This report describes the implementation and validation of the water quality module in the bioeconomic optimisation model Flame. The module focuses on the calculation of groundwater nitrate concentration in relation to agricultural practices on individual dairy farms on sandy soil. Within the framework of the Minerals Policy Monitoring Programme (LMM), the National Institute for Public Health and the Environment (RIVM) carries out measurements on agricultural farms. The measured nitrate concentration of 55 dairy farms has been compared with FLAME’s calculated nitrate concentration. The average farm-level concentration calculated by FLAME is higher than the measured average concentration. The difference is 2.8 mg/L, which is a deviation of 9%. Averaged over the 55 dairy farms, this difference is not significant. Based on this and on a reliability analysis of the population average, it is concluded that the water quality module is well suited for operational evaluations of the manure policy for groups of farms.