Mitigating particle and nutrient losses via subsurface agricultural drainage using lightweight aggregates
Introduction
Transport of soil particles and dissolved nutrients from agricultural fields to surface water is one of the most important factors affecting the water quality in Scandinavian water systems (Gustavsson et al., 1992; Rekolainen et al., 1996). Other studies are also showing that agriculture is a major contributor to the load of nutrients in rivers and other water bodies in industrialised countries (Carpenter et al., 1998; Fisher et al., 2000; Lenat and Crawford, 1994; Ulén and Folster, 2007). An overenrichment of nutrients in a water body leads to eutrophication. Phosphorus is an essential input in all bio-productions and is generally the limiting element for biological growth in fresh water systems (Shindler, 1977; Sharpley et al., 1994; Sundareshwar et al., 2003). In addition to increased growth of aquatic weeds and algae that, when decomposed, can consume most of the available oxygen in the water, leading to suffocation of e.g. fish, eutrophication can also lead to production of toxic compounds (e.g. H2S) and growth of toxic algae (e.g. Cyanobacteria) that kills other life in the water (Carpenter et al., 1969; Gray, 1992; Kotak et al., 1993). Eutrophication produces an undesirable water quality and makes the water unsuitable for use in industry and agriculture and for other humane use including recreation and consumption.
Climate change is expected to lead to increased annual precipitation. It is also expected an increase in the number of rain incidents with high intensity in Scandinavia and other northern European countries (Alfnes and Førlan, 2006; Hanssen-Bauer et al., 2017; IPCC, 2014). Higher average temperature may lead to an increased number of freezing and thawing cycles during winter. These changes may cause increased runoff and erosion, causing increased loss of soil and Phosphorus. These changes may require further measures to reduce these losses.
Mitigation efforts to reduce loss of particles and nutrients from agriculture in Norway and other countries have mainly focused on surface runoff. The most common methods have been reduced tillage, vegetation filter strips, grass covered overland flow courses for water and detention ponds (eg. Schoumans et al., 2014). Reduced use of phosphorus fertiliser have also been recommended. However, despite these efforts, the national Norwegian environmental monitoring show no clear improvement of water quality in agricultural catchments over the last years (Beckmann et al., 2017). Despite similar efforts within the EU the situation is very much the same regarding water quality (EEA, 2017)
In Norway approximately 60% (about 600,000 ha) of the agricultural land is artificially drained (Bjerkholt, 2002). In North-Western European countries typically between 30 and 90% of the agricultural land is artificially drained (Brown and van Beinum, 2009). Several Norwegian studies have shown that considerable amounts of particles, Phosphorous and Nitrogen are lost through subsurface drainage systems (Kværner, 1991; Lundekvam, 1993; Øygarden et al., 1996, 1997). Studies in other countries has also shown the significance of subsurface drains as conveyers of sediments and nutrients from the field to the watercourse (Chapman et al., 2001, 2005; Foster et al., 2003; Grant et al., 1996; Hooda et al., 1999, Kronvang1997; Ulén and Persson, 1999; Wesström et al., 2001). Losses via subsurface drainpipes might even be higher than those from surface runoff (Øygarden et al., 1997). Kværnø and Bechmann (2010) summarised results for long-term field experiments in Norway and reported that 50–90 % of the total runoff from a field is directed through the drainpipes. The loss of SS through the drainpipes was in the range of 30–4000 kg/ha/yr, representing 5–95 % of the total loss. The loss of Phosphorus was in the range of 0.2–5 kg/ha/yr representing 10–90 % of the total loss. The loss of Nitrogen through the drainpipes was in the range of 17–48 kg/ha/yr representing 70–90 % of the total loss of Nitrogen. All losses depending on soil type, land levelling, type of crop, precipitation intensity, management practice and other factors. The increasing awareness of the importance of nutrient loss in drainage outflow has brought the need to reduce nutrient via drainage into focus. In Scandinavia Ulén and Folster, (2007) and Ulén et al., (2010) has shown that there is a potential for reducing P-losses from agricultural fields by focusing more on the function and design of the artificial drainage system, especially for clay and silt soils with a risk of high P-loss, and in recent experiments use of a ash material as drain backfill (McDowell et al., 2008) and Fe-coated sand enveloping pipe drain (Groenenberg et al., 2013) have been tested as a mean of reducing P-loss from drainage water. Based on the idea that new types of agricultural drainage filters might reduce P-losses from agricultural tiles, several recent laboratory studies have looked at hydrophysical and P-retention processes and properties of potential new filter materials including Leca® (e.g. Canga et al., 2016a, 2016b). Replacing the traditional drainpipe and filter system with Leca as a conveying and filtering material has been given little attention.
Previous laboratory experiments have shown that Lightweight Aggregates (LWA), Leca®, a lightweight expanded clay aggregate with a silicate matrix produced from pure clay, have a good ability to remove Phosphorus from water and also perform well as a filtering material, holding back fine soil particles (Adam et al., 2007; Jenssen et al., 1991). The laboratory results also indicated an acceptable hydraulic conductivity for drainage purpose. Based on the results from the laboratory tests and the need to reduce the losses of particles and nutrients through agricultural drainage systems, a full-scale field experimental project was initiated to assess the effects on drainage water quality and hydrological performance of drainage systems based on LWA in comparison to traditional drainpipes systems.
Section snippets
General description of the research drainage field
A field experiment was established in 1991 at Grimsrud farm in Souteastern Norway, 43 km SE of Oslo (latitude 59°36ʹ50ʺ N, longitude 11°59ʹ56ʺ E). The criteria for selection of the site were i) the soil and climate should be typical for the region and ii) previous artificially drains that might influence drainage outflow should not occur.
Annual mean temperature in the area is 5.7 °C (Table 1). Average annual precipitation is 829 mm (Table 2). Average potential evapotranspiration in different
Results and discussion
Contents of suspended solids (SS), Total-P, PO4-P, Total-N, NO3-N, NH4-N, pH in water samples from the different drain types are shown in Table 3, Table 4. The components are discussed separately in the following. Due to traffic by heavy machinery close to the instrumental hut in the beginning of February 2000, the collector pipe for LWA drain type A was damaged. This caused water to enter the system directly without filtering, causing high values of SS and Total-P.
Conclusions
In this experiment the annual drainage volume for the LWA drains were about 75% compared to pipe drain with an envelope of wood shavings. In addition to conveying the water the LWA drains also acted as a treatment system for the drainage water, removing nutrients and particles and increasing the pH of the drainage water between 1–1.5 units. The measurements in 1999 and 2000 showed that LWA drains removed between 40 and 90% of the Phosphorus in drainage water compared to standard pipe drains.
The
Declaration of interest
None.
Acknowledgments
The authors gratefully acknowledge the technical assistance of colleagues at the Norwegian University of Life Sciences and at the Norwegian Institute of Bioeconomy Research (NIBIO).The authors also want to extend sincere thanks to farmer Mr Øistein Johansen for hosting the experiments. Thanks also go to the Civil Engineering students (at the time) Mr Gunnar R. Lindseth and Mr Sven Morten Klungland for running the experiments during 1999/2000. This work was funded by: The Norwegian Ministry of
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