Retention performance and hydraulic design of constructed wetlands treating runoff waters from arable land

Agriculture is the main source of nitrogen (N) and phosphorus (P), which are the nutrients
accelerating the eutrophication of waters in Finland. Hence, mitigation measures are needed to reduce
the nutrient loading from the arable land. Since Finland’s accession to the EU in 1995 and the
subsequent adaptation to its agri-environmental policy, constructed wetlands (CWs) have been one
of the mitigation measures for which farmers may receive agri-environmental subsidies. The aim of
this study was to find out how efficiently such CWs are able to retain the loading and how they should
be designed and dimensioned in order to optimize their performance. Particular attention was paid to
CW hydrology and hydraulics, since the dynamics of the water flowing through a CW is the major
factor governing retention. Water quality and flow measurements were made in three CWs located in
agricultural watersheds in southern Finland during 1999-2002. Hydraulic properties were examined
in 2 of the CWs by simulations with 2-dimensional hydrodynamic and water quality models.
According to the calculations of material fluxes, the maximum annual retention was 72% for solid
material (TSS), 67% for total P and 40% for total N. The lowest retentions were slightly negative,
because the CW with the smallest CW-to-watershed area ratio (0.5% in the Alastaro CW) sometimes
acted rather as a source than a sink of nutrients. The highest percent retentions were found in the Hovi
CW with the largest CW-to-watershed area ratio (5%). In terms of mass per CW area, the Hovi CW
retained 25 kg of total P and 300 kg of total N per one hectare per one year. In the Hovi CW also
dissolved reactive P retention was high (49% in situ and 34% in laboratory microcosm experiments),
obviously due to high contents and low P saturation of Al and Fe oxides of the CW soil. The basic
underlying reason behind the high retentions of both dissolved nutrients and particulate matter in the
Hovi CW was the long water residence time coupled with high hydraulic efficiency. In the deep part
of the Hovi CW, near-bottom increase of dissolved O2 was found in phase with diurnal temperature
changes. The oxygen transport by this kind of convective circulation of CW water inhibited nearbottom
anoxia and thus decreased the risk of P desorption. According to the hydrodynamic
simulations coupled with simulated tracer tests made for the Hovi CW, a 40% improvement in
hydraulic efficiency was achieved by baffles directing the main flow to optimally exploit the CW
area. The rectangular, elongated shape of the Alastaro CW also showed fairly high hydraulic
efficiency. Hydrodynamic simulations were also coupled with a sediment transport model, which
proved to be a useful method in predicting the change of TSS concentrations in CWs. Hourly datasets
of inflow and outflow revealed high attenuation of runoff peaks in the well-designed and –
dimensioned Hovi CW. The hourly outflow modeled with the reservoir routing method corresponded
to the observed with a reasonable accuracy. When carefully designed, painstakingly implemented and
wisely located, CWs may – even in cold climate – efficiently contribute to agricultural water pollution
control.