It has been noted that lead concentrations continued to fall in the first few years following implementation of treatment . Continued monitoring has now revealed that compliance has continued to steadily improve over the last decade since delivering the optimised solution despite phosphate concentrations remaining static (Figure ). Indeed, lead non-compliance has fallen from 3.0 % in 2004 to 0.3 % in 2016 which represents a further 90 % reduction in lead non-compliance since 2004. Improvements are also seen at much higher lead concentrations where lead concentrations are determined by particulate lead. For example, the percentage of samples exceeding 100µg/l has fallen from 0.14% in 2005 to 0.017 % in 2015, which represents a 90% reduction in non-compliance. These results indicate that the structure of the corrosion layer has been steadily evolving over the decade since optimised treatment was delivered.