Groundwater quality in the Guanzhong Basin, China: Novel Approaches of Comprehensive Appraisals, Prediction, and Management

Résumé

The Guanzhong Basin, which covers an area of over 20,000 km², is a strategic economic zone of the “Belt and Road” and is the People's Republic of China's fourth largest alluvial plain. It is situated in a semiarid region with scarce water resources and a significant reliance on groundwater resources for economic operations. Agricultural land covers 60% of the basin's total area, implying a high demand for irrigation water. In addition, groundwater in this basin is being contaminated by an increase in domestic sewage, industrial effluents, and intensive agricultural practices. The overall groundwater quality, its trend, and influencing factors were investigated in the present thesis. The research methodology consisted of literature collection, field investigations, groundwater and surface sampling, in-laboratory sample analyses, numerical analyses, and geochemical modeling. The innovative methods were adopted to determine groundwater quality indices for household, industrial, and irrigation water purposes. To address the parameter weighting issues, the commonly used entropy water quality index (EWQI) was improved and developed to create the integrated-weight water quality index (IWQI). A novel industrial water quality index (IndWQI) and a comprehensive irrigation water quality index (IrWQI) are proposed and applied to solve problems of scattered results from traditional indices. Moreover, the development and application of the automatic exponential smoothing model (AESM) to forecast water quality is another new approach for comprehending the water quality evolution in the Guanzhong Basin. On this basis, the hydrogeochemical characteristics of groundwater in the Guanzhong Basin, its evolution patterns, pollution source apportionment, infiltration of surface water into phreatic water, and the influencing factors of hydrogeochemistry were comprehensively analyzed. The evolution processes of various groundwater flow paths were quantified by geochemical simulations. Finally, according to the current situation and simulation prediction results of groundwater quality in the Guanzhong Basin, the risk of groundwater pollution in the Guanzhong Basin was quantified by the identification of potential pollution sources and the groundwater pollution index. The results aided in developing a model for making decisions on the implementation of groundwater quality management and protection measures. The following findings of this research have been achieved: (1) New methods such as the IWQI, IrWQI, and IndWQI for assessing the status of groundwater quality were proposed, and health risk assessments were carried out for major pollutants such as NH₄⁺, NO₃⁻, NO₂⁻, F⁻, and As. The overall assessments showed that groundwater in the Guanzhong Basin was excellent and good at 70.80%, poor at 18.01%, and very poor and unsuitable at 11.18% for drinking purposes, and TDS greatly influenced this quality. The non-carcinogenic risk caused by water ingestion and the cutaneous pathway was found to be 91.93% for adults and 95.03% for children. NO₃⁻ contributed significantly to the potential health risk at 77.62% for adults and 79.29% for children. A negligible carcinogenic risk (CRT) was exhibited for adults, whereas for children, it was moderately significant. A magnesium hazard was discovered in 55.28% of all irrigation groundwater samples, and a salinity hazard was discovered in 33.16% of all samples. According to the IrWQI, only 54.65% of the Guanzhong Basin's total area could be used for irrigation without restrictions. Groundwater quality was classified as excellent and good at 78.16%, poor at 6.28%, very poor at 7.28%, and unsuitable for industrial use at 8.28%. The most influential parameters of groundwater quality for drinking, irrigation, and industrial purposes are TDS, TH, EC, Na⁺, Mg²⁺, Cl⁻, F⁻, SO₄²⁻, NO₃⁻, total alkalinity, and pH, essentially discovered in the middle-north and northeastern parts of the Guanzhong Basin. (2) Hydrogeochemical studies revealed that groundwater types change and water quality deteriorates from Beishan (north) and Qinling (south) Mountains’ piedmont to the lower reaches of the Wei River valley. In the upstream, the dominant hydrogeochemical facies is HCO₃-Ca·Mg, while downstream, water is mixed and combines HCO₃-Ca·Mg, SO₄·Cl-Ca·Mg, and SO₄·Cl-Na types. Concentrations of Na⁺, NH₄⁺, Cl⁻, SO₄²⁻, and NO₃⁻ seriously increase from upstream to downstream. The reasons behind this are rock weathering, river water infiltration, and anthropogenic inputs, as confirmed by positive matrix factor modeling (PMF) and chemical tracer tests. The PMF model allowed us to conclude that higher concentrations of TDS, Na⁺, Mg²⁺, Cl⁻, SO₄²⁻, NO₂⁻, and NO₃⁻ are at >50% caused by human activities. Surface and groundwater interaction test results using chemical tracers, including Cl⁻ and isotopes such as ²H and ¹⁸O, revealed that shallow aquifers are recharged by meteoric precipitations and river waters in some places. The inverse mass balance simulation for mineral phases including calcite, dolomite, gypsum, and halite, as well as Ca/Na and Mg/Na exchange, provided predicted phases and mass transfer through simulated groundwater flow paths. Mineral dissolution and precipitation unevenly varied from upstream to downstream, while for ion exchange, Ca²⁺ and Mg²⁺ were generally adsorbed, and Na⁺ was generally released downstream. (3) The temporal evolution of groundwater quality analysis for monitored wells in the Guanzhong Basin demonstrated that water quality deteriorated in Xian, Baoji, and Xianyang cities and improved in Weinan city over 16 years. Groundwater quality predictions for 15 years followed the same trend. The water quality prediction results showed that: in Baoji area, the IWQI values ranged from 41.47 to 87.49 with an increasing pattern determined by R² between 0.525 and 0.891. Around the Xi’an area, the IWQI forecasts range from 47.93 to 191.83 with an increasing coefficient R² ranging from 0.008 to 0.254. In Xianyang area, the IWQI prediction values vary from 36.12 to 259.88 with an increasing trend determined by R² varying from 0.004 to 0.128. Finally, around the Weinan region, the IWQI forecasts are between 4.17 and 108.56 with a decreasing trend determined by R² between 0.248 and 0.797. The decreasing trend of the IWQI indicates groundwater quality improvement, while the increasing trend of the IWQI signifies groundwater quality deterioration. (4) The field survey and geospatial analysis showed that 42.24% of the total sampled wells are located in the vicinity of potential pollution source points such as industries, landfills, and sewage treatment plants. In addition, 26.08% of the sampled wells are located on agricultural lands, and 22.09% are near urbanized areas. Moreover, the higher pollution index found at 30% is similar to the groundwater quality degradation level revealed by drinking, irrigation, and industrial water quality indices. Based on these results, the decision-making of groundwater quality management measures is promoted, and an alert standard for quantifying the deterioration of groundwater quality by 1 to 4 levels is proposed.

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