Freshwater seepage flow to salt lakes of Rottnest Island, Western Australia

Abstract

Rottnest Island, a popular tourist and holiday destination, is situated about 30 km west of Perth (Western Australia) in the Indian Ocean. Total area is about 2000 ha, of which 10% is made up by salt lakes. The main freshwater resources at Rottnest Island are the two shallow unconfined aquifers known as Oliver Hill and Wadjemup mounds. These aquifers may feed freshwater seepages along swamps and lake edges for many island faunas that are dependent on freshwater. The freshwater seepage flow to the salt lakes primarily depends on the hydraulic gradient of water table but currently it is unknown at Rottnest Island. This study investigates the freshwater seepage flow to the salt lakes and its quality for the use of island’s fauna. Two seepage meters were designed based on Lee (1977) and Brodie et al. (2009) by cutting a drum into two pieces and installed in the margins of two selected lakes-Lake Baghdad and Lake Negri. These two sites were chosen because of numerous visits of the animals and birds and close proximity to the Wadjemup aquifer. The visit of animals was ensured from their footprints and poos. Several site visits were made during the wet period (June–September) of 2012 for water sampling from the seepage meter, lake water and nearby piezometer. This time was selected because of winter/rainy season of the year when the seepage water is prominent in the margin of the salt lakes. The water samples from seepage meter, respective lakes and nearby groundwater was collected and measured for different water quality parameters such as, electrical conductivity, pH, dissolved oxygen, total dissolved solids, total nitrogen and total phosphate. The average seepage flow to Lake Baghdad (204 ml.m-2.min-1) was found much higher than Lake Negri (8.95 ml.m-2.min-1). This may be because of some preferential flow to Lake Baghdad which was also confirmed from visible seeps. Water quality parameters revealed that the pH is within the range of drinking water quality standard and dissolved oxygen of groundwater was found lower than the lake water. But all other parameters fall above the standard values. The salinity effect may be due to hypersaline salt lakes and the sea-water intrusion and the nutrients are found higher because of vegetative waste and fauna poos near the salt lakes where the freshwater seeps are available.