Accumulation of phosphorus was investigated in the substrata and surface deposits of pilot-scale gravel-bed constructed wetlands after 2 (planted and unplanted) and 5 (planted only) years operation. Four different hydraulic loading rates of farm dairy wastewaters provided mean cumulative total P (TP) loadings ranging from ∼520 to 2000 g m−2 after 5 years operation. Mean substratum TP accumulations after 2 years operation ranged from 52 to 100 g m−2 in the planted wetlands and from 40 to 51g m−2 in the unplanted wetlands, both showing a general increase with wastewater loading. After 5 years operation, associated with declining wetland TP removal performance, substratum TP accumulations in all the planted wetlands converged at similar values of 115 to 128 g m−2. This represented 6 to 23% of their cumulative loadings.

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