High Recharge Rates Research Articles

Richards Equation Model of a Rain Garden

Traditional stormwater management does not mitigate groundwater depletion resulting from groundwater pumping and reduction in recharge. Infiltration practices, such as rain gardens, offer a potentially effective approach for addressing groundwater depletion. A rain garden is a landscaped garden in a shallow depression that receives the stormwater from nearby impervious surfaces, focusing recharge. We have developed a numerical model that can be applied in rain garden design and evaluation. Water flow through the rain garden soil is modeled over three layers: a root zone, a middle storage layer of high conductivity, and a subsoil lower layer. To continuously simulate recharge, runoff, and evapotranspiration, the model couples the Richards Equation with a surface water balance. The model was applied to the climate of southern Wisconsin. Simulation results show that very high recharge rates are possible during the non-snowfall season. (The model does not handle snowmelt.) A rain garden with an area of about ...

Effects of Stormwater Infiltration on Quality of Groundwater Beneath Retention and Detention Basins

Infiltration of storm water through detention and retention basins may increase the risk of groundwater contamination, especially in areas where the soil is sandy and the water table shallow, and contaminants may not have a chance to degrade or sorb onto soil particles before reaching the saturated zone. Groundwater from 16 monitoring wells installed in basins in southern New Jersey was compared to the quality of shallow groundwater from 30 wells in areas of new-urban land use. Basin groundwater contained much lower levels of dissolved oxygen, which affected concentrations of major ions. Patterns of volatile organic compound and pesticide occurrence in basin groundwater reflected the land use in the drainage areas served by the basins, and differed from patterns in background samples, exhibiting a greater occurrence of petroleum hydrocarbons and certain pesticides. Dilution effects and volatilization likely decrease the concentration and detection frequency of certain compounds commonly found in background groundwater. High recharge rates in storm water basins may cause loading factors to be substantial even when constituent concentrations in infiltrating storm water are relatively low.

Perched Water Bodies in Arid Environments and Their Role as Hydrologic Constraints for Recharge Rate Estimation: Part 2. The Case of Yucca Mountain

The formation and temporal evolution of perched water bodies in the Yucca Mountain (YM) site is the focus of this article. Following the approach proposed by Bagtzoglou (2003), recharge rates that would be required to form and sustain perched water bodies with characteristics similar to the known occurrences at YM are estimated through a series of detailed flow simulations. The results were tested for plausibility by comparing with hydrological and geochemical data. The proposed approach is applied with reasonable discriminatory success to a specific geologic cross-section at the YM site, and recharge rates of 0.13, 0.51, and 6.2 mm/yr are calculated for a set of three hydraulic property realizations. However, only the highest recharge rate (6.2 mm/yr) satisfies both hydrological and geochemical constraints. This is in excellent agreement with current estimates by the US Department of Energy.

Renewal rate estimation of groundwater based on radioactive tracers ( 3H, 14C) in an unconfined aquifer in a semi-arid area, Iullemeden Basin, Niger

Estimation of groundwater recharge in arid and semi-arid areas is difficult due to the low amount and variability of recharge. A combination of radiotracers investigation based on simple mixing models allows direct investigation of relatively long-term renewal rates of an aquifer. The recharge process of the shallow Continental Terminal aquifer in the Iullemeden basin (Niger) was investigated using a geochemical and isotopic approach. This study investigates the area in the one degree square of Nianey (13–14°N, 2–3°E). In this area, recharge is highly heterogeneous and mainly occurs through a drainage system of temporary streams and pools during the rainy season. Heterogeneity of the recharge is reflected through the wide variation in electrical conductivity and oxygen-18 content of the groundwater. The carbon-14 activity range for most of the groundwater falls between 69 and 126 pmc showing pre and post-aerial thermonuclear test recharge. Two renewal rate models have been investigated: the first one models a well-mixed reservoir and the second one is derived from a piston flow model, in which mixing is in equal proportions. Major ions in tritium data analyses allow exclusion of non-representative samples and confirm the carbon-14 renewal rate estimations. Both models give similar results for the relatively low renewal rate investigated in the area. Using carbon-14, the mean annual rates of groundwater renewal range from 3 to 0.03% of the aquifer volume with a median of 0.1%. Assuming the median is representative of the overall renewal rate of the area, the recharge rate is in the order of 5 mm a −1. The shallow aquifer recharge extends from the last small humid period (around 4000 a) up to now. High recharge rates are found in depressions whereas low recharge occurs below the plateaux.

Field study of spatial variability in unsaturated flow beneath and adjacent to playas

We quantified unsaturated flow beneath playa and adjacent interplaya settings at a site in the Southern High Plains (United States) to resolve issues related to where and how water moves through the unsaturated zone. This is the first study in which the data density (39 boreholes) and the variety of techniques used (physical, chemical, and isotopic) were sufficient to quantify spatial variability in unsaturated flow. Water contents, water potentials, and tritium eoneentrations were much higher and chloride eoneentrations were much lower beneath playas than in interplaya settings, which indicated that playas focus recharge. These results refute previous hypotheses that playas act as evaporation pans or that recharge is restricted to the annular region around playas. Water fluxes estimated from environmental tracers ranged from 60 to 120 mm yr−1 beneath playas and were ≤0.1 mm yr−1 during the past 2000–5000 years beneath natural interplaya areas not subjected to ponding. To evaluate the apparent inconsistency between high recharge rates and thick clay layers beneath playas, we applied bromide and FD&C blue dye to evaluate flow processes. These applied tracer experiments showed preferential flow along roots and desiccation cracks through structured clays in the shallow subsurface in playas.

Analytical Solutions of the Nonlinear Groundwater Flow Equation in Unconfined Aquifers and the Effect of Heterogeneity

Using the method of decomposition, new analytical solutions of the nonlinear Boussinesq flow equation and of the exact two‐dimensional groundwater flow equation subject to a nonlinear free‐surface boundary condition are presented and tested with respect to the linearized Boussinesq equation. It is found that for mild regional gradients and for the range of recharge values usually encountered in the field, the extensively used linearized equation with Dupuit assumptions is a reasonable approximation to the exact solution for the hydraulic heads and the regional flow velocities. Discrepancies occur in the presence of high regional hydraulic gradients, unusually high recharge rates, or regions of low hydraulic conductivity. With this result a new nonperturbation solution of the two‐dimensional plan view groundwater flow equation in a heterogeneous (heterogeneity represented as a spatial random field) aquifer is presented, and expressions for the statistical properties of the longitudinal and transverse velocities are derived. The results suggest that scale dependency, or spatial variability, in the flow velocity arises naturally as a result of recharge from rainfall and aquifer heterogeneity and may help explain the scale dependency of aquifer dispersion parameters.

The chemical and hydrologic structure of Poa´s Volcano, Costa Rica

Comparison of the chemical characteristics of spring and river water draining the flanks of Poa´s Volcano, Costa Rica indicates that acid chloride sulfate springs of the northwestern flank of the volcano are derived by leakage and mixing of acid brines formed in the summit hydrothermal system with dilute flank groundwater. Acid chloride sulfate waters of the Rio Agrio drainage basin on the northwestern flank are the only waters on Poa´s that are affected by leakage of acid brines from the summit hydrothermal system. Acid sulfate waters found on the northwestern flank are produced by the interaction of surface and shallow groundwater with dry and wet acid deposition of SO 2 and H 2SO 4 aerosols, respectively. The acid deposition is caused by a plume of acid gases that is released by a shallow magma body located beneath the active crater of Poa´s. No evidence for a deep reservoir of neutral pH sodium chloride brine is found at Poa´s. The lack of discharge of sodium chloride waters at Poa´s is attributed to two factors: (1) the presence of a relatively volatile-rich magma body degassing at shallow depths (< 1 km) into a high level summit groundwater system; and (2) the hydrologic structure of the volcano in which high rates of recharge combine with rapid lateral flow of shallow groundwater to prevent deep-seated sodium chloride fluids from ascending to the surface. The shallow depth of the volatile-rich magma results in the degassing of large quantities of SO 2 and HCl. These gases are readily hydrolyzed and quickly mix with meteoric water to form a reservoir of acid chloride-sulfate brine in the summit hydrothermal system. High recharge rates and steep hydraulic gradients associated with elevated topographic features of the summit region promote lateral flow of acid brines generated in the summit hydrothermal system. However, the same high recharge rates and steep hydraulic gradients prevent lateral flow of deep-seated fluids, thereby masking the presence of any sodium chloride brines that may exist in deeper parts of the volcanic edifice. Structural, stratigraphic, and topographic features of Poa´s Volcano are critical in restricting flow of acid brines to the northwestern flank of the volcano. A permeable lava-lahar sequence that outcrops in the Rio Agrio drainage basin forms a hydraulic conduit between the crater lake and acid chloride sulfate springs. Spring water residence times are estimated from tritium data and indicate that flow of acid brines from the active crater to the Rio Agrio source springs is relatively rapid (3 to 17 years). Hydraulic conductivity values of the lava-lahar sequence calculated from residence time estimates range from 10 −5 to 10 −7 m/s. These values are consistent with hydraulic conductivity values determined by aquifer tests of fractured and porous lava/pyroclastic sequences at the base of the northwestern flank of the volcano. Fluxes of dissolved rock-forming elements in Rio Agrio indicate that approximately 4300 and 1650 m 3 of rock are removed annually from the northwest flank aquifer and the active crater hydrothermal system, respectively. Over the lifetime of the hydrothermal system (100's to 1000's of years), significant increases in aquifer porosity and permeability should occur, in marked contrast to the reduction in permeability that often accompanies hydrothermal alteration in less acidic systems. Average fluxes of fluoride, chloride and sulfur calculated from discharge and compositional data collected in the Rio Agrio drainage basin over the period 1988–1990 are approximately 2, 38 and 30 metric tons/day. These fluxes should be representative of minimum volatile release rates at Poa´s in the last 10 to 20 years.

Dating of shallow groundwater: Comparison of the transient tracers 3H/3He, chlorofluorocarbons, and 85Kr

This paper describes a direct comparison of apparent ages derived from 3H/3He, chlorofluorocarbons (CCl3F and CCl2F2), and 85Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficial aquifers in late Cenozoic Atlantic Coastal Plain sediments of the Delmarva Peninsula, on the east coast of the United States. Most of the apparent tracer ages agree within 2 years of each other for recharge dates between 1965 and 1990. Discrepancies in apparent tracer ages usually can be explained by hydrological processes such as mixing in a discharge area. Recharge rate calculations based on apparent tracer age gradients at multilevel well locations agree with previous recharge estimates. High recharge rates on the Delmarva Peninsula result in nearly complete dissolved‐gas confinement in the groundwater. The remarkable agreement between the different tracer ages indicates negligible mixing of waters of different ages, insignificant dispersion, minimal gas loss to the atmosphere, and insignificant sorption‐desorption processes at this location.

Rates and patterns of groundwater flow in the Cascade Range Volcanic Arc, and the effect on subsurface temperatures

The central Oregon section of the Cascade Range volcanic arc is characterized by relatively high Quaternary volcanic extrusion rates and hot‐spring discharge rates, and by high conductive heat flow. However, a large area of near‐ zero near‐surface conductive heat flow occurs in the younger volcanic rocks, due to downward and lateral flow of cold groundwater. Alternate models for the high heat flow observed in older rocks on the flanks of the Cascade Range involve (1) a laterally extensive midcrustal heat source or (2) a narrower, spottier deep heat source that is confined to the Quaternary arc and is flanked by relatively shallow conductive heat flow anomalies caused by regional groundwater flow. We simulated groundwater flow and heat transport through two cross sections west of the Cascade Range crest: one in the Breitenbush area, where there is no major arc‐parallel normal faulting, and one in the McKenzie River drainage, where major graben‐bounding faults exist. Measured temperature profiles, hot‐spring discharge rates, and geochemical inferences constrain the results. The numerical simulations provide some estimates of regional‐scale permeabilities; simulated bulk permeabilities of ∼10−14 m2 in the youngest (0–2.3 Ma) rocks and ∼10−17 m2 in the oldest (18–25 Ma) rocks allow the thermal observations to be matched. In general, permeability decreases downsection, but for rocks of any age, permeability at very shallow (&lt;50 m) depths is probably much higher than the bulk permeability values required by the thermal observations: this is indicated by high recharge rates in 0–7 Ma rocks (&gt;1 m yr−1) and well‐test data from domestic wells in rocks older than 7 Ma (which indicate permeability values of about 10−14 to 10−12 m2). In the simulations, the alternate conceptual models for the deep thermal structure were represented as wide or localized deep heat sources. We found that either model can satisfy the observations. Thermal observations in the Breitenbush area seem to require significant advective heat transfer, whereas the sparser observations in the McKenzie River area can be satisfied with either advection‐ or conduction‐dominated simulations. Available regional gravity, magnetic, and electrical geophysical data do not clearly favor either of the two alternate models. Deep drilling in areas of high heat flow in the older rocks would be the most definitive test. The actual thermal structure is probably more complex than either of the models considered here.

Meteorologic and isotopic characteristics of precipitation events with implications for groundwater recharge, Southern High Plains

Recharge estimates for the Ogallala aquifer in the Southern High Plains vary by two orders of magnitude (0.01 to 1.6 inches/yr or 0.25 to 41 mm/yr) because of differing interpretations of the contribution of the area's numerous playa lakes to the recharge of the aquifer. Higher rates are suggested by studies in which the playa lakes are assumed to be a major source of recharge. To reevaluate the recharge potential of precipitation, we measured meteorologic and isotopic properties of single precipitation events at five stations for one year. Values of precipitation for δ 18O (−22.7‰ to −4.7‰) and δD (−162% to +35%) plot along the world meteoric water line. Mean annual weighted values of δ 18O and δD are −7.5‰ and −48‰, respectively. Air masses from the Gulf of Mexico account for most summer precipitation, which is more isotopically enriched than winter precipitation, mainly derived from air masses from the eastern Pacific Ocean. Precipitation is more depleted in δ 18O and δD toward the northwestern part of the Southern High Plains, becoming isotopically enriched toward the southeast. These isotopic differences reflect temperature, continental, and altitude effects. Isotopic composition of Ogalla ground water has a spatial distribution similar to that of precipitation. Ground water, however, is consistently more enriched than precipitation sampled in the same area, suggesting evaporation during recharge. Evaporation can occur either in playa lakes prior to infiltration or in the vadose zone. Tritium in precipitation ranges from 1.1 to 14.7 TU, and in groundwater it ranges from zero where the vadose zone is thick, to 73 TU, where the zone is thin. The calculated recharge rate (using tritium as a tracer) ranges from 0.5 to 3.24 inches/yr, or 12.7 to 82 mm/yr. Such high recharge rates were only measured below playa lakes. Because of the fast recharge rates and the slightly enriched values of δ 18O and δD in ground water, the Ogalla a aquifer is most likely recharged by focused percolation of partly evaporated playa lake water, rather than by slow regional diffusive percolation of precipitation.

Model study of some factors influencing the resaturation of spoil following mining and reclamation

This study demonstrates how groundwater flow models can be used to study the long-term response of groundwater flow in and around strip mines, particularly once mining and reclamation are complete. The two-dimensional, finite-element model used in this study is able to simulate the actual steps in mining and reclamation, and to model groundwater flow using a transient, free surface approach. Simulations presented here are carried out on two significantly different scales. In the mine-scale simulations, the mine occupies a major portion of the system. These simulations show that spoil resaturation depends on the magnitude and distribution of recharge and the hydraulic conductivity of spoil and geological units. During the early phase of spoil resaturation, high gradients along the water table can develop due to, first a lack of downward drainage through units below the mine or, second due to spoil with a low hydraulic conductivity. Conversely when conditions opposite to these develop, the gradients along the water table are relatively low. The time over which these changes in water level occur depends upon recharge rates and the storativity of the spoil. At an even smaller scale, individual hills formed during reclamation are the features of interest. Once reclamation is complete, the flow system will return to a new steady state with the position of the water table again dependent upon recharge rates and hydraulic conductivity but modified by hill size. A high water table is favoured by low hydraulic conductivities, high recharge rates, or hills with a wide base. While the addition of widely spaced surface drains is ineffective in controlling water levels, discrete open drains which are closely spaced within the spoil appear much more useful. Generally, the effect of a drain is independent of the size of the hill.

Preliminary evaluation of the groundwater resources of the western part of the Vale of Pickering

Summary An investigation of the water resources of the fissured limestone/sandstone Corallian Series aquifer was carried out between 1970 and 1974. The techniques employed and the interpretation of flow mechanisms and development potential may be relevant to other similar areas of the United Kingdom as well as further afield. Groundwater movement and chemistry are shown to be strongly related to geological structure. The aquifer is characterized by a relatively high recharge rate and, in some zones, high transmissivity; storage is very limited. This latter characteristic leads to difficulties in the formulation of development proposals, particularly if the aquifer is to be employed in river regulation.

Evaluation of a Shallow Leaky Water Table Aquifer in a Subhumid Region of France

ABSTRACTThe Arcachon Bay Plio‐Quaternary aquifer, located in a flat poorly‐drained region which receives one m annual precipitation, has a shallow water table which impedes agricultural development. It is composed of a 50‐100 m thick body of well‐sorted clean sand and gravel which has excellent hydrodynamic properties and much fresh ground water in storage. This affluent aquifer is underlain by, and interconnected with, a relatively poor calcareous aquifer of Miocene age.Leakage to the Miocene aquifer, inferred from geologic and hydrogeologic observations, was calculated by water budgeting and Darcy's law to be on the order of 1.5 m3/s. This rate, about 15 percent of the annual precipitation, is nearly half the recharge rate to the Plio‐Quaternary aquifer.Schoeller's chloride method was used to estimate recharge to the Plio‐Quaternary aquifer, and indicated that this recharge amounts to more than 35 percent of the annual precipitation. The calculation of the water budget supported this finding. The evapotranspiration rate, based on Thornthwaite's method, is more than 55 percent of the annual precipitation. High recharge and evapotranspiration rates, a direct consequence of the relief, permeability of soils, and occurrence of hot summers, are coupled with a low surface runoff.Exploitation of the Plio‐Quaternary aquifer can be planned confidently for irrigation and water‐supply purposes. Drainage improvement, needed for extensive farming of the lands, will also result from such exploitation. These development measures are suggested to be tailored and staged in the framework of the master plan aimed to develop and conserve the Arcachon Bay area and the region it is part of.Descriptors: subhumid environment, shallow water table, multiple‐aquifer systems, leaky aquifers, Darcy's law, aquifer recharge, Schoeller's chloride method, water budget, ground‐water quality, environmental hydrogeology, development and conservation.Identifiers: shallow leaky aquifers in subhumid environment.