Elm Park -Trimleston Streams

Dun Laoghaire Area Committee


Motion: Councillor M. Baker
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 what measures are in place to remedy any problems with sewage entering the streams.
Both the Elm Park and the Trimleston Streams are very typical of urban streams in Ireland affected by pollution from their urban environments, namely:
1. Misconnections
2. Old leaking foul sewers
3. Overflows from the foul and combined sewer systems from blockages and /or heavy
4. Runoff from roads and footpaths where in effect, rainfall cleans the streets and puts the dirt into the gullies and then the river.

Both streams have been studied as part of UCD’s EU funded Acclimatise Project. DLR contributed to the project by sampling a number of locations on both streams over the course of a year and a half. The Acclimatise Project’s brief is to study the impact of climate change on seawater quality in urban areas. The part of the study that focused on our County, found
that poor water quality in the Elm Park Stream could have a negative impact on bathing water quality at Merrion Strand and Sandymount, particularly in heavy rain conditions. This leads to the conclusion that the 3 overflows from the foul and combined sewers (which can operate
in heavy rain) that discharge to the river are significant. PCR testing of the bathing waters, by UCD (PCR testing can identify the source of E.coli in
bathing water samples i.e. human, ruminant, bird, dog) has shown that dog and bird excrement are significant sources of pollution to the bathing waters on the strands.

In dry weather, both streams show signs of moderate pollution. Of the two streams the Elm Park stream appears to have the greater influence on bathing water quality at the two strands. In their paper “Identifying Sources of Faecal Contamination in a Small Urban Q12 Stream Q13
Catchment: A Multiparametric Approach” a multidisciplinary team led by UCD (including DLR staff) identified three areas in the Elm Park catchment that were sources of pollution for the stream.
1. The overflow from the foul sewer network to the surface water network
which drains into the Elm Park Stream at Larchfield Road (see below)
In 2020, Irish Water increased the capacity of the foul sewer network on the Clonskeagh Road which made the overflow at Larchfield Road unnecessary. It has since been blocked up and is no longer a source of pollution for the stream.

For image see Link

The yellow icon indicates an overflow from the foul sewer to the surface water pipe that drains to the river
2. Upper Mulvey Catchment
Sampling identified the area within the yellow circle as a source of pollution for the stream. This area is on one of the three tributaries of the Elm Park Stream upstream of UCD (the Upper Mulvey Stream Catchment). The DLR Water Pollution Control team is active in this area and to date has worked with property owners to remove two full properties all of which’s foul drainage was entering the stream. The drainage system in the mini catchment is being divided into smaller areas to trace which properties that are contributing pollution to the stream. This
work is slow as it involves making appointments to assess individual properties for misconnected drainage – where the foul drainage is wrongly connected to the surface water system.
3. UCD Campus
The study highlighted the campus in UCD as a source of pollution for the Elm Park Stream which passes beneath the Campus in a concrete box culvert. DLR has engaged with the UCD Facilities Team to highlight the College’s responsibilities under the Water Pollution Act 1977
(amended 1990). Since then, the College has commissioned an investigation with a Consulting Engineer to find sources of pollution for the stream within the Campus. Phase 1 of the investigation is complete and included a CCTV survey of the complete length of the culvert.
We understand from UCD that Phase 2 of the study is awaiting funding and should begin before the end of the year.

Other Works
1. In general, pollution in the stream when the weather is dry is caused by
misconnections or leaking foul sewers which allow foul sewage to enter ground water and then enter the river. In the drawing above, the areas shaded in various colours show the Elm Park and Trimleston catchments and the sub catchments of their tributaries. The properties highlighted by green, blue and purple dots have been inspected for misconnections. This work was carried out some years ago.
2. The DLR Water Pollution Engineer has walked the stream and identified pipes entering the stream which contain pollution. Various property owners have been engaged to remove the pollution.

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