Saturated buffer design flow and performance in Illinois

There are few peer-reviewed studies documenting saturated buffer annual nitrate (NO₃) removal or that have assessed the federal practice standard design criteria. Drainage flow, NO₃, and dissolved reactive phosphorus (DRP) were monitored at three saturated buffers in Illinois, USA, for a combined 10 site-years. Nitrate loss reduction averaged 48 ± 19% with removals of 3.5–25.2 kg NO₃–N ha⁻¹ annually. Median DRP concentrations at all sampling locations were at the analytical detection limit of 0.01 mg L⁻¹. The current design paradigm (i.e., USDA practice standard) prescribes there should be no flow bypassing the saturated buffer at flow rates that are ≤5% of the peak drainage system flow rate. The drainage coefficient–based and Manning's equation-based peak flow estimates were higher and lower, respectively, than the observed annual peaks in all years. This illustrated inherent uncertainty introduced early in the design process, which can be further compounded by dynamic in-buffer hydrology. The percentage of the observed peak flow rate at which bypass initiated ranged across an order of magnitude between sites (4.4–8.1% of peak flow rate at one site and 42–49% of peak at another) despite the buffers providing relatively similar NO₃ removal. Bypass at one site (SB2) was related to the concept of “antecedent buffer capacity filled,” which was defined as the 5-d average water depth in the middle control structure chamber expressed as a relative percentage of the bypass stop log height. This design flow analysis serves as a call to further evaluate predictive relationships and design models for edge-of-field practices.