GROUND WATER/SURFACE WATER INTERACTIONS JULY 1-3 AWRA SLIMMER SPECIALTY CONFERENCE 2002

3 . L 6 3 f2F.

DEVELOPMENT OF AN INEGRATED HYDROLOGIC MODEL FOR ROCKY FLATS. COLORADO

Robert H. F’ruchal. Christine M. Hawley2. Torsten V. Jacobsens. Christine S. Dayton4. Ian B. Patons, Tissa H. Illangaselcare6

ABSTRACT: The Rocky Flats Environmental Technology Site (WETS) in Golden. Colorado, a former Department of Energy nuclear weapons manufacturing facility. is currently undergoing closure. The hydrologic conditions at RFETS are complex and further complicated by anthropogenic modifications in an Industrialized Area. To study effects of these changes on the site hydrology and water balance and to evaluate possible future water management scenarios. a fully-integrated hydrologic model of the surface- subsurface system was developed and assessed using a substantial dataset. An integrated model with this detail has not been previously developed in a semi-arid area. A new step-wise process-coupling approach was developed and sub-scale integrated models were used to parameterize and better understand complex areas of the WETS model. This approach has signiilcantly improved the understanding of the integrated hydrologic behavior of the Site. Observed continuous integrated system response was reproduced well. Pre- and post-calibration year validation simulations further support the callbration. KEY ’IERMS Integrated hydrologic model, semi-arid hydrology, management decision modeling tool

INTRODUCTION

?he Rocky Flats Environmental T&hnologv Site (RFETS or Site) is being closed and converted from its former use as a nuclear weapons production facility into a National Wildlife Refuge. Closure activities anticipated for the Site involve reco&guring an existing Industrial Area from its current condition into a more natural state. These future changes are expected to impact the current Site hydrology and require assessment. In addition, other closure management concems exist, including 1) wildlife habitats: 21 wetlands: 3) surface water quality: and 4) groundwater contaminant transport.

Assessing the potential short- and long-term impacts to the system hydrology requires a thorough understanding of the complex RFETS hydrologic system. I t also requires comprehension of the integrated surface-subsurface flow conditions further complicated by Industrial Area modifications to the system. Developing a numerical model of the hydrologic flow system was considered the best available approach to for quantitatively assessing effects of Site closure on system flows.

A variety of hydrologic models have been used previously at RFETS to study the hydrologic behavior of the Site. These have ranged from single process models. such as the U.S. Geologic Survey WGS1

Senior Water Resources Engineer, RMC Consultants Inc.. Lakewood CO 80228, Phone: (303) 642-

Environmental Engineer. RMC Consultants Inc.. Lakewood CO 80228. Phone: (303) 966-6865, E-mail:

Senior Hydrologist, DHI Inc.. Stockingworks, 301 South State Street, New Town, PA 18940. phone.

Program Manager, Environmental Systems and Stewardship, Kaiser-Hill Company, L.L.C., Rocky Flats

0366, E-mail: prucha@rtt.colorado.edu.

Christine.Hawlev@rfets.~ov.

fax. E-mail: tvj@dhigroup.com.

Environmental Technology Site.10808 Highway 93, Unit B, T130C, Golden, CO 80403-8200. Phone: 303-966-9887, Fax: 303-966-7991 E-mail: Christine.Davton@rfets.eov.

Senior Project Engineer, Wright Water Engineers, Inc.. 2490 W. 26* Ave.. Suite 100-A, Denver. CO 8021 1, Phone: 303-966-4457. FAX: 303-966-7991, E-mail: ian.paton@rfets.gov.

AMAX Distinguished Chair of Environmental Science and Engineering and Professor of Civil Engineering. Director, Center for Experimental Study of Subsurface Environmental Processes (CESEPI. Phone: (303) 384-2126, Fax: (303) 273-331 1, E-mail: Tillanaa@mines.edu.

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MODFLOW code (McDonald and Harbaugh, 1984) or the U.S. Army Corps of Engineers HEC codes, to more complex coupled-process models using the U.S. Environmental Protection Agency code SWMM, or the USGS HSPF code. More recently, the U.S. Department of Agriculture code WEPP has been used for hfflslope erosion and transport modeling (Kaiser-Hill, 2000 and Kaiser-Hill, 2002). None of these efforts considered the fully-integrated system, coupling surface flows and subsurface flows. including the unsaturated and saturated zones.

Application of a fully-integrated hydrologic code was considered essential to create a tool to assess the type of flow conditions that occur at WETS. A detailed literature review was performed (Kaiser-Hill, 2001) and several integrated codes were identifled that could simulate the integrated hydrologic behavior at WETS. In this detailed model comparison, the MIKE SHE code, developed by DHI (DHI. 2001). was considered the most applicable for use in the Site-Wide Water Balance (SWWB) modeling. I t is noted that fully-integrated codes remain largely untested In semi-arid/arid areas. This is because detailed spatial and temporal climate data are required to simulate the more non-linear flow conditions in these areas (Prucha, 2002). The MIKE SHE code, however, has been tested in a large-scale semi-arid/arid area application (F'rucha and Illangasekare, 2002). Further, Illangasekare et al. (2001) evaluated the physical and numerical flow equations in MIKE SHE, developed a detailed code verification procedure to test the code, and then evaluated its performance using this procedure. Results showed the code was capable of simulating the important hydrologic processes and their interactions at WETS.

The general MIKE SHE code capabilities are described in Abbott et al. (1986) and more recently in Storm and Refsgaard (1996). Key processes that are simulated in this code include: (1) two-dimensional overland flow based on a kinematic wave solution: (2) 1-dimensional channel flow using the St. Venant equations: (3) one-dimensional Richards based unsaturated zone flow: and (4) !idly three-dimensional saturated flow based on the Bousinesq equation. In addition, evapotranspiration, based on the KristensenJensen method and Penman-Monteith equations, and a simple degree-day snowmelt model are also simulated. The code is quite flexible, allowing for simulation of numerous combinations of process couplings. and various process simplifications to the full flow equations.

This paper describes the design and development of a fully-integrated hydrological model of the WETS system. Through its development, the model has been used to better understand the complex system response. This is important because previous modeling was not able to provide this level of insight into the integrated system behavior. The main objective of the Site-Wide Water Balance (SWWB) was to develop a management tool to evaluate how the Site-wide hydrology is likely to change when the present Site configuration is modified.

The steps required to develop this model are discussed below in more detail. The study area, general background and Site history are described first, followed by a discusslon of the conceptual flow system at WETS. The general modeling approach used to develop the integrated model is presented next, followed by the design of the model. Model performance I s described last. The performance discussion begins with model calibration against observed and semi-quantitative data. continues with a sensitivity analysis, and ends with validation using data available from years other than the calibration period.

BACKGROUND AND STUDY AREA

WETS (shown on Figure 1) is located 16 miles northwest of Denver in Jefferson County, Colorado. The former nuclear weapons facility is owned by the United States Department of Energy (DOE) and encompasses approximately 6,585 acres of federally-owned land (EG&G. 1993). Major plant structures, including all former production buildings, are located within a centraked 385-acre Industrial Area that is surrounded by a 6,200 acre Buffer Zone (EG&G, 1993). Construction at the Site began in 1951 and durhg the next four decades, evolved into more than 440 permanent and tempomy structures used for manufacturing, chemical processing. laboratory, support and administrative facilities (DOE, 1995).

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Industrial Area B Model Grid

.

Figure 1. Study Area and Model Boundary

Environmental Setting

climate

The WETS climate is characteristic of Colorado’s Front Range, temperate and semi-arid. The dry atmosphere of the Site, at 1,830 meters (m) elevation above mean sea level (MSL), often causes wide temperature fluctuations between daytime and nighttime. Summer high temperatures are typically in the upper-20 degrees Centigrade (“C), with nighttime lows falling to approximately 16°C (EG&G, 1993). During the winter, temperatures typically range from 4°C to 7°C during the day and -9°C to 4°C at night. Arctic and Siberian air masses occasionally bring frigid air during the winter when low temperatures may drop to between -21°C and -24°C (EG&G, 1993).

The average annual precipitation, based on 30 years of record, is approximately 368 millimeters (mm) (14.5 inches) (DOE, 1995). Roughb half of the precipitation occurs as rain and half as snow, with precipitation falling primarily as snow from late October through early April and as rain during the remaining months (RMRS, 1996). Annual snowfall averages approximately 178 mm, with the highest monthly snowfall average (approximately 406 mm) occurring in March (EG&G, 1993). Rainfall is highest from April through June, with nearly 42 percent of the average annual precipitation occurring during those months (EG&G, 1993).

Surface Hydrology

Streams and seeps at WETS are largely ephemeral. Streams gain or lose flow, depending on the season and precipitation amounts. Surface water at the Site flows generally from west to east, with three major drainages traversing the Site. Two of these drainages capture runoff from the Industrial

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Area. They are: 1) Walnut Creek, which drains the northem portion of the Site, including the majority of the Industrial Area: and 2) Woman Creek, which drains the southem portion of the Site, including southem Industrial Area runoff (after it has been diverted by the South Interceptor Ditch through Pond C-2). Pond C-2 is one of 12 Site detention ponds within the study area, 10 of which are actively managed. The third major drainage at the Site, Rock Creek, does not receive runoff from the Industrial Area, has not been impacted by Site activities, and is not within the SWWB model boundaries (Figure 1).

Geology and Hydrogeology

The Site is situated approximately two miles east of the Front Range of Colorado, on the westem margin of the Colorado Piedmont section of the Great Plains Physiographic Province (Spencer. 1961). Geologic units at WETS are grouped into two general categories: unconsolidated surficial deposits and underlying consolidated bedrock (RMRS. 1999). The Site is entirely covered by these surflcial deposits,

ir(i/?* . ~v;n.;n:."Crlrll

Figure 2. Geologic Units and Profile

except for several small isolated bedrock outcrops. Suficial deposits include artificial fd, colluvium, alluvium and landslide material and range in thickness from 0 to 30.5 meters (EG&G, 1995). These deposits, combined with the weathered portion of subcropping bedrock formations, are the most important geologic units in terms of groundwater flow at the Site (RMRS, 1999).

The Site is located along the westem margin of the Denver Basin, an asymmetric basin with a steeply east-dipping westem flank and a gentle eastem flank. The interpretation of the subsurface structure is presented in the west-east generalized geological cross section of the Site area shown on Figure 2. A monoclinal fold limb exposed west of the Site is the most significant surficial geologic feature in the Site area. Along the west limb of the fold, an angular unconformity extsts between the Upper Cretaceous bedrock and the base of the Quatemary Rocky Flats Alluvium. Though no active faults have been identified at the Site, several high-angle bedrock faults have been inferred based on available geologic data. These faults likely have limited hydrologic significance on subsurface flows (RMRS, 1996).

Consolidated Bedrock Deposits

Bedrock from the Arapahoe, Laramie. Fox Hills and uppermost Cretaceous Pierre Formations are present at WETS (EGBrG, 1995). Only the weathered portions of the Arapahoe Formation transmit significant groundwater flows. Therefore. only the weathered portion is incorporated into the SWWB study. The Arapahoe Formation is generally less than 8 meters (25 feet) thick at the Site, occurring as claystone and silty claystone with lenticular sandstone in the basal portion of the formation (EG&G. 1995).

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Below thk Araphoe Formation, the unweathered Laramie Formation is approxhately 180 to 250 meters (600 to 800 feet) thick. It is composed of an upper, thick claystone intend and a lower sandstone/claystone/coal interval. The claystones with low hydraulic conductivity inhibit downward groundwater flow. Saturated zone flow in the unweathered bedrock is considered insignificant compared to flow in the overlying unconsolidated material and weathered bedrock.

Vegetation

The Site's topography and close proximity to the mountains support a unique, diverse array of prairie and foothilIs plant communities that have been extensively characterized in multiple studies (Kaiser-Hill. 1997a. Kaiser-Hffl 199% Kaiser-Hill 1997~). Six hundred plant species were reported to grow at the Site through the 2001 field season (Murdock. 2002). Plant communities range from xeric (dry) grassland communities to more hydric (wet) communities such as wet meadows and marshes. Vegetation controls the transpiration. which dominates system evapotranspiratlon losses during wanner months.

The most signiAcant plant communities in the study area are: The Xeric tallgrass prairie, which comprises approximately 28% of the total area, occurs on flat upland areas and ridges on the westem half of the Site: The Mesic mixed grasslands cover approximately 34% of the area and occur on more damp hillsides primarily in the eastem half of the Site: The Great Plains riparian community, mapped as riparian (stream channel) woodland and shrubland, is found along streams. It comprises approximately l0h of the Site area. Cottonwood trees and willows predominate in this plant community: and Wetlands are most common on north-facing hillsides. The combined area of wet meadow. short marsh, and tall marsh comprise approximately 6% of the Site area. The largest Site wetland is Antelope Springs. located south of the Industrial Area.

CONCEPTUAL FLOW MODEL

The three primary components of the conceptual flow model developed for the MEIS system are shown on Figure 3. Several types of extemal hydrologic stresses act upon the surface and subsurface model framework and produce a hydrologic response. Hydrokgic stresses include time-varying processes like precipitation (as rain or snow), meteorological changes, imported water, surface inflows, and pond control operations. The basic structure and hydraulic properties associated with different hydrologic processes d e b e the modelframework. For example. the topographic surface is the dominant feature that deflnes the overland and channel flow network, while subsurface hydrostratigraphy deflnes the subsurface flow structure. Hydrologic responses include overland and channel flows and unsaturated and saturated zone flows.

Framework

Figure 3. Conceptual Flow Model Components

Water flows through the Site via both the surface and subsurface systems. Six hydrologic processes govem how water flows through the system. The two primary surface flow processes include overland flow and channelized flow in. for example, streams or culverts. Flow in the subsurface occurs as either unsaturated flow, or saturated flow (Le., groundwater). Evapotranspiration, defined by plant transpiration and surface evaporation. represents the interaction between the unsaturated zone, saturated zone and the atmosphere. The last process. snowmelt, is affected by air temperature and

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incident solar radiation. It affects both surface and subsurface response, but is more notable in the surface water response.

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sllhuulam conponenb;

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Figure 4. Industrial Area Conceptual Model

To simulate flow conditions at WETS, many surface and subsurface features that affect the hydrologic system response were considered in the model. Many of these features affect flows in the Industrial Area. The Important features are included in the conceptualized flow diagram shown on Figure 4. SubsurEzce flows are dected by permeable backfill in utility trenches, inffitmtion and inflow in subsurface pipes, variable subsurface geologic structure, and several active remediation systems. In contrast, surface flows are aected by impervious areas (defined by buildings and pavement) drain inflows, surface depressions,'and numerous surface channels and culverts. The paved, or roofed areas prevent infiltration and promote rapid runoff to subsurface storm drains or surface channels in the Industrial Area. These areas also reduce groundwater recharge and cause the water table to drop. The local water table also adjusts to leaky water supply pipes and extraction by storm, sanitary and footing drains. The drains discharge groundwater into channels, which af€ects surface flow response. A conceptual model developed for the Buffer Zone (Figure 5) is different than the conceptual model for the Industrial Area. Several Site ponds are key features of the Buffer Zone. They control runoff from the Industrial Area and interact kith groundwater. Region-, 'both surface and subsurface water flows from west to east according to the surface topography. Locally, the bedrock and topographic surfaces, characteristic of the hillslope configuration at WETS, route surface and subsurface flows toward Site streams. Within the hillslope structure, a number of groundwater seeps occur where shallow, less permeable bedrock outcrops. The outcrops exist because of slumping or landslides along steeper, upper hillslope areas.

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Figure 5. Buffer Zone Conceptual Model

Groundwater is recharged mainly through direct precipitation. Recharge occurs preferentially in mesa or hilltop areas where more permeable Rocky Flats Alluvium is typically present. Although groundwater flows toward nearby streams as shown on Figure 5, the slow rates and shallow groundwater table cause most of the recharge to be lost locally through evapotranspiration before reaching the streams. As a result. stream-aquifer interactions are dominated by local groundwater conditions adjacent to the stream, and are generally not strongly influenced by groundwater levels on hilltops or hillslopes.

Near-stream areas become saturated at the surface because groundwater is shallow (saturation excess). These variable source areas (Kirkby, 1988) cause additional fast runoff during precipitation events that influence stream hydrographs. In hillslope and mesa hilltop areas, surface soils are too permeable to allow Horton-type overland flow to occur. As a result. stream flow response within the WEIS model area is dominated by near-stream, surface-groundwater conditions and controlled pond discharges.

MODELING APPROACH

The approach used to construct the MIKE SHE model for the Site was developed considering the spec& modeling objectives, the integrated behavior of the flow system, and code capabilities. A distributed, fully-integrated model was considered essential to achieving modeling objectives given the integrated behavior of the flow system. (The term fdyintegrated, as used in this paper, refers to a system where all important hydrologic flows are linked and simulated.) For the WETS system, these include: (1) overland and channel surface flows: and (2) unsaturated and saturated zone subsurface flows. In addition, the evapotranspiration and snowmelt processes were also simulated. The term distributed indicates that model input and output are fully distributed in space and time, instead of being lumped or simplifted as in some models.

A substantial amount of data was required to develop the fully-integrated model. Of the entire set of possible model parameters, only a subset was identifed that would be adjusted during the calibration process. Storm and Refsgaard (1996) indicate that this approach is appropriate for calibration of complex integrated models to reduce the number of degrees of freedom.

Initial integrated model simulations of the entire Site proved computationally inefllcient and produced output that was too complex to interpret. It was difllcult to associate a specific model

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response with a given parameter change. Changing a parameter in one process in an integrated model aRects flows in other processes. This added level of complexity in integrated modeling, combined with initial numerical instabilities, made improving calibration more challenging.

The modeling approach developed for the WETS flow system involved two important steps illustrated on Figure 6 to overcome the initial calibration difficulties. These steps were dehed by m c h a (20021 to model a large arid/semi-arid basin flow system. The flrst step, shown at the top of Figure 6, starts by simulating sub-regional models to reduce flow complexity and help determine appropriate model parameters. The sub-regional models were computationally much more efficient than the regional model. The First sub-regional scale models were single-column models. These were developed fist to evaluate local groundwater recharge dynamics by coupling unsaturated and saturated zone flow with evapotranspiration. Next, more complex sub-regional scale models, like hillslopes or individual catchments, were developed and used to determine reasonable parameter values.

Perhaps the most attractive feature of the MIKE SHE code is its capability of simulating flow in individual processes in addition to simulating flow in more complex process couplings. These processes are presented on Figure 6 using the following acronyms: groundwater (Sa, unsaturated zone [UZ) flow, overland flow (OF), channel flow (CF). evapotranspiration [ET) and snowmelt (SM). The lower part of Figure 6 illustrates how this feature was utilized in developing and calibrating the fully-integrated model. Instead of beginning simulations with the fully-integrated Site model, single-process models were evaluated First. Once basic flow dynamics were understood and appropriate model parameter values were determined using the single-process models, more complex models were simulated. The more complex models that couple the individual processes, shown in Figure 6, allowed simulation of the integrated system flow.

This systematic process-coupling approach, like the systematic development of sub-regional scale models, was useful in understanding the behavior of individual processes and their sensitivity on the integrated system behavior. A problem with not performing these types of simulations I s that the more complex hydrologic response of the fully-integrated model can be incorrectly attributed to the wrong input stress or parameter settings.

Lmp Term si"

SZ - SBIuraled Zwe F lw UZ . UnraWaled Zme Flow OF. Ov&and F l w SM . S n h Fmm h c h a , Z W ) CF - Cham4 Flow ET - Evawlmns+allan

Figure 6. Modeling Approach

System behavior over multiple years was also evaluated using this approach because simpler process couplings are significantly more computationally efficient than the fully-integrated model. Long-term simulations were used to determine the length of time required by the fully-integrated model to reach a state of dynamic equilibrium from initial conditions. Partially-coupled process models provide a more efficient way to determine this time period.

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MODEL DESIGN

Integrated codes. We MIKE SHE. are complex and data intensive. The integrated model developed here was the result of a comprehensive effort that included important surface and subsurface features atreCting the Site hydrology. Early modeling studies at the Site had been Umited in their ability to simulate the entire system hydrology. As such, it was important to understand how the basic model framework was structured. and how the numerous input model parameters were spatially and temporally distributed. The text below provides a brief summary of the important model features.

Model Boundary and Discretization

The model boundary is shown on Figure 1. Both subsurface and surface flows enter the westem boundary and exit the eastem boundary. The northem and southem boundaries are considered no-flow boundaries for both surface and subsurface flows. From north to south, the channelized inflow points along the westem study area boundary are Upper Church Ditch, McKay Ditch, Woman Creek and Owl Branch. Along the eastem boundary, surface flows exit principally at Walnut and Woman Creeks. The relatively stable groundwater flow gradients at the eastem and westem boundaries, combined with the relatively low saturated hydraulic conductivities. suggest groundwater flows are generally constant across these boundaries.

Spatial and temporal discretization are important modeling considerations as they dictate the accuracy of the model. The numerical kameworks for the overland flow, unsaturated zone, and saturated zone all share the same regularly-spaced, two-dimensional 200-foot by 200-foot horizontal grid. Simulated channel flow is based on a different numerical grid to account for one-dimensional flows. Because the channel flows are more dynamic in nature than overland or subsurface flows, the spatial and temporal discretjzations are much finer. Vertically, the saturated zone model considers four layers: two define the unconsolidated material, and two defme the weathered bedrock. Unsaturated mne columns at each of the 4,035 horizontal grid cells are discretized into 117 vertical nodes; a finer grid is specifled at the groundsurface through the root zone (-1 meter) and increases to a constant value of 0.4 meters to the groundwater table.

In the MIKE SHE model, three different time steps must be specified for channel flow, overland and unsaturated zone flow, and saturated zone flow. Timesteps are specifled at 0.5 minutes for channel flow, 30 minutes for overland and unsaturated zone flows, and every 6 hours for the saturated zone [the saturated zone responds much more slowly than the channel flow).

Model Parameterization and Boundary Conditions

Surface channel and pond profiles were carefully constructed from available detailed topographic data. Channel resistances were spatially distributed based on former Site modeling and available field observations. Overland flow areas were deflned based on observed surface flow pattems that address pavement and building routing. Surface depression storage and resistance vary spatially based on differences between paved, graveled, and vegetated surfaces. Observed surface inflow boundary conditions were specified at the westem boundary, and very shallow constant stage heights were set for downstream stream boundaries. The downstream constant stage heights did not affect upstream flow conditions based on observed conditions. Intemal upstream network branches were assigned zero discharge conditions. The model calculates only lateral downstream inflows.

Spatially distributed saturated hydraulic conductivities for the saturated and unsaturated zones were deked based on available surficial and bedrock material delineations by the USGS. Values ranged from 2e-4 to 3e-5 m/s for unconsolidated material and 8e-6 to le-10 m/s for the underlylng weathered bedrock material. Vertical anisotropy ranged from 1 to about 5 for the four saturated zone model layers. Spatially constant speciflc yields were specifled for each layer, ranging from 0.1 for unconsolidated material. 0.2 for weathered bedrock. Three primary zones were used to d e b e the distribution. Rocky Flats Alluvium in mesa areas, colluvium and landslide material on hillslopes and eastem areas, and Valley Fill Alluvium in the main near-stream areas. The near-stream soils exhibited the highest permeabities and the colluvium the lowest.

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Precipitation data was spatially distributed every 15-minutes over the model area using .a truncated Gaussian weighting filter method developed by Thomton et al (1997). The method accounts for complex topography, and variable station data. Hourly temperature data from a single on-Site meteorological station was used to drive snowmelt calculations. The REF-ET Program (Allen, 2000) was used to calculate the potential evapotranspiration (PEn required in the KristensenJensen method in MIKE SHE. Several meteorological data types were used to calculate PET including: (1) solar radiation; (2) air temperature: (3) humidity: (4) wind speed; (5) soil heat flux; (6) atmospheric pressure; and (7) precipitation.

MODEL PERFORMANCE

Model performance was established and evaluated in three steps: (1) calibration; (2) sensitivity analysis; and (3) validation. In the model calibration process, key hydrologic parameters that control system response were adjusted to reproduce observed system response. In the sensitivity analysis. the calibrated model was used to identify which parameters have the most impact on simulated responses in areas of interest. The sensitivity analysis was also used to establish the level of accuracy needed for some of the model parameters. The model validation further tested the performance of the calibrated model using climate and boundary condition data other than that used to develop the calibrated model. These three steps are described in greater detail below.

Calibration Approach

Current literature indicates that typical approaches to calibrating a fully-integrated hydrologic model in a semi-arid area like WEXS are either unavailable, too general [Storm and Refsgaard, 19961. or too reliant on the approach for calibrating traditional groundwater flow models. The traditional groundwater calibration approach is inappropriate for an integrated model mainly because there are substantially more parameters that can be adjusted. Another traditional approach involves assuming steady-state conditions in groundwater modeling to facilitate calibrating specific parameters, like hydraulic conductivity (Andersson and Woesnner, 1992). However. this is also not possible in the integrated flow modeling conducted for the WETS model because the saturated zone flow is coupled to the surface flow system, which is much more sensitive to transient effects than the saturated zone. Automated parameter estimation methods are also infeasible because of the large number of parameters that control the complex integrated flow in a system at WETS. Given the large number of calibration parameters, it was not expected that a unique set of values, that produce a unique model solution, would be determined. Instead, the set of calibration model parameter values represent one of many possible combinations that satisfy the specified calibration target criteria.

The speciflc calibration approach developed for the WETS SWWB model considered several factors that included project objectives, focus areas, grid resolution, and the quantity and qualily of calibration parameters and targets. Although a substantial amount of spatial and temporal output is produced for each simulation. different levels of accuracy were defined for different areas in the model. This is because of differences in the quality and quantity of calibration parameters and targets within the model. Specific focus areas, where key management decisions will be required in the future, or where pronounced hydrologic effects are likely due to Site closure, were required to have higher levels of accuracy in the model. Available measured calibration target data include the following:

Surface water flow rates (15-minute record); Quarterly groundwater levels (quarterly water level data); and Groundwater levels recorded at 4-hour intervals.

Other more semi-quantitative data, like observations of seep locations or losing/gaining reaches, were also used to help constrain the model calibration.

The amount of spatially distributed streamflow data alone is substantial (more than 40 gages with 15-minute record). The rapid streamflow response to precipitation effectively constitutes an independent calibration target data set that can be used to calibrate the integrated model. However, instead of

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considering the response of each gage, the approach used for this project prioritized gages by importance to calibration of the regional model response.

Figures 7 and 8 are used to show how the model calibration process was spatially and temporally prioritized. Surface flow at the eastern model boundary. along Walnut and Woman Creeks (gages GS03 and GSO1, respectively), are important to the SWWB because most discharge (except evapotranspiration) from the model occurs through these points. They represent the primary possible route for contaminant transport off-Site. Streamflow hydrographs for these gages are complex. They are influenced by the combined effects of all upstream inflows or losses, including contributions from groundwater. As a result, simulating accurate annual and seasonal flow response at the GSOl and GS03 gages was given a higher priority than other gages.

Other important flow data at the Site include response from Industrial Area gages SW093. GSlO and SW027. Annual stream flow hydrographs for these gages indicate flows result from the combination of several types of upstream inflows, including: direct groundwater interaction. drain flows (storm, sanitary and footing drains). and rapid runoff from paved and building areas. These gages are also important because they are used to closely monitor Industrial Area contaminant transport. An attempt was made to calibrate the response at other gages with available data, though this was considered less important than simulating flow response at the eastem model boundary and from the Industrial Area.

Figure 7. Spatial Calibration Features

Figure 8 summarizes the calibration priorities for surface water flows. The highest priority was simulating annual and seasonal surface flows along Woman and Walnut Creeks at the model boundary (GSO1 and GS03). Simulating annual and seasonal surface water response for the GS10. SW093 and SW027 gages from the Industrial Area was considered next most important. Simulating individual runoff events for these and other internal model gages was considered at the next lower level of importance. Finally. correctly simulating individual events was considered less important than simulating annual or seasonal flows. because individual events are more subject to spatial and temporal accuracy of climate data and other averaging in development of the numerical model.

Calibrating model parameters to accurately reproduce groundwater flow response is important to the model, but considered secondary to simulating surface water flows accurately at key locations (discussed above). The saturated zone takes much longer to respond to external stresses and discharges far less water at the eastern model boundary than does the surface flow system. Although continuously- monitored groundwater level data (recorded every 4 hours) were available, simulating quarterly groundwater levels was given a higher priority. Spatially, these data are more comprehensive than the continuously-monitored water level data. Reproducing average groundwater levels in focus areas were given a higher priority.

The continuous data were useful in determining unsaturated zone and evapotranspiration parameters with single column models because of the clear recharge response to precipitation events.

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Single column simulations also demonstrated the importance of local evapotranspiration on groundwater levels. even where levels were well below the root zone. Despite reasonably good simulation of groundwater levels in individual wells (achieved by adjusting evapotranspiration, unsaturated and saturated zone parameters), it was difficult to reproduce the response in all wells located within the same regional geologic material (i.e., Rocky Flats Alluvium). This is probably caused by local variations in soil properties. As a result, only the seasonal average levels for continuous well data were used as calibration targets.

A full year was used to calibrate the fully-integrated model. The calibration year coincides with WY2000 (October 1, 1999 to October 1, 2000). This year was selected because model input and response data were the most comprehensive. Furthermore, the annual precipitation for this year was close to the average annual amount and considered reasonable for calibration purposes. These calibration year climate data were used as input to the model for two years to allow the subsurface system to reach a dynamic state of equilibrium. Results show that the more important shallow groundwater areas (near streams) equilibrate quickly, while deeper. mesa areas take years. Therefore. the two-year simulation provided more realistic initial conditions for the actual calibration year.

sw At Model Boundary

sw sw AnnuaUSeasonal AnnuaVS easonal Internal (OS IO) Internal (SW093)

SW Event SW Event SW Event Internal Internal (SW093) (SW027)

/ Quanerly Heads (Water levels)

Continuous Heads (Water Levels) \

Figure 8. Temporal Prioritization of the Calibration Process

Model Calibration Results

The following sections present comparisons of observed and simulated responses with discussion of model performance and applicability.

Surface water - Annual Volume Comparison

In the Industrial Area, simulation of flow volumes compared well to observations for the main subdrainages of GS10. SW027 and SW093. Differences in the annual values ranged from 2% for SW093 to 26% for SW027. In general. simulated seasonal volumes also followed observed trends. The Buffer Zone simulations varied more from the observations. Woman Creek (at GSO1) exhibited a difference of roughly 30%. while Walnut Creek (at GS03) produced a 40% difference.

A 20% error was assumed for all observed cumulative volume estimates. ?his error was based on manufacturer-reported accuracy for the monitoring equipment (ISCO. Inc., 1996) and recommendations from the Site hydrologist responsible for estimation of missing data. Estimation is required to AU in data gaps when flume capacities are exceeded, equipment fails or freezing conditions produce questionable results.

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Surface Water - Comparison of 15-Minute Responses

A more detailed comparison of observed and simulated response. using the 15-minute record, was also performed for gages in the Industrial Area and Buffer Zone. This comparison allowed for a more detailed assessment of system response.

Consideration of the high-resolution flow rate response proved particularly important to model calibration within the Industrial Area, where the model structure is complex and response to precipitation is rapid. System responses at gages SW093. GSlO and SW027 were assessed for flow conditions ranging from low-flow (no precipitation) conditions. to snowmelt events, to a range of rainfall runoff events. Simulated response for these gages compared well in all conditions. The following images, Figures 9 through 12. provide examples of calibration performance for varlous flow conditions.

South Walnut Creek Industrial h a Runoff- GS10 Simlated VS. Obaerved . W M W

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€ 1.1

ii 1.0

2 0.6

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0.2

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North Walnut Creek industrial k e a Runoff- SWWJ Sirmlated VI. Obwwed - W Z O M l

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Figure 9. Simulated vs. Observed Flow: Large Precipitation Event

Walnut Creek industrial Area Runoff . OS10 Simulated VS. Obsewed -WYZOOO

Walnut Creek Industrial Area Runoff. SWOSS Simulaled VI. Obaewed -WY2000

Figure 10. Simulated vs. Observed Flow: Moderate Precipitation Events

283

Walnut Creek IndustTI.4 Area Runoff- OS10 Simulated VS. Observed - W Y Z O O O

Walnut Creek indusmal A n a Runoff - SWOb3 Simulated VS. Observed -WYZOOO

Figure 11. Simulated vs. Observed Flow: Snowmelt Event

Buffer Zone responses to system stresses are generally less rapid than the Industrial Area responses. Evaluation of high-resolution flow rate information, however. still provides signiRcant insight into the behavior of the model as it attempts to simulate the observed behavior in accordance with the conceptual model. Buffer Zone responses are discussed below for both Woman (GSO1) and Walnut (GS03) Creeks.

In the conceptual model for the Site. surface flow response at GSOl is largely seasonal, due to the signillcant summer-time effects of evapotranspiration on near-stream groundwater levels. The simulated and observed annual stream flow responses at GSOl are shown on Figure 12. The lack of observed flow response from June through October results from increased evapotranspiration rates that eliminate baseflow observed earlier in the year. Simulation of this loss period was challenging. The combination of narrow channel profiles. with respect to the ZOO-foot x ZOO-foot model cell dimensions, and the shallow hillslope-aquifer configuration caused the model to over-estimate near-stream groundwater levels in some areas. As a result, the model simulated groundwater discharge into the stream during dry periods. Although evapotranspiration-vegetation parameters and saturated hydraulic conductivities were adjusted to reduce these flows, discharge was not entirely eliminated within the range of possible parameter values.

Despite the noted flow differences. simulated flow conditions along lower Woman Creek are considered reasonable for the purposes of the SWWB objectives. Simulating the difference between current (calibrated) and future conditions is considered more important than simulating the exact flow conditions at a point within the system. I t is assumed that if the correct seasonal processes and approximate flows are simulated well, then it is less necessary to simulate single points accurately.

The Buffer Zone portion of the Walnut Creek drainage includes the entire Walnut Creek drainage within the study area except the GSlO and SW093 subdrainages (which reflect the Industrial A r b response). The annual GS03 hydrograph for WYZOOO is dominated by 10 pond discharges throughout the year (apparent as eight distinct events). Also. the surface flow system lost water between the discharge points (terminal ponds) and the end of the study area (measured at GS03) for all discharges in WY2000.

The simulated and observed annual hydrographs for GS03 are presented in Figure 12. Stream losses are correctly simulated for each pond discharge event as obsewed. Low flow periods are also simulated well.

284

Woman Creek - OS01 Simulated vs. Observed - WY2000

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Walnut Creek - OS03 Simulated vs. Observed - WYZOOO

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Figure 12. Simulated vs. Observed Flow: Woman and Walnut Creeks at Model Boundary

Surface Water - Pond Performance

Limited observed data sets from Site ponds were available for comparison to simulated results. Observed and simulated levels compared well for the major Site ponds for which data were available (Ponds A-3, A-4, B-5 and C-2). As an example. the observed and simulated pond level records for Pond A-4 (WY2000) are shown below in Figure 13. Discharges from and transfers to Pond A-4 are apparent in the record as sharp decreases and increases in the water level.

1753

1752

1751

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I I I -Observed

Figure 13. Comparison of Observed and Simulated Pond Levels for Pond A-4

Simulated Groundwater

Simulated groundwater levels were compared against observed quarterly groundwater data for January, April, July and October 2000. Results show that simulated levels in focus areas and near- stream areas compare well against observed data. The largest deviations occurred in the northwestern model area and within hillslope areas where depths to groundwater are greater. Average simulated levels over the Industrial Area were slightly higher than observed levels by about 1.6 meters in January 2000 to 1.2 meters in October 2000. Average simulated levels for all quarters were slightly over- predicted (though locally, some areas were over-predicted, and others under-predicted). For example,

285

levels in a small sub-drainage in the southwestem Industrial Area, are over-predicted by more than 2 meters, while, levels were under-predicted along the main east to west road through the Industrial Area. These deviations are attributed mostly to averaging Industrial Area features on the 200-foot by 200-foot

These results are considered reasonable for the purposes of the SWWB model. To better compare observed and simulated response, groundwater depths for each quarter were spatially interpolated onto all MIKE SHE grid cells containing at least one well. Although this is a reasonable way to compare simulated and observed data over the effective grid cell dimension, cells containing multiple wells, particularly in steep hillslope areas, may not represent the local water table conditions well. Observed water levels within a single quarter for some cells varied several meters, which could have contributed to differences between observed and simulated responses.

grid.

Additional Simulated System Response

The following sections briefly describe additional system response for which no quantitative observed Nevertheless, the simulated system response demonstrated several important data were available.

findings that result from the calibration of the model to quantitative data discussed above.

Simulated Overland Flow

Annual total simulated overland flow entering nearby streams is shown on Figure 14. Although, the areas actually contributing overland flow to streams varies from event to event, the annual distribution clearly showed that only cells immediately adjacent to stream channels provide overland flow. The relatively high, saturated vertical hydraulic soil conductivities compared to precipitation intensities did not produce Horton-type overland flow. Instead, most of the overland flow occurred near stream areas as saturation excess (shallow groundwater reached the ground surface). This is consistent with the hillslope flow conceptualization.

Simulated Evapotranspiration Figure 14. Cells Contributing to Overland Flow

The distribution of simulated annual actual evapotranspiration (in/year] is shown on Figure 15. The highest rates occurred along stream areas where riparian vegetation occurs in a higher density. Rates were also higher in the mesa area north of the Present Landfill, which was likely due to the higher annual precipitation amounts in this area. The lowest rates occurred in impervious areas within the Industrial Area. Evapotranspiration in these areas occurred only as surface evaporation. Simulated evapotranspiration rates were lower over Rocky Flats Alluvium areas compared with Colluvium, or Valley Fill alluvium areas. This results mostly from the lower water use (xeric) vegetation combined with relatively high surface infiltration rates. Although annual rates were not monitored at WETS. the simulated distribution appeared consistent with the conceptual understanding of where actual evapotranspiration was the highest.

Simulated Losing/Gaining Stream Areas

The simulated groundwater discharge along Woman Creek correlated well with the gaining/losing stream segments identified in the Fedors and Warner (1993) study. Figure 16 shows gaining and losing stream areas for the model area. Conceptually the distribution also makes sense: gaining stream

286

segments were simulated in steeper hillslope areas to the west, and losing segments occurred in flatter, less steep hillslope areas to the east. The losing segment on eastem Walnut Creek, below the A- and B- enes ponds, is probably affected by pond operations. The western part of North Walnut Creek is a gaining stretch, also due to steeper hillslopes.

Figure 15. Simulated Actual Evapotranspiration (in/yr)

Figure 16. Simulated losing and gaining stream segments

Model Sensitivity Analysis

Sensitivity Analysis Approach

The purpose of the sensitivity analysis was to determine parameters that affect integrated system behavior the most in key decision areas. It is an important step in demonstrating performance of the integrated model because the general system response was also evaluated. Sensitivity analysis did not

287

consider effects of all possible parameter combinations, or sensitivity to climate variability, though individual parameter adjustments to these eight yield valuable insight into model performance.

Through model calibration, eight of the calibration parameters were identiiled for the sensitivity analysis. The range of parameter values used in the sensitivity analysis was based on approximate observed ranges. The eight parameters used in the analysis were: 1) saturated hydraulic conductivity: 2) soil vertical hydraulic conductivity: 3) soil retention function: 4) unsaturated hydraulic conductivity function shape exponent: 5) evapotranspiration leaf area index (W); 6) evapotranspiration crop coefficient (Kc): 7) evapotranspiration root depth function (RDF): and 8) channel leakance.

Sensitivity Analysis - Results

Simulated sensitivity output results were normalized against the calibration model results in areas of key management decisions or future land use changes. Results showed that the calibrated model performs well in predicting system response to given parameter adjustments. The three most sensitive model parameters included saturated hydraulic conductivity, W, and the vertical saturated hydraulic conductivity for the unsaturated zone. For example, increasing saturated hydraulic conductivity values, decreased groundwater levels in mesa areas, while in stream areas, levels initially increased, then dropped as a result of enhanced hillslope drainage toward the streams. Stream flow appeared to be most sensitive to evapotranspriation parameters &AI and Kc). A decrease of either parameter resulted in higher groundwater levels, which provided higher baseflow to streams, and hence increased stream flow.

Model Validation

Validation Approach

The two chosen validation simulation periods were the comparatively large spring precipitation event of 1995, and the entire year of WY2001. These two periods were chosen primarily based on the range of climatic conditions represented, relative to the calibration period of WY2000. This section briefly discusses the climate of the periods, development of the models, and the data limitations.

The May 17. 1995 event corresponded to roughly a 15-year retum-frequency precipitation event at the Site (EGSrG. 1992 and Kaiser-Hill, 2002). Further, the 1995 months of Aprll and May were exceptionally wet: 12.1 inches of precipitation fell in these two months, compared to 13.8 inches for all of WY2000. These 1995 precipitation events were long-duration, low-intensity events. WY2001 (15.6 inches of precipitation) was also wetter than average (14.8 inches). In contrast with the 1995 event: however, WY2001 had several high-intensity events.

The model structure was modified in an attempt to reproduce the conditions of each simulation period. This amounted to a number of routing and control structure changes for 1995 but very few changes were required for WY2001. Climatic input, boundary inflows and intemal transfers and discharges were all updated to reflect the period of record.

The data set for WY2001 was as complete as that for WY2000. The data set for the 1995 simulation was more limited. For example, some inflow and system response data and peak flow data were not recorded, and spatially-distributed precipitation data were not available. These limitations were taken into consideration when assessing results.

Validation Results

In general, both validation models performed well. Simulated results compared well to observations, with trends similar to those for the calibration year. Observed Industrial Area hydrographs and annual cumulative volumes were reproduced very well. In the Buffer Zone, both the Walnut Creek and Woman Creek simulated responses compared well to observed data, though grid resolution effects were still present along Woman Creek. Groundwater level comparisons to quarterly data showed good matches in near-stream areas. Figure 17 shows a comparison between observed and simulated annual surface flows for the 1995 and WY2001 validation simulations. The comparison shows under-predictions of flow for 1995 at GSOl (woman Creek) and GS03 (Walnut Creek), as expected based on exclusion of unknown boundary inflow volumes.

288

Jan 1396 tttrough May $SSS

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Figure 17. Simulated vs. Observed Stream Flow (Pre- and Post-Calibmtion Period Validation)

SUMMARY AND CONCLUSIONS

A physically-based, fully-integrated model was developed for the semi-arid RFETS hydrologic system. Complex Site features incorporated into the model included: (1) Industrial Area subsurface utilities, basements and impervious areas, and (2) Buffer Zone seeps, streams and 10 Site-managed ponds. High temporal resolution climate data (including temperature, potential evaptranspiration and spatial distributed precipitation) were specified as external stresses to the model. Simulated output from the model was evaluated against observed data in a calibration approach that gives higher priority to focus areas ofthe Site where critical management decisions must be made. The performance of the model was also evaluated through sensitivity analyses and a validation exercise using pre- and post-calibration year model input.

Sub-regional scale models were used to improve the understanding of flow within smaller, less complicated ateas, while partially-coupled processes were used to understand the sensitivity of system response to individual processes. Both types of models were used to help parameterize the fully- integrated calibrated model. Results of single column simulations showed that evapotranspiration and precipitation recharge dominated seasonal groundwater level fluctuations, wen where groundwater is deep. Hillslope and subregional catchment models supported the conceptual model of seep flow and also showed that stream-aquifer flow dynamics were dominated by near-stream groundwater levels rather than by groundwater conditions in mesa or hillslope areas.

Integrated model results demonstrated that simulated surface flows for key locations compared well with observed flow data. Annual, seasonal and event-level responses were reproduced well in each of these locations for the intended purpose of using the model as a decision tool. Groundwater levels in key near-stream areas and the Industrial Area were also reproduced well, though, in mesa areas, deeper levels were more diffcult to accurately simulate. This occurs because the deeper unsaturated zones in these areas cause greater Iag times in precipitation recharge events, which result in greater sensitivity to imposed initial conditions.

ACKNOWLEDGEMENTS

The authors would like to thank the Department of Energy- Rocky Flats Field Oflice and the Kaiser- Hill Company, L.L.C. for supporting this modeling project. In addition, invaluable help was provided by WETS technical support staff from other organizations in collecting and providing Site data These individuals included Brain Blaser, Wendell Cheeks, Nick Demos, Craig Hoffman, Marcia Murdock, Jeff Pietsch, Stwe Singer, Rob Smith, George Squibb, and Diana Woods.

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