The influence of envelope material on sedimentation and water quality in subsurface drainage

Sammanfattning

A well-functioning drainage system is an essential component of crop production. With climate change bringing increased precipitation, higher demands are placed on our drainage systems. Throughout the past decades, little research has been conducted on drainage materials, and our most common drain envelope, gravel, has become expensive. However, new materials are now available. These new materials must be examined to improve the functionality of our drainage systems in agriculture. The overall aim has been to use measured data in a field trial to investigate how various drainage envelope materials in clay soils affect subsurface drainage, sediment accumulation, subsurface runoff, turbidity and nitrogen and phosphorus concentrations. Furthermore, it sought to provide insight into whether using an envelope can help to reduce sediment accumulation and nutrient leaching compared to drainage pipes without an envelope. The field trial, performed in central Sweden during the hydrological years 2023-2025, examined treatments with no envelope, synthetic envelope and crushed rock envelope. During an excavation, 40 cm of the pipe was dug up. In general, a small amount of sediment was found in all pipes. Most sediment was found in the treatment without an envelope (5.94 g per 40 cm pipe), while the synthetic envelope had the significantly lowest amount (0.89 g). Pipes with crushed rock envelopes had the largest number of particles in the size range of 0.06-0.2 mm (21.3%), which pose the highest risk of clogging the pipes. The results showed that the use of an envelope decreases the sediment accumulation (p = 0.0009) and the concentration of nitrogen (p < 0.02). In the first hydrological year, the average nitrogen concentration was reduced from 10.91 mg/l (no envelope) to 7.86 mg/l (synthetic envelope). The result for phosphorus concentrations was not significant. The findings showed no effect on turbidity when using an envelope. To verify these results, a follow-up study examining sedimentation in the pipes and nitrogen and phosphorus concentrations after about a decade is recommended to determine if the long-term results of sediment accumulation and nutrient concentrations in the pipes are consistent. Further research should also explore whether nitrogen and phosphorus concentrations decrease with the use of an envelope on soils other than clay. The decrease of nutrient concentrations with an envelope is positive for the future climate with increases in precipitation. The use of an envelope could therefore be a strategy to reduce nitrogen leaching in drainage, but more research is needed to confirm these findings. Keywords: envelope, filter, drainage, nutrient leakage, turbidity A well-functioning drainage system is an essential component of crop production. With climate change bringing increased precipitation, higher demands are placed on our drainage systems. Throughout the past decades, little research has been conducted on drainage materials, and our most common drain envelope, gravel, has become expensive. However, new materials are now available. These new materials must be examined to improve the functionality of our drainage systems in agriculture. The overall aim has been to use measured data in a field trial to investigate how various drainage envelope materials in clay soils affect subsurface drainage, sediment accumulation, subsurface runoff, turbidity and nitrogen and phosphorus concentrations. Furthermore, it sought to provide insight into whether using an envelope can help to reduce sediment accumulation and nutrient leaching compared to drainage pipes without an envelope. The field trial, performed in central Sweden during the hydrological years 2023-2025, examined treatments with no envelope, synthetic envelope and crushed rock envelope. During an excavation, 40 cm of the pipe was dug up. In general, a small amount of sediment was found in all pipes. Most sediment was found in the treatment without an envelope (5.94 g per 40 cm pipe), while the synthetic envelope had the significantly lowest amount (0.89 g). Pipes with crushed rock envelopes had the largest number of particles in the size range of 0.06-0.2 mm (21.3%), which pose the highest risk of clogging the pipes. The results showed that the use of an envelope decreases the sediment accumulation (p = 0.0009) and the concentration of nitrogen (p < 0.02). In the first hydrological year, the average nitrogen concentration was reduced from 10.91 mg/l (no envelope) to 7.86 mg/l (synthetic envelope). The result for phosphorus concentrations was not significant. The findings showed no effect on turbidity when using an envelope. To verify these results, a follow-up study examining sedimentation in the pipes and nitrogen and phosphorus concentrations after about a decade is recommended to determine if the long-term results of sediment accumulation and nutrient concentrations in the pipes are consistent. Further research should also explore whether nitrogen and phosphorus concentrations decrease with the use of an envelope on soils other than clay. The decrease of nutrient concentrations with an envelope is positive for the future climate with increases in precipitation. The use of an envelope could therefore be a strategy to reduce nitrogen leaching in drainage, but more research is needed to confirm these findings.