Denitrification is a significant process for the removal of nitrate transported in groundwater drainage from agricultural croplands to streams and has important ecological consequences. In this paper, analytical solutions are developed for advective-reactive and nonpoint sources contaminant transport in a two-layer unconfined aquifer with a sloping bed. In particular, we investigate the impact of a sloping bed of denitrifying sediments on nitrate in groundwater discharge to streams. Nitrate discharge to streams is considered to be the result of mixing of waters from two geochemically distinct zones in the aquifer. Groundwater flow is assumed to be steady, and the effect of dispersion on solute transport is neglected. First, the problem of steady state groundwater flow in an unconfined aquifer with a sloping bed subject to uniform recharge is solved exactly and approximately using a regular perturbation technique. The transport equations, which describe advective-reactive transport in a two-layer aquifer, are then developed based on mass balance and solved in the Lagrangian viewpoint for the case of complete denitrification in the lower layer (or redox zone), using the perturbation technique. Application of the methodology to paired agricultural watersheds in the mid-Atlantic coastal plain predicts that the sloping bed of the surficial aquifer affects both groundwater discharge and nitrate loading to two creeks in the watersheds. Because of the sloping bed of the surficial aquifer, groundwater discharged to Morgan Creek is estimated to be dominated by discharge from the lower layer and therefore shows greater potential for losses by denitrification at the base of the aquifer than those predicted at the Chesterville Branch. Simulation results show that a relatively low nitrate discharge to Morgan Creek may be also the result of historically low input of nitrogen at the source. Further, a numerical experiment based on the solutions shows that dilution with nitrate-free groundwaters in a riparian buffer strip could be misinterpreted as nitrate losses. Lumped parameter indices are derived, which estimate the capacity of watersheds and riparian zones for nitrate removal by denitrification and/or other loss pathways, and their variation with aquifer bed slope, stream-aquifer hydraulic and geometric properties, and groundwater recharge is illustrated. ¿ 2001 American Geophysical Union