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Copyright 1999 by SME1

SME Annual Meeting

March 1-3, 1999, Denver, Colorado

Preprint 99-32DEMONSTRATING PASSIVE HYDRAULIC CONTAINMENT FOR AN OPEN PIT COPPER MINE

R. D. Bartlett

Dames & Moore

Phoenix, AZ

J. L. Moreno

Dames & Moore

Denver, CO

A. L. WilliamsonPhelps Dodge Morenci, Inc.

Morenci, AZ

Abstract

Large open pit mines in desert climates, such as the Phelps

Dodge Morenci copper mine in Arizona, can create

groundwater cones-of-depression of considerable size and

stability. The hydraulic containment provided by the

groundwater depression prevents migration of process

solutions into the surrounding regional aquifer. Copper leach

operations that fall within the groundwater depression may

not require costly lining if passive containment can be

demonstrated.

An approach is presented in this paper that was successfully

used to demonstrate passive containment. The approach

includes a combination of groundwater modeling and a water

balance analysis of leaching operations failing within or

immediately near the open pit.

INTRODUCTION

The Phelps Dodge Morenci Mining District lies in eastern

Arizona and is one of the largest open-pit copper mines in the

world. The mining District covers nearly 75 square miles and

includes several open pit-mining areas, leach operations, and

an extensive tailing deposition area. The State of Arizonarequires that all mines within the state obtain an Aquifer

Protection Permit (APP) for continued mine operation. Phelps

Dodge and Dames & Moore prepared an APP application for

the mining District and submitted it for review by the Arizona

Department of Environmental Quality (ADEQ) in 1996. One

of the requirements of an APP is the demonstration of Best

Available Demonstrated Control Technologies (BADCT) for

facilities at the mine that may discharge a process solution to

groundwater. Arizona's extremely dry climate results in

significant evaporation from open water bodies, including pi

lakes formed after mine closure. Arizona state law recognizes

that, under the right conditions, a post-closure mine pit lake

can become a groundwater sink thereby providing

containment of discharged solutions. The demonstration of

passive containment is the subject of this modeling study.

All leaching operations are considered discharging by

Arizona statue. New leach facilities in mine pits wouldrequire expensive lining technologies unless it is

demonstrated that a "passive containment capture zone"

(PCCZ) exists such that all discharged solutions remain

within the PCCZ even after closure of the mine. The

demonstration of the PCCZ for the Morenci District became a

critical component of the APP application.

The primary modeling objective was to predict the near and

long-term future PCCZ of the mine, given the ultimate mine

plan. Secondary objectives included:

Use regional USGS data, Phelps Dodge hydrogeologic

data, and mine water balance information to generate agroundwater conceptual model consistent with observed

groundwater data.

Predict lake flow balances and ultimate lake levels after

mining.

Assess the effects of unusual storm events on the

long-term PCCZ.

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HYDROGEOLOGIC SYSTEM

The Morenci District is located in Greenlee County, Arizona

near the towns of Clifton and Morenci, between Eagle Creek

and the San Francisco River. The active mining and

ore-processing operations encompass an area of

approximately 72 square miles. The open pit mining area is

located in the northern part of the District, within the MiddleChase Creek watershed. The tailing dams are located in the

southern part of the District, along with most of the ore and

solution processing facilities.

The Morenci District is located in the southern part of the

Morenci groundwater basin, which comprises approximately

1,645 square miles of the Central Highlands physiographic

province in eastern Arizona. The boundaries of the basin are

defined by the Arizona-New Mexico state line and the

boundaries of the San Francisco River and Eagle Creek

watersheds. The physiography of the basin consists of rugged

terrain dissected by steep-walled canyons. The hydrogeologic

system within the Morenci basin is characterized almostentirely by fracture flow in consolidated bedrock.

Groundwater originates as infiltration from rainfall and

snowmelt in the higher elevations of the basin, and flows

toward the lower elevations where it issues from springs and

provides underflow to perennial streams.

Hydrogeologic information for the APP application was

obtained by planning and carrying out two hydrogeologic

field investigations designed to obtain sufficient groundwater

data to complete the hydrogeologic characterization. The

Phase 1 investigation initially consisted of compiling and

evaluating existing groundwater data and data relating to

facilities, and assessing data needs. This was followed by a

field investigation that consisted of monitor well installation,

development and sampling, and aquifer testing. The Phase 2

field investigation included monitor well installation,

development and sampling, piezometer installation, bedrock

coring and pressure testing, aquifer testing, and District-wide

water level measurements.

There are currently 90 monitor wells, including 87 water-

level monitoring points, in the District that are available for

collecting groundwater quality samples or water level

measurements. Monitor wells are defined, for present

purposes, as wells constructed and used primarily formonitoring groundwater quality. Monitor wells in the District

range in depth from approximately 200 to 1,200 feet.

Geologic units in the Morenci District include rocks of

Precambrian through Quaternary age, and Quaternary

alluvium.

Aquifer System

The regional aquifer can be subdivided into two distinct

hydrogeologic units: (1) the older bedrock complex, and (2

the Gila conglomerate. The older bedrock complex includes

the Precambrian- through Laramide-age rock units, and

encompasses most of the northern part of the District

including the open pit mining areas. The Gila conglomeratecomprises most of the southern part of the District, and is

separated from the older bedrock complex by two major

faults: the San Francisco Fault and the Eagle Creek Fault.

Water level data from the District-wide water leve

measurement event were conducted in September and October

1995. Groundwater elevations in the Morenci District tend to

follow topography. Elevations are highest in the mountainous

area in the northern part of the District, and lowest near the

perennial streams on the eastern, southern and western

boundaries of the District. Measured groundwater elevation

range from more than 6,300 feet above rnsl on Copper King

Mountain to less than 3,400 feet above msl in the wells southof the tailing impoundments.

Two distinct groundwater flow patterns can be delineated in

the District. Outside of the active mining area and the Upper

Chase Creek watershed, groundwater flows southeast to

southwest. Within the active mining area, the open-pi

mining operation has altered the natural hydraulic gradient

forming a sink. As a result, all groundwater within the active

mining area and the Upper Chase Creek watershed flows

toward the Morenci Pit, and is ultimately collected in a sump

at the bottom of the pit known now as the Dispatch Hill

Sump. The elevation of the bottom of the Dispatch Hill Sump

is 3,350 feet above msl.

In the northern part of the District, there are no wells with

water level data prior to 1992. Observed changes in water

level elevation in wells since 1992 are typically less than 10

feet, and in some wells show a consistent rise or decline.

Hydrologic Boundaries

The Morenci District is located within the Gila River

drainage basin, which includes most of southern and central

Arizona. The District is partially surrounded by three

perennial rivers: the Gila River, the San Francisco River, and

Eagle Creek. These perennial rivers are hydraulicallyconnected to the groundwater system and are assumed to

represent hydrologic boundaries of fixed water-leve

elevation. The other primary river to be considered is Chase

Creek, which bisects the open-pit mining area. Chase Creek

is divided into three reaches; upper, middle, and lower. The

middle reach lies within the open pit area. Surface flows from

upper Chase Creek have been diverted around the middle

reach to lower Chase Creek. The upper and lower reaches of

Chase Creek act as groundwater sinks. Sumps and

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evaporation control groundwater elevations in the open-pit

area. The average base elevation of the Morenci Pit is at 3800

ft. with the groundwater elevations at the bottom of the

Morenci Pit at 3,700 ft msl. Groundwater flow to the Morenci

Pit has been estimated at 300 gpm.

Hydraulic PropertiesThe hydraulic properties discussed in this section include the

saturated and unsaturated hydraulic conductivities in the

horizontal and vertical, anisotropy of conductivity, and

storativity.

Hydraulic Conductivity

Hydraulic conductivity values for the Morenci District were

obtained from the results of aquifer testing, well development

testing, core hole pressure testing, and reclaim wellfield

aquifer testing. Estimates of hydraulic conductivity were

obtained for both the older bedrock complex and the Gila

conglomerate. The hydraulic conductivity values obtained for

these units span several orders of magnitude, from as low as0.0017 ft/d in the older bedrock complex to as high as 400

ft/d in the Gila conglomerate. In general, the hydraulic

conductivity of the Gila conglomerate is approximately two

orders of magnitude greater than the hydraulic conductivity of

the older bedrock complex.

The saturated hydraulic conductivity of the older bedrock

complex ranges from 0.0017 to 42 ft/d, although most values

are less than 10 ft/d. The low hydraulic conductivity values

obtained are typical of massive, low porosity, fractured

bedrock. Some monitor wells show hydraulic conductivity

values that exceed the general range of hydraulic conductivity

probably due to the presence of fractures associated with

faulting. The saturated hydraulic conductivity of the older

bedrock complex can vary vertically and laterally within a

short distance, as shown by the results of core hole pressure

testing and the results of multiple well aquifer tests.

The local variability of hydraulic conductivity is attributed to

preferential flow paths within the older bedrock sequence

formed by fractures. The mineralized area of the mine

workings, which is of higher hydraulic conductivity than the

surrounding rocks, is assumed to extend to a depth of 2,000 ft

MSL beneath the Morenci pit, and to a depth of 3,000 ft MSL

beneath the rest of the mine. These assumed depths are basedon deep exploration drilling. The unsaturated hydraulic

conductivity of the older bedrock complex, as determined

from core hole pressure testing, ranges from 0.003 to 22

ft/day The mean unsaturated hydraulic conductivity of the

unit is 0.88 ft/day.

For modeling purposes, a generalized geologic map was used

to assign zones of materials with similar hydraulic properties.

In each zone, available data were collated and the range of

observed data used in model calibration.

Anisotropy

A qualitative assessment of the anisotropy of Laramide

monzonite porphyry and Laramide granite porphyry was

performed using the relative drawdown in the observationwells during the Phase 2 aquifer tests of monitor wells MP3

MP-5 and MP-8. Two observation wells were located at each

site, approximately 15 feet from the pumping well, and at an

angle of 900 from each other.

The results of the aquifer tests show substantial differences in

drawdown in the observation wells at each test site. The

results from the tests indicate the aquifer is at least locally

anisotropic due to the complex nature of fracture flow in

groundwater. The areal extent of aquifer anisotropy at each

site is unknown. Regional groundwater flow patterns in the

vicinity of the open pit mining area do not suggest anisotropy

on a regional scale. Anisotropic hydraulic conductivity valueswere not supplied to the model, except in the Precambrian

granite and granodiorite zone east of the mine, where many

dikes were observed. In this region, the northwest - southeast

component of hydraulic conductivity was set equal to

one-tenth that of the northeast -southwest component.

Storativity

The storativity of Laramide monzonite porphyry and

Laramide granite porphyry was assessed using observation

well measurements collected during pumping tests. The

pumping storativity ranged from 1.122X10-9 to 0.0217. The

calculated storativity values are generally outside the accepted

range for storativity values for confined aquifers of 5 x 10 -5to

5 x 10-3 (Kruseman and deRidder, 1970) and reflect the

fractured nature of the rock and the difficulties in testing

these types of rocks. Storativity values of 5 x 10-5 ft-1 were

supplied to the model

Sources and Sinks

The climate of the Morenci District is typical of the

southeastern Arizona desert. The average annual rainfall is

12.6 inches with approximately half the precipitation falling

as light showers during the Writer and half during summer

thunderstorms, Data collected at the mine between 1949 and

1990 show average precipitation as 13.04 in/yr. Temperaturesin Morenci range from summer highs of 90 Fahrenheit (F) to

115 F, to winter lows of 20 to 30 F .

Infiltration occurs due to natural infiltration, infiltration due

to stockpile leaching, tailing impoundment seepage, and

reservoir seepage. Natural infiltration is assumed to occur at a

rate of 4 to 10 percent of the incident precipitation rate

varying with elevation. Sensitivity analyses were used to

assess the effect of varying infiltration rates with slope as wel

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as elevation. Detailed infiltration data are presented in the

section titled Water Balance.

Springs occur throughout the Morenci District. The springs

are all located along ephemeral and intermittent drainages in

the northern part of the District, and represent points of

groundwater surfacing either from the regional aquifer orfrom perched aquifers. Springs also occur along the San

Francisco River, the Gila River, and Eagle Creek. Spring

elevations were used to add calibration data points for the

model.

The discharge of seepage water has been controlled to varying

degrees through reclaim water pumping and passive

containment. Historical reclaim water pumping dates to the

1950s and has occurred in two principal areas of the mine:

(1) between the San Francisco River and the tailing

impoundments, and (2) in Lower Chase Creek. There are 11

operational reclaim wells included in the model, four in

Lower Chase Creek and 7 south of the tailing impoundments.Reclaim wells range in depth from approximately 300 to

more than 1,000 feet. Reclaim pumping averaged 4,185 gpm

in 1994. Deepening the Morenci Open Pit to below the

regional water table has created passive containment. The

resulting depression in the water table, now estimated to be

more than 300 feet below the pre-mining water table, has

created hydraulic capture of groundwater in the vicinity of the

open pit operations. These hydrogeologic discharge controls

are a key component of BADCT for the entire District and

were therefore evaluated using numerical groundwater flow

modeling.

Water Balance

Water balances were prepared for 1994 operating conditions,

post-closure, and for the pit lakes under equilibrium

conditions. Water balance components were derived from

field data, to the extent possible. The most uncertain

components are the flows to/from the rivers and their

channels, which cannot readily be confirmed.

After closure, groundwater flow to pit lakes driven by

evaporative loss from the lake surfaces will be significantly

greater than presently occurring in existing mine pits and

sumps. Therefore, the groundwater hydrosink presently

observed is likely to be maintained or enlarged under post-closure conditions.

MODEL DEVELOPMENT

Three different models were used to address the modeling

objectives one at a time thereby avoiding the need to develop

a single highly complex model. Data conversion programs

developed at Dames & Moore were used to translate data

between models efficiently. The basic modeling approach is

summarized in Figure 1. This figure shows the primary

outputs from and conclusions used in the subsequent model a

each step. Dames & Moore used a combination of

public-domain model codes and in-house codes to conduc

this analysis because no single, public-domain code available

in 1996 had all of the features needed to perform this

analysis.

Regional 2D Model - FLOWPATH

The purpose of this model was to predict the regional flow

balance. Since the future zone of influence of the mine was

uncertain at the start of the modeling analyses, the regional

model was designed to be large and extend to known

boundaries. That is, rivers and significant creeks located a

sufficient distance from the mine that the hydraulic heads at

these locations could be assumed to be constant throughou

the model during and after mining. USGS regional data and

hydrogeologic data developed by Phelps Dodge were

compiled and supplied to the model. The model was setup to

test predictions of steady water levels under current operating

conditions. Model-predicted water levels were compared toseveral rounds of field data. Uncertain input data (infiltration

rates mainly) were varied until a satisfactory match was

obtained. The conclusions of this model in terms of head and

flow boundary conditions, calibrated material properties, and

tested assumptions were supplied to the 2D regional lake

model.

Regional 2D Lake Model - TARGET 2DH

The purpose of this model was to predict the lake flow

balance. This model was developed to simulate lake water

balances and average water quality for complex lake

geometries, such as multiple man-made pits or underground

mines, on a more detailed scale than that of the model cells

The model was validated against literature data for

lake-groundwater flows under steady and transient conditions

then applied to this mine setting. Initially, the FLOWPATH

dataset was translated into a TARGET 2DH dataset and the

model rerun to confirm that the model predictions matched

those of the FLOWPATH model. The results matched

identically. Then, details of the ultimate mine-pit volume

stockpile areas, evaporation rates, and surface-water run-on

were supplied to the model. Transient calculations o

groundwater and lake elevations for the mine at closure and

during the post-closure period were made. It was found that

some lakes overflowed into others, but that the primary lakeof interest was the large lake produced by the combination o

three open pit mines in the District, the Metcalf, Western

Copper and Morenci Pits. Due to the volume of the pits, and

the slow groundwater inflow rates, it was predicted to take

more than 100 years for the pit-lake elevations to equilibrate

The effect of a sequence of storms on the predicted lake

elevations was also evaluated. The conclusions of the mode

in terms of predicted heads around the mine pits area, and

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predicted lake levels over time, were supplied to the mine pits

area 3D model

Mine Pits Area 3D Model - TARGET 3DS

The purpose of this model was to predict the mine pits area

flow balance. It was believed that a finer grid of model cells

and a substantial model depth were important to accuratelysimulate the 3D-capture zone of the mine. Model cells were

subdivided to 500 ft by 500 ft. Since ft was not clear initially

to what depth underflow of the lakes might occur, the model

was extended to 7,000 to 10,000 ft below the water table. The

boundaries of the model were also drawn in to accommodate

the influence of the mine pits area without requiring an

excessive number of model cells. There were 96,600 cells in

the 3D model. Horizontal and vertical flow paths and the

corresponding 3D-capture zone of the mine were predicted

for the time after which lake levels stabilize, i.e. 100 years

after the end of mining. Predicted lake levels were compared

to the lowest point on the capture zone to establish that lake

overflow will not occur under future flow conditions. Thepredicted lake levels, together with the ultimate mine layout,

are shown in Figure 2.

CONCLUSIONS

The conclusions from the model predictions are as follows:

The groundwater outflow due to evaporation from the

lakes is predicted to be about 2,000 gallons per minute

(gpm). This outflow is calculated to be greater than the

groundwater outflow under current operating conditions

and so the corresponding area within the capture zone

will be as large, or larger than, the currently-observed

capture zone.

The future groundwater divide around the open pit

mining area will be at a higher elevation than that

observed in the present-day capture zone because lake

levels will be above the elevation of the current pit bases

and groundwater levels will rebound toward pre-mining

levels.

The predicted capture zone, considering horizontal flow

only, is predicted to be of similar area, but at about 100 ft

higher in elevation than the current capture zone.

The three-dimensionally-predicted capture zone is

similar to the capture zone for two-dimensional flow

since the pit lakes are predicted to capture groundwater

vertically upward through the pit bottom.

REFERENCES

1. Dames & Moore, 1987. Report on Hydrologic Modeling

Study of Chase Creek Basin near Morenci Arizona

Prepared for Phelps Dodge Morenci, Inc.

2. Kruseman, G. and N. deRidder, 1970. Analysis and

Evaluation of Pumping Test Data. International Institute

of Land Reclamation and Improvement Bulletin, 11

Wagenin, The Netherlands.

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Figure 1

Groundwater Modeling

Approach

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Figure 2Post Closure Mine

Documents

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