Hydrology and physical setting of northern Stradbroke island

Abstract

Recent groundwater investigations involving the drilling of additional monitoring bores have lead to a revised conceptual model of the groundwater hydrology and groundwater surface interaction. The 2002 groundwater fl ow model has also been revised and calibrated to account for the groundwater surface water interaction. North Stradbroke Island (NSI) is composed mostly of fi ne aeolian sand of Cainozoic age which has undergone weathering resulting in the deposition of ferricrete layers and carbonaceous indurated layers. Peat layers are also present. NSI achieves a maximum height of 219 metres above mean sea level (msl) and the maximum depth to bedrock of 89 metres below msl. A feature of the hydrology of NSI is the costal wetland systems that virtually surround the island. There are two groundwater systems on the island; a regional interconnected system and localised disconnected perched systems. The regional groundwater system forms an elongated mound. The depth to this mound varies from 180 metres near Mt Hardgrave to zero metres near the coastal wetlands. Since the commencement of groundwater observations in 1965, the maximum observed height of the groundwater mound was 44 metres above msl in 1975, and minimum observed height of 34 metres above msl occurred in 2007. The corresponding estimated volume of the groundwater mound above msl was 1300 Gigalitres (GL) and 800 GL, respectively. Groundwater discharge occurs as fl ow into coastal wetlands, some creeks, Blue Lake, and as evapotranspiration where the depth to the groundwater table is less than 10 metres, and as submarine discharge. Recharge is from infi ltration of rainfall below the root zone. While average annual rainfall decrease from 1644 mm/year at Point Lookout in the north, to 1484 mm near Swan Bay in the south, the annual rainfall can vary from 545 mm/year to 2780 mm/year. In order to model the hydrology of the island, the entire island was discretised into 100x100 metre cells. A soil moisture model at the day scale was developed to account for the variation in daily rainfall across the island, the different vegetation communities and associated root depths, and the difference to the regional groundwater table. While the exact location and extent of the perched groundwater system remains unknown, where the observation bore response to rainfall suggested there was a perched watertable, their presence was incorporated into the soil moisture model as a small retention storage. This allowed for the modelling of antecedent conditions, especially after extended dry periods. The results showed that recharge entering the regional groundwater systems in the more elevated areas could be delayed by up to 200 days and, annual recharge varied from 40 mm/year to 1600 mm/year. The weighted average recharge is approximately 400mm a-1. Regional groundwater fl ow was modelled using the MODFLOW-2000 software, and a transient calibration was done using the PEST-ASM software for the period 1960 to 2007. Model results show that in the period 1998 to 2007 the average annual infl ow to the model was 103 Gigalitres (GL) and the average annual outfl ow was 129 GL compared to an average total groundwater volume of 930 GL. Of the 129 GL discharge, approximately 16% was groundwater extraction, 55% was discharge to wetlands, 2% was discharge to Blue Lake, and 27% was submarine discharge at the coast line.