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EO Regulations Review - Simple assimilative capacity model for transitional waters
Background
This model is used indicate a given licence’s compliance with Article 7 of the European
Communities Environmental Objectives (Surface Waters) Regulations 2009.
The model estimates, for a given discharge parameter, the resultant concentration in a
receiving waterbody which has freshwater and saltwater inputs. Using the salinity values of
the ‘open’ water and in the vicinity of the discharge, it estimates the flow of dilution water
(i.e. seawater and freshwater inputs) within the estuary. This figure is then divided in to the
mass flow rate of the substance being discharged to calculate its concentration in the
receiving waters.
The salinity value in the vicinity of the discharge point may be determined directly by the
applicant from sampling analysis. Otherwise this salinity value may be taken as the salinity
of the entire WFD ‘transitional’ waterbody within which the discharge is located. The open
water salinity value may be taken as the salinity value for the WFD ‘coastal’ waterbody
adjacent to transitional waterbody.
Please Note: This model should only be used in the absence of more comprehensive
modelling that may have already been completed for the current licensed discharges.
Sources of data
To determine the transitional and coastal waterbodies relevant to your discharge, you may
use the EPA’s Envision website (using the WFD status layer).
http://maps.epa.ie/InternetMapViewer/mapviewer.aspx
Salinity data for most waterbodies in question should be available from the following EPA
download file: http://www.epa.ie/downloads/data/water/name,30857,en.html (If using this
download file, please use an average of the summer and winter median values.) More
accurate salinity values and other information on estuaries is available from Shane O’Boyle,
EPA Regional Inspectorate, Dublin (01 268 0100). For instances where salinity values for
open water are not available from the Agency, the licensee may assume that the salinity
value of the open water is 35 p.s.u.
River flow data (95%ile) is also required for this model. This data is available from a number
of sources, as listed below:
� Long term flow records -Low Flow Statistics at selected hydrometric stations
http://www.epa.ie/downloads/pubs/water/flows/name,30473,en.html
� EPA HydroNet -online hydrometric data http://hydronet.epa.ie
� Hydro-Data provided by the OPW http://www.opw.ie/hydro/

� For non-gauged locations use the EPA Hydrotool. This system provides an estimate
of flows exceeded for 5% -95% of time. Register as a user at:
http://watermaps.wfdireland.ie/HydroTool/Authentication/Register.aspx
General queries on the use of this model should be directed to Gavin Clabby, EPA Regional
Inspectorate, Inniscarra, County Cork. (021 487 5540)
Model
The model formula for calculating the flow of available dilution water (Qd) is as follows:
Qd = (Qe + Qf) So/ (So-S) where Qe is the flow rate of licensed discharge (m 3 /s)
Qf is the flow rate of the river (m 3 /s)
So is the salinity of the open water
S is the salinity of the water in the vicinity of licensed
discharge
The licensee will be required to measure, or otherwise obtain values, for Qe, Qf, S and So.
Please see appendix for full derivation of the above equation.
Once the value for Qd is established it can be substituted into the following formula to
calculate the concentration (C) of a given discharged substance in the receiving water:
C = Cb + [ (Ce - Cb)/ (1 + (Qd / QL)) ] where QL is the maximum flow of the discharged
substance allowable under the licence.
Cb is the background concentration.
Ce is the maximum effluent discharge concentration
allowable under the licence.
NB When dividing Qd by QL, ensure units are consistent. If no max flow limit is specified in the
licence, please use your declared maximum hourly rate or contact the reviewing inspector to
discuss.
Worked example
• An industrial plant is licensed to discharge 0.5m 3 /s of effluent into an estuary (good
status). This licensed discharge has and ELV of 5mg/l P for Orthophosphate (MRP).
The mean background concentration of MRP is 0.02mg/l P. The 95%ile flow of the
river entering the estuary is 10m 3 /s.
• Salinity values for the discharge point (transitional) waterbody and the coastal
waterbody were determined to be 19 p.s.u. and 33 p.s.u. respectively.

• Estimate the concentration of MRP of the receiving water in the vicinity of the
discharge.
Solution:
i) Compute flow rate of available dilution water
Qd = (Qe+Qf)So/(So-S)
= (0.5 + 10) x 33 / (33 – 19)
= 24.75 m 3 /s
ii) Compute mean concentration of MRP in receiving water
C = Cb + [(Ce - Cb)/ (1 + (Qd / QL))]
= 0.02 +[ (5 - 0.02)/ (1 + (24.75/0.5))]
= 0.02 + [4.98/ (1 + 49.5)]
= 0.119 mg/l P
This value is greater than the relevant MRP limit for 19 p.s.u. (0.058 mg/l P).
� Therefore, the licence ELV, as set, does not appear compliant with Surface Waters
Regulations 2009.
(Please Note: EO Regulation limits for MRP in Transitional Waters are dependent on the
salinity and WFD status of the receiving water. For convenience a table with the
salinity/status dependent limits is available on the EPA website.)

Appendix
1. Illustration of a typical estuary with indicated waterbodies and parameters relevant
to equation.
coastal Water body transitional waterbody river waterbody
2. Derivation of model formula
From salt mass balance:
Qo So = (Qo + Qe + Qf)S => Qo So = QoS + QeS + QfS
Qo So - QoS = + QeS + QfS => Qo (So – S) = + (Qe + Qf)S
Therefore Qo = (Qe + Qf)S/(So - S)
Qd = Qo + Qe + Qf => Qd = (Qe + Qf)S/(So - S) + Qe + Qf
Qd(So - S) = (Qe + Qf)S + Qe(So - S) + Qf(So - S) => QdSo - QdS = Qe So + Qf So
Therefore Qd = (Qe + Qf) So/ (So - S)