Using GIS and mass balance techniques to investigate how hillslope and tributary proximity influence the transport of nitrate from cultivated land to surface water

Abstract:

The agricultural use of fertilizer often results in loss of excess nitrate to streams and groundwater via overland flow and infiltration respectively. Flowpaths in the shallow subsurface facilitate transport of high loads of nitrate to surface water. Likewise, groundwater flowpaths move nitrate to streams through aquifers. In this study we investigate the transport of nitrate at two scales: 1) the scale of a cultivated field within a hillslope farm next to a stream, and 2) on a HUC-10 watershed scale. A series of shallow wells (2.5 m - 4.5 m) were drilled throughout the farm hillslope to investigate nitrate loads and evidence for denitrification. A synoptic sampling event for the watershed utilized 50 community members, landowners, and students to collect 52 surface water samples and 19 discharge measurements at the same time throughout the watershed. This data was used to create a high-resolution chemical “snapshot” of the watershed. Mass balance techniques were used to determine groundwater discharge and analyte loads along the main stem, and GIS techniques were used to map landscape-related risk factors related to landuse, slope, and proximity to a tributary. An ephemeral rivulet on the farm was observed to transport high loads of nitrate during higher rainfall periods. Nitrate concentrations are highest at the location where a perennial spring that drains a steep cultivated field discharges into the rivulet, and nitrate concentrations decrease along the length of the rivulet before entering a pond. At the larger scale, nitrate loads peak ~15 km down the longitudinal profile of the main stem of the creek after two tributaries draining highly cultivated catchments discharge high loads of nitrate into the main stem. Three subcatchments identified by the risk assessment model contribute the highest loads of nitrate to the main stem. Nitrate loads from groundwater also peak downstream of the two at-risk catchments.