Ffynnon Leinw was noted as a tidal well in which the water ebbed and flowed on a regular basis. The name Leinw is supposedly derived from the Welsh Llanw meaning tide. Gerald of Wales, travelling through the area in 1188, wrote
There is a spring not far from Ruthlan, in the province of Tegengel,
which not only regularly ebbs and flows like the sea, twice in
twenty-four hours, but at other times frequently rises and falls
both by night and day. Trogus Pompeius says, “that there is a town
of the Garamantes, where there is a spring which is hot and cold
alternately by day and night.”
Although it in by no means certain that this is Ffynnon Leinw that was being referred to. This ebbing and flowing characteristic had already ceased by the time Thomas Pennant visited the site in the late eighteenth century.
“…on the side of the turnpike-road, not far from Kilken hall, is the noted Ffynnon Leinw, or the flowing well; a large oblong well with a double wall round it. This is taken notice of by Camden for its flux and re-flux; but the singularity has ceased since his time, according to the best information I can receive.” 
Lewis noted in 1849 that:
It is a copious and limpid spring, and is much resorted to for bathing, for which purpose it has been inclosed, and is said to possess properties fully equal, if not superior, to those of the far-famed spring at Holywell. 
In more recent times the water sources appear to have largely stopped. The well basin, built of dressed stone with steps leading down into it, measures about 18 feet by 10 feet and is 2-3 feet deep. The water source is a deep cut towards the east of the well, which appears to have an underground tunnel leading from it. When we visited there was still water at the bottom of the cut, about two to three feet lower than the floor level of the well. The Royal Commission visit in 1910 noted that the water in the well had dried up even at that time, except during periods of significant rainfall.
Ffynnon Leinw is at the side of the A541 between Mold and Denbigh, just through Hendre.
From the 24/7 Roller Shutter factory walk along the road towards Denbigh with a small woodland on the left hand side. Just before the end of the wood the well can be seen on the left hand side about 5 yards from the roadside.
The visit reported above was in June 2011, passing the well again in December 2011 and January 2012 we have noticed that it was full of water.
The water drains out along a stone lined channel in the south western corner of the well, flooding a wide area beside the well.
From this we must assume that the dry periods are seasonal, and that the well is still functioning when the water table rises at certain times of the year.
 Pennant, Thomas (1780) Tours In Wales. Volume 2. Re-printed by H Humphreys Caernarfon, 1883
 Lewis, Samuel(1849) A Topographical Dictionary of Wales. (see references page)