Geology topics

Michael L. Merritt

Publications and source records attributed to Michael L. Merritt.

11 recordsLinked to original sources

USGS2005

Rewetting approximation for a simulator of flow in a surficial aquifer overlain by seasonally inundated wetlands

An approximation is presented to rectify situations occurring as part of the three-dimensional simulation of surficial aquifer flows in which surface grid cells become dry (zero saturated thickness) and then cannot readily receive flows that would again partially saturate them. The approximation permits the cells to be rewetted by adjusting dry cell pressures by the same amount as the pressure change computed for the uppermost nondry cells in the vertical column. This is continued in successive timesteps until the dry cells once again become partially or fully saturated. The rewetting procedure was tested in a recent transient simulation of flows in the surficial Biscayne aquifer of Dade County, Florida, which is partly covered by seasonally inundated wetlands. The uppermost layer of the model (layer 1) was used to represent overland sheetflow and grid cells of high equivalent hydraulic conductivity in this layer became dry and were rewetted seasonally, requiring a relatively robust representation of rewetting. The rewetting procedure made possible a simulation of overland sheetflow stages and aquifer water-table altitudes between 1945 and 1989 that was generally correct to within about 1 foot in inundated and noninundated regions. In a 6-year time period (1962–67) in which the behavior of the rewetting procedure was studied in detail, there were 4,723 simulated cell rewettings, of which 3,172 were in the overland sheetflow layer and 1,551 were in the uppermost aquifer layer (layer 2). The average head adjustment in a cell rewetting was 0.10 foot in layer 1 and 0.15 foot in layer 2. This study demonstrates an empirical approach to the problem of cell rewetting that might prove useful in other models of flow in surficial aquifers.

Groundwater↗

USGS2004

Estimating hydraulic properties of the Floridan Aquifer System by analysis of earth-tide, ocean-tide, and barometric effects, Collier and Hendry Counties, Florida

Aquifers are subjected to mechanical stresses from natural, non-anthropogenic, processes such as pressure loading or mechanical forcing of the aquifer by ocean tides, earth tides, and pressure fluctuations in the atmosphere. The resulting head fluctuations are evident even in deep confined aquifers. The present study was conducted for the purpose of reviewing the research that has been done on the use of these phenomena for estimating the values of aquifer properties, and determining which of the analytical techniques might be useful for estimating hydraulic properties in the dissolved-carbonate hydrologic environment of southern Florida. Fifteen techniques are discussed in this report, of which four were applied. An analytical solution for head oscillations in a well near enough to the ocean to be influenced by ocean tides was applied to data from monitor zones in a well near Naples, Florida. The solution assumes a completely non-leaky confining unit of infinite extent. Resulting values of transmissivity are in general agreement with the results of aquifer performance tests performed by the South Florida Water Management District. There seems to be an inconsistency between results of the amplitude ratio analysis and independent estimates of loading efficiency. A more general analytical solution that takes leakage through the confining layer into account yielded estimates that were lower than those obtained using the non-leaky method, and closer to the South Florida Water Management District estimates. A numerical model with a cross-sectional grid design was applied to explore additional aspects of the problem. A relation between specific storage and the head oscillation observed in a well provided estimates of specific storage that were considered reasonable. Porosity estimates based on the specific storage estimates were consistent with values obtained from measurements on core samples. Methods are described for determining aquifer diffusivity by comparing the time-varying drawdown in an open well with periodic pressure-head oscillations in the aquifer, but the applicability of such methods might be limited in studies of the Floridan aquifer system.

Florida↗

USGS2000

Documentation of a computer program to simulate lake-aquifer interaction using the MODFLOW ground water flow model and the MOC3D solute-transport model

Heads and flow patterns in surficial aquifers can be strongly influenced by the presence of stationary surface-water bodies (lakes) that are in direct contact, vertically and laterally, with the aquifer. Conversely, lake stages can be significantly affected by the volume of water that seeps through the lakebed that separates the lake from the aquifer. For these reasons, a set of computer subroutines called the Lake Package (LAK3) was developed to represent lake/aquifer interaction in numerical simulations using the U.S. Geological Survey three-dimensional, finite-difference, modular ground-water flow model MODFLOW and the U.S. Geological Survey three-dimensional method-of-characteristics solute-transport model MOC3D. In the Lake Package described in this report, a lake is represented as a volume of space within the model grid which consists of inactive cells extending downward from the upper surface of the grid. Active model grid cells bordering this space, representing the adjacent aquifer, exchange water with the lake at a rate determined by the relative heads and by conductances that are based on grid cell dimensions, hydraulic conductivities of the aquifer material, and user-specified leakance distributions that represent the resistance to flow through the material of the lakebed. Parts of the lake may become "dry" as upper layers of the model are dewatered, with a concomitant reduction in lake surface area, and may subsequently rewet when aquifer heads rise. An empirical approximation has been encoded to simulate the rewetting of a lake that becomes completely dry. The variations of lake stages are determined by independent water budgets computed for each lake in the model grid. This lake budget process makes the package a simulator of the response of lake stage to hydraulic stresses applied to the aquifer. Implementation of a lake water budget requires input of parameters including those representing the rate of lake atmospheric recharge and evaporation, overland runoff, and the rate of any direct withdrawal from, or augmentation of, the lake volume. The lake/aquifer interaction may be simulated in both transient and steady-state flow conditions, and the user may specify that lake stages be computed explicitly, semi-implicitly, or fully-implicitly in transient simulations. The lakes, and all sources of water entering the lakes, may have solute concentrations associated with them for use in solute-transport simulations using MOC3D. The Stream Package of MODFLOW-2000 and MOC3D represents stream connections to lakes, either as inflows or outflows. Because lakes with irregular bathymetry can exist as separate pools of water at lower stages, that coalesce to become a single body of water at higher stages, logic was added to the Lake Package to allow the representation of this process as a user option. If this option is selected, a system of linked pools (sublakes) is identified in each time step and stages are equalized based on current relative sublake surface areas.

Water-Resources Investigations Report↗

USGS1997

Tests of subsurface storage of freshwater at Hialeah, Dade County, Florida, and numerical simulation of the salinity of recovered water

Injection and observation wells were drilled in late 1974 for the purpose of conducting tests of storage and recovery of potable water in the brackish Upper Floridan aquifer. Three tests, involving storage and recovery cycles of varying volumes and storage period lengths, were performed between July 1975 and January 1980. Recovery was by natural artesian flow, and recovery efficiencies were 32.9, 47.8, and 38.5 percent. Wellbore plugging occurred during the injection stages, but injectivity was restored by periodic 2- to 3-hour backflushes at the natural artesian flow rate. An interval of shelly limestone between 1,015 and 1,050 feet below land surface contained the flow zone. Data from an analysis of 18 spinner flowmeter logs indicated that the principal part of the flow zone extended from 1,024 to 1,036 feet below land surface and that minor amounts of flow occurred to a depth of about 1,047 feet. A neutron porosity log indicated the bulk porosity of both the flow zone and confining layers to be 35 percent. Chloride and dissolved-solids concentrations of water in the flow zone were 1,200 and 2,700 milligrams per liter, respectively. A three-dimensional, finite-difference flow and solute-transport code was used to simulate pressure data measured during an aquifer test and observed salinity increases in recovered water during storage and recovery cycles. The aquifer test conducted in February 1975 was simulated by using a hydraulic conductivity estimate of 800 feet per day and a rock compressibility estimate of 0.0000400 (pound per square inch)-1. The equivalent transmissivity and storage coefficients were 9,600 cubic feet per day per square foot times foot of aquifer thickness and 7.8×10-5, respectively. Simulation of observed salinity increases during the three recoveries required dispersivities of 65 feet, a molecular diffusivity of 0.0002 foot squared per day, and a regional pore velocity of 260 feet per year. Central differencing in space and time was used for the solute-transport computations as well as an experimental method of computing vertical dispersion that used a scaling factor of 0.013. Additional simulations of the aquifer-test data and recovery salinities were obtained based on assumptions that (1) the flow zone was 21 feet thick, (2) flow-zone effective porosity was 20 percent, and (3) flow-zone hydraulic conductivity was bipolar anisotropic by a ratio of 10:1. The four sets of simulation values were used in model runs in which 10 years of annual injection, storage, and recovery cycles were simulated. Computed recovery efficiencies increased from 40 percent in the first year to 68 percent in later cycles. The high regional pore velocity required for model calibration substantially limited the recovery efficiency achieved in later cycles.

Florida↗

USGS1997

Computation of the time-varying flow rate from an artesian well in central Dade County, Florida, by analytical and numerical simulation methods

Simulation of the development of a plume of brackish water in a surficial aquifer caused by discharge from a flowing artesian well required an accurate estimate of the rate of flow during a 40-year period. The rate of flow from the well, constructed with 12-inch casing in 1944, was measured to be 2,350 gallons per minute 2 months after completion and 1,170 gallons per minute in 1964, 1965, and 1969. The rates measured 20 years apart appeared to be mutually inconsistent unless the difference could be explained by the: (1) drawdown of the aquifer over time, (2) raising of the altitude at which the water was discharged, (3) installation of 80 feet of 8-inch liner, or (4) deterioration of the condition of the well over time. The latter possibility implies losses of flows through holes in the casing and increased friction losses. Application of an analytical solution relating the time-varying rate of flow to a constant opposing head (a rewriting of the "constant drawdown" formula) failed to reconcile the measured rates, though estimates provided by use of the formula differed by only about 15 percent. Numerical simulation techniques were also used to estimate the rate of artesian flow from the source aquifer, a permeable zone at about 1,200 feet below land surface, near the top of the Upper Floridan aquifer in southeastern Florida. The selected simulation code contained a well-riser model that was used to account for friction losses in the well. Construction of a highly generalized model of the Floridan aquifer system for the flow-rate analysis, and the determination of a set of predevelopment head values for use as an initial condition, permitted simulation of the effects of recharge through leaky confining layers or from aquifer boundaries at a finite distance from the well. The conceptual model supported by the calibration of the model of the Floridan aquifer system is that a hydraulically uniform Lower Floridan aquifer ("Boulder Zone") provides recharge to the Upper Floridan aquifer through a leaky middle confining unit, and head variations in the Upper Floridan aquifer are related to variations in the thickness of the zone of fresh and brackish water extending downward into the confining unit. Results of the flow-rate analysis indicated that the flow rate should reach equilibrium after about 1 week because the Upper Floridan aquifer received recharge from the Boulder Zone through the intervening middle confining unit. The well modifications probably only decreased the rate of flow by 18 to 19 percent. A sensitivity analysis indicated that variation in the roughness coefficient of a degree that could represent severe deterioration of the well casing decreased the estimated flow rate by about 17.5 percent. Another sensitivity analysis indicated that installation of 80 feet of 8-inch liner had only a slight effect on the flow rate. The flow-rate inconsistency was not fully resolved by the analysis, but could be explained as a combination of the result of well modification, deterioration of the condition of the well, and other factors not amenable to analysis (inaccuracy in one or more of the flow-rate measurements or greater than estimated losses through the casing). The transmissivity of the source aquifer was estimated to be 11,125 feet squared per day at the well site. Besides the construction of a generalized model of the Floridan aquifer system, the most significant result of the analysis was the demonstration of a simulation approach for accurately showing the relation between aquifer characteristics and the rate of flow from artesian wells. However, this approach requires considerably more analytical effort and data describing aquifer properties than does application of the constant drawdown formula.

Florida↗

USGS1995

Computation of the time-varying flow rate from an artesian well in central Dade County, Florida, by analytical and numerical simulation methods

To construct a digital simulation of a plume of brackish water in the surficial Biscayne aquifer of central Dade County, Florida, that originated from a flowing artesian well, it was necessary to quantify the rate of spillage and the consequent point-source loading of the aquifer. However, a flow-rate measurement (2,350 gallons per minute) made 2 months after drilling of the well in 1944 was inconsistent with later measurements (1,170 gallons per minute) in 1964, 1965, and 1969. Possible explanations were the: (1) drawdown of the aquifer over time; (2) raising of the altitude at which the water was discharged; (3) installation of 80 feet of 8-inch liner; (4) an increase in the density of the flowing water; and (5) gradual deterioration of the well casing. The first approach to reconciling the measured flow rates was to apply a form of the equation for constant-drawdown analysis often used to estimate aquifer transmissivity. Next, a numerical simulation analysis was made that provided the means to account for friction loss in the well and recharge across vertically adjacent confining layers and from lateral boundaries. The numerical analysis required the construction of a generalized model of the subsurface from the surficial Biscayne aquifer to the cavernous, dolomitic Boulder Zone at a depth of 3,000 feet. Calibration of the generalized flow model required that the moddle confining unit of the Floridan aquifer system separating the artesian flow zone in the Upper Floridan aquifer from the Lower Floridan aquifer (the Boulder Zone) have a vertical hydraulic conductivity of at least 1 foot per day. The intermediate confining unit separating the flow zone from the surficial Biscayne aquifer was assigned a much lower hydraulic conductivity (0.01 foot per day or less). The model indicated that the observed mounding of Upper Floridan aquifer heads along the axis of the Florida Peninsula was related to the variable depth of the freshwater and brackish-water zone overlying deeper saline water. The analyses only partly reconciled the two rates. The second rate was accepted as representative of the conditions prevailing at the time of its measurement. On the basis of flowmeter logging, it was assumed that an additional 230 gallons per minute escaped through the corroded casing at that time. Factors not amenable to analysis, such as the inherent inaccuracy of the method of estimating flow from the well and possible error in estimating losses through the casing, could easily account for the remainder of the difference between the two measured rates.

Florida↗

USGS1989

Review of factors affecting recovery of freshwater stored in saline aquifers

A simulation analysis reported previously, and summarized herein, identified the effects of various geohydrologic and operational factors on recoverability of the injected water. Buoyancy stratification, downgradient advection, and hydrodynamic dispersion are the principal natural processes that reduce the amount of injected water that can be recovered. Buoyancy stratification is shown to depend on injection-zone permeability and the density contrast between injected and saline native water. Downgradient advection occurs as a result of natural or induced hydraulic gradients in the aquifer. Hydrodynamic dispersion reduces recovery efficiency by mixing some of the injected water with native saline aquifer water. In computer simulations, the relation of recovery efficiency to volume injected and its improvement during successive injection-recovery cycles was shown to depend on changes in the degree of hydrodynamic dispersion that occurs. Additional aspects of the subject are discussed.

Conference Paper↗

USGS1986

RECOVERY OF FRESHWATER STORED IN SALINE AQUIFERS IN PENINSULAR FLORIDA.

Subsurface freshwater storage has been operationally tested at seven sites in central and south Florida. Injection was into a high chloride water aquifer at six sites, and into a high sulfate water aquifer at the seventh. Recovery efficiency has ranged from 0 to 75 percent in high chloride water aquifers, and has exceeded 100 percent in the high sulfate water aquifer. Computer modeling techniques were used to examine the geohydrologic, design, and management factors governing the recovery efficiency of subsurface freshwater storage. The modeling approach permitted many combinations of geohydrologic and operational conditions to be studied at relatively low cost.

Conference Paper↗

USGS1985

Subsurface storage of freshwater in South Florida; a digital model analysis of recoverability

As part of a study of the feasibility of recovering freshwater injected and stored underground in south Florida, a digital solute-transport model was used to investigate the relation of recovery efficiency to the variety of hydrogeologic conditions that could prevail in brackish artesian aquifers and to a variety of management alternatives. The analyses employed a modeling approach in which the control for sensitivity testing was a hypothetical aquifer considered representative of permeable zones in south Florida that might be used for storage of freshwater. Parameter variations in the tests represented possible variations in aquifer conditions in the area. The applicability of the analyses to south Florida limestone aquifers required the assumption that flow nonuniformities in those aquifers are small on the scale of volumes of water likely to be injected, and that their effect could be represented as hydrodynamic dispersion. Generally, it was shown that a loss of recovery efficiency is caused by (1) processes causing mixing of injected freshwater with native saline water (hydrodynamic dispersion), (2) processes causing the more or less irreversible displacement of the injected freshwater with respect to the well (buoyancy stratification, background hydraulic gradients, and interlayer dispersion), or (3) processes causing injection and withdrawal flow patterns to be dissimilar (directionally biased well-bore plugging, and dissimilar injection and withdrawal schedules in multiple-well systems). Other results indicated that recovery efficiency improves considerably with successive cycles, providing that each recovery phase ends when the chloride concentration of withdrawn water exceeds established criteria for potability (usually 250 milligrams per liter), and that freshwater injected into highly permeable or highly saline aquifers (such as the 'boulder zone') would buoy rapidly. Many hydrologic conditions were posed for model analysis. To have obtained comparable results with operational testing would have been more costly by orders of magnitude. The tradeoff is that the validity of results obtained from computer modeling is somewhat less certain. In particular, results must be qualified with observations that (1) the complex set of processes lumped as hydrodynamic dispersion is represented with a somewhat simplified mathematical approximation, and (2) other flow processes in limestone injection zones are as yet incompletely understood. Despite such reservations, the study is considered a practical example of the use of transport models in ground-water investigations.

Water Supply Paper↗

USGS1983

Subsurface storage of freshwater in South Florida; a digital analysis of recoverability

As part of a feasibility study of cyclic freshwater injection, digital models were implemented to analyze the relation of recovery efficiency to various hydrogeologic conditions which could prevail in brackish aquifers and to various management regimes. The analyses implemented an approach in which the control for sensitivity testing was a hypothetical aquifer representative of potential injection zones in south Florida, and parameter variations in sensitivity tests represented possible variations in aquifer conditions in the area. The permeability of the aquifer determined whether buoyancy stratification could reduce recovery efficiency. The range of permeability leading to buoyancy stratification became lower as resident fluid salinity increased. Thus, recovery efficiency was optimized by both low permeability and low resident fluid density. High levels of simulated hydrodynamic dispersion led to the lowest estimates of recovery efficiency. Advection by regional flow within the artesian injection zone could significantly affect recovery efficiency, depending upon the storage period, the volume injected, and site-specific hydraulic characteristics. Recovery efficiency was unrelated to the rate of injection or withdrawal or to the degree of penetration of permeable layers, and improved with successive cycles of injection and recovery. (USGS)

Florida↗

USGS1979

Evaluation of a digital model for estuarine water quality simulation in waste allocation studies

Hydrologic and water-quality data were collected on 4 estuaries in Pasco, Citrus, and Pinellas Counties, Florida, to evaluate modeling results. Current and predicted waste loading of the four estuaries was simulated by use of a two-dimensional steady-state, intertidal-condition model. Concentrations of DO, carbonaceous and nitrogenous BOD, and chloride were simulated as averages over a tidal cycle. General equations for the model are based on the law of conservation of mass. Assumption of steady-state required that water-quality data for calibration be averaged over an appropriate time cycle with respect to volume and cross-section. Diurnal DO fluctuation was determined in 2 estuaries for evaluating the influence of photosynthesis and respiration. The estuary model is best applied by calibrating it for a particular set of observed conditions, and then using this calibrated model for sensitivity analyses without attempting to verify the chosen parameter values against a second set of conditions. Sensitivity analyses included dispersion coefficient , decay rates, photosynthesis, and respiration. (Woodard-USGS)

Water-Resources Investigations Report↗