Nitrate loss in Saylorville Lake reservoir in Iowa

•We conduct mass balance analyses for riverine nitrate passing through a reservoir.•Our results are consistent with others spanning a wide range of retention times.•Percent nitrate removal is a predictable function of hydraulic loading rate.•Mass nitrate removal is a function of percent removal and mass loading.•These results apply to reservoirs having moderate to high input concentrations.

SummaryFew studies have quantified nitrate-nitrogen reductions in rivers flowing through large flood control reservoirs despite their potential role in helping to meet nitrate-N load reduction targets. In this study, we evaluated a 26-year record of nitrate-N monitoring in Saylorville Lake reservoir that impounds the Des Moines River in central Iowa to estimate the annual mass of nitrate-N removed and evaluate controls on inter-annual patterns of nitrate-N removal rates. Results from mass balance calculations indicate a nitrate mass loss of 4.9% over the 26 year period from 1986 through 2011. The annual percent nitrate mass removed is related to hydraulic loading rate with low flow years having longer retention times removing a significantly greater percentage of nitrate than is removed during high flow years with low retention times. Average annual residence times varied from 3.5 to 74 days with a 26 year average of 9.7 days. Nitrate removal results for Saylorville Lake agree well with published work for several other reservoirs which together encompass a broad range of nitrate removal rates. Combining data from Saylorville Lake with other published studies, a relationship between nitrate-N removal rate and hydraulic loading rate was developed resulting in a predictive equation with a model R2 = 0.92.