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Constructed wetlands as a treatment system are widely explored in different climate conditions and established to be effective in pollution removal from water environment. This study aims to demonstrate the performance of pilot-scale subsurface flow constructed wetland for storm water treatment in Latvia. The catchment basin was located in a farmyard of agricultural area and storm water was collected from the impermeable pavements. Storm water was accumulated in an open pond and periodically pumped above the filter part of the subsurface flow constructed wetland. Grab samples were collected once or twice per month at the inlet and outlet of the treatment system during a period of 73 months from year 2014 to 2020. Water quality parameters as nitrate nitrogen (NO3-N), ammonium nitrogen (NH4–N), total nitrogen (TN), orthophosphate phosphorus (PO4-P), and total phosphorus (TP), total suspended solids (TSS), biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) were monitored. Water level at the inlet structure was automatically measured and flow rate was calculated based on the Manning equation for partially filled circular pipes. Results showed the reduction of average concentrations for all parameters during the study period. However, in some sampling cases concentrations increased at the outlet of the treatment system and can be explained by influencing factors of farming and maintenance. The treatment efficiency of NO3-N, NH4-N, TN, PO4-P, TP, TSS, BOD5 and COD concentrations was 17 %, 68 %, 55 %, 78 %, 80 %, 57 %, 80 % and 74 %, respectively. The study site demonstrated a potential to improve water quality in the long term.

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Artificial subsurface drainage systems are a common water management practice to remove excess water from agricultural land in the Baltic and Nordic countries. Unfortunately, these systems also serve as nutrient transport pathways from agricultural fields to downstream waterbodies. The objective of this study was to determine the status of subsurface drainage water in the context of implementation of the European Union (EU) Water Framework Directive (WFD) and EU Nitrate Directive (ND), to investigate the impact of hydrology and mineral fertilizer application on nutrient losses through subsurface drainage systems and to evaluate the possible effects of climate change on drainage and nutrient loss. This research was conducted from 1995 through 2013 at the agricultural run-off–monitoring sites in Berze and Mellupite. Continuous-flow measurement data and composite water samples were collected on a monthly basis for assessment of total nitrogen (TN), nitrate–nitrogen (NO3–N) and total phosphorus (TP) in drainage water. Results from the study indicated that the mean annual NO3–N concentration was slightly below the threshold level defined by ND, while TN concentrations exceeded the threshold level for good chemical status of water according to WFD. Differences in TN losses were mainly driven by the hydrologic behaviour at the study sites rather than by mineral fertilizer application rates, whereas TP losses under similar application rates were affected by open inlets for surface water accumulation. The comparison of meteorological data for the research period (1995–2013) with the long-term mean (1950–1994) showed a pronounced evidence of climate change. The mean annual precipitation amount has increased at both Berze and Mellupite monitoring sites by 36 mm and 42 mm, respectively, compared to the long-term mean for 1950–1994. Meanwhile, the mean annual temperature during the research period was 7.3°C for Berze and 6.4°C for Mellupite, which is 1.4°C and 0.6°C warmer than long-term mean. [ABSTRACT FROM AUTHOR]

With population growth, demand for agricultural products has increased, which affects agriculture, fertiliser use and land management. Due to nutrient inputs from arable areas, the water quality of rivers and lakes near agricultural lands has deteriorated. Constructed wetlands (CWs) are one of the measures used to improve water quality through natural water treatment processes. We studied phosphorus removal in relation to environmental characteristics in five in-stream free surface flow CWs in Finland, Estonia and Latvia. Wetland/catchment area ratios varied from 0.1% to 5%. Three CWs were covered in dense vegetation. On average, the Vända CW and Hovi CW TP removal efficiency throughout the study period were highest (32.1 ± 3.6% and 34.9 ± 4.4%, respectively) among all studied CWs. Nummela Gateway CW, Rantamo-Seitteli CW and Mezaciruli CW showed lower TP removal efficiency and on many occasions the outflow concentration exceeded the inflow concentration in Rantamo-Seitteli and Mezaciruli CW. Vegetation cover emerged as the most important factor in the treatment efficiency of in-stream CWs, reducing flow rate, increasing water retention time and decreasing probability of the CW becoming a source of phosphorus during flood events. • Flow rate controls phosphorus removal efficiency in in-stream constructed wetlands. • High flow values often resulted in significant reduction in phosphorus removal. • Wetlands with dense vegetation cover had higher and more stable phosphorus removal efficiency. • Shallow meandering wetlands benefit vegetation spread, water retention time, and phosphorus removal. [ABSTRACT FROM AUTHOR]