A reassessment of the Lower Namoi Catchment aquifer architecture and hydraulic connectivity with reference to climate drivers

Abstract

We demonstrate the need for better representations of aquifer architecture to understand hydraulic connectivity and manage groundwater allocations for the ∼140 m-thick alluvial sequences in the Lower Namoi Catchment, Australia. In the 1980s, an analysis of palynological and groundwater hydrograph data resulted in a simple three-layer stratigraphic/hydrostratigraphic representation for the aquifer system, consisting of an unconfined aquifer overlying two semi-confined aquifers. We present an analysis of 278 borehole lithological logs within the catchment and show that the stratigraphy is far more complex. The architectural features and the net-to-gross line-plot of the valley-filling sequence are best represented by a distributive fluvial system, where the avulsion frequency increases at a slower rate than the aggradation rate.

We also show that an improved understanding of past climates contextualises the architectural features observable in the valley-filling sequence, and that the lithofacies distribution captures information about the impact of climate change during the Neogene and Quaternary. We demonstrate the correlation between climate and the vertical lithological succession by correlating the sediment net-to-gross ratio line-plot with the marine benthic oxygen isotope line-plot – a climate change proxy. Pollens indicate that there was a transition from a relatively wet climate in the mid–late Miocene to a drier climate in the Pleistocene, with a continuing drying trend until present. Groundwater is currently extracted from the sand and gravel belts associated with the high-energy wetter climate. However, some of these channel belts are disconnected from the modern river and flood zone. We show that the cutoff between the hydraulically well- and poorly connected portions of the valley-filling sequence matches the connectivity threshold expected from a fluvial system.

我们认为，有必要更好地。在20世纪80年代，孢粉和地下水水文数据的分析导致一个简单的代表含水层系统的三层色层/水文地层单元，由非承压含水层上覆两个半承压含水层组成。我们提呈流域内278钻孔岩性日志的分析，显示地层学特征复杂得多。填谷序列的结构特色和净毛比线图最好是由一个分配的河流系统来代表，其撕脱频率以比加积速率更慢的速度增加。我们还表明，对过去气候更好的理解可用来更好地解释在山谷中填充序列中观察到的特征。岩相分布捕获关于晚第三纪和第四纪气候变化的影响的信息。我们通过将沉积物净毛比率线图和海相底面氧同位素线图（气候变化指标）相对比，说明气候和垂直岩性序列之间的关系。花粉表明，有一个从中新世后期相对潮湿的气候到更新世干燥气候的过渡，干旱化趋势持续到现在。目前，地下水提取自与高能量的多雨气候相关的砂及砾石带。但是，其中的一些遂道带从现代河流和洪水区断开。我们认为，峡谷充填序列中水力连接好的部分和连接不好的部分之间的界线与河流系统应具有的连接状态相吻合。

KEY WORDS:

• groundwater
• hydrostratigraphy
• global climate change
• Neogene
• distributive fluvial system
• Namoi Catchment

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support from the Cotton Catchment Communities CRC, National Water Commission, Australian Research Council and The University of New South Wales. The first author also thanks Dr Gary Nichols for some enjoyable conversations on re-evaluating how we think about the development and classification of inland fluvial systems. We also acknowledge the constructive comments by the reviewers.