Stratigraphic sequence map for groundwater assessment and protection of unconsolidated aquifers: A case example in the Abitibi-Témiscamingue region, Québec, Canada

Figures & data

Figure 1. Study area. (a) Bedrock geology based on a compilation of geological maps at the 1:20,000 scale from the government of Québec, Système d'information géominière [SIGEOM] from the Ministère de l'Énergie et des Ressources naturelles du Québec [MERN]) (b) Surficial geology adapted from maps produced by the Geological Survey of Canada (Veillette 2004; Thibaudeau and Veillette, 2005; Paradis 2005, 2007; Veillette, 1986a, 1986b, 1987a, 1987b, 1987c); (c) elevation and continental water divide; (d) stratigraphic boreholes from geological and hydrogeological government databases.

Figure 2. Regional stratigraphic column (unconsolidated sediments) and associated simplifications used in the stratigraphic sequence map. For simplification purposes, the till unit is considered here to be composed of silt, sand and gravel.

Figure 3. Conceptual representation of the approach for constructing the stratigraphic sequence map.

Figure 4. Average range of surface elevation (referring to surface roughness, SR) for groups of geological units within predefined diameters.

Table 1. Average range (in meters) of surface elevation (surface roughness, SR) and associated standard deviations (σ) within a predefined circle diameter (distance) for groups of geological units.

Distance (m)	Group 1			Group 2			Group 3
	SR	σ	SR+2σ	SR	σ	SR+2σ	SR	σ	SR+2σ
30	1.4	1.4	4.26	0.6	0.8	2.1	0.55	0.71	1.97
50	2.8	2.8	8.32	1.2	1.4	4.1	1.07	1.33	3.73
90	5.7	5.2	16.11	2.4	2.7	7.7	2.11	2.37	6.85
250	16.1	11.6	39.34	6.7	5.9	18.6	5.41	4.41	14.23
490	28.4	17.2	62.73	12.9	9.3	31.4	9.12	5.67	20.46

Group 1: Bedrock outcrops and shallow till patches; Group 2: Glaciofluvial, sublittoral and deltaic deposits; Group 3: Fine-grained deep-water glaciolacustrine sediments.

Table 2. Data sources and number of boreholes (n) for evaluating unconsolidated sediment thickness (UST).

Data types	Data sources	n	Preliminary GIS data treatments
Bedrock outcrops	- Government database^1	132,651	A value of zero is attributed to overburden thickness for each point-specific bedrock outcrop. In the case of bedrock/thin till outcrops defining polygons on the GSC maps, points of zero overburden thickness are created at vertices of the contour and in the center of corresponding 100 m × 100 m cells.
	- Point-specific outcrops from GSC maps^2	27,912
	- Surface and contours of polygons corresponding to bedrock and shallow till outcrops in the GSC maps^2	717,345
Boreholes	- Wells from the Hydrogeological Information System (HIS) government database^3	3918	A systematic approach is applied in order to subtract inconsistent data: - Boreholes identified within bedrock/thin till outcrops of the GSC maps; - Boreholes revealing a zero unconsolidated sediment thickness; - Duplicate data; - Boreholes suggesting unrealistic unconsolidated thicknesses (i.e. >100 m); The position (x, y, z) of data are corrected based on borehole dip and direction.
	- Mining diamond drillings^1	30,195
	- Overburden drilling management (ODM) from government database^1	1282

¹ Ministère de l’Énergie et des Ressources naturelles du Québec.

² Geological Survey of Canada.

³ Ministère du Développement durable, de l’Environnement et de la Lutte contre les changements climatiques du Québec.

Figure 5. GIS-based procedures for evaluating (a) the unconsolidated sediment thickness (UST) and (b) the fine-grained glaciolacustrine unit thickness (FGGUT). DNBO: distance to nearest bedrock outcrop; MF: multiplication factor; MUST: maximum unconsolidated sediment thickness; DEM: digital elevation model; DNL: distance to nearest limit of the clay plain.

Figure 6. Regional trend of average unconsolidated sediment thickness (UST, solid line) vs. distance to nearest bedrock outcrop (DNBO) and maximum plausible unconsolidated sediment thickness (MUST) equal to the average UST, plus surface roughness of bedrock (SR), plus twice the standard deviation on the SR ( in m), both with reference to the left-side y-axis. The number of available borehole data (dashed line) with respect to each class of DNBO is shown, with reference to the right-side y-axis.

Table 3. Data sources and number of boreholes (n) for evaluating the fine-grained glaciolacustrine unit thickness (FGGUT).

Data types	Data sources	n	Preliminary GIS data treatments
Stratigraphic drilling	- Wells from Hydrogeological Information System (HIS) government database^1	3061	Selection of boreholes for which (1) the FGGU is identified as the dominant facies for at least one layer and (2) the total unconsolidated sediment thickness is known. Calculation of the total FGGUT for each of the boreholes selected above. Suppression of duplicate data.
	- Overburden drilling management (ODM) from government database^2	989
	- Technical reports and work by Cloutier et al. (2013, 2015)	199

¹ Ministère de l’Énergie et des Ressources naturelles du Québec.

² Ministère du Développement durable, de l’Environnement et de la Lutte contre les changements climatiques du Québec.

Figure 7. Conceptual GIS-based method to define the reference surface delineating the maximum elevation of the fine-grained glaciolacustrine unit (FGGU). The surface is obtained by interpolating the highest elevations reached by the FGGU.

Figure 8. Regional trend of average fine-grained glaciolacustrine unit thickness (FGGUT, solid line) vs. distance to the nearest limit of reference plan (DNL), with reference to the left-side y-axis. The number of available borehole data (dashed line) for each class of DNL is shown, with reference to the right-side y-axis.

Table 4. Description of the criteria for defining the 15 regional stratigraphic sequences.

SS#	Criteria for defining the stratigraphic sequences
	Surface deposits (based on GSC maps)	UST (m)	FGGUT (m)	UST − FGGUT (m)	Legend scheme
1	Bedrock and shallow till	= 0	= 0	= 0
2	Till	> 0	= 0	> 0
3	Glaciofluvial	> 0	= 0	> 0
4	Glaciolacustrine deep-water	> 0	> 0	> 0
5	Glaciolacustrine deep-water	> 0	> 0	= 0
6	Glaciolacustrine littoral and deltaic	> 0	> 0	≥ 5
7	Glaciolacustrine littoral and deltaic	> 0	> 0	< 5
8	Glaciolacustrine littoral and deltaic	> 0	= 0	> 0
9	Organic	> 0	> 0	> 0
10	Organic	> 0	> 0	= 0
11	Organic	≥ 5	= 0	≥ 5
12	Organic	< 5	= 0	< 5
13	Alluvium	> 0	> 0	> 0
14	Alluvium	> 0	> 0	= 0
15	Alluvium	> 0	= 0	> 0

SS#: Stratigraphic sequence number; GSC: geological survey of Canada; UST: unconsolidated sediment thickness; FGGUT: fine-grained glaciolacustrine unit thickness; UST − FGGUT: difference between UST and FGGUT values.

Figure 9. Main regional-scale outputs of the GIS-based calculations. (a) Unconsolidated sediment thickness (UST); (b) fine-grained glaciolacustrine unit thickness (FGGUT); (c) stratigraphic sequence map; (d) calculated aquifer thicknesses. The black rectangle corresponds to the area covered in Figure 10 which provides local-scale data for a sector located west of the city of Amos.

Figure 10. Local-scale illustration of the consistency between the maps and datasets supporting the construction of the stratigraphic sequence map. (a) Unconsolidated sediment thickness (UST). (b) Fine-grained glaciolacustrine unit thickness (FGGUT). (c) Surface geology map. (d) A–A` cross section showing the bedrock (red) and overlying coarse-grained (orange) and fine-grained (purple) sediments. The numbers reported above the cross section correspond to the different stratigraphic sequences, as identified in Table 4. (e) Corresponding local-scale stratigraphic sequence map with numbers corresponding to the different stratigraphic sequences identified in Table 4.

Table 5. Descriptive statistics and volume estimates in relation to the 15 regional stratigraphic sequences.

Stratigrpahic sequence	Area (km^2)	% of study area ^i	Elevation					UST (m)				FGGUT					Confined granular aquifers					Unconfined granular sediments				Unconfined granular aquifer
			Min (m)	Max (m)	Range (m)	Mean (m)	SD (m)	Max (m)	Mean (m)	SD (m)	Volume (km^3)		Max (m)	Mean (m)	SD (m)	Volume (km^3)	Max (m)	Mean (m)	SD (m)	Volume (km^3)	Groundwater volume ii (km^3)	Max (m)	Mean (m)	SD (m)	Sediment volume (km^3)	Max (m)	Mean (m)	SD (m)	Volume (km^3)	Groundwater volume ii (km^3)
0	1774
1	3101	17.6	180	567	387	315	34
2	1313	7.5	176	556	380	333	32	67	8	7	10											67	10	6	11
3	808	4.6	175	426	251	326	29	91	23	13	18											91	23	13	19	90	15	10	6	2
4	4844	27.5	180	381	201	296	22	87	17	7	80		66	10	5	49	71	7	6	32	11
5	2469	14.0	180	377	197	293	25	64	7	6	17		64	7	6	17
6	361	2.1	180	366	186	304	31	78	23	10	8		65	9	6	3	63	8	8	3	1
7	351	2.0	180	364	184	285	41	69	13	9	4		65	11	9	4
8	1184	6.7	175	428	253	328	22	74	12	9	14											74	13	8	14	66	11	8	6	2
9	1319	7.5	180	370	190	303	18	83	17	7	23		58	10	4	13	64	8	6	10	4
10	366	2.1	180	357	178	297	23	59	11	7	4		59	11	7	4
11	905	5.1	229	428	199	324	13	56	15	7	13											56	15	7	13	56	11	7	8	3
12	234	1.3	185	439	255	314	21	5	1	2	-											5	4	1	-	5	2	1	-	-
13	234	1.3	180	346	166	294	23	64	17	8	4		47	10	5	2	57	7	6	2	1
14	83	0.5	180	335	154	291	24	55	10	7	1		55	10	7	1
15	53	0.3	183	457	274	313	25	42	6	7	-											42	11	6	-	38	9	7	-	-
Total	19,397	100	175	567	392			91	11		198		66	9		93	71	7		46	16	91	15		57	90	7		21	8

ⁱ The areas corresponding to lakes are excluded.

ⁱⁱ Groundwater volumes are calculated assuming a porosity of 35%. UST: Unconsolidated sediment thickness. FGGUT: Fine-grained glaciolacustrine unit thickness. SD: Standard deviation.

Figure 11. Average unconsolidated sediment thickness (UST, solid line) and glaciofluvial sediment thickness (dashed line) against bedrock surface elevation. The Cadillac Fault, separating the Abitibi (north) and Pontiac (south) geological sub-provinces and roughly corresponding to the position of the water divide between lakes Ojibway (north) and Barlow (south), is used for partitioning the data.

Figure 12. Conceptual representation of regional distribution of stratigraphic sequences and their position among regional flow paths. The stratigraphic sequences are depicted for illustrating their general distribution and elevation within the regional hydrogeologic environment and the numbers correspond to those found in Tables 4 and 5. The conceptual model suggests increasing unconsolidated sediment thickness (UST) toward the right side of the cross section.

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