List of Tables... ii. List of Figures... ii. Executive Summary... ES - 1. Section 1: Introduction

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2 Table of Contents List of Tables... ii List of Figures... ii Executive Summary... ES - 1 Section 1: Introduction Modeling Objective Work Flow Section 2: Develop Hydrogeological Conceptual Model Data Compilation and Assessment Hydrostratigraphic Units Hydrogeologic Budget Inflows Outflows Aquifer Parameters Geochemical Analysis Data Quality Assessment Section 3: Groundwater Flow Model Development Selection of Flow Modeling Tools Model Domain Groundwater Flow Model Setup Groundwater Flow Model Groundwater Flow Model Calibration Procedure Section 4: Solute Transport Model Development Selection of Solute Transport Modeling Tools Solute Transport Model Setup Solute Transport Model Solute Transport Model Calibration Procedure Section 5: Remedial Alternatives Monitored Natural Attenuation Extraction and Treatment with Reinjection to the Upper Aquifer Extraction, Treatment and Offsite Discharge Sensitivity and Uncertainty Analysis Section 6: Report References... References - i Work Plan for Groundwater Modeling, i

3 List of Tables 1 Responses to CVRWQCB (2004) Guidelines 2 Project Schedule List of Figures 1 Site Location Map 2 Site Plan 3 Wastewater and Primary Land Use Timeline ( ) 4 Former Primary Land Reuse Area Application Timeline (December 2010 September 2011) ii Work Plan for Groundwater Modeling,

4 Executive Summary Hilmar Cheese Company (HCC) operates a milk-processing facility (the Site) near the town of Hilmar in Merced County, California. Facility process wastewater was discharged to a holding/percolation pond from 1985 to 1989 and then to areas identified as Primary Lands through The discharge of partially treated wastewater to the Primary lands occurred from 1987 through December The maximum area of the Primary Lands was 97 acres. From 2001 to present, HCC s highly treated process wastewater (reclaimed water) has also been discharged for agricultural irrigation to locations identified as the Reuse Areas. The Primary Lands have been returned to agricultural use and are irrigated with HCC s highly treated process wastewater (reclaimed water) or water from the Turlock Irrigation District (TID), although there is also an option to irrigate with dairy wastewater. Portions of the former Primary Lands were designated as Reuse Areas in March and April The past discharge of process wastewater and partially-treated process wastewater contributed to a TDS groundwater plume that has reached a state of equilibrium. A Remedial Investigation (RI) and Feasibility Study (FS) were completed by Jacobson James & Associates (JJ&A, 2010a, b) in response to Cleanup and Abatement Order (CAO) No. R , issued to HCC by the Regional Water Quality Control Board (RWQCB). The FS identified, screened, and evaluated potentially applicable remedial alternatives. The FS stated that estimates of cleanup time could not be developed for inclusion in the FS, because discharges to the Primary Lands were being terminated, which represented a change in land use conditions, and no representative groundwater quality data were available to support development and calibration of a model. The FS recommended that modeling be performed at a later date. In a letter dated 4 October 2011, the RWQCB concurred that time estimates were necessary and requested that groundwater modeling be conducted to estimate the time required to reach or achieve the remedial action objective for each alternative considered for the Upper Aquifer (RWQCB, 2011). This Work Plan provides a roadmap for a groundwater modeling effort to address the RWQCB request and estimate cleanup timeframes associated with three of the remedial alternatives: (1) monitored natural attenuation, (2) groundwater extraction with onsite reinjection, and (3) groundwater extraction with offsite discharge. The fourth remedial alternative evaluated in the FS was groundwater extraction with onsite reuse, which is not being modeled because yearround onsite reuse is not a viable option. The Work Plan approach is consistent with relevant technical and regulatory guidance for groundwater modeling, including the Draft Guidelines for Submittal of Information Developed from Models to the Central Valley Regional Board (RWQCB, 2004). The approach includes development of a detailed hydrogeological conceptual model (HCM) based on existing data, followed by development of a numerical groundwater flow and solute transport model using the MODFLOW family of codes. The model will be rigorously calibrated using site-specific data, industry-standard modeling codes, and parameter estimation techniques. Statistical methods will be used to evaluate the goodness-of-fit of the model compared with observed data. Three separate scenarios will be modeled to estimate timeframes required to achieve RAOs for three remedial alternatives. A rigorous sensitivity and uncertainty analysis will be conducted for Work Plan for Groundwater Modeling, ES - 1

5 the remedial alternative simulations. Key aquifer parameters and boundary conditions will be varied within acceptable ranges based on the HCM to quantitatively evaluate the simulation outcomes relative to model input values. ES - 2 Work Plan for Groundwater Modeling,

6 Section 1: Introduction Hilmar Cheese Company (HCC) operates a milk-processing facility (the Site) near the town of Hilmar in Merced County, California. The site location is shown on Figure 1 and Site features are shown on Figure 2. As shown on Figure 3, HCC process wastewater was initially discharged to a holding/percolation pond (1985 to 1989) and then to an area identified as the Primary Lands through The discharge of partially treated process wastewater to the Primary Lands occurred from 1987 through December 2010, with a maximum Primary Land area of 97 acres. The Primary Lands have now been returned to agricultural use and are irrigated with HCC s highly treated process wastewater (reclaimed water) or water from the Turlock Irrigation District (TID), although there is the option to irrigate with dairy wastewater. As shown on Figure 4, portions of the former Primary Lands were designated as Reuse Areas S-63 and S-64 in March 2011 and as Reuse Area S-65 in April The past discharge of partially-treated process wastewater contributed to a TDS groundwater plume that has reached a state of equilibrium. A Remedial Investigation (RI) and Feasibility Study (FS) were completed by Jacobson James & Associates (JJ&A, 2010a, b) in response to Cleanup and Abatement Order (CAO) No. R , issued to HCC by the Regional Water Quality Control Board (RWQCB). The FS identified, screened, and evaluated potentially applicable remedial alternatives for the Upper Aquifer. The FS stated that estimates of cleanup time could not be developed for inclusion in the FS, because discharges to the Primary Lands were being terminated, which represented a change in land use conditions, and no representative groundwater quality data were available to support development and calibration of a model. The FS recommended that modeling be performed at a later date. In a letter dated 4 October 2011, the RWQCB requested that groundwater modeling be conducted to estimate the time required to reach or achieve the remedial action objectives (RAOs) for each alternative (RWQCB, 2011). This Work Plan provides a roadmap for a groundwater modeling effort to address the RWQCB request and estimate timeframes associated with three remedial alternatives. As illustrated in Table 1, the Work Plan approach is consistent with relevant technical and regulatory guidance for groundwater modeling, including the Draft Guidelines for Submittal of Information Developed from Models to the Central Valley Regional Board (RWQCB, 2004). 1.1 Modeling Objective The objective of the modeling effort is to provide quantitative estimates of the timeframes required to achieve RAOs for three remedial alternatives for the Upper Aquifer at the Site, namely: 1. Monitored natural attenuation 2. Groundwater extraction and treatment with onsite reinjection 3. Groundwater extraction with offsite discharge Work Plan for Groundwater Modeling, Page 1-1

7 A numerical groundwater flow and solute transport model for the Site will be developed. The model will be developed with the necessary detail and resolution to serve as a quantitative tool for evaluating the fate and transport of constituents of concern (COCs) at the Site. 1.2 Work Flow The transient calibrated numerical groundwater flow and solute transport model developed for this effort will focus on the Upper Aquifer and will not include the Lower Aquifer below the Corcoran Clay. The Corcoran Clay will be incorporated in the model as a boundary condition. The groundwater flow model will be based on refinement of the existing qualitative hydrogeologic conceptual model (HCM), which was developed from previously collected field and laboratory data. The numerical groundwater flow model will be calibrated with respect to site-specific groundwater elevation data. The calibrated groundwater flow model will also be evaluated against new data as they become available. During development of the HCM, an evaluation of geochemical conditions beneath the Site will be conducted to assess the relative impact on simulated cleanup times caused by oxidation/reduction (redox)-sensitive constituents. If it is determined that the relative impact is minor, then it will be unnecessary to consider the impact of geochemical reactions on simulated cleanup timeframes, and solute transport modeling of conservative constituents (i.e., one or more Site COCs) will be sufficient to achieve the modeling objective. The solute transport model will be based on the HCM and will be calibrated with respect to concentrations of selected constituents. The calibrated solute transport model will also be evaluated against new data as they become available. The calibrated flow and transport models will be used to evaluate the three remedial alternatives. Sensitivity and uncertainty analyses will be conducted during development of the groundwater flow and solute transport models and for simulations of the three remedial alternatives. The schedule for project execution is designed to meet project milestones (Table 2). Four principal overlapping Tasks are anticipated for this project: Task 1: Develop the HCM from existing data Task 2: Develop and calibrate groundwater model Task 3: Perform model evaluations Task 4: Prepare report and present results to the RWQCB Subtasks have been defined that delineate work flow activities (Table 2). Project milestones are the initial completion of each of these subtasks. Post-milestone time is allotted for the first three subtasks to account for potential additional work on model complexity, which will be guided by the model calibration process. As an example, consider the potential impact of offsite groundwater pumping by other parties. Our objective is to construct a reliable model that includes the major processes necessary to simulate groundwater flow and solute transport at the Site. Initially, an aquifer stress such as offsite groundwater pumping may not be included in the model, because the need to include offsite pumping, and the degree to which it needs to be included (i.e., distance from Site), cannot be determined a priori. Rather, the need to include offsite pumping by other parties will Page 1-2 Work Plan for Groundwater Modeling,

8 become apparent during model calibration and it would be added to the model if its inclusion increases the reliability of the model in terms of achieving the model objective. Work Plan for Groundwater Modeling, Page 1-3

9 Section 2: Develop Hydrogeological Conceptual Model The HCM will describe elements of the groundwater flow system that will be translated into the numerical models of flow, solute transport, and potentially, geochemical processes. The model domain for this modeling effort is restricted to the Upper Aquifer. 2.1 Data Compilation and Assessment A thorough understanding of Site hydrogeology and key processes affecting concentrations of Site COCs will be developed. In collaboration with JJ&A, Kennedy/Jenks will review available reports and information pertinent to the project, including hydrogeologic and water quality data. Groundwater elevation and quality data have been collected and analyzed by JJ&A and others; these data are compiled in a database. Data compilation and assessment includes the following: Compile existing geological and hydrogeological reports, maps, and geologic crosssections. Assemble available data necessary for development of conceptual and numerical models. Possible data sets include groundwater elevations, aquifer tests, geologic logs, geophysical logs, surface water gauge records, precipitation, well locations, well pumping records, and water quality data. Generate electronic-format Site base maps in an appropriate coordinate system such as AutoCad or ArcMap. Review reports, maps, and data that are integral components of the HCM including the occurrence and movement of groundwater, hydrogeologic budget including estimates of recharge and discharge, aquifer geometry and hydraulic characteristics, hydrogeologic stresses, and spatial and temporal trends in groundwater quality. The application of process wastewater to the Primary Lands has varied over the years, in terms of water quality and volume. The history of process wastewater discharges has been summarized by JJ&A (2010b; 2011) in terms of TDS concentration (Figures 3 and 4). These data will be reviewed from a model-development perspective, to gain an understanding of the TDS concentrations present groundwater at the Site. 2.2 Hydrostratigraphic Units In support of numerical model development, a range of reasonable values is defined for aquifer properties and the hydrogeologic budget based on measured field data (groundwater elevations and quality) and hydrogeological analysis (e.g., geologic cross-sections, well logs, and geologic interpretation). The general procedure for this process is to define values for a representative elementary volume (REV), as described by Bear and Verruijt (1987). These values represent the major physical features of the groundwater flow system, recharge and discharge components, definition of model layers, and the distribution of aquifer properties. A comprehensive review of these elements will lead to the definition of hydrostratigraphic units (HSUs) designed to facilitate development of the numerical model. Work Plan for Groundwater Modeling, Page 2-1

10 HSUs can be thought of as a convenient way to package small-scale geologic features into larger aquifers and aquitards. For example, within an HSU hydraulic conductivity may vary but the HSU is treated as one unit for the purposes of inverse modeling (automated calibration using Groundwater Vistas and PEST), sensitivity analysis, and displaying depth-averaged contours of head, drawdown, and concentration (ESI, 2007). HSUs facilitate the modeling effort by providing a hydrogeologic framework at the appropriate scale for conducting simulations and interpreting simulation results. The definition of HSUs will guide decisions regarding appropriate model grid resolution and layering, with respect to achieving the modeling objective of estimating cleanup timeframes for the remedial alternatives. 2.3 Hydrogeologic Budget The hydrogeologic budget forms the basis for defining boundary conditions. Known inflows (recharge) and outflows (discharge) to and from the model domain will be quantified. Differentiating between inflows and outflows is generally straightforward. However, irrigation canals adjacent to the Site have the potential to be both a recharge and discharge source, depending on relative water levels. During calibration of the flow model, the overall impact of irrigation canals on Site hydrogeology will be assessed Inflows Recharge from applied water includes known and estimated applications of process wastewater, supplemental irrigation water, and dairy wastewater. COC concentrations in applied water sources will be determined and estimated as needed based on facility records. Subsurface inflow upgradient of the Site is a major component of the water balance. For the Site HCM, these flows will be incorporated by applying specified head boundary conditions on the upgradient boundary of the model domain. Return flows resulting from percolation of water applied to the surface for irrigation or other programs may be a significant source of recharge to the model domain. These flows will be based on available data or estimated based on standard practices. Usage of reclaimed water, TID water, and dairy wastewater will be based on data where available, and estimated where unavailable, due to the impact of these different water sources on groundwater quality. Precipitation recharge rates will be based on an assumed percolation rate of rain that falls directly onto unpaved surfaces. These rates will be based on precipitation records, soil types, or other data and will be consistent with the values previously used by Kennedy/Jenks in water balance modeling for the Site. However, local precipitation may not be significant, compared to the volume of irrigation return flows and applied water. Therefore, an assessment of the relative importance of precipitation will be conducted Outflows Subsurface outflow downgradient of the Site is a major component of the water balance. For the Site HCM, these flows will be incorporated by applying specified head boundary conditions on Page 2-2 Work Plan for Groundwater Modeling,

11 the downgradient boundary of the model domain and a downward flux across the Corcoran Clay to represent leakage through the bottom boundary. Networks of TID tile drains are present in the area to lower the water table for agricultural purposes. These tile drains are a potentially significant source of discharge (outflow from the model domain). Some of the previously active tile drains have been plugged. The history of the tile drain networks will be reviewed and flows to past and present TID tile drain systems will be estimated. Evapotranspiration is the loss of water through evaporation and uptake by vegetation. It can be locally important and generally impacts only shallow groundwater. Evaporation rates will be estimated based on standard practices and will be consistent with the values previously used by Kennedy/Jenks in water balance modeling for the Reuse Areas. Onsite groundwater pumping occurs primarily from below the Corcoran Clay, and is therefore outside of the model domain (see Section 3.2). Offsite groundwater pumping will be included in the model as needed, based on data review and model calibration. Pumping records for smaller wells are commonly not obtainable and, therefore, will be estimated as needed. The potential impact of wells that are screened in both the Upper and Lower Aquifers will also be considered. 2.4 Aquifer Parameters Aquifer parameters represent properties of the aquifer that are relevant to groundwater flow and solute transport. A numerical model requires that aquifer parameters be defined everywhere within the model domain. Because aquifer property datasets are often insufficient, methods to extrapolate aquifer parameters into areas with insufficient data will use science-based assumptions. Initial values of hydraulic conductivity, specific yield, specific storage, effective porosity, and dispersivity will be based on aquifer pump tests, laboratory tests, well logs, and soil characterization (Kennedy/Jenks, 2005; JJ&A, 2010a, b). These initial values will be evaluated for consistency with aquifer parameter data available in United States Geological Survey (USGS) reports (Burow et al., 2004; Phillips et al. 2007a, b) as well as other available reports. 2.5 Geochemical Analysis A geochemical analysis will be conducted during the development of the HCM to address the relative impact of redox-sensitive Site COCs on simulated cleanup timeframes for the three remedial alternatives. In addition, water quality trends, based on previous work submitted to the RWQCB (JJ&A, 2011), and more recent work as it becomes available, will be assessed to determine the number of chemical constituents required for reliable solute transport simulations. The first objective of the geochemical analysis is to evaluate the impact on simulated cleanup times caused by redox-sensitive constituents. If the impact is minor, then it will be unnecessary to simulate reactive solute transport. The USGS geochemical simulator PHREEQC (Parkhurst and Appelo, 1999) will be used to determine the relative importance of coupling individual reactions or a reaction network to the solute transport model, because it provides flexibility in defining geochemical reaction networks. Additionally, one-dimensional solute transport can be simulated with PHREEQC, which facilitates testing of the reaction network. Furthermore, Work Plan for Groundwater Modeling, Page 2-3

12 PHREEQC can perform inverse modeling, in which applicable reaction networks are deduced from water quality data. The second objective of the geochemical analysis is to determine the number of individual Site COCs that need to be considered for the solute transport simulations (Sections 4 and 5). Water quality trend analyses (JJ&A, 2011), as updated by more recent sampling results, will be evaluated to determine whether a single conservative constituent (e.g., TDS) is sufficient for simulating the solute transport of all Site COCs, or if the fate and transport of multiple COCs need to be simulated. 2.6 Data Quality Assessment The objective of the data quality assessment is to develop an understanding of the uncertainty associated with the different data and data types. Statistical methods of data assessment will be used where appropriate. These methods are more fully described in Section 3.5. Page 2-4 Work Plan for Groundwater Modeling,

13 Section 3: Groundwater Flow Model Development A numerical groundwater flow and solute transport model will be developed for the Site. The model will include the necessary detail and resolution to serve as a quantitative tool to evaluate the cleanup timeframes of remedial alternatives. The first step is to develop the groundwater flow model, which forms the foundation for the solute transport model. Development of the groundwater flow model begins with numerically quantifying the conceptual model and ends with model calibration and uncertainty analysis. A key element of this effort is to incorporate the HCM into the numerical groundwater model. Reasonable ranges of values from the Site database and local area hydrogeologic reports will be defined for aquifer properties and the hydrologic budget. These values represent the major physical features of the groundwater flow system, recharge and discharge components, definition of model layers, and the distribution of hydraulic conductivity and storage coefficients. During model calibration, these values will be varied within the defined ranges to determine the values to be used to simulate the three remedial alternatives. 3.1 Selection of Flow Modeling Tools The industry-standard groundwater simulator MODFLOW-SURFACT (Hydrogeologic, 2001), which was developed from the USGS s MODFLOW groundwater flow model (Harbaugh et al., 2000), will be the primary modeling code for this project. MODFLOW- SURFACT has the capability to handle the Site HCM, which may require the inclusion of the unsaturated zone. If data review indicates that the unsaturated zone can be excluded, MODFLOW with the multi-node well package (MNW) will be used. These models have advanced features for handling resaturation of model cells and the ability to dynamically apportion and/or limit pumping in model layers as the water table fluctuates. MODFLOW-SURFACT has the same capabilities as the standard USGS MODFLOW, with the addition of a built-in reactive solute transport simulator and enhanced features for simulating unsaturated-zone processes, resaturation of dry model cells, and wells across multiple model layers. These additional features provide flexibility in simulating a subsurface domain in which the water table fluctuates. EPA-approved models and the standard MODFLOW available from the USGS do not simulate reactive solute transport coupled with saturated-unsaturated subsurface fluid flow in a rigorous and efficient manner. Modeling results from MODFLOW- SURFACT simulations have been rigorously evaluated in peer-reviewed journals and numerous consulting reports. The underlying fundamentals and assumptions of MODFLOW-SURFACT are the same as those found in MODFLOW. For example, both models are process-based models that are founded on conservation of mass and conservation of energy principles, as described by the well-known groundwater flow equation and Darcy s law. Two primary assumptions for groundwater models such as these are that: (1) flow of water in the subsurface is laminar and (2) Darcy s law is applicable at all scales (e.g., pore-scale phenomena are insignificant). These are standard assumptions and are appropriate for the scale and modeling objectives of the subject groundwater flow system. Work Plan for Groundwater Modeling, Page 3-1

14 3.2 Model Domain The model developed for this project will cover an area of sufficient extent to achieve the model objective. The spatial extent will be designed so that model boundaries are far enough away from the area of interest so that groundwater flow dynamics in the area of interest will not be influenced by the boundaries. For calibration purposes, the simulation period will begin in 2005 and run through the end of The simulations of remedial alternatives (Section 5) will be conducted in incremental durations to determine the amount of simulated time needed to achieve the RAOs. The model will simulate groundwater flow and solute transport in the Upper Aquifer. Therefore, the bottom boundary of the model domain will be the top of the Corcoran Clay. Data analysis will guide the decision of including the unsaturated zone in the model domain. Inclusion of the unsaturated zone may be necessary because residual COCs above the water table may constitute an ongoing source to groundwater. The grid cell size will be sufficiently small to achieve the model objective. Fine spatial resolution will facilitate accurate numerical solution of the solute transport equations without creating excessive numerical dispersion and/or numerical instability. Model layers will be designed to accommodate the HSUs and provide sufficient resolution for reliable solute transport simulations. 3.3 Groundwater Flow Model Setup The HCM will be numerically quantified in the Groundwater Vistas modeling environment (ESI, 2007), which allows flexibility with regard to code selection and model refinements. Aquifer parameters will be defined based on evaluation of aquifer properties. As described above for development of the HCM, numerical models require that aquifer parameters be defined everywhere within the model domain. Because aquifer property datasets are often insufficient, methods to extrapolate aquifer parameters into areas with insufficient data will use assumptions supported by sound scientific principles. Appropriate value ranges will be defined for each parameter. These ranges will be used as guidance during model calibration. The spatial distribution of these parameters will be evaluated during model calibration. Boundary conditions represent the hydrologic budget. The upgradient and downgradient boundaries will be defined as specified head boundaries. Lateral no-flow boundaries will be designed to represent horizontal flow lines far enough away from the Site to avoid influencing flow dynamics in the area of interest. The upper boundary of the model will preliminarily be defined as a free-surface boundary to represent the water table in the Upper Aquifer. The lower boundary of the model will be the Corcoran Clay. If it is determined through data analysis or model calibration that the unsaturated zone needs to be included, the upper boundary will be the ground surface. Initial conditions (groundwater elevations) will be based on data evaluation and steady-state modeling. These initial groundwater elevations will represent conditions at the end of Stress periods will be defined to represent the appropriate time scale for groundwater flow and solute transport. The simulation period will be increased (or decreased) based on preliminary Page 3-2 Work Plan for Groundwater Modeling,

15 simulation results. Stress periods need to account for the schedule of discharges and any local pumping that occurred during the calibration period. Furthermore, modeling of the remedial alternatives may require additional temporal discretization. Lastly, definition of the stress-period duration needs to consider the data-collection interval, to facilitate model calibration. All of these constraints must be accounted for, thus the ultimate stress-period durations will be determined during the model development process. It is likely that during the model calibration period, stress periods on the order of one month may be required for certain, but not necessarily all, time intervals. For the simulation of remedial alternatives (Section 5), stress period duration may be increased to decrease model execution times, in a manner that maintains numerical stability. Calibration targets will be chosen based on length of data record and data quality. For groundwater flow modeling, the calibration targets will be groundwater elevations through time at select wells identified during initial data review and development of the HCM. 3.4 Groundwater Flow Model Implementation of the groundwater flow model will occur in three successively more comprehensive steps. The basic philosophy is to start with simplicity and add complexity as needed to achieve model calibration. In the first step, a preliminary groundwater flow model will be implemented. This will be a steady-state model, to be used for model testing and refinement of initial conditions. The principal objectives of the steady-state model are to: (1) test the model to identify potential numerical difficulties related to model construction and (2) visually match observed groundwater elevations at the start of the simulation period (i.e., the end of 2005). In the second step, an initial transient groundwater flow model will be implemented. Stress periods, the applied water recharge schedule, and the pumping schedules will be added to the steady-state model. Simulation results will be evaluated statistically to identify needed improvement and refinements. The third and final step is to incorporate the modifications indicated by the initial transient groundwater flow model. Refinements to the final groundwater flow model will be guided by the parameter estimation techniques described below using optimization-based inverse modeling techniques. 3.5 Groundwater Flow Model Calibration Procedure For groundwater flow, the model performance objective is to minimize the residual between observed and simulated heads at select observations points identified during initial data review and development of the HCM. The modeling process will include iterative modification within appropriate ranges of aquifer parameters and boundary conditions to achieve this performance objective. Model performance will be rigorously evaluated using quantitative statistical techniques. The accuracy of simulation results will be improved by analyzing the statistical results and identifying aquifer parameters that need to be modified or additional processes that need to be considered. Work Plan for Groundwater Modeling, Page 3-3

16 Hill and Tiedeman (2007) have succinctly described a set of guidelines for effective model calibration. These guidelines are shown below for illustration purposes and will be applied as appropriate during the model calibration process: 1. Apply the principle of parsimony (start with simplicity; build complexity slowly) 2. Use a broad range of system information (soft data) to constrain the problem 3. Maintain a well-posed, comprehensive regression problem 4. Include many kinds of observations (hard data) in the regression 5. Use prior information carefully 6. Assign weights that reflect errors 7. Encourage convergence by making the model more accurate and by evaluating the observations 8. Consider alternative models 9. Evaluate model fit 10. Evaluate optimized parameter values 11. Identify new data to improve simulated processes, features, and properties 12. Identify new data to improve predictions 13. Evaluate prediction uncertainty and accuracy using deterministic methods 14. Quantify prediction uncertainty using statistical methods After qualitative calibration by trial-and-error, the final groundwater flow model will be calibrated quantitatively using optimization-based inverse modeling techniques, such as those found in PEST (Doherty, 2005), UCODE (Poeter et al., 2005), and MODFLOW-2000 (Harbaugh et al., 2000). These techniques facilitate quantification of: (1) the quality of calibration, (2) data shortcomings and needs, and (3) uncertainty of parameter estimates and predictions (Hill and Tiedeman, 2007). In particular, for the groundwater flow modeling effort described in this Work Plan, the 14-step process outlined above can be generalized and summarized as: Qualitative Calibration: Groundwater elevation maps will be compared visually by overlying measured and simulated groundwater elevations for the preliminary flow model and the initial flow model for selected time periods. Hydraulic gradients will be compared with respect to both magnitude and direction to ensure that the model is accurately simulating existing conditions. Statistical Evaluation: Weighted and unweighted residuals between measured and simulated groundwater elevations will be evaluated using parameters such as the residual mean, the absolute residual mean, the standard deviation of the residual mean, and others. These statistical comparisons will be made over both the entire and localized portions of the model to show the spatial variation in the calibration. Time Series Analysis: Hydrographs from a selected set of high-quality monitoring wells will be compared with simulated values to verify that the magnitude and overall trends in groundwater elevation are properly simulated by the model. Page 3-4 Work Plan for Groundwater Modeling,

17 Model Accuracy: Mass-budget data will be compared over each stress period to verify that the model results are reliable. This also provides a quality assurance check that the model input data were entered correctly. Parameter Estimation: Linear and nonlinear regression will be used for calibration of the final groundwater flow model. An objective function will be defined using simulation residuals. The value of this objective function will be minimized with the calibration tools described above. Model Refinements: Calibration results will be used to guide model refinements that may be required, as indicated in the project schedule (Table 2). Work Plan for Groundwater Modeling, Page 3-5

18 Section 4: Solute Transport Model Development The solute transport model will be developed after the calibrated final groundwater flow model is complete. The modeling process will include iterative modification within appropriate ranges of aquifer parameters and boundary conditions. The model will include the necessary detail and resolution to serve as a quantitative tool to evaluate the timeframe required to achieve RAOs for the three remedial alternatives. The calibration process may indicate that refinements to the groundwater flow model would be beneficial, as indicated in the project schedule (Table 2). 4.1 Selection of Solute Transport Modeling Tools As described in Section 3.1, the industry-standard groundwater simulator MODFLOW- SURFACT (Hydrogeologic, 2001), which was developed from the USGS s MODFLOW groundwater flow model (Harbaugh et al., 2000), will be the primary modeling code for this project. MODFLOW-SURFACT has built-in reactive solute transport capabilities. Solute transport is simulated with the advection-dispersion equation, which is based on conservation of mass principles. The principle features of the transport modules are advective-dispersive transport of multiple chemical species in steady-state or transient flow fields, linear or nonlinear retardation for each species, first-order decay and biochemical degradation of contaminants in water and soil, and generation of transformation products. If MODFLOW with the MNW package is selected as an alternate groundwater flow simulator, MT3DMS will be used for the solute transport portion of the model (Zheng and Wang, 1999). However, if it is determined during the geochemical analysis that reactions need to be included, RT3D (Clement, 1997) would be used. These solute transport simulators are well-established, industry-standard codes. 4.2 Solute Transport Model Setup The Site COCs that need to be evaluated in the context of solute transport are defined here as transport constituents. Evaluation of water quality trends at the Site (see Section 2.5) will determine the number of required transport constituents. Each transport constituent needs to be defined separately for the transport model (i.e., one solute transport equation for each constituent). Initial concentrations within the model domain need to be specified for each transport constituent. These initial concentrations will be assigned based on measured concentrations at the end of If the geochemical analysis indicates that interactions between dissolvedphase COCs and aquifer solids are significant with regard to estimating cleanup timeframes for the remedial alternatives, geochemical characteristics of the solids will be estimated based on data and literature review and assigned as an initial condition. Inflow concentrations to the model domain will be defined as specified flux and specified concentration boundary conditions. At the appropriate locations, these concentrations will reflect Work Plan for Groundwater Modeling, Page 4-1

19 background groundwater concentrations, estimates of irrigation water quality, and process wastewater concentrations as they changed through time (e.g., see Figures 3 and 4). In general, the spatial resolution for solute transport simulations often needs to be finer than the spatial resolution for groundwater flow simulations. This is because the solute transport equation and the groundwater flow equation are different types of partial differential equations (hyperbolic versus parabolic, respectively). The accepted criteria for grid spacing for solute transport simulations is based on the grid Peclet number, which relates length scale of dispersion to the grid cell dimensions. The horizontal and vertical spatial resolution will be revisited to ensure numerical stability. Time steps within each stress period will be determined based on the selected modeling tools. Although appropriate time-step size is important for both groundwater flow and solute transport modeling, it is more relevant to solute transport modeling. It is not unusual for time steps in a solute transport model to be on the order of days or hours. An advantage of MODFLOW- SURFACT is that time-step size is determined dynamically as the model is running, based on the convergence behavior within the numerical solver. For other solute transport simulators, transport time-steps would be determined so as not exceed a reasonable Courant number, which relates time-step size to solute velocity, which will ensure model stability and prevent excessive numerical dispersion. Transport parameters reflect the physical properties of the porous media that influence the movement of solutes. These parameters are the dispersivity, which is a scale-dependent parameter that describes the spreading of a solute as it is transported, and the effective porosity, which affects the average linear velocity of a solute particle and thus has a direct impact on estimates of cleanup timeframes. Initial values for these parameters will be estimated based on Site data, and will be adjusted within a reasonable range as necessary during model calibration. 4.3 Solute Transport Model Similar to the groundwater flow model, the solute transport model will be developed in steps that successively incorporate additional complexity. The preliminary solute transport model will be based on the final (calibrated) groundwater flow model. Transport parameters will be generalized and homogenous for the preliminary model. The principal objectives of the preliminary solute transport model are to: (1) test the model to identify potential numerical difficulties related to model construction and (2) evaluate the spatial resolution of the model grid in terms of grid Peclet number. The principal objective of the initial solute transport model is to perform a qualitative calibration. Once a reasonable visual match between observed and simulated concentrations is achieved, the simulation results will be evaluated statistically to identify needed improvement and refinements. The third and final step is to incorporate the modifications indicated by the initial solute transport model. Refinements to the final solute transport model will be guided by parameter estimation techniques that use optimization-based inverse modeling techniques. Page 4-2 Work Plan for Groundwater Modeling,

20 4.4 Solute Transport Model Calibration Procedure For solute transport, the model performance objective is to minimize the residual between observed and simulated concentrations at selected observations points. The modeling process will include iterative modification within appropriate ranges of transport parameters and boundary conditions to achieve this performance objective. Modeling and calibration issues that may need to be revisited include the number of simulated transport processes, characterization of the Primary Lands source, scale issues, numerical accuracy and execution time, representation and weighting of transport observations used for model calibration, and the need for additional model inputs. The calibration process may also indicate that additional modifications to the groundwater flow model may be advantageous. Model performance will be rigorously evaluated using quantitative statistical techniques. The accuracy of simulation results will be improved by analyzing the statistical results and identifying aquifer parameters that need to be modified or additional processes that need to be considered. The calibration process for the solute transport model is similar to the process for the groundwater flow model (Section 3.5) and can be generalized and summarized as: Qualitative Calibration: Groundwater concentration maps will be compared visually by overlying measured and simulated concentrations for the preliminary and initial solute transport models for selected time periods. Statistical Evaluation: Weighted and unweighted residuals between measured and simulated groundwater concentrations will be evaluated using parameters such as the residual mean, the absolute residual mean, the standard deviation of the residual mean, and others. These statistical comparisons will be made over both the entire and localized portions of the model to show the spatial variation in the calibration. Time Series Analysis: Concentration histories from a selected set of high-quality monitoring wells will be compared with simulated values to verify that the concentrations are properly simulated by the model. Model Accuracy: Mass-budget data will be compared over each stress period to verify that the model results are reliable. This also provides a quality assurance check that the model input data were entered correctly. Parameter Estimation: Linear and nonlinear regression will be used for calibration of the final solute transport model. An objective function will be defined using simulation residuals. The value of this objective function will be minimized with the calibration tools described in Section 3.5. Model Refinements: Calibration results will be used to guide model refinements that may be required, as indicated in the project schedule (Table 2). Work Plan for Groundwater Modeling, Page 4-3

21 Section 5: Remedial Alternatives The groundwater flow and solute transport model will be used to simulate three remedial alternatives: 1. Monitored Natural Attenuation (MNA) 2. Extraction and treatment with reinjection to the Upper Aquifer 3. Extraction and treatment with offsite disposal/discharge The remedial alternatives will be simulated as future what-if scenarios, based on hydrological conditions developed from the recent past (e.g., climate, pumping, irrigation, facility operations etc.). The results of this analysis will provide a basis for evaluating the timeframes required to meet the RAOs. Cleanup timeframes associated with each scenario will be compared and contrasted to estimate the relative differences in time required to meet the RAOs. The calibrated simulations described in Sections 3 and 4 of this Work Plan end in December 2010, when the discharge of partially-treated process wastewater to the Primary Lands ceased. The final, calibrated groundwater flow and solute transport model represent initial conditions for simulation of the remedial alternatives. 5.1 Monitored Natural Attenuation At the Site, MNA essentially started after the discharge of partially-treated process wastewater to the Primary Lands ceased in December Year one of the MNA simulation is therefore Groundwater elevation and concentration data will be used to evaluate the year-one simulation results. The simulation will be run forward to evaluate the effects of MNA through timeframes of sufficient duration to simulate achieving the RAO. 5.2 Extraction and Treatment with Reinjection to the Upper Aquifer Well locations, pumping, and reinjection rates for this remedial alternative will be based on the capture zone simulations reported in the FS. In this scenario, it is assumed that extracted water is returned to the Upper Aquifer, and it remains in the model domain. The simulation will be run forward to evaluate the effects of groundwater extraction and reinjection through timeframes of sufficient duration to simulate achieving the RAO. 5.3 Extraction, Treatment and Offsite Discharge Well locations and pumping rates for this scenario will be based on the capture zone simulations reported in the FS. In this scenario, extracted water is not returned to the subsurface, and is removed permanently from the model domain. The simulation will be run forward to evaluate the effects of groundwater extraction through timeframes of sufficient duration to simulate achieving the RAO. Work Plan for Groundwater Modeling, Page 5-1

22 5.4 Sensitivity and Uncertainty Analysis The prediction sensitivity and uncertainty analysis will be conducted within the Groundwater Vistas modeling environment with PEST and/or UCODE using fit-independent statistics (e.g., composite scaled sensitivities and parameter correlation coefficients). Estimated parameters will be evaluated with parameter variance-covariance matrices. Parameter and prediction uncertainty will be quantified using inferential statistics such as linear and nonlinear individual confidence intervals and statistics similar to those described in Sections 3 and 4, plus prediction scaled sensitivity, and a parameter-prediction statistic (Tiedeman et al., 2003). The key parameters for the sensitivity and uncertainty analysis are hydraulic conductivity, dispersivity, effective porosity, and inflow boundary conditions. The parameter and boundary condition values will be varied within appropriate ranges based on the HCM. The analysis will also include modifying the boundary conditions to simulate the potential impact of higher or lower pumping and regional changes in groundwater elevations. The suite of statistics employed for the sensitivity and uncertainty analysis will facilitate an understanding of the range of expected outcomes regarding the estimated timeframes required to achieve RAOs for the remedial alternatives. Page 5-2 Work Plan for Groundwater Modeling,

23 Section 6: Report Documentation of the modeling for the three remedial alternatives will be presented in a report to be submitted to the RWQCB. The report will present the estimated timeframes required to achieve RAOs for each of the three alternatives and will include a discussion of data sources and application, detailed descriptions of all tasks performed, methodologies, results, and conclusions. Appropriate tables, maps, charts, and figures will be presented that illustrate the analyses and clearly support the conclusions. The methods of analyses, assumptions, hydrogeologic and mathematical methods will be cited and explained in the context of this investigation. The logic and methods used to evaluate the data will be clearly related to the findings and conclusions. To support the findings and interpretation methods used for analyses, calculations or interpretations will be cited with appropriate scientific references. A list of references will be included in the report. The groundwater modeling results and conclusions will be used to complete the evaluation of remedial alternatives presented in the FS. A revised FS or FS Addendum will be subsequently submitted to the RWQCB, with the modeling report included as an attachment. Work Plan for Groundwater Modeling, Page 6-1

24 References Bear, J. and A. Verruijt Modeling Groundwater Flow and Pollution. D. Reidel Publishing Company, Boston, 414 p. Burow, K.R., Shelton, J.L., Hevesi, J.A., and Weissmann, G.S., Hydrogeologic Characterization of the Modesto Area, San Joaquin Valley, California. U.S. Geological Survey Scientific Investigation Report , 54 p. Clement, T.P RT3D - A Modular Computer Code for Simulating Reactive Multi-Species Transport in 3-Dimensional Groundwater Aquifers. Pacific Northwest National Laboratory, Richland, WA, USA. PNNL Doherty, J PEST Version Watermark Computing, Corinda, Australia. Environmental Solutions International (ESI) Guide to Using Groundwater Vistas Version 5. Environmental Simulations, Inc., Reinholds, Pennsylvania, 366 p. Harbaugh, A.W., E.R. Banta, M.C. Hill and M.G. McDonald MODFLOW 2000, The U.S. Geological Survey Modular Ground-Water Model User Guide to Modularization Concepts and the Ground-Water Flow Process. U.S. Geological Survey Open-File Report Hill, M.C. and Tiedeman, C.R., Effective Groundwater Model Calibration, with Analysis of Data, Sensitivities, and Uncertainty. John Wiley and Sons, Inc., Hoboken, NJ, 455 p. Hydrogeologic, MODFLOW-SURFACT Software (Version 3.0) Documentation, Hydrogeologic, Inc., Herndon, VA. JJ&A, 2010a. Remedial Investigation Report. Prepared for Hilmar Cheese Company by Jacobson James and Associates, Inc., dated 18 June JJ&A, 2010b. Draft Upper Aquifer Feasibility Study and Proposed Remedial Action Approach. Prepared for Hilmar Cheese Company by Jacobson James and Associates, Inc., dated 19 December JJ&A, Former Primary Land Use and Upper Aquifer Groundwater Quality Trend Update: 1Q2008-2Q2011. Prepared for Hilmar Cheese Company by Jacobson James and Associates, Inc. Kennedy/Jenks, Expert Report and Prepared Direct Testimony Regarding: Nature, Extent, Gravity, Toxicity, and Susceptibility to Cleanup, in the Matter of Hilmar Cheese Company, Inc. and Hilmar Whey Protein, Inc. Prepared by Kennedy/Jenks Consultants in Response to California Regional Water Quality Control Board Central Valley Region ACL Complaint No. R Work Plan for Groundwater Modeling, References - i

25 Parkhurst, D.L. and Appelo, C.A.J., User s Guide to PHREEQC (Version 2): A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations. United States Geological Survey Water-Resources Investigations Report Phillips, S.P., Burow, K.R., Rewis, D.L., Shelton, J.L., and Jurgens, B., 2007a. Hydrogeologic Setting and Ground Water Flow Simulations of the San Joaquin Valley Regional Study Area, California. In Hydrogeologic Settings and Ground-Water Flow Simulations for Regional Studies of the Transport of Anthropogenic and Natural Contaminants to Public- Supply Wells Studies Begun in 2001, edited by S.S. Paschke. U.S. Geological Survey Professional Paper 1737A, 31 p. Phillips, S.P., Green, C.T. Burow, K.R., Shelton, J.L., and Rewis, D.L., 2007b. Simulation of Multiscale Ground Water Flow in Part of the Northeastern San Joaquin Valley, California U.S. Geological Survey Scientific Investigation Report , 43 p. Poeter, E.P., Hill, M.C., Banta, E.R., Mehl, S, and Christensen, S UCODE_2005 and Six Other Computer Codes for Universal Sensitivity Analysis, Calibration, and Uncertainty Evaluation. U.S. Geological Survey Techniques and Methods Report TM 6-A11. RWQCB, Draft Guidelines for Submittal of Information Developed from Models to the Central Valley Regional Board, dated 16 August RWQCB, Remedial Action Feasibility and Proposed Remedial Actions for the Upper Aquifer, Hilmar Cheese Company, Hilmar, Merced County, letter from Mr. Russell Walls (RWQCB) to Mr. Burt Fleischer (Hilmar Cheese Company), dated 4 October Tiedeman, C.R., Hill, M.C., D'Agnese, F.A., and Faunt C.C., Methods for using groundwater model predictions to guide hydrogeologic data collection, with application to the Death Valley regional groundwater flow system. Water Resources Research 39(1), Zheng, C. and Wang, P MT3DMS: A Modular Three-Dimensional Multispecies Transport Model for Simulation of Advection, Dispersion, and Chemical Reactions of Contaminants in Groundwater Systems; Documentation and User s Guide. U.S. Army Corps of Engineers. December References - ii Work Plan for Groundwater Modeling,

26 Tables

27 Table 1: Responses to CVRWQCB (2004) Guidelines CVRWQCB Guideline No. (a) Description Work Plan Section 1 What is the modeling objective? What is being modeled, why that information is needed, and how will the results be used? What model has been chosen? (Name, author, version, etc.) Why is the chosen model applicable to this situation? Is the model open or proprietary? What level of peer review has been done on the model? Provide examples of previous use of the model. If the model is proprietary, explain why open models or EPA-approved models are not appropriate for the application Describe the underlying fundamentals and assumptions of the model and why they are appropriate. What are the critical conditions that you are trying to model? What is the model s performance objective or target? How will model performance be demonstrated, and what efforts will be taken to improve the model s accuracy? 2.5; 3.1; ; 3.1; ; 3.1; ; ; 4.1 1, 1.1, ; ; 4.4; Modeling reports should provide: 6 (a) Draft Guidelines for Submittal of Information Developed from Models to the Central Valley Regional Board, dated 16 August Work Plan for Groundwater Modeling, G:\IS-Group\Admin\Job\07\ _Hilmar\09-Reports\GW Modeling Work Plan\Tables\Tables Page 1 of 1

28 Table 2: Project Schedule Project Tasks Task 1 Task 2 Task 3 Task 4 Weeks Work Flow Subtasks Hydrogeological Conceptual Model (HCM) * Data Compilation and Assessment Hydrogeological Budget Hydrostratigraphic Units Aquifer Parameters Geochemical Analysis Data Quality Assessment Groundwater Flow Model Development * Model Domain Groundwater Flow Model Setup Groundwater Flow Model Groundwater Flow Model Calibration Solute Transport Model Development * Solute Transport Model Setup Solute Transport Model Solute Transport Model Calibration Simulate Remedial Alternatives * Monitored Natural Attenuation Groundwater Extraction Groundwater Extraction with Reinjection Wells Sensitivity and Uncertainty Analysis Report * Draft Report Internal Review Final Report * Project milestone; green shading indicates potential additional work required after initial subtask completion as determined during the model calibration process. Week 1 starts after RWQCB approval of Work Plan. Work Plan for Groundwater Modeling, G:\IS-Group\Admin\Job\07\ _Hilmar\09-Reports\GW Modeling Work Plan\Tables\Tables Page 1 of 1

29 Figures

30 Path: Z:\Projects\Hilmar_Cheese\Events\ _WorkPlan\Fig_1-1_SiteMap.mxd Site Location ^ Site Location Source: ESRI ³ Miles Kennedy/Jenks Consultants Hilmar Cheese Company Merced County, California Site Location Map K/J *11 January 2012 Figure 1

31 Path: Z:\Projects\Hilmar_Cheese\Events\ _WorkPlan\Fig_2_SitePlan.mxd Kennedy/Jenks Consultants Hilmar Cheese Company Merced County, California Site Plan (JJ&A, 2010b) K/J *11 January 2012 Figure 2

32 Path: Z:\Projects\Hilmar_Cheese\Events\ _WorkPlan\Fig_3_Timeline.mxd Kennedy/Jenks Consultants Hilmar Cheese Company Merced County, California Wastewater and Primary Land Use Timeline (JJ&A, 2010b) K/J *11 January 2012 Figure 3

33 Path: Z:\Projects\Hilmar_Cheese\Events\ _WorkPlan\Fig_4_Timeline.mxd LEGEND Hilmar Cheese Company (HCC) Reuse Water and Turlock Irrigation District (TID) Freshwater Application Area TID Freshwater Application Area HCC Wastewater Application Area in December 2010 (discontinued December 2010). 115,872 gallons of treated wastewater was applied to Area C in December Application data and history includes parcels and Reuse Areas within the extent of the former Primary Lands. Source: Hilmar Cheese Company Quarterly Monitoring Report dated January 28, 2011, April 25, 2011, July 20, 2011 and October 24, Kennedy/Jenks Consultants Hilmar Cheese Company Merced County, California Former Primary Land Reuse Area Application Timeline (December 2010 September 2011) K/J *11 January 2012 Figure 4