(2)acardis_ground water model report 09-28-07

c:\projects\asarco-el paso\model\final report 9-19-07\asarco el paso modeling report 09-07.docPage:

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MEMO

To:

Thomas Klempel, PE – ASARCO, LLCLairy Johnson, PG- ASARCO, LLC

Copies:

Randy Huffsmith, PE, DEE – CDMKent Whiting – CDMMichael Smith - CDM

From:

Gastón Leone, PE - ARCADISRobert Mongrain, RG - ARCADIS

Date: ARCADIS Project No.:

September 28, 2007 AZ001022.0008

Subject:

Groundwater Model Report, ASARCO , LLC El Paso Smelter Site

Introduction and Objectives

This memorandum is intended to present the status of a groundwater model developed for the ASARCO, LLC (ASARCO) smelter site located in El Paso, Texas. This model was constructed following several years of remedial investigation and refinement of the conceptual site model(CSM). The primary objectives of this modeling effort are to:

• Accurately represent groundwater flow behavior;

• Evaluate groundwater and surface water interaction;

• Estimate potential arsenic loading to surface water;

• Identify potential data gaps;

• Evaluate relative performance of various active remedial strategies; and

• Aid in the specific design of remedial alternatives.

ARCADIS-US, Inc.

630 Plaza Drive

Suite 200

Highlands Ranch

Colorado 80129

Tel 720 344 3500

Fax 720 344 3535

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To date, ARCADIS has built and calibrated the groundwater flow model and completed the first four items listed above. This memorandum is intended to present a description of the numericalmodel, calibration results, and estimates of flow and arsenic loading to the Rio Grande River and American Canal surface water bodies. Going forward, the model will be used to evaluate and aid in the design of remedial alternatives.

Description of Numerical Model

The model domain is approximately 4,700 ft long by 4,000 ft wide, encompasses the entire smelter site area and extends northeast along the Parker Brothers arroyo at the base of the Franklin Mountains (Figure 1). The domain is bounded to the southwest by the Rio Grande River where groundwater that flows through the site discharges. Grid spacing ranges between 50 and 100 ft, the closer spacing was set between the Rio Grande and American Canal to allow for better representation of groundwater/surface water interaction. Two layers are used to represent the upper alluvium unit and the lower bedrock. Layers thicknesses were estimated based on available geologic data. The alluvium hydrogeologic unit encompasses the Rio GrandeRiver alluvial sediments and the Picacho and Santa Fe Bolson deposits. Bedrock groundwater flow is considered to be negligible but it was incorporated to account for areas where bedrock highs are observed and alluvium saturated thickness is significantly reduced (less than 10 ft).

Groundwater generally flows from the upland mountain front, where the majority of recharge occurs, towards the Rio Grande where groundwater discharges. To simulate this groundwater flow pattern, constant head boundary conditions are applied along the Rio Grande River. The American Canal is simulated by using river nodes to best represent the interaction with groundwater.

Recharge from precipitation was estimated in historical reports and the CSM to be approximately 91 acre feet per year (acf/year) based on groundwater discharge to the Rio Grande River during baseflow conditions. This equates to a general recharge of 1.6 inches per year (in/year) for the entire model domain which is equivalent to a 16% recharge value considering that the average precipitation at the site is approximately 10 in/year. Along the upper reaches of the Parker Brothers arroyo and along the south eastern boundary of the model, recharge is increased to 11 in/year to represent the effects of mountain front recharge that takes place in the upland areas (Figure 1).

Model Calibration

The model was calibrated for steady-state conditions using the February 2006 water level measurements as calibration targets. Transient calibration was not deemed necessary at this time due to the fact that seasonal water level fluctuations are small (generally less than 3 ft) and

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the overall groundwater flow pattern and gradient do not significantly change between the wet and dry seasons. Transient analysis may be conducted in the future if seasonal loading patternsare required.

The spatial distribution of hydraulic conductivity values was modified until a good fit between model-simulated and observed water levels was achieved. To assist in this process the code MODAC was used. MODAC is an inverse parameter estimator that uses hydraulic gradients over the entire model domain as opposed to the traditional inverse modeling techniques that use point measurements only. MODAC results were used to assist in identifying zoning patterns for hydraulic conductivity values. The calibrated hydraulic conductivity distribution is presented on Figure 2. Hydraulic conductivity values are higher along the Rio Grande floodplain (30 feet per day [ft/day]) and the buried arroyos (15 ft/day). Hydraulic conductivity values range from 0.1 ft/day to 5 ft/day across the remainder of the site. The calibrated values are within the range of observed values measured during slug and aquifer tests as can be seen on Figure 2.

In general the model-simulated water levels are in good agreement with observed water levels as presented on Figure 3. The root mean square error is 2.55 ft, the standard deviation of residuals of 2.44 ft, and the observed range of heads is 81.2 ft across the site. These parameters and the overall agreement between model and observed groundwater flow patterns indicate a reasonable calibration. The majority of the groundwater flow occurs along the buried arroyos and discharges into the Rio Grande floodplain alluvium. Reduced flow is observed along the south and east portions of the site where saturated thickness decreases due to higher bedrock elevations, reduced hydraulic conductivity, and lower mountain front recharge.

Estimates of Arsenic Loading to Surface Water – Low Flow Conditions

One of the main objectives of this groundwater flow model is to quantify groundwater discharges to the Rio Grande River and American Canal to assist in the evaluation of arsenic loading to these surface water bodies.

Increased arsenic concentrations as the Rio Grande River and American Canal flow through the site have been observed in recent years. Grab samples are collected along both surface water bodies on a semiannual basis and analyzed for several constituents including arsenic. Figure 4presents the locations of the surface water monitoring stations currently sampled for arsenic as well as the distribution of arsenic in groundwater. Figures 5 and 6 present a summary of the average measured arsenic loads between 2004 and 2006 for low flow conditions for the Rio Grande River and American Canal.

Along the Rio Grande River the arsenic load increases between stations SEP-1 and SEP-10 by about 0.23 kilograms per day (kg/day) and then again between SEP-11 and SEP-2 by about

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0.08 kg/day. Smaller load increases are observed downstream. Along the American Canal an increase in the load of 0.17 kg/day is observed between stations SEP-1 and SEP-7 and by 0.26 kg/day between SEP-7 and SEP-3. This increase in arsenic load is attributable to arsenic-impacted groundwater discharging to the Rio Grande River.

Model estimates of arsenic loading along the Rio Grande River were calculated using the model calculated fluxes and the average arsenic concentrations observed where the groundwater discharge occurs. Between stations SEP-1 and SEP-10 the model estimated groundwater discharge flux is approximately 28 gallons per minute (gpm). As can be seen on Figure 4 the average arsenic concentration on the downstream side of the American Dam is approximately 2 milligrams per Liter (mg/L), which results in an arsenic load increase of 0.3 kg/day (28 gpm = 152 cubic meters [m3] per day [m3/day] * 2 kg/1000 m3 = 0.3 kg/day). The model-calculated load of 0.3 kg/day is similar to the measured load increase of 0.23 kg/day for this segment of river. The agreement between these estimates further verifies model results, increasing our confidence in future model-assessment of remedial alternatives.

The majority of the groundwater flux is due to higher heads behind the American Dam which results in a localized discharge of groundwater to the Rio Grande River. The ground water level behind the American Dam was assumed to be four ft higher than the water level in the Rio Grande River immediately downstream of the dam. This head difference is not known at this time due to lack of additional monitoring wells near the upstream side of the dam. The 4-ft head difference is considered to be conservative considering that the top of the dam is more than 10 ft above the Rio Grande River water level. This data gap is identified in the following section and should be addressed to confirm that the actual arsenic loading in this area is occurring along a short stretch of the river. This information will have significant importance when designing a remedial system.

Loading between SEP-11 and SEP-2 takes place along a 600-ft stretch of river where elevated arsenic concentrations are observed in groundwater. The model-calculated flux is approximately 10 gpm. Considering an average arsenic concentration of 2 mg/L results in an arsenic load of 0.1 K/day (10 gpm = 54 m3/day * 2 kg/1000 m3 = 0.1 kg/day), which is similar to the arsenic load of 0.08 kg/day measured for this segment.

Model estimates of arsenic loading along the American Canal could not be conducted because the elevation of the bottom of the canal is above the water table elevation, not allowing the model to discharge groundwater into the canal. Discharge into the canal has been observed in the past, particularly at the outlet of the first closed conduit where Diesel Spill No. 1 took place in the late 1980’s. Based on this information it is likely that the bottom elevation used in this model, which was reported in the canal re-habilitation report (MWH, 2002), is higher than the true

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elevation. Therefore, this is considered an important data gap and ARCADIS recommends that the bottom of the canal be re-surveyed.

Arsenic loading to surface water was not evaluated for high flow periods (September through February) because arsenic concentrations during high flows at the American Canal and Rio Grande are typically very low and close to the detection limit of 5 micrograms per Liter (µg/L),which does not allow for a clear identification of load quantities and discharge areas. During high flow periods it is probable that significant leakage along the American Canal may occur due to cracking of the concrete liner as identified in the rehabilitation report (MWH-Montgomery Watson Harza, Inc., 2002). This leakage can potentially increase the groundwater discharge into the Rio Grande River, also increasing arsenic loads during high flow periods.

Conclusions and Recommendations

Model calibration results indicate that the model provides an accurate representation of the groundwater flow patterns at the site and potential arsenic discharge areas to the Rio GrandeRiver. There is good agreement between model calculated and measured arsenic loads. The greatest arsenic load occurs right beneath the American Dam as ponded water flows around the dam and discharges to the Rio Grande River picking up arsenic along the way. Model-estimated fluxes need to be verified by installation additional monitoring wells in the vicinity of the AmericanDam to measure groundwater levels.

The groundwater interaction with the American Canal could not be evaluated due to probable inaccuracies in the canal bottom elevation.

The following list of recommendations is aimed at addressing data gaps:

• Topographic Survey. Conduct a survey of the American Canal bottom, American Dam, Rio Grande riverbed and MW wells. This information will be used to assess groundwater interaction with the American Canal and provide additional data for model calibration targets. This recommendation will require access with the International Water & Boundary Commission (IWBC).

• American Dam Groundwater Elevation. Install additional monitoring wells to measure groundwater levels in the vicinity of the dam. These water levels will be used to evaluate head differences induced by the ponded water behind the dam which could potentially be increasing groundwater discharge into the Rio Grande River downstream of the dam. Additionally, install a stilling well with a pressure transducer to measure the surface water elevation of the ponded water at the dam. This recommendation will require access with the IWBC. However, four new monitoring wells (PZ-1 through PZ-4) were installed

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earlier this year near the American Dam, in areas where access was available. Water level data collected from these monitoring wells during the upcoming low-flow period will be used to update this groundwater model and may provide further understanding of flow patterns in the vicinity of the dam.

• Tracer Test. This test would consist of releasing a tracer into the upstream side of the dam and measuring breakthrough at the downstream and nearby monitoring wells. The purpose of this is to identify flow paths, travel time, and discharge areas of surface water infiltrating on the upstream side of the dam and reporting as groundwater downstream. This recommendation will require access with the IWBC.

• Groundwater/Surface Water Interaction. Install pressure transducers with continuous recorders (such as miniTrolls) to measure groundwater level fluctuations and hydraulic gradients between the American Canal and Rio Grande River. Up to four transducers should be installed in wells located along the Rio Grande River and up to four installed in wells along the American Canal. Additionally, install four stilling wells with pressure transducers along the Rio Grande River to measure the surface water elevation in the vicinity of the instrumented monitoring wells. These data will be used to obtain a better understanding of the surface and groundwater interaction as well as the potential for leakage from the American Canal to discharge into the Rio Grande River. Access to the Rio Grande River is currently not available so no stilling wells will be installed. ARCADIS is currently in the process of installing up to 12 pressure transducers and dataloggers distributed across the Parker Brothers arroyo and floodplain area to obtain a better understanding of seasonal groundwater level fluctuations.

Attachments:

Figure 1 – Site Map and Model Domain

Figure 2 - Calibrated and Measured Hydraulic Conductivity Values

Figure 3 - Model Simulated and Observed Water levels

Figure 4 - Arsenic Distribution in Groundwater

Figure 5 – Rio Grande Arsenic Load - Low Flow

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Figure 6 – American Canal Arsenic Load - Low Flow

References:

MWH-Montgomery Watson Harza, Inc., 2002. Conceptual Design Report, American Canal Lining Replacement Study. Prepared for International Boundary and Water Commission, United States and Mexico, United States Section, June.

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INTERSTATE 10

RIO GRANDE

RIO G

RANDE

SEP-9

SEP-10

SEP-11

SEP-7SEP-2

SEP-12

SEP-13

SEP-3

SEP-14

SEP-1AMERICAN CANAL

AMERICAN CANAL

AMER

ICAN

CAN

AL

Site Map and Model Domain

Legend

2500 500 Feet250

Treated as a Constant Head Boundary

Treated as a River Boundary

Mtn. Front Recharge

Monitoring Wells

Monitoring Wells

Surface Water Stations

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EM-6 0.28

EP-105 0.18

EP-108 0.79

EP-109 0.69

EP-112 44.29

EP-114 3.83

EP-119 6.28

EP-12 0.22

EP-120 1.30

EP-121 3.63

EP-122 26.33

EP-123 18.31

EP-124 3.18

EP-125 1.95

EP-128 66.40

EP-131 17.82EP-131 4.58

EP-132 1.57

EP-133 29.58

EP-134 19.73

EP-135 0.49

EP-14 1.51

EP-29 6.69

EP-49 3.22EP-51 4.70

EP-53 10.90

EP-54 0.26

EP-56 5.30

EP-59 3.17

EP-61 1.36

EP-67 3.20

EP-68 2.18

EP-73 3.92

EP-76 4.70EP-78 17.80

EP-79 2.77

EP-80 2.95

EP-81 1.39

EP-82 6.63

EP-83 3.19

EP-90 0.72

EP-93 1.32

EP-94 3.10

EP-95 18.08

EP-97 5.31

EP-98 0.73

MW-132 17.91

INTERSTATE 10

RIO GRANDE

RIO G

RANDE

AMERICAN CANAL

AMERICAN CANAL

AMER

ICAN

CAN

AL

0.65

-3.46

Calibrated and Measured Hydraulic Conductivity Values

Legend

2500 500 Feet250

Historic ArroyoModel Calibrated K=30 f/d

Model Calibrated K=15 f/d

Model Calibrated K=5 f/d

Measured K [f/d]EP-134 19.73

Model Calibrated K=0.1 f/d

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3746.28

3780

3770

3760

3750

3740

3730

3720

3715

INTERSTATE 10

RIO GRANDE

RIO G

RANDE

SEP-9

SEP-10

SEP-11

SEP-7SEP-2

SEP-12

SEP-13

SEP-3

SEP-143780

3790

3770

3760

3750

3740

3730

3720

37153710

3710

3790

3800

3810

SEP-1AMERICAN CANAL

AMERICAN CANAL

AMER

ICAN

CAN

AL

0.58

1.43

4.08

10.30

-0.38

-2.40

-4.68

-3.93

-2.59

-2.50

3.92

4.35

3.40

-0.38

0.54

0.20

0.65

0.01

-1.53

0.34

-0.09

0.90

4.02

-0.43

3.29

8.77

-2.60

-2.15

0.36

0.46

-2.67

-0.71

-0.84

0.56

-0.47

0.36

0.44

0.44

-0.84

-0.07

-5.14

0.19

3.39 3.230.62

2.63

0.34

-3.71

-0.20

0.62

1.29 -0.07

0.20

2.81

-2.81

-0.26

-0.84

1.30

0.630.00

0.11

0.71

-0.04

0.41

0.40

0.05

0.14

0.40

4.21

-0.28

0.00

4.05

-0.06

0.61

-0.89

-2.31

-0.06

2.93

-1.30

1.39

-0.74

-1.74

1.96

0.56

-2.81

-3.46

2.58

-4.33

-1.09

-1.69

-0.88

2.68

February 2006Model Simulated and Measures Water Levels

Legend

2500 500 Feet250

Groundwater Elevation Contour3744.13 Measured Groundwater Elevation

Monitoring Wells

Monitoring Wells


Model Simulated

Model Higher than MeasuredModel Lower than Measured

-0.47

0.38

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INTERSTATE 10

RIO GRANDE

RIO G

RANDE

0.1

1.0

10

1.0

0.1

0.1

1.0

1.0

0.1

10100

1.0

1.01.0

0.1

0.1

0.1

1.0

SEP-9

SEP-10

SEP-11

SEP-7SEP-2

SEP-12

SEP-13

SEP-3

SEP-14

0.1

SEP-1AMERICAN CANAL

AMERICAN CANAL

AMER

ICAN

CAN

AL

0.1

February 2006Arsenic Distribution in Groundawter

Legend

2500 500 Feet250

Arsenic Concentration [mg/L]Monitoring Wells

Monitoring Wells


Former Impoundment Location

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(2)acardis_ground water model report 09-28-07

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