During the conceptualisation and calibration stages of a new groundwater model currently under development, issues related to transition from confined to unconfined state were identified in specific reaches of the study area. This gradual transition from confined to unconfined state has implications for the water balance, integrity of aquifers and groundwater quality.
Groundwater development in the Dumaresq River Alluvium commenced in 1950s; the current groundwater monitoring network consists of 102 bores on both sides of the border, of which, about 57 pipes are currently read. There are nine monitoring bores in QLD that are equipped with loggers and telemetered (https://water-monitoring.information.qld.gov.au/host.htm). This subset of the monitoring network provided vital information on groundwater dynamics for the study reaches.
Based on the available hydrogeological information, the study area was conceptualised into three layers: an upper aquifer directly underlying the ground surface, a semi-confining layer that becomes confining further downstream, and a lower aquifer, comprising the most productive zone. The Texas Beds and the sediments of the Great Artesian Basin represent the bedrock in the study area.
Between Mingoola and Roseneath, the groundwater system exhibits predominantly unconfined to semi-confined characteristics. From Roseneath to Texas, the system transitions to semi-confined/ confined characteristics, as the thickness and extent of the confining layer increases. However, downstream of Texas, the groundwater system becomes predominantly confined and there is a clear distinction in the hydraulic response between the upper and lower aquifer.
Groundwater extractions increased substantially during the millennium drought (2002-2010), when groundwater level falls below the confining unit in some reaches. After the millennium drought, groundwater levels have partially recovered. However, conversion from confined to unconfined state may have led to physico-chemical changes such as a change in groundwater quality.