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 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

Knowledge gaps and opportunities for understanding water-quality processes affecting water availability for beneficial uses

“…Anthropogenic N inputs that can lead to nitrate contamination include fertilizer applications, animal waste infiltration (manure spreading, lagoons), wastewater disposal (including septic systems), enhanced biologic N fixation, and atmospheric deposition (van der Schans and others, 2009; Bellmore and others, 2018). While anthropogenic sources of N have increased dramatically since industrialization (Galloway and others, 2004), natural sources of nitrate also can affect water quality in some areas, especially in arid regions (Walvoord and others, 2003;Jackson and others, 2015;Linhoff and Lunzer, 2021). For example, nitrate concentrations in groundwater may be elevated where natural recharge rates are low and where long-term nitrate accumulations in soils are leached by flooding, irrigation, or artificial recharge.…”

Recognizing the ephemeral stream floodplain: Identification and importance of flood zones in drylands

“…Following flooding, concentrations of nitrogen (nitrate and ammonium) increase in riparian groundwater reservoirs along ephemeral streams (Harms et al, 2009; Marti et al, 2000). Ephemeral stream floodplains are recognized as nitrogen sinks; for example, confined alluvial floodplains can have over 100 times higher volumes of nitrate compared with surrounding uplands (Linhoff & Lunzer, 2021). Ephemeral stream floodplains can therefore also be disproportionately important for denitrification, emitting both NO and N 2 O following flow pulses (Figure 5; Marti et al, 2000; Harms & Grimm, 2012).…”