Interaction of soil water and groundwater during the freezing–thawing cycle: field observations and numerical modeling

Freezing-induced groundwater-level decline is widely observed in regions with a shallow water table, but many existing studies on freezing-induced groundwater migration do not account for freezing-induced water-level fluctuations. Here, by combining detailed field observations of liquid soil water content and groundwater-level fluctuations at a site in the Ordos Plateau, China, and numerical modeling, we showed that the interaction of soil water and groundwater dynamics was controlled by wintertime atmospheric conditions and topographically driven lateral groundwater inflow. With an initial water table depth of 120 cm and a lateral groundwater inflow rate of 1.03 mm d−1, the observed freezing and thawing-induced fluctuations of soil water content and groundwater level are well reproduced. By calculating the budget of groundwater, the mean upward flux of freezing-induced groundwater loss is 1.46 mm d−1 for 93 d, while the mean flux of thawing-induced groundwater recharge is as high as 3.94 mm d−1 for 32 d. These results could be useful for local water resources management when encountering seasonally frozen soils and for future studies on two- or three-dimensional transient groundwater flow in semi-arid and seasonally frozen regions. By comparing models under a series of conditions, we found the magnitude of freezing-induced groundwater loss decreases with initial water table depth and increases with the rate of groundwater inflow. We also found a fixed-head lower boundary condition would overestimate freezing-induced groundwater migration when the water table depth is shallow. Therefore, an accurate characterization of freezing-induced water table decline is critical to quantifying the contribution of groundwater to hydrological and ecological processes in cold regions.

A parsimonious model of longitudinal stream DOC patterns based on groundwater inputs and in-stream uptake

The supply of terrestrial dissolved organic carbon (DOC) to aquatic ecosystems affects local in-stream processes and downstream transport of DOC in the fluvial network. However, we have an incomplete understanding on how terrestrial DOC inputs alter longitudinal variations of DOC concentration along headwater stream reaches because groundwater discharge, groundwater DOC concentration and in-stream DOC uptake vary at relatively short spatial and temporal scales. In the riparian zone, the convergence of subsurface flow paths can facilitate the inflow of terrestrial DOC from large upslope contributing areas to narrow sections of the stream. We refer to these areas of flow path convergence as discrete riparian inflow points (DRIPs). In this study, we ask how longitudinal patterns of stream DOC concentrations are affected by DRIPs, as they are major inputs of terrestrial DOC and important locations for in-stream processes. We used a mixing model to simulate stream DOC concentrations along a 1.5 km headwater reach for fifteen sampling campaigns with flow conditions ranging from droughts to floods. Four sets of model scenarios were used to compare different representations of hydrology (distributed inputs of DRIPs vs diffuse groundwater inflow), and in-stream processes (passive transport vs in-stream biological uptake). Results showed that under medium (10–50 l/s) and high flow conditions (> 50 l/s), accounting for lateral groundwater inputs from DRIPs improved simulations of stream DOC concentrations along the reach. Moreover, in-stream biological uptake improved simulations across low to medium flow conditions (< 50 l/s). Only during an experimental drought, longitudinal patterns of stream DOC concentration were simulated best using diffuse groundwater inflow and passive transport scenarios. These results show that the role of hydrology and in-stream processes on modulating downstream DOC exports varies over time. Importantly, we demonstrate that accounting for preferential groundwater inputs to the stream is needed to capture longitudinal dynamics in mobilization and in-stream uptake of terrestrial DOC. The dominant role of DRIPs in these transport and reaction mechanisms suggests that consideration of DRIPs can improve stream biogeochemistry frameworks and help inform management of near-stream areas that exert a large influence on stream conditions.

Evaluation methods for groundwater inflows into rock tunnels: a state-of-the-art review

Groundwater inflow into tunnels is always a salient topic in Hydrology, Hydraulic Engineering, Hydrogeology, Rock Engineering and allied sciences. In fact, tunnels particularly built below the groundwater table, often face groundwater inflows during their excavation, and even sometimes after they are put into operation. These inflows, habitually regarded as unpredictable geological hazards, cause instabilities in the surrounding rocks of tunnels, and lead to considerable damages such as injuries, loss of lives, and huge-scaled economic expenses. It is argued that groundwater conditions are of decisive significance for the design and running of tunnels. Therefore, accurate prediction or evaluation of groundwater inflows into tunnels is of paramount importance. Such prediction, although it is still challenging, has been broached by many researchers with diverse methods. However, a state-of-the-art review of these methods has not yet been presented. This paper reviews the assessment methods of groundwater inflows into tunnels built in rocky media. The results mainly include analytical, semi-analytical, empirical, semi-empirical, numerical, machine learning, and other methods used in the field. This was made possible by selecting and analysing relevant scientific articles published by various worldwide Journals. In addition, some recommendations and future trends are pointed out. This paper can provide useful references in understanding groundwater inflows prediction in different points of view and their limits in terms of applicability and accuracy.

Validation of a Semi-Empirical Procedure for Estimating Steady-State, Groundwater Inflows in Shallow Rock Tunnels Through Case Study Analyses

The construction of underground excavations and tunnels can only be done safely and economically when the subsurface conditions are adequately understood. Excessive groundwater inflows into rock tunnels under construction can injury personnel and terminate the construction of the tunnel project. Therefore, it is important to accurately estimate groundwater inflows into rock tunnels. A semi-empirical procedure/method for estimating steady-state, groundwater inflows in shallow rock tunnels is presented and discussed in this paper. In addition, this paper presents two case study analyses which include the Elizabethtown tunnel in New Jersey and the Toledo tunnel in Ohio. Packer test (i.e., pressure test) data was analyzed for both case studies utilizing this semi-empirical procedure. This paper reviews the theory behind the procedure, summarizes validates the procedure through case study analyses. It also describes previous proposed modifications and clarifies the need for any such modifications. In general, good groundwater inflow estimates were derived for shallow rock tunnels utilizing this semi-empirical procedure/method.