Performance of Saturated Riparian Buffers in Iowa, USA

Nitrate from artificial drainage pipes (tiles) underlying agricultural fields is a major source of reactive N, especially NO₃, in surface waters. A novel approach for reducing NO₃ loss is to intercept a field tile where it crosses a riparian buffer and divert a fraction of the flow as shallow groundwater within the buffer. This practice is called a saturated riparian buffer (SRB), and although it is promising, little data on the performance of the practice is available. This research investigated the effectiveness of SRBs in removing NO₃ at six sites installed across Iowa, resulting in a total of 17 site‐years. Water flow and NO₃ in the tile outlets, diverted into the buffers, and NO₃ concentration changes within the buffers were monitored throughout the year at each site. Results showed that all the SRBs were effective in removing NO₃ from the tile outlet, with the average annual NO₃ load removal ranging from 13 to 179 kg N for drainage areas ranging from 3.4 to 40.5 ha. This is NO₃ that would have otherwise discharged directly into the adjoining streams. The annual removal effectiveness, which is the total NO₃ removed in the SRB divided by the total NO₃ draining from the field, ranged from 8 to 84%. This corresponds to an average removal rate of 0.040 g N m⁻³ d⁻¹ with a range of 0.004 to 0.164 g N m⁻³ d⁻¹. Assuming a 40‐yr life expectancy for the structure and a 4% discount rate, we computed a mean equal annual cost for SRBs of US$213.83. Given the average annual removal of 73 kg for all site‐years, this cost equates to $2.94 kg⁻¹ N removed, which is very competitive with other field‐edge practices such as denitrification bioreactors and constructed wetlands. Thus, SRBs continue to be a promising practice for NO₃ removal in tile‐drained landscapes. CORE IDEAS: Saturated riparian buffers (SRBs) removed an average of 73 kg N yr⁻¹ of NO₃ from tile water. A buffer with established perennial vegetation is important for optimum NO₃ removal. The percentage of NO₃ removed to NO₃ delivered to a SRB ranged from 8 to 84%. The cost of N removal for this practice is similar to other field‐edge practices.