ANTHROPOGENIC SULFATE LOADS FROM FERTILIZER USE IN THE RIO GRANDE VALLEY

The Rio Grande is the fourth longest river system in United States and is the primary source of irrigation water for the Rio Grande Valley. Evaporation, groundwater recharge associated with salt-rich sedimentary rocks, and flood irrigation increase the solute content of the river. Salt loading in the Rio Grande gains particular importance in downstream locations in southern New Mexico and western Texas where salt loads can exceed 3,000 mg/L. Although irrigation is thought to contribute to salt loading in the Rio Grande, almost no information has been acquired on the potential role of fertilizers in salinization. To assess the effect of agricultural on water resources in the Rio Grande Valley, we have been working to determine the chemistry and S isotope composition (δ³⁴S of SO₄) for the Rio Grande, its shallow aquifers, and the commonly used fertilizers in the local irrigation districts. SO₄ is the major component of N-P-K fertilizers used in the Rio Grande Valley, constituting about half of the total fertilizer by mass. Low δ³⁴S values of fertilizers (-2.1 to +1.6 ‰) follow accordingly low δ³⁴S of SO₄ in the Rio Grande (-1.6 to +0.9 ‰). The δ³⁴S of SO₄ from deep groundwater sources and nearby evaporite deposits is much higher (+7 to +13 ‰), suggesting that a significant SO₄ flux may come from irrigation canals, drains and/or return flows of irrigated water. In addition to SO₄ from fertilizers, we identified SO₄ loads derived from the weathering of sulfide minerals in the Questa mining district (-4.3 to -3.2 ‰) in Northern New Mexico. This investigation demonstrates that the δ³⁴S value of SO₄ is a valuable tracer for studying the effects of agricultural on water resources in semi/arid environments. In the Rio Grande, SO₄ loading from fertilizers appears to increase the salinity of the river and its shallow aquifers.