Electrical Resistivity Imaging of a Brine Plume under the Margin of a Sodium Sulphate Lake 1

E.P. Zimmerman 1, L. R. Bentley 1, M Hayashi 1, L.l. Kelley. and C. Holmden 3

Zimmerman, E.P., Bcntky, L.R .. Hayashi. M .. Kelley, L.I. , and I Iolmden, C. (2000): Electrical resistivity imaging of a brine plume under the marg in of a sodium sulphate lake; in Summary oflnvestigations 2000. Volume 2. Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2000-4.2.

Abstract

Electrical resistivity tomography (ERT) is a modem technique for imaging two-dimensional resistivity cross-sections in areas with complex subsurface geology. We used ERT to investigale the interface between relatively f resh groundwater and dense lake brine at the margins of a· modern lacustrine evaporile deposit of sodium sulphate in west-central Saskatchewan. Six ERT prqfiles were oriented perpendicular to the shoreline of l y dden Lake and three parallel to the shore. A nomalously low resist ivity is (.,-'Vident at depths of 5 to 25 m along the lake shore. We interpret the source qf the anomalous~v low resistivity to be dense lake brine seeping.from the lake bed under the influence qf gravity. The =ones of anomalously low resistivity intrude over 60 m inshore in areas where the lakeshore slopes gently, and a much shorter distance in areas where the shoreline is f lanked by clifj~·- The difference in length of the brine p lume is interpreted to he related to a lower hydraulic gradient in areas where the shoreline slopes gently

I . Introduction

Saline lakes are common hydrogeolog ical features in a rid and semi-arid c limatic zones. In most saline lakes in the prairie region, lake water contains sulphates o f Na and Mg (Salama et al. , 1999) . The tota l disso lved solids (T DS) content in some of these lakes exceeds 200,000 ppm. High concentration of dissolved sa lts leads to the formation of crysta lline salt beds, which are mined for Na2 S04 - a valuable industrial min era I. Fro m the hydrogeological viewpo int, sodium sulphate lakes are the focus of groundwater discharge and the high TDS concentration is the result of evaporation . Subsurface resistivity imaging provides ins ight into the interaction between saline and fresh waters on the marg ins of the lake. The study of these processes is essent ia l to our understanding of the genesis of evaporite deposits in the saline lakes.

2. Site Description and Methodology

a) Lydden Lake Site

Lydden Lake is situated 35 km west of Biggar, Saskatchewan (Figure I). The area is underlain by approximately I 00 m of unconsolidated glac ia l. glac iofluvial. and glac iolacustrine deposits. Beneath the glacial drift, Mesozoic and Paleozo ic rocks form a sequence of nearly horizonta l sedimentary layers approaching 2000 m in th ickness (Macdonald and Slimmon, 1999). Like most of the Na2S04-producing lakes of the reg ion, Lydden Lake is in a narrow river­like valley (Last and Slezak, 1987). It is approx imate ly 5000 m long and 300 to 500 m wide. Most of the shoreline is marked by 5 to 15 m high cl iffs, but on the southern shore, a bay with a very gentle slope is present. Lydden Lake is underla in by a well-developed crysta lline salt bed, estimated to contain 800,000 short tons (700 000 tonnes) of anhydrous sodium and magnesium sulphates. In mid-summer, the lake brine reaches a maximum concentration of200,000 ppm TDS, wi th Na ·, SO/ . and Mg~· be ing the dominant ion species.

b) Electrical Resistivity Tomography

Electrical resist ivity tomography (ERT) is a modern technique for imaging two-dimens ional res ist iv ity cross-sect ions in areas with complex subsurface geology (Lokc and Barker. 1996). The electrical current is injected through two current e lectrodes and the resulting voltage d ifference is then measured at two potentia l e lectrodes. The depth of penetration (and therefore imaging) depends on the spacing between the electrodes and the type of the array. For th is investigat ion we used the Wenner array, wh ich resulted in the least amount o f noise wi th the deepest penetration. From the current (I) and voltage (V) values. the apparent resistiv ity (p.) is calcu lated by:

p, = kV I I (I )

where k (m) is a geometric factor, wh ich depends on the a rrangement and spacing of the electrodes (Loke, 1999). The ERT unit consists of a large number of e lectrodes (in o ur case up to 56) connected to a single cable and set out at a constant spacing.

I Partially funded by the Saskatchewan St rategic Initiati ves Fund. 1 lJnivcrsity o f Calgary. Department of Geology and Geophysics. 2~00 University Dri ve N W. Calgary. AH T2N 1N4 . ' Departme nt of Geological Sciences. University of Saskatchewan. I I 4 Scicncc !'lace. Saskatoon. SK S 7N 5 E2 .

218 Summary ollnvestigatio11s 2000. l'o/ume 2

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Figure I - Location of the study area.

The cable is la id out in a straight line and a computer­controlled system is then used to automatically select four act ive electrodes for each measurement ( individual array). Measurements are then taken and stored in the computer. The m inimum distance between neighbouring electrodes in an individual array is the set-up spacing. As the d istance between neighbouring electrodes increases, the number of ind ividual arrays that can be used decreases and so does the number of measurements at a given depth level. Therefore the interpretation is most accurate for the top half of the profile (Loke, 1999).

The data obtained is commonly arranged and contoured in the form of a pseudo-section. The relatio n between the true resistivity of the subsurface and the apparent resistiv ity is complex. An inversion algorithm ( Loke and Barker, 1996) was used to est imate the true soil resistiv ity from the apparent resistivity values and to produce a model of resistivity cross-section.

3. Results

a) General

Six ERT profiles were run perpendicular to the shoreline of the Lydden Lake and three were oriented paralle l to the shore (figure 2).

Saskatchewan Geo/01,u·cal S11n·ey

To characterize the marginal processes, the ERT profi les were carried out on both steep and gentle shores ofLydden Lake. Four lines ( 12, 13, 14 , and 15) were run where the shoreline was marked by 5 to 10 m highcliffs andfive(lines 1, 2,9, 10, and l l )wererun from a bay on the southern shore that had a gentle slope (Figure 2). The elevation of the bay ' s slope gradually increases inland from 663 to 665 m. The shoreline extends for about 500 m with in th is bay (Figure 2).

The depth of imaging reached 40 to 50 m, with electrodes spaced between 4 and 6 m . The images obtained were of good quality. The amount of noise increased for the profiles run across complicated topographical features (e.g. cliffs, valleys).

The images from the profiles run perpendicular to the shoreline showed an area o f lower resistiv ity starting from the lake ·s marg in and extending inland in some cases for more than 60 m.

h) Response Along Gently-sloping Shoreline

On line I, the resistiv ity values near the shore and at S to 25 m depth are consistently below 3 Q -m wh ile the background resistivity values exceed 20 !.1-m . We interpret the source of these anomalies to be brine intrusions from the lake. The brine plume has a tongue shape in long itudinal section and significant inland

219

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- - .J Om I OOm 200m JOOm 400m N

A Shoreline cliff

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LYDDEN LAKE

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1 ! / / / / ,, -~ ~ddtn L~ke i

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Figure 2 - The ERT survey scheme.

extension in the case of line I where the shore slopes ge.ntly (Figure 3). Lines 2 and 9 (not shown) were 0!1ented su~-parallel to line I and y ie lded images s1m1lar to F1gur~ 3. The area of lower resistivity on top of the plume ts interpreted as groundwater discharge having much lower salt content than the lake wate r.

The plume occurs between 660 and 640 m and the water level in Lydden Lake is at 663 m. A layer of lower hydrau lic conductivity may control the location of the bottom of the plume. A single drill ho le completed east o f the study area penetrated fine sand b~tween 5 and_ 15 m below surface, rough ly co inc ident wtth the elevatio n o f the anomalously low resistivity.

The sand is underlain by clay­rich glacial till.

Line 1: Subsurface Resistivity, Ohm-m ~i~e 10+ +une 11

The tongue-like shape of the brine plume is evidently three­dimensional , based on pseudo­sections oriented parallel to the shore and perpendicular to line I. Figure 4 shows the image o bta ined a long line 10.

0 60 80 100 120 140 lliO 180 200

DistBe,m Figure 3 - ERT p1·eudo-section for line I.

220

The cross-section of brine plume on line 11 (not shown), that was run farther inland than line IO is smaller, thus confirming that the plume thins away from the lake.

Summary of Investigations 2000, J "ofume 2

Line 10: Subsurface Resistivityt Obm-m Line 1 + Line 9+

100 1;o 200

Diim,m Figure 4 - ERT pseudo-section for line I 0.

0 50 100 150 200

Dialce,m Figure 5 - ERT pseudo-section for line 12.

w

w c) Response Along Shoreline Cliffs

Where a cliff forms the lakeshore, the plume becomes shorter, and appears not to advance inland beyond the cliff break (Figure 5).

Line 14 was run parallel to the shore on the bluff of the cliff (Figure 6). This image does not have an area of lower resistivity, which is consistent with the conclusions that the plume does not advance past the top of the bluff on the shores formed by a cliff (Figure 5).

NE 4. Discussion

300

E

A shallow saline-freshwater interface was described along the margins of the Dead Sea in Quaternary deltaic sandy gravel overly ing upper cretaceous carbonates (Yechieli, 2000). Our study suggests that a similar process of saltwater intrusion is occurring on the margins of a significantly smaller saline lake. The tomography at Lydden Lake is interpreted to indicate two water bodies under the margins of the lake. The uppermost is relatively fresh groundwater, flowing into the lake from the watershed. The hydraulic head decreases towards the lake that is the focus of this groundwater discharge. Beneath the freshwater

Line 14: Subsurface Resistivity, Ohm-m

is a body of saline groundwater. This denser brine seeps from the lake bed and pushes inshore in the opposite direction to the flow of relatively fresh groundwater. On the margins of Lydden Lake, the hydraulic gradient is lower under the gentle shore flanking the bay and the brine plume extends farther inshore. In contrast, where the lake is flanked by cliffs, the hydraulic gradient is steeper and the brine plume does not extend far inland.

0 50 100 150

Ditm,m Figure 6 - £RT pseudo-section for line 14.

Saskatchewan GeoloKicaf Survey

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5. Conclusions

Electrical resistivity tomography was successfully used to image subsurface resistivity under the margins of a sodium sulphate lake.

221

Drilling and groundwater sampling are planned to quantify the groundwater salinity variations presumed to be the cause of the low-resistivity plumes imaged in the ERT survey.

6. References Last, W.M. and Slezak, L.A. ( 1987): Sodium sulphate

deposits of western Canada: Geology, mineralogy, and origin; in Gilboy, C.F. and Vigrass, L.W. (eds.), Sask. Geo!. Soc ., Spec. Pub!. No. 8, pl 97-205.

Loke, M.H. ( 1999): Electrical imaging surveys for environmental and engineering studies, a practical guide to 2-D and 3-D surveys; http://www.agiusa. com/literature.shtml, accessed 10 Feb 2000.

Loke, M.H. and Barker, D. (1996): Rapid least­squares inversion of apparent resistivity pseudosections by a quasi-Newton method; Geophys. Prospect., v44, p 131-152.

Macdonald, R. and Slimmon, W.L. (compilers) (1999): Geological Map of Saskatchewan; Sask. Energy Mines, I : I 000 000 scale.

Salama, R.B., Otto, C.J. , and Fitzpatrick, R.W. (1999): Contribution of groundwater conditions to soil and water salinization; Hydrogeol. J. , v7, p46-64.

Yechieli , Y. (2000): Fresh-saline groundwater interface in the Western Dead Sea area; Ground Water, v38, no4, p615-623.

222 Summary of Investigations ]()()(), l 'olume 2