DRAINAGE DITCH WITH A VIEW: DELIVERY OF NITRATE FROM CROPLAND VIA EMERGING GROUNDWATER

Most of the biologically reactive N that humans add to the biosphere hits the ground in croplands. N loss from croplands to streams begins mainly as biologically produced nitrate in the soil leaching through the root zone to groundwater. Subsequent nitrate delivery from streams to estuaries such as Chesapeake Bay is an important driver of eutrophication, causing harmful algal blooms, depletion of dissolved oxygen, and demise of submerged aquatic vegetation. However, some of the nitrate may be removed from groundwater by microbial reduction to N₂. We investigated groundwater nitrate delivery to channelized streams draining a 4.8 km² watershed where agriculture was the dominant N source. We sampled emerging groundwater with piezometers drawing from 0.5 m depth mid-channel in the stream bed, in essence using drainage ditches as a window into the aquifer. We used membrane inlet mass spectrometry to measure N₂, oxygen, and argon concentrations in groundwater and estimated biogenic N₂ by comparisons to argon concentration. Concentrations of nitrate and biogenic N₂ in emerging groundwater varied markedly along the stream channel: nitrate ranged from near 0 to 1.5 mM and biogenic N₂ ranged from 0.05 to 0.59 mM. Large spatial differences often occurred over tens of meters of channel length and much of the nitrate delivery was focused in short sections of the stream with no clear relation to changes in the adjacent land cover. Biogenic N₂ increased as nitrate decreased but by less than equimolar amounts, suggesting that groundwater flow paths to the stream differed in initial nitrate concentration as well as in biogenic N₂ production. Concentrations of dissolved iron, oxygen, nitrate, and biogenic N₂ followed expected oxidation-reduction relationships. As oxygen declined, nitrate declined and biogenic N₂ increased. Iron concentrations generally remained below 0.01 mM but ranged up to 0.6 mM when nitrate dropped below 0.01 mM. Sulfate concentrations increased with decreases in nitrate, suggesting that nitrate reduction might be coupled to sulfide oxidation in the aquifer. Thus the capacity for nitrate reduction may partly depend on the pool of sulfide minerals in the aquifer.