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SFUND RECORDS CTR

SCOPE OF WORK 88134296

GROUNDWATER MODELING AR0055SAN FERNANDO VALLEY BASIN rt

SFUND RECORDS CTR. 2166-04875. .-

INTRODUCTION

The modeling program described in this scope of work is designed to providethe numerical tools needed to evaluate the occurrence, movement, andsources of groundwater contamination in the San Fernando Basin. The sheersize of the contaminated area, complexity of the aquifer system, and heavylocal reliance on the basin as a potable supply source, dictate the need toaugment traditional field methods (i.e., exploratory drilling, wellconstruction, sampling, etc.) with state-of-the-art computerized analysis.CDM's DYNFLOW/DYNTRACK modeling system provides this capability and hasbeen used successfully on numerous contamination projects throughout the ,United States. '*

-N The simulation modeling would be used to assist all phases of the RemedialInvestigation. Specific modeling objectives include:

r• o providing guidance in defining the scope and execution of initialr data collection activities;i

o identification of major deficiencies in available data, thereby

providing guidance for all phases of field activities;

f" o simulation of the flow of groundwater in the basin, includingrepresentation of major geohydraulic features;

o identification of potential sources of contamination;

o simulation of contaminant movement (both forwards and backwards irvtime), and

o prediction of future groundwater quality in supply wells.

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The simulation models developed during the RI could also be used in thesubsequent Feasability Study to assist in the formulation and evaluation ofalternative remedial measures.

To meet the specific objectives outlined above and the more comprehensivegoals of the overall RI, it will be necessary for COM and LADWP to closelycoordinate their respective activities and analyses. As proposed here,this coordination would be accomplished through regularly scheduledtechnical meetings and an exchange of data generated during their

[" respective work tasks.

Figure 1 summarizes the proposed simulation modeling process and identifiesthe major links or channels of data exchange between the three main parts

P of the Remedial Investigation: Data Collection and Analyses, Source! ; Investigation, and Simulation Modeling Program.

MODELING STRATEGY 1

Based on our analysis of data available during preparation of the work planentitled "Work Plan, Remedial Investigation/Feasibility Study for the SanFernando Groundwater Basin, California," August 1985, we propose to useCDM's DYNFLOW/DYNTRACK modeling system to simulate the hydraulic and

•~ contaminant transport characteristics of the San Fernando GroundwaterBasin.

F—i Due to the spatial variations in key parameters (ex. hydraulic

permeability) that are found in nature, it is necessary to use average orP typical parameter values over a wide area in order to model groundwater

problems in a timely and cost-effective manner. The size of the area,p however, over which variations are represented by a single value must be

consistent with the accuracy required to satisfy the study objectives. Inp zones where contamination exists, it will be necessary to reduce the size• of these individual areas to better reflect actual conditions. At greater'

r. distances from problem zones, the importance of localized variations in[ geologic, hydraulic or contaminant characteristics is reduced andf~^ approximations over a larger area are acceptable.

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DATA COLLECTIONAND ANALYSES

SIMULATIONMODELING

SOURCEINVESTIGATION

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COLLECT UNO RNRLVZE BASIC DATA, REPORTSMAPS, CROSS

SECTIONS, ETCEXISTING RRSIC DRTR

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IDENTIFY DATADEFICIENCIES CM.LECTINOflNM.VZE

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•EFINE SCiPE OFPMKE I FIEll STHDIES

IDENTIFY POTENTIALREGIONAL SOURCtSOF CONTAMINATION

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CONTAMINANTDISTRIBUTIONS

IDENTIFY POTENTIALLOCAL SOURCES

OF CONTAMINANTCONMCTPIMSE

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PREPARE REMEDIAL INVESTIGATION REPORT

FIGURE 1

T •r The size of the San Fernando study area and the number of water supply/ "N wells to be considered may require development of a "large" model. If

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necessary, COM will minimize computer and labor costs by conducting modelsimulations at two levels of detail. First, a regional model would bedeveloped which covers the entire study area, I.e. the area bounded by theSan Diego Freeway (1-405) on the west, the Santa Monica Mountains on thesouth and the Verdugo Mountains on the north and east. Within the regionalmodel, aquifer characteristics would be averaged over relatively largeareas to simulate regional piezometric surfaces and groundwater gradients.

Within the area simulated by the regional flow model, we anticipate that upto four local models may be constructed 1n areas where groundwater pumpingis extensive and contaminant levels are known or suspected to be high. Theareas considered to be potential candidates for construction of local modelare shown on Figure 2. The actual number of local models constructed (1f ':at all) and the detail undertaken would be determined under Tasks 2 and 3.,-v.The goal would be to construct the local models with enough detail to tracic ?major individual plumes of contaminants. Once constructed, two types of

( "^ simulations would be run using DYNTRACK (the contaminant transport*

simulation model). Forecasting techniques would be used to developsimulations which would attempt to duplicate known contaminant patternsgiven suspected locations and strengths of contaminant sources. Hind-casting techniques would be used in an attempt to track known contaminantplumes back to their respective sources.

The local models would be coupled to the regional model defined by nodesalong common boundaries. By doing this, simulation runs using the regionalmodel would be used to establish the boundary conditions for subsequentruns to be made with the local models.

TASK DESCRIPTIONS

The work tasks described in this section are generic to each model to beconstructed, whether it be on a regional or local scale. As indicated byFigure 1, construction of a simulation model 1s an iterative process.During this process, the conceptual model (I.e., a geologist's description

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SAN FERNANDO VALLEYOROUNOWATER BASIN BOUNDARY

PROJECT REGIONAL BOUNDARY

SITE AREA BOUNDARY

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SAN FERNANDO GROUNDWATER BASIN

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of the groundwater system) used in configuring the original computer modelwill be re-evaluated, revised and verified as the computer model iscalibrated.

TASK 1 - EDIT MODEL INPUT DATA

During this task, COM will review the inventory of data compiled by LADWPin order to flag the data necessary for development of the simulationmodel(s). These data will Include:

lithologic logsr - geophysical logs (electric logs, gamma logs, etc.)' well design datar- - well specific capacity and aquifer test data'• ' - groundwater level and water quality data_ - surface flows and water quality1 well production data

recharge datarainfall dataprevious geologic hydrologic and modeling reports and maps fromState, Federal and Local agenciesliterature search on potentially applicable dispersivity values

p for San Fernando Basin Aquifers

^ The objective will be to identify common periods of record for use in theI. calibration effort, to eliminate data not considered reliable, and to focus

the most intensive data-gathering effort on the most valuable items.r~r '

Meetings with the technical staff of LADWP will be held at reguarlyP scheduled intervals to facilitate the exchange of data and focus the data

collection effort.

r• Deliverables: Inventory of existing data to be used in the modelP confiugration process.

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r TASK 2 - DEVELOP CONCEPTUAL MODEL

[• The objective of this task will be to develop a thorough understanding of' the local and regional geology, hydrogeology, subsurface hydrology, water_ quality and existing recharge and extraction operations. DetailedI . hydrogeologlc studies are requisite to evaluate the contamination problems

and to develop viable and effective physical remedial measures and| management solutions. COM will utilize, to the maximum extent possible,

the basic data collected and analytical work completed by LADWP toj eliminate any duplication of effort. The data and products assumed to be

available from LADWP will include:

r1 o base maps showing well locations, surface topography and land use;

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o regional and local scale geologic cross-sections and fencediagrams identifying the major aquifers and aquitards in the study, v.area; '* -:

F1^ o a contour map of the base of the aquifer system in the study area;

p o maps showing the area! variation in aquifer and aquitard hydraulicconductivity, storativity /specific yield, effective porosity;

r' o water level contour maps;

r-j o contaminant concentration maps;

f->o monthly summary of well production;

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P ° monthly summary of volume of water spread in recharge basins;

o monthly summary of surface flows which traverse the study area inunlined channels; and

o monthly summary of rainfall.

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r The maps are intended to summarize available data, consequently there may/ "~N be maps which do not cover the entire study area due to lack of reliablef data. The monthly summaries listed above will cover calibration and' simulation periods identified in Task 1.

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Meetings will be held with the technical staff of LADWP to review andfinalize the conceptual model proposed by COM.

Deliverables: Technical memorandum describing conceptual model.

TASK 3 - CONFIGURE COMPUTER MODELS

The objective of this task is to construct the regional and local flow (asnecessary) simulation models needed to analyze the contamination problem.

The work to be completed under this subtask is divided into three subtasks* Vas described below: • r

Subtask 3-1: Develop Finite Element Nodal Pattern

A finite element nodal pattern will be developed for both the regional andlocal models to represent the major aquifers of the study area. Thelocation of nodes will delineate the aquifer(s) both vertically andareally. The density of the nodes will be governed by the scale of the

P model (whether regional or local), the number and location of individualI extraction wells and potential source areas and the need to accurately

model the piezometric variations within a given well field. For at least{ the regional model, the density of nodes will be less near the model

boundaries.

r1 ' Subtask 3-2: Estimate Initial Values of Aquifer Properties forElements in the Finite Element Nodal Patterns

p Using data collected and compiled during Tasks 1 and 2, COM will preparei initial estimates of aquifer properties for each node including:

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P - hydraulic conductivity with respect to the x, y, and z directionst

(S - storativity/spedfie yieldf - effective porosity< - disperslvity coefficients 1n the x, y, and z directions

rI , Subtask 3-3: Select Calibration Criteria, Initial and Boundary

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. Conditions

COM will review the historical water level, water quality, artificialp recharge, well production and other hydrologlc data supplied by LADWP.

Based on this review, COM will select calibration criteria for the DYNFLOWj-. model(s). The calibration period will be as long as consideredi practicable, given the available historic data. The minimum calibration

period anticipated is five years.

Based on the data review, COM will develop the initial conditions to besimulated at the beginning of the calibration period. Boundary conditions-*will be assigned to those nodes located along the perimeter of the area?modeled. The boundary conditions will be either constant head, constantdischarge or impermeable (no-flow). Using available historic data, wellextraction and recharge data will be developed for the calibration periodand for subsequent simulation runs.

After reviewing the data compiled in Tasks 1 and 2, COM will select keywells with long histories of water level and water quality monitoring, foruse in the calibration process. Using historic data from these wells,hydrographs and piezometric levels will be selected to serve as calibrationtargets to be compared with values calculated during model calibration.

Deliverables: Technical Memorandum summarizing initial configuration ofsimulation models.

TASK 4 - INITIAL CALIBRATION OF FLOW MODELS

Using the exisiting data developed in the previous tasks, flow models willbe calibrated to reproduce, as closely as practical, observed piezometric

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r levels (I.e. heads). Model calibration will be an Iterative process (see

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/ -> Figure 1). In areas of the models where calculated heads are not con-r sidered sufficiently close to observed heads, adjustments will be made in

the assigned aquifer parameters, boundary conditions and source/sink terms _(i.e., recharge and extraction).

The adjustments made will be restricted to a range of values consideredresonable for each model parameter. Initially, these ranges will bedetermined from existing data. Areas of the models which cannot be

P calibrated using values within these ranges will be targeted for additional1 data collection during the RI field studies.

r' Deliverables: Calibrated flow simulation models (usingv existing data)._ Technical memorandum on calibration process.I Identification of data deficiencies.

r- i 'I TASK 5 - SIMULATE REGIONAL CONTAMINANT TRANSPORT *

r*-\ Using historical water quality and land use data, COM will develophypothetical scenarios which identify possible pollutant source locations

r" and estimate the historic level of contamination. DYNTRACK will be used tosimulate the movement of contaminants away from the hypothetical sources

p. and predict contaminant concentrations at specific wells. Comparison ofi simulated versus observed levels of contamination will provide an

indication of the validity of each hypothesis and help identify areas of

. , high or low probability as contaminant sources.

[ Simulation runs will be conducted also using DYNTRACK1s "back-tracking"feature. With this option, contaminant movement will be simulated

r backwards in time from known locations of contamination to possiblehistoric source areas. Together, the back-tracking and hypothesis-test

p results will assist in guiding subsequent field activities to evaluate1 potential source areas.

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Deliverables: Technical Memorandum which summarizes results of simulationruns and include the following items:

- preliminary location of potential sources of contamination^.

- identification of major deficiencies in available data

- recommendations for additional data collection activitiesdesigned to more fully characterize existing contaminentplumes and verify these suspected sources.

TASK 6 - RECALIBRATE FLOW MODELS USING PHASE I FIELD DATA;SIMULATE LOCAL CONTAMINANT TRANSPORT

Hydrologic data collected during the Phase I field program including well :.logs, aquifer test data, and, to some extent, zone specific water leveldata, will be used to recalibrate the DYNFLOW models and improve their -\ Asimulation accuracy. Following this, soil quality and water quality datagenerated from the field program will be used to both refine the hypo-thetical scenarios simulated during Task 5 and generate new scenarios foradditional simulation runs.

Simulation runs conducted during this task will focus on recalibration ofthe flow model, as necessary, and on refining the boundaries of potentialsources of contamination identified in Task 5.

COM will meet with LADWP at regularly scheduled intervals to discuss theprogress of the Phase I field studies, exchange data and develop simulationscenarios.

Deliverables: Technical memorandum describing recalibration of the flowmodels and results of additional simulation runs includinglocations of potential local sources, and recommendationsfor Phase II (Source Investigation) field studies.

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TASK 7 - CONDUCT FINAL SIMULATION RUNS BASED ON DATA GENERATEDDURING PHASE II (SOURCE INVESTIGATION) FIELD STUDIES

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Work completed under Task 7 will be similar to that completed under Task 6."Prior to conducting the final simulation runs, new hydrogeologic datacollected during Phase II (Source Investigation) field studies will beIncorporated into the flow models as appropriate. All pertinentinformation collected during the Source Investigation, Including potentialcontamination sources, soil gas data and groundwater quality data, will beused to improve the accuracy with which DYNTRACK Is able to simulate themovement of contaminant plumes and identify their respective sources.

COM will meet with LADWP to develop various scenarios for the finalsimulation runs.

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Deliverables: Technical memorandum describing final calibration of DYNFLOVjmodels and results of simulation runs.Recommendations as to how the DYNFLOW/DYNTRACK models couldbe used during subsequent feasibility studies to evaluateremedial alternatives or proposed operable units.

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