In weathered bedrock aquifers, groundwater is stored in pores and fractures that open as rocks are exhumed and minerals interact with meteoric fluids. Little is known about this storage because geochemical and geophysical observations are limited to pits, boreholes, or outcrops or to inferences based on indirect measurements between these sites. We trained a rock physics model to borehole observations in a well-constrained ridge and valley landscape and then interpreted spatial variations in seismic refraction velocities. We discovered that P-wave velocities track where a porosity-generating reaction initiates in shale in three boreholes across the landscape. Specifically, velocities of 2.7 ± 0.2 km/s correspond with growth of porosity from dissolution of chlorite, the most reactive of the abundant minerals in the shale. In addition, sonic velocities are consistent with the presence of gas bubbles beneath the water table under valley and ridge. We attribute this gas largely to CO₂ produced by 1) microbial respiration in soils as meteoric waters recharge into the subsurface and 2) the coupled carbonate dissolution and pyrite oxidation at depth in the rock. Bubbles may nucleate below the water table because waters depressurize as they flow from ridge to valley and because pores have dilated as the deep rock has been exhumed by erosion. Many of these observations are likely to also describe the weathering and flow path patterns in other headwater landscapes. Such combined geophysical and geochemical observations will help constrain models predicting flow, storage, and reaction of groundwater in bedrock systems.

在风化的基岩含水层中，地下水被储存在孔隙和裂缝中，裂缝随着岩石的挖掘以及矿物质与陨石的相互作用而打开。对于这种存储方式知之甚少，因为地球化学和地球物理观测仅限于坑，井眼或露头，或者仅限于基于这些站点之间的间接测量得出的推论。我们训练了一个岩石物理模型，以在约束良好的山脊和山谷景观中进行井眼观测，然后解释地震折射速度的空间变化。我们发现纵波速度跟踪了在整个景观的三个钻孔中的页岩中产生孔隙的反应的起始位置。具体而言，速度为2.7±0.2 km / s对应于页岩中丰富矿物中最活泼的亚氯酸盐溶解后孔隙度的增长。此外，声速与山谷和山脊下方地下水位下方的气泡一致。我们将这种气体主要归因于一氧化碳₂是由于1）大气中的水补充到地下时微生物在土壤中的呼吸作用，以及2）岩石深处碳酸盐的溶解和黄铁矿氧化的耦合。气泡可能会在地下水位以下成核，这是因为水从山脊流向山谷时会降低压力，并且由于深层岩石因侵蚀被侵蚀而使孔隙膨胀。这些观察中的许多都可能也描述了其他源头景观中的风化和流径模式。结合地球物理和地球化学观测，将有助于约束预测基岩系统中地下水的流量，存储和反应的模型。