appendix j – mixing zone assessment · the meander river. this change will alter the location and...

GHD | Report for TasWater - Carrick STP New Discharge Pipeline and Outfall , 32/17413

Appendix J – Mixing Zone Assessment

TasWater

Carrick STP New Discharge Pipeline and Outfall

Appendix J – Mixing Zone Assessment

October 2014

GHD has prepared this report on the basis of information provided by TasWater and the EPA who provided information to GHD which GHD has not independently verified or checked beyond the agreed scope of work. GHD does not accept liability in connection with such unverified information, including errors and omissions in the report which were caused by errors or omissions in that information.

This report is prepared as an attachment to the Carrick DPEMP and must only be read in conjunction with the full DPEMP report.

Tas Water - Carrick STP New Discharge Pipeline and Outfall Appendix J Mixing Zone Assessment, 32/17413 | i

Table of Contents 1. Background and Introduction ......................................................................................................... 1

2. Physical Setting .............................................................................................................................. 1

3. River Flow Regime ......................................................................................................................... 1

4. Source Data .................................................................................................................................. 4

5. Dilution Requirement for Toxicants ................................................................................................ 6

5.1 Mixing Requirements of Individual Toxicants ...................................................................... 6

5.2 Selection of a Combined Dilution Requirement ................................................................... 9

6. River Flow Requirements ............................................................................................................. 10

7. Mixing Zone .................................................................................................................................. 10

7.1 Low Plant Discharge Cases (5 L/s) ................................................................................... 11

7.2 High Plant Discharge Cases (10 L/s) ................................................................................. 14

8. Dilution of non-toxicants ............................................................................................................... 18

8.1 Heterogeneous Zone Extent .............................................................................................. 18

8.2 Water Quality in the Heterogeneous Zone ........................................................................ 19

8.3 Travel Time ........................................................................................................................ 21

9. Interpretation of Results and Conclusions ................................................................................... 22

10. References ................................................................................................................................... 23

Table Index Table 1 Data Source Used for Each Analysis ................................................................................... 4

Table 2 Comparison of Ambient Water Quality Data Sources ......................................................... 5

Table 3 Total Copper Effluent Concentration Data ........................................................................... 7

Table 4 Total Zinc Effluent Concentration Data ................................................................................ 7

Table 5 Velocity and Depth Estimates for a Range of Flows ......................................................... 10

Table 6 Mixing Zone Extent – Low Discharge Cases ..................................................................... 13

Table 8 Mixing Zone Extent – High Discharge Cases .................................................................... 17

Table 8 Distance to Cross Section of Relative Homogeneity ......................................................... 18

Figure Index

Figure 1 Historical Flows – Logarithmic Vertical Scale ...................................................................... 1

Figure 2 Summer Flow Statistics ....................................................................................................... 2

Figure 3 Autumn Flow Statistics......................................................................................................... 2

ii | Tas Water - Carrick STP New Discharge Pipeline and Outfall –Appendix J Mixing Zone Assessment, 32/17413

Figure 4 Winter Flow Statistics ........................................................................................................... 3

Figure 5 Spring Flow Statistics........................................................................................................... 3

Figure 6 Mixing Zone Extent, 1.0 m3/s (7Q10) river flow case......................................................... 11

Figure 7 Mixing Zone Extent, 2.0 m3/s (intermediate low flow) river flow case................................ 11

Figure 8 Mixing Zone Extent, 3.45 m3/s (summer median) rive flow case ....................................... 12

Figure 9 6.33 m3/s (autumn median) river flow case ....................................................................... 12

Figure 10 Mixing Zone Extent, 1.0 m3/s (7Q10) river flow case......................................................... 14

Figure 11 Mixing Zone Extent, 2.0 m3/s (intermediate low flow) river flow case................................ 14

Figure 12 Mixing Zone Extent, 3.45 m3/s (summer median) river flow case ..................................... 15

Figure 13 Mixing Zone Extent, 6.33 m3/s (autumn median) river flow case ...................................... 15

Figure 14 Mixing Zone Extent, 11.08 m3/s (spring median) river flow case ....................................... 16

Appendix Appendix A – Carrick STP Effluent Data 2009-2014)

Appendix B – Summary of Dilution Calculations

1 | Tas Water - Carrick STP New Discharge Pipeline and Outfall Appendix J – Mixing Zone Assesment, 32/17413

1. Background and Introduction As part of changes to the Carrick Sewage Treatment Plant (STP), it is proposed that the

discharge point be relocated from its current location on the Liffey River to a nearby location on

the Meander River. This change will alter the location and size of the mixing zone (the zone in

which the discharge is to be diluted). This document summarises an investigation into the

mixing requirements and mixing zone size for the proposed discharge location.

2. Physical Setting The proposed discharge location is on the Meander River downstream of the confluence

between the Liffey River and approximately upstream of the confluence with the South Esk

River. The Conservation of Freshwater Ecosystem Values (CFEV) Database indicates a low

mean slope of 0.00106 (rise/run) and this is consistent with an examination of available

topographic information. The CFEV database indicates a maximum slope of 0.00409 (rise/run),

although this maximum slope is based on coarse longitudinal data and it is possible that steeper

sections exist. A relatively steep section of this reach (riffles) is present immediately

downstream of the proposed discharge location.

3. River Flow Regime The seasonal flow regime has undergone change since the completion of the Meander Dam in

late 2007. The flow at the proposed discharge location has been estimated by combining data

from Meander River (upstream at Strathbridge, station 852) and Liffey River (station 164).

Figure 1 shows the available data (7 day average flow rate) with a logarithmic vertical scale in

order to emphasise changes in the low flows. The data is available from 1985 to present and

missing data is not shown.

Figure 1 Historical Flows – Logarithmic Vertical Scale

The 7Q10 flow (the lowest 7-day average flow that occurs on average once every 10 years)

before the creation of Meander Dam was 0.2 to 0.25 m3/s, based upon 17 full years of data. The

current 7Q10 flow (post Meander Dam completion) is more uncertain given only 5 full years of

data is available. For this analysis, a value of 1.0 m3/s is assumed. This figure is approximate

and is considered conservative in the context of dilution estimation (the data indicates that the

flow has not decreased below this value since February 2008). This 7Q10 is sensitive to the

operating rules of Meander Dam and it is likely that this low flow estimate can be refined in the

future.

2 | Tas Water - Carrick STP New Discharge Pipeline and Outfall –Appendix J Mixing Zone Assessment, 32/17413

The seasonal flow characteristics for summer (December to February), autumn (March to May),

winter (June to August) and spring (September to November) are indicated in Figure 2, Figure

3, Figure 4 and Figure 5 respectively. The flow data are plotted separately for the periods

before and after the construction of Meander Dam. These graphs show the percentage of the

time (within the given season) that the flow is below a given flow rate. For instance, in the case

of Figure 2 (summer), the flow is less than 1.0 m3/s approximately 10% of the time post 2008,

while it is less than 1 m3/s around 30% of the time pre 2008.

Figure 2 Summer Flow Statistics

Figure 3 Autumn Flow Statistics

Tas Water - Carrick STP New Discharge Pipeline and Outfall Appendix J Mixing Zone Assessment, 32/17413 | 3

Figure 4 Winter Flow Statistics

Figure 5 Spring Flow Statistics

This data shows that on average, flow rates are substantially higher in winter and spring than in

summer and autumn. It also indicates that the addition of the Meander dam upstream has the

expected impact, making the low flows higher and high flows lower. This effect is more

pronounced in summer and autumn. The flow rate data is discussed in more details in the

following sections.


4. Source Data The following sources of data were used in this mixing zone assessment:

Daily flow data from 1985 to 2013 for Meander River (measured at Strathbridge, station

852) and Liffey River (station 164). Flow data for the Meander River at the discharge

point was calculated by combining the upstream values from both stations.

Effluent data from Carrick WWTP Outfall: 61 samples from July 2009 to June 2014

(Appendix A).

Annual effluent metals monitoring data: 4 samples from 2011 to 2014.

Ambient monitoring data from upstream site P1 (approximately 700 m upstream from the

proposed discharge): eight samples from March 2013 to February 2014.

Ambient monitoring data from Strathbridge (approximately 10 km upstream from

discharge): 55 samples from October 2003 to July 2009 (as per EPA draft WQOs)

Table 1 outlines which sources were utilised for each analysis in this report, and why.

Table 1 Data Source Used for Each Analysis

Analysis/section Source Reasoning

Mixing requirements for

individual toxicants,

excluding metals

Effluent data

Upstream

Strathbridge data

Strathbridge ambient data has been used

instead of P1 ambient data because there

are more data points (longer period of

sampling) and thus more meaningful

percentiles can be calculated. See below

this table for a comparison of these

ambient data sources.

Mixing requirements for

individual toxicants –

copper and zinc

Annual effluent

metals monitoring

data

Assumed zero

upstream

concentration

Main effluent data does not include

metals.

No upstream metal concentration

monitoring available.

Water quality in the

heterogeneous zone

(non-toxicant

parameters)

Effluent data

Upstream

Strathbridge data

(except

microbiological)

Upstream P1 data

(thermotolerant

coliforms and

enterococci)

Daily flow data

Strathbridge ambient data has been used

instead of P1 ambient data because there

are more data points (longer period of

sampling) and thus more meaningful

percentiles can be calculated.

The exception is for microbiological

quality - data for Strathbridge is not

available and so P1 upstream data (8

samples) was used instead.

See below this table for a comparison of

these ambient data sources.


Comparison of Upstream Data Sources

Upstream data from P1, though they are likely to be more representative of the discharge site

than the data collected from further away at Strathbridge, only contain eight values, and so

meaningful percentiles cannot be calculated. While Strathbridge data is collected some distance

upstream, the summary comparison presented in Table 2 demonstrates that they may be

considered reasonably representative of ambient water quality immediately upstream (as

measured at P1). Strathbridge values have been used for all ambient quality analyses in this

report. After a longer period of monitoring has occurred at P1, this is likely to provide a good

reliable data source for future such studies.

Table 2 Comparison of Ambient Water Quality Data Sources

Parameter Strathbridge (2003-2009) P1 (Mar 2013- Feb 2014)

Median 90th # samples Median 90th # samples

Dissolved Oxygen (mg/L) 9.6 11.9 55 9 10.42 7

Field Conductivity (µs/cm) 79.5 137.1 58 66.7 460 6

pH field 7 7.6 55 6.7 7.5 7

Ammonia as N (mg/L) 0.009 0.022 58 0.005 0.011 8

Nitrate as N (mg/L) 0.024 0.338 58 0.091 0.329 8

Nitrite as N (mg/L) 0.001 0.004 58 0.002 0.004 8

Total Nitrogen as N (mg/L) 0.285 0.619 58 0.28 0.659 8

Dissolved Reactive Phosphorus

as P (mg/L)

0.003 0.005 58 0.005 0.012 8

Total Phosphorus as P (mg/L) 0.015 0.025 58 0.0195 0.0296 8

Enterococci (cfu/100mL) N/A N/A N/A 50 154 8

Thermotolerant coliforms

(cfu/100mL)

N/A N/A N/A 225 392 8


5. Dilution Requirement for Toxicants The required dilution at the edge of the mixing zone has been estimated based on the following assumptions:

The required dilution is based upon the concentrations of toxicants, specifically Ammonia, Nitrate, and recorded metals species.

Where available the assumed ambient (river) concentration was based on the median locally monitored concentration.

Where local data is not available locally the ambient (river) toxicant concentration was estimated on a case by case basis.

In the absence of local toxicity data, the target toxicant concentration in all cases was assumed to be the ANZECC 95% species protection default trigger levels (non bioaccumulating toxicant) or 99% species protection default trigger levels (bioaccumulating toxicant).

The required dilution for the ultimate mixing zone is set by the toxicant requiring the highest individual dilution.

5.1 Mixing Requirements of Individual Toxicants

5.1.1 Total Ammonia

The 90th percentile Total Ammonia concentration for discharges from the Carrick plant is 9.4

mg/L based upon 60 measurements over 6 years. The median receiving water Total Ammonia

concentration was 0.009 mg/L, based upon 58 measurements at Strathbridge. The target

concentration for Total Ammonia was assumed 2.8 mg/L (ANZECC 95% species protection

level, adjusted for a median pH of 7.5).

Given these inputs, the required dilution for Total Ammonia was 2.4 times.

5.1.2 Nitrate

The 90th percentile Nitrate concentration for discharges from the Carrick plant is 3.8 mg/L

based upon 61 measurements over five years. The median receiving water Nitrate

concentration was 0.024 mg/L, based upon 58 measurements at Strathbridge. The target

concentration for Nitrate was assumed 2.4 mg/L (revised ANZECC 95% species protection

level, pers. comm. Dr Rick van Dam, ERISS).

Given these inputs, the required dilution for Nitrate was 1.0 times.

5.1.3 Total Copper

Total Copper is typically a toxicant of interest in STP effluent however very little data is available

at present. Effluent characterisation is limited to four samples taken over four years as part of

annual monitoring (Table 3).


Table 3 Total Copper Effluent Concentration Data

Annual Monitoring Period Total Copper concentration (µg/L)

2010-2011 3

2011-2012 11

2012-2013 <15

2013-2014 3

Insufficient ambient Total Copper concentration data is available, and as such the concentration

in the river was assumed to be zero. The target concentration adopted was the ANZECC 99%

species protection default trigger level (1 µg/L).

With only four Total Copper measurements (one of which is below the minimum measurement

threshold) it is extremely difficult to define a characteristic effluent concentration. If the

maximum measured Total Copper concentration (15 µg/L) is adopted as the effluent

concentration, the required mixing is 14.0 times.

To summarise, if:

The ambient Total Copper concentration is assumed to be zero.

The target concentration is the default 99% species protection trigger level (1 µg/L).

The effluent concentration is 15 µg/L (the maximum on record, three samples).

A dilution of 14.0 times is required.

The coarse nature of the assumptions above reflects the lack of data available for this analysis.

The dilution estimate is not considered precise and this is discussed in greater detail in a

following section.

5.1.4 Total Zinc

Zinc is another toxicant of interest in STP effluent and, again, there is very little effluent data

available at present. Effluent characterisation is limited to four samples taken over four years as

part of annual monitoring (Table 4).

Table 4 Total Zinc Effluent Concentration Data

Annual Monitoring Period Total Zinc Concentration (µg/L)

2010-2011 9

2011-2012 33

2012-2013 42

2013-2014 13

Insufficient ambient Total Zinc concentration data is available and as such the concentration in

the river was assumed to be zero. The target concentration adopted was the ANZECC 99%

species protection default trigger level (2.4 µg/L).

With only four Total Zinc measurements it is extremely difficult to define a characteristic effluent

concentration. If the maximum measured Total Zinc concentration (42 µg/L) is adopted as the

effluent concentration the required mixing is 16.5 times.


To summarise, if:

The ambient Total Zinc concentration is assumed to be zero.

The target concentration is the default 99% species protection trigger level (2.4 µg/L).

The effluent concentration is 42 µg/L (the maximum on record, three samples).

A dilution of 16.5 times is required.

The coarse nature of the assumptions above reflects the lack of data available for this analysis.

The dilution estimate is not considered precise and this is discussed in greater detail in a

following section.

5.1.5 Miscellaneous Measured Toxicants for which ANZECC Guidelines are Available

The following toxicants were monitored as part of the annual monitoring program and were

found to be relatively low in concentration when compared with their associated ANZECC 95%

species protection guideline value:

Total Arsenic

Total Boron

Total Cadmium

Chromium (VI)

Total Lead

Total Manganese

Total Nickel

Total Selenium

Total Silver

5.1.6 Miscellaneous Measured Toxicants for which ANZECC Guidelines are not Available

The following toxicants were monitored as part of the annual monitoring program but do not

have ANZECC guideline values and were therefore not addressed in this analysis:

Total Barium

Total Calcium

Total Chromium

Total Cobalt

Total Iron

Total Magnesium

Total Molybdenum

Total Potassium

Total Silver

Total Sodium

Total Dissolved Sodium

Total Tin


5.2 Selection of a Combined Dilution Requirement

Under ideal conditions the dilution requirement at the edge of the mixing zone is determined

simply by one toxicant which requires the largest dilution. In this case, this is likely to be one of

the two key metals analysed, Total Copper or Total Zinc. The dilution requirements for Total

Copper and Total Zinc as calculated are 14 times and 16.5 times for respectively, however

these calculations are based on very small effluent datasets and coarse assumptions about the

receiving water quality. Consequently, these estimates cannot be considered reliable and a

conservative approach to defining the mixing zone is advocated.

TasWater proposes that the mixing zone be defined by an 80 times dilution of effluent. This

dilution, although arbitrary, is in line with the minimum dilution considered acceptable in the

emission limit guidelines for sewage treatment plants that discharge into fresh waters (EPA

2001), and is about 5 times the dilution requirements based on the (very limited) data available

now. A dilution of 80 times also exceeds that required for Enterococci by a factor of 16 (refer

Section 8.2.6).


6. River Flow Requirements Discharges to the river can be controlled. Two flow rates have been analysed, a low flow rate of 5 L/s and a high flow rate of 10 L/s. With a required dilution of 80 times the minimum river flow rate the river flow rate must be 400L/s to 800 L/s in order to achieve the required mixing for the low and high flow cases respectively. Given the 7Q10 flow rate has been estimated at 1.0 m3/s (1000 L/s) it is likely that the required dilution will be met in all but the most extreme dry periods.

7. Mixing Zone In the context of this analysis the mixing zone is defined as a region around the outlet where the water quality objectives are not achieved. Outside the mixing zone, the effluent is sufficiently diluted to meet water quality objectives.

The size of a mixing zone for a point discharge into a river is dependent on the required dilution, the depth and width of the river as well as the turbulent river mixing characteristics. In this analysis the extent of the mixing zone is estimated using the Gaussian dispersion model described in Fischer et al (1979). In this model, the speed of horizontal mixing is dependent on the transverse mixing coefficient ( ). For gently meandering channels, the following equation for the transverse mixing coefficient is recommended by Fischer:

0.6 ∗

where:

is the average river depth, and

∗ is the friction velocity.

The friction velocity is a fluid mechanics parameter which is used to describe the amount of bed shear (and therefore turbulence) present in a flow. In order to calculate the average depth and shear velocity for a range of flows, some basic open channel calculations were undertaken. In these calculations the velocity and depth were calculated assuming the river energy slope follows the bed slope, and assuming a bed roughness characteristic of rivers with cobble bed. For the purposes of these calculations, a river width of 25 m and the average slope was assumed. The results of this analysis are shown in Table 5.

Table 5 Velocity and Depth Estimates for a Range of Flows

Flow Rate (m3/s)

Flow Case

Average Depth (m)

Average Velocity (m/s)

Friction Velocity (m/s)

(m2/s)

1 7Q10 0.19 0.21 0.04 0.01

2 Intermediate Low Flow 0.29 0.28 0.05 0.01

3.45 Summer Median 0.40 0.35 0.06 0.02

6.33 Autumn Median 0.58 0.44 0.08 0.03

11.08 Spring Median 0.81 0.54 0.09 0.04

15.22 Winter Median 0.99 0.61 0.10 0.06

25 Intermediate High Flow 1 1.35 0.74 0.11 0.09




The spatial extent of the mixing zone was evaluated using the Gaussian model for the nine flows shown in Table 5. In the first set of results it was assumed that the effluent discharge rate was 5 L/s. The effluent discharge rate for the second set of flow scenarios was assumed to be 10 L/s. These mixing zone calculations assume that the river water quality is the median value, and the effluent discharge quality is given by the adopted values derived above.

7.1 Low Plant Discharge Cases (5 L/s)

In the figures below, the output of the Gaussian modelling is shown. In all of these figures, point of origin of the effluent is the middle of the river [0,12.5] coordinate on the graph. The horizontal axis shows distance along the river from the point of discharge, while the vertical axis represents the distance across the river. In these figures, the solid black line represents the extent of the zone where a dilution of 80 times is achieved.

Figure 6 Mixing Zone Extent, 1.0 m3/s (7Q10) river flow case

Figure 7 Mixing Zone Extent, 2.0 m3/s (intermediate low flow) river flow case


Figure 8 Mixing Zone Extent, 3.45 m3/s (summer median) rive flow case

Figure 9 6.33 m3/s (autumn median) river flow case

As the mixing zone for the latter cases it so small that its shape cannot be accurately plotted

using the model, the graphs for the following flows have not been presented:

11.08 m3/s (spring median) river flow case

15.22 m3/s (winter median) river flow case

25 m3/s (intermediate high flow) river flow case



The modelled dimensions of the mixing zones for these flows are shown in Table 6.


Table 6 shows data extracted from the model results, focusing on the distance downstream of

the outlet where the WQOs are not met (i.e. the size of the mixing zone). The distances

presented in this table are relative to the point of discharge.

Table 6 Mixing Zone Extent – Low Discharge Cases

Flow Scenario Length of Mixing

Zone

Maximum distance

downstream where

dilution is less than

80 times

Width of Mixing

Zone

Maximum width of

the zone with a

dilution less than

80 times

Volumetric

(far-field)

Dilution

Ratio of river

flow rate to

discharge

flow rate 1 Port 4 Ports 1 Port 4 Ports

1.0 m3/s

(5 L/s discharge)

7Q10

340 m < 1 m 4.5 m < 1 m 200

2.0 m3/s

(5 L/s discharge)

Intermediate Low Flow

60 m < 1 m 2.5 m < 1 m 400

3.45 m3/s

(5 L/s discharge)

Summer Median Flow

15 m < 1 m 1.5 m < 1 m 690

6.33 m3/s

(5 L/s discharge)

Autumn Median Flow

3 m < 1 m < 1 m < 1 m 1266

11.08 m3/s

(5 L/s discharge)

Spring Median Flow

< 1 m < 1 m < 1 m < 1 m 2216

15.22 m3/s

(5 L/s discharge)

Winter Median Flow

< 1 m < 1 m < 1 m < 1 m 3044

25.0 m3/s

(5 L/s discharge)

Intermediate High Flow 1

< 1 m < 1 m < 1 m < 1 m 5000

30.0 m3/s

(5 L/s discharge)


< 1 m < 1 m < 1 m < 1 m 6000

35.0 m3/s

(5 L/s discharge)


< 1 m < 1 m < 1 m < 1 m 7000


7.2 High Plant Discharge Cases (10 L/s)

The result for the high discharge cases are presented in the following figures, and summarised

in Table 7.

Figure 10 Mixing Zone Extent, 1.0 m3/s (7Q10) river flow case

Figure 11 Mixing Zone Extent, 2.0 m3/s (intermediate low flow) river flow case


Figure 12 Mixing Zone Extent, 3.45 m3/s (summer median) river flow case

Figure 13 Mixing Zone Extent, 6.33 m3/s (autumn median) river flow case


Figure 14 Mixing Zone Extent, 11.08 m3/s (spring median) river flow case

As the mixing zone for the latter cases it so small that its shape cannot be accurately plotted

using the model, the graphs for the following flows have not been presented;

15.22 m3/s (winter median) river flow case





The modelled dimensions of the mixing zones for these flows are shown in Table 7.

Table 7 Mixing Zone Extent – High Discharge Cases

Flow Scenario Length of Mixing

Zone

Maximum distance

downstream where

dilution is less than

80 times

Width of Mixing

Zone

Maximum with of the

zone with a dilution

less than 80 times

Volumetric

(far-field)

Dilution

Ratio of river

flow rate to

discharge

flow rate 1 Port 4 Ports 1 Port 4 Ports

1.0 m3/s

(10 L/s discharge)

7Q10

1400 m 190 m 9.5 m < 1 m 100

2.0 m3/s

(10 L/s discharge)

Intermediate Low Flow

240 m 14.5 m 4.5 m < 1 m 200

3.45 m3/s

(10 L/s discharge)

Summer Median Flow

60 m < 1 m 2.5 m < 1 m 345

6.33 m3/s

(10 L/s discharge)

Autumn Median Flow

13 m < 1 m 1.5 m < 1 m 633

11.08 m3/s

(10 L/s discharge)

Spring Median Flow

3 m < 1 m < 1 m < 1 m 1108

15.22 m3/s

(10 L/s discharge)

Winter Median Flow

< 1 m < 1 m < 1 m < 1 m 1522

25.0 m3/s

(10 L/s discharge)


< 1 m < 1 m < 1 m < 1 m 2500

30.0 m3/s

(10 L/s discharge)


< 1 m < 1 m < 1 m < 1 m 3000

35.0 m3/s

(10 L/s discharge)


< 1 m < 1 m < 1 m < 1 m 3500


8. Dilution of non-toxicants The previous section provides an estimate of the size of the mixing zone for toxicants under a

range of river and effluent flow rate conditions. Separate from any potential toxic impacts,

discharges will also modify concentrations of nutrients downstream of the point of discharge.

This section investigates this issue.

The impact of effluent discharges on general water quality can be divided into two zones: a

heterogeneous zone immediately downstream of the outlet where plumes of effluent are present

at higher than average concentration, and a homogenous zone downstream where effluent is

evenly mixed across the river and the water quality is effectively even.

The heterogeneous zone / homogenous zone terminology has been adopted to avoid confusion

with the mixing zone, which in this document refers exclusively to meeting the toxicant related

mixing threshold.

The physical location of the boundary between the heterogeneous zone and homogenous zone

has practical application in the selection of monitoring locations and in the discussion of non-

toxicant related water quality. In order for a monitoring location to be representative of the total

flow, it should be located in the homogenous zone.

The design of a diffuser structure has an impact on the extent of the heterogeneous zone. A

more efficient diffuser will result in a smaller heterogeneous zone.

8.1 Heterogeneous Zone Extent

In addition to modelling the size of the mixing zone, the Gaussian dispersion model was used to

estimate the distance downstream of the outlet at which the water quality reaches relative

homogeneity. This is required in order to locate monitoring stations for downstream water

quality. The criteria for relative homogeneity were defined as:

A location where the peak concentration at a river cross-section

is less than 10% greater than the average cross-section concentration.

Table 8 Distance to Cross Section of Relative Homogeneity

Flow Scenario Distance Downstream To Relative Homogeneity

Single Port Four Port

1.0 m3/s (7Q10) > 1500 m 1100 m

2.0 m3/s (Intermediate Low Flow) 1430 m 770 m

3.45 m3/s (Summer Median Flow) 1100 m 580 m

6.33 m3/s (Autumn Median Flow) 800 m 430 m

11.08 m3/s (Spring Median Flow) 600 m 320 m

15.22 m3/s (Winter Median Flow) 500 m 270 m

25.0 m3/s (Intermediate High Flow 1) 390 m 200 m




Table 8 shows the modelled extent of the heterogeneous zone. The use of a four port diffuser

approximately halves the distance to reach homogeneity assuming the high flow (10 L/s)

effluent discharge.

8.2 Water Quality in the Heterogeneous Zone

The potential for sustained changes in water quality in the reach of the Meander River between

the point of discharge and the confluence with the South Esk River have been investigated by

comparing the quality of the effluent discharged with the background river water quality. The

effluent water quality assessed in this section is the median effluent quality across all available

data, for the species of interest. The selection of the median quality in this analysis reflects the

interest in the potential for prolonged changes in water quality.

The key background water quality levels considered for each species were the median and 80th

percentile across all available data. For each species the dilution required to bring the effluent

concentration to be below the 80th percentile background was calculated (assuming the effluent

was diluted by water of median background quality). The required dilution for each species was

then placed into context by calculating how frequently this dilution would be achieved, on a

seasonal basis.

8.2.1 Total Ammonia

The median total ammonia concentration in discharges from the Carrick plant is 5.36 mg/L

based upon 61 measurements over 6 years. The median background concentration of total

ammonia in the Meander River is assumed to be 0.009 mg/L, based on 58 measurements at

Strathbridge. The target maximum concentration in the river is 0.016 mg/L, based on the 80th

percentile of background data. To keep the river nitrate concentration below this target, a

dilution of 764 times is required.

At a discharge rate of 5 L/s, the river provides this dilution:

>50% of the time in Summer

>65% of the time in Autumn

>95% of the time in Winter

>95% of the time in Spring






8.2.2 Nitrate

The median nitrate concentration in discharges from the Carrick plant is 1.31 mg/L based upon

61 measurements over 6 years. The median background concentration of nitrite in the Meander

River is assumed to be 0.024 mg/L, based on 58 measurements at Strathbridge. The target

maximum concentration in the river is 0.146 mg/L, based on the 80th percentile of background

data.

To keep the river nitrate concentration below this target, a dilution of 10 times is required. The

river provides this level of dilution greater than 95% of the time in all seasons for both a 5 and

10 L/s discharge rate.


8.2.3 Nitrite

The median nitrite concentration in discharges from the Carrick plant is 0.201 mg/L based upon 61 measurements over 6 years. The median background concentration of nitrite in the Meander River is assumed to be 0.001 mg/L, based on 58 measurements at Strathbridge. The target maximum concentration in the river is 0.003 mg/L, based on the 80th percentile of background data. To keep the river nitrate concentration below this target, a dilution of 100 times is required.











8.2.4 Total Nitrogen

The median total nitrogen concentration in discharges from the Carrick plant is 11.4 mg/L based upon 61 measurements over 6 years. The median background concentration of total nitrogen in the Meander River is assumed to be 0.285 mg/L, based on 58 measurements at Strathbridge. The target maximum concentration in the river is 0.511 mg/L, based on the 80th percentile of background data.

To keep the river total nitrogen concentration below this target, a dilution of 50 times is required. At discharge rates of both 5 L/s and 10 L/s the river provides this greater than 95% in all seasons.

8.2.5 Total Phosphorus

The median total phosphorus concentration in discharges from the Carrick plant is 4.50 mg/L based upon 61 measurements over 6 years. The median background concentration of total phosphorus in the Meander River is assumed to be 0.015 mg/L, based on 58 measurements at Strathbridge. The target maximum concentration in the river is 0.002 mg/L, based on the 80th percentile of background data. To keep the river total phosphorus concentration below this target, a dilution of 900 times is required.












8.2.6 Enterococci

The median Enterococci concentration in discharges from the Carrick plant is 454 CFU/100 mL,

based upon 62 samples collected over 6 years. The median background concentration of total

Enterococci in the Meander River is assumed to be 50 CFU/100 mL, based on 8 measurements

over the 2013/14 year. The target maximum concentration in the river is 122 CFU/100 mL,

based on the 80th percentile of background data.

To keep the river Enterococci concentration below this target, a dilution of 5 times is required.

The river provides this level of dilution greater than 95% of the time in all seasons for both a 5

and 10 L/s discharge rate.

8.2.7 Thermotolerant coliforms

The median Thermotolerant coliforms concentration in discharges from the Carrick plant is 800

CFU/100 mL, based upon 64 samples collected over 6 years. The median background

concentration of Thermotolerant coliforms in the Meander River is assumed to be 225

CFU/100 mL, based on 8 measurements over the 2013/14 year. The target maximum

concentration in the river is 352 CFU/100 mL, based on the 80th percentile of background data.

To keep the river Thermotolerant coliform numbers below this target, a dilution of 4 times is

required. The river provides this level of dilution greater than 95% of the time in all seasons for

both a 5 and 10 L/s discharge rate.

8.3 Travel Time

The travel time for flow (and suspended material including algae) between the point of

discharge and the confluence with the South Esk River is relatively short, less than one day for

all flows analysed (including the 7Q10 flow). Discharges to the Meander River may result in

elevated nutrient concentrations however the short travel time means the impacts that elevated

nutrient concentrations has on biota that travel with the flow (including algae) is considered to

be extremely limited. Impacts are therefore considered to be restricted to species that are

permanently located in the reach of the river impacted.


9. Interpretation of Results and Conclusions Analysis of the effluent and ambient water quality indicates that Total Copper and Total Zinc are

likely to determine the required dilution, but data on these metals is very limited at this site. The

uncertainty in the estimated dilution requirement for these species has been incorporated in the

selection of a dilution requirement of 80 times.

The flow data available indicate that this dilution will be achieved under almost all flow

conditions (the 7Q10 flow is sufficient to achieve this dilution).

Modelling of the mixing zone shows the expected dependence of mixing zone size on river flow

rate, discharge rate, and the number of mixing zone ports. Under typical winter and spring

conditions the mixing zone can be expected to be very small (less than 3 m long and 1 m wide

under the high effluent flow scenario for a single port outlet). Under unusually low flow

conditions such as the 7Q10 flow rate, the mixing zone extents increase dramatically (to 1400 m

in length for a single port or 190 m for four ports), however these conditions are rare and

discharges of effluent are unlikely as demand for reuse is expected to be high.

Monitoring of water quality downstream of the discharge location will need to be undertaken

more than 1100 m downstream of the point of discharge in order for the water quality at a point

to be reliably representative of the mean (under most flow conditions for a single port outlet).

The use of a four port diffuser decreases the distance downstream (by approximately half) at

which the water quality becomes mixed and therefore allows for monitoring closer to the point of

discharge.


10. References EPA 2001. Emission Limit Guidelines for Sewage Treatment Plants that Discharge Pollutants in Fresh and Marine Waters, 2001.

Fischer, H. B., et al. 1979. Mixing in Inland and Coastal Waters, Academic Press.


Appendix A – Carrick STP Effluent Data 2009-2014)


Appendix B – Summary of Dilution Calculations

GHD

Level 2, 102 Cameron Street Launceston Tasmania 7250 T: (03) 6332 5500 F: (03) 6332 5555 E: [email protected]

© GHD 2014

This document is and shall remain the property of GHD. The document may only be used for the purpose for which it was commissioned and in accordance with the Terms of Engagement for the commission. Unauthorised use of this document in any form whatsoever is prohibited.

G:\32\17413\WP\21499.docx

Document Status

Rev No.

Author Reviewer Approved for Issue Name Signature Name Signature Date

0 Thomas Ewing

J. Woodworth On file S. McLeod

15/04/2013

1 Thomas Ewing

J.Woodworth On file S.McLeod

04/12/2013

2 J. Errey T. Ewing On file S.McLeod

01/10/2014

appendix j – mixing zone assessment · the meander river. this change will alter the location and...

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