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Ballymore Eustace WTP Spillway Chemistry and Impact on River Liffey A Report to Irish Water 7 June 2018 Martin McGarrigle Limnos Consultancy

allymore Eustace WTP Spillway hemistry and Impact on River

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Page 1: allymore Eustace WTP Spillway hemistry and Impact on River

Ballymore Eustace WTP Spillway Chemistry

and Impact on River Liffey

A Report to Irish Water 7 June 2018

Martin McGarrigle

Limnos Consultancy

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Contents Introduction ............................................................................................................................................ 3

Methods and Datasets –Flows, Water Chemistry, Mass-Balancing ....................................................... 4

River Liffey Flows ................................................................................................................................ 4

WTP Spillway Flows ............................................................................................................................. 4

Water Chemistry Datasets .................................................................................................................. 5

Mass-Balance Methodology ............................................................................................................... 5

Results ..................................................................................................................................................... 6

Aluminium ........................................................................................................................................... 6

pH ...................................................................................................................................................... 15

Suspended Solids .............................................................................................................................. 18

Total Nitrogen (TN) ........................................................................................................................... 21

Total Phosphorus (TP) ....................................................................................................................... 24

Total Organic Carbon (TOC) .............................................................................................................. 27

Turbidity ............................................................................................................................................ 30

Summary and Conclusions .................................................................................................................... 33

Appendix 1. Summary of water chemistry results. ............................................................................... 34

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Introduction This report examines the water chemistry of the River Liffey at Golden Falls and Ballymore Bridge

plus that of the Spillway from the Ballymore Eustace Water Treatment Plant. Flow data for the Liffey

at Golden Falls and the Spillway are used in conjunction with the chemistry data to mass-balance the

Spillway with the River Liffey in order to compare observed with expected mass-balanced results. A

detailed set of statistical comparisons are made comparing:

• upstream v downstream

• mass-balanced v observed downstream

• left bank v right bank,

and the concentrations of a range of quality elements are plotted over time.

Conclusions are drawn regarding the possible cause of filamentous algal growths that are observed

during the growing season in the Liffey in the Ballymore Eustace stretch between Golden Falls and

Ballymore Bridge (Figure 1. Map showing location of spillway from water treatment plant and River

Liffey from Golden Falls to Ballymore Bridge. © OpenStreetMap contributors.Figure 1).

Figure 1. Map showing location of spillway from water treatment plant and River Liffey from Golden Falls to Ballymore Bridge. © OpenStreetMap contributors.

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Methods and Datasets –Flows, Water Chemistry, Mass-Balancing

River Liffey Flows

Available daily flows for Golden Falls for the period from 1 January 2013 to 31 August 2017 (Figure

2). These are taken as equivalent to the discharge for the main River Liffey between the

hydroelectric power station and Ballymore Bridge, and are used for mass balancing with the Spillway

from the WTP. The river is flow controlled with a minimum flow of 1.5 m3/s and this flow accounts

for almost 50% of days measured over the 2013–2017 period. Winter flows can exceed 40 m3/s on

occasion, but the median flow is 1.55 m3/s and the mean flow is 5.8 m3/s. The flows were matched

against the dates for which chemistry sampling was available for the purposes of mass-balancing.

Figure 2. Flows in the River Liffey at Golden Falls Jan 2013 – Aug 2017.

WTP Spillway Flows

Spillway flows for the period January to early April 2018 are shown in Figure 3. The average flow of

0.142 m3/s is less than 10% of the 1.5 m3/s low flow most commonly observed in the main River

Liffey. The discharge from the WTP is relatively stable ranging from 0.124 to 0.157 m3/s with a low

coefficient of variation (SD/Mean) of 5.4%. This is seen as expected because spillway flows are

dependent on the process within the plant which delivers a steady daily volume of treated water to

Dublin consumers.

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Figure 3. Average daily flows in the spillway from the Ballymore Eustace Water Treatment Plant for Jan to early April 2018.

Water Chemistry Datasets

WTP Spillway, River Liffey at Golden Falls and Ballymore Bridge Summary details of the water chemistry available for Golden Falls, Ballymore Bridge (left (S) and

right (N) side) and the WTP spillway are shown in Appendix 1. Summary of water chemistry results.

Note that Total Nitrogen (TN) and Total Phosphorus (TP) data were available for January to March

2018 while the other quality elements, aluminium, etc., were measured over the period 2016–2017.

Mass-Balance Methodology

Mass-balancing combines two (or more) water bodies to predict the downstream concentration of a

relevant quality element such as phosphorus or nitrogen when the water bodies combined and are

mixed completely. Both water flow (discharge) in terms of volume flowing past a point in a given

time, e.g. m3/s, and concentration of the quality element are needed – the formula used in this case

for the WTP spillway discharging into the River Liffey downstream of Golden Falls is:

𝑑𝑜𝑤𝑛𝑠𝑡𝑟𝑒𝑎𝑚 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 = (𝑓 ∗ 𝑐 + 𝐹 ∗ 𝐶)/(𝑓 + 𝐹)

Where f is the flow of water in the spillway (m3/s) and c is the concentration (mg/l) of the relevant

quality element in the spillway discharge into the Liffey – aluminium, suspended solids, phosphorus,

nitrogen, etc., F is the flow in the River Liffey and C is the corresponding concentration of the quality

element of interest in the River Liffey.

The flows and concentrations must match as closely as possible – typically the daily average flow and

a matching concentration for a sample of water taken on the same day as the average flow value is

used. In the analysis below: flows and concentrations are available on the same day in the Liffey at

Golden Falls and Ballymore Bridge for the period 2016–2017. Corresponding concentrations are

available for the spillway over this period – but flows were not available. However, available flow

measurements in the spillway from January to early April 2018, showed a small range of variation

(compared with a natural river) – with mean flow of 0.142 m3/s. This spillway flow was, therefore,

used in mass-balancing the spillway into the Liffey for the 2016-2017 chemistry data. Nutrient data

for Total Nitrogen (TN) and Total Phosphorus (TP) were available for Jan to Mar 2018.

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Results

Aluminium

Al – Comparison of River Liffey: Upstream and Downstream of Ballymore WTP discharge.

Welch Two Sample t-test data: Upstream and Downstream t = -2.2186, df = 84.238, p-value = 0.0292 p-value significance * <0.05

Figure 4. Aluminium concentrations 2016-2017 comparing Golden Falls (upstream) with Ballymore Bridge (downstream).

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Al – Year to Year Reduction 2016 v 2017

Figure 5. Time series comparison of aluminium concentrations upstream and downstream

of WTP.

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Welch Two Sample t-test data: Years20162017 by Chm$Year t = 3.1017, df = 26.775, p-value = 0.004497 p-value significance ** <0.01

Figure 6. A statistically significant reduction in aluminium concentrations at Ballymore Bridge comparing 2016 and 2017.

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Figure 7. Aluminium concentrations in the WTP Spillway from July 2016 to July 2017

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Al – Comparison of Mass-Balanced Concentrations and Observed Values

Welch Two Sample t-test data: MassBalanced and Observed t = 4.6639, df = 80.576, p-value = 0.00001215 (****) p-value significance **** <0.0001

Figure 8. Mass-balanced aluminium (WTP Spillway with Liffey) compared with observed upstream (Golden Falls) and downstream values (Ballymore Bridge).

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Al – Left Bank v Right Bank Comparison at Ballymore Bridge

Welch Two Sample t-test data: LeftBank and RightBank

t = 1.0325, df = 67.948, p-value = 0.3055 p-value ns: no significant difference

Figure 9. Aluminium – Left Bank v Right Bank comparison at Ballymore Bridge.

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Al – Concentrations in Relation to Flow in the River Liffey

Figure 10. Difference in Aluminium concentration between Golden Falls and Ballymore Bridge on R. Liffey at increasing river flow rates. Positive values mean that the

concentration has increased. See text for details.

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Interpretation of Aluminium Results Aluminium shows a statistically significant increase when Golden Falls is compared directly with

Ballymore Bridge for average concentrations over the period 2016-2017 (Figure 4). When a year to

year comparison is made, however, a significant decline in concentration is observed when

comparing 2016 against 2017. The Al concentrations in 2017 are lower than in 2016 (Figure 5. Time

series comparison of aluminium concentrations upstream and downstream of WTP.Figure 5, Figure

6).

Figure 5 compares aluminium concentrations upstream and downstream of the WTP at Golden Falls

and Ballymore Bridge for the period July 2016 to June 2017. A statistically significant increase is seen

from a mean of 136 to 172 µg/l. When this is further broken down it is apparent that there is a

significant difference between the 2016 and 2017 results (Figure 6). The reduction is due to a lower

mean concentration of aluminium in the WTP spillway – from 2447 µg/l to 1704 µg/l – rather than a

year on year increase in river flow, providing greater dilution (Figure 7 ). Thus, the statistical

difference between upstream and downstream disappears when the 2017 results are treated

separately.

The mass-balanced predicted concentration for aluminium shown in Figure 8 is significantly greater

than the average concentration measured at Ballymore Bridge. This suggests that deposition of

aluminium is occurring between the spillway discharge point and Ballymore Bridge. As indicated

above in Figure 6, aluminium concentrations in the spillway had dropped significantly from 2016 to

2017. The time series in Figure 8 shows little difference between the upstream and downstream

aluminium concentrations in 2017; it is obvious, however, that the predicted mass-balanced

concentration is significantly greater than the observed downstream concentration on a sampling

date basis.

There is no apparent difference in aluminium concentrations comparing left bank samples with right

bank samples over the 2016-2017 period (Figure 13).

Figure 10 compares the increase (or decrease) in aluminium concentration at Ballymore Bridge

compared with that at Golden Falls upstream of the WTP spillway. The concentrations changes are

here categorised by flow category – see Table 1 for the ranges for each category – note that these

are selected based on the distribution of flows during the period under study. The flows in the River

Liffey are controlled by the ESB for power generation and thus, most flows are at 1.5 m3/s (Figure 2).

The highest flow measured during 2016-2017 that corresponds to a chemistry sampling date was

15.66 m3/s on 20 October 2016. An interpretation of the data underlying Figure 10 is as follows: at

low flows the aluminium entering the river via the spillway at the WTP gives rise to an increase in

concentrations of some 18 µg/l at 1.5 m3/s (Flow Category 1 in the boxplot). At mid flows between

1.55 and 2 m3/s there is little, or no increase compared with concentrations in the river at Golden

Falls. At higher flows 4 – 16 m3/s, however, there appears to be an increase again – albeit based on a

limited number of results. This latter is believed to be due to a scouring effect, with the currents

lifting deposits of aluminium from the substratum during higher flows.

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Table 1. Flow categories for Liffey River 2016-2017.

Flow Category

Flow range (m3/s)

1 1.50 1.55

2 1.55 1.80

3 1.80 1.85

4 1.85 2.00

5 2.00 4.00

6 4.00 10.0

7 10.0 16.0

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pH

pH – Upstream v Downstream variation

Welch Two Sample t-test data: Upstream and Downstream t = 0.1746, df = 87.299, p-value = 0.8618 p-value ns: no significant difference

Figure 11. Comparison of pH upstream and downstream of the WTP spillway. No significant difference was observed.

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pH – Mass-balanced v Observed

Welch Two Sample t-test data: MassBalanced and Observed t = -4.0629, df = 88.008, p-value = 0.000105 (***)

Figure 12. Mass-balanced pH (WTP Spillway with Liffey) compared with observed upstream (Golden Falls) and downstream values (Ballymore Bridge).

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pH – Left Bank v Right Bank at Ballymore Bridge

Welch Two Sample t-test data: LeftBank and RightBank t = 0.0853, df = 89.951, p-value = 0.9322 p-value ns: no significant difference

Figure 13. Comparison of left and right bank for pH.

Interpretation of pH The spillway median pH was 7.2 compared with 7.8 for Golden Falls for the 2016–2017 period under

consideration. There was, however, no appreciable or statistically significant change in pH when

Golden Falls is compared with Ballymore Bridge, (see Figure 11). This indicates that there is sufficient

buffering capacity in the system to maintain acidity levels close to the expected pH for the River

Liffey at this point.

The predicted mass-balanced pH values (Figure 12) are significantly lower than the observed mean

pH at Ballymore Bridge. This is closely related to the behaviour of aluminium concentrations above –

the alum depositing on the river bed will have a much lower pH than the main water column. This

may mean that there are microhabitats in the stretch between Golden Falls and Ballymore Bridge

with a lower than average pH.

There was no left right bank difference in pH noted at Ballymore Bridge (Figure 13).

Note that because pH is a log scale, in all cases where pH values were mass-balanced or averaged,

the pH values were converted to hydrogen ion concentrations before averaging and then logged

back as negative log of the hydrogen ion concentration as per the definition of pH.

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Suspended Solids

SS – Upstream v Downstream

Welch Two Sample t-test data: Upstream and Downstream t = -1.02, df = 78.662, p-value = 0.3108 p-value ns: no signficant difference

Figure 14. Comparison of suspended solids upstream and downstream of the WTP spillway. No significant difference was observed.

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SS – Mass-balanced v Observed

Welch Two Sample t-test data: MassBalanced and Observed t = 0.9988, df = 78.187, p-value = 0.321 p-value ns: no signficant difference

Figure 15 Mass-balanced suspended solids (WTP Spillway with Liffey) compared with observed upstream (Golden Falls) and downstream values (Ballymore Bridge).

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SS – Left Bank v right Bank at Ballymore Bridge

Welch Two Sample t-test data: LeftBank and RightBank t = 0.9069, df = 65.903, p-value = 0.3677 p-value ns: no signficant difference

Figure 16. Comparison of suspended solids left v right side at Ballymore Bridge

Interpretation of Suspended Solids Suspended solids in the Liffey at Ballymore Eustace were generally very low for an Irish river system,

and this is due to the impoundment at Golden Falls; allowing settling to take place. Many values

were below the limit of detection of 5 mg/l. (Note that in calculating mean values for these, the

convention of using half the limit of detection was used, i.e. 2.5 mg/l). Downstream of the WTP

spillway no statistically significant increase in concentration was observed, taking the 2016–2017

dataset as a unit (Figure 14). Occasional spikes occurred, reaching a maximum mean value of 15.75

mg/l on 1 Sep 2016. This latter is based on a left-hand side sample of 29 mg/l and a right-hand

sample of < 5 mg/l (taken as 2.5 mg/l) possibly indicating a localised disturbance on the day of

sampling – it was not obviously related to increased flow or a change in the spillway concentration.

There was no significant difference between the predicted mass-balanced SS and the observed

Ballymore Bridge concentrations (Figure 15). There are, however, some anomalously high spikes in

SS concentration at Ballymore Bridge. These may be associated with resuspension of deposited

material in higher than normal flow rates or due to other disturbance.

There was no statistical difference between left and right samples at Ballymore Bridge (Figure 16).

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Total Nitrogen (TN)

TN – Upstream v Downstream

Welch Two Sample t-test data: Upstream and Downstream t = 0.8776, df = 21.809, p-value = 0.3897 p-value ns: no signficant difference

Figure 17. Comparison of total nitrogen (TN) upstream and downstream of the WTP spillway. No significant difference was observed.

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TN – Mass-balanced v Observed

Welch Two Sample t-test data: Mass-Balanced and Observed t = 0.6086; df = 21.832 p-value = 0.5491 p-value ns: no signficant difference

Figure 18. TN (mg/l) - Spillway & Mass-Balanced compared with Upstream (Golden Falls)

and Downstream (Ballymore Bridge).

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TN – Left Bank v Right Bank

Welch Two Sample t-test data: Left Bank and Right Bank t = 0.1323, df = 21.95, p-value = 0.896 p-value ns: no significant difference

Figure 19. Comparison of left and right bank for Total Nitrogen.

Interpretation of TN There was no statistical difference between TN at Golden Falls and Ballymore Bridge (Figure 17).

The TN concentrations in the spillway are lower than the TN concentrations at Golden Falls and

Ballymore Bridge (Figure 18 ). The mean and median concentrations of just under 1.0 mg/l would be

likely to classify Ballymore Bridge and Golden Falls as high status for nitrogen in terms of WFD

nutrient conditions. There is a suggestion of a seasonal cycle in the time series, with all sites

increasing towards April – but nutrient data are only available to date for the January to early April in

2018 and there is insufficient data to confirm this trend. A more normal trend would be for nitrogen

concentrations to decrease as typically they will be higher in winter than summer.

There was no statistical difference between left and right samples at Ballymore Bridge (Figure 16).

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Total Phosphorus (TP)

TP – Upstream v Downstream

Welch Two Sample t-test data: Upstream and Downstream t = 2, df = 21.82, p-value = 0.0581 p-value ns: no signficant difference

Figure 20. Comparison of total phosphorus (TP) upstream and downstream of the WTP spillway. No significant difference was observed.

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TP – Mass-balanced v Observed

Welch Two Sample t-test data massbalanced and Downstream t = 1.5485 df = 21.935 p-value = 0.1358 p-value ns: no signficant difference

Figure 21. TP (mg/l) - Spillway & Mass-Balanced compared with Upstream (Golden Falls)

and Downstream (Ballymore Bridge).

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TP – Left Bank v Right Bank

Welch Two Sample t-test data: Left Bank and Right Bank t = 0.8424, df = 21.64, p-value = 0.4088 p-value ns: no signficant difference

Figure 22. Comparison of left and right bank for total phosphorus.

Interpretation of TP TP concentrations are low at Golden Falls and Ballymore Bridge with no significant difference

between the two locations upstream and downstream of the spillway from the WTP. The mean TP

values of 0.020 and 0.017 mg/l at Golden Falls and Ballymore Br would suggest that the river would

meet high status for phosphate under the WFD Regulations (SI 272 of 2009). As with TN, the TP

concentrations in the spillway are lower than those of the main river (Figure 20, Figure 21). There

was no significant difference for TP between left and right sides at Ballymore Bridge (Figure 22).

There is a suggestion of P uptake between Golden Falls and Ballymore (Figure 20. Comparison of

total phosphorus (TP) upstream and downstream of the WTP spillway. No significant difference was

observed.), but this was not statistically significant. Phosphorus is normally the limiting nutrient in

freshwater systems and at the low concentrations observed in the Liffey any additional available P

would be rapidly taken up by plants during the growing season. The quite abundant growths of algae

observed in June 2018, however, are denser than would be expected for the nutrient levels

measured. While Cladophora, which is indicative of enriched systems, is present, there are several

other algal species present, including the red alga, Lemanea, an indicator of high quality, is present

in significant quantities in this stretch. The fact that the flow is controlled and normally flows at 1.5

m3/s in this wide stretch of river is undoubtedly an important factor in allowing the proliferation of

algae. A natural river would experience flood events much more frequently and these would scour

out filamentous algae such as Cladophora on a regular basis.

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Total Organic Carbon (TOC)

TOC – Upstream v Downstream

Welch Two Sample t-test data: Upstream and Downstream t = -0.4234, df = 89.405, p-value = 0.673 p-value ns: no signficant difference

Figure 23. Comparison of total organic carbon (TOC) upstream and downstream of the WTP spillway. No significant difference was observed.

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TOC – Mass-balanced v Observed

Welch Two Sample t-test data: MassBalanced and Observed t = -1.1671, df = 89.436, p-value = 0.2463 p-value ns: no signficant difference

Figure 24. Mass-balanced TOC (WTP Spillway with Liffey) compared with observed

upstream (Golden Falls) and downstream values (Ballymore Bridge).

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TOC – Left Bank v Right Bank

Welch Two Sample t-test data: LeftBank and RightBank t = 0.4411, df = 89.839, p-value = 0.6602 p-value ns: no signficant difference

Figure 25. Comparison of left and right bank at Ballymore Bridge for total organic carbon.

Interpretation of TOC No significant increase in total organic carbon (TOC) was observed (Figure 23). The spillway TOC

concentrations were generally lower than those in the main River Liffey.

There was close agreement between the mass-balanced prediction and the observed TOC at

Ballymore Bridge (Figure 24). This is not unexpected as the TOC in the spillway discharge is quite low.

The time series shows an interesting (and expected) seasonal cycle with peaks in autumn and spring

and minima in summer. This phenomenon is not related to the WTP. There was no difference in TOC

between left and right sides of the river at Ballymore Bridge.

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Turbidity

Turbidity – Upstream v Downstream

Welch Two Sample t-test data: Upstream and Downstream t = 0.0877, df = 89.689, p-value = 0.9303 p-value ns: no signficant difference

Figure 26. Comparison of turbidity upstream and downstream of the WTP spillway. No significant difference was observed.

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Turbidity – mass-balanced versus observed

c

Welch Two Sample t-test data: MassBalanced and Observed t = 1.0735, df = 89.951, p-value = 0.2859 p-value ns: no signficant difference

Figure 27. Mass-balanced Turbidity (WTP Spillway with Liffey) compared with observed

upstream (Golden Falls) and downstream values (Ballymore Bridge).

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Turbidity – Left Bank v Right Bank

Welch Two Sample t-test data: Left Bank and Right Bank t = 0.8054, df = 80.765, p-value = 0.4229 p-value ns no signficant difference

Figure 28. Turbidity – left bank v right bank comparison at Ballymore Bridge.

Interpretation of Turbidity No significant change in turbidity was observed comparing Golden Falls with Ballymore Bridge

(Figure 26). While the spillway mean Turbidity was 3.66 NTU compared with 1.60 at Golden Falls, the

turbidity at Ballymore was slightly lower at 1.59 NTU but not statistically significant. The mass-

balanced calculations predicted a downstream turbidity of 1.76 NTU slightly higher than observed

(Figure 27) and this may indicate some settlement of particulates between the spillway discharge

point and Ballymore Bridge; similar to aluminium as discussed above. Occasional spikes in turbidity

are noted in the data but these seem unrelated to flow rates or spillway discharge concentrations.

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Summary and Conclusions The nutrient levels in the spillway discharge from the Ballymore Eustace Water Treatment Plant are

low – lower than the main river into which it discharges. The main river has nutrient levels that

would normally be indicative of good or high status. Total P concentrations of 0.020 mg/l are lower

than the 0.025 mg/l high status threshold for phosphate in the WFD Regulations (Table 2). Total N

concentrations of approximately 1 mg/l N are also suggestive of good- or high-status conditions.

Table 2. Environmental Quality Standards for phosphate in WFD Regulations SI 272 of 2009.

The aluminium concentration in the spillway has declined significantly between 2016 and 2017

(Figure 6). The mass-balanced predicted aluminium concentration is higher than those observed at

Ballymore Bridge (Figure 8). This indicates that settlement is occurring during the normal low flows

observed downstream of Golden Falls. This is in keeping with observations of white flocs made in the

field during low flows. Flushing appears to occur during higher flows (Figure 10).

None of the left right bank comparisons showed any statistically significant difference for any

measured quality element. This contrasts with the historic situation here where in the past the right-

hand side of the river was of much poorer quality than the left due to streaming along the right side.

The current regime appears to be much improved in this respect; with no obvious left/right

differences in water chemistry.

The algal growths in the river are unexpected if nutrient levels were the only driving factor. TN and

TP concentrations are generally low in the River Liffey between Golden Falls and Ballymore Bridge.

The flow regime, with long periods of low flow velocity (Figure 2) undoubtedly contribute to the

luxuriant growths seen – as in June 2018, for example. The lack of flushing allows the algae to

continue to build up biomass over many weeks without the scouring that would be expected more

frequently in an unregulated river. (The author has, however, noted abundant filamentous algae in

some high-quality rivers in the west of Ireland in early June 2018 due to a long warm dry spell.)

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Appendix 1. Summary of water chemistry results. Results for Golden Falls, Ballymore Bridge (L&R), Ballymore Eustace WTP Spillway results plus

mass-balanced predictions for Ballymore Bridge.

Spillway Golden

Falls

Ballymore Br Left (S)

Bank

Ballymore Br Right (N)

Bank

Ballymore Br (mean)

Mass-Balanced

Aluminium (µg/l) Min 195 26 54 60 57 70

Median 1,910 122 136 141 140 266

Mean 2,043 136 183 161 172 277

Max 4,400 257 808 315 517 557

pH

Min 6.90 7.50 7.60 7.60 7.60 7.40

Median 7.20 7.80 7.80 7.80 7.80 7.70

Mean 7.19 7.80 7.79 7.79 7.79 7.69

Max 7.50 8.20 8.20 8.10 8.15 7.90

Suspended Solids (SS) mg/l

Min 5.00 2.50 2.50 2.50 2.50 2.60

Median 13.50 2.50 2.50 2.50 2.50 3.90

Mean 15.09 3.41 4.16 3.53 3.85 4.28

Max 33.00 7.00 29.00 11.00 15.75 8.60

Total Organic Carbon (TOC) (mg/l)

Min 2.95 5.40 5.47 5.48 5.52 5.30

Median 5.30 6.59 6.62 6.62 6.62 6.60

Mean 5.39 6.58 6.65 6.60 6.63 6.50

Max 7.89 7.81 7.81 7.70 7.60 7.70

Turbidity (NTU)

Min 1.60 0.80 0.73 0.86 0.83 1.00

Median 3.61 1.39 1.33 1.34 1.32 1.57

Mean 3.66 1.60 1.66 1.52 1.59 1.76

Max 7.03 5.07 5.96 4.33 4.56 5.17

Total Nitrogen (TN)

Min 0.60 0.73 0.64 0.70 0.57 0.59

Median 0.87 1.01 0.96 0.98 0.97 0.99

Mean 0.81 1.03 0.96 0.96 0.93 0.98

Max 0.98 1.27 1.25 1.17 1.21 1.24

Total Phosphorus

Min 0.005 0.010 0.010 0.010 0.010 0.010

Median 0.010 0.020 0.020 0.020 0.018 0.019

Mean 0.011 0.020 0.017 0.016 0.017 0.019

Max 0.020 0.030 0.020 0.020 0.020 0.029