Isotopic studies of the Upper and Middle Rio Grande. Part 2 — Salt loads and human impacts in south New Mexico and west Texas
Isotopic studies of the Upper and Middle Rio Grande. Part 2 — Salt loads and human impacts in south New Mexico and west Texas
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DOI：
10.1016/j.chemgeo.2015.05.023
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发表时间：
2015-09
期刊：
影响因子：
3.9
通讯作者：
A. Szynkiewicz;D. Borrok;G. Ganjegunte;G. Skrzypek;Lin Ma;M. Rearick;G. Perkins
中科院分区：
文献类型：
--
作者：
A. Szynkiewicz;D. Borrok;G. Ganjegunte;G. Skrzypek;Lin Ma;M. Rearick;G. Perkins
地下水和土壤盐分的增加对干旱地区的土地和水资源构成威胁。全球变暖可能会增加旱地河流系统的盐度。为了表征新墨西哥州南部和德克萨斯州西部格兰德河半干旱部分的盐负荷，我们对河流、农业排水沟和地下咸水进行了季节性（2009-2011年）采样。除主要元素化学外，还分析了这些样品中溶解SO 4的硫和氧同位素组成（δ 34 S和δ 18 O），在某些情况下还分析了溶解NO3的氮和氧同位素组成（δ 15 N和δ 18 O）。铀同位素（234 U/238 U活度比）也被测量为选定的样品。半干旱格兰德河研究区流域卤水/地下水（δ 34 S为+ 8至+ 11‰）的自然流入量在调查季节较小，无法通过δ 34 S质量平衡检测到。然而，我们确实发现格兰德河中δ 34 S（+ 2至+ 5‰）的局部增加，这归因于农业排水沟与天然盐坪地下水位相交处的盐负荷以及浅层地下的相关蒸发盐水（δ 34 S为+ 12‰）。在土地灌溉用水量较高的地区，河流和排水的δ 34 S相对一致（~ 0 ~+ 2‰），而δ 18 O相对一致（~+ 2 ~+ 6‰）。最有可能的是，这是由于施用富含硫的肥料（例如，硫酸铵、元素硫、硫酸），δ 34 S低（− 2至+ 4‰），δ 18 O高（+ 9至+ 16‰）。此外，我们观察到格兰德河和农业排水沟中的δ 18 O（SO 4）（< 7‰）比地质和人为SO 4源（+ 9至+ 16‰）低得多，这可能是由于灌溉土地土壤中SO 4的微生物再循环与同化硫酸盐还原有关。格兰德河的浅层补给也可以从较低的234 U/238 U活度比（1.62至1.88）（与深层地下水（2.54至2.64）相比）和独特的硝酸盐δ 15 N和δ 18 O值（分别为+ 5至+ 25‰和-5至+ 15‰）推断出来，这是进行反硝化的化粪池污水的典型特征。漫灌期间的农业做法加剧了格兰德河地表水的蒸发，并大大增加了水的盐度。这一过程也是重要的水化学的演变走向Na-SO 4-Cl丰富的组合物和沉淀次生方解石在土壤剖面。
Increasing groundwater and soil salinity is a threat to the land and water resources in arid regions. Global warming will likely increase salinity of dryland river systems. In order to characterize salt loading into the semi-arid portion of the Rio Grande in south New Mexico and west Texas, we sampled seasonally (2009–2011) the river, agricultural drains, and saline groundwater. In addition to major element chemistry, these samples were analyzed for sulfur and oxygen isotope compositions (δ34S and δ18O) of dissolved SO4and in some cases for nitrogen and oxygen isotope compositions (δ15N and δ18O) of dissolved NO3. Uranium isotopes (234U/238U activity ratio) were also measured for selected samples. The natural inflow of basinal brines/groundwater (δ34S of + 8 to + 11‰) in the semi-arid Rio Grande study area was minor in the investigated seasons and could not be detected by the δ34S mass balance. However, we did find localized increases of δ34S (+ 2 to + 5‰) in the Rio Grande that were attributable to salt loads from the intersections of agricultural drains with the water table of a natural salt flat and associated evaporative brine (δ34S of + 12‰) in the shallow subsurface. In the areas, with higher water use for land irrigation, the δ34S of the river and drain water was relatively consistent (from ~ 0 to + 2‰) compared to the δ18O (from ~+ 2 to + 6‰). Most likely, this resulted from application of S-rich fertilizers (e.g., ammonium sulfates, elemental S, sulfuric acid) with low δ34S (− 2 to + 4‰) and high δ18O (+ 9 to + 16‰). Additionally, we observed considerably lower δ18O (SO4) in the Rio Grande and agricultural drains (< 7‰) compared to geologic and anthropogenic SO4sources (+ 9 to + 16‰), which likely resulted from microbial recycling of SO4in soil of the irrigated land related to assimilatory sulfate reduction. Shallow recharge to the Rio Grande was also inferred from the lower234U/238U activity ratios (1.62 to 1.88) compared to deeper groundwater (2.54 to 2.64) and the distinctive δ15N and δ18O values of nitrates (+ 5 to + 25‰ and − 5 to + 15‰, respectively) typical for septic effluents that are undergoing denitrification. Agricultural practices during flood irrigation intensify evaporation of the Rio Grande surface water and considerably increase water salinity. This process is also important in the evolution of water chemistry toward a Na–SO4–Cl-rich composition and precipitation of secondary calcite in soil profiles.