U.S. Geological Survey, Sacramento, CA, United States.
U.S. Geological Survey, Sacramento, CA, United States.
Sci Total Environ. 2018 Nov 15;642:125-136. doi: 10.1016/j.scitotenv.2018.05.333. Epub 2018 Jun 14.
Total dissolved solids (TDS) concentrations in groundwater tapped for beneficial uses (drinking water, irrigation, freshwater industrial) have increased on average by about 100 mg/L over the last 100 years in the San Joaquin Valley, California (SJV). During this period land use in the SJV changed from natural vegetation and dryland agriculture to dominantly irrigated agriculture with growing urban areas. Century-scale salinity trends were evaluated by comparing TDS concentrations and major ion compositions of groundwater from wells sampled in 1910 (Historic) to data from wells sampled in 1993-2015 (Modern). TDS concentrations in subregions of the SJV, the southern (SSJV), western (WSJV), northeastern (NESJV), and southeastern (SESJV) were calculated using a cell-declustering method. TDS concentrations increased in all regions, with the greatest increases found in the SSJV and SESJV. Evaluation of the Modern data from the NESJV and SESJV found higher TDS concentrations in recently recharged (post-1950) groundwater from shallow (<50 m) wells surrounded predominantly by agricultural land uses, while premodern (pre-1950) groundwater from deeper wells, and recently recharged groundwater from wells surrounded by mainly urban, natural, and mixed land uses had lower TDS concentrations, approaching the TDS concentrations in the Historic groundwater. For the NESJV and SESJV, inverse geochemical modeling with PHREEQC indicated that weathering of primary silicate minerals accounted for the majority of the increase in TDS concentrations, contributing more than nitrate from fertilizers and sulfate from soil amendments combined. Bicarbonate showed the greatest increase among major ions, resulting from enhanced silicate weathering due to recharge of irrigation water enriched in CO during the growing season. The results of this study demonstrate that large anthropogenic changes to the hydrologic regime, like massive development of irrigated agriculture in semi-arid areas like the SJV, can cause large changes in groundwater quality on a regional scale.
地下水的总溶解固体（TDS）浓度在过去 100 年中，加利福尼亚州圣华金河谷（SJV）用于有益用途（饮用水、灌溉、淡水工业）的地下水平均增加了约 100mg/L。在此期间，SJV 的土地利用从自然植被和旱地农业转变为以灌溉农业为主，城市面积不断扩大。通过比较 1910 年（历史时期）采样井的 TDS 浓度和主要离子组成与 1993-2015 年（现代时期）采样井的数据，评估了百年尺度的盐度趋势。使用聚类方法计算了 SJV 的南部（SSJV）、西部（WSJV）、东北部（NESJV）和东南部（SESJV）等子区域的 TDS 浓度。所有地区的 TDS 浓度都有所增加，其中 SSJV 和 SESJV 的增加幅度最大。对 NESJV 和 SESJV 的现代数据进行评估后发现，浅层（<50m）井中最近补给（1950 年后）的地下水 TDS 浓度较高，这些浅层井周围主要是农业用地，而较深井中较老的地下水（1950 年前）和周围主要是城市、自然和混合用地的最近补给地下水的 TDS 浓度较低，接近历史时期地下水的 TDS 浓度。对于 NESJV 和 SESJV，使用 PHREEQC 进行的反向地球化学模拟表明，硅酸盐矿物的风化作用是 TDS 浓度增加的主要原因，其贡献超过了肥料中的硝酸盐和土壤改良剂中的硫酸盐。主要离子中增加最多的是碳酸氢盐，这是由于生长季节灌溉水补给 CO2 导致硅酸盐风化增强所致。本研究结果表明，像 SJV 这样的半干旱地区大规模发展灌溉农业等对水文条件的人为重大改变，可能会导致地下水质量在区域范围内发生重大变化。
