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

In an area of elevated nitrate (NO₃) groundwater concentrations in the northern Chihuahuan Desert in central New Mexico (United States), a large reservoir of nitrate was found in the subsoil of an arroyo floodplain. Nitrate inventories in the floodplain subsoils ranged from 10,000 to 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). Chloride mass balance calculations of subsoils indicate arroyo floodplain subsoils have undergone negative recharge since 2600–8600 yr ago, while the surrounding desert has had negative recharge since 13,000–17,000 yr ago. Compared to the adjacent desert, plant communities are larger and more abundant in the floodplain, though subsoil NO₃ is apparently not utilized. We demonstrate that NO₃ accumulates in the subsoil of the floodplain through evaporation of monsoon season precipitation funneled into the arroyo. Through a one-dimensional vadose zone model, we show that the NO₃ inventories in the arroyo floodplain could be acquired 8 to 75 times faster than through atmospheric deposition through the lateral movement