Discovery 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 Chihua-huan 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 of water from the arroyo channel to the adjacent unsaturated zone. As aquifer recharge occurs through the arroyo, channel migration across the floodplain likely flushes subsoil NO3 to the aquifer. High NO3 concentrations and molar ratios of NO3 to Cl in monitoring wells beneath the arroyo floodplain indicate a subsoil NO3 source. These results have major implications for land-use planning in arroyo and ephemeral stream floodplains as well as arid region soil biogeochemistry.