Howard W. Reeves

The role of groundwater flow in controlling the spatial distribution of soil salinity and rooted macrophytes in a southeastern salt marsh, USA

Groundwater flow is an important factor in governing botanical zonation in the salt marsh at North Inlet, SC. Areas of the marsh adjacent to upland forest are characterized by upward flow of fresh groundwater. This inhibits the infiltration and evapoconcentration of saline tidal water and the development of a habitat for hypersaline-tolerant fugitive species such as Salicornia europaea. Areas of high marsh that are not adjacent to extensive upland forest are characterized by downward gradients in hydraulic head. This allows the infiltration and evapoconcentration of tidal water and the development of hypersaline conditions that are suitable for salt-tolerant fugitives.

How biofilm clusters affect substrate flux and ecological selection

We use three-dimensional mathematical modeling to represent key substrate phenomena in a prototypical cluster-and-channel biofilm. Clusters are represented as cylinders in which diffusion and reaction occur simultaneously. The geometry of the effective diffusion layer surrounding the cluster is manipulated to simulate different degrees of advection within the channels. The cluster-and-channel configuration can increase the average substrate flux per substratum area only if the channels have advection and the surface coverage is high enough. The modeling also suggests that sub-cluster niches for slower growing species, such as nutrifiers, are reduced in size when substrate penetrates the cluster from all sides. Thus, the cluster-and-channel configuration might create a competitive disadvantage for slow-growing species.

Groundwater dynamics along forest-marsh transects in a southeastern salt marsh, USA: Description, interpretation and challenges for numerical modeling

Long-term and spatially dense time series of total head measured alongthree forest-marsh piezometer transects across a finger marsh basin,together with bimonthly groundwater salinity measurements, reveal dynamicfeatures that present challenges to the interpretation and modeling of thisshallow water table aquifer. These include:1. Rapid response of forest water table to rain events.2. Daytime lowering (drawdown) of the water table in the forest and highmarsh due to evapotranspiration and its recovery due to rain, seepage ortidal inundation.3. Upward gradients in head along the western margin of the marsh butdownward gradients in the eastern, more seaward, high marsh and forest.4. Rapid head responses to the tide in parts of the high marsh andforest that are not actually inundated.5. Asymmetrical distribution of salinity with higher values in theeastern marsh and forest than in the west.6. Sharp salinity gradients across the tidal creek that bisects the basin. We discuss modeling issues related to these features because acomprehensive understanding of them may have a bearing on patterns ofbotanical zonation and primary production, the transport of nutrients andcontaminants, and the response of this system to sea level rise.

A New Capture Fraction Method to Map How Pumpage Affects Surface Water Flow

All groundwater pumped is balanced by removal of water somewhere, initially from storage in the aquifer and later from capture in the form of increase in recharge and decrease in discharge. Capture that results in a loss of water in streams, rivers, and wetlands now is a concern in many parts of the United States. Hydrologists commonly use analytical and numerical approaches to study temporal variations in sources of water to wells for select points of interest. Much can be learned about coupled surface/groundwater systems, however, by looking at the spatial distribution of theoretical capture for select times of interest. Development of maps of capture requires (1) a reasonably well-constructed transient or steady state model of an aquifer with head-dependent flow boundaries representing surface water features or evapotranspiration and (2) an automated procedure to run the model repeatedly and extract results, each time with a well in a different location. This paper presents new methods for simulating and mapping capture using three-dimensional groundwater flow models and presents examples from Arizona, Oregon, and Michigan.

Using prediction uncertainty analysis to design hydrologic monitoring networks: Example applications from the Great Lakes water availability pilot project

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Spatial patterns in soil water fluxes along a forest-marsh transect in the southeastern United States

Abstract. Time series of water levels in piezometer nests along a forest-marsh transect near North Inlet, SC, show fluctuations that are attributable to recharge by precipitation and tidal flooding and to removal by evapotranspiration (ET) and seepage out of the soil. Volumes of water associated with these water level changes were estimated by correlating rain-induced water level increases with measured rainfalls. In the forest the ratio of water table rise to rainfall is about 10. This ratio increases with decreasing elevation to about 40 in the mid marsh where the antecedent soil moisture is generally higher. The relative influence of removal by ET and seepage and recharge by rain and tides varies systematically along the transect. In the mainland forest, loss of water by ET is somewhat less than infiltration, leading to a net recharge of fresh water which eventually discharges in the adjacent mid marsh. With decreasing elevation, the relative importance of recharge by rain decreases as recharge by tidal flooding increases. In the low marsh, however, these mechanisms of loss and recharge can not be discerned in the water level time series because the water table rarely, if ever, drops below the marsh surface.

Linking MODFLOW with an agent-based land-use model to support decision making.

The U.S. Geological Survey numerical groundwater flow model, MODFLOW, was integrated with an agent-based land-use model to yield a simulator for environmental planning studies. Ultimately, this integrated simulator will be used as a means to organize information, illustrate potential system responses, and facilitate communication within a participatory modeling framework. Initial results show the potential system response to different zoning policy scenarios in terms of the spatial patterns of development, which is referred to as urban form, and consequent impacts on groundwater levels. These results illustrate how the integrated simulator is capable of representing the complexity of the system. From a groundwater modeling perspective, the most important aspect of the integration is that the simulator generates stresses on the groundwater system within the simulation in contrast to the traditional approach that requires the user to specify the stresses through time.

An iterative compositional model for subsurface multiphase flow

The governing equations describing multiphase flow with multicomponent solute transport may be cast in a number of forms. While mathematically equivalent, the numerical models based on each formulation differ in flexibility and efficiency. The set-iterative compositional formulation, a mathematical formulation that separates the solution of the phase-balance equations from the species balance is described herein. A mathematical model based on this formulation describes the flow of two mobile phases. Each phase may transport multiple chemical components. Mass exchange between phases is expressed by a linear kinetic equation. The numerical model based on the set-iterative compositional formulation is shown to model non-equilibrium phase partitioning, to provide a flexible framework that may be applied to organic mixtures with differing numbers of components and to yield a more efficient solution in comparison with models arising from standard compositional formulations.

SEASONAL PATTERNS IN THE SOIL WATER BALANCE OF A SPARTINA MARSH SITE AT NORTH INLET, SOUTH CAROLINA, USA

Time series of ground-water head at a mid-marsh site near North Inlet, South Carolina, USA can be classified into five types of forcing signatures based on the dominant water flux governing water-level dynamics during a given time interval. The fluxes that can be recognized are recharge by tides and rain, evapotranspiration (ET), seepage into the near surface soil from below, and seepage across the soil surface to balance either ET losses or seepage influxes from below. Minimal estimates for each flux can be made by multiplying the head change induced by it by the measured specific yield of the soil. These flux estimates are provide minimal values because ET fluxes resulting from this method are about half as large as those estimated from calculated potential evapotranspiration (PET), which place an upper limit on the actual ET. As evapotranspiration is not moisture-limited at this regularly submerged site, the actual ET is probably nearly equal to PET. Thus, all of the other fluxes are probably twice as large as those given by this method. Application of this method shows that recharge by tides and rain only occurs during spring and summer when ET exceeds upward seepage from below and is thereby able to draw down the water table below the marsh surface occasionally. During fall and winter, seepage of fresh water from below is largely balanced by seepage out of the soil into overlying tidal water or into sheet flow during tidal exposure. The resulting reduction in soil water salinity may thereby enhance the growth of Spartina in the following spring.

Axi-symmetric simulation of soil vapor extraction influenced by soil fracturing.

Fracturing, either pneumatic or hydraulic, is a method to improve the performance of soil vapor extraction (SVE) in relatively low permeability soils (< 10(-5) cm/s). A two-dimensional model is presented to simulate trichloroethylene (TCE) soil vapor extraction modified by fracturing. Flow and transport is modeled using mobile macropore and micropore networks, which also have been identified in the literature as dual porosity, dual permeability, or heterogeneous flow models. In this model, fluids can flow in both the macropore and micropore networks. This represents a more general model compared to immobile micropore, mobile macropore models presented thus far in the literature for vapor flow and transport in two dimensions. The model considers pressure- and concentration-driven exchange between the macropore and micropore networks, concentration-driven exchange between the gas and sorbed phases within each network, and equilibrium exchange between the gas and water and a sorbed phase within each network. The parameters employed in an example simulation are based on field measurements made at a fractured site. Considered in the simulations were the influence of the volume percentage of fractures, the length of fractures, the relative location of the water table, and the influence of pulsed pumping. For these simulations, internetwork concentration-driven exchange most significantly affected mass removal. The volume percentage of fractures more significantly influence flow and mass removal than the length of fractures. The depth of the water table below the contamination plume only significantly influenced flow and mass removal when the water table was within 60 cm of the bottom of the contaminated soil in the vadose zone for the parameters considered in this study. Pulsed pumping was not found to increase the amount of mass removed in this study.