A paleolimnological approach for interpreting aquatic effects monitoring at the Diavik Diamond Mine (Lac de Gras, Northwest Territories, Canada)

Figures & data

Figure 1. Study site locations. Map showing the location of Lac de Gras within the Northwest Territories of Canada (inset A), and the locations of the Diavik Diamond Mine and the 3 sediment coring locations within Lac de Gras (inset B). Yellow stars represent the locations of major population centers in the Northwest Territories, including the city of Yellowknife (northernmost star).

Table 1. Lac de Gras sediment coring sites.

Site	Latitude (°N)	Longitude (°W)	Depth (m)	pH	Specific conductivity (µS/cm)	TN (mg/L)	TP (mg/L)	TDS (mg/L)	Alkalinity (mg/L CaCO3)
NF3	64°30.586'	110°14.741'	19.1	6.6	27.3	0.177	0.003	20.0	8.2
MF 2–3	64°31.562'	110°08.688'	20.7	7.0	26.2	0.128	0.005	17.3	8.8
MF 3–4	64°26.837'	110°19.857'	18.2	7.4	10.6	0.143	0.002	13.3	6.4

Lake parameters and water chemistry data from Lac de Gras sampling sites collected by the Dominion Diamond Corporation Diavik Aquatic Effects Monitoring Program. Data were collected in Aug 2014, with the exception of pH and conductivity, which were collected in Aug 2011. Data are accessible from the Wek’èezhı̀i Land and Water Board public registry.

TN = total dissolved nitrogen; TP = total dissolved phosphorus; TDS = total dissolved solids.

Figure 2. Results of ²¹⁰Pb gamma dating of Lac de Gras cores. Radioisotopic activities for ²¹⁰Pb, ²¹⁴Pb, and ¹³⁷Cs in sediment cores collected from 3 sites (NF3, MF2–3, MF3–4) in Lac de Gras. Chronology and inferred sedimentation rates were determined using the constant flux constant sedimentation model (Appleby and Oldfield 1978).

Figure 3. Temporal trends in sedimentary metal concentrations in Lac de Gras sediment cores. Stratigraphy showing downcore changes in several metals at 3 sites in Lac de Gras along a gradient of impact from diamond mining operations. Concentrations are in µg/g dry weight. The gray box represents the current mining period.

Figure 4. Temporal changes in organic matter composition in Lac de Gras cores. Stratigraphy showing downcore changes in nitrogen stable isotope composition, percent total nitrogen (TN), percent total organic carbon (OC), percent sulfur (S), carbon:nitrogen ratio (C/N), and chlorophyll a inferred using visible reflectance spectroscopy (mg/g dry weight) in sediment cores from Lac de Gras along a gradient of impact from diamond mining operations. The gray box represents the current mining period. The dashed line represents the estimated lower method detection limit for VRS–chlorophyll a (Michelutti et al. 2005).

Table 2. Rates of change in geochemical proxies in Lac de Gras sediment cores.

	Premining rate of change		Postmining rate of change
Element	Percent change/yr	mg/L/yr	Percent change/yr	mg/L/yr
NF3 (near-field)
Mn	2.2 ± 1.6 (0.8–4.1)	56.5 ± 44.6 (−7.0 to 100)	4.0 ± 3.5 (0.0–5.0)	190.4 ± 163.4 (0.0–393)
Sr	1.2 ± 0.6 (0.7–2.2)	0.1 ± 0.1 (0.1–0.2)	1.7 ± 2.3 (0.0–5.0)	0.2 ± 0.3 (0.0–0.7)
Zn	0.6 ± 1.0 (−0.3 to 2.0)	0.4 ± 0.6 (−0.2 to 0.6)	1.5 ± 1.2 (0.0–2.8)	1.2 ± 0.9 (0.0–2.0)
Pb	2.2 ± 1.4 (0.3–4.6)	0.2 ± 0.1 (0.0–0.4)	0.5 ± 1.6 (−1.5 to 1.8)	0.1 ± 0.2 (−0.2 to 0.2)
Chl-a	11.4 ± 20.8 (−4.5 to 44.8)	—	17.8 ± 16.4 (6.5–41.9)	—
MF3–4 (midfield)
Mn	1.0 ± 0.9 (0.2–2.3)	35.0 ± 37.0 (−6.0 to 114)	0.4 ± 0.6 (0.0–0.9)	25.0 ± 36.0 (0.0–51.0)
Sr	0.6 ± 0.3 (0.3–1.0)	0.1 ± 0.0 (0.0–0.1)	0.9 ± 1.3 (0.0–1.8)	0.1 ± 0.2 (0.0–0.3)
Zn	0.2 ± 0.2 (0.0–0.6)	0.1 ± 0.1 (0.0–0.4)	0.4 ± 1.8 (−0.8 to 1.7)	0.3 ± 1.3 (−0.6 to 1.1)
Pb	0.4 ± 0.3 (−0.1 to 0.9)	0.0 ± 0.0 (0.0–0.1)	−0.1 ± 0.9 (−0.8 to 0.5)	0.0 ± 0.1 (−0.1 to 0.1)
Chl-a	4.0 ± 5.7 (−2.1 to 12.2)	—	7.7 ± 11.0 (0.0–15.5)	—
MF2–3 (midfield)
Sr	0.5 ± 2.3 (1.3–0.8)	0.1 ± 0.1 (0.0–0.2)	1.6 ± 1.5 (0.0–3.1)	0.2 ± 0.2 (0.0–0.4)
Pb	0.6 ± 0.3 (0.3–1.1)	0.0 ± 0.0 (0.0–0.1)	0.8 ± 0.4 (0.5–1.3)	0.1 ± 0.0 (0.0–0.1)
Chl-a	11.7 ± 2.8 (−7.6 to 51.0)	—	3.5 ± 9.6 (−3.3 to 10.2)	—

The rate of change in geochemical proxies exhibiting long-term increases, presented as mg/L change per year, and as percent change per year. Data are presented as the mean ± standard deviation, with the range of values in parentheses.

Figure 5. Temporal changes in diatoms at site NF3. Stratigraphic profile of the most abundant diatom taxa (as relative abundances) in a sediment core collected from site NF3 in Lac de Gras, located near the effluent discharge site at the Diavik Diamond Mine. ²¹⁰Pb-inferred dates are shown as a secondary y-axis. Principal components analysis axis 1 and axis 2 scores for diatom assemblages are also shown, as well as the ratio of chrysophyte scales to diatoms, and the ratio of chrysophyte cysts to diatoms. The gray box represents the current mining period.

Figure 6. Temporal changes in diatoms at site MF2–3. Stratigraphic profile of the most abundant diatom taxa (as relative abundances) in a sediment core collected from site MF2–3 in Lac de Gras, located midfield from the Diavik Diamond Mine. ²¹⁰Pb-inferred dates are shown as a secondary y-axis. Principal components analysis axis 1 and axis 2 scores for diatom assemblages are also shown, as well as the ratio of chrysophyte scales to diatoms, and the ratio of chrysophyte cysts to diatoms. The gray box represents the current mining period.

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