Phosphorus forms in urban and agricultural runoff: Implications for management of Danish Lake Nordborg

Abstract

The catchment of eutrophic Lake Nordborg, Denmark, consists of 25% urban areas with separate sewer systems and 63% agricultural areas with clayey soil and grain crops. Diffuse runoff is the only external phosphorus (P) source (∼ 550 kg/yr), but the lake suffers from high internal loading (∼ 1300 kg/yr). In-lake aluminum treatment is suggested if external P-loading can be brought down by ∼ 30% to an annual average inlet concentration of < 0.100 mg/L. We examined the share and bioavailability of particulate P (PP) in 14 tributaries during a winter season to evaluate if the reduction in bioavailable P-load can be obtained by construction of precipitation/retention ponds in major inlets. One-third of the P-load occurred as PP, independent of catchment type. For urban runoff, 62% of PP was surface adsorbed P, iron-bound P, and extractable organic P, all of which may be considered bioavailable. The corresponding value for agricultural tributaries was 76%. For both types of catchments more than 70% of total P (PP and dissolved P) was bioavailable, but total concentrations were much higher (0.174 ± 0.032 mg/L) in agricultural runoff than in urban runoff (0.082 ± 0.019 mg/L). Measurements of PP settling velocity revealed that ∼ 30% settled slower than 1 cm/h whereas ∼ 50% settled faster than 1 m/h. Therefore, water retention time in precipitation ponds to Lake Nordborg should exceed 8 h to reach the target reduction in P-load. Focus should be on the agricultural catchment that contributes 140 kg to PP load, rather than the urban catchment that only contributes 48 kg.