Responsive ■ Resourceful ■ Reliable 1 05157640l12 rev south imp. well cert

GROUNDWATER MONITORING SYSTEM CERTIFICATION -

REVISED

WALTER SCOTT JR. ENERGY CENTER

COAL COMBUSTION RESIDUAL SOUTH SURFACE IMPOUNDMENT

POTTAWATTAMIE, IOWA

Terracon Project No. 05157640

January 30, 2018

1.0 INTRODUCTION

The South Surface Impoundment is an existing coal combustion residual (CCR) surface

impoundment that receives materials from the Walter Scott Jr. Energy Center (WSEC) facility.

Both the WSEC facility and the South Surface Impoundment are located in Pottawattamie

County, Iowa. The location of the WSEC facility and the South Surface Impoundment are

depicted on the attached Figure 1. The impoundment receives and stores wet sluiced CCR

materials which include bottom ash, economizer ash, and boiler slag. Wastewater streams

that are discharged to the impoundment include ash transport water, polisher wastewater,

demineralization neutralizer wastewater, economizer ash hydrovactor water, boiler tube and

air heater cleaning water, and pH adjusted demineralizer water. The impoundment has no

spillways, but water from the impoundment is circulated back to the WSEC facility. The South

Surface Impoundment is classified as an existing, unlined CCR surface impoundment.

On April 17, 2015, the United States Environmental Protection Agency (USEPA) issued the

final version of the CCR rule for regulation and management of CCR materials at coal-fired

units under subpart D of the Resource Conservation and Recovery Act (RCRA). The Federal

CCR rule (40 CFR, Part 257) became effective on October 19, 2015 and applies to the WSEC

South Surface Impoundment.

As required by the Federal CCR rule 40 CFR 257.91 the owner or operator of the CCR unit must install a groundwater monitoring system that consists of a sufficient number of wells, installed at appropriate locations and depths, to yield groundwater samples from the uppermost aquifer that meet the following requirements:

▪ Accurately represent the quality of background groundwater that has not been affected

by leakage from a CCR unit,

▪ Accurately represent the quality of groundwater passing the waste boundary of the

CCR unit.

On October 15, 2017, a Groundwater Monitoring System Certification for the South Surface

Impoundment CCR Unit was placed in the operating record, which included nine monitoring

Groundwater Monitoring System Certification - Revised WSEC CCR South Surface Impoundment ■ Council Bluffs, Iowa January 30, 2018 ■ Terracon Project No. 05157640

Responsive ■ Resourceful ■ Reliable 2 05157640l12 rev south imp. well cert

wells MW-5R, MW-9R, and MW-18 through MW-24. Since submittal of the Groundwater

Monitoring System Certification, evaluation of groundwater analytical data indicate the

possibility that MW-9R may not accurately represent the quality of background groundwater.

Monitoring well MW-133, which is a background well associated with the nearby Monofill CCR

unit, was selected to be incorporated into the monitoring system of the South Surface

Impoundment CCR unit. Incorporation of monitoring well MW-133 into the site monitoring

system meets the requirements of 40 CFR 257.91(a) for a background well for the South

Surface Impoundment CCR unit.

The Groundwater Monitoring System Certification - Revised incorporates the monitoring well

network (MW-5R, MW-9R, MW-18 through MW-24, and MW-133) that will be used during

groundwater sample collection events for monitoring the South Surface Impoundment CCR

unit. The monitoring well network at the South Surface Impoundment meets the general

requirements of the Federal CCR Rule as outlined in 40 CFR 257.91 for groundwater

monitoring system certification. The following sections in this report provide support of the

certification.

2.0 SITE HYDROGEOLOGY SUMMARY

The WSEC South Surface Impoundment (site) is located in the Missouri River Valley near

Council Bluffs, Iowa. A Hydrogeologic Investigation Report (HIR) dated September 2006 was

prepared for the nearby Monofill CCR site (located approximately a half mile to the southwest

of the South Surface Impoundment) which details geology and hydrology for the Monofill CCR

site. As indicated in the HIR, layers of sands, silts, and clays are found at the site which are

consistent with alluvial flood plain deposits. There were no confining units identified in site

borings, and vertical hydraulic gradients were minor in the aquifer. The alluvial aquifer is

considered a contiguous unit with a single water table. Bedrock was not encountered in a

boring that was drilled at the Monofill CCR site to a depth of approximately 117 feet below

ground surface (bgs), however, bedrock at the site is believed to be Pennsylvanian-age shale

or limestone. Based on previous groundwater monitoring event data collected from May 2016

through October 2017, the water table has an average elevation of approximately 960 feet

above mean sea level (amsl) at the site. Groundwater flow at the site is generally to the west

based on interpolation of groundwater elevation data collected during monitoring events from

May 2016 through October 2017.

Groundwater Monitoring System Certification - Revised WSEC CCR South Surface Impoundment ■ Council Bluffs, Iowa January 30, 2018 ■ Terracon Project No. 05157640

Responsive ■ Resourceful ■ Reliable 3 05157640l12 rev south imp. well cert

3.0 GROUNDWATER MONITORING SYSTEM

Based on the hydrogeologic information collected from the site and the location of the CCR

South Surface Impoundment, the groundwater monitoring system consists of one up-gradient

well (MW-9R), three down-gradient wells (MW-5R, MW-18, and MW-19), one cross-gradient

well (MW-21), two cross- to up-gradient wells (MW-22 and MW-23), two cross- to down-

gradient wells (MW-20 and MW-24), and one background well (MW-133). The location of

monitoring wells MW-5R, MW-9R, and MW-18 through MW-24 are depicted on the attached

Figure 2. The location of monitoring well MW-133 relative to the South Surface Impoundment

CCR unit is depicted on the attached Figure 1. Details regarding the groundwater monitoring

system for the South Surface Impoundment CCR unit are summarized on the attached Table

1.

TABLE 1

WALTER SCOTT JR. ENERGY CENTERCOAL COMBUSTION RESIDUAL SOUTH SURFACE IMPOUNDMENT

POTTAWATTAMIE COUNTY, IOWATERRACON PROJECT NO. 05157640

GROUNDWATER MONITORING WELL SYSTEM

Top Bottom

MW-5R 984.46 34.9 959.6 - 949.6 Down-gradientMW-9R 972.26 17.8 964.5 - 954.5 Up-gradientMW-18 978.89 37.7 956.2 - 941.2 Down-gradientMW-19 980.08 37.9 957.2 - 942.2 Down-gradientMW-20 980.59 34.0 961.6 - 946.6 Cross- to Down-gradientMW-21 979.76 40.0 954.8 - 939.8 Cross-gradientMW-22 980.24 27.0 968.8 - 953.8 Cross- to Up-gradientMW-23 986.69 38.6 963.1 - 948.1 Cross- to Up-gradientMW-24 982.30 38.8 958.5 - 943.5 Cross- to Down-gradient

MW-133 970.71 25.5 955.3 - 945.3 Background

Notes:TOC = Top of casing.BTOC = Below top of casing.ft = Feet.ID = Identification.NAVD88 = North American Vertical Datum, 1988.1Installed Depth

Screened Interval

(ft NAVD 88) Classification

Well ID TOC(ft NAVD 88)

TotalDepth (ft

BTOC)1

Page 1 of 1