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The Impact of Hydraulic Load and Aggregation on Sedimentation of Soil Particles in Small Constructed Wetlands

JORDFORSK (Norwegian Center for Soil and Environmental Research), N-1432 Aas, Norway;

H. Lundekvam and T. Krogstad

Dep. Soil and Water Sci., Agric. Univ. Norway, N-1432 Aas, Norway.

^* Corresponding author (bent.braskerud{at}jordforsk.no).

ABSTRACT

Loss of soil particles from arable land to streams and lakes negatively affects water quality. When initiatives to mitigate soil erosion are insufficient or fail, constructed wetlands (CWs) could be a last buffer to mitigate pollution. The objectives in this study were to (i) determine the influence of aggregation on clay sedimentation in CWs and (ii) evaluate the prediction performance of two commonly used retention models, based on hydraulic load and particle sedimentation velocity. Retention was measured three ways, with (i) water flow proportional sampling systems in the inlet and in the outlet, (ii) sedimentation traps, and (iii) sedimentation plates. Surface area of the CWs was 0.03 to 0.07% of the watershed, which consisted of silty clay loam (18–33% clay). Some runoff episodes, usually at high runoff rates, accounted for a relatively high proportion of total sedimentation. Thus 80% of the particles were retained from less than 44% of the total runoff. Constructed wetland performance increased with increased hydraulic load or decreased detention time. The clay content in the CW sediment reflected the clay content in the arable soil. Actual CW sediment exceeded model estimates 2.5 to 8.2 times, depending on CW size and runoff. The probable reason for the prediction error is clay particles entering the CWs as aggregates. Constructed wetlands should be located in small streams to avoid break up of aggregates and a reduction in retention efficiency.

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