Integrated Surface and Ground Water Management

Aquifers are currently used in the United States primarily for water supply purposes. In recent years, aquifer storage recovery (ASR) technology has been developed and implemented widely, utilizing aquifers for cost-effective water storage. Experience with these ASR systems has demonstrated that aquifers are also capable of providing treatment of the stored water for many constituents of great interest to U.S. water managers and utilities. Conveyance of water from points of recharge to points where the water is needed for recovery, is also beginning to be practiced in some areas. Accordingly, a new paradigm is needed that recognizes the ability of aquifers for water storage, treatment and conveyance. Changes in our regulatory framework are needed to accommodate this new paradigm.

The city of West Palm Beach, Florida, has developed a reuse program to maximize water conservation that includes an innovative approach for the reuse of treated wastewater and augmentation of the drinking water supply—a Wetlands-Based Water Reclamation Project (WBWRP). Implementation of the program will provide maximum flexibility for management of all water resources. The innovative technology of indirect potable reuse maximizes water conservation by taking treated wastewater and cleansing it to a very high level through several additional processes, including wetland treatment systems. This process will allow the city to augment their drinking water supply, create valuable wetlands, and protect existing wetland and water supply resources. To simulate the hydrologic interaction between surface water and groundwater at specific wetland locations within the model area, an integrated hydrologic model was needed. By using the WETLANDS module created by the South Florida Water Management District for use in the USGS MODFLOW, model the quantification of surface water mounding and preferential flow through the wetland sloughs could be accounted. The robust nature of the integrated surface water/groundwater flow model was confirmed by simulating groundwater and surface water elevations measured throughout the area for the period January 1994 through July 1997. The resulting model provided an excellent representation of the surficial aquifer system in east-central Palm Beach County and onsite surface water features. The model was then used for evaluating different pumping and recharge options associated with the WBWRP.

To understand why the City of Chandler has developed the Tumbleweed Recharge Facility, there must be a need to understand Chandler's history, where it began, and its relationship to the community, the industries, and its citizens. Beginning with the definition of our great City, from our economic development division: City with a Vision, it is this vision that laid the foundation to support the use of creative and innovative ideas like the use of aquifer storage and recovery wells. This paper will present a brief background and history of the City of Chandler and its experience with growth and its wastewater treatment capacity. The ability to expand the wastewater treatment capacity to meet the growth needs of the City is currently limited by the ability to reuse or recharge the effluent. The main focus of this paper is the use, design, and operation of recharge wells at the Tumbleweed Facility to handle the effluent recharge. The next generation and expansion of the Tumbleweed Recharge Facility includes the newer design of aquifer storage and recovery wells, which is also presented as a part of the latest and most innovative plan for managing effluent.

Water resources planning, development and management in Washington has become increasingly complex as a result of many factors. These include: rapid population growth and increasing demands on existing water supplies; the recent Federal listing of some salmon species under the Endangered Species Act; regulatory complexities surrounding water rights and in-stream flow needs; water supply planning uncertainties due to the historical political establishment of agencies; and a lack of regional conveyance facilities to move water between water supply sources. A proposed water supply, known as the Snoqualmie Aquifer Project, offers a unique opportunity for Central Puget Sound through conjunctive use of surface water and groundwater in the Snoqualmie River Basin. The project is conjunctive in two respects: 1) it utilizes both surface and groundwater, and 2) it increases the water supply during critical demand periods for both human and fish populations. The proposal is to pump the Snoqualmie Aquifer during high summer water supply demand periods (August–October) and disperse the pumped water directly into the Snoqualmie River. This groundwater would then flow downstream through critical salmon habitat during the summer low-flow period to a downstream location where it would be withdrawn, treated, and put into the regional water supply conveyance system. River withdrawals to the treatment plant would be carefully regulated in response to streamflow conditions and the amount of augmentation provided from the upstream well field. The project is unique in many respects: the configuration and prolific nature of the Snoqualmie Aquifer offer the opportunity for substantial seasonal pumping with real and measurable benefits to streamflow in the Snoqualmie River; high winter recharge rates provide a sufficient recovery period to minimize long-term impacts to streamflows and groundwater levels in the upper watershed; the timing of augmentation coincides with critical fish migration and spawning periods; and the system can be further optimized through conjunctive management of surface water reservoirs located within and adjacent to the Snoqualmie watershed.

Historically the delivery of water rights in the Boise River Basin in the State of Idaho has focused on flows in the Boise River proper. Water District 63 has for decades employed a Watermaster who has measured diversions from the river, and who has curtailed these diversions on a priority basis. Wells located near the river have never been considered to be part of the delivery system, even when delivery of pre-1900 water rights from the river is curtailed. Creation of conjunctive management rules in Idaho, and a growing awareness of the relationship between surface and ground water systems, have led regulators to consider the impacts of the diversion of ground water that is tributary to the Boise River upstream from and within the regulated reaches of the river. Proper and equitable long-term delivery of this ground water mandates that delivery be monitored in conjunction with Water District 63. Researchers are assembling data and technical analyses that will enhance the capability to determine the interconnectedness and flow patterns of ground water throughout the basin. More than 100 distinct GIS data layers have been assembled. However, a difficult question that remains is how these data and analyses will enable decision-making and implementation of decisions relative to ground water pumpage at specific locations throughout the basin. This paper proposes a methodology to conduct a one-day collaborative spatial decision-making session to be attended by the significant stakeholders relative to conjunctive management within the basin, including irrigators, municipalities, government and environmental representatives. The paper details a technique whereby commercially available software such as GeoChoicePerspectives^TM is used to assist the stakeholders in developing a conjunctive management implementation plan. The stakeholder session will utilize a Collaborative Spatial Decision Making (CDSM) approach to identify the significant variables and to derive an optimal process for determining the extent of allowable pumping at wells in the basin. The effectiveness, of the CSDM process will be compared against the effectiveness of a control group that uses conventional maps and charts. The stakeholder sessions are scheduled for May 2001, and the results of the sessions are anticipated to be reported at the 2001 Specialty Symposium on Integrated Surface and Ground Water Management.

Conjunctive use for surface water and groundwater management began in the 1930s and is an increasingly popular method to optimize water resources. Major Northwest cities west of the Cascade mountain range (Seattle, Tacoma, Portland) all rely primarily on surface supplies that are dependent on winter rainfall and/or snow packs to provide seasonal storage. While each city operates supplemental groundwater supplies, Portland's Columbia South Shore Well Field (CSSW) is the largest with a capacity approaching 380,000 cubic meters per day (m³/d). The groundwater system has served in two critical roles since the mid-1980s: 1) to augment the primary (Bull Run) surface supply during the peak season and 2) to replace Bull Run when storm and other events make it impossible to serve unfiltered surface water. This paper examines the evolving conjunctive management of two sources and some of the challenges associated with meeting expectations that are associated mainly with the primary source. The current and expected future policy is that the CSSW is Portland's secondary and backup source of water, and that Portland will use the well field to meet water demands that cannot be met with the primary Bull Run supply. A transition from the current conjunctive management approach (separate operation of surface and groundwater) to a combined conjunctive operation using aquifer storage and recovery (ASR) is being tested.

Fairbanks, Alaska soils are highly transmissive with a very shallow water table. Consequently, the interaction of ground-water and surface-water can be linked or correlated. A neural network model was developed to predict the ground-water elevation at user specified sites of interest based on the discharge data of a nearby river, the Chena river. The neural network model was trained using the samples provided by a field-calibrated, finite element model of ground-water flow for the Fort Wainwright region, adjacent to Fairbanks. The United States Geological Survey has maintained a discharge stage gage for the Chena river since 1948. Once trained with available field data, the neural network model can predict the ground-water elevation at sites within the study domain throughout this time period. This strategy and implementation can reduce ground-water monitoring costs and labor requirements substantially. This system also provided the necessary underpinnings to model contaminant fate and transport situations over extended periods of time.

The weighted implicit finite volume approach for 2-D flow is extended to model canal flow in the South Florida Regional Simulation Model (SFRSM). St Venant equations with the diffusion approximation are used as governing equations. The water body and water mover base classes are used to represent canal segments and junctions in the 1-D model in an object oriented framework. Ground water and surface water flow is integrated with canal flow through flow functions attached to segment walls. The model uses an external sparse solver to solve overland, ground and canal flows simultaneously. The method is stable because of the implicit formulation. This paper describes the theory and provides a sample application.

Interaction of canal flow with groundwater flow is an important part of South Florida hydrology. Practically every regional and sub-regional model in South Florida considers interaction between streams and aquifers or streams and wetlands in some approximate form. In the current paper, an analytical solution for the idealized stream aquifer interaction problem is used with results of a numerical error estimation study to determine the spatial and temporal discretizations needed for a model. A test case is used to demonstrate that the results obtained using this method are accurate.

As part of the Comprehensive Everglades Restoration Plan (CERP) initiated by the U.S. Army Corps of Engineers and the South Florida Water Management District, numerous structural improvements to the water management system of the Lower East Coast Service Areas of southern Florida have been proposed. As part of the conceptual design and evaluation process, a high-resolution ground water flow model of northern Miami-Dade County, Florida, was developed using MODFLOW96 along with a set of recently developed modules that allow the model to also simulate flow within wetlands, interactions between ground water and reservoirs, and the transfer of water throughout the model domain based on computed water levels. These modules include the Wetlands Package, a modified Lake Package the Reinjection Drain Flow Package, and the Operations Package. The primary objective for this model was to evaluate the effectiveness of the proposed system of improvements in achieving the goals identified by the CERP for the management of environmental and public water supplies. The specific model features that are of primary interest include proposed improvements such as subsurface reservoirs and surface impoundments used for the detention and treatment of surface water flows. Reservoir stages were computed using the Lake package while seepage from and stages within surficial impoundments were successfully simulated using the Wetlands package. In the latter case, the option to include user-specified water diversions was used to account for certain pumped inflows and outflows required for regional water supply purposes. At selected impoundments, the Reinjection Drain Flow package was applied to simulate the backpumping of seepage that was intercepted by perimeter borrow canals. Finally, other water transfers between surficial impoundments, reservoirs and ASR wells that are a function of reservoir stage were incorporated into the model using the Operations package.

When the surface water/groundwater interaction is important, applications of single-regime hydrologic models often lead to inaccurate simulations of hydrologic systems. By coupling the surface water model HSPF with the groundwater model MODFLOW, major limitations of each model can be overcome, allowing the user to conduct long-term simulations lasting decades with varying climatic and stress conditions. The integrated hydrologic model has been successful in simulating Florida's shallow water environment, even where there is significant interaction with deeper aquifers. The comparative strengths and weaknesses of single-regime and integrated hydrologic models are discussed. Results of case studies illustrate the advantages of the integrated hydrologic model, and how the results of the model can be used to help predict future impacts of groundwater production on wetland conditions in those areas where wetlands interact with the shallow groundwater environment.

The South Florida Regional Simulation Model (SFRSM) is used in this study to simulate flow-dynamics in the Everglades National Park (ENP). The SFRSM, a model under development by the South Florida Water Management District (SFWMD), was designed to simulate major components of the hydrologic cycle as well as the complex water management rules and policies in South Florida. This paper focuses only on the physical processes such as overland and ground water flow, evapotranspiration, levee seepage, and canal flow that dominate the hydrology within the study area. The current working version of the SFRSM, referred to as the Hydrologic Simulation Engine (HSE) is a weighted, implicit, finite-volume, rainfall-runoff model, capable of simulating two-dimensional flow using a variable mesh with arbitrarily shaped triangular elements. One-dimensional canal flow and two-dimensional overland flow are simulated in the model using the diffusive wave approximation of the Saint Venant equation. The Darcy equation is used for the one-dimensional canal seepage and two-dimensional groundwater flow calculations. Overland and groundwater flow components are fully coupled for a more realistic representation of runoff generation. This feature makes the HSE ideally suited for simulating high water tables and relatively flat terrains associated with the Everglades. Calibration results show that the HSE provides stage history-matching capabilities comparable to the South Florida Water Management Model — the more thoroughly-tested regional simulation model for the same region.

The Río Salado catchment covers 170,000km² of the Buenos Aires province, Argentina. The area suffers from persistent flooding situations that affect a large proportion of the catchment and hence imposes a severe constraint in the existing production levels and in the realisation of the fall economic potential of the region. The complexity of the landscape and the delicate groundwater-surface water interaction in a large dune field area made it essential to develop different approaches to model such interaction, as part of the study for an Integrated Master Plan for the whole basin. An approach using MODFLOW proved to be useful at a regional and prefeasibility level to generate flood risk maps and estimate net benefits for different drainage schemes. However ISISMOD, a groundwater model coupled with a surface water model proved to be a key water resource management tool to support feasibility studies and the detailed design of a drainage network capable of removing flood waters without detriment to the wetlands and fresh groundwater lenses of the area during dry periods.

Using established petroleum reservoir engineering techniques as applied to a geopressured system, this paper presents results from analysis on the Gladys McCall geopressured aquifer, located in Cameron Parish, Louisiana, U.S.A. Analysis of the flow data has led to the assessment of important formation parameters, evaluation of aquifer drive mechanism, and development of conceptual aquifer model that accounts for pressure behavior observed during both depletion and recovery phases. This paper contains results from studies conducted in two parts. First purpose of this study is to summarize and analyze results of the eight transient tests. This analysis provided data needed for a quantitative description on the characteristics of the aquifer, including: a) connected pore volume, b) formation transmissivity and skin, c) the size and possible shape of the main producing aquifer, and d) characteristics of the pressure support mechanism. The second part is an evaluation of the aquifer drive mechanism using long-term pressure data. Based on results obtained from the pressure transient analysis and material balance calculations, a fault-controlled influx model has been proposed. The model was derived from a conceptual model of water leakage through a partially sealing fault into the aquifer under steady-state conditions. In addition, the stress-induced hysteresis in the formation rock was highlighted by the differences between aquifer unloading (depletion) and loading (recovery) fault conductivities. An influx calculations based on the model provides an excellent match to the detailed downhole pressure buildup measurement made during aquifer tests.

Holocene sands of Ocean Bight in Exuma, Bahamas, contain a Ghyben-Herzberg freshwater lens, the source of a 39,000 gallons per day water supply. The sands are underlain by Pliestocene limestone, which contains the basement of the aquifer at the salt-water/freshwater interface. The perimeter of the lens is the Atlantic ocean to the east and saline natural ponds to the north, west, and south. These ponds were formed in low-lying areas, where the water table is just below ground and there is direct evaporation from the water table. In Exuma, potential evapotranspirtion exceeds precipitation, and in areas where the water table is above surface, evapotranspiration occurs at the potential rate and ponds become hypersaline. Freshwater lenses become thin to non-existent where these lakes are formed. Three small ponds were excavated into the freshwater part of the lens, as part of a development. Based on the fact that evaporation exceeds rainfall, there is potential for saline intrusion at the site of these ponds. This paper presents the results of the numerical and analytical techniques that were used in assessing the integration of the surface water ponds with the groundwater lens.

Ground-water/surface-water interactions in of the largest aquifer systems in the United States were analyzed in this article. The Edwards karst aquifer of Texas exhibits unique ground-water recharge processes. It is also located in a region of pronounced precipitation variability, the dominant controlling factor of ground-water recharge. The evolution of ground-water storage effected by recharge, pumping, and spring flow in the Edwards Aquifer has provided new evidence about the role of hydrologic, climatic, ecologic, and social factors in the determination of sustainable aquifer management policies. Historical data and numerical simulations were used to analyze pumping impacts within an integrated framework of sustainable aquifer production.

As populations grow and demand on water resources for urban use increases, effective integrated management to maintain sustainable water supplies becomes essential. In many regions, such as South Florida, available potable water supplies are limited by the effects ground and surface water drawdown may have on other legal users and/or environmental resources such as wetlands or lakes. This paper describes a management model that combines the advantages of nonlinear, distributed parameter groundwater flow simulation modeling (S) with linear programming optimization (O) in order to optimize water supply withdrawals. The model includes an optimization matrix that takes about 10 seconds per scenario to run using common spreadsheet software. To create this matrix, several simulations are run to develop a relationship between the hydraulic stresses and responses at predetermined constraint locations. This model was applied in a case study seeking ways to meet present as well as future needs of a region in South Florida, while taking into account the limitations imposed from other legal users and environmental resources. The end result was a simple resource management tool for surface water and groundwater optimization.

We present a multi-dimensional finite element numerical model (COSFLOW) to simulate 1D canal, 2D overland, and 3D subsurface flow in South Florida. The developed model was designed for use in both regional scale analyses of various water resources projects and for the detailed design of specific projects. The diffusion wave approach was used to determine water flows on overland and in each canal reach controlled by hydraulic structures at its two ends. The Richard's equation was solved to compute the subsurface flow in both saturated/unsaturated zones. The flow governing equations were discretized with the Gelerkin finite element method, where the interaction between surface and subsurface waters is handled numerically through a coupling process. The developed numerical code has been incorporated into a modified GMS (Groundwater Modeling System) graphical user environment to allow accurate construction of computational domains and efficient use of the code for the user. From the developed hydrogeologic conceptual model, the South Dade Model that covered the area from just west of L-67 Extension eastward to the coast in South Florida was calibrated with water elevation data from observation wells in 1995. In the South Dade Model, the canal system consisted of all major canals in South Dada County where each canal reach contained many canal elements whose lengths were about 600 meters. The 2D overland flow mesh consisted of 4,720 nodes, which was formed by the surface layer of nodes and elements in the 3D subsurface mesh. The 3D finite element mesh for the subsurface flow consisted of 37,760 nodes and 65,429 triangular prism elements which were distributed over 7 discretized layers. The model calibration started with a steady 3D subsurface flow simulation using consistent boundary conditions; followed by overland/subsurface flow simulations; and finally the fully coupled 1D/2D/3D mode. Partial calibration results are presented. Suggestions to improve computer efficiency are also provided for future works.

With the population growth and increase in land development, comprehensive management of sustainable water resources related to water supply, water quality, wetlands, and recreational issues becomes increasingly important. Hydrologic modeling in regions with flat topography and shallow water table is complex because of strong surface water — ground water interactions, reverse flow and backward effects. In order to address these issues, it is necessary to have an integrated ground water and surface water (IGSW) model that is capable of accurately evaluating the impacts and the effects of various water management alternatives. These alternatives often involve operation of surface water systems to alter ground water flows and levels and vice versa. Most of the current evaluation processes involve using either a ground water model or a surface water model. The accuracy of such model results is often questioned or challenged because the models simulate only one of two important components of the hydrologic system. Some development towards need and use of integrated ground and surface water models has been initiated by various agencies (e.g., South Florida Water Management District, Department of Defense for Site Remediation, among others). An IGSW model should be capable of simulating all major components in the hydrologic cycle along with the water use and management features such as various water control structures, wellfields, lakes, canals, water quality, etc. The IGSW model should also be capable of calculating evapotranspiration, recharge, and other hydraulic and hydrologic components based on land use, soil type, hydrometeorological information, and water management operations.

The St. Johns River Water Management District's (SJRWMD) is one of Florida's five water management districts. It is located in northeast Florida with a jurisdiction covering approximately 12,400 square miles. The Florida Legislature has established water supply planning requirements with the intent that sufficient water be available for all existing and future reasonable-beneficial uses and the natural systems, and that the adverse effects of competition for water supplies be avoided. The two primary elements of these requirements are (1) the preparation of districtwide water supply assessments and (2) the preparation of regional water supply plans. These assessments and plans are to be performed every five years and are to be based on 20-year planning horizons. SJRWMD's first regional water supply planning process, the Water 2020 process, was recently completed with the approval and publication of the District Water Supply Plan (DWSP). This process was a structured process designed to meet the water supply planning requirements imposed by the Florida Legislature. This process was based on two primary elements: 1) strong and defensible technical evaluations and 2) a consent-based, inclusive public process. According to the legislative mandate, the Water 2020 process was an open public process, conducted in coordination and cooperation with local governments, regional water supply authorities, government-owned and privately owned water utilities, self-suppliers, and other affected and interested parties. SJRWMD focused its water supply planning efforts within water supply planning work group areas. This paper describes SJRWMD's water supply planning process as well as future directions in planning for and implementing water supply solutions for the future.

East central Florida, including the Orlando metropolitan area, has traditionally used fresh groundwater to provide a safe, reliable, and inexpensive water supply source to meet both public supply and agricultural irrigation needs. Approximately 1.99 billion liters per day (526 mgd) of fresh groundwater was withdrawn from the Floridan aquifer in this region, in 1995. By the year 2020, average day water supply demands are expected to increase by about 61 percent, to approximately 3.21 billion liters per day (848 mgd). Investigations conducted by St. Johns River Water Management District (SJRWMD), as part of the Water 2020 planning process, indicated that all future water supply needs cannot be fully met by fresh groundwater alone without incurring unacceptable harm, including reduction in spring flow discharge, and de-watering of sensitive isolated wetland systems resulting in changes in wetland vegetation and wetland type. These investigations, conclude that substantial new alternative water supply sources and new water management strategies, will likely need to be developed, to supplement existing fresh groundwater withdrawals, if the potential adverse impacts are to be avoided while meeting future water supply needs. Alternative water supply sources and management strategies investigated include; optimization of groundwater withdrawals, brackish groundwater development, surface water development, artificial recharge, increased water conservation and reuse, interconnection of water supply systems, aquifer storage recovery (ASR), and avoidance or mitigation of adverse wetland impacts resulting from increased groundwater withdrawals. This paper provides an overview of the Water 2020 process as well as a summary of the major regional planning tools developed. Each of the water supply sources and water management strategies investigated is discussed, and the concept of adaptive management is introduced. The Water 2020 process provides an informative case study in integrated water resources planning and management that may prove useful in other regions facing increased demands placed on finite and varied water resources.

The natural hydrology of southern Florida has been greatly altered through drainage, canalization, urbanization and agriculture, to the point where the Everglades is now one of the most threatened ecosystems in the nation. The Comprehensive Everglades Restoration Plan is an effort to preserve the remnant Everglades and restore more natural hydrological conditions while maintaining adequate water supply and flood protection in south Florida. The South Florida Water Management Model was used in the development of the plan to simulate the hydrologic performance of regional water management alternatives. Simulation models have become the only feasible means to evaluate proposed region-wide modifications to the south Florida water resource system. The South Florida Water Management Model is probably the best available tool to simulate the complexities of the south Florida water control system and its operational rules. It is an integrated surface water — groundwater model that simulates the major components of the hydrologic cycle together with current or proposed operations. Extensive output, in the form of target-based hydrologic performance measures generated from the model, provides adequate information for comparing alternatives, permitting the evaluation of proposed regional-scale water management options. In this paper, the South Florida Water Management Model is described in more detail and its application in the simulation of water resource alternatives for development of the $7.8 billion Comprehensive Everglades Restoration Plan is presented.

The South Florida Water Management District (SFWMD) manages the water resources of South Florida for the benefit of the region, balancing the needs of present and future generations. The complexity of the system and the coexistence of competing water needs such as flood control, water supply and water for environmental needs, necessitate the use of appropriate water management decision tools. The SFWMD is using position analysis, a special form of risk analysis, which also has the capability of incorporating climatic forecasts into planning the seasonal operation of the system. The South Florida Water Management Model and its application in position analysis mode to investigate current and proposed operational scenarios are described in this article.

Regional-scale water resources planning and management are critical in south Florida. Balancing the competing needs of the environment, agriculture and urban communities has been the central theme among the numerous water-related issues pertinent to the area. Specifically, water resources planning involves the enhancement and restoration of natural flow and water levels in the Everglades, and the provision of water supply and flood protection to the residents of south Florida. Computer simulation modeling is perhaps the only feasible way of assessing system-wide impacts of the various proposed modifications to the water resources system in the region. The South Florida Water Management Model is used as the primary regional-scale modeling tool for water resources planning in south Florida. This paper provides an overview of the South Florida Water Management Model, its theoretical basis, areas of applicability and a brief description of significant projects where the model has been successfully used as the analytical tool for evaluating the technical merits of various water resources alternatives.

Sound land-use activities must be implemented in accordance with the ambient environmental situation as well as social needs. The psychologist Abraham Maslow stipulated that social needs conform to a pyramidal hierarchy of human needs, and cautioned that a population will only begin to focus seriously upon higher-level needs when lower level needs are fulfilled. Similarly, effective groundwater management can only be carried out step-by-step, fulfilling lower level needs before addressing higher level needs. Operational remediation activities must be backed by full public participation, in compliance with the concerns and needs of the resident population. Only then can sustainable groundwater management succeed. This paper suggests a pyramidal scheme, integrating groundwater resources management and environmental land-use planning with the population's needs. The Northern region of Israel's Coastal aquifer has been addressed as a case in point.

Present environmental resource permitting requirements incorporate criteria for water quality treatment, environmental protection and discharge. The current criteria do not directly address the potential increase in volume discharged due to increased impervious coverage, reduced groundwater seepage or loss of surface water storage. Total volume discharged to lakes or tidal waters is increasingly important from the standpoint of water supply/conservation and potential flood impacts to downstream areas. The following paper presents a pre-development vs. post-development runoff volume analysis to estimate the actual difference in discharge volumes. Seven permitted sites are modeled for a typical annual year and the 25-year, 72-hour storm event. The results are tabulated below and indicate that continued research is needed to understand the degree to which runoff volume affects estuaries, groundwater supply and water conservation efforts. In addition, a "theoretical" redesign, based on not exceeding annual or design storms pre-development discharge volumes, is compared to the existing design. Impacts associated with the theoretical redesign are quantified. Potential impacts include: enlarged pond size, raised control structure, modified pond shape to increase effective perimeter, increased excavation depth thereby increasing transmissivity, swales, reduced directly connected impervious area, storm water reuse, overexcavate to reach more permeable layer, and use of regional storm water facilities.

The Orange County Subdivision Regulations require that the peak flow resulting from a storm shall be calculated using Orange County rainfall distribution, which was derived using three severe storms in 1956 and 1960. The Federal Emergency Management Agency (FEMA) previously accepted the use of Orange County rainfall distribution in establishing the 100-year flood elevations in Orange County. Recently, FEMA requested Orange County to provide justifications for the use of the Orange County rainfall distribution. Since 1986, Orange County has been collecting rainfall data in 15-minute increments. Fourteen storms were extracted from these rainfall data with depths ranging from 4.6 to 7.6 inches and with durations ranging from 14 to 25 hours. The peak flows from these storms were calculated using the SCS method for various times of concentration and runoff curve numbers. They were compared with the peak flows calculated for the same depths and duration but using different storm distributions such as Soil Conservation Service (SCS) Type II, SCS Type II Florida Modified, Florida Department of Transportation (FDOT), and South Florida Water Management District distributions. The analysis showed that for practical values of times of concentration and curve numbers, Orange County rainfall distribution and SCS Type II Florida Modified, distribution predicted peak flows closer to the peak flows generated by the actual storms. The 100-year elevations of Shingle Creek located in an 82 square mile watershed in Orange County were calculated using these distributions. The differences in these elevations are within the computational accuracy proving that Orange County rainfall distribution can be used in determining the 100-year elevations.

Simulation models are commonly used for watershed planning and pollution control. While even the most extensive monitoring programs can only provide watershed information at a few locations at a few discrete points in time, well-developed and applied models provide valuable watershed information at many locations at many points in time, including the future. Models, therefore, are often used to characterize impacts of existing and future land uses on water quality, and then to develop a technical framework such as Total Maximum Daily Loads (TMDLs) to control pollution. This paper presents and compares various computer models that are currently used for TMDL development along with their suitability for application. The paper also discusses important gaps and development needs for modeling tools to address the needs of the TMDL program.

Constructed wetlands have been used for over two decades for the treatment of acid mine drainage (AMD). Through a variety of physical, chemical, and biological processes, these wetlands are effective in reducing acidity and removing up to 99+ percent of iron and aluminum from AMD, but they only remove between 20 to 30 percent or less of the manganese load. The Slippery Rock Creek watershed in northwest Pennsylvania has been adversely impacted by acid mine drainage (AMD) for over 10 years. In the Slippery Rock Creek watershed, aerobic and vertical flow wetlands, along with limestone drains and vertical flow systems, are being employed to restore the watershed at a total cost of $8,929,500.

San Diego Creek Watershed forms the southern end of the Coastal Plain that covers much of Los Angeles County and Orange County, California. Prior to 1900, the central part of the watershed was marshland, and the edges of the watershed were used for grazing sheep and cattle. After 1900, cattle and sheep grazing was displaced by irrigated agriculture. Drainage ditches and channels were constructed to drain the marshes to "reclaim" the land for agriculture. Today, the drainage ditches and channels still exist in the watershed, which is undergoing massive urban growth. Shallow groundwater discharges into these channels and ditches, and the surface water eventually flows into Upper Newport Bay, a thriving ecological habitat. Surface flows in these channels usually exceed the USEPA chronic criterion for selenium of 5 μg/L (micrograms per liter) for protection of aquatic life. Selenium increases in surface flows as a result of baseflow seepage of selenium-laden groundwater. The highest concentrations of selenium in shallow groundwater in the watershed coincide with the marshland areas that were displaced by agriculture. Concentrations of selenium in groundwater in the former marsh areas often exceed 25 μg/L, and are as high as 478 μg/L. In areas where marshes were absent, concentrations of selenium in groundwater are usually less than 25 μg/L. We hypothesize that the elevated concentrations of selenium in groundwater where the marshes once existed are a direct result of the destruction of the marsh. Today, oxygenated groundwaters flow through the soils where the marshes once existed, remobilizing selenium as selenate, a water soluble and oxidized form of selenium that is highly mobile in aquifer systems.

A study was conducted to assess water quality in the shallow aquifer of the San Diego Creek Watershed, Orange County, California. The objectives of the study were to determine concentrations of nitrate in surface water and shallow groundwater, and to study the hydraulic relationship between shallow groundwater and surface streams. San Diego Creek Watershed is located on the southern end of the Coastal Plain that covers a considerable portion of Los Angeles and Orange Counties, California. The aquifers within the watershed are divided into shallow zones, which are not pumped, and deep zones, which are pumped. The shallow aquifer, which is in the upper 30 m of sediments, is composed of interbedded and discontinuous layers of silt, silty sand, sandy clay, clay, and sand. The shallow aquifer is hydraulically connected to a number of drainage ditches and channels that were constructed when the extensive marshland areas of the watershed were drained in the early 1900's. Some of these channels are lined with cement, and some are unlined and have sandy bottoms. Stream gaging along unlined channels indicates that flow accumulates as a result of groundwater seepage. Nitrate in shallow groundwater varies from about 40 mg/L to as much as 300 mg/L NO₃. Sources of nitrate include contemporary and historical agriculture. Nitrate-laden groundwater seeps into and contaminates the surface streams in the watershed. Large commercial nurseries in the upper part of the watershed are another important source of nitrate. These nurseries release water with as much as 700mg/L NO₃ into unlined surface channels.

This paper reports on experiences with infiltration basin and trenches constructed and operated as part of the Caltrans Best Management Practice (BMP) Retrofit Pilot Study Program. Fifty-two sites adjacent to highways and maintenance facilities in the Los Angeles and San Diego areas were evaluated and two infiltration basins and two infiltration trenches were constructed. One basin and one trench successfully met design expectations, which in California includes a 3-day drain time to prevent mosquito breeding. The drain time of the trench at the Carlsbad Maintenance Station in San Diego County was about twice as long as intended. The infiltration basin at I-5 at La Costa Ave., also in San Diego County, required months to drain. Based on post-construction analysis, contributing causes of these problems were inaccurate estimates of infiltration rates and groundwater depth, and application of inappropriate siting and design criteria. The short-term in-hole permeability tests resulted in higher estimates of infiltration rates than were later observed. Fluctuations in groundwater depth were larger than anticipated. Based on this experience and on computer modeling of one site, Federal Highway Administration recommendations for minimum infiltration rate and depth to groundwater can result in infiltration BMPs that do not meet performance requirements in California.

Riparian zones in arid regions have high groundwater tables that allow a variety of plant species to thrive. These zones are critical habitat due to the mix and density of vegetation as compared to the surrounding areas that have limited water supplies and deep groundwater tables. Many riparian zones have been impacted by human intervention. Often, groundwater levels have been lowered due to groundwater pumping. As a result, riparian vegetation has been detrimentally affected. Thus, management of riparian zones requires consideration of groundwater and surface conditions. These considerations are particularly important during restoration planning of disturbed zones. Here, restoration is defined as replanting of natural vegetation. However, as vegetation is planted evapotranspiration (ET) rates change as a function of the type and location of vegetation. ET rates, in turn, affect groundwater levels and plant survivability. Due to the strong interaction between vegetation and groundwater levels, the selection and survivability of different plant species depends upon the type and their distribution in the riparian zone. A decision support system (RRZ-DSS — Restoration of Riparian Zones — Decision Support System) has been developed to assist managers make decision regarding the plant distribution for a riparian restoration project. The goal of the DSS is to assist the modeler in identifying a distribution of plant species that will survive and maximize their internal objective. Alternatively, it can be used to study the impact of groundwater pumping on a prescribed plant distribution. The DSS consists of visualization, groundwater simulation, and knowledge of plant characteristics. The water/riparian zone management decision process begins with the development and calibration of a groundwater model that reflects field conditions and can model impacts of changes in evaporation losses and groundwater pumping scenarios. MODXX, a finite element groundwater model, is incorporated in the DSS to meet this need. The groundwater model is linked with other analysis tools using a Visual Basic interface. This interface provides push button execution of the various tools. To analyze the impact and viability of vegetation distributions in the riparian zone, a graphical user interface that is built around ArcView GIS (Geographic Information System) is used to provide the layout and types of vegetation within the riparian zone. Once defined, evaporation rates and extinction depths for each vegetation type are passed to the groundwater model. The groundwater response is then computed. The resulting groundwater levels are then passed back to the GIS. Based on the root depth and the groundwater levels, a graphic display of plant viabilities is presented. The modeler may then modify the layout to examine alternative planting configurations. The system has been structured to study restoration alternatives in Bingham Cienega in Southern Arizona.