University of Illinois, Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL, 61820.
Ground Water. 2020 May;58(3):453-463. doi: 10.1111/gwat.12922. Epub 2019 Aug 5.
Faults can act as flow barriers or conduits to groundwater flow by introducing heterogeneity in permeability. We examine the hydrogeology of the Sandwich Fault Zone, a 137 km long zone of high-angle faults in northern Illinois, using a large-scale historic aquifer test. The fault zone is poorly understood at depth due to the majority of the faults being buried by glacial deposits and its near-vertical orientation which limits geologic sampling across faults. The aquifer test-perhaps one of the largest in terms of overall withdrawal in North American history-was conducted in 1942 at a facility adjacent to the fault zone. More than 34,000 m /day was pumped for 37 days from nine multiaquifer wells open to the stratified Cambrian-Ordovician sandstone aquifer system. We modeled the aquifer test using a transient MODFLOW-USG model and simulated pumping wells with the CLN package. We tested numerous fault core/damage zone conceptualizations and calibrated to drawdown values recorded at production and observation wells. Our analysis indicates that the fault zone is a low-permeability feature that inhibits lateral movement of groundwater and that there is at least an order of magnitude decrease in horizontal hydraulic conductivity in the fault core compared to the undeformed sandstone. Large head declines have occurred north of the fault zone (over 300 m since predevelopment conditions) and modifying fault zone parameters significantly affects calibration to regional drawdown on a decadal scale. The flow-barrier behavior of the fault zone has important implications for future groundwater availability in this highly stressed region.
断层可以通过引入渗透率的非均质性来充当地下水流动的阻隔或通道。我们利用一项大规模的历史含水层测试，研究了伊利诺伊州北部一条 137 公里长的高角度断层带——三明治断层带的水文地质情况。由于大部分断层被冰川沉积物掩埋，而且其近乎垂直的走向限制了断层两侧的地质采样，因此该断层带在深部的情况了解甚少。含水层测试——也许是北美历史上总抽水量最大的测试之一——于 1942 年在该断层带附近的一个设施中进行。从九个多含水层井中每天抽取超过 34000 立方米/天，持续 37 天，这些井都与分层的寒武纪-奥陶纪砂岩含水层系统相通。我们使用瞬态 MODFLOW-USG 模型对含水层测试进行建模，并使用 CLN 包模拟抽水井。我们测试了许多断层核/损伤带的概念化，并对生产和观测井记录的水位下降值进行了校准。我们的分析表明，断层带是一个低渗透性特征，它抑制了地下水的侧向流动，与未变形的砂岩相比，断层核中的水平水力传导率至少降低了一个数量级。断层带以北地区的水头下降幅度很大（自开发前条件以来超过 300 米），并且显著改变断层带参数会对十年尺度的区域水位下降校准产生重大影响。断层带的阻隔作用对该高度承压地区未来地下水的可用性具有重要意义。
