Hydrogeological Modelling of the Esperance Drinking Water Aquifer Subjected to Seawater Intrusion

Abstract

Saltwater intrusion in groundwater is a concern in many aquifers around the world. It results in deterioration of potable and irrigation water quality and degradation of ecosystems. The problem is exacerbated by fresh groundwater abstraction, drying climates and a rising seawater level.
Esperance is a small town situated on the south coast of Western Australia. The drinking water is supplied by a borefield located on the coastal plain in between the Southern Ocean and the saline lakes: Lake Warden and the terminal Pink Lake. Saltwater intrusion has been monitored for over two decades in this aquifer.
Managing Saltwater intrusion has been a priority for the local water company and the regulator for decades yet a numerical model of the aquifer capable of quantifying flows and salinity distributions does not exist.
In this study, a numerical flow model was developed as a first step towards a model that also includes saltwater movement, i.e. density-dependent flow. A site-specific literature review is documented in this report, as well as relevant saltwater intrusion studies.
A 3-dimensional hydrogeological model was created with a kriging geostatistical algorithm with data from Airborne Electromagnetic (AEM) surveys and borehole logs. A database including climatological fluxes, abstraction rates, groundwater heads, lake stages and lithographic data was created. The salinity distribution in the catchment was analysed using borehole observations and AEM data. Groundwater salinity is more prevalent in the east and northeast of the domain. Salts from inland sources are transported to the coastal lakes by a combination of groundwater and surface water.
The flow model was calibrated with PEST++ and a linear parameter error and sensitivity reduction was carried out with GENLINPRED from the PEST++ suite. The result of the study is a groundwater flow model that is able to quantify the approximate fluxes and heads within the catchment. However, it must be noted that the calibration can be improved. The relative error reduction for some parameters is large, for example groundwater recharge in areas with sparse and dense vegetation (0.91, 0.84 respectively), the runoff/quickflow factors towards the lakes (0.87, 0.95 and 0.88 for the Pink Lake, Lake Warden and Windabout/Woody lakes respectively) and reasonable for some of the hydraulic conductivities. The identifiability for other parameters (such as specific yields and vertical hydraulic conductivity) is low.
Zone budget calculations show that groundwater annual recharge and evaporation are the largest fluxes in the catchment and are equal in magnitude. The annual average evaporation rate over the entire catchment is 0.74 mm/d. The annual average groundwater abstraction is the same order of magnitude of the flow that enters the ocean, 5,656 m3/d vs. 8,169 m3/d respectively.
Modelling indicates that abstraction has a large effect on groundwater levels. The decline is the largest (> 1 m) a couple of kilometres west from the town area where abstraction decreases the water table. The effect is less south of the Pink Lake (~0.40 m).
(Hyper) salinity is the result of the net transport of saline groundwater entering into the lakes in combination with net evaporation. Water budget calculations show that the groundwater inflow in the hypersaline lakes exceeds groundwater outflow by a factor 8.1 for the Pink Lake and 1.7 for Lake Warden. Freshwater entering the lakes originates from direct precipitation and a calibrated runoff/quickflow component. This results in an effective lake area for precipitation of 1.6 times for the Pink Lake and 2 times for Lake Warden. In the lakes the evaporation exceeds the freshwater inflow by a factor of 1.5 and 1.1 for the terminal Pink Lake and Lake Warden respectively.
The low flow velocities towards the ocean in the eastern part of the catchment underline the risk of saltwater intrusion in this area. Flow field modelling suggests that groundwater abstracted in areas directly south of the Pink Lake is saline.
Finally, the current model is able to support recommendations for expansion of the borefield towards the area in the west of the catchment.