This paper analyses time series of nitrate concentration for 35 sites throughout the Frome and Piddle catchments in southern England for the period 1978--2007. The Piddle catchment lies wholly within the Chalk aquifer in its upper reaches, whereas the headwaters of the Frome are a mixture of Chalk and other lithologies; both rivers flow across Palaeogene sands and clays in their lower reaches, before discharging into Poole Harbour. At each site sources of observed variation arise from differences between sample month (seasonality) and year (underlying trend). Considering water year mean concentrations, there is a strong and highly significant positive correlation with year for all but two sites. The annual rate of increase tends to be higher in the Piddle than the Frome, and in both catchments the annual rate of increase tends to decrease downstream; both these effects relate to the degree of groundwater influence. In addition to the strong upward trend, there is also a clear seasonal cycle in nitrate concentration at most sites; amplitude generally increases downstream. Laying aside the assumption of linearity, solute breakthrough curves (S-curves) are fitted to 34 of the 36 nitrate time series. Results show that application of S-curves is an appropriate description of changes in annual mean concentrations over time and provides improvements over linear models in most cases. The interpretation of rising nitrate concentrations in these groundwater-dominated catchments moves from a statistical to a hydrological process base, providing valuable insight into lowland catchment function.