Rapid accumulation of a large subsoil nitrate reservoir in an arroyo floodplain and associated aquifer contamination

Abstract:

In an area of elevated nitrate (NO₃) groundwater concentrations in the northern Chihuahuan Desert, an extremely large reservoir of nitrate was found in the subsoil zone in the floodplain of an arroyo. Nitrate inventories in the floodplain subsoils ranged from 10,000–38,000 kg NO₃-N/ha—over twice as high as any previously measured arid region. The floodplain subsoil NO₃ reservoir was over 100 times higher than the adjacent desert (59–95 kg NO₃-N/ha). Soil ages calculated from chloride mass balance suggest significantly younger soils in the floodplain relative to the adjacent desert. Plant communities were larger and more abundant in the floodplain. Hence, neither plant nitrogen uptake nor older soil can explain the large subsoil NO₃ inventories in the arroyo floodplain. We demonstrate that NO₃ accumulates in the subsoil zone of the floodplain through extensive evaporation of monsoon season precipitation funneled into the arroyo. Through a 1-D vadose zone water transport model, we show that the observed NO₃ inventories could be quickly acquired through the lateral movement of water from beneath the stream channel to the adjacent dry unsaturated zone. As aquifer recharge occurs through the arroyo, channel migration across the floodplain likely mobilizes subsoil NO₃ . High molar ratios of NO₃ to chloride (Cl) in monitoring wells beneath the arroyo floodplain suggest subsoil NO₃ has contaminated the groundwater. Groundwater beneath a second arroyo 15 km away also had high NO₃/Cl ratios and NO₃ implying NO₃ accumulation in arroyo floodplain subsoils may not be an isolated phenomenon. These results have major implications for land use planning in arroyo and ephemeral stream floodplains.