<p>Wales has a long history of coal and metal mining, with over 1,300 abandoned metal mines across the country. These mines pollute a number of rivers and lead to the failure of Water Framework Directive (WFD) standards for heavy metals. This includes Nantymwyn, an abandoned lead mine, which has two streams that flow into the River Tywi, and metal loads can be traced downstream for 65km. Nantymwyn, in common with many of Wales’ lead mines, has complex and poorly understood underground workings, which have degraded in the 90 years since closure. The mine is believed to have been worked during pre-Roman times and, is noted historically from AD 1530. The Nant y Bai stream flows through extensive spoil heaps, as well as over old workings, and has inflows from adits, over ground and subterranean inputs. Consequentially, conventional spot sampling of the metal concentrations as has been historically carried out by government agencies does not accurately convey the problems caused by the mine.</p><p>Tracer dilution and synoptic sampling was determined at Nantymwyn in July 2019 to calculate stream flow and heavy metal loadings and their variation downstream. Sodium bromide was injected upstream of the mine site, and once it reached a ‘plateau state’ in the stream it was sampled at 34 points over the 2km of stream. These samples were analysed for metal and bromide concentrations using Inductively coupled plasma mass spectrometry (ICP-MS), which allowed One-Dimensional Transport with Inflow and Storage (OTIS) modelling to predict pollutant flows. Additionally, monthly sampling and salt dilution flow gauging at 12 sites along the stream has been conducted since February 2019. The second, smaller stream at Nantymwyn is monitored monthly at six points.</p><p>Preliminary results show a large temporal variation in flows and concentrations across both streams, with varying loads of lead, but a consistent zinc load in the smaller stream. Initial results from the synoptic sampling show flows consistent with the salt gauging carried out after the stream had been sampled. This research will determine the extent of unseen inflow and outflows upon metal pollution on the River Tywi and allow mitigation strategies to be evaluated.</p>