To get an idea of the total nitrate load contribution (kg/day) from waste water in the entire catchment, one of the largest WWTPs in the Thur catchment at Weinfelden (avg. Q = 14,617 m3/day in 2013) is taken into consideration. The total nitrate load from the WWTP is compared to the population it serves (29,480) and extrapolated to the entire population of the urban areas within the whole catchment (1,210,000). Thus the estimated loads from the WWTP considering the entire population of the Thur catchment were 3400 kg/day and 3860 kg/day (due to difference in the amount of effluent released on both the sampling days), respectively in SC1 and SC2. The WWTP contribution to the total nitrate load calculated at the catchment outlet (at T(E)) was 76% in SC1 and 35% in SC2. This is consistent with the earlier finding that during low-flow periods, there is a higher relative contribution of nitrate from the WWTP compared to the high flow season. This is also consistent with the conclusion obtained from Section 4.3 that there is clearly a nitrate isotopic signature of soil nitrogen in the samples from SC2, while samples from SC1 had a distinct waste-water signature.