Hydro G Final report - Kildare.ie

Hydro-G FINAL REPORT
4. Conceptual Model of Leixlip Spa Flow Patterns
The observed flow patterns throughout the site were detailed in Section 3.2. Based on the
physiochemical monitoring, hydrochemical analyses and flow rate assessments at the Leixlip Spa
site, the following model of the hydrological regime is proposed:
Water at the Leixlip Spa site originates from a complex groundwater system combining two
sources: a deeper, older and warmer groundwater system, discharging at the top of the first
terrace through the Spa Well; and a more recent, shallow groundwater that flows through the
karstified limestone bedrock with the main groundwater discharge located in the vicinity of the
‘filtering ponds’. Rainfall runoff also plays a part.
Hydrochemical analysis suggests that groundwater discharging from the warm spring at the
Spa Well is highly mineralised and is the dominant source of water within the site. It is also
evident that warm water discharging from the Spa Well drains northwards and flows into and
through the shallow system, resulting in water being mixed with more recently recharged water.
Groundwater flow in the shallow system also travels through conduits, one of which is evident of
the east side of the face of the terrace bounding the 2nd and 3rd terraces. Due to the steep
topography of the site, shallow depth of subsoil (if any) and karstic nature of bedrock,
groundwater can easily discharge through rock faces and later flow over ground toward the river
Rye Water.
The main source of water for the site is that from the warm spring known as the Spa Well.
This water flows out of the well in northern direction through an underground drain. It is possible
that this drain originally ran in a north westerly direction directly to the Roman Bath. However,
this design was modified before 1981 and the drain was diverted towards the north east direction
towards the two ‘filtering ponds’.
It is possible that the original positioning of the underground drain linking the Spa Well and
the Roman Bath, in a north western direction, was designed to bypass the cold springs (‘filtering
ponds’) to avoid mixing with cooler water discharging from the upper bedrock. However, lack of
any evidence of these cold springs on the 6 inch maps, which are usually very accurate for spring
discharges, suggests that the cold spring could have resulted from digging out the substrate,
which subsequently enhanced seepage and created the fen wetland habitat.
Site layout drawings of the 1980’s (Meagher) suggest hat the drying out of the grass wetland
on the Intel side of the 1 st terrace may be attributed to repositioning of underground drains and
pipes rather than construction of the drain within Intel’s boundary. Further, water quality in the
Intel drain shows different hydrochemical signature than samples collected within the Leixlip Spa
site suggesting that this water originates from surface runoff. Very little or no water in the drain
suggests that this depression does not act as a drainage channel and as such is unlikely to draw
water from the grassland reported to be drying out.
A cold spring is also located at the ‘filtering ponds’. Based on the disperse distribution of
precipitated calcite at the ground surface around the cold spring, it is likely that there are many
groundwater discharge points in this area, however the most pronounced is that at the northern
end of the Lower Pond. Analysis of flow volumes suggested also that some seepage occurs into
the bedrock in the area of the ‘filtering ponds’, reappearing at lower terraces. Seepages and
interflows in the subsoil and upper limestone move throughout the site and mix with other
groundwater sources, cold springs and rainfall to create a variable habitat controlled by
hydrology.
Project No.: 07_136 -13-
Leixlip Spa