Nature Of Groundwater Flow Research Articles

Non‐Darcian flow toward an injection well fully penetrating a leaky confined aquifer with clogging‐induced permeability reduction

AbstractManaged aquifer recharge (MAR) using aquifer injection wells is generally developed with the validity of Darcy's law and assumption of a constant hydraulic conductivity, hence ignoring the non‐Darcian nature of groundwater flow and the spatiotemporal permeability reduction owing to well and aquifer clogging. In this study, an Izbash's law‐based analytical model is proposed for constant‐rate recharge (CRR) and constant‐head recharge (CHR) with special consideration of well and aquifer clogging using a time‐dependent hydraulic conductivity (K) function. By means of linearization approximations, variable substitutions and Hankel and Weber transforms, approximate analytical solutions for the model are derived. The early‐time and late‐time solutions and the steady‐state solutions are also derived using the developed approximate solutions. A comparison with numerical solutions indicates that the approximate solutions are highly accurate except during the short period of initial recharge stage. Then, a semi‐log graphic approach based on the developed solutions is tendered for estimating parameters associated with the clogging effect and non‐Darcian aquifer property. The proposed model offers a straightforward way for the quantitative assessment of the non‐Darcian flow dynamics in confined aquifers experiencing clogging and can be adopted for the estimation and prediction of clogging‐related permeability reduction in MAR using aquifer injection wells.

Economic and spatial modelling of groundwater extraction

This essay examines the underlying spatial assumptionsprevalent in economic models of groundwater extraction.Due to the common pool nature (see Glossary at the end)of aquifers, economists have long claimed that whengroundwater is extracted under competition—in theabsence of regulations in terms of the location and ratesof extraction—a series of externalities prevent the efficientexploitation of the resource (Provencher and Burt 1994).Specifically, extraction of an additional unit of groundwa-ter reduces the total stock, which creates two consequen-ces. First, a lower stock reduces future availability andextraction alternatives; thus, a stock externality ariseswhenever the stock constraint is binding. Second, apumping cost externality arises because a lower stockincreases the depth to groundwater and, consequently, theextraction costs of other users. Other types of externalitiesincluding third-party effects in the form of groundwaterquality deterioration and greater income risk may alsoarise with extraction. Given the spatial-dynamic nature ofgroundwater flow, the extent of all these externalitiesdepends on the quantity, location and time of extractionand on the type of strategic behaviour under competition(Negri 1989).In the presence of a competitive and unregulatedextraction regime, the temporal and spatial profile ofexternal effects results in inefficient pricing and misallo-cation of the resource. Users extract too much, too quicklyand from the wrong locations. Aquifers are depletedfaster, pumping costs increase and water quality maydeteriorate, resulting in lower profits from groundwaterextraction and resilience of the groundwater system.Gisser and Sanchez (1980) were the first to provide acounter-intuitive result contrary to the prevailing view thatgroundwater extraction is inefficient in the absence ofregulations or ways to internalise the externalities of waterwithdrawal. Under restrictive economic, hydrologic andagronomic assumptions, they found that there is nosubstantive quantitative difference between optimal rulesfor pumping water and competitive/unregulated rates. Thishas become known as the Gisser-Sanchez effect (GSE)and still dominates economic thinking in terms ofgroundwater management. Namely, that the welfare lossfrom inter-temporal misallocation of pumping effort isnegligible.This essay briefly reviews the underlying assumptionsof the GSE and then presents alternative model specifica-tions that allow for spatial heterogeneity and both spatialand endogenous decision making by aquifer extractors.The conclusion describes two key findings of spatial andendogenous location (SEL) models that are in contrast tothe GSE.

Cave exploration as a guide to geologic research in the Appalachians

Cave exploration and mapping can provide considerable insight into the nature of groundwater flow and geologic processes in soluble rocks. The Appalachian Mountains provide an ideal setting for this exchange of information because their geology varies greatly over short distances. Caves reveal the way in which groundwater flow is guided by geologic structure, and they help to clarify aquifer test data, well yield, and contaminant dispersion. Well tests in karst aquifers often reveal confined or unconfined conditions that make little sense stratigraphically, but which can be explained with the aid of cave mapping. With regard to geologic mapping, many caves reveal structures that are not visible at the surface. Caves also show evidence for underground geochemical processes that cannot be detected from well data. Subtle mineralogical clues are generally erased by weathering and erosion at the surface, but persist in many caves. The information that caves have provided about subsurface geology and water flow is now being used by explorers, even those with no geologic background, to help them find new caves. DOI: 10.4311/jcks2008es0042

Inferring source waters from measurements of carbonate spring response to storms

We infer information about the nature of groundwater flow within a karst aquifer from the physical and chemical response of a spring to storm events. The spring discharges from the Maynardville Limestone in Bear Creek Valley, Tennessee. Initially, spring discharge peaks approximately 1–2 h from the midpoint of summer storms. The initial peak is likely due to surface loading, which pressurizes the aquifer and results in water moving out of storage. All of the storms monitored exhibited recessions that follow a master recession curve very closely, indicating that storm response is fairly consistent and repeatable, independent of the time between storms and the configuration of the rain event itself. Electrical conductivity initially increases for 0.5–2.9 days (longer for smaller storms), the result of moving older water out of storage. This is followed by a 2.1–2.5 day decrease in conductivity, resulting from an increasing portion of low conductivity recharge water entering the spring. Stable carbon isotope data and the calcite saturation index of the spring water also support this conceptual model. Spring flow is likely controlled by displaced water from the aquifer rather than by direct recharge through the soil zone.

SULPHATE DYNAMICS IN RELATION TO GROUNDWATER–SURFACE WATER INTERACTIONS IN HEADWATER WETLANDS OF THE SOUTHERN CANADIAN SHIELD

The spatial and temporal distribution of sulphate (SO4) concentrations in peat pore water and the outlet streams of two forested swamps was related to variations in the magnitude of upland runoff, wetland water levels and flow path. The swamps were located in headwater catchments with contrasting till depths typical of the southern Canadian Shield. Inputs of SO4 from shallow hillslope tills and streams showed little seasonal variation in either source or concentration in both swamps. Sulphate dynamics at the outlet stream reflected hydrological and biogeochemical processes within the valley wetlands, which in turn were partly controlled by catchment hydrogeology. During high runoff, maximum water table elevations and peak surface flow in the swamps resulted in upland inputs largely bypassing anoxic peat. Consequently, SO4 concentrations of 8–10 mg/l at the swamp outlets were similar to stream and groundwater inputs. During periods of low flow, concentrations of SO4 at the swamp outlets declined to less than 3 mg/l. At this time lower water table elevations resulted in increased interaction of input water with anoxic peats, and therefore, SO4 reduction. Contrasts in till depth and the nature of groundwater flow between catchments resulted in differences in SO4 dynamics between years and swamps. In dry summers the absence of groundwater inputs to the swamp in the catchment with thin till resulted in a large water table drawdown and re-oxidation of accumulated S, which contributed to maximum SO4 concentrations (up to 35 mg/l) during storm runoff. Continuous groundwater input to the swamp in the catchment with deeper till was critical to maintaining saturated surfaces and efficient SO4 retention during both dry and wet summers. A conceptual model of wetland SO4 retention and export, based on catchment hydrogeology, is developed to generalize the SO4 dynamics of valley bottom wetlands at the landscape scale. © 1997 by John Wiley & Sons, Ltd.

Groundwater flow, velocity, and age in a thick, fine-grained till unit in southeastern Wisconsin

Piezometer nests were installed at study sites in each of five north-south-trending end moraines of the late Pleistocene Oak Creek Formation in southeastern Wisconsin. The formation is composed primarily of a fine-grained glacial diamicton (till) and laterally continuous and discontinuous, coarse-grained lake and meltwater stream sediment. It overlies the Silurian dolomite aquifer, which is a source of drinking water to rural areas. The average vertical linear velocity and age of ground water in the Oak Creek Formation were estimated by three methods: Darcy's Law, environmental isotopes including 3H, δ 2H, δ 18O, and 14C (dissolved inorganic carbon), and solute transport modeling of 18O. The F-1 and Metro sites in the Tinley moraine showed excellent agreement among the three estimates of vertical velocity and showed the lowest velocity values (0.3–0.5 cm year −1 downward), which suggests that diffusion controls vertical mass transport at these sites. Although the extrapolated maximum age of ground water is 35 000 years, ground water below 75 m at these sites is probably not older than 15 000 years, which is the maximum age of the formation. Estimates of velocity showed less agreement at study sites in the Lake Border moraine system to the east and ranged from about 0.2 to 20.7 cm year −1; maximum groundwater age could range from 213 to 6000 years. Higher and more variable velocities, perhaps owing to thinner and more heterogeneous sediment in these areas, suggest that diffusion may not dominate vertical mass transport. Heterogeneity and fractures may also promote the development of groundwater flow systems dominated by lateral flow. Because of the uncertainty about the nature of groundwater flow, velocity, and age in the formation east of the Tinley moraine, future waste-disposal activity in southeastern Wisconsin should be confined to the thickest parts of the Tinley moraine near the present F-1 and Metro sites.

The nature of groundwater flow in fractured rock: Evidence from the isotopic and chemical evolution of recrystallized fracture calcites from the Canadian Precambrian Shield

The isotope geochemistry of fracture calcites in three Precambrian plutons on the Canadian Shield has been investigated in order to understand the paleohydrogeological conditions in fractured crystalline rock. Fracture calcites of ancient hydrothermal origins in the Chalk River and East Bull Lake plutons exhibit 18O enrichment and 13C depletion trends resulting from recent low-temperature calcite recrystallization under open-system conditions for oxygen, but semiclosed for carbon, and under extremely variable time-integrated, water/rock (calcite) ratios. This process causes recycling of elements with calcite distribution coefficients > 1 (rare earths, manganese, and possibly iron) from the precursor calcite to younger calcites as well as calcite control over the 87Sr 86Sr ratio of the groundwater within the Chalk River pluton. The large but variable water/rock (calcite) mole ratios calculated from the shifts in the stable isotopic compositions of fracture calcites are compatible with fracture flow models that invoke flow channeling within single fractures that also contain regions of immobile porosity. Quantification of isotopic shifts resulting from recrystallization requires that the initial isotopic composition of the precursor hydrothermal calcite be well constrained. Unlike the Chalk River and East Bull Lake plutons, hydrothermal fracture calcites in the White Lake pluton, which intrudes marble-rich country rocks of the Grenville Supergroup, have highly variable δ 13C and δ 18O values. This is attributed to mixing of carbon from magmatic and sedimentary reservoirs, and to oxygen isotopic exchange between hydrothermal fluids and carbonate country rocks at the time of intrusion. Convective circulation of meteoric groundwater from the surrounding 13C-rich carbonate rocks (up to 6.6%.) resulted in δ 13C values as heavy as 3.0%. for the fracture calcites in the pluton.