Trimleston Stream

Dun Laoghaire Area Meeting



That the Chief Executive bring a report on the water quality and condition of Elm Park and Trimleston Streams, that flow through the County and enter the sea at Merrion Strand. The report should include information on the reports of sewage in the water and on any task force that may have been set up by DLR and Dublin City Council.
1. Elm Park
The Gledswood and Mulvey Streams come together in the grounds of the Carmelite Convent on Roebuck Rd to form the Elm Park Stream. From there, the stream runs in open channel to UCD through which it passes in a concrete culvert, passing beneath the N11. Crossing into Dublin City Council, it once more opens to an open natural stream through Elm Park Golf Club. It discharges to sea to the rear of the Texaco garage on Merrion Road.
The primary pressures on the stream are domestic misconnections and overflows from the Irish Water foul sewer system. There are 3 such overflows which during heavy rain or blockages in the foul system can overflow to the stream. These are located at Larchfield Avenue, Roebuck Castle Estate and Elm Park Golf Club. Irish Water is currently working on a Drainage Area Plan (DAP) for the area. The purpose of the DAP is to increase the capacity of the foul system thereby reducing the need
for it to overflow. The DAP will cover the DCC and DLRCC areas of the Elm Park Catchment. Within the DLRCC area of the catchment, there are 4,500 properties. After representations from the DLRCC Drainage Dept, Irish Water had informed that it intended to increase capacity of the foul system on Goatstown Road this summer.
However, the project has been postponed. The most recent estimate from Irish Water for the completion of the work is late Spring 2021. This will enable the Larchfield overflow to be removed permanently. This overflow has been a significant contributor to pollution in the Elm Park Stream in heavy rain, so it’s removal will have a beneficial impact on water quality in the stream.
2. Trimleston
The Trimleston Stream catchment area is split almost evenly by the N11. From the N11 to the sea, the stream is open and enters the sea via a culvert that forms the northern edge of Booterstown Marsh. On the west side of the N11, what was once an open stream has long since been piped. The total catchment contains 3,200 properties. There are 3 Irish Water overflows within the catchment. The most upstream is at St Thomas Meade (off Foster’s Ave). The other 2 overflows are on either side of the Rock Road. As with the Elm Park Stream, Irish Water is engaged on
a DAP for the Trimleston Catchment. It is hoped that the resulting works from the DAP will reduce or remove completely operation of these 3 overflows. Water Quality The Elm Park and Trimleston Streams are regularly sampled for water quality. In dry conditions, both streams show signs of moderate urban pollution. However, in heavy rain, water quality can be poor showing increased sewage contamination. Weekly
samples taken at the discharge point of the Elm Park Stream in 2019 show a range for E.coli from the low 100s to 47,000 CFU/100ml. At the discharge point of the Trimleston Stream for the same period the range is from the low 100’s to 70,000 CFU/100ml. Likely sources of sewage contamination for these streams are the following.
1. Domestic misconnections
2. Sewage Overflows
3. Leaking Sewers
Sandymount and Merrion Strand
Both streams drain to Merrion Strand in DCC. Sandymount Strand is to the north of Merrion Strand. Both water bodies are official bathing areas and sampled twice every week during the bathing season.
Results at both beaches have been disappointing. It is likely that Merrion Strand will be delisted as a bathing area this year by the EPA due to poor results over the past 4 years. Sandymount Beach has had a history of poor results also. However, it is not currently at risk of losing its bathing water status. Due to the poor water results at Merrion and Sandymount Strand, in November 2019, DCC initiated a Task Force to investigate causes of these results and to prepare plans to improve water quality. The Task Force meets on a monthly basis and is comprised of:
 Irish Water
 Dept of Housing, Environment and Local Government.
 DLR Water Pollution Control Section
 DCC Water Pollution Control Section

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