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

-Anna Szynkiewicza,b, James C. Witcherc, Magdalena Modelskad, David M. Borrokb, Lisa M. Pratt

The sources of SO4along a ~550 km stretch of the Rio Grande in New Mexico and western Texas wereinvestigated using stable S isotopes. During 2007 and 2008, theδ34S of dissolved SO4in the Rio Grande surfacewater varied over a narrow range from−1.6 to +0.9‰, which was consistent with theδ34S of local fertilizers(−2.1 to +1.6‰) and was not consistent with Paleozoic evaporite sources of SO4in regional bedrock (+7.6to +12.9‰). This is likely due the fact that SO4is the major component of N–P–K fertilizers used in the RioGrande Valley, constituting about half of the total fertilizers by mass. The SO4/Cl ratios of the groundwatersystem are relatively low (0.06 to 3) compared to the fertilizer source, suggesting that more Cl is added to theRio Grande from geological sources as compared to SO4. In the Mesilla Basin in southern New Mexico, weidentified 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 thatNa–K–Cl concentrations were largely attributable to geological sources and SO4–Mg–Ca concentrations toanthropogenic sources. Here, an additional anthropogenic source of SO4(with aδ34Sof−2.7‰) was linked toanaerobic decomposition of manure on a horse farm. In this case SO4concentrations (800 mg/L) increased byabout three times compared to background SO4concentrations in groundwater (b300 mg/L). Because of thecommon application of H2SO4in fertilizer manufacturing, anthropogenic SO4fluxes to rivers and shallowaquifers from irrigation waters can be significant worldwide.

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