Regolith influence on surficial water chemistry at Hovells Creek, central west NSW: dryland salinity hazard mitigation in high-relief landscapes

Hovells Creek Catchment is located in the central tablelands of New South Wales, approximately 40 km south east of Cowra and 35 km north of Boorowa (Figure 1). Hovells Creek is a sub-catchment of the larger Boorowa River catchment, which itself covers 1,820 km2 (Evans 1994). A 200 km2 study area in the upper Hovells Creek catchment is the focus of this study. In this area Hovells Creek flows north for approximately 16 km through high-relief granitic rock of the Wyangala Batholith. Anecdotal evidence from landholders indicates that the runoff and shallow groundwater coming off the local ranges in the study area appears to be diluting the slightly saline water of Hovells Creek. Surficial water was sampled at three points: as it enters the granitic section of the catchment; prior to it leaving the study area; and below the confluence of a local spring-fed tributary (Oakey Creek) and Hovells Creek. This study explores whether it is in the best interest of the downstream catchments to allow water flow from Hovells
Creek to flow freely and dilute the salt flux into the Boorowa River. Understanding the regolith is imperative to understanding the shallow groundwater flow system of any area. Preliminary studies at Hovells Creek have highlighted the complex relationship between regolith distribution and surface and groundwater properties, occurring at a local scale. The regolith plays an important role in the infiltration, movement and storage of near surface water and source/storage of salts (Wilford et al. 2002).
The location of recharge and discharge zones and the amount of time water has to interact with regolith materials depends on the distribution and physical characteristics of the regolith materials present. In the
study area the residence time of water in the local groundwater system is approximately 2 months. At Hovells Creek, transport in the alluvial sediment is a lot slower as the fluid migrates via primary porosity in the
unconsolidated silty sands, while in the slightly weathered bedrock water movement is faster and its direction is controlled by the orientation of the fractures and joints. The slow movement through the alluvial sediments
does cause salinity problems elsewhere in the Boorowa shire. As salt is dominantly found in the shallow soil and weathered bedrock, the slow movement of water in these regolith types may mobilise salt as there is both
time and the capacity to interact (Evans 1998).