The use of hydrochemical sections to identify recharge areas and saline intrusions in alluvial aquifers, southeast Queensland, Australia

Journal of Hydrology, ! 35 (1992) 259-274

259

Elsevier Science Publishers B.V., Amsterdam

[1}

The use of hydrochemical sections to identify recharge areas and saline intrusions in alluvial aquifers, southeast Queensland, Australia William Dixon a and Barry Chiswell b :Department of Geology of Mineralogy, Universityof Queensland, Qld. 4072, Australia bDepartment of Chemistry. Universityof Queensland, QId. 4072, Australia (Received 24 October 1990; revision a :¢epted 29 September 1991)

ABSTRACT Dixon, W. and Chiswell, B., 1992. The use of hydrochemical sections to identify rechar2e areas and saline intrusions in alluvial aquifers, southeast Queensland, Australia. J. Hydrol., 135: 259-274. Alluvial ground waters used for irrigation in the southwest Lockyer Valley, Queensland, experience varying sali~ity p~oblem~. Samples were taken on three occasions along transects in the valleys of the Tenthill and Ma Ma creeks during the dry season (August 1984), the wet season (February 1985), and during a period of drought (January 1987). The waters are categorized by chemical type, and hydrochemical sections are plotted for major cation data (magnesium, calcium and sodium). The interpretation of the sections permits the identification of recharge areas for the alluvial aquilers, and the location of saline intrusions into the alluvium from underlying sandstone aquifers, it is also possible, with the assistance of chemical type, to correlate some saline intrusions in adjacent valleys as issuing from the same sandstone aquifers. A diagram to summarize the locations and magnitudes of salinity changes along transects is given. INTRODUCTION

The Lockyer Valley in southeast Queensland is a rural area, whose economy is dominated by the growing of crops; in particular the area produces over a third of Queensland's vegetables. The average rainfall in the Lockyer region of 820 mm year-i is insufficient to sustain crop yields without spray irrigation, which is largely supplied from boreholes sunk iato Quaternary alluvium along cceek banks and the valley floor. From approxiraately 500 bores using an average of 47.5 x 106 m 3 of ground water each year, 12 000 ha of farm land are irrigated, although the safe withdrawal rate from the alluvium has been estimated at only 25 x 10 6 m 3year-I (Queensland Water Resources Commission (QWRC), 1982). Correspondence to: W. Dixon, Department of Geology of Mineralogy, University of Queensland, Qld. 4072, Australia.

260

W. DIXON AND B. CHISWELL

The salinity of alluvial ground wa,ers varies markedly over the Lockyer Valley catchment, and in places reaches levels unsuitable for irrigation; the deeper aqrifers in Marburg Formation sandstones also yield saline waters. The evaporation rate in the area is double that of the rainfall, and it would be expected that the use of saline ground waters for irrigation without sufficient flushing should lead to increase in soil salinity, with a detrimental effect on crop yields. This study examines hydrochemical sections plotted for major cation data of ground waters from two tributary creeks, the Tenthill and Ma Ma creeks, which lie adjacent in the southwest Lockyer Valley, to identify recharge areas, and sources of saline intrusions into the alluvial aquifers. It forms part of a broader project (Dixon, 1989) undertaken to examine the hydrochemistry of groundwater salinity in these catchments.

DESCRIPTION OF THE STUDY AREA

Geology and topography The study area, whose geological features have been described by Grimes (1968), and are shown in Fig. l, lies in the Moreton basin. The valleys of the Tenthill and Ma Ma creeks iie between steep sided, but rounded hills of Jurassic Marburg Formation sandstones. These hills rise in the east to the Mistake Mountains, and in the south and west to the Great Dividing Range, where the Marburg sandstones ar,'~ overlain by the Walloon Coal Measures, and are finally capped by Tertiary basal#.~. The valleys contain Quaternary alluvial deposits, up to 30 m deep and I km wide, which consist of many layers of sated, gravel, clay, and stones. Superficially, the alluvium has a low gradient and ~ fertile soil containing smectite clays. The alluvial aquifer volumes are very different for the two creeks, with Tenthill Creek having an aquifer volume of 182930 x 106m3, and Ma Ma Creek only 19790 x 10t'm3: the volumes of available water are 17720 x 106ma, and 1780 x 106m3, respectively. The creeks have incised channels up to 5 m deep in the alluvium, and stream water contributes to the recharge of alluvial ground waters by percolation through the sides and bottom of the channels. A recharge weir, shown in Fig. 2, has been constructed by the QWRC on each of the creeks to assist this process. Alluvial aquifer recharge could also be derived directly from rain, from hillside runoff, and from ~eepage of saline ground water from underlying and adiacent sandstones.

ALLUVIALAQUII'ERS IN SOUTHEASTQUEENSLAND

261

Fig. !. Geology of the study area.

Climate The climate of the region is sub~ropical, and the bulk of the annual rainfall occurs during the summer months. However, over the period of the sampling programme rainfalls were light, and even during the summer of 1985 stream flows were low, which diminished recharge to the alluviual aquife~'s During February 1987, when samples for this study were taken, the drought was so great that both Tenthill and Ma Ma creeks had ceased to flow at all. Farmers

W. DIXON AND B. CHISWELL

262

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kilometers

1

Fig. 2. Topographic locations q~f sampling sites and transect lines.

ALLUVIALAQUIFERSIN SOUTHEASTQUEENSLAND

263

TABLE 1 G r o u n d w a t e r salinity hazard classification Salinity hazard

Concentration o f chloride ion (mEq I - i )

Low Medium High Extreme

< 10 10-20 20-36 > 36

experienced then a critical lowering of groundwater levels, and some bores could not draw any water.

Hydrochemistry of ground waters The following notes on the hydrochemistry of the study area are derived from Dixon (1989). Both the Tenthill and Ma Ma creeks flow from south to north over very similar geology (see Fig. 1), yet they exhibit pronounced differences in groundwater quality. Roberts and Talbot (1980) found the following levels of salinity hazard, as defined in Table 1 (QWRC, 1982): (1) for the Tenthill Creek catchment, 38% of alluvial ground waters had low hazard, 60% had medium hazard, and 2% had high salinity hazard; while (2) for the Ma Ma Creek catchment 15% had high salinity hazard, and 85% extreme hazard. Ground waters obteined directly from sandstone aquifers (Dixon, 1989) were of high or extreme salinity hazard. The c~n~apositions of the major ions in waters sampled in 1987 from sites marked on Fig. 2, are presented graphically (Piper, 1944) in Fig. 3. It can be seen that chloride is the dominant anion in all the ground waters of the study area, and all the samples analysed could be typed by the Durov system of classification (Chillingar, 1956) as Class IV (chloride) waters, Comparisons, however, of the levels of the three major cations, magnesium, sodium, and calcium (as mEq 1-l ) showed a linear trend in Fig. 3. The waters were then classified as three types, (see Table 2) where: Mg > Ca > Na = typeI Mg > Na > Ca = t y p e l I Na > Mg > Ca = t y p e l I I Of alluvial ground waters type I were: most common, and type III waters

264

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Fig. 3. Trilinear (Piper) diagram of groundwater and slream-water compositions in the study area (from Dixon, 1989).

were least common. There were no calcium-dominant waters, and the series represents a progressive rise in the proportion of sodium. Sampling sites S l, $2, and $4, drawing from depths of approximately 50 m in sandstone aquifers, yielded type III waters, whereas sit(,' $3, drawing from sandstone at a shallow depth near a spring, produced type II water. H Y D R O C H E M I C A L SECTIONS

It is a common practice in hydrochemical studies to plot contour diagrams for relevant parameters in order to identify sa!ient features, such as recharge areas, ar, d sites of polhltion. These diagrams are particularly useful when data

ALLUVIALAQUIFERSIN SOUTHEASTQUEENSLAND

265

TABI_E 2 Chloride data for groundwater samples Dry season (1984)

Wet season (1985)

Drought (1987)

S:te

CI (mEq 1 - t )

Site

CI (mEq I - t )

Site

CI (mEq 1- t )

M2 M3A M4 M5 M6 M8 Tl T2 T3 T4 T5 T6 T8 T9 TII TI3 T14 S! $2 $3

29.5 22.7 31.6 32.9 26.7 49. l 9.6 9.3 26.4 ~.3 36.7 13.9 14.1 7.5 8.0 4.7 4.3 26.9 40.6 44.8

M1A M2 M3 M6 M7 M8 TI T3 T4 T5 T7 T8 T9 TI0 TI2 T13 TI4 SI $2 $3 $4

43.2 25.4 16.6 28.0 41.9 42.2 10.5 28.6 14-! 34.8 11.4 12.3 9.3 13.3 8.0 5.8 5.0 26.5 34.2 34.8 ~4 !

M! M2 M3 M4 M5 M7 M8 T! T2 "I"3 T4A T5 T6 T7 T8 T9 TII TI2 1"13 1'!4 S! $2 $3 $4

96.1 27.1 35.5 51.2 43.4 49.6 69.4 12.6 11. I 40. ! 19.2 45.7 ! 7.9 8.1 13.4 10.3 14.4 9.5 9.5 10.7 32.7 54.8 45.9 36.8

have been obtained from an area which extends in two dimensions. The data for this study, however, have largely been obtained from almost linear transects along two isolated alluvial aquifer systems, and consequently contour diagrams are of minimal use. In this situation the plotting of the major cation data (i.e. magnesium, calcium, and sodium) is most usefully presented as hydrochemical sections, plotted for alluvial g:'ound waters. Chloride data for lhe ground waters are presented in Table 2. The Tenthill and Ma Ma creeks flow northward, separated for most of their length by a range of hiils, but in their lower reaches they converge to within 300m, before separating again. Their junctions with Lockyer Creek are ultimately 5.5 km apart. The place of closest convergence has been selected as the datum point (A, see Fig. 2) for the hydrochemical ~,ections, as at this point,

266

W. DIXON AND B. CHISWELL

TABLE 3 Distances of boreholes along the transect lines Tenthill Creek line

Ma Ma Creek line Bore no.

Distance from datum (km)

Bore no.

Distance from datum (km)

MI M2 M3 M4 M5 M6 M7 M8

2.5 3.4 4.5 5.5 6.0 7.1 8.2 10.1

T! T2 T3 T4 T5 T6 T7 T8 T9 TI0 T11 TI2 TI3 T14

1,2 i,7 3.9 4.6 5.3 6.0 6.8 '?,2 8.7 9.4 !).4 10.2 12.2 13.8

when there are no apparent physical barriers segregcting waters recharged to the alluvium from the streams, the separate influence of the creeks on the alluvial aquifers is likely to be at a minimum. The transect lines are shown in Fig. 2, and the distances listed in Table 3 are those at whici: the sampling points are perpendicular to the transect lines. During the period of the study some sites were unavailable: due to pump breakdowns, etc. Where an alternative could be found close by, this new site was identified from the original by the suffix letter A. On both the Tenthii! and Ma Ma creeks the alluvial deposits extend much farther upstream than the farthest sampling poir, ts, i.e. past T14 and M8, respectively, but these points represent the limits of extensive use of alluvial ground water for irriga~i~.~. The hydrochemical s,,:ctions are shown in Figs. 4-9. The coordinates used for both creeks, in all y,',ars, are unchanged for ease of corrparison, and the cation data are present, el with a common ordinate scale, so that changes in water type, or in catio~ 0roportions can be viewed directly.

De:;cription and interpreh,tion o[ the hydrochemical sections Commencing with the Tefithill sections (see Figs. 4, 6 and 8), it is apparent that the alluvium at T i 4 (13.8 kin) has received good quality recharge of type

ALLUVIALAQUIFERS IN SOUTHEASTQUEENSLAND

267

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DISTANCE FROM DATUM(km) Fig. 4. Hydrochemical section for Tenthill Creek alluvial ground waters, dry season, August 1984.

I water from stream runoff. Stream flow had ceased at all points when sampling was undertaken in January 1987, and there was a small increase in the salinity of ground water taken from TI4 when compared with previous years', characterized by a greate~ proportional increase in the magnesium and calcium levels than for the sodium level. This indicates that as stream flow diminishes in this region of the Tenthill Creek there is a proportiona.l increase 40 .--ID--

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268

W. DIXON AND B. CHISWELL

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in contribution to alluvial groundwater recharge from a source of type I water. Progressing along the sections downstream from TI4, there is no change in water type and only a slight increase in salinity until TI 1 and TI0 (9.4 km) are reached (Fig. 8 shows a slight decrease in salinity for TI3 and TI2 compared

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Fig. 7. Hydrochemical sectiop for Ma Ma Cre~'k alhjvial ground waters, wet season, February 1985.

A.LLU~/IALAQUIFERSIN SOUTHEASTQUI.c.NSLAND

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Fig. 8. Hydrochemical section fer Tenthill Creek alluvial ground waters, drought period, January 1987.

with TI4). TI 1 was unavailable for sampling in 1985, and as no close alternative could be found, a bore on the opposite side of the creek, T 10, at the same transect distance was used. TI 0 on the east bank receives a small input of type II saline water, possibly from the Wonga Creek system, while T11 on the west bank receives a different input of type III ground water. The Ma Ma Creek sections (see Figs. 5, 7 and 9), show the alluvial aquifer as providing type III ground waters of extreme and high salinity hazard in the 50 m

45

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Fig. 9. Hydrochemica! section for Ma Ma Creek alluvi:d gr~L,n~ wa~crs, arougttt period. Jar~uary 1987.

270

W. DIXON AND B. CHISWELL

region 10-6 km, decreasing to a minimum at M6 (7.1 km) before increasing again at M5 (6.0 km). The recharge in this region must come, in considerable proportion, from sandstone aquifers. The Ma Ma Creek itself has lower flow than the Tenthill Creek, and is frequently dry. Also, the aquifer volume is far lower than the Tenthill's, and is, therefore, less able to retain good-quality stream recharge. It can be seen from the hydrochemical sections that the Ma Ma Creek alluvium at M8 (10.1 km) receives type III saline ground water at approximately the same distance along the transects as the Tenthill Creek receives a small contribution of type III water at T l l (9.4km). This correlation is interpreted as an aquifer of the Ma Ma Creek Sandstone, conveying saline type III water, being traversed by, and interfacing with, the alluvia of both the Tenthill and Ma Ma creeks at approximately 9-10 km along the transect lines. The small input of t~pe III water to the Tenthill alluvium at 9.4 km does not greatly affect the quality of the main alluvial aquifer, and at T9 (8.8 kin), the aquifer provides water of low salinity hazard. Farther downstream, however, ground water sampled from T8 at 7.2 km shows a marked increase in salinity; notably, however, the sodium levels in all the 3 years' samples of T8 ground water are lower than those of the T9 samples, while both calcium and magnesium levels increase. This is the result of an input of type I saline water. The Tenthill sections show that T7 (6.8 km) ground watc~s (not sampled in 1984) decreased in all levels of cations compared with T8 ground waters. The minima in cation concentrations of the Ma Ma Creek type I11 ground waters at M6 occurred at a similar distance along the transect lines. While this may be coincidental, as T7 and M6 l~,round waters are of very different character, it may indicate the presence of a good-quality sandstone aquifer lying across the transect lines; although given the quality of waters in other Ma Ma Sandstone aquifers this is considered unlikely. It is considered more likely that stream recharge is responsible for the observed decreases in dissolved ions at T7 and M6. Ground waters sampled at T6 (6.0 km) are similar in type and quality t_o those sampled at T8 (7.2 km), but they are followed by a sharp rise in the salinity of waters at T5 (5.3 kin). In 1984 a1~d 1987 the rise in salinity was accompanied by a change from type I to type II. However, this change was transitional, and the 1985 T5 sample did not vary greatly in composition from those of the other years. Considering the Tenthill hydrochemical sections, it would be tempting to project the lines between samples adjacent to T5 (5.3 km), and to treat T5 as an anomaly, except that this occurrence of a type 1i alluvial ground water at 5.3 km is echoed strongly on the Ma Ma Creek sections at 5.5 km (M4). The Ma Ma Creek sections in all years show a transition from type III to type It

ALLUVIAL AQUIFERS IN SOUTHEAST QUEENSLAND

271

waters at approximately 5.6 km, and in 1984 the composition of sample M4 is very similar to that of T5. This occurrence of type II ground waters on both the Tenthill and Ma Ma creek sections at approximately 5.5 km is interpreted as an input to the alluvia of type II ground water of extreme salinity hazard from a sandstone aquifer. Correlation with the geological map suggests that this aquifer is in the transitional section between the Ma Ma Creek Sandstone and the Winwill Conglomerate. Recharge weirs were constructed by the QWRC on both creeks in 1973. Their relative positions are at 5.0 km along the Tenthill transect, and at 4.6 km along the Ma Ma Creek transect. The effect of the recharge is evident on the hydrochemical sections at T4 and "I'4A (4.6 km) (see Figs. 4, 6 and 8), and at M3 (4.5 kin) and M2 (3.4 km) (see Figs. 5, 7 and 9). Figure. 4 shows a marked improvement in water quality at T4 (4.6 km)~ The ground water there is type I, and has a low salinity hazard. This effect diminishes in subsequent years, with the saliaity hazard of T4 rising to medium in 1985, ar, d the salim~y hazard of T4A becoming moderately high in 1987. Presumably the recharge to the alluvial aquifer from the light rains of early summer 19,35 was insufficient to provide the same improvement in water quality as w~s evident in 1984, and this ' situation declined fiirther as climatic conditions changed tc- drought in the summer of 1987. The Ma Ma Creek sections (see Figs. 5, 7 and 9) show that the recharge from the Ma Ma Creek weir also had an effect on the local ground water. When sampled during 1984 and 1985, M3A and M3, both immediately downstream from the weir, provided water with the lowest salinity of all the Ma Ma Creek boreholes. During the 1987 droughl, however, the salinity of M3 ground water rose markedly to approximately double the 1985 level, ~nowing a decrease in the dilution of saline g~ound waler by better quality stream recharge. Samples from T3 at 3.9 km on the Tenthill transect show a progressive increase in salinity. Here the diluting effect of recharge from the weir is much less apparent than at T4 or T4A (4.6 km). Ground waters sampled at M2 (3.4 kin) were type 1I, with medium to high salinity hazard, and from 1984 to 1987 the level~ of total dissolved solids rose progressively (1984 = 2130mgl -~, 1985 = 2411mgl -~, 1987 = 2576 mg l-k). This increase did not occur, however, at the same rate as the increase in the salinity of M3 samples, i.e. M2 ground waters are buffered from the effects of drought, either by the consequence of drawing on a larger alluvial aquifer volume, or by receiving recharge from a sandstone aquifer which provides type II ground water of high, but not extreme salinity hazard. The salinity of Ma Ma Creek ground waters sampled at M I and M1A

272

W. DIXON AND B. CHISWELL

(2.5km) rose sharply compared with samples from M2. This rise was accompanied by a change in water type from type II to type I, indicating that the alluvium around 2.5 km on the Ma Ma Creek transect receives recharge from a Winwill Conglomerate aquifer transmitting type I ground water of an extreme salinity hazard. At T2 (1.7 km) on the Tenthill transect there is also a change in water to type I, although the ground water from T2 has much lower salinity than that from M1 or M IA. Therefore, the alluvium around T2 may receive some ground water from the same Winwili Conglomerate aquifer that recharges M l; however, if this is so then the saline ground water is di!uted by goodquality recharge from the stream. The closest sampling outlet to the datum point, TI at 1.2 km on the Tenthill transect, provides type II water (type I in 1984) of lower medium salinity hazard. A stream-water sampling point, TCI (Dixon, 1989), was sited close to TI at 1.5 km along the Tenthill transect, and comparisons of analyses of TI ground waters and TCI stream waters showed them to be very similar. It is inferred, therefore, that direct recharge from Tenthill Creek exerts the major influence on the quality of alluvial ground water at TI. CONCLUSIONS AND SUMMARY

It can be seen from Figs. 5, 7 and 9 that along Ma Ma Creek there was little difference in salinity levels between waters sampled in winter, August 1984, and summer, February 1985. There was, however, a notable increase in salinities for all waters sampled in January 1987 compared with the previous years' salinities, especially in the vicinity of the recharge weir (4.6 km). Figures 4 and 6 also show very little difference between the salinities of samples taken in winter, August 1984, and summer, February 1985. In January 1987, however, and in contrast to the experience of Ma Ma Creek alluvial ground waters noted above, Fig. 8 shows that in general the salinities of the Tenthill Creek alluvial ground waters increased only slightly compared with the previous years' salinities. The only noticeable increase in 1987 was close to the Tenthill Creek rech~rge weir, sited at 5.0 km. The dry conditions of January 1987 produced a larger deterioration in groundwater quality along the Ma Ma Creek than along the Tenthili Creek, presumably because the much larger aquifer volume (a factor of 10) of the Tenthill Creek alluvium buffered the waters during the drought against the effects of an increasing proportional input e,!" saline ground water from sandstone aquifers, and evapotranspiration. The hydrochemical sections were productive in identifying areas where good-quality water was recharged to the alluvial aquifers, and in identifying

ALLUVIALAQUIFERSIN SOUTHEASTQUEENSLAND

273

ALLUVIALGROUNDWATERS Tonthill Creek Me Ma Creek ~. Salinity.~ t Sdnlty... aecroaN fncmmse ~km

decreaee

Incroaso km

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Fig. 10. Summary diagram showing the positions aJong the transect lines of recharge areas (decreases in salinity) and saline intrusions into the alluvial aquifers (increases in salinity).

areas where saline water intrudes from sandstone aquifers into the alluvium. In conjunction with the categorization of ground waters by chemical type, it was also possible to correlate saline intrusions in adjacent valleys as issuing from the same sandstone aquifers.

274

W. DIXON AND B, CHISWELL

The hydrochemical sections also demonstrated the effectiveness of the recharge weirs constructed across the creeks by the QWRC. In times of stream flow, the weirs clearly improve the quality of local alluvial ground waters, but this effect declines with the cessation of stream flow. especially on the Ma Ma Creek. Figure 10 summarizes the discussions above by illustrating the iocatlan and extent of good-quality recharge and saline intrusions, together with correlations between creeks for the latter. The degree of salinity change is based on a comparison with the last sample upstream (see Table 3). However, this diagram does not include data on changes of water type. It may be used to assist the siting of irrigation boreholes, and the construction of recharge weirs. REFERENCES

Chillingar, G.V., 1956, Durov's classification of natural waters and chemical composition of atmospheric precipitation in the U.S.S.R. Trans. Am. Geophys. Union, 37(2): 193. Dixon, W., 1989. Hydrochemistry of groundwater salinity in the S.W. Lockyer Valley, Queensland, M.Sc. Thesis, University of Queensland. Geological Survey of Queensland, 1973. Sheet SG56-14, Department of Mines. Ipswich. Geological Map, I : 250 000. Grimes, K.G., 1968. The geology of the Lockyer Valley. B.Sc. Hons. Thesis, University of Queensland. Piper, A.M., 1944, A graphic procedure in the geochemical interpretation of water-analyses. Tran~. Am. Geophys. Union, Hydrology, p. 914. Queensland Water Resources Commission (QWRC), 1982. Further Progress Report on Lockyer Valley Water Resources Investigation. QWRC, pp. 1-2. Roberts, M. and Talbot, R., 1980. Groundwater quality. In: Salinity in the Lockyer Valley .... A Preliminary Evaluation. QId. Divn. Land Util. Rep. 80/4, p, 12.