Nitrate Loss from Wetlands: Mass balance analysis and modeling were used to calculate observed and predicted nitrate removal for the monitored wetlands. The monitored wetlands performed as expected with respect to nitrate removal efficiency (expressed as percent mass removal) and mass nitrate-N removal (expressed as Kg N ha-1 year-1). Wetland performance is a function of hydraulic loading rate (HLR), hydraulic efficiency, nitrate concentration, temperature, and wetland condition. Of these, HLR and nitrate concentration are especially important for CREP wetlands. The range in hydraulic loading rates expected for CREP wetlands is significantly greater than would be expected based on just the four fold range in wetland/watershed area ratio approved for the Iowa CREP. In addition to spatial variation in precipitation (average precipitation declines from southeast to northwest across Iowa), there is substantial annual variation in precipitation. The combined effect of these factors means that hydraulic loading rates to CREP wetlands can be expected to vary by more than an order of magnitude, and will to a large extent determine nitrate loss rates for individual wetlands (figure above).
The relationship in the figure above represents the annual percent nitrate removal as a function of the annual average hydraulic loading rate. However, since so much of the annual nitrate load comes during elevated springtime flows it is also important to compare nitrate removal efficiency during those periods. Although potential denitrification rates can be expected to increase with increasing summer temperatures, the percent nitrate mass loss for the period April-May-June (AMJ) is very similar to the annual percent loss (figure to right, R2 = 0.98). This is in in large part because the bulk of the nitrate load is delivered during AMJ (about 60% on average) and during late October-November (about 20% on average). Considerably less of the load is delivered during July and August (about 12% on average) when temperatures are high but discharge is generally low.
The expected long term average annual nitrate removal capacity for CREP wetlands constructed between 2004 and 2018 was estimated based on actual wetland acreage, watershed acreage, an assumed long term average water yield of 0.25 m/yr, and a wetland inflow flow-weighted average (FWA) concentration of 14 mg N/L, which is the observed FWA nitrate concentration of monitored CREP wetlands (figure to right). The expected nitrate mass removal for these calculations is based on the removal function derived for monitored wetlands illustrated in the top figure. The figure below illustrates the increase in nitrate removal capacity with the annual addition of new CREP wetlands. For the 87 CREP wetlands completed by 2018, the estimated long term average annual nitrate removal rate is about 1800 kg N ha-1 yr-1 (1600 lb N acre-1 yr-1).