fresh and marine waters (DPIWE 2001). These performance parameters are presented in Table

2-1. These guidelines are applicable to all new and existing sewage treatment plants that discharge into fresh or

marine waters with wastewater flows of less than 500 kL/day average dry weather flow (ADWF). The mixing

zone is calculated using the 90th percentile limits.

Table 2-1 AMT effluent quality limits. Highlighted cells are the numbers used to determine the mixing zone

Parameter Median 90%ile Maximum

BOD (mg/L) 5 10 15

Non-Filterable Residue (mg/L) 10 15 20

Thermotolerant Coliforms (cfu/ 100mL) - - 200

Oil and Grease (mg/L) 2 5 10

Total Nitrogen (mgN/ L) 7 10 15

Ammonia (mg N/L) 1 2 5

Total Phosphorus (mgP/L) 0.5 1 3

pH - - 6.5 – 8.5

An average daily flow (ADF) of approximately 0.41 ML/d and an ADWF of approximately 0.24 ML/day is

expected from the Rosebery STP outfall, based on inflow volumes (Table 2-2). Dry weather flows only occur for

four to five weeks per year as the system is significantly impacted by infiltration during frequent wet weather

(CMW 2013, p.5). The mixing zone is calculated using the ADWF of 0.24 ML/day as the effluent volume, as this

would be the outfall volume expected under the 7Q10 flow conditions in Stitt River.

Table 2-2 Existing Rosebery influent data (from the DRAFT Rosebery STP DPEMP). Highlighted cells are the flow used to

determine the mixing zone

Description Rate ML/day How Determined Comments

ADWF 0.24 Calculated Tenement based excluding infiltration into the

network

Wet Weather Flow from SPS 1.6 Estimated This equated to 18.5 l/s over 24 hours

ADF 0.41 Estimated This equated to 4.7 l/s over 24 hours

Minimum ADF 0.13 Observed Measured once on site during summer dry

weather.

Current maximum pump deliverable

capacity

3.2 Estimated This equated to 37.0 l/s over 24 hours


Final PAGE 3

3. Water quality objectives for Stitt River

According to EPA Tas (2013), the preferred WQOs for assessing outfall impacts to slightly to moderately

disturbed ecosystems are the EPA published WQOs (EPA Tas, nd). Alternative, but less desirable, WQOs are

the 80th percentile of site-specific data (CMW 2013) or the default Australian Water Quality Guidelines

(ANZECC & ARMCANZ 2000). These WQO options are outlined in this section.

3.1 EPA water quality objectives

Site specific water quality and biological condition objectives have been developed for the monitoring sites at

Stitt River at Rosebery (site code: PIEM22) (EPA Tas nd). The WQOs were set based on the 80th percentile

values of ambient data collected between 2004 and 2009 (EPA Tas nd). However, regional values were used to

set WQOs for nutrients because there was no background nutrient data available for this site (Table 3-1). The

annual WQOs will be used to define the boundary of the mixing zone.

Table 3-1 Site specific WQOs for Stitt River at Rosebery (PIEM22) (EPA Tasmania, nd). Highlighted cells are those used to

determine the mixing zone

Water Quality Indicator Annual WQ obj Summer Winter Autumn Spring

Percentile 20th 80th 20th 80th 20th 80th 20th 80th 20th 80th

Ammonia as N mg/L* 0.012 0.012 0.014 0.011 0.012

Total N as N mg/L* 0.381 0.370 0.485 0.340 0.376

Total P mg/L* 0.021 0.021 0.025 0.017 0.020

Total Suspended Solids mg/L 5.00 5.00 10.00 5.00 5.00

pH 5.9 6.7 6.2 6.7 5.7 6.4 5.9 6.9 6.2 6.6

*based on regional values, ~ based on statewide values

3.2 Ambient water quality data

Water quality data has been collected at the proposed outfall site (downstream) on Stitt River as part of the

ambient water quality monitoring program for the Rosebery STP (CMW 2013) from January to August 2013

(Table 3-2). This data shows that ammonia and total nitrogen levels are higher than the EPA WQOs, whereas

total phosphorus and total suspended solids levels are lower than the EPA WQOs (Table 3-2). As there are only

eight months of data within one year, it cannot be assumed that the data is representative of the background

variability of Stitt River. However, as the data is specific to Stitt River at the outfall location, the 80th percentile

concentration is likely to be a more suitable objective than the regional-based nutrient objectives. Therefore, the

80th percentile concentration from Stitt River will also be modelled to compare with the results from using the

WQOs to define the extent of the mixing zone.

Table 3-2 Ambient data for Stitt River downstream of Rosebery (CMW 2013). Highlighted cells are used for the mixing zone

WQ indicator Median of ambient data

collected Jan-Aug 2013 (15

samples)

80th

percentile of ambient data

collected Jan-Aug 2013 (15

samples)

EPA WQO for comparison

*based on regional values

Ammonia (mg/L) 0.010 0.025 0.012*

Total N (mg/L) 0.350 0.480 0.381*

Total P (mg/L) 0.008 0.010 0.021*

Total Suspended Solids (mg/L) 1 2.4 5.00

pH 6.4 6.6 6.7


Final PAGE 4

3.3 Australian Water Quality Guidelines

ANZECC & ARMCANZ (2000) provides a set of regional default low-risk trigger values for slightly to moderately

disturbed ecosystems for south-eastern Australia. The default trigger values are classified, based on altitude, as

upland (>150m) and lowland (<150m) rivers. However, due to Tasmania’s mountainous landform and relatively

small catchment sizes, this definition may be less appropriate for Tasmania than for other south-eastern states.

Therefore, the more stringent default trigger values (those for upland rivers) are typically applied for the

assessment of Tasmania’s rivers (Table 3-3; DPIW 2008). Rosebery has an altitude of 165m ASL (at the

weather sub-station).

The trigger values for total nitrogen and total phosphorus are lower than the EPA WQOs (Table 3-3). The trigger

value for ammonia is marginally higher than the EPA WQOs.

Toxicant trigger values are also given for ammonia for the protection of 95% of species in freshwater

ecosystems. Ammonia has a toxicant trigger level of 0.9 mg/L.

Table 3-3 ANZECC & ARMCANZ (2000) default low-risk trigger values for physical and chemical stressors in slightly disturbed

ecosystems in Tasmania

Indicator Units Upland River (>150m)

Ammonium (NH4) mg/L 0.013

Total N mg/L 0.250

Total P mg/L 0.013

Turbidity NTU 2-25

pH - 6.5-7.5 (humic rich lakes

and rivers 4.0-6.5)

Total ammonia (toxicant) mg/L 0.9

*toxicant guideline for ammonia for the protection of 95% of species.


Final PAGE 5

4. Stitt River flows

4.1 7Q10 Flow in Stitt River

Section 20.3 of the State Policy of Water Quality Management 1996 requires the mixing zones in rivers to be

designated with regard to low flow conditions. EPA Tas (2013) specifies that the dilution provided by stream

water should be calculated under 7Q10 flows. The 7Q10 flow is the lowest 7 day flow rate which can be

expected to occur in a 10 year period and will be used for the analysis. Flow data collected between 1991 and

2013 was analysed by GHD (2013) to determine the river flow rates required for a mixing zone analysis. A total

of 6240 daily flow records were available for analysis. The results of the flow rate analysis are shown in Table

4-1.

Table 4-1 Flow parameters

Parameter Flow Rate

7Q10 flow rate 7.8 ML/d


Final PAGE 6

5. Mixing zone assessment

5.1 Dilution of effluent

The maximum level of dilution (assuming a zero-background pollutant concentration) under the 7Q10 flows in

Stitt River and the ADWF effluent volume, is calculated using the following equation:

Where, S = Dilution factor

Qup = Flow in Stitt River (7Q10) upstream (i.e. 7.8 ML/d)

Qeff = Effluent flow (i.e. 0.24 ML/d)

Using this equation, the maximum dilution of the effluent provided by the 7Q10 flows in Stitt River is 34-fold. No

other tributaries or drainage lines join the Stitt River between the outfall location and Lake Pieman. Therefore,

no additional dilution is provided until the confluence with Lake Pieman.

5.2 Dilutions of pollutant concentrations

Water quality indicators of interest for the mixing zone calculations are:

Ammonia (mg/L);

Total Nitrogen (TN, mg/L);

Total Phosphorus (TP, mg/L).

Total Suspended Solids (TSS, mg/L);

Thermotolerant Coliforms (cfu/100ml); and

pH (pH units).

For pollutants with a non-zero background concentration, the concentration of pollutants downstream of the

outfall (assuming complete mixing), given the maximum level of dilution (34-fold) under 7Q10 flows is as follows:

Where, Cds = pollutant concentration downstream of the outfall (mg/L)

Ceff = effluent pollutant concentration (mg/L)

Cus = median background pollutant concentration upstream (mg/L)

S = Dilution factor

The required dilutions of effluent pollutant concentrations to meet the WQOs are calculated as follows (EPA Tas 2013):

Where, Sreq = required dilution factor to meet WQO

Ceff = effluent pollutant concentration (mg/L)

Cwqo = WQO (mg/L)

Cus = median background pollutant concentration upstream (mg/L)


Final PAGE 7

Under 7Q10 flows, the effluent TSS, thermotolerant coliforms and pH concentrations are diluted sufficiently to

meet the objectives (Table 5-1). However, the nutrient objectives are not met. See Appendix A for the

calculation data.

Ammonia is the limiting pollutant concentration. The 90th percentile effluent ammonia concentration needs to be

diluted 995-fold to achieve the EPA WQOs and 662-fold for the ANZECC trigger level (ecosystem stressor)

(Table 5-1). Ammonia concentrations downstream of the outfall may present a risk to environmental values from

nutrient enrichment, but they are well below chronic or acute toxicity levels (Table 5-1).

Total phosphorus and total nitrogen also require significant dilutions to meet WQOs.

Table 5-1 Summary of required dilutions of 90th percentile AMT standard effluent to meet EPA WQOs, the 80th percentile of

ambient data and the ANZECC trigger levels. Bold numbers indicate that the dilution is not met under 7Q10 flows, where the

maximum dilution is 34-fold.

WQ indicator Dilution factor

required to

meet EPA

WQO

Dilution factor

required to

meet 80th

%ile

ambient

Dilution factor

required to

meet ANZECC

trigger levels

Upstream

conc. in Stitt

River, median

(mg/L)

Downstream

conc. in Stitt

River under

7Q10 flows

(mg/L)

Ammonia (stressor) 995 132 662 0.01 0.07

Ammonia (toxicant) - - 1.2 0.01 0.07

TN 321 73 ID 0.35 0.64

TP 75 495 197 0.008 0.038

TSS 3 9 - 1 1.4

Thermotolerant

coliforms

- 1 - 29 34

pH 1 1 1 6.4 6.6

ID= cannot be determined as median ambient water quality is higher than trigger level.

5.3 Extent of the mixing zone

The dilutions required to meet the EPA WQOs, 80th percentile ambient concentrations or the ANZECC trigger

levels for nutrients are not achieved under 7Q10 flows in Stitt River (Table 5-1). A maximum of dilution of 34-fold

is provided by the 7Q10 flows. This implies that the mixing zone extends to the confluence of the Stitt River with

Lake Pieman regardless of the water quality objective used. This is a distance of approximately 2 km.


Final PAGE 8

6. Conclusion

An analysis of the 90th percentile Rosebery STP effluent and background pollutant concentrations in Stitt River

(and regionally where data was insufficient) indicates that the effluent will need to be diluted approximately 995-

fold to attain the preferred EPA WQOs at the edge of the mixing zone. A maximum dilution of 34-fold is provided

by 7Q10 flows in Stitt River. Therefore, the size of the mixing zone extends about 2 km downstream of the

outfall to the confluence with Lake Pieman.

The nutrient concentrations in the mixing zone may present a risk to environmental values from nutrient

enrichment, but are well below chronic or acute toxicity levels.


Final PAGE 9

7. References

ANZECC & ARMCANZ (2000). Australian and New Zealand Guidelines for Fresh and Marine Water Quality,

National Water Quality Management Strategy Paper No. 4, Australian and New Zealand Environment and

Conservation Council (ANZECC) and Agriculture and Resource Management Council of Australia and New

Zealand (ARMCANZ).

DPIWE (2001). Emission limit guidelines: Sewage treatment plants that discharge pollutants into fresh and

marine waters. Tasmanian Department of Primary Industries, Water and Environment

EPA Tas (nd). Stitt River at Rosebery. Water quality and biological condition objectives. EPA Tasmania.

EPA Tas (2013). Wastewater ambient monitoring program - key parameters for assessing outfall impacts. EPA

Tasmania.

GHD (2013). Rosebery STP DPEMP Stitt River mixing assessment. A report to TasWater by GHD.

Tasmanian Government (1997). State policy on water quality management 1997.


Final PAGE 10

Appendix A. Mixing zone calculations

Table 7-1 Dilutions required for the EPA WQOs for Stitt River (PIEM22; EPA tas, nd), where nutrient objectives are based on

regional values. ADWF (0.24 ML/d) from Rosebery STP in 7Q10 flows (7.8 ML/d) in Stitt River.

WQ indicator Ceff

90th

percentile

effluent quality

(mg/L)

Cwqo

EPA WQO

(annual) (mg/L)

Cus

50th

percentile

ambient conc.

(mg/L)

Cds

Diluted conc.

under 7Q10

flows (mg/L)

Sreq

Dilution factor

required to

meet WQO

Ammonia 2 0.012 0.01 0.069 995

TN 10 0.38 0.35 0.64 321

TP 1 0.021 0.008 0.038 75

TSS 15 5 1 1.4 3

Thermotolerant

coliforms

200* ND 29 34 -

pH 6.5-8.5* 6.7 6.4 6.6 1

*Maximum range

Table 7-2 Dilutions required to meet the 80th percentile of Stitt River data, sampled from January to August 2013 at the

proposed outfall location as part of the ambient monitoring program for the Rosebery STP for receiving waters (CMW 2013).

ADWF (0.24 ML/d) from Rosebery STP in 7Q10 flows (7.8 ML/d).

WQ indicator Ceff

90th

percentile

effluent quality

(mg/L)

Cwqo

80th

percentile

ambient conc.

(mg/L)

Cus

50th

percentile

ambient conc.

(mg/L)

Cds

Diluted conc.

under 7Q10

flows (mg/L)

Sreq

Dilution factor

required to

meet 80th

%ile

Ammonia 2 0.025 0.01 0.069 132

TN 10 0.48 0.35 0.64 73

TP 1 0.01 0.008 0.038 495

TSS 15 2.4 1 1.4 9

Thermotolerant

coliforms

200* 132 29 34 1

pH 6.5-8.5* 6.6 6.4 6.6 1

*Maximum range

Table 7-3 Dilutions required to meet the ANZECC WQ trigger levels. ADWF (0.24 ML/d) from Rosebery STP in 7Q10 flows (7.8

ML/d)

WQ indicator Ceff

90th

percentile

effluent

quality (mg/L)

Cwqo

ANZECC

trigger level

(mg/L)

Cus

50th

percentile

ambient conc.

(mg/L)

Cds

Diluted conc.

under 7Q10

flows (mg/L)

Sreq

Dilution factor

required to

meet ANZECC

Ammonia (stressor) 2 0.013 0.01 0.069 662

Ammonia (toxicant) 2 0.9 0.01 0.069 1.2

TN 10 0.25 0.35 0.64 ID


Final PAGE 11

TP 1 0.013 0.008 0.038 197

TSS 15 - 1 1.4 -

Thermotolerant

coliforms

200* - 29 34 -

pH 6.5-8.5* 6.5-8.5 6.4 6.6 1

*Maximum range; ID= cannot be determined as median ambient water quality is higher than trigger level.