Measuring horizontal groundwater flow with distributed temperature sensing along cables installed with direct-push equipment

The pressure on groundwater systems, especially in coastal regions, increases as the population rapidly grows. In these regions, management of water tables and fluxes is important to minimize droughts, salt-water intrusion, and flooding. Proper management of such groundwater systems requires knowledge of how groundwater responds to water entering and leaving the system. Groundwater models can translate changes in inflow and outflow into changes in the groundwater table and flow. Proper calibration of these models depends on measurements of the flow and the groundwater table. While the groundwater table can be measured relatively easily, flow can only be measured either when it enters or exits groundwater systems (e.g., wells, infiltration, seepage), indirectly with tracers (solutes or heat), or with a variety of geophysical techniques. In this dissertation, a new approach is presented to measure horizontal groundwater in the aquifer with distributed temperature sensing (DTS) along cables that are inserted with direct-push equipment...Water Resource

Simulating heat transfer through the unsaturated zone of MAR systems: With an application to the MAR system at PWN

Consider a managed aquifer recharge system in the Netherlands where approximately 25 million cubic meters of pre-treated water is infiltrated and recovered annually for the purpose of disinfection, buffering, and attenuation of water quality. The system consists of a sequence of alternating, parallel elongated recharge basins and rows of recovery wells. The basins and wells are spaced 70 m apart. Groundwater velocities are on the order of 1-2 m/d. SEAWAT was used to simulate combined groundwater flow and heat transport, both in the saturated as in the unsaturated zone. The thermal and hydraulic properties in the unsaturated zone were approximated using the van Genuchten relation and by using a constant water table. Both Distributed Temperature Sensing (DTS) measurements along vertically installed fiber optic cables, and temperature sensors in pumping wells, were modeled and matched for the year 2014.Water ManagementCivil Engineering and Geoscience

Estimation of temperature and associated uncertainty from fiber-optic raman-spectrum distributed temperature sensing

Distributed temperature sensing (DTS) systems can be used to estimate the temperature along optic fibers of several kilometers at a sub-meter interval. DTS systems function by shooting laser pulses through a fiber and measuring its backscatter intensity at two distinct wavelengths in the Raman spectrum. The scattering-loss coefficients for these wavelengths are temperature-dependent, so that the temperature along the fiber can be estimated using calibration to fiber sections with a known temperature. A new calibration approach is developed that allows for an estimate of the uncertainty of the estimated temperature, which varies along the fiber and with time. The uncertainty is a result of the noise from the detectors and the uncertainty in the calibrated parameters that relate the backscatter intensity to temperature. Estimation of the confidence interval of the temperature requires an estimate of the distribution of the noise from the detectors and an estimate of the multi-variate distribution of the parameters. Both distributions are propagated with Monte Carlo sampling to approximate the probability density function of the estimated temperature, which is different at each point along the fiber and varies over time. Various summarizing statistics are computed from the approximate probability density function, such as the confidence intervals and the standard uncertainty (the estimated standard deviation) of the estimated temperature. An example is presented to demonstrate the approach and to assess the reasonableness of the estimated confidence intervals. The approach is implemented in the open-source Python package “dtscalibration”.Corrigendum: DOI 10.3390/s21030912. The original article has been updated.Water Resource

Small-Scale ASR Between Flow Barriers in a Saline Aquifer

Regular aquifer storage recovery, ASR, is often not feasible for small-scale storage in brackish or saline aquifers because fresh water floats to the top of the aquifer where it is unrecoverable. Flow barriers that partially penetrate a brackish or saline aquifer prevent a stored volume of fresh water from expanding sideways, thus increasing the recovery efficiency. In this paper, the groundwater flow and mixing is studied during injection, storage, and recovery of fresh water in a brackish or saline aquifer in a flow-tank experiment and by numerical modeling to investigate the effect of density difference, hydraulic conductivity, pumping rate, cyclic operation, and flow barrier settings. Two injection and recovery methods are investigated: constant flux and constant head. Fresh water recovery rates on the order of 65% in the first cycle climbing to as much as 90% in the following cycles were achievable for the studied configurations with constant flux whereas the recovery efficiency was somewhat lower for constant head. The spatial variation in flow velocity over the width of the storage zone influences the recovery efficiency, because it induces leakage of fresh water underneath the barriers during injection and upconing of salt water during recovery.Water Resource

Estimation of the Variation in Specific Discharge Over Large Depth Using Distributed Temperature Sensing (DTS) Measurements of the Heat Pulse Response

An approach is presented to determine groundwater flow in unconsolidated aquifers with a heat pulse response test using a heating cable and a fiber-optic cable. The cables are installed together using direct push so that the cables are in direct contact with the aquifer. The temperature response is measured for multiple days along the fiber-optic cable with Distributed Temperature Sensing (DTS). The new approach fits a two-dimensional analytical solution to the temperature measurements, so that the specific discharge can be estimated without knowledge of the position of the fiber-optic cable relative to the heating cable. Two case studies are presented. The first case study is at a managed aquifer recharge system where fiber-optic cables are inserted 15 m deep at various locations to test the fitting procedure. Similar and relatively large specific discharges are found at the different locations with little vertical variation (0.4–0.6 m/day). The second case study is at a polder, where the water level is maintained 2 m below the surrounding lakes, resulting in significant groundwater flow. The heating and fiber-optic cables are inserted to a depth of 45 m. The specific discharge varies 0.07–0.1 m/day and is significantly larger in a thin layer at 30-m depth. It is shown with numerical experiments that the estimated specific discharge is smoother than in reality due to vertical conduction, but the peak specific discharge is estimated correctly for layers thicker than ∼1.5 m.Water Resource

A passive heat tracer experiment to determine the seasonal variation in residence times in a managed aquifer recharge system with DTS

Targeted provisional session N°8.01 The seasonal variation in residence times is determined in a managed aquifer recharge system using a passive heat tracer test. The managed aquifer recharge system consists of a sequence of alternating elongated recharge basins and rows of recovery wells. The temperature of both the water in the recharge basin and the surface influence the temperature in the aquifer. The flow field changes when the temperature changes, as the hydraulic conductivity is a function of the temperature. Fiber optic cables were inserted up to a depth of 20 meters with direct push equipment to measure vertical temperature profiles with DTS. In this fashion, the fiber optic cables are in direct contact with the aquifer and the disturbance of the aquifer is minimal. The measured spatial and temporal temperature variations in the subsurface were modeled with SEAWAT, a coupled flow and heat transport model. MODPATH was used to compute flow paths and residence times. During the winter, a larger fraction of the water moves through the warmer lower part of the aquifer, thereby increasing the residence time. The opposite happens during the summer, when most of the water moves through the warmer upper part of the aquifer, resulting in shorter residence times.Water Resource

Thermodynamics of a fast-moving Greenlandic outlet glacier revealed by fiber-optic distributed temperature sensing

Measurements of ice temperature provide crucial constraints on ice viscosity and the thermodynamic processes occurring within a glacier. However, such measurements are presently limited by a small number of relatively coarse-spatial-resolution borehole records, especially for ice sheets. Here, we advance our understanding of glacier thermodynamics with an exceptionally high-vertical-resolution (∼0.65 m), distributed-fiber-optic temperature-sensing profile from a 1043-m borehole drilled to the base of Sermeq Kujalleq (Store Glacier), Greenland. We report substantial but isolated strain heating within interglacial-phase ice at 208 to 242 m depth together with strongly heterogeneous ice deformation in glacial-phase ice below 889 m. We also observe a high-strain interface between glacial- and interglacial-phase ice and a 73-m-thick temperate basal layer, interpreted as locally formed and important for the glacier's fast motion. These findings demonstrate notable spatial heterogeneity, both vertically and at the catchment scale, in the conditions facilitating the fast motion of marine-terminating glaciers in Greenland. Water ResourcesGeoscience and Remote Sensin