**1. Introduction**

In many parts of the world, water scarcity has posed a great threat to socio-economic development and the ecological environment [1,2]. Due to the limited surface water resources and precipitation, groundwater resources have become indispensable freshwater resources for domestic drinking, irrigation water and industrial activities [3–6]. With largescale industrial activities, groundwater pollution has become a serious problem affecting human health and life in many countries and regions, and as of 2018, the number of declared contaminated sites in 31 provincial capitals in China reached 174. In the process of industrial development, chromium has been used in a growing number of industries, and chromium salt production has subsequently developed to a certain extent [7–11]. At

**Citation:** Chen, W.; Zhang, Y.; Shi, W.; Cui, Y.; Zhang, Q.; Shi, Y.; Liang, Z. Analysis of Hydrogeochemical Characteristics and Origins of Chromium Contamination in Groundwater at a Site in Xinxiang City, Henan Province. *Appl. Sci.* **2021**, *11*, 11683. https://doi.org/10.3390/ app112411683

Academic Editor: Bing Bai

Received: 29 October 2021 Accepted: 22 November 2021 Published: 9 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**<sup>\*</sup>** Correspondence: 3005190010@cugb.edu.cn

the same time, a large amount of chromium slag was produced, and after its long-term open piling, Cr(VI) will seep into the soil and groundwater with surface runoff, thus causing pollution to the surrounding soil and groundwater, and since Cr(VI) is a highly migratory and toxic pollutant, chromium pollution in groundwater has therefore become a serious worldwide problem [12–15]. Cr(VI) has been identified as highly toxic and one of the carcinogens, posing a threat to human health when chromium exceeds drinking water standards.

Groundwater contamination investigation and remediation is urgent, and the analysis of hydrogeochemical parameters are important to grasp the groundwater quality status [16–21]. The study of hydrogeochemical composition and its origins is the basis of groundwater protection and restoration. There are many methods, with the most common ones being multivariate statistical analysis [22,23], the isotope labeling method and the hydrogeochemical simulation method [24–26]. Xiao Yong et al. studied the characteristics and controlling factors of groundwater chemistry in the long-term use area of reclaimed water in Beijing by Piper's trilinear diagram, ionic proportional relationship and saturation index method, and concluded that the changes in the types of groundwater chemistry and water–rock action during the rainy and dry season, and believed that the hydrogeological conditions of the area should be fully considered when reclaimed water was used for agricultural irrigation and ecological landscape [27]. Zhang Jingtao et al. explored the hydrogeochemical characteristics and evolution of groundwater in the Dachaidan area from the pre-mountain alluvial fan to the salt lake by ion ratio analysis, Gibbs plot and hydrogeochemical simulation, and concluded that the water–rock interaction was dominated by the dissolution of halite and gypsum and positive cation exchange [28]. Ashwani used a multivariate statistical and hydrogeochemical approach to analyze Cr(VI) concentration levels, pollutant sources and groundwater geochemistry in an industrial town in Italy to provide an effective aid for water resource management in the region [29]. By testing chemical parameters related to fluoride in groundwater during the periods of abundance, flatness and depletion, D. Laxmankumar concluded that weathering, ion exchange and anthropogenic activities played an important role on the chemical composition of groundwater using Gibbs plot and principal component analysis [30].

Chromium exists in groundwater mainly as trivalent chromium and hexavalent chromium [31]. The migration and transformation of chromium in groundwater is influenced by the redox potential, acidity and concentration [32,33]. Under acidic conditions with low redox potential, chromium mainly exists as Cr(III). As pH increases, the solubility of Cr(III) decreases and Cr(OH)3 precipitation is formed. Under alkaline conditions with high redox potential, chromium is mainly present as Cr(VI) [34,35]. Most of the research focused on the chromium migration and transformation by performing batch and column experiments indoors. The chromium presence pattern was analyzed by varying different pH and concentration values by the controlled variable method. Column experiments usually take a lot of time to complete. In this study, the presence pattern of Cr in representative samples was characterized by numerical simulation using saturation index.

Xinxiang City is a significant industrial base in the North China Plain. To date, there are many left industrial contaminated sites. Thus, it is necessary to get insight into the influence of contaminants on groundwater. Although it has been determined that the groundwater quality at the contaminated site exceeds the Class III water standard, the groundwater hydrogeochemical characteristics and their governing mechanisms are still poorly known. The purpose of this work was to investigate the level of contamination in groundwater, hydrogeochemical characteristics and their origins at a contaminated site in Xinxiang by using mathematical statistics, correlation analysis, ion ratio analysis and saturation index, and to study the hydrogeochemical processes controlling the evolution of groundwater chemistry and the influencing factors of chromium-containing mineral by means of hydrogeochemical simulation. This study will provide effective assistance for groundwater remediation in the region.
