Field drains, septic tanks and streams

Our soil research (SOWAP and MOPS) is improving our understanding of the influence on surface runoff and the movement of soil and nutrients within fields, with implications for watercourses adjoining them. 

These projects also provide a guide for appropriate management to reduce this problem.  However, field drains also carry sediment and associated phosphorus from the field directly to ditches and streams and run almost continuously during the winter. 

Our monitoring reveals that the phosphorus concentration in field drain water exceeds that likely to have an ecological impact on 60 - 100% of sampling occasions.  An MSc project carried out at Loddington showed that sediment concentrations were six times higher following heavy rain than under base flow conditions.  Further sampling carried out within the PARIS project reveals similar levels of sediment and nutrients in our field drain water.  Comparison of field drain water from split fields with half minimum tillage and half plough suggests that minimum tillage may be one in-field management practice that could reduce levels of sediment and phosphorus. 

Our research on detention ponds at the field edge demonstrates that this can be another way of addressing this issue, while also minimising the impact on the cropped area.

Agriculture is not the only source of phosphorus in rural streams.  Our monitoring suggests that phosphorus concentrations in the water of small tributaries can be five times higher downstream than upstream of septic tanks.  Such concentrations are three times higher than those known (from our PARIS research) to cause an ecological impact. 

Given the widespread distribution of septic tanks in rural areas, this finding has wide implications for the chemical and ecological improvement of watercourses.