Heat as a groundwater tracer in shallow and deep heterogeneous media: Analytical solution, spreadsheet tool, and field applications

Effects of Soil Type and Thermal Boundary on Predicting Temperature Profiles and Groundwater Fluxes

In the last few years, several articles have studied heat as a groundwater tracer and developed analytical geothermal solutions to predict the subsurface temperature and groundwater fluxes. These solutions can be sorted into steady-state and transient solutions. The steady-state solutions cannot describe the time-varying subsurface temperature, while the transient solutions ignore subsurface thermal boundary effects. Moreover, soil type may be another crucial factor significantly affecting the prediction results. This study compares six existing classical analytical solutions to examine the effects of soil types and subsurface thermal boundaries on simulating temperature-depth profiles and estimating groundwater fluxes. Several synthetic cases are built by considering the common soil types, sand and clay, to demonstrate their effects on predicting the profiles. A field case is used to show the effect of subsurface thermal boundaries on the groundwater flux estimated by an inverse approach. The study results indicate that the soil types have significant influences on simulating the profiles, and the influences grow with time. Some existing solutions may give inaccurate estimations of the field groundwater flux since they merely consider the heat source from the temperature variations on the ground surface but ignore possible thermal boundary effects in the subsurface. These findings will be valuable to those applying heat as a tracer to investigate infiltration.

Using Heat as a Tracer to Detect the Development of the Recharge Bulb in Managed Aquifer Recharge Schemes

Managed Aquifer Recharge (MAR), the intentional recharge of aquifers, has surged worldwide in the last 60 years as one of the options to preserve and increase water resources availability. However, estimating the extent of the area impacted by the recharge operations is not an obvious task. In this descriptive study, we monitored the spatiotemporal variation of the groundwater temperature in a phreatic aquifer before and during MAR operations, for 15 days, at the LIFE REWAT pilot infiltration basin using surface water as recharge source. The study was carried out in the winter season, taking advantage of the existing marked difference in temperature between the surface water (cold, between 8 and 13 °C, and in quasi-equilibrium with the air temperature) and the groundwater temperature, ranging between 10 and 18 °C. This difference in heat carried by groundwater was then used as a tracer. Results show that in the experiment the cold infiltrated surface water moved through the aquifer, allowing us to identify the development and extension in two dimensions of the recharge plume resulting from the MAR infiltration basin operations. Forced convection is the dominant heat transport mechanism. Further data, to be gathered at high frequency, and modeling analyses using the heat distribution at different depths are needed to identify the evolution of the recharge bulb in the three-dimensional space.

Spreadsheet Tools for Quantifying Seepage Flux Across the GW-SW Interface

Identifying the spatial distribution and magnitude of seepage flux across the groundwater-surface water (GW-SW) interface is critical for assessing potential impairments and restoration alternatives for water bodies adjacent to sites with groundwater contamination. Measurement of the vertical distribution and time-varying characteristics of temperature in sediments provides an indirect way to map out spatial and temporal patterns of seepage flux into surface water. Two spreadsheet-based calculation tools are introduced that implement four one-dimensional analytical solutions to calculate the magnitude and direction of seepage flux based on measurement of steady-state vertical temperature profiles or transient diel temperature signals at two depths within sediment. Performance of these calculation tools is demonstrated for a pond receiving contaminated groundwater discharge from an adjacent landfill. Transient versus steady-state model performance is compared, and limitations of transient modelsare illustrated for a situation with unfavorable sediment characteristics and inadequate sensor spacing. The availability of a range of analytical solutions implemented within Microsoft Excel^® is intended to encourage practitioners to explore use of this seepage flux characterization method and develop greater insight into best practices for model selection and use.

Geophysical Assessment of a Proposed Landfill Site in Fredericktown, Missouri

In cooperation with the U.S. Environmental Protection Agency (EPA), the U.S. Geological Survey (USGS) collected numerous types of geophysical data at a site in Fredericktown, Missouri, in June 2018. Various geophysical surveys were collectively used to help evaluate the overall suitability of the site for use as a mine waste-soil and sediment repository, and to evaluate the suite of geophysical methods for rapid collection and preliminary assessment of sites with shallow sediments. Land-based geophysical methods, which included frequency-domain electromagnetic induction (FDEM), electrical resistivity tomography (ERT), horizontal-to-vertical spectral ratio passive seismic (HVSR), and shear-wave refraction, were used to determine the depths to crystalline bedrock and characterize the overlying unconsolidated sediments (or regolith). Water-borne FDEM profiles and forward-looking infrared (FLIR) thermal image surveys were conducted along the Fredericktown City Lake shoreline to identify locations of potential interactions between groundwater and surface water. Sediment temperature profilers were installed at two locations along the shoreline to characterize shallow unconsolidated sediment thermal properties and support the interpretation of the other geophysical surveys. Geophysical reconnaissance methods including the FDEM and HVSR methods, were used to rapidly evaluate the vertical and lateral extent of overburden, or unconsolidated sediments, overlying the bedrock at the site. The results of these methods were compared to reference geophysical methods of ERT and shear-wave refraction surveys that have greater accuracy and are more labor intensive and time-consuming. A goal of the project was the evaluation of the validity and reliability of this suite of reconnaissance geophysical methods as a means by which shallow (less than 3 meters (m)) sediments can be rapidly assessed. Two orthogonal ERT survey profiles, which used 28 electrodes spaced 1 m apart in dipole-dipole and combined Wenner-Schlumberger configurations, were collected to determine the subsurface resistivity. The results were inverted to produce electrical resistivity profiles that were compared to the FDEM and HVSR survey results. The FDEM data were collected along cleared paths through the proposed disposal cell locations. The data were inverted to generate depth-dependent estimates of electrical conductivity along the transects. An analysis of the depth of investigation (DOI) indicated the FDEM imaged to depths of about 3 m below land surface. The ERT, FDEM, and HVSR indicated the depth to crystalline bedrock was approximately 1.5 m below land surface with shallower and deeper areas. Results from this investigation indicate this suite of methods will likely perform well at sites with shallow depths to bedrock and strong conductivity and acoustic impedance contrasts, where the FDEM and HVSR methods can provide estimates of the depth to bedrock, and ERT and shear-wave refraction surveys might not be worth the added time and expense.

Comparison of Applications to Evaluate Groundwater Recharge at Lower Kelantan River Basin, Malaysia

Groundwater has supported 70% of the water supply at the Lower Kelantan River Basin (LKRB) since the 1930s and demand for groundwater increases annually. Groundwater has been abstracted from shallow and deep aquifers. However, a comprehensive study on groundwater recharge estimation has never been reported. This study evaluated various methods to quantify recharge rate using chloride mass balance (CMB), water table fluctuation (WTF), temperature–depth profiles (TDP), and groundwater modelling coupled with water balance (GM(WB)). Recharge estimation using CMB, WTF, TDP, and GM(WB) showed high variability within 8% to 68% of annual rainfall. CMB is range from 16% to 68%, WTF 11% to 19%, TDP 8% to 11%, and GM(WB) 7% to 12% of annual rainfall, respectively. At 11%, recharge from GM(WB) was the best method for estimation because the model was constructed and calibrated using locally derived input parameters. GM(WB) is the only method involved with calibration and validation process to reduce the uncertainty. The WTF method based on long-term hydrological records gives a reasonable recharge value, in good agreement with GM(WB) and these methods can be paired to ensure the reliability of recharge value approximation in the same ranges. Applying various methods has given insight into methods selection to quantify recharge at LKRB and it is recommended that a lysimeter is installed as a direct method to estimate recharge.

Evaluation of Temperature Profiling and Seepage Meter Methods for Quantifying Submarine Groundwater Discharge to Coastal Lagoons: Impacts of Saltwater Intrusion and the Associated Thermal Regime

Surface water-groundwater interactions were studied in a coastal lagoon performing 180 seepage meter measurements and using heat as a tracer in 30 locations along a lagoon inlet. The direct seepage meter measurements were compared with the results from analytical solutions for the 1D heat transport equation in three different scenarios: (1) Homogeneous bulk thermal conductivity (Ke); (2) horizontal heterogeneity in Ke; and (3) horizontal and vertical heterogeneity in Ke. The proportion of fresh groundwater and saline recirculated lagoon water collected from the seepage experiment was used to infer the location of the saline wedge and its effect on both the seepage meter results and the thermal regime in the lagoon bed, conditioning the use of the thermal methods. The different scenarios provided the basis for a better understanding of the underlying processes in a coastal groundwater-discharging area, a key factor to apply the best-suited method to characterize such processes. The thermal methods were more reliable in areas with high fresh groundwater discharge than in areas with high recirculation of saline lagoon water. The seepage meter experiments highlighted the importance of geochemical water sampling to estimate the origin of the exchanged water through the lagoon bed.

Automated Laboratory Infiltrometer to Estimate Saturated Hydraulic Conductivity Using an Arduino Microcontroller Board

This paper describes the design, calibration and testing processes of a new device named Automated Laboratory Infiltrometer (ALI). It allows to determinate in laboratory, under controlled conditions the saturated hydraulic conductivity (Ks) of altered or unaltered soil samples which is a key parameter to understand the movement of water through a porous medium. The ALI combines the advantages of three different approaches: measures vertical infiltration rates in a soil column, measures the actual volumes of vertically drained water through the soil column, and finally, uses heat as a natural tracer to determinate water flux rates through the porous medium; all those parameters are used to determinate Ks. The ALI was developed using the popular Arduino microcontroller board and commercially available sensors that give the whole system a low cost. Data from the ALI are recorded in a microSD memory so they can be easily read from any spreadsheet software helping to reduce time consuming and avoiding reading errors. The performance of this device was evaluated by comparing the water flow rates determined by the three approaches for which is designed; an excellent correlation among them was observed (worst correlation: R2 = 0.9826 and r-RSME = 0.94%).

Hydrogeochemical controls on brook trout spawning habitats in a coastal stream

Brook trout (Salvelinus fontinalis) spawn in fall and overwintering egg development can benefit from stable, relatively warm temperatures in groundwater-seepage zones. However, eggs are also sensitive to dissolved oxygen concentration, which may be reduced in discharging groundwater (i.e., seepage). We investigated a 2 km reach of the coastal Quashnet River in Cape Cod, Massachusetts, USA, to relate preferred fish spawning habitats to geology, geomorphology, and discharging groundwater geochemistry. Thermal reconnaissance methods were used to locate zones of rapid groundwater discharge, which were predominantly found along the central channel of a wider stream valley section. Pore-water chemistry and temporal vertical groundwater flux were measured at a subset of these zones during field campaigns over several seasons. Seepage zones in open-valley sub-reaches generally showed suboxic conditions and higher dissolved solutes compared to the underlying glacial outwash aquifer. These discharge zones were cross-referenced with preferred brook trout redds and evaluated during 10 years of observation, all of which were associated with discrete alcove features in steep cutbanks, where stream meander bends intersect the glacial valley walls. Seepage in these repeat spawning zones was generally stronger and more variable than in open-valley sites, with higher dissolved oxygen and reduced solute concentrations. The combined evidence indicates that regional groundwater discharge along the broader valley bottom is predominantly suboxic due to the influence of near-stream organic deposits; trout show no obvious preference for these zones when spawning. However, the meander bends that cut into sandy deposits near the valley walls generate strong oxic seepage zones that are utilized routinely for redd construction and the overwintering of trout eggs. Stable water isotopic data support the conclusion that repeat spawning zones are located directly on preferential discharges of more localized groundwater. In similar coastal systems with extensive valley peat deposits, the specific use of groundwater-discharge points by brook trout may be limited to morphologies such as cutbanks, where groundwater flow paths do not encounter substantial buried organic material and remain oxygen-rich.

Heat as a groundwater tracer in shallow and deep heterogeneous media: Analytical solution, spreadsheet tool, and field applications

Groundwater flow advects heat, and thus, the deviation of subsurface temperatures from an expected conduction-dominated regime can be analysed to estimate vertical water fluxes. A number of analytical approaches have been proposed for using heat as a groundwater tracer, and these have typically assumed a homogeneous medium. However, heterogeneous thermal properties are ubiquitous in subsurface environments, both at the scale of geologic strata and at finer scales in streambeds. Herein, we apply the analytical solution of Shan and Bodvarsson (2004), developed for estimating vertical water fluxes in layered systems, in 2 new environments distinct from previous vadose zone applications. The utility of the solution for studying groundwater-surface water exchange is demonstrated using temperature data collected from an upwelling streambed with sediment layers, and a simple sensitivity analysis using these data indicates the solution is relatively robust. Also, a deeper temperature profile recorded in a borehole in South Australia is analysed to estimate deeper water fluxes. The analytical solution is able to match observed thermal gradients, including the change in slope at sediment interfaces. Results indicate that not accounting for layering can yield errors in the magnitude and even direction of the inferred Darcy fluxes. A simple automated spreadsheet tool (Flux-LM) is presented to allow users to input temperature and layer data and solve the inverse problem to estimate groundwater flux rates from shallow (e.g., <1 m) or deep (e.g., up to 100 m) profiles. The solution is not transient, and thus, it should be cautiously applied where diel signals propagate or in deeper zones where multi-decadal surface signals have disturbed subsurface thermal regimes.