Vertical Groundwater Velocities Research Articles

Analytical models for heat transfer around a single ground heat exchanger in the presence of both horizontal and vertical groundwater flow considering a convective boundary condition

Ground heat exchanges (GHEs) are widely applied in the ground source heat pump (GSHP) system for exploration of shallow geothermal energy. Heat transfer around a single GHE in the presence of both horizontal and vertical groundwater flow under a convective boundary condition is investigated. To describe the heat transfer process, three typical GHE models including the solid cylindrical heat source (SCS) model, the ring coil heat source (RCS) model and the finite line source (FLS) model are improved by incorporating the effect of horizontal and vertical groundwater flow and considering convection between the air and the ground surface. After introducing a new variable, analytical solutions for the three improved models are established using the Green's function method. Comparison between the analytical solution and a numerical solution of COMSOL is made, and the agreement between the two solutions confirms the derivation and programming process of the analytical solution. Effects of horizontal and vertical groundwater flow on the temperature increment caused by a SCS are investigated using the analytical solution. For situations with high velocities of horizontal groundwater flow, the curve relating the temperature increment and the depth has a depth-independent segment, and the temperature increment for locations near the mid-depth of heat source remains unchanged with the velocity of vertical groundwater flow changing; the variation of average temperature increment of pipe wall with the variation of vertical groundwater velocity is negligible. For situations with low velocities of horizontal groundwater flow (Pex = 10), strong vertical groundwater flow (Pez = 100) makes the aforementioned depth-independent segment disappear, and the temperature increment at a downstream location increases from 0.58 °C to 7.79 °C with the depth increasing from 0 to lc. With Pez increasing from 10 to 100, the average temperature increment of pipe wall decreases from 10.14 °C to 6.92 °C, which reduces the thermal resistance of ground and improves the economic performance of the GSHP system.

Spatial Variability of Groundwater Arsenic Concentration as Controlled by Hydrogeology: Conceptual Analysis Using 2-D Reactive Transport Modeling.

Combined geological, hydrogeological, and geochemical controls on the arsenic concentration of contaminated aquifers in SE Asia were explored by two‐dimensional (2‐D) reactive transport modeling of data sets from Bangladesh, Cambodia, and Vietnam. For each site, the field data are summarized and used to create a conceptual 2‐D reactive transport model that elucidates characteristic features influencing the groundwater arsenic concentration. Comparison of models for Bangladesh and Vietnam indicates that fine‐grained layers overlying young sandy aquifers generate shallow high arsenic groundwater because low vertical groundwater velocities allow sufficient time for kinetic As release from the sediment. The low vertical groundwater velocity below major river channels, predicted by the model, also creates long groundwater residence times, leading to high arsenic groundwater. Young aquifer sediments release more arsenic than older sediments, and alternating young and older sediments create complex patterns of high and low arsenic groundwater. Over time, floodplain basins will subside, and river channels migrate, causing sedimentation and erosion on the floodplain while creating local environments with evolving hydrogeology and groundwater geochemistry. We have developed a three‐step model for the evolution of the Red River floodplain with sedimentation and shifting channels over the last 6000 years. The results show comparable timescales between the dynamics of arsenic release and of river migration, causing complex groundwater As distributions, comprising geochemical palinopsia of long vanished rivers.

Mangroves buffer marine protected area from impacts of Bovoni Landfill, St. Thomas, United States Virgin Islands

One of the many ecosystem services that mangrove systems provide is their ability to act as buffers between the land and sea, protecting human development from storm surges while also trapping terrestrial pollutants. In St. Thomas, United States Virgin Islands, an ecologically-important mangrove system sits between Bovoni Landfill and a marine protected area, the St. Thomas East End Reserves. To characterize the physical processes driving this mangrove system, groundwater hydraulic head, sediment cores, sediment surface temperatures, and water and sediment chemistry were analyzed. Hydraulic head data from January to November 2014 were used to determine vertical and horizontal groundwater flow directions. Water and sediment samples were tested for heavy metals potentially originating from Bovoni Landfill. Stratigraphic context was provided by the sediment cores and used to infer past environmental conditions. Subsamples were taken from these cores and analyzed for dry bulk density, organic matter content (through loss on ignition), and heavy metals using electron microscopy. Vertical groundwater velocity and sediment porosity were determined by calibrating a one-dimensional finite difference heat transport model to near surface temperature data from depths of 0, 7, 14, and 21 cm. Groundwater was found to flow from the terrestrial upland, through the mangroves, and toward the ocean for the majority of the study. Flow reversal was seen after long periods of little precipitation. In the surface and shallow groundwater samples, trace metal concentrations were measured from 23 to 105 μg/L for Cr, Ni, Sn, and Zn. Sediment samples collected near the landfill contained Bi, Cr, Sn, Ti, and Zn. Very slow flushing of sediment pore water was indicated by the vertical groundwater velocities produced from the heat transport model, which ranged from ±10–7 to ±10–9 m/s. This study revealed that the mangrove system is an important buffer system protecting the outer lagoon of the marine protected area from terrestrial contaminants via sediment trapping and slowing of water fluxes from the upland area into the lagoon. The results presented here can be used as a baseline for future studies and are relevant to local managers and to landfill closure plans.

Shallow subsurface temperature in the environs of El-Nubaria canal, northwestern Nile Delta of Egypt: implications for monitoring groundwater flow system

Borehole temperatures from 47 wells were utilized to comprehend the spatial subsurface thermal regime in the study area. The recharge regions are situated in reclaimed desert lands (southern and southwestern parts) and have high subsurface temperatures, whereas the discharge areas are situated in the old agricultural lands (northern and northeastern parts) and have lower subsurface temperatures. This thermal system is attributed to differences in the surface air temperature where higher values are recorded in the developed desert (Wadi El-Natrun station average annual 23.15 °C) and the lower values in the old agricultural land (Damanhour station average annual 20.37 °C). The geothermal gradient is low in the recharge area (average 0.0198 °C/m) and high in the discharge area (average 0.0343 °C/m). The effect of the irrigation canals as gaining or losing streams on the thermal system decreases gradually and disappears at 90 m below sea level. Two-dimensional groundwater flow and heat transport models were simulated to estimate the vertical groundwater flow velocity and predict the factors controlling the groundwater flow system. Permeability was the most sensitive parameter during the simulation process. The assessed vertical linear groundwater velocities are, up to 3.2 mm/day in the main recharge region toward the southwestern course, up to 2.4 and 2 mm/day under El-Bustan and El-Hager canals, respectively, and up to 1.4 mm/day in the contact between the reclaimed desert and the old agricultural lands. The average vertical linear groundwater discharge velocities are, more than 2.4 mm/day in the southwestern corner, 0.8 and 0.6 mm/day in the northeastern and northern directions, respectively and 1.2 and 2.4 mm/day under El-Nubaria and Ferhash canals, respectively.

Paleohydrogeology of the Cretaceous sediments of the Williston Basin using stable isotopes of water

Hydraulic and isotopic data collected from aquifers are routinely used to characterize hydrogeological conditions within sedimentary basins, but similar data from confining units are generally not collected despite their ability to provide insights on important water/solute transport controls. We characterized paleogroundwater flow and solute transport mechanisms across 384 m of Cretaceous shale (aquitard) in the Williston Basin, Canada, using high‐resolution depth profiles of water isotopes (δ18O, δ2H). Water samples were also collected from wells installed in the underlying regional sandy aquifer (Mannville Fm; 93 m thick) and from seepage inflows into potash mine shafts (to 825 m below ground). The 1‐D numerical transport modeling of δ18O profiles provided insight into large‐scale/long‐term solute transport in both Cretaceous sediments and the basin. Despite the potential for significant advective migration during glaciations, molecular diffusion appears to be the dominant solute transport mechanism through the aquitard. Simulations suggest average vertical groundwater velocities of <0.05 m/10 ka and an average excess hydraulic head of <10 m; these values are much less than anticipated by successive glaciations. The dominant paleoevent reflected in present‐day profiles is introduction during the Pleistocene of glaciogenic meteoric water to the aquifer underlying the shale, likely along an aquifer outcrop area east of the site or through local vertical conduits. Simulations suggest these recharge events occurred during one or more glacial periods. The isotopic profile over the upper 25 m of Pleistocene till and shale is consistent with glacial deposition and transport processes within these units over the Holocene (past 10 ka).

Salt marsh ecohydrological zonation due to heterogeneous vegetation–groundwater–surface water interactions

Vegetation zonation and tidal hydrology are basic attributes of intertidal salt marshes, but specific links among vegetation zonation, plant water use, and spatiotemporally dynamic hydrology have eluded thorough characterization. We developed a quantitative model of an intensively studied salt marsh field site, integrating coupled 2‐D surface water and 3‐D groundwater flow and zonal plant water use. Comparison of model scenarios with and without heterogeneity in (1) evapotranspiration rates and rooting depths, according to mapped vegetation zonation, and (2) sediment hydraulic properties from inferred geological heterogeneity revealed the coupled importance of both sources of ecohydrological variability at the site. Complex spatial variations in root zone pressure heads, saturations, and vertical groundwater velocities emerged in the model but only when both sources of ecohydrological variability were represented together and with tidal dynamics. These regions of distinctive root zone hydraulic conditions, caused by the intersection of vegetation and sediment spatial patterns, were termed “ecohydrological zones” (EHZ). Five EHZ emerged from different combinations of sediment hydraulic properties and evapotranspiration rates, and two EHZ emerged from local topography. Simulated pressure heads and groundwater dynamics among the EHZ were validated with field data. The model and data showed that hydraulic differences between EHZ were masked shortly after a flooding tide but again became prominent during prolonged marsh exposure. We suggest that ecohydrological zones, which reflect the combined influences of topographic, sediment, and vegetation heterogeneity and do not emphasize one influence over the others, are the fundamental spatial habitat units comprising the salt marsh ecosystem.

Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses

Using groundwater age determination done through CFC analysis and geochemical data obtained from seven sites in Brittany (France), a hydrogeochemical model for hard-rock aquifers is presented. According to the geological structure, three zones can be defined: the weathered layer, about 30 m thick; the weathered-fissured layer (fractured rock with a high density of fissures induced by weathering), which represents a transition zone between the weathered zone and the lower fractured zone; and the unweathered part of the aquifer. (1) The weathered layer (alterites) is often considered as a porous medium and is the only part frequently used in hard-rock aquifers. Recent apparent ages (0­10 a) are observed in the groundwater fluctuation zone in a thin layer, which is from 1­2 m-thick in the lower parts and 10­15 m-thick in the upper parts of the catchments. Below this thin layer, the groundwater apparent age is high (between 10 and 25 a) and is unexpectedly homogeneous at the regional scale. This groundwater apparent age contrast, which also corresponds to a Cl- concentration contrast, is attributed to rapid lateral transfers in the fluctuation zone which limit water transfer to the underlying weathered zone. Groundwater chemistry is characterized by and Cl- concentrations related to land uses (high in agricultural areas, low in preserved ones). (2) At the interface between the weathered and the weathered-fissured layers a strong biogeochemical reactivity is observed. Autotrophic denitrification is enhanced by a higher availability of sulfides. (3) Under this interface, in the weathered-fissured layer and the underlying fractured deep part of the aquifer, groundwater apparent age is clearly correlated to depth. The vertical groundwater velocity is estimated to be 3 m/a, whatever be the site, which seems to indicate a regional topographic control on groundwater circulation in the deep part of the aquifer. In this deep part, groundwater chemistry is modified by water­rock interaction processes as indicated by Ca and Na concentrations, and a slight sea-water contribution (from 0.1% to 0.65%) in the sites close to the seacoast. One site inland shows a saline and old end-member. The global hydrogeochemical scheme is modified when the aquifer is pumped at a high rate in the fissured-weathered layer and/or the fractured layer. The increase in water velocity leads to a homogeneous groundwater apparent age, whatever be the depth in the weathered-fissured and fractured layers.

Groundwater flow and velocity in a 500 ka pre-Illinoian till, eastern Iowa

Few hydrology studies have investigated glacial till older than Illinoian time (> 300,000 BP) despite these older tills overlying a large portion of North America. An 8- and 6-well monitoring well nest installed into a 31 m thick pre-Illinoian till sequence near Cedar Rapids, Iowa was characterized using traditional hydrologic methods and chemical tracers. The aquitard system consists of about 9 m of fine-grained oxidized pre-Illinoian till overlying 22 m of unoxidized till and Devonian dolomite bedrock. Hydraulic conductivity ranged from 10−7 m/s in oxidized till and 10−10 m/s in unoxidized till. Hydraulic head relations indicated downward groundwater flow through the till profile with hydraulic gradients steepest near the unoxidized till/bedrock interface. Tritium and nitrate concentrations indicated recent (< 50 years old) recharge to a depth of 9–12 m below land surface. 18O and 2H results ranged between −6.2 to −7.9% and −38.0 to −50.9%, respectively, and plotted near the local Meteoric Water Line. A 1 per mil shift toward less negative 18O values with depth may suggest a climate change signal contained in the till water but more data are needed to verify this trend. Vertical groundwater velocity through the unoxidized till was estimated to range from 0.4 to 5.7 cm/year. The thickness of unoxidized pre-Illinoian till in Linn County was estimated from available records and contoured against vertical travel times to evaluate the effectiveness of pre-Illinoian till in preventing nitrate migration to underlying bedrock aquifers.

Using solver to determine vertical groundwater velocities by temperature variations, Purdue University, Indiana, USA

Vertical groundwater velocities can be estimated based on directions of groundwater thermal gradients. Temperature-depth profiles were obtained from 12 monitoring wells at 3 different times of the year (Fall, Winter, and Spring) in West Lafayette, Indiana (USA) mainly on the Purdue University campus. Microsoft Excel Solver was used to match plots of groundwater temperature distribution in the wells with published type curves in order to find a dimensionless parameter β, from which vertical groundwater velocities were obtained. The vertical groundwater velocities found in the monitoring wells ranged from 0.92 to 4.53 cm/yr. Clay-rich aquitards presented greater vertical groundwater velocities than outwash aquifers. The highest groundwater velocities occurred in the Spring while the lowest were during the Winter. This method was found to be especially useful in glacially-derived materials with varying hydraulic conductivities for estimating vertical groundwater velocities in-situ.

Temporal and spatial variations of shallow subsurface temperature as a record of lateral variations in groundwater flow

In the present paper it is shown how profiles consisting of closely spaced (≤10 m) temperature measurements at shallow depth, obtained at several instances during one season, provide a detailed record of lateral variations in vertical groundwater flow. This is illustrated by a field study around the Peel Boundary Fault zone that cuts through the unconsolidated, siliciclastic deposits that occur in the southeastern part of Netherlands. This regionally important fault forms at many locations a strong barrier to horizontal groundwater flow and therefore induces complex groundwater flow patterns. Temperature anomalies (over 2°C) are observed over short distances. These anomalies reverse over the season. Numerical modeling of coupled groundwater flow and heat transport demonstrates how the temporal and spatial variations of subsurface temperature are the result of the interaction between seasonal fluctuations in surface temperature and spatial variations in groundwater flow. In addition to the horizontal profiles, temperature‐depth profiles obtained in groundwater observation wells were used to constrain the larger‐scale characteristics of the groundwater flow system. In order to simulate the observed geothermal patterns it appeared to be essential to account for the long‐term changes in surface temperature. Although groundwater temperature data are commonly used to constrain groundwater flow fields on regional scale or to calculate vertical groundwater velocities at point locations beneath small streams, the present study is one of the first to integrate these different scales and incorporate the impact of recent climate change.

A validation of the 3H/3He method for determining groundwater recharge

Tritium and He isotopes have been measured at a site where groundwater flow is nearly vertical for a travel time of 100 years and where recharge rates are spatially variable. Because the mid‐1960s 3H peak (arising from aboveground testing of thermonuclear devices) is well‐defined, the vertical groundwater velocity is known with unusual accuracy at this site. Utilizing 3H and its stable daughter 3He to determine groundwater ages, we compute a recharge rate of 0.16 m/yr, which agrees to within about 5% of the value based on the depth of the 3H peak (measured both in 1986 and 1991) and two‐dimensional modeling in an area of high recharge. Zero 3H/3He age occurs at a depth that is approximately equal to the average depth of the annual low water table, even though the capillary fringe extends to land surface during most of the year at the study site. In an area of low recharge (0.05 m/yr) where the 3H peak (and hence the vertical velocity) is also well‐defined, the 3H/3He results could not be used to compute recharge because samples were not collected sufficiently far above the 3H peak; however, modeling indicates that the 3H/3He age gradient near the water table is an accurate measure of vertical velocities in the low‐recharge area. Because 3H and 3He have different diffusion coefficients, and because the amount of mechanical mixing is different in the area of high recharge than in the low‐recharge area, we have separated the dispersive effects of mechanical mixing from molecular diffusion. We estimate a longitudinal dispersivity of 0.07 m and effective diffusion coefficients for 3H (3HHO) and 3He of 2.4×10−5 and 1.3×10−4 m2/day, respectively. Although the 3H/3He age gradient is an excellent indicator of vertical groundwater velocities above the mid‐1960s 3H peak, dispersive mixing and diffusive loss of 3He perturb the age gradient near and below the 3H peak.

Origin, age and movement of pore water in argillaceous Quaternary deposits at four sites in southwestern Ontario

Hydrogeologic information in the form of textural properties, field- and laboratory-determined hydraulic conductivity values, vertical hydraulic gradients from piezometer nests, and major-ion and isotopic contents ( 18O, 2H, 3H, 13C, 14C), was obtained from four sites located in southwestern Ontario on thick deposits of clayey till and glaciolacustrine clay. At each site the piezometers were installed from the water-table zone, situated 1–4 m below the surface, to a maximum depth between 20 and 35 m below the surface. Values of hydraulic conductivity determined in the laboratory by consolidometer and triaxial methods are in the order of 10 −8–10 −7 cm/s and are similar to values calculated from rates of water-level rise in piezometers. Mean values of average vertical groundwater velocity, calculated with the measured values of hydraulic conductivity, porosity and hydraulic gradient, using the Darcy equation for a saturated non-fractured porous medium, range from 0.13 to 0.26 cm/yr. Tritiated pore waters only occur within 3–6 m of ground surface, thereby indicating that groundwaters below this depth recharged prior to at least 1952. 18O in the pore waters exhibit a distinctive regular shift with depth from water-table values of −9 to −10‰ (SMOW), characteristic of present-day precipitation, to values between −14 and −17‰ at depths of 20–40 m, which are characteristic of much cooler waters. Corrected 14C ages of the groundwater are greater than 8000 yr. B.P., thereby suggesting that the groundwaters at depth entered these deposits during or after the formation of these deposits approximately 11,000 to 14,000 yr. ago. 18O, 2H and Cl − concentration profiles were simulated with a one-dimensional model for transport by advection and diffusion in a saturated porous medium. Reasonable agreement between the model and the field profiles was obtained with values of effective diffusion coefficients of 3.0 · 10 −6 cm 2/s for 18O, 2H and Cl − and with groundwater velocities between 0.03 to 0.05 cm/yr. Based on these results, it is concluded that the pore water in these deposits is a mixture of late Pleistocene and modern waters and that the distribution of 18C, 2H and Cl − in these deposits is influenced predominantly by molecular diffusion, apparently more so than by hydraulic flow.

Determination of recharge rates using temperature‐depth profiles in wells

Recharge rates to the shallow dolomite aquifer in northeastern Illinois are determined using a theory which describes the simultaneous transfer of heat and fluid in a porous medium. Temperature‐depth profiles in a nonpumping well are used to determine vertical groundwater velocities of the downward leakage of water recharging the aquifer which is overlain by semiconfining beds of clay‐rich glacial drift. Calculated and observed depths of penetration (19 m) of the annual surface temperature wave, and demonstration of the thermal stability of the well, validate application of the theory, which uses a dimensionless parameter β to relate curvature in temperature‐depth profiles to the magnitude of vertical leakage. Generally, the magnitude of β decreases as the depth interval L within the aquitard increases. The corresponding decrease in the vertical component of groundwater velocity accompanies a gradual coarsening with depth of the aquitard materials. Temperature differences over time at a given depth within a thermally stable well are found to be within the accuracy of the temperature measuring system. Analysis of variations in β Over time shows that the variability can be due to the limitations of the resolution (0.01°C) of the temperature measuring system. Recharge rates determined by analyses of temperature‐depth profiles range from 2.47×10−9 to 9.86×10−9 m/s and agree with estimates resulting from water‐budget method.

Forced convective heat transfer in the monroe hot springs geothermal system

A knowledge of convective heat transfer is essential to understanding geothermal systems and other systems of moving groundwater. A simple, kinematic approach toward convective heat transfer is taken in this paper. Concern is not with the cause of the groundwater motion but only with the fact that the water is moving and transferring heat. The mathematical basis of convective heat transfer is the energy equation which is a statement of the first law of thermodynamics. The general solution of this equation for a specific model of groundwater flow has to be done numerically. The numerical algorithm used here employs a finite difference approximation to the energy equation that uses central differences for the heat conduction terms and one-sided differences for the heat convection terms. Gauss-Seidel iteration is then used to solve the finite difference equation at each node of a non-uniform mesh. The Monroe and Red Hill hot springs, a small hydrothermal system in central Utah, provide an example to illustrate the application of convective heat transfer theory to a geophysical problem. Two important conclusions regarding small geothermal systems follow immediately from the results of this application. First, the most rapid temperature rise in the convecting part of a geothermal system is near the surface. Below this initially rapid temperature increase the temperature increases very slowly, and thus temperatures extrapolated from shallow boreholes can be seriously in error. Second, the temperatures and heat flows observed at Monroe and Red Hill, and probably at many other small geothermal areas, can easily result from moderate vertical groundwater velocities in faults and fracture zones in an area of normal heat flow.

Measurement of Vertical Groundwater Velocity from Temperature Profiles in Wells

Vertically moving groundwater transports heat by convection and causes curvature in the earth's thermal profile. Dimensionless plots of the temperature distribution in wells can be matched with published type curves to obtain solutions for vertical groundwater velocity, if the thermal conductivity of the solid‐fluid complex is known or can be estimated. Rates of upward movement through semiconfining beds determined from temperature studies in the San Luis valley of Colorado and the Roswell basin of New Mexico were in good agreement with rates computed from pumping tests and water budget methods. Limitations of the method result from instability in borehole fluids, measurement detail required, and magnitudes of flow that can be detected.