Evaluating water distribution in merokarst system using nuclear magnetic resonance

Abstract

Merokarst system, which is a composite of thin limestone and shale layers, constitutes a significant portion of global carbonate terrain and water supply and remains understudied. Water distribution is one of the most critical parameters to develop understandings of flow paths in any karst aquifer. However, it is challenging to gather water distribution data in merokarst system due to its significant spatially heterogeneous geological structure. In particular, the lacking of direct moisture and water table measurements especially during the dry season builts obstacles to understand the drainage process and water holding capacity of the merokarst system. The main objectives of this work are using nuclear magnetic resonance (NMR) as a tool to 1) quantify the change of water distribution under drought condition and, 2) infer the water holding capacity of limestone and shale units at different depth of a catchment under drought condition in the Flint Hills merokarst region of Kansas, USA. This study is formed by two parts of experiments: field borehole NMR (bNMR) measurements in N04d catchment to obtain in-situ water content and water distribution, while laboratory NMR based drainage tests on selected core samples are designed to link NMR signals with varaing saturation degrees in a simulated dring condition. Six boreholes targeting Eiss limestones, Stearns shales, and Morrill limestones on the hillslope on both sides of the streams are used for borehole NMR measurements during persistent dry season. Overall, shale layers exhibit higher water content (25 to 40%) than limestone (10 to 30%) and wells located in upstream hold more mobile water. The lab NMR tests show that mobile water (T2 > 100 ms) depleted faster than bound water (T2 < 10 ms) and consequently, shale has higher water holding capacity than limestone which is consistent with the higher bNMR derived water content in shale. Evidently, both NMR measurements can distinguish shale (unimodal T2 distribution) from limestone (multimodal T2 distribution). Furthermore, both measurements reveal a characteristic T2 peak around 1 ms for shale but 50 ms for limestone at low saturation degree. Additionally, lab drainage test shows that Morrill limestone tends to lose bulk water slower than Eiss limestone which contributes to its well-known high hydraulic conductivity. This study suggests NMR has a unique advantage in understanding the discharge and water holding capacity of merokarst system and provides inferences which can be used to develop conceptual model of groundwater-surface water interactions in such complex terrain.