3.1 Groundwater chemistry
Descriptive statistical data of the hydrochemical compositions of 85 Quaternary unconfined water (shallow 0–30 m) samples, 80 Quaternary confined water (intermediate 30–90 m) samples, and 60 Neogene confined water (deep 90–220 m) samples are presented in Table 1.
Table 1
Main statistics for the considered parameters of groundwater samples
Parameters | Phreatic water (Shallow, n = 85) | Quaternary confined water (Intermediate, n = 80) | Neogene confined water (Deep, n = 60) |
Min | Max | Mean | Min | Max | Mean | Min | Max | Mean |
Depth(m) | 8 | 30 | 21 | 32 | 90 | 61 | 90 | 220 | 133 |
pH | 7.2 | 8.3 | 7.7 | 7.3 | 8.1 | 7.8 | 7.4 | 11.6 | 8.0 |
TDS(mg/L) | 216 | 4340 | 950 | 160 | 1730 | 591 | 163 | 2030 | 513 |
K+(mg/L) | 0.3 | 5.5 | 1.1 | 0.3 | 2.2 | 1.0 | 0.5 | 39.3 | 3.1 |
Na+(mg/L) | 9.2 | 710.0 | 152.4 | 7.6 | 483.0 | 106.7 | 16.9 | 358.0 | 105.0 |
Ca2+(mg/L) | 19.9 | 530.0 | 99.3 | 20.8 | 168.0 | 61.0 | 6.6 | 174.0 | 48.4 |
Mg2+(mg/L) | 10.4 | 287.0 | 50.2 | 7.2 | 111.0 | 29.8 | 1.4 | 122.0 | 20.4 |
Cl−(mg/L) | 3.9 | 1090.0 | 105.1 | 3.1 | 623.0 | 48.4 | 2.6 | 714.0 | 63.0 |
SO42−(mg/L) | 0.9 | 659.0 | 81.7 | 0.6 | 596.0 | 48.0 | 1.7 | 270.0 | 35.8 |
HCO3−(mg/L) | 170 | 1900 | 570 | 168 | 953 | 495 | 2 | 950 | 378 |
NO3−(mg/L) | 0.01 | 298.00 | 21.42 | 0.01 | 35.40 | 1.71 | 0.01 | 31.30 | 1.11 |
F−(mg/L) | 0.16 | 7.99 | 1.62 | 0.19 | 5.90 | 1.31 | 0.26 | 6.49 | 0.94 |
As(µg/L) | 1 | 310 | 13 | 1 | 148 | 13 | 1 | 104 | 12 |
The pH values in the shallow and intermediate groundwater of the study area were between 7.2–8.3 and 7.3–8.1, respectively, and mainly consisted of neutral water. The pH of deep groundwater ranges from 7.4 to 11.6, with 13.3% of the samples being alkaline. The total dissolved solids (TDS) content in the shallow groundwater was generally higher than that in the mid-deep groundwater. The TDS content ranges in the shallow, intermediate, and deep groundwater were 216–4340, 160–1730, and 160–2030 mg/L, respectively. According to the TDS content range, this study defined groundwater with TDS < 1000 mg/L as freshwater, and groundwater with TDS > 1000 mg/L as brackish water (Wang et al., 2021). Among these, 31.8% of the shallow groundwater samples were brackish, 7.5% of the intermediate groundwater samples were brackish, and 5% of the deep groundwater samples were brackish. In general, an increase in the salinity of groundwater is associated with high concentrations of As and F, which limits the wider use of groundwater (Alarcón-Herrera et al., 2013).
In the shallow groundwater samples, the low TDS was primarily of the Ca-HCO3 type, whereas the high TDS groundwater was mainly of the Na-HCO3, Na-HCO3/Cl, and Na-SO4/HCO3 types (Fig. 2). In the intermediate groundwater samples, low-TDS groundwater was primarily of the Ca-HCO3 and Na-HCO3 types, while high-TDS groundwater was usually of the Na-HCO3/Cl and Na-SO4/HCO3 types (Fig. 2). In the deep groundwater samples, low-TDS groundwater was mainly of the Ca/Na-HCO3 type, whereas high-TDS groundwater was of the Na-HCO3 and Na-HCO3/Cl types (Fig. 2). The hydrochemical types of the aquifers at the three depth intervals showed clear similarities.
Significant differences were observed in the spatial distribution of the main anions in the groundwater, primarily HCO3−, SO42−, and Cl−. The content of NO3− varies greatly, from 0.01 mg/L to 298 mg/L, with the highest concentration appearing in shallow groundwater, indicating that it is significantly influenced by human factors. The main cations present were Na+, Ca2+, and Mg2+. The contents of the characteristic components varied inconsistently with depth (Fig. 3), which may indicate that hydrogeochemical processes dominated vertically (Zabala et al., 2016). In general, the Ca2+ and Mg2+ contents varied within the same range at all depths, except for a few brackish water samples; however, the highest concentrations were found in the shallow groundwater samples (Fig. 3a, 3b). At the three groundwater sample depths, the Na+ content showed a decreasing trend with increasing depth (Fig. 3c). The content of HCO3− varied widely (2–1900 mg/L); however, the trend with increasing depth was not evident (Fig. 3d). Higher concentrations of HCO3− were also observed in the intermediate and deep aquifers, which may have been caused by the precipitation of calcite or dolomite (Li et al., 2015). The content of SO42− ranges from 0.6 mg/L to 659.0 mg/L, decreasing with the increase in aquifer depth (Fig. 3e). The content of Cl− varies by three orders of magnitude (2.6–1090.0 mg/L), with no obvious change trend with increasing depth (Fig. 3f).
3.2 Co-occurrence of As and F in the groundwater
The concentration of F and As in the 225 groundwater samples from the study area ranged from 0.16 to 7.99 mg/L and from 1 to 310 µg/L, with average concentrations of 1.33 mg/L and 12 µg/L, respectively. The median concentration of F (1.13 mg/L) was less than its average value, whereas the median As concentration (16 µg/L) exceeded its average, with more outliers for both F and As. Among these, 75 samples (33%) had F concentrations above the drinking water limit (1.5 mg/L), and 45 samples (20%) exceeded the As limit for drinking water (10 µg/L). Co-contamination with As and F was observed in groundwater from different depths. The co-contamination was more severe in shallow and intermediate groundwater, whereas deep groundwater exhibited more serious As contamination than F contamination (Fig. 4a). Contaminated groundwater was primarily found at depths of 12–128 m (Fig. 4b, 4c).
Among the 85 shallow groundwater samples collected, the F concentration ranged from 0.16 to 7.99 mg/L, with an average of 1.62 mg/L. Among these, 35 samples (41.2% of the shallow samples) had F concentrations above the drinking water limit. The concentration of F in 80 intermediate groundwater samples ranged from 0.19 to 5.90 mg/L (the mean value was 1.31 mg/L). Among these, 33 samples (accounting for 57.5% of the intermediate samples) exceeded the F limit for drinking water. In the 60 deep groundwater samples, the F concentration spanned from 0.26 to 6.4 9 mg/L, with an average of 0.94 mg/L. Among these, seven samples (11.7% of the deep samples) had fluoride levels above the potable water limit (Fig. 4b). In general, the F concentration in the groundwater decreased with increasing depth (Fig. 4b).
In the shallow groundwater samples, the As concentration ranged from 1 to 310 µg/L, with an average of 13 µg/L. Among them, 16 samples (18.8% of the shallow samples) had As concentrations greater than 10 µg/L (Fig. 4c). The As concentration in the intermediate groundwater samples varied from 1 to 14 µg/L, with an average of 13 µg/L. Among them, 15 samples (18.8% of the intermediate samples) exceeded the 10 µg/L (Fig. 4c). In the deep groundwater samples, the As concentration spanned from 1 to 104 µg/L, with an average of 12 µg/L. Among these, 14 samples (23.3% of the deep samples) had As concentration above 10 µg/L (Fig. 4c). There were 15 samples (7% of the total) with As concentrations exceeding 50 µg/L, indicating the potential existence of high-arsenic groundwater within a depth of 128 m. High-arsenic groundwater was found in shallow, intermediate, and deep layers and is currently being used for domestic consumption and agricultural irrigation. This issue requires significant attention and should be the focus of further investigations.
The spatial distributions of F in the aquifers at different depths showed similar characteristics (Fig. 5). The upper area of the plain had the lowest F concentrations, whereas higher concentrations were observed in the middle and lower areas of the plain. High F concentrations are concentrated in two areas in the middle of the plain, one near Qian'an County (1.20–7.99 mg/L) in the eastern part of the plain, and the other near Tongyu County (1.30–5.19 mg/L) in the western part of the plain. The highest F concentration was measured in samples collected from shallow groundwater at a depth of 12 m. Overall, the F concentration in the groundwater of the central plain area was higher than that in other parts of the plain. F in shallow groundwater (Fig. 5a) was higher than that in mid-deep groundwater. The F concentration in the intermediate groundwater (Fig. 5b) was higher than that in the deep groundwater (Fig. 5c), but the highest F concentration in the intermediate groundwater was lower than the highest value in the deep groundwater.
The regional distribution of As in the aquifers at different depths is shown in Fig. 6. There was an obvious gradient decrease in As content within the plain, gradually increasing from the surroundings towards the southwestern part of the plain near Taonan-Tongyu. The maximum concentrations of As in the three different depth aquifers were 310 µg/L, 148 µg/L, and 104 µg/L, respectively. These were found in the middle and lower parts of the plain, south of Tongyu County (Fig. 6a), north (Fig. 6b), and west (Fig. 6c). Overall, the groundwater As concentration near Tongyu County in the middle and lower parts of the plain was higher than that in the other parts of the plain. The highest As concentration was measured in the samples collected from shallow groundwater at a depth of 17 m.
On a regional scale, the peak distribution trend of As concentrations in groundwater at different depths (Fig. 6) is generally similar to that of F concentrations (Fig. 5) but does not completely overlap. This suggests that an increase in the concentration of one substance does not necessarily imply a proportional increase in the concentration of another. In the lower part of the plain, south of Tongyu County, there was an abnormal zone of As concentration in shallow groundwater (Fig. 6a), but no abnormal zone of F concentration (Fig. 5a). In the upper part of the plain north of Zhenlai County, abnormal zones of F were found in the shallow and intermediate groundwater (Fig. 5a, 5b), but no abnormal zones of As were found (Fig. 6a, 6b). In the area west of Tongyu County, there was an abnormal zone of As in the deep groundwater (Fig. 6c) but no abnormal zone of F (Fig. 5c).
3.3 Geochemistry of high-F and As groundwater
Groundwater with high F/Cl and As/Cl ratios was found in aquifers at different depths and in large areas of the study area (Fig. 7). This indicated a large-scale enrichment of F and As concentrations in the regional groundwater. High F/Cl and As/Cl ratios indicate that the enrichment of F and As is not related to evapotranspiration (Currell et al., 2011). Some brackish water samples (TDS > 1070 mg/L) in the shallow groundwater had relatively low F/Cl and As/Cl ratios (Fig. 7). This suggests that evapotranspiration plays a role in the enrichment of F and As concentrations. However, this process is limited to local shallow groundwater.
The residence time of groundwater increases with depth in the Songnen Plain. The residence time of shallow groundwater is less than 50 years, that of Quaternary-confined water ranges from modern times to 19,500 years, and that of Neogene-confined water ranges from 3,100 to 24,900 years (Chen et al., 2011). The wide age range of groundwater at different depths, that is, the large time range of concentration enrichment of F and As, suggests that natural processes may be the main reason for the enrichment of F and As in the regional groundwater. The lack of correlation between the F, As, and NO3− concentrations and anthropogenic pollution in groundwater (Fig. 8a, 8b) also indicates that the use of agricultural chemicals is not the main source of F and As in regional-scale groundwater. NO3− pollution in shallow groundwater was severe in the study area, and its regional distribution is shown in Fig. 8c. In general, shallow groundwater samples with higher As and F contents (Fig. 5a, Fig. 6a) had lower NO3− contents (Fig. 8c). As the NO3− content decreases, the F concentration increases, and as the NO3− content increases, the As concentration decreases. The eastern area of the plain had a higher NO3− content, which was mainly related to the overuse of fertilizers and chemicals in agricultural processes (Wang et al., 2023).
The high concentrations of F and As in the groundwater in the study area have significant chemical characteristics. The concentration of Na+ and HCO3− is generally enriched, Ca2+ tends to be depleted, and pH value (> 7.5) is relatively high. The content of F and As in groundwater is positively correlated with pH value: When the content of F (< 1.5 mg/L) and As (< 10 µg/L) is low, the pH value of local deep groundwater changes greatly (8.4–11.6); As the content of F and As gradually increases, the pH value increased with the increase in the content of F and As (Fig. 9a, 9b). This indicates that pH may be an important factor in controlling the migration of F and As. In addition to the local shallow groundwater samples, the contents of F and As in other samples were also positively correlated with the contents of HCO3− and Na+ (Fig. 9c–9f) and negatively correlated with the content of Ca2+ (Fig. 9g, 9h). Other related studies have also found that F and As are positively correlated with HCO3− and Na+, and negatively correlated with Ca2+ (Yan et al., 2020; Currell et al., 2011; Zabala et al., 2016). These data suggest that changes in the content of major ions in groundwater as well as changes in the pH value may be controlled by the water-rock interaction process in the aquifer and have a significant impact on the migration of F and As.