Lower catchment groundwater

Intermediate and regional groundwater systems are recharged at a number of topographic highs and discharged at the major topographic low (e.g., Stuyfzand 1998). The groundwater chemistry of discharging intermediate and regional systems is not necessarily related to the geology of discharge sites (Chambers et al. 1996; Uliana et al. 2007), with abrupt changes in groundwater chemistry along flow paths indication of groundwater discharge from these systems (e.g., Love et al. 1994). Group three

represented groundwater samples dominated by Na, Mg and HCO3, with these samples

collected from different sub-catchments in the mid and lower catchment (Table 3.3). The other four groups which represented groundwater bodies in the lower catchment were also comprised of groundwater samples collected from these areas, with groups partitioned according to Cl-HCO3 co-dominance, and dominance by Na and/or Mg.

Groundwater samples collected from bores screened into the same aquifer, such as Site Four and Site Two, which were screened into alluvium; and Hannah’s and Roach’s #1 which were screened into the Narrabeen Group; did not necessarily belong to the same groundwater group. The significant difference in chemistry between groundwater samples collected from the same aquifer indicates the occurrence and/or mixing of groundwater bodies with different solute sources, and therefore different spatial extents, within the same aquifer. The dominance of groundwater by Cl in many mid-lower catchment groups is anomalous with silicate weathering of the alluvium and the silica dominated Narrabeen Group aquifers the groundwater was sampled from, and is indication of groundwater discharge from intermediate and/or regional groundwater systems.

Groundwater discharged from regional groundwater bodies is typically saline and Na-Cl dominated (Genereux and Jordan 2006), with water evolving from Ca-HCO3 to

Chapter Three – Aquifers and groundwater bodies in the Wybong Creek catchment

80

Jacobson 1989). The Narrabeen Group occurs throughout the Wybong Creek catchment, with most groundwater samples in the mid and lower catchment yielded from bores screened into this aquifer or into alluvium derived from weathering of the Liverpool Volcanics or Narrabeen Group. Chemical weathering of these silicate dominated

materials produces groundwater with HCO3-dominated solutes. The dominance of Cl in

many samples, and differing 87Sr/86Sr, and δ35S signatures than occurs in the upper catchment, indicated that groundwater in the lower catchment has not evolved from groundwater sourced in the Narrabeen Group nor the Liverpool Range Volcanics. Dominance by Cl instead indicated solutes in many mid-lower catchment samples were acquired from another source. Cl dominated groundwater is known to occur in the Permian strata which underlie the Narrabeen Group and Liverpool Range Volcanics (Kellett et al. 1987). Cl dominated salt stores may also occur in the regolith (e.g., Ruprecht and Schofield 1991), with groundwater also evolving to Cl dominance with increasing distance down flow paths due to the precipitation of HCO3 and SO4 minerals

(e.g., Eugster 1984; Jankowski and Jacobson 1989).

Valleys act as groundwater discharge regions with the major topographic low of a catchment receiving discharge from local, intermediate, and regional groundwater systems (e.g., Domenico and Schwartz 1990). Two samples (Dart’s Pivot and Roach’s #2) had HCO3-Cl facies in the mid-catchment area, with all other HCO3-Cl samples

occurring at the top of the basalt-capped sandstone escarpments (Rossgole #2, Coffin Gully, Dip Spring, Springs Spring), within the gravel alluvium of Wybong Creek (Wybong Bridge, Rockhall Riverflats, Rockhall Hayshed, Yarraman Well) or as groundwater seeps (TSR Seep). Although the two HCO3-Cl dominated groundwater

samples from the mid-catchment area may represent groundwater evolving from HCO3

to Cl dominated facies, mixing of water bodies with Cl and HCO3 facies may also give

rise to water dominated by these anions, with further geochemical investigation required in order to isolate which of these is more likely.

The potential for groundwater discharge is indicated by hydraulic heads rising above the water-bearing strata in an aquifer and by the occurrence of springs (Freeze and Cherry 1979), with discharge typically occurring in catchment valleys (e.g., Domenico and Schwartz 1990). Discharging and pressurised groundwater in the lower Wybong Creek catchment was indicated by SWLs generally above the level of water- bearing strata in all non-alluvial aquifers (Table 3.3). Static water levels were instead below water-bearing strata in the alluvial aquifer on most sample dates, which indicates the alluvial aquifer can be recharged by Wybong Creek (Praamsa et al. 2009). A

Chapter Three – Aquifers and groundwater bodies in the Wybong Creek catchment 81

number of permanent and ephemeral springs and groundwater seeps occurred in the mid-catchment area, with springs indicating points where groundwater is pressurised to the extent that it is forced to the Earth’s surface. Springs in this area were both fresh (e.g. Whip Well) and saline (e.g. Dry Creek Road Seep). In addition, one of two salt scalds that occur in the Wybong Creek catchment also occurred in the mid-catchment area of Wybong Creek, with this possibly indicating saline groundwater discharge (Morgan and Jankowski 2004). The incidence of both saline, and fresh (<500 mg L-1) springs and groundwater in the mid-catchment area indicates that this area is possibly a site where groundwater discharge from localised, intermediate and/or regional

groundwater systems occurs.

Local, intermediate and regional groundwater systems will develop in areas of hummocky terrain similar to that which occurs across the Wybong Creek catchment, with discharge of all these systems possible in the major topographic low of a

catchment (Domenico and Schwartz 1990). The mid-catchment area of Manobalai is not the lowest point in the catchment, however, with Wybong Creek itself incised

approximately 20 m below the regolith surface where saline seeps and springs occur. Discharge of all groundwater systems in the Manobalai area rather than at lower points in the catchment may occur due to vertical groundwater flow, with a number of

mechanisms by which vertical flow occurs.

Recharge and discharge may be vertical in areas where the hydraulic conductivity of a unit is higher than that of the unit above (Domenico and Schwartz 1990). This is almost certainly the case in the Wybong Creek catchment where smectitic clays occur above fractured Narrabeen Group sandstones and conglomerates on both the

escarpments and in the Wybong Creek valley. Discharge may also occur in the mid- lower catchment area due to strata in the catchment dipping north and west (Brunton and Moore 2004), whereas the surface topography decreases from north to south. The dipping of beds in the opposite direction to the topographic drive causes groundwater to flow up-dip (Domenico and Schwartz 1990). This cannot be proven in the Wybong Creek catchment, although strata dip north and west while topography dips south. Vertical flow may also occur due to fracturing and faulting, with deep fractures connecting the Permian Coal Measures to the Narrabeen Group sandstones and

conglomerates in the lower catchment (Umwelt Environmental Consultants 2006). The conceptual model of a saline regional groundwater system in the Permian Coal

Chapter Three – Aquifers and groundwater bodies in the Wybong Creek catchment

82

Group adjacent to locally derived fresh groundwater systems is consistent with observations in the catchment.

4. Conclusions

Most outcropping rock in the Wybong Creek catchment is fractured sandstones and conglomerates of the Narrabeen Group. The termination of many bores within this formation indicates that it is an important aquifer within the catchment, with the termination of many bores within the alluvium in the mid-lower catchment indicating this is also an important aquifer. Regional groundwater systems are known to occur within the Permian Coal Measures, however, this groundwater is not utilised within the catchment.

Numerical analyses provided no added benefit in the grouping of groundwater samples despite using many more major and minor ions than visual analyses. Visual and numerical analyses of groundwater samples indicated that groundwater can be divided into two groups, constituting groundwater systems in the upper and lower catchment. Groundwater flow in the upper catchment occurs as localised systems which recharge on isolated topographic highs in the Liverpool Ranges and discharge in adjacent topographic lows. A number of groundwater samples from the lower catchment were also placed in the upper catchment groundwater group. This was due to the similar groundwater chemistry of these and upper catchment groundwater samples, and indicated localised groundwater flow at these sites.

Chloride dominated groundwater in the lower catchment did not source solutes from chemical weathering of the Narrabeen Group. Saline and Na-Cl dominated springs, scalds and seeps in the mid-catchment are instead indicative of discharge from intermediate and/or regional groundwater systems. Abrupt and significant changes in groundwater chemistry within the alluvium and fractured Narrabeen Group were also indication of intermediate and/or regional groundwater systems. Groundwater discharge and recharge within the alluvium and fractured Narrabeen Group was likely to occur as vertical groundwater flow due to fracturing and higher hydraulic conductivity of these formations than the smectitic clays occurring above. This vertical groundwater flow makes it difficult to predict groundwater flow paths. The dipping of strata and topography in opposite directions, indicates groundwater recharging in the Liverpool Ranges may be forced to flow up dip before discharging into the fractured Narrabeen Group. Solutes occurring in this groundwater may be acquired from the deeper Permian Coal Measures in the mid-lower catchment area between recharge and discharge, with

Chapter Three – Aquifers and groundwater bodies in the Wybong Creek catchment 83

further geochemical investigation necessary to identify the source of solutes to the groundwater described.

The findings of this research were in contrast to Kellett et al.’s (1987) work which found HCO3 dominated groundwater was sourced from the weathering of the Triassic

Narrabeen Group. The Cl dominated groundwater could not be related to any of the surficial aquifers in the catchment, with further research required in order to corroborate Kellett et al.’s (1987) findings that solutes in this groundwater group are sourced from to the Wittingham Coal Measures. The research presented in this chapter was limited by a lack of data on the geology of the catchment and long term groundwater height data. A detailed description of the aquifers could not be undertaken, therefore, with only a conceptual understanding of confining layers and flow directions within the aquifers described. Further research is required in order to clarify the conceptual models put forward in this chapter, specifically the extent of groundwater bodies. This can be done by conducting tracer studies and pumping tests, with further dating of groundwater in addition to the initial 14_{C dates presented also useful for this purpose.}