Anthropogenic sulfate loads in the Rio Grande, New Mexico (USA)

The sources of SO4 along a ~ 550 km stretch of the Rio Grande in New Mexico and western Texas were investigated using stable S isotopes. During 2007 and 2008, the δ34S of dissolved SO4 in the Rio Grande surface water varied over a narrow range from − 1.6 to + 0.9‰, which was consistent with the δ34S of local fertilizers (− 2.1 to + 1.6‰) and was not consistent with Paleozoic evaporite sources of SO4 in regional bedrock (+ 7.6 to + 12.9‰). This is likely due the fact that SO4 is the major component of N–P–K fertilizers used in the Rio Grande Valley, constituting about half of the total fertilizers by mass. The SO4/Cl ratios of the groundwater system are relatively low (0.06 to 3) compared to the fertilizer source, suggesting that more Cl is added to the Rio Grande from geological sources as compared to SO4. In the Mesilla Basin in southern New Mexico, we identified zones of mixing between recharging irrigation water with groundwater within the depth range of ~ 50–200 m below the ground surface. For this aquifer, Principal Component Analysis (PCA) indicated that Na–K–Cl concentrations were largely attributable to geological sources and SO4–Mg–Ca concentrations to anthropogenic sources. Here, an additional anthropogenic source of SO4 (with a δ34S of − 2.7‰) was linked to anaerobic decomposition of manure on a horse farm. In this case SO4 concentrations (800 mg/L) increased by about three times compared to background SO4 concentrations in groundwater (< 300 mg/L). Because of the common application of H2SO4 in fertilizer manufacturing, anthropogenic SO4 fluxes to rivers and shallow aquifers from irrigation waters can be significant worldwide.