Nutrient fluxes regulation in an in-stream constructed wetland treating polluted agricultural runoff

Abstract

Ühiskonna suurenenud toiduvajadus on suurendanud põllumaade osakaalu ning väetiste kasutamist. Intensiivne või halvasti ajastatud väetamine mõjutab pinnavett ja suurendab veekogude eutrofeerumist. Põllumajandusliku hajukoormuse leevendamiseks ja vee kvaliteedi parandamiseks on tõhus meede tehismärgalade rajamine. Need aitavad parandada vee kvaliteeti looduslike protsesside abil ning pakuvad muidki ökosüsteemi hüvesid (suurendavad bioloogilist mitmekesisust, puhverdavad sademevett ja toimivad veereservuaarina). Doktoritöö uuris vahetult voolusängi rajatud Vända avaveelise tehismärgalargala toimet: toitainete vähenemist ja kasvuhoonegaaside (KHG) heitkoguseid mõjutavaid parameetreid. Varasemast on teada, et tehismärgalades sõltuvad toitainete eemaldamise tõhusus ja KHG heitkogused vee parameetritest ja märgalade kujundusest. Vända märgala vähendas tõhusalt fosforit, millel oli selge hooajaline dünaamika. Tõhusamalt eemaldas fosforit soojal perioodil ja külmal perioodil oli tõhusus väiksem. Fosfori eemaldamise efektiivsus oli suurem väikese vooluhulga korral, sest siis aeglane vool soodustas settimis- ja filtreerimisprotsesse. Lämmastiku eemaldamine oli ootustele vastupidine, kuna märgalas suurenes lämmastiku hulk väljavoolu suunas. Lämmastiku lisandumine toimus tõenäoliselt nitraadirikka põhjavee sissevoolu tõttu. Fosfori puhastusefektiivsus sõltus ka taimkattest – mida rohkem taimi, seda tõhusam. Taimestik levis märalal aastate jooksul ning see soodustas fosfori eemaldamist. Metaani (CH4) ja dilämmastikoksiidi (N2O) puhul ilmnes sesoonne dünaamika – soojal perioodil oli lendumine suurem ja külmal perioodil väiksem. N2O heitkogused sõltusid vee sügavusest, voolukiirusest ja vee temperatuurist. Suvel kui mikroobne aktiivsus on suur, täheldati oluliselt suuremaid N2O emissioone.

Water quality has degraded due to increased proportion of agricultural lands in some regions, if more fertilizer is used. Intensive or poorly timed fertilization affects surface waters and increases eutrophication in water bodies. Diffuse agricultural pollution is difficult to locate and control due to its diffuse nature, climate conditions and strong anthropogenic influence. In order to maintain or improve surface water quality, it is important to find and use effective measures. One of the best measures for improving water quality is using constructed wetlands. Constructed wetlands help to improve water quality through natural treatment processes, among several other benefits. They increase biodiversity, buffer storm water and act as a water reservoir. The aim of this thesis was to investigate the efficiency of in-stream free water surface flow Vända constructed wetland and study the parameters affecting nutrient reduction and greenhouse gas (GHG) emissions. Long-term monitoring results show that in in-stream CWs, nutrient removal efficiency and GHG emissions are highly dependent on water parameters and wetland design. Vända CW effectively reduced phosphorus that had clear seasonal dynamics. Higher removal efficiency occurred during the warm period and lower values were seen during the cold period. Higher removal efficiency values for phosphorus were seen when the flow rate was lower. With a lower flow rate, sedimentation and filtration processes are favoured. The results for nitrogen removal were contrary to expectations because CW acted more as a source of nitrogen. The addition of nitrogen was suspected to come from groundwater that was contaminated with nitrate. Phosphorus removal had a strong positive correlation with vegetation coverage. Through years’ vegetation spread over the wetland and that promoted phosphorus removal processes. For methane (CH4) and nitrous oxide (N2O), clear seasonal dynamics were seen, with higher emissions during the warm period and lower emissions during the cold period. N2O emissions were strongly related to water depth, flow rate and water temperature. During summer, with high microbial activity, significantly higher N2O emissions were seen.