Hydrogeochemical evolution and groundwater mineralization of shallow aquifers in the Bas-Saint-Laurent region, Québec, Canada

Figures & data

Figure 1. A, Location of the study area in the Bas-Saint-Laurent region, Québec, Canada, with the geological and hydrogeological context of the study area. B, Schematic drawing of the principal hydrostratigraphic units present along cross section 1–2, from the coastal plain to the highlands.

Figure 2. Piper diagram showing water chemistry based on major ions in the granular and bedrock aquifers. The four geochemical groups identified by hierarchical cluster analysis (HCA) are reported. Characteristics of seawater (X) are presented in the diagram.

Table 1. Contribution of the different geochemical facies as revealed by the Piper diagram.

Geochemical facies	No. of samples	Contribution (%)
Ca-HCO3	96	66
Na-HCO3	29	20
Na-Cl	10	7
Ca-Cl	6	4
Mx cation-HCO3	2	1
Mx cation-SO4	1	< 1
Mx cation-anion	1	< 1
Total	145	100

Table 2. Mean geochemical and physical parameter values characterizing the four geochemical groups identified by hierarchical cluster analysis (HCA).

Group	C1^*	C2	C3	C4
No. of samples	44	40	16	42
HCO3 (mg/L)	284.0 ± 76.8	159 ± 49	319.6 ± 156.9	133.0 ± 74.1
TAlk(mgCaCO3/ L)	232.5 ± 62	130.7 ± 41	259.2 ± 136	108.0 ± 61
Ba^2+ (mg/L)	0.02 ± 0.2	0.10 ± 0.1	0.5 ± 1.0	0.0
Ca^2+ (mg/L)	78.6 ± 33.6	42.3 ± 17.1	6.3 ± 6.4	37.8 ± 24.0
Cl^- (mg/L)	117 ± 313	22.3 ± 19.9	172.9 ± 267.2	5.6 ± 5.0
Na+ (mg/L)	91.1 ± 207.1	23.9 ± 22.1	236.4 ± 191.6	7.0 ± 5.4
Mg^2+ (mg/L)	16 ± 15	7.1 ± 2.5	4.2 ± 7.4	6.0 ± 3.9
Mn^2+ (mg/L)	0.1 ± 0.4	0.3 ± 0.6	0.0	0.0
K^+ (mg/L)	6.5 ± 11.6	4.0 ± 5.8	5.0 ± 3.7	1.3 ± 0.8
Si^2+ (mg/L)	5.1 ± 9.2	4.2 ± 1.2	2.9 ± 1.4	3.4 ± 1.1
Sr^2+ (mg/L)	0.7 ± 0.7	0.4 ± 0.3	0.1 ± 0.1	0.2 ± 0.1
SO4^2- (mg/L)	35.3 ± 47.6	21.9 ± 10.7	42.5 ± 40.8	10.6 ± 8.8
pH	7.4 ± 0.5	7.3 ± 0.7	8.8 ± 0.6	7.2 ± 0.6
TDS (mg/L)	641 ± 585	315 ± 93.1	792 ± 545	210.9 ± 98.5
Depth (m)	28 ± 22	23 ± 18	50 ± 20	21.2 ± 17.1
Geochemical facies	Ca-HCO3	Ca-HCO3	Na-HCO3	Ca-HCO3
δ^2H ‰^**	−84.6 ± 8.2	−83.8 ± 7.2	na	−85.5 ± 7.8
δ^18O ‰^**	−12.1 ± 1.1	−12.3 ± 0.8	na	−12.3 ± 1
δ^13C ‰^***	−19.7 ± 4.3	−21.4 ± 4.7	na	−17.5 ± 11
Confinement	unconfined to confined	unconfined	semi-confined toconfined	unconfined
Hydrogeological context	highlands	highlands	coastal plain	highlands

Note:

na: no data available.

* Mean values calculated by including the most salinized sample located on the St. Lawrence shore.

** Only 123 samples were analyzed.

*** Only 34 groundwater samples were analyzed.

Figure 3. Locations of the four geochemical water groups based on 11 parameters as revealed by a hierarchical cluster analysis (HCA). The two main hydrological contexts (the coastal plain and the highlands) and the degree of confinement of Bas-Saint-Laurent (BSL) aquifers are also reported.

Figure 4. Distribution of δ¹⁸O and δ²H (V-SMOW in ‰) in groundwater facies. The dotted line represents the line linking the stable isotopes of solid and liquid precipitation, and the black line is the linear regression of groundwater samples. The mean values of δ¹⁸O and δ²H are also reported for the regional rain and snow precipitation and seawater (data from Buffin-Bélanger et al. 2015 and Chaillou et al. in press).

Figure 5. Relationships between different ions and Cl content (in mg/L) in Bas-Saint-Laurent (BSL) groundwater. The grey lines represent the theoretical mixing line between fresh groundwater and seawater for Na (A), Mg (B), Ca (C), and NO₃ (D). The high-mineralized sample is reported in parentheses.

Figure 6. A, Sodium concentrations; B, bromide to chloride; and C, sulphate to chloride mass ratios reported as a function of chloride concentrations. Marine mass ratios are also reported. The high-mineralized sample is not reported.

Figure 7. Processes related to water–rock interaction reported for the four water groups: Ca (in meq/L) (A) and pH (B) in relation to Na (in meq/L). The high-mineralized sample is not reported.

Figure 8. A, Calcium content (in meq/L) in relation to bicarbonate (HCO₃ in meq/L). B, Sum of major cations (Ca + Mg in meq/L) in relation to bicarbonate (HCO₃ in meq/L). C, Sum of major ions (Ca + Mg − (HCO₃+SO₄) in meq/L) in relation to Na-Cl (in meq/L). The high-mineralized sample is not reported.

Figure 9. Saturation indexes (SI) of carbonate minerals. A, SI of calcite in relation to pH; and B, relationship between calcite and dolomite SI. The high-mineralized sample is not reported.

Figure 10. Conceptual model of the hydrogeochemical evolution of groundwater in the Bas-Saint-Laurent (BSL) bedrock aquifers along cross section 1–2 (see location in Figure 1A and geological details in Figure 1B). Concentrations of major anions (A) and cations (B) along the section (concentrations are reported in mg/L). C, Geology, hydrological settings, and expected groundwater flow paths from the recharge area to the coastal discharge zone. The water groups are reported as a function of the ion concentrations.

Buffin-Bélanger, T., G. Chaillou, C-A. Cloutier, M. Touchette, B. Hétu, and R. McCormack. 2015. Programme d’acquisition de connaissance sur les eaux souterraines du nord-est du Bas-Saint-Laurent (PACES-NEBSL): Rapport final, 199. UQAR, Rimouski, Qc, Canada.

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Chaillou, G., F. Lemay-Borduas, M. Larocque, and M. Couturier. in press. Flow and discharge of groundwater from a snowmelt-affected sandy beach. Submitted to Journal of Hydrology.

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