Can provide a bit of older background.
As well as the highly visible iron pollution, there are considerable inputs of zinc and acidity, plus lesser amounts of lead and cadmium (the last being unusually high for this part of the world). The site is one of the three largest polluting mines in the county.
The filter beds inside the chain link fenced compound between the road and the river were installed when the hydro-electric power station was built a couple of miles down the valley (around 1960? I don't have the date to hand). The rartionale being that because the power station abstracted naturally acidic upland water from the headwaters of the river (from the Nant y Moch Reservoir) and returned it to the river in a locatio where the river water was less acidic, some attempts at offsetting this input of acid water had to be made. As the mine site was imputting a grest deal of acidity just upstream of the power station, it was an obvious candidate for an alleviation scheme. Apparently the filter beds worked tolerably well until the late 1960s when the then occupier of the mine site attempted to reopen Level 9 (the one in the photos earlier on this thread), apparently to either extract ochre or to commence prospecting work (I'm hazy on this aspect). Disturbance of the natural ochre dam in the Level resulted in a 'blow-out' of a large volume of polluted water that overwhelmed the filter beds with ochre. A movement of ochre and/or deads inside the workings very promtly cut off the flow but the damage was done. No official body (at that time, presumably the Central Electricity Generating Board, the Welsh National Water Development Authority, and Cardiganshire County Council) was eager to rejuvenate the filter beds, so they remained untouched for many years (until about 2000, I believe).
In the mid 1980s, to stop the outflows from Level 6 (the adit at the head of the tips) and Level 9 (only Levels 6 and 9 came to surface) from percolating through the tips and leaching out pollutants, their flows were piped down the western edge of the tips to flow into the filter beds. Also in the mid 1980s, there were some works to limit streams and surface water sinking into the back of the lode in the vicinity of Ystumtuen village, located beyond the top of the hillside. The pipes from Levels 6 and 9, I believe, became choked with rocks and ochre and in the early/mid 1990s the project described earlier on this thread by Rheidol38 was carried out to replace the pipes and to carry out works to the entranxce to Level 9.
I recall being told that the pH of the water in Level 9 is around 3 - i.e. similar to battery acid. The level is stated to have oxygen deficiency to a markedly dangerous degree. Plans assocaited with the late 1960s attmept at reopening show it blocked at the lode. This blockage holds back around 150 feet head of ochre and water - most of the outflow of the mine is from Level 6, around 150 feet higher up the hillside.
Although the Cwmrheidol Levels have a hydrological connection with the Ystumtuen, Penrhiw, Bwlchgwyn and Llwynteifi Mines, Temple Mine appears to drain entirely independently.