5.1 Recognition of arsenic spatial distribution characteristics by different ion ratios
Since the occurrence of arsenic is significantly affected by hydrogeochemical processes in the groundwater environment, the distribution characteristics of different ion ratios can not only help to identify hydrogeochemical processes in groundwater, but also effectively facilitate identifying the distribution mechanism of arsenic in the study area. In the light of the concentration characteristics and spatial distribution of different ion ratios in the foregoing sections, the groundwater in the study area was divided into six zones according to the final boundary that was set according to boundaries of overlapping areas of the four ion ratios (Fig. 8).
The Manhan Mountain piedmont alluvial plain zone (Zone I) was characterized by Na+/Ca2+<1.1, Ca2+/(HCO3−+CO32−) > 0.7, and Ca2+/Mg2+>1.4, indicating that groundwater was mainly distributed in this area (Fig. 8A). The median value of Ca2+ concentration in this zone was 69.7mg/L, which was the highest among the five zones (Table S1). The high Ca2+ concentration reflected that groundwater was strongly influenced by surface water. This was mainly caused by strong piedmont lateral recharge considering the geomorphological characteristics of this area. The low Na+ content indicated a weak cation exchange and a good groundwater runoff condition in this area. The mean value of arsenic concentration in this region is 1.08µg/L and the arsenic concentration of all samples is less than 10µg/L, which is the lowest among the six regions.
The Horinger platform zone (Zone II) was characterized by groundwater with ion ratio 0.4 < Ca2+/Mg2+<1.4, 2<(HCO3−+CO32−)/SO42−<5, and 0.1 < Ca2+/(HCO3−+CO32−) < 0.5 in the northern front of Horinger platform (Fig. 8B).The median values of Na+, Ca2+and Mg2+ concentration in this zone were 165, 40.7 and 45.2mg/L respectively (Table S1). The values of Ca2+/Mg2+ and Ca2+/(HCO3−+CO32−) indicated that the Ca2+ content in this area was relatively lower than that in the vicinity of the Hasuhai Lake and the Manhan Mountain, but higher than that in the Dahei River alluvial plain, representing that the groundwater was weakly recharged from the platform region. The value of (HCO3−+CO32−)/SO42− indicated that the redox condition of groundwater is in a weak reducing state. The average arsenic concentration of groundwater in this area was 3.33µg/L, and only one water sample with As > 10µg/L was distributed in Zone II.
The Daqing Mountain piedmont alluvial plain zone (Zone III) is located in the alluvial plain in front of Daqing Mountain (Fig. 8B) with ion ratios of 2<(HCO3−+CO32−)/SO42−<5 and 0.5 < Ca2+/(HCO3−+CO32−) < 0.7. The ion ratio indicated that the groundwater in this place was under weak reducing and runoff conditions. The groundwater was affected by the lateral recharge in front of Daqing Mountain, which is stronger than the lateral recharge in the Horinger platform. The mean value of arsenic concentration in this area was 17.8µg/L, and the groundwater had a low arsenic level near the mountain front but a high arsenic level near the Dahei River alluvial plain.
The transition zone (Zone IV) was represented by ion ratios of 0.4 < Ca2+/Mg2+<1.4, 5<(HCO3−+CO32−)/SO42−<10, and 0.1 < Ca2+/(HCO3−+CO32−) < 0.5 in the transition zone between the piedmont alluvial plain and the Dahei River alluvial plain(Fig. 8C). The ion ratios demonstrated that the groundwater in Zone IV was affected by a weak lateral recharge and medium reducing environment that was stronger than Zone II and Zone III. The median value of arsenic concentration was 17.9µg/L and 47.4% of water samples had arsenic levels higher than 10µg/L.
The Dahei River alluvial plain zone (Zone V) was mainly situated in the groundwater discharge area of Dahei River alluvial plain (Dongguang et al. 2013) in view of the distribution range of (HCO3−+CO32−)/SO42 > 10 and Na+/Ca2+>13, Ca2+/(HCO3−+CO32−) < 0.1 (Fig. 8D). The ratio of groundwater with (HCO3−+CO32−)/SO42−>10 was the highest in the study area, indicating the strongest reducibility of groundwater in this area. The median values of Ca2+ and Na+ concentration were 28.2 and 215mg/L, which respectively were the minimum and maximum values in all zones (Table S1). The ratios of Ca2+/(HCO3−+CO32−), Na+/Ca2+ and ion content jointly indicated that the surface water recharge effect of Zone IV is the weakest in the whole study area, and neither the Hasuhai Lake nor the piedmont groundwater can recharge to the groundwater of this area. At the same time, the cation exchange between Ca2+ in the aquifer and Na+ in the groundwater is sufficient. The average arsenic concentration in the groundwater of Zone IV reached 91.6µg/L, which is significantly higher than that in other zones.
The drainage canal zone (Zone VI) had groundwater with Na+/Ca2+<1.1 and (HCO3-+CO32−)/SO42−<2 that was mainly distributed in the east of the Hasuhai Lake and the drainage canal (Fig. 8C). The mean value of arsenic concentration in the groundwater of this area is 10.0µg/L. The ratio of (HCO3−+CO32−)/SO42− in Zone VI is the smallest in the study area, indicating the strongest oxidation degree of groundwater. Studies have shown that the groundwater was affected by the quality of its nearby lakes and canals (Huaming et al. 2011, Søren et al. 2012). Groundwater will be mixed with surface water of higher oxygen content, resulting in increased oxidation of the groundwater. Besides, the Na+/Ca2+ ratio of Zone VI is the smallest in the study area, indicating a weak cation exchange between Ca2+ and Na+ in the aquifer and a good runoff condition. It is worth noting that although the Na+/Ca2+ value in Zone VI is the smallest in the whole study area, the median and mean value of Na+ concentration were 203 and 298mg/L respectively, which were only lower than that in Zone V among the six zones. Meanwhile, its TDS, Mg2+ and SO42− concentrations were the highest in the study area, and the median and average values of Ca2+ were only lower than that of Zone I (Table S1).
It can be concluded that the distribution of arsenic and ion ratio in groundwater had significant consistent characteristics. The median value of arsenic concentration in the six zones were 0.785, 2.76, 2.87, 4.30, 54.3 and 2.48µg/L respectively (Fig. 9a). It can be found that the total arsenic content of Zone V was significantly higher than that of other zones, while the total arsenic content in Zone I was the lowest. The arsenic concentration of Zone II, III, IV and VI were roughly similar, between Zone V and IV. The result of ion ratios revealed that Zone V had the weakest lateral recharge but the strongest reduction and cation exchange. In contrast, Zone I had the strongest piedmont lateral recharge effect and the best runoff condition among the six zones.
Therefore, the occurrence characteristics of arsenic can be identified by the value distribution of the four kinds of ion ratios (Table 2). High arsenic groundwater was distributed in the groundwater discharge zone of the Dahei River alluvial plain. This zone was in a strong reducing environment with high (HCO3−+CO32−)/SO42−, low Ca2+/(HCO3−+CO32−) and low Ca2+/Mg2+, indicating that it was not affected by surface water recharge. Meanwhile, under the poor runoff conditions of the groundwater, the increase of Na+/Ca2+ ratio represented a strong cation exchange. The low-arsenic area, affected by recharge from piedmont and the Hasuhai Lake, had good groundwater runoff, which was not conducive to arsenic release.
5.2 Hydrogeochemical processes in different zones
In Zone V, which had the highest arsenic concentration, the depth to the water table is shallow, ranging between 1.33 and 7.83m (Fig. 9c), with a median value of 2.68m. The median TDS in this zone reached 901.3mg/L. The median depth to the water table and TDS in Zone VI were 2.7m and 1416mg/L respectively (Table S1). Zone V and VI showed the shallowest water level in all zones (Fig. 9a, b). There was a significant negative correlation between arsenic content and water level (Fig. 10A), in other words, shallower groundwater had higher arsenic levels. Therefore, it suggested that strong evaporation and concentration occurred in Zone V and VI (L. and G. 2002, Liu et al. 2017), and arsenic was further enriched under evaporation concentration (Dongguang et al. 2013). This also explained that although cation exchange was strong in Zone IV, the Na+ content was still significantly higher than that in the piedmont plain and platform. On the contrary, the median value of the groundwater table and TDS reached 14.09m and 477.1mg/L respectively in Zone I, meaning that the groundwater was not affected by evaporation and TDS also showed desalination characteristics (Fig. 9C and Table S1).
Arsenic and iron concentration in groundwater were positively correlated (Fig. 10b), indicating that the release of arsenic was related to the increase of iron content. It showed that the groundwater in Zone V had the strongest reducing condition and also the highest concentration of iron in the basin, with a median concentration of 0.48mg/L (Table S1). Under the reducing condition, iron oxide gained electrons and dissolved into ferrous ions into groundwater through reductive dissolution. Arsenic adsorbed on iron oxide was released into the groundwater (L. et al. 2003, Ting et al. 2012). The low arsenic areas were affected by the recharge of oxygen-bearing surface water and had a low iron content.
Ammonia-nitrogen and nitrate-nitrogen concentrations were greatly affected by redox conditions hence they could be considered as sensitive indicators of redox. The transformation of nitrogen in the groundwater was also significantly different. Arsenic concentration was negatively correlated with nitrate concentration but positively correlated with ammonium concentration (Fig. 10c, d). In other words, nitrate concentration was significantly lower in high arsenic groundwater while ammonium concentration was higher in low arsenic groundwater (Qi et al. 2014, Shuangbao et al. 2013) (Fig. 9a, e and f). The NO3− concentration was found to be the highest in Zone I and IV, where oxidation was the strongest, with the median concentration reaching 28.8 and 42.2mg/L respectively. Meanwhile, the median ammonium concentration in these two zones (0.02mg/L and 0.04mg/L) was significantly lower than that of other zones. The groundwater in Zone VI and I were affected by oxygenated water recharge from the Hasuhai Lake and the drainage canal as well as lateral recharge from the Manhan Mountain, and therefore the oxidation of Zone I and IV were significantly higher than that in other zones (Huaming et al. 2010). Ammonia-nitrogen in oxidized groundwater would be converted to nitrate nitrogen by nitrification (Marco et al. 2017). Low nitrate-nitrogen (median 3.08mg/L) and high ammonium nitrogen (median 0.16mg/L) were found in Zone V. Compared with the Manhan Mountain piedmont plain, the concentration of ammonia nitrogen in Zone V significantly increased while the concentration of total inorganic nitrogen decreased (Figure S2). Groundwater was in an anaerobic state under the strong reducing condition of Zone V. In the presence of organic compounds, nitrate-nitrogen not only released N2 through denitrification but also transformed into ammonium nitrogen through dissimilatory nitrate reduction to ammonium (DNRA) reaction (T. et al. 2011, Zhipeng et al. 2021).
Meanwhile, the median concentration of HPO42− in Zone V reached 1.2mg/L, which was significantly higher than that in other zones (Fig. 9d and Table S1). The release of phosphorus (P) under anoxic conditions was mainly controlled by the reductive dissolution of iron oxide (Yao et al. 2022). The high phosphate concentration promoted the release of arsenic in groundwater by competing for adsorption sites (L. and G. 2002). There was no evident correlation between F and As in the study area (Fig. 10e). Highly fluoridated groundwater was mainly found in Zone II and V, where the Na+ level was high. Because Ca2+ and F− in groundwater were able to form fluorite precipitation, high Ca2+ was not conducive to the enrichment of fluoride. Arsenic enrichment was enhanced by weak evaporation and concentration due to the shallow water level, but the weak runoff and oxidation environment inhibited the release of arsenic into groundwater in Zone II and III. The position and ion ratios of Zone IV manifested that the environment of the transition zone slightly promoted the release of arsenic. Thus, as the aquifer environment altered along the direction of groundwater runoff, the arsenic release mechanism and the hydrogeochemical action model changed accordingly (Fig. 11).