The effectiveness of sediment and phosphorus removal by a small constructed wetland in Norway: 18 years of monitoring and perspectives for the future

Highlights

• Small constructed wetlands (0.05% of catchment area) capture significant amounts of TSS and TP.

• Conditions in the catchment are most important for CW performance.

• Constructed wetland relative removal efficiency is highest in summer, with low runoff and limited erosion.

• The highest loads are captured in autumn (for TSS) and spring (for TP)

• Constructed wetlands are a good supplement to best management practices in erosion-prone catchments.

Abstract

Constructed wetlands (CWs) are a widely recognised measure for reducing pollution loads and improving the quality of surface waters. The removal efficiency of CWs varies considerably depending on system type and design as well as residence time, hydraulic load, particles and nutrient loading rates. Therefore, there is a need to closely monitor the efficiency of existing measures, look at their efficiency in practice and be able to foresee potential implications for their efficiency in light of climate change and land management intensification.

This study presents 18 years of data from a typical Norwegian small CW established in the Skuterud catchment. The main objective of this study was to look at the impact of hydraulic load, particles and nutrient loads (depending on climatic factors such as temperature and precipitation) on CW effectiveness. The results showed an average of 39 % and 22 % annual removal efficiency for sediment and phosphorus, respectively. It appears that good CW effectiveness coincides with a combination of high sediment or phosphorus loads to the CW and a stable runoff of low to moderate intensity.

At the seasonal level, the highest sediment and phosphorus removal efficiency is observed in the summer seasons (47% for sediment and 29% for phosphorus), when the sediment and phosphorus loads and runoff are at their lowest, and the lowest in autumn (23% for sediment) and in winter (4% for phosphorus). The relationship between removal efficiency and loads to the CW is not that straightforward, as other seasonal differences, such as erosion patterns, vegetation development, also become important.

The conclusion based on the results presented is that establishing CWs can be a good supplement to best management practice in erosion-prone catchments with sensitive recipients.

Introduction

Constructed wetlands (CWs) are a widely recognised measure for reducing pollution loads and improving the water quality of rivers and streams. The removal efficiency of CWs varies considerably depending on the system type and design, as well as current conditions in the catchment, including flow characteristics, residence time, particles and nutrient loading rates and vegetation stage (EPA 1995).

In Norway, free water surface CWs, with emergent vegetation treating agricultural runoff, have been used since the early 1990 s as an important supplement to best management practices for water quality improvements (Direktoratsgruppa, 2009). In 2015, there were already more than a thousand CWs established in Norway and that number is constantly growing (Blankenberg et al., 2015). Many studies have shown that CWs are effective also in cold climate regions (e.g., Koskiaho et al., 2003, Braskerud, 2001, Braskerud et al., 2005, Carstensen et al., 2023, Hauge et al., 2008, Rozema et al., 2016). However, climate change and a constantly growing need for intensification of agriculture may influence hydrological catchment responses like runoff patterns, and potentially increase erosion and nutrient and sediment losses. Consequently, the need for mitigation measures in the agricultural landscape, such as CWs, will continue to increase (Øygarden et al., 2011, Deelstra et al., 2011, Blankenberg et al., 2013).

Runoff and diffuse pollution from agricultural areas are the main sources of phosphorus and sediment input to Norwegian surface waters (Selvik et al., 2006, Øgaard et al., 2016). Therefore, Norwegian CWs are mainly designed to remove suspended sediment and phosphorus through sedimentation and filtration, and, to a lesser extent, plant uptake (e.g., Kadlec, 2005). A typical CW in Norway consists of a deeper sedimentation chamber (1.0–2.0 m deep), followed by one or more shallow vegetation filters (0.1–1.0 m deep), often divided by thresholds (e.g., dams, stones, baffles). Due to the typical small-scale Norwegian agricultural fields and the topography, CWs often comprise less than 0.1 % of the catchment area and are usually constructed by expanding the width of natural streams (e.g., Braskerud, 2002a, Braskerud and Hauge, 2008). In this way, they receive the total runoff from the catchment, which includes surface runoff and drainage discharge. In many cases, the ditch runoff is at least as important for the total runoff as the surface runoff, and it can also be an important contributor in sediment and phosphorus transport to the watercourses.

Factors that influence sediment and phosphorus removal efficiency in free water surface CWs are sediment and phosphorus loads at the inlet to the CW, runoff characteristics, residence time and the age of the CW (e.g., Braskerud, 2001, Kadlec, 2005, Mustafa et al., 2009). The loads of sediment and nutrients from agriculture fields to surface waters, and consequently inputs to the CW, depend strongly on weather factors (i.e., temperature and precipitation) and source factors (i.e., crop management). According to nationally scaled climate scenarios, the annual average temperature in Norway will increase by 2.7 °C from the period 1971–2000 to the period 2071–2100, and the increase will be more prominent during winter and less during summer (Hanssen-Bauer et al., 2017). Increased annual precipitation and more frequent episodes of heavy precipitation are also expected (Øygarden et al., 2011, Deelstra et al., 2011, Blankenberg et al., 2013). Additionally, increased agricultural land-use intensification leads to increased and intensified runoff, erosion, and loss of nutrients and other pollutants from agricultural areas to watercourses and downstream rivers, where flooding might occur.

There is therefore a need to closely monitor the efficiency of existing CWs, look at CW efficiency in practice and be able to foresee potential implications for CW efficiency in light of climate change and land-use intensification. Such monitoring should include adequate monitoring of hydrological processes, potentially accounting for the effects of seasonality (especially for CWs used to treat agricultural and urban runoff; Land et al., 2016), periods of extreme weather conditions or other extraordinary circumstances. However, comprehensive, high-temporal resolution monitoring of CW efficiency is often a challenge and, consequently, the long-term performance of CWs is poorly investigated in the field (Johannesson et al., 2011, Land et al., 2016). As regards small CWs, with loading rates driven by rainfall and snowmelt, it is necessary to focus on the relationship between the current condition in the catchment and the retention performance of the CW (Braskerud, 2001, Land et al., 2016).

This study presents the results of 18 years of measurements from a typical small Norwegian agricultural catchment and a CW established at the catchment outlet (the Skuterud CW). The evaluation is based on analyses of water samples taken at the inlet and outlet of the Skuterud CW. The objective of this study is to look at both the long-term and seasonal variation in sediment and phosphorus removal efficiency in the Skuterud CW and consider this in light of hydrological conditions in the catchment.