Connectivity between groundwater and surface water largely occurs via alluvial aquifers and varies in space and time. For example, the Macquarie River has a combination of gaining and losing reaches that changes with flood recharge and drought events. A connectivity map for the Central West subregion identifying zones where streams are dominantly gaining or losing, and zones where seasonal variability occur, is given in Figure 29. This map was produced using relative water levels for groundwater and surface water at a single point in time, and has not been further verified by analyses at multiple points in time, or by identifying losses and gains between stream gauging stations. CSIRO (2008) note that while their assessment is generally consistent with previous hydrogeological interpretations of the catchment and numerical modelling results for the Lower Macquarie Alluvium, an exception is the Lower Macquarie groundwater model (Bilge, 2007) predicting that the Macquarie River around Narromine is more highly losing than shown in the assessment, and that the Bogan River is ‘low losing’ rather than ‘maximum losing’.
For the Upper Macquarie Alluvium between Wellington and Dubbo, the upper and lower aquifers are hydraulically connected. Water levels throughout the aquifer responded to the main river flood events of 1990, 1998 and 2000 with near-river parts of the aquifer showing a rapid response. Smithson (2010) concluded that the Macquarie River in this area had a combination of gaining and losing reaches, and that this changed with flood recharge and drought events. Field-based connectivity studies carried out over a 30 km stretch of the Upper Macquarie River centred on Narromine showed the river to be losing disconnected, with a small proportion showing losing disconnected characteristics (Brownbill et al., 2011).
Figure 30 shows a hydrograph from the Baroona gauging station (25 km east of Narromine) on the Macquarie River superimposed on water levels in the Quaternary aquifer from a monitoring bore 1.2 km from the river. This hydrograph shows that the river here is potentially gaining, as indicated by the higher elevation of the watertable compared to the river stage during low-flow conditions. The high degree of interaction is highlighted by the strong correlation between rises in the river hydrograph and groundwater levels. At this scale, the response in groundwater level to high-flow events appears to be instantaneous for the 1990 and 1998 events; however the actual response in groundwater level lags the flood peak by a few days, and by up to a month for the December 2000 flood event.
Groundwater from Cenozoic Basalts and Gunnedah Basin strata is likely to provide baseflow to surface water in the southern parts of the subregion, where Gunnedah Basin rocks abut Great Artesian Basin (GAB) and alluvial units (EMGA Mitchell McLennan Pty Ltd, 2012). Discharge to springs and seeps is likely to be via short run, local flow systems, and not from the deeper, regional flow systems of the Gunnedah Basin.
Kellett and Stewart (2013) show that there is potential leakage from the alluvium of the Castlereagh River to the Pilliga Sandstone where CSIRO (2008) identifies the river as being ‘medium losing’. This supports the contention of riverine recharge to the Pilliga Sandstone in the Castlereagh River valley proposed by Radke et al. (2000).
Significant flow losses from the Macquarie River further downstream in the reach between Narromine and Warren have been consistently recorded since river discharge measurements began.
Figure 29 Surface water - groundwater connectivity in the Central West subregion
Note that many streams are not classified. Maximum losing conditions occur where the watertable is separated from the streambed by an unsaturated zone, the additional classification of high, medium or low losing for these reaches referring to the flux from streambed to watertable.
Source data: modified from CSIRO (2008)
Source data: modified from Smithson (2010)
Figure 31 shows deuterium levels in groundwater of flowing artesian water bores in the Pilliga Sandstone aquifer in the Coonamble Embayment. This shows a plume of depleted water (–45 0/00 ≤ δ2H ≤ –42 0/00) in the southern region of the Coonamble Embayment basinward from where the Macquarie and Castlereagh rivers cross the intake beds. Waters with depleted isotopic signatures such as this are characteristic of heavy rainfall at high altitudes. Therefore the isotopically light patterns in Figure 31 are interpreted as having been generated by heavy rainfall in the headwaters of both of these streams, which was sufficient to produce river flows greater than certain threshold values and aquifer recharge by river leakage during these high-flow periods. Aquifer recharge occurs by downward leakage from the streams at some point where the Macquarie and Castlereagh rivers traverse the intake beds. When river stage is high, water percolates laterally through permeable alluvium exposed in the river banks and recharges the Cenozoic aquifers. If the hydraulic gradient between the Cenozoic aquifers and the Pilliga Sandstone is downward, then the Pilliga Sandstone aquifer will also be recharged from these high-flow events that occur periodically in the Macquarie and Castlereagh river systems.
Source data: Radke et al. (2000)
A study using airborne electromagnetic surveying identified many areas where the Macquarie River is connected to groundwater in the alluvial aquifers (Macaulay and Kellett, 2009). Several airborne electromagnetic conductivity-depth sections show potential river leakage from the Macquarie River to the adjacent alluvial aquifer systems, indicating a highly connected system.
Figure 25 shows the watertable elevation in the Quaternary alluvial aquifer of the Coonamble Embayment in the Central West subregion. The stippled brown regions demarcate areas where water levels in the Quaternary alluvial aquifer are higher than the regional watertable in the uppermost GAB partial aquifer and yellow areas indicate areas where the GAB regional watertable is coplanar with or higher than the water levels in the alluvium. A higher water level in the alluvium than that in the GAB aquifer is a necessary but not sufficient condition for downward leakage and recharge from the alluvium to the GAB formations. A second condition which must be satisfied is that a pathway exists for inter-formational groundwater flow. During the Late Cretaceous to the early part of the Paleogene, the exposed GAB rocks were subjected to prolonged and intense weathering that produced a thick, basin-wide layer of saprolite. The lower part of the saprolite is clay-rich and of very low permeability. This is the zone of leachate accumulation of silica, clay minerals and salts. This layer prohibits vertical fluid flow.
The same airborne electromagnetic survey detected the lower impermeable saprolite as a layer of high bulk conductivity which forms a continuous blanket over the GAB rocks and prevents hydraulic connection between the Quaternary alluvium and the GAB sequence. However, in places where the saprolite has been removed by erosion, hydraulic connection is possible and highly likely. Such areas occur beneath Neogene paleochannels where erosion during their formation was sufficient to completely remove the saprolite. There are many such paleochannels in the Surat Basin and they either underlie modern stream channels (e.g. the Warrego and Castlereagh rivers) or are buried adjacent to them (for example, the Namoi and Dirranbandi paleovalleys).
Figure 29 shows two areas in the Central West subregion where it is highly likely that significant surface water – groundwater interaction occurs. In both cases the rivers are losing streams during periods of high flow. The first is the Castlereagh River from Mendooran to 25 km north of Curban. This is most likely the source of river leakage from the Castlereagh River to the Pilliga Sandstone aquifer shown in Figure 29.This reach of the Castlereagh River is underlain by a paleochannel at least 100 m deep and the basal alluvial beds contain Neogene pollens (Martin, 1981). The deep alluvium lies directly on unweathered Early Cretaceous rocks of the GAB.
The second area in Figure 29 is a small but important site of surface water – groundwater interaction in the Macquarie River at the Buddah kink, 20 km north of Narromine. There is no Neogene paleochannel here but the saprolite on the GAB beds has been partially eroded by Cenozoic avulsion and channel switching by the Macquarie River (Macaulay and Kellett, 2009).
Figure 25 shows extensive areas where the watertable in the Quaternary alluvium is higher than the hydraulic head of the uppermost GAB partial aquifer. Three of these areas are located around Warren, to the west of Narromine and between Walgett and Brewarrina. There is unlikely to be any downward leakage from the alluvium to the GAB as Macaulay and Kellett (2009) report that the impermeable saprolite forms a continuous aquitard. The higher watertables in the alluvium of the two southern areas (Warren and west of Narromine) have been generated by irrigation water perched on top of the saprolite. There is negligible surface water – groundwater interaction in these areas.
Flight line 26181 of the airborne electromagnetic survey (Figure 32) reported by Macaulay and Kellett (2009), is an east – west oriented line intersecting the Macquarie River in the vicinity of the Buddah kink. This cross-section shows thick (25 to 40 m) conductive saprolite on the Keelindi beds, underlying 5 to 10 m of resistive soil sloping down towards the Macquarie River. A 25 m thick band of conductive material to the west of 610000 mE (Figure 32) was interpreted by Macaulay and Kellett (2009) to be saprolite on the Drildool beds. A bore on the western side of the river at Buddah intersected 28 m of Quaternary sediments overlying a thin saprolite. This is represented in Figure 32 as a moderately conductive band between 10 and 15 m thick. Saprolite underlying the Macquarie River has been partially eroded by incision; this is shown as a depression extending from 610000 mE to 618000 mE in Figure 32. This was subsequently filled with alluvium. The upper confined sandy aquifer intersected in drilling from 12 to 21 m depth was interpreted as being the Bugwah Formation channel fill sediments, and the lower confined sandy aquifer (35 to 48 m depth) as the Carrabear Formation channel fill, both of which contain fresh groundwater (Macaulay and Kellett, 2009).
The clay layer between these two sandy aquifers forms a mostly continuous band, but is eroded between 612000 mE and 613000 mE and infilled by Bugwah Formation sediments. This erosion feature may permit vertical leakage between these two aquifers. Material between 50 and 80 m below the Macquarie River was interpreted to represent unweathered Keelindi beds overlying Pilliga Sandstone, however some of this material, below 130 m depth, could be granite. This material was interpreted to be gently dipping basinward, to the north-west (Macaulay and Kellett, 2009).
Possible leakage pathways from the river to the alluvium and then into the underlying Mesozoic sequence were examined by Macaulay and Kellett (2009). Keshwan’s (1995) estimates of aquifer recharge from river leakage were based on hydraulic parameters derived from control bore GW036977. On 5 and 6 October 1991, Keshwan estimated that the river stage was 2.5 metres higher than the potentiometric surface of the Carrabear Formation aquifer in the bore, creating the potential for water to flow from the river to the aquifer.
The airborne electromagnetics indicate a high degree of connectivity between the river and the alluvium in its banks from the highly resistive 10 metre-thick surface band of alluvium adjacent to the channels. Furthermore, the airborne electromagnetics indicates hydraulic connection between the western-most channel in the section (at Buddah) and both sand aquifers in bore GW036977. At the time of flying, the wetted resistive zone of the top aquifer extended 2 km westward and 4 km westward for the bottom aquifer. Importantly, the underlying saprolite appears to be breached and displaced in the intervals from the bore to 616000 mE, 616500 to 617000 mE, 618000 to 618500 mE and possibly 612000 to 613000 mE. These breaches were identified as potential pathways for groundwater flow from the alluvium to the Mesozoic aquifers (Macaulay and Kellett, 2009).
The airborne electromagnetic survey reported by Macaulay and Kellett (2009) depicts the hydrological situation at Buddah after several years of drought. A survey flown during high river stage, at a time when maximum river leakage to the alluvium is occurring (as postulated by Keshwan, 1995) may show more pronounced hydraulic connectivity between the river and the alluvium, and the Mesozoic aquifers. Keshwan (1995) estimated a maximum potential leakage from the river channel to the aquifer of about 50 ML/day under conditions of high river flow (> 2500 ML/day) for the Buddah kink of the Macquarie River, and potential aquifer recharge from river leakage in the reach from Baroona to Gin Gin of 450 ML/day.
Therefore the two necessary conditions for downward leakage from the river to the alluvium and thence to the GAB regional aquifers were satisfied for the Macquarie River at Buddah. Firstly, the work by Keshwan (1995) established the existence of the downward hydraulic gradient from the alluvium to the GAB aquifer at a certain threshold river flow, and secondly, the airborne electromagnetic (AEM) demonstrated the existence of pathways for fluid transfer (Macaulay and Kellett, 2009).
Comparable analysis and estimation of river leakage losses to groundwater have not as yet been undertaken for the Castlereagh River.
Figure 32 Airborne electromagnetic flight line 26181, perpendicular to the Macquarie River at Buddah
Source data: modified from data presented in Figure A1.5 from Macaulay and Kellett (2009)
Macaulay and Kellett (2009) report several instances of flush zones in the Quaternary alluvium as a result of periodic flooding of the Macquarie River. These can be seen in cross-sections as resistive plumes in the alluvium adjacent to the river channel. Figure 33 shows one such feature at ‘Elengarah’, 20 km upstream of Warren. The river channel is at 594000 mE, and is about 10 m deep. The flush zone was interpreted to extend to about 20 m depth on the western side of the river, as indicated by a resistive wetted perimeter (Macaulay and Kellett, 2009). Drilling intersected 14 m of Marra Creek Formation sediments overlying Bugwah Formation gravel, which overlie weathered Drildool beds at 27 m depth. Groundwater in the gravels was interpreted to represent a mix between river water and more saline regional groundwater (Macaulay and Kellett, 2009). The possible maximum lateral extent of the flooding-related flush zone is shown in Figure 33.
Source data: modified from data presented in Figure A1.2 from Macaulay and Kellett (2009)
The Macquarie Marshes are an internationally recognised Ramsar-listed wetland complex occupying an area of about 220,000 ha within the Macquarie River floodplain in the Central West subregion. It was thought that the Macquarie Marshes received upward leakage from the Pilliga Sandstone to supplement floodwater from the Macquarie River in sustaining vegetation (e.g. Brereton, 1994; Wolfgang, 2001).
A comprehensive multi-disciplinary study of the Macquarie Marshes was reported by Macaulay and Kellett (2009). This system is hosted within the Quaternary sediments of the Lower Macquarie Alluvium. Macaulay and Kellett (2009) identified high groundwater salinity levels of up to 44,600 mS/cm at depths of 4 to 8 m below the Macquarie Marshes. This provides evidence that water in the marshes is not sustained by upwards flow from the GAB. Macaulay and Kellett (2009) also concluded that there is no hydraulic connection between the fresh water lens from flooding and the shallow groundwater system; that is, there is not surface water – groundwater connection in the Macquarie Marshes.
The airborne electromagnetic results also showed the importance of the Rolling Downs Group as a spatially extensive unit, acting as a highly effective aquitard. Additionally, clay-rich weathered zones in the units immediately underlying the alluvial aquifers act as effective barriers to upward leakage. This is highlighted in flight line section 20990, which traverses the northern Macquarie Marshes (Figure 34). This cross-section clearly shows the continuous aquitard formed by the saprolite on top of the Rolling Downs Group which precludes any vertical upward transmission of groundwater from the Pilliga Sandstone. Inspection of every flight line through the Macquarie Marshes revealed no breaches in the saprolite anywhere although there is clearly vertical displacement.
This feature is continuous everywhere beneath the Macquarie Marshes, precluding any upward leakage from the Pilliga Sandstone. Source: Figure 6.5 from Macaulay and Kellett (2009)
