The alluvial plain in the middle and lower reaches of the Yellow River is an important agricultural production base that affects groundwater quality. Groundwater quality in the region is related to the residential and production uses of water by local residents. Samples of shallow groundwater and river water were collected from the right bank of the middle and lower reaches of the Yellow River to determine the evolution and causes of hydrochemical characteristics, and the relationship between the hydrochemical evolution of river water and groundwater was explored. The results showed that the shallow groundwater in the area received lateral recharge from the Yellow River water. The closer to the Yellow River the groundwater was, the higher the SO₄²⁻, Cl⁻, and Na⁺ concentrations and the lower the HCO₃⁻ and Mg²⁺ concentrations were. Agriculture and aquaculture has influenced and complicated the hydrochemical types of shallow groundwater in recent decades. The groundwater in the area was jointly affected by water–rock interactions and evaporation concentrations; a strong cation exchange effect was detected. Arsenic exceeded the limit in some shallow groundwater, which was mainly distributed in the Yellow River alluvial plain and caused by the reductive sedimentary environment of the Yellow River alluvial plain. The “three nitrogen”, NH₄⁺-N, NO₂⁻-N, and NO₃⁻-N, demonstrated sporadic local excesses in shallow groundwater, which were related to human activities, such as aquaculture.

Based on the groundwater chemistry data from Karamay City, Xinjiang Province, this study examines seven major components: K⁺, Ca²⁺, Na⁺, Mg²⁺, SO₄²⁻, Cl⁻, and HCO₃⁻. The sampling was conducted during two periods: flood and dry periods. On the basis of analyzing the regional geologic background and hydrogeologic conditions, the study area was divided into calculation units and the test data validity was screened. Then, the outliers were eliminated by Grubbs’ method and Piper’s trilinear plot method, and the effectiveness of the elimination was evaluated by the box-and-line diagram. After that, the distribution types of the groundwater chemistry data in each calculation unit were judged to obtain the background values of the seven test indexes for the groundwater in different calculation units. The results show that Grubbs’ method and Piper’s trilinear plot method are effective in removing outliers. Secondly, the background values of Na⁺ and Cl⁻ in the groundwater of Karamay City are mostly higher than those of other anions and cations during the two sampling periods. This phenomenon may be due to the concentration effect of evaporation. Additionally, the groundwater background values of Ca²⁺, Na⁺, and Cl⁻ showed more significant differences between the two sampling periods compared to other ions.