W94613D

DRAFTCONCEPTUAL WETLAND MITIGATION PLAN

REMEDIAL INVESTIGATION/FEASIBILITY STUDY

NEW HAMPSHIRE PLATING COMPANY SITEMERRIMACK, NEW HAMPSHIRE

ForU.S. Environmental Protection Agency

ByHalliburton NUS Corporation and

Badger Engineers, Inc.

EPA Work Assignment No. 33-1LG1EPA Contract No. 68-W8-0117

HNUS Project No. 0772

August 1994

f&\ Halliburton NUS"" C O R P O R A T I O N

W94613D

DRAFT CONCEPTUAL WETLAND MITIGATION PLAN

REMEDIAL INVESTIGATION/FEASIBILITY STUDY

NEW HAMPSHIRE PLATING COMPANY SITE MERRIMACK, NEW HAMPSHIRE

For U.S. Environmental Protection Agency

By Halliburton NUS Corporation and

Badger Engineers, Inc.

EPA Work Assignment No. 33-1LG1 EPA Contract No. 68-W8-0117

HNUS Project No. 0772

August 1994

Peyfoji Doub George til/ Gardner, P.E. Wetlands Ecologist Program Manager

DRAFT

TABLE OF CONTENTS DRAFT CONCEPTUAL WETLAND MITIGATION PLAN

REMEDIAL INVESTIGATION/FEASIBILITY STUDY NEW HAMPSHIRE PLATING COMPANY SITE

MERRIMACK, NEW HAMPSHIRE

SECTION PAGE

E.O EXECUTIVE SUMMARY E-l

1.0 INTRODUCTION 1-1 1.1 Site Description 1-3 1.2 Historical Site Descriptions for NHPC Site 1-7 1.3 Functional Assessment of Wetlands on NHPC Site 1-8 1.4 Overview of Wetland Mitigation Objectives 1-13

for NHPC Site

2.0 OVERVIEW OF ALTERNATIVE WETLAND MITIGATION 2-1 CONCEPTS 2.1 Concept 1: Restoration of Wetlands in Entire 2-6

Meander Scar 2.2 Concept 2: Restoration of Wetlands in Part of 2-7

Meander Scar; and Creation of Compensatory Wetlands to Offset Upland Capping of South Wetland and Lagoon 1 Area

2.3 Wetland Mitigation Concepts Not Considered Further 2-8

3.0 RESTORATION OF WETLANDS IN THE MEANDER SCAR 3-1 3.1 Design Objectives - Functions and Values 3-1 3.2 Design Objectives - Hydrology 3-2 3.3 Design Objectives - Soils 3-5 3.4 Design Objectives - Vegetation 3-6

4.0 CREATION OF ONSITE COMPENSATORY WETLANDS (OPTION 2 . . 4-1 ONLY) 4.1 Alternative Onsite Wetland Creation Scenarios 4-1 4.2 Design Objectives 4-3

5.0 REFERENCES 5-1

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TABLE OF CONTENTS DRAFT CONCEPTUAL WETLAND MITIGATION PLAN

REMEDIAL JNVESTIGATION/FEASffilLrrY STUDY NEW HAMPSHIRE PLATING COMPANY SITE

MERRIMACK, NEW HAMPSHIRE

TABLES

NUMBER PAGE

3-1 ILLUSTRATIVE PLANTING SCHEDULE, PERMANENTLY 3-9 INUNDATED WATER AREAS

3-2 ILLUSTRATIVE PLANTING SCHEDULE, PERMANENTLY 3-10 SATURATED/SEASONALLY INUNDATED MARSH AREAS

3-3 ILLUSTRATIVE PLANTING SCHEDULE, SEASONALLY 3-11 SATURATED/SHAIXOWLY INUNDATED MARSH AREAS

3-4 ILLUSTRATIVE PLANTING SCHEDULE, SEASONALLY 3-12 SATURATED/SHRUB-SAPLING FRINGE AREA

3-5 ILLUSTRATIVE PLANTING SCHEDULE, UPLAND 3-14 REFORESTATION AREAS

FIGURES

NUMBER PAGE

1-1 SITE LOCATOR MAP 1-2 1-2 SITE FEATURES MAP 1-4 1-3 EXISTING VEGETATION AND EXISTING WETLAND 1-6

LOCATIONS (1994) 2-1 WETLAND MITIGATION CONCEPT 1 OVERVIEW 2-2 2-2 WETLAND MITIGATION CONCEPT 1 CONCEPTUAL FINISHED . . . 2-3

GRADING SCHEME 2-3 WETLAND MITIGATION CONCEPT 2 OVERVIEW 2-4 2-4 WETLAND MITIGATION CONCEPT 2 CONCEPTUAL FINISHED . . . 2-5

GRADING SCHEME 3-1 CONCEPTUAL SECTION VIEW OF RESTORED MEANDER SCAR . . 3-8

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E.O EXECUTIVE SUMMARY

This Conceptual Wetland Mitigation Plan presents and compares alternative concepts for

restoring wetlands impacted by remediation of the New Hampshire Plating Company Site in

Merrimack (Hillsborough County), New Hampshire. Wetlands on the site are located hi a

narrow topographic depression (meander scar) on the floodplain of the Merrimack River. The

surrounding area is a mix of woodland and industrial and commercial development. Part of the

original wetlands in the meander scar had been excavated prior to 1984 to create a series of 4

lagoons for disposal of electroplating waste, which resulted in soil contamination by heavy

metals, cyanide, and volatile organic compounds. One of the lagoons was capped in 1992, and

other parts of the meander scar were shallowly covered, as part of the 1992 removal action.

However, high concentrations of the contaminants still remain on the soil surface, and

subsequent remedial activity may be necessary.

Two wetland mitigation concepts for the meander scar are presented in detail. The first involves

restoring the meander scar to create an integrated system of open water, permanently saturated

or shallowly inundated marsh, seasonally saturated marsh, and a seasonally saturated scrub-shrub

fringe that transitions to forested uplands on the side slopes. The second involves a similar

restoration of most of the meander scar, but leaving the southernmost lagoon capped as an

upland. Onsite wetlands would be created from uplands to offset these uplands. Both wetland

mitigation concepts principally emphasize terrestrial wildlife habitat quality.

Information presented in this conceptual wetland mitigation plan builds upon data collected in

a series of ecological site characterizations performed between 1989 and 1994. The plan will

complement a proposed Feasibility Study intended to investigate remedial design alternatives for

the site. Based on information presented in this plan and in the Feasibility Study, one wetland

mitigation concept will be selected as the basis for preparing detailed drawings and specifications

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(the Final Wetland Mitigation Plan). Conceptual design recommendations developed for the

selected concept in this plan will be incorporated into the Final Wetland Mitigation Plan.

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INTRODUCTION

This Conceptual Wetland Mitigation Plan outlines alternative concepts for restoring wetlands

impacted by the New Hampshire Plating Company (NHPC) Site in Merrimack (Hillsborough

County), New Hampshire (Figure 1-1). An electroplating operation was conducted in a building

on the southern part of the NHPC Site between 1965 and 1985. During this time, spent plating

solutions were discharged to a series of lagoons in a long, narrow topographic depression

(meander scar) on the northern part of the NHPC Site. The lagoons had been excavated from

vegetated wetlands that had previously occupied the bottom of the meander scar. The meander

scar is thought to be a remnant river channel that had become hydrologically isolated in

prehistoric times. Bottom sediments and soils in the lagoons were suspected of being

contaminated with a number of heavy metals, cyanide, and volatile organic compounds (U.S.

EPA, 1990a).

The U.S. EPA listed the NHPC Site on the National Priority List (NPL) of contaminated sites

in 1992. An Ecological Assessment completed by the U.S. EPA hi 1990 concluded that wildlife

habitats in the meander scar had been significantly degraded by contamination and proposed that

functioning wetland habitats be restored in conjunction with future site remediation (U.S. EPA,

1990b). It recommended that the meander scar be restored to a self-perpetuating wetland system

that provides maximal wildlife habitat and performs the hydrological functions of the meander

scar prior to excavation of the lagoon. The report emphasized a functions-oriented approach to

restoration design rather than exact replication of whatever vegetation and hydrology existed

prior to development.

A removal action performed in 1992 involved excavating contaminated soil from the lagoon

system and stockpiling soil that was not remediated at the south end of the meander scar.

Although the excavated areas were covered by a layer of clean soil and the stockpile location

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N

APPROXIMATE SCALE 1 ln= 2600 ft

FIGURE 1-1 SITE LOCATOR MAP

CONCEPTUAL WETLAND MITIGATION PLAN NEW HAMPSHIRE METAL PLATING SITE - MERRIMACK, NH

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was capped, excessive concentrations of contaminants remain throughout much of the meander

scar. A Remedial Investigation (RI) is underway to determine the present levels and extent of

contamination. Wetlands and other habitats in the meander scar were recharacterized in 1993

(Halliburton NUS, 1993a), and a new Ecological Assessment is in preparation to determine the

potential ecological risks from current levels of contamination.

Information presented in this conceptual wetland mitigation plan builds upon data collected in

the various site characterizations discussed above. The plan will complement a proposed

Feasibility Study intended to investigate remedial alternatives for the entire site, including the

wetlands. Based on information presented in this plan and in the Feasibility Study, one wetland

mitigation concept will be selected as the basis for preparing detailed drawings and specifications

(the Final Wetland Mitigation Plan). Conceptual design recommendations developed for the

selected concept in this plan will be incorporated into the Final Wetland Mitigation Plan.

Site Description

The NHPC Site encompasses approximately 13 acres within an urban, industrially developed

area on a broad stream terrace along the west bank of the Merrimack River. The NHPC Site

is bordered to the south by Wright Avenue, to the east by the Boston and Maine Railroad and

by industrial development, to the north by industrial development, and to the west by industrial

and commercial properties facing the Daniel Webster Highway and Wright Avenue. The

southern part of the NHPC Site, at the corner of Wright Avenue and the railroad, comprises

disturbed upland soils and ruderal vegetation and formerly supported the electroplating building.

The northern part of the NHPC Site comprises the meander scar and surrounding forested

uplands (Figure 1-2).

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Wetlands on the NHPC Site are limited to bottom lands in the meander scar (Figure 1-3). The

side slopes of the meander scar and surrounding uplands support a mixed forest of northern

hardwoods and conifers, with small areas of early successional deciduous forest,

scrub/shrub/sapling vegetation, and early successional grasslands (ruderal vegetation) (U.S.

FWS, 1994). Areas of the NHPC Site outside of the meander scar have been determined not

to be wetlands (Halliburton NUS, 1993a; U.S. FWS, 1994).

A small area of seasonally saturated wetlands, designated as the South Wetland, exists at the

"south end of the meander scar. Vegetation in this area has not been characterized in detail but

appears on recent aerial photographs (April 1992) to comprise herbaceous perennial and is

described in a recent report (U.S. FWS, 1994) as highly degraded.

That part of the meander scar that formerly supported the lagoon system was covered with a

shallow layer of loam soil as part of the removal action, but much of this area still appears to

contain wetlands (Halliburton NUS, 1993a). The lagoon basins have thus been partially

obscured, and are shown in Figure 1-2 as Lagoon 1 Area, Lagoon 2 Area, and Lagoon 3&4

Area. The southernmost lagoon (Lagoon 1) had been used to stockpile contaminated soil

excavated from the other lagoons to the north (Lagoons 2,3, and 4) and was capped. Portions

of the meander scar in the former locations of Lagoons 2 and 3 (Figure 1-2) support standing

water (U.S. EPA, 1992; U.S. FWS, 1994). Other areas support herbaceous vegetation

characterized by sedges (Carex sp.), rushes (Juncus effusus and Eleocharis obtusd), cattail

(Typha angustifolia), grasses (Poa sp. and Festuca sp.), and unidentified annual plants. The

sedges, rushes, and cattail tend to be dominant closest to the standing water, and the grasses tend

to be dominant toward to edges and northern part of the former lagoon system (U.S. FWS,

1994). The northernmost 350 feet of the meander scar (northern wetland area) was never

excavated to construct the lagoon system and contains seasonally saturated wetlands supporting

a dense monoculture of reed canarygrass (Phalaris arundinaced) (U.S. FWS, 1994). Dense

monocultures of invasive plant species such as reed canarygrass are generally indicative of

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stressful conditions, suggesting that conditions in this area are not reflective of the pre­

development conditions in the meander scar.

Historical Site Descriptions for NHPC Site

The following sections outlines available descriptive data for the NHPC Site for periods prior

to lagoon excavation and prior to the 1992 removal action. Following that, the meander scar

has remained largely unchanged in its present condition.

Prior to Lagoon Excavation by NHFC

No description exists of the wetlands in their original condition, prior to excavation of the

lagoons. National Wetland Inventory (NWI) maps dated 1977 designate the entire bottom of the

meander scar as Palustrine scrub-shrub, broad-leaved deciduous wetlands (PSS1) (U.S. EPA,

1990a). This designation could either indicate permanently saturated or shallowly inundated

wetlands dominated by inundation-tolerant hardwood shrubs, or seasonally saturated wetlands

dominated by hardwood tree seedlings and saplings that established following an earlier

disturbance activity. Aerial photos flown in 1979 for a county soil survey suggest that the

meander scar was covered by woody vegetation (scrub-shrub or forest) (SCS, 1981). The soil

survey maps the bottom lands in the meander scar in the Rippowam soil series and states that

most such areas in Hillsborough County support woodland.

Prior to the 1992 Removal Action

The wetland delineation performed in 1990 identified the four lagoons as shallow basins

separated by berms and supporting sparse herbaceous vegetation. Vegetation in the lagoons was

dominated by sedges (Scirpus sp., Carex scabrata, and Eleocharis sp.), blue flag (Iris

versicolof), steeple-bush (Spiraea tomentosa), sensitive fern (Onoclea sensibilis), purple

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loosestrife (Lyihrum salicaricd), and unidentified grasses (U.S. EPA, 1990). Wetlands

supporting this vegetation would be classified as Palustrine emergent (PEM) rather than as

Palustrine scrub-shrub (PSS), suggesting that the NWI map reflects conditions prior to

excavation of the lagoons. Upland forests on the side slopes of the meander scar were described

as dominated by quaking aspen (Populus tremuld), white pine (Pinus strobus), and gray birch

(Betula populifolid). The forest on the west side slope was described as being in an earlier

successional stage than that on the east side (U.S. EPA, 1990a).

Functional Assessment of Wetlands on NHPC Site

No functional assessment, using procedures such as the Wetland Evaluation Technique (WET

2.0) or Habitat Evaluation Procedures (HEP), has been performed for the NHPC Site wetlands.

The following brief functional assessment was performed subjectively using the same functions

and values assessed by WET 2.0 (Adamus et aL, 1991). The assessment is used to define

functional goals for wetlands restored or created under the wetland mitigation concepts presented

in Chapter 2.

Groundwater Recharge

Unless the Feasibility Study calls for capping contaminated soils beneath the wetlands, retaining

the ability of the meander scar to contribute to recharging the underlying aquifer will be an

important wetland mitigation consideration for the NHPC Site. The meander scar has served

to intercept surface runoff that would have otherwise entered the Merrimack River as

floodwater. The original Rippowam soils in the meander scar were of moderate to moderately

rapid permeability (SCS, 1981), and thus allowed water trapped in the meander scar to infiltrate

down to the underlying aquifer. The removal action effort has topographically eliminated the

southernmost part of the meander scar, but fill soils placed in the rest of the meander scar are

loam (Halliburton NUS, 1993a), which is relatively permeable. Keeping soils in the restored

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wetlands from becoming too compacted and avoiding the use of low permeability substrates such

as clay should help ensure that the groundwater recharge functions of the meander scar are not

degraded.

Groundwater Discharge

Groundwater discharge will not be a wetland mitigation consideration for the NHPC Site.

Available hydrological data suggests that water in the meander scar is perched above a water

table more than 5 feet below the surface even during the wettest seasons of the year (Goehlert,

1993). Groundwater discharge by perched wetlands is not considered to be possible (Adamus * ** „ f C t *s ' " ' •• 1 ~ t*iil"Vi_i_ ^ i- * %' " I * * •*. s_

et aL, 1991). Furthermore, the mearidef'scar lacks an"outlet through which groundwater can

be discharged to any river or stream.

Floodflow Alteration

Unless the Feasibility Study calls for capping contaminated soils beneath the wetlands, retaining

the ability of the meander scar to contribute to the attenuation of floodwaters in the Merrimack

River will be an important wetland mitigation consideration for the NHPC Site. The meander

scar has served to abate the intensity of floodwaters in the Merrimack River by intercepting

runoff which would have otherwise contributed to floodflows. The relative value of wetlands

in reducing floodflow intensity is considered greater when wetlands and lakes occupy less than

10 percent of the upstream watershed (Adamus et al., 1991). A cursory review of areas along

the Merrimack River upstream of the NHPC Site using aerial photographs in a soil survey (SCS,

1981) suggests that wetlands and lakes do indeed occupy less than 10 percent of the landscape.

Additionally, a Flood Insurance Rate Map dated July 16, 1979 shows the meander scar within

the 100-year floodplain of the Merrimack River (FIA, 1979). Any floods that raise the height

of the river by more than (approximately) 25 feet will cause floodwater to enter the meander

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scar. Floodwater trapped in the meander scar will not enter downstream floodflows, and will

instead recharge the groundwater or be lost by evaporation. Unless the topography of the

meander scar or surrounding lands is radically altered, capacity for the meander scar to affect

floodwaters of the Merrimack River would not likely be affected by any wetland mitigation

effort.

Sediment Stabilization

Bank sediment stabilization will not be a wetland mitigation consideration for the NHPC Site.

Because the meander scar is topographically isolated from the Merrimack River and other nearby

surface waters, it has never served to stabilize sediment sources subject to scouring by river

flows. Grading and the duration of soil exposure during wetland mitigation construction

activities will be minimized, but only because it will be necessary to protect the restored

meander scar wetlands from sedimentation.

Sediment/Toxicant Retention

Retaining the ability of the meander scar to trap and sequester toxicants derived from the

contaminated NHPC Site and from surrounding urban areas, and thereby reduce contamination

of underlying groundwater, will be an important wetland mitigation consideration for the NHPC

Site. Although the meander scar can not serve as a sink for sediment loads carried by the

Merrimack River or other surface water flows, it contains wetland vegetation and organic

sediments capable of permanently binding heavy metals and other toxicants that could otherwise

contaminate groundwater. Isolated palustrine wetlands in cool climates are recognized as being

exceptionally capable of sequestering heavy metals, binding them permanently to organic soil

(Adamus et aj., 1991).

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The high concentrations of cadmium and zinc found in the lagoon soil (Halliburton NUS, 1993b)

likely reflects the ability of organic matter in the soil to bind these metals. Had wetlands never

been present in the lagoons, more cadmium and zinc might have reached underlying

groundwater. Unless the NHPC Site is completely remediated to background concentrations of

heavy metals and organics, any wetland mitigation effort must ensure that dense wetland

vegetation and deep organic soils remain throughout the meander scar to protect underlying

groundwater from future contamination.

Nutrient Removal/Transformation

Nutrient removal/transformation will be only a minor wetland mitigation consideration for the

NHPC Site. Although the meander scar may be serving as a sink for small quantities of

nitrogen and phosphorus borne by surface runoff, its isolation from the Merrimack River and

other surface waters has likely prevented it from removing water-borne nutrients carried by

downstream flows. Furthermore, the surrounding landscape is primarily forested and urban,

with few agricultural areas where chemical fertilizers are used.

Production Export

Production export will not be a wetland mitigation consideration for the NHPC Site. The

meander scar lacks an outlet to the Merrimack River or other surface water bodies. Therefore

there is no opportunity for carbon export to downstream deepwater habitats.

Aquatic Diversity/Abundance

Aquatic diversity/abundance will only be a very minor wetland mitigation consideration for the

NHPC Site. The wetlands and shallow open waters presently or historically present hi the

meander scar are likely of low value as habitat for fish and other aquatic habitat. The meander

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scar is not contiguous to the Merrimack River or other areas of deeper water, and therefore can

not serve as an area for spawning, feeding, or predator avoidance by fish species in these

waters. Furthermore, shallow, isolated open waters are too susceptible to lethal oxygen

stagnation to support a sustained fishery (Adamus et al., 1991). Creating suitable fish habitat

would require devoting the majority of the meander scar to construct a deepwater pond rather

than to establish wetland vegetation, and the resulting fishery would still be isolated and not

improve the fisheries of the Merrimack River or other nearby surface waters.

Wildlife Diversity/Abundance

Wildlife diversity/abundance will be the most important wetland mitigation consideration for the

NHPC Site. Although wetlands may have once been abundant on the Merrimack River

floodplain, most of the area surrounding the meander scar is presently either upland forest or

urban. When restored, the meander scar will be the largest vegetated wetland in the local

region. It will provide an oasis of wetland habitat in a riverine floodplain comprised mostly of

uplands. Together with the forested side slopes, the meander scar will provide an oasis of

natural vegetation in an urban setting.

The Merrimack River and Horseshoe Pond are abutted by steep embankments rather than

wetlands. Historically, the meander scar was likely one of the largest areas of vegetated

wetlands in the surrounding region. It provided habitat for wildlife that inhabit the floodplain

of the Merrimack River.

Recreation

Recreation will be a secondary wetland mitigation consideration for the NHPC Site. Because

it is fenced to exclude humans from hazardous contamination, the meander scar is presently of

no recreational value. If the NHPC Site is remediated until completely safe for casual human

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entry, the restored wetlands and forested side slopes could provide an opportunity for

birdwatching, botanizing, or other ecologically-based hobbies for residents of the surrounding

urban area. Any open waters potentially created in the meander scar would not likely be large

or deep enough for recreational boating, swimming, or fishing.

Uniqueness/Heritage

Any project which restores the meander scar to a naturalistic system of vegetated wetlands will

contribute to restoring the unique heritage of the Merrimack River floodplain. The floodplain

likely included many more wetland areas prior to its extensive urbanization. The meander scar

is also a geologically interesting landscape feature.

Overview of Wetland Mitigation Objectives for NHPC Site

In decreasing order of priority, the overall objectives for restoring wetlands (and for creating

wetlands, if necessary) are as follows:

(1) Maximizing the Habitat Value of the Finished Wetlands for Terrestrial and

Semi-Aquatic Wildlife

As indicated in the functional assessment in Section 1.3, the most important wetland mitigation

objective will be the development of the best possible habitat for terrestrial and semi-aquatic

wildlife and aquatic invertebrates. Restored to provide an integrated system of wetland habitats

in the bottom lands and upland forest habitat on the side slopes and surrounding uplands, the

meander scar could provide a valuable oasis of terrestrial wildlife habitat in an urban location.

Aerial photos included in the soil survey for Hillsborough County indicate that much of the

former Merrimack River floodplain has been subject to urban development, and that large areas

of forested and scrub-shrub habitat are not common (SCS, 1981). Provision of high quality

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natural habitat on floodplains and other riparian lands near rivers is important for terrestrial

wildlife species requiring habitat near the edge of open waters or that use these lands as

migration corridors.

Meeting this objective will require establishing a high diversity of plant species favored by

terrestrial and semi-aquatic wildlife for food and cover and a high diversity of habitat types.

Optimal positioning of open water, marsh, and scrub-shrub habitats and enhancement of

adjoining upland forests will further this objective.

(2) Maximizing the Ability of the Wetlands to Sequester Contaminants

As indicated in the functional assessment in Section 1.3, the wetlands in the meander scar may

be serving to reduce the quantity of heavy metals and other contaminants reaching underlying

groundwater. This objective will be of particular importance if the Feasibility Study supports

a decision to cover contaminated soils in the meander scar instead of removing them. Even if

the contamination is removed, the wetlands could provide back-up protection against relict

contamination. Meeting this objective will require the establishment of a thick layer of high

organic matter topsoil in the wetlands and the establishment of dense vegetation wherever

possible.

(3) Maintaining the Groundwater Recharge and Floodflow Abatement Functions

of the Meander Scar

As indicated in the functional assessment in Section 1.3, the meander scar appears to recharge

the underlying groundwater by trapping surface runoff and allowing it to infiltrate slowly into

the underlying soils. The meander scar is also located in the 100-year floodplain of the

Merrimack River and may serve to trap and hold water during very high flows that would

otherwise be carried downstream. Unless the extent of the watershed contributing runoff to the

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meander scar, or the elevation of lands separating the meander scar from the river, are changed,

little design effort will be needed to maintain these functions. Use of liners, clays, or other

impervious substrates in the meander scar would have to be avoided.

If the Feasibility Study supports a decision to cap contaminated soils in place in the meander

scar, it may not be desirable to allow groundwater recharge to continue. It may then be

necessary to consider methods to minimize the perviousness of surface soils and to move excess

standing water out of the meander scar.

The ability to restore or create wetlands that can perform this function will depend upon

remedial design decisions made following review of the Feasibility Study. Localized grade

changes could alter the quantity of surface runoff reaching the meander scar. Furthermore, it

may not be desirable to encourage the interception of surface runoff by the meander scar unless

all soil-borne contamination is completely removed from it.

(4) Maximizing the Resemblance of the Finished Wetlands to the Predevelopment

Condition of the Wetlands in the Meander Scar

Restoring wetlands to their original condition, and creating wetlands that are "in-kind" to those

that are lost, is a traditional objective of most wetland mitigation efforts. Attempting to

successfully recreate original elements vegetation, soils, and hydrology can indirectly contribute

to restoring the original ecological functions of a site (Kruczynski, 1990). But the U.S. EPA

has placed a higher priority on restoring the ecological functions of the NHPC Site wetlands

rather than on duplicating their historical species composition and soil profile (U.S. FJPA,

1990b). As indicated in Section 1.2, the original wetlands in the meander scar were never

adequately characterized to support exact replication. Furthermore, the meander scar may not

have originally contained the optimal diversity of habitat types and plant species for terrestrial

wildlife.

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Meeting this objective will involve removal of remaining vestiges of the lagoon system and

restoration of a contiguous, integrated system of wetland habitats in the meander scar bottom.

However, some open water habitat will be created in the meander scar despite its apparent

absence prior to excavation of the lagoons. Only noninvasive, indigenous plant species will be

selected for revegetation, but no attempt will be made to create a species composition resembling

that most likely to have been originally present.

(5) Maximizing the Probability of Successful Habitat Creation Despite Uncertain

Hydrological Conditions Following Site Remediation

Available evidence suggests that hydrological conditions in the bottom lands of the meander scar

are primarily the result of surface runoff. Recent hydrological data collected in March 1993

indicated that the water table was between 5 and 15 feet below the bottom surface of the

meander scar (Goehlert, 1993). The hydrology of the restored wetlands will therefore be subject

to rapid and extreme fluctuation in response to storms and droughts. Even in more stabile

groundwater driven systems, establishment of exact hydrological conditions is one of the most

technically challenging elements of wetland mitigation design. Meeting this objective will

require planting species that are tolerant of a wide range of hydrological conditions. Unless

excavation activities result in the meander scar being dry or becoming too deeply inundated to

support persistent vegetation, revegetation specifications will be subject to interim adjustments

to best reflect the exact hydrological conditions that ultimately result.

(6) Maximizing Compatibility with Potential Land Use Objectives for the NHPC

Site

Although lands within the meander scar will never be used for urban development, other parts

of the NHPC Site, especially the disturbed uplands facing Wright Avenue, could be suitable for

future industrial or commercial development. All or part of the site, especially the forested

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uplands surrounding the meander scar, could be suitable for future use as a community park

(Palermo, 1994). Meeting this objective will involve ensuring that the wetlands and surrounding

uplands are aesthetically pleasing and that any area committed to wetland creation does not

conflict with potential development objectives.

(7) Minimizing Any Potential Requirement for Post-Construction Maintenance,

Repair, or Revegetation.

The Interagency Committee on Wetland Restoration and Creation has established a priority for

wetland restoration and creation projects that become self sustaining as soon as possible

following establishment (Interagency Committee for Wetland Restoration and Creation, 1992).

Failure of restored or created wetlands to meet their design objectives leads to increased project

costs to perform repairs and/or revegetation and delays successful achievement of their

ecological functions. Meeting this objective will involve keeping the mitigation design simple;

avoiding the use of high-maintenance water control structures; and planting only indigenous plant

species that establish and spread rapidly, are tolerant of variable hydrology, and not highly

subject to pests.

(8) Minimizing Construction Cost

Meeting this objective will primarily depend on successfully meeting the other design objectives

without having to perform a second round of excavation and without having to replant dead

vegetation. It will also be necessary to minimize the extent of excavation and ensure that all

specified plant material is produced in large quantities by wetland nurseries.

W94613D 1-17

2.0

DRAFT

OVERVIEW OF ALTERNATIVE WETLAND MITIGATION CONCEPTS

Two alternative wetland mitigation concepts are being considered for the NHPC Site:

Concept 1: Restoration of Wetlands in Entire Meander Scar (Figure 2-1 and 2-2)

Concept 2: Restoration of Wetlands in Part of Meander Scar; and Creation of Compensatory

Wetlands to Offset Upland Capping of South Wetland and Lagoon 1 Area (Figure

2-3 and 2-4)

Concepts 1 and 2 differ only with respect to the relative extent of wetland restoration versus

wetland creation. Both emphasize restoration over creation, although Concept 2 has been

developed to include a limited area of creation to offset wetlands which may not be restorable

following certain potential remedial designs.

Wetland restoration, when possible close to the area of wetland losses, is generally preferable

to creation because it is usually less expensive and usually has a higher likelihood of success

(Kruczynski, 1990). Wetland restoration has been defined as the reestablishment of a damaged

or converted wetland ecosystem to its pre-degradation condition, as nearly as practical, so that

either:

1) the original wetland community is reestablished to the condition that probably

existed before degradation or conversion of the wetland, including wetland

hydrology and hydrophytic vegetation where the original wetland was vegetated

or

W94613D 2-1

DRAFT

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2) a wetland community is established, exhibiting wetland hydrology and

hydrophytic vegetation where the original wetland was vegetated, different from

what probably existed prior to degradation, but at least partially replacing original

functions and values (Interagency Committee on Wetlands Restoration and

Development, 1992).

Consistent with the overall wetland mitigation objectives outlined for the NHPC Site in Section

1.4, the latter approach will be followed. This latter approach emphasizes the restoration of

ecological functions, and is consistent with the highest priority objectives established for the

NHPC Site.

A detailed wetland mitigation design will be developed in accordance with one of these

alternative concepts following review of this Conceptual Wetland Mitigation Plan and the

Feasibility Study. These reviews may indicate a need to modify the selected concept prior to

development of the design. Both concepts can be readily adapted to remedial approaches

emphasizing either complete removal of all contaminated material from the meander scar or

approaches which involve covering rather than removing contaminated material from the

meander scar.

2.1 Concept 1: Restoration of Wetlands in Entire Meander Scar

Concept 1 involves restoring all bottom lands in the meander scar to support an integrated

system of open water, permanently saturated or shallowly inundated marsh, seasonally saturated

marsh, and a seasonally saturated scrub-shrub fringe that transitions to forested uplands on the

side slopes (Figure 2-1). The hydrological gradient would trend from deeper and more extended

inundation near the center of the meander scar to seasonal soil saturation near the edge, and

from open water and sparse vegetation near the center to dense herbaceous vegetation and a

W94613D 2-6

2.2

DRAFT

scrub-shrub fringe near the edge. Forested wetlands would not be created hi the meander scar.

They likely never occurred there, and the extreme hydrological fluctuations likely to result from

runoff to the meander scar would place forested wetlands at a high risk of failure. Instead,

upland forests on the side slopes would be enhanced to provide forested habitats in close

proximity to the wetlands.

Concept 1 would be consistent with the U.S. EPA's policy of no net loss of wetlands, restoring

the original extent of wetlands that existed on the NHPC Site prior to its industrial development.

As depicted in Figure 2-1, Concept 1 would result hi the restoration of approximately 2.8 acres

of wetlands on the NHPC Site, including approximately 2.1 acres of Palustrine emergent (PEM)

wetlands, 0.5 acres of Palustrine scrub-shrub (PSS) wetlands, and 0.2 acres of Palustrine open

water (POW) wetlands. The PEM wetlands would be dominated by persistent vegetation that

retains a yearlong presence (PEM1 wetlands). Noninvasive, nonpersistent aquatic vegetation

would be established as possible in this open water, concerting all or part of it to a Palustrine

emergent nonpersistent (PEM2) wetland.

Concept 2; Restoration of Wetlands in Part of Meander Scar; and Creation

of Compensatory Wetlands to Offset Upland Capping of South Wetland and

Lagoon 1 Area

Concept 2 differs from Concept 1 hi that the South Wetland and Lagoon 1 Area would be

capped to create an upland, and an equivalent area of upland on the NHPC Site would be used

to create compensatory wetlands. The remaining bottom lands within the meander scar would

be restored to wetlands following the same approach outlined for Concept 1. The compensatory

wetlands would be created either by widening the meander scar (as depicted in Figure 2-3) or

by creating a separate wetland system elsewhere on the NHPC Site. If the former approach

were taken, the compensatory wetlands would complement the integrated system of hydrological

and vegetation gradients used to restore the meander scar bottom. If the latter approach were

W94613D 2-7

2.3

DRAFT

taken, the compensatory wetlands would be hydrologically isolated from the meander scar,

although they could complement the meander scar and intervening uplands to create an integrated

system of upland and wetland wildlife habitats.

Because compensatory wetlands would be created to offset the permanent conversion of the

South Wetland and Lagoon 1 Area to uplands, Concept 2 would be consistent with the U.S.

EPA's policy of no net loss of wetlands. As depicted in Figure 2-3, Concept 2 would restore

approximately 2.4 acres of wetlands in bottom lands of the original meander scar and widen the

meander scar to accommodate an additional 0.6 acres of compensatory wetlands. The integrated

wetland system in the widened meander scar would encompass approximately 2.4 acres of

Palustrine emergent (PEM) wetlands, 0.4 acres of Palustrine scrub-shrub (PSS) wetlands, and

0.2 acres of Palustrine open water (POW) wetlands. As under Concept 1, the PEM wetlands

would be dominated by persistent vegetation that retains a yearlong presence (PEM1 wetlands).

Noninvasive, nonpersistent aquatic vegetation would be established as possible in this open

water, concerting all or part of it to a Palustrine emergent nonpersistent (PEM2) wetland.

Figure 2-3 illustrates only one of several potential ways to create compensatory wetlands to

offset the permanent loss of part of the meander scar. Other possibilities, which would likewise

be consistent with the U.S. EPA's policy of no net loss of wetlands, are discussed further in

Chapter 4.

Wetland Mitigation Concepts Not Considered Further

The concept of converting the entire meander scar to uplands following its remediation and

offsetting this loss by creation of compensatory wetlands was rejected because of the relative

ease of restoring the South Wetland and Lagoons 2 through 4, the higher cost, and the potential

for exceeding the availability of suitable land on the NHPC Site for wetland creation. The

concept of offsetting the losses of the South Wetland and Lagoon 1 through offsite (rather than

W94613D 2-8

DRAFT

onsite) wetland creation was rejected because it would require land acquisition and because it

would not meet the goal of restoring wetland functions as close as possible to their original

location.

The concept of creating a large pond throughout the meander scar was rejected because it would

not provide habitat for a high diversity of terrestrial wildlife, because it would not restore any

wetland habitat similar to that which originally occurred on the NHPC Site, and because it may

not be hydrologically possible. While a large area of open water, Horseshoe Pond, exists

roughly 1,000 feet south of the meander scar, large areas of marsh do not.

Creating a large, deep pond would be the only way to create suitable habitat for sportfish species

such as bass. But because the pond would be fed principally by runoff, its depth would fluctuate

substantially, threatening the fish with periodically anoxic conditions. At least 10 feet excavation

would likely be necessary to create a groundwater-fed pond. It would also be difficult to

stabilize the resulting steep side slopes. Furthermore, Horseshoe Pond, a large pond occupying

another former meander channel of the Merrimack River, already exists less than 1,000 feet

away.

The concept of creating only upland habitat within the meander scar was rejected because it

would conflict with the U.S. EPA's policy of no net loss of wetlands. However, this concept

would be substantially lower in cost than any other approach to habitat creation and would still

provide important habitat for certain wildlife species. Allowing upland old field vegetation to

establish in the meander scar would provide valuable habitat for forest edge dwelling wildlife.

This vegetation would become gradually replaced by upland forest through natural succession,

a process which could be hastened by planting tree and shrub seedlings. The existing forest on

the side slopes and regenerated forest in the meander scar bottom would provide increasingly

scarce habitat for forest-interior dwelling birds such as vireos, warblers, and thrushes.

W94613D 2-9

DRAFT

3.0 RESTORATION OF WETLANDS IN THE MEANDER SCAR

Restoration of wetland habitats in the bottom of the meander scar is a key component of both

Wetland Mitigation Concepts for the NHPC Site introduced in Chapter 2. Under Concept 1,

a continuous wetland would be restored throughout the original bottom area of the meander scar.

Under Concept 2, a continuous wetland would be restored in the original bottom area of all parts

of the meander scar except for the South Wetland and Lagoon 1 Area.

3.1 Design Objectives - Functions and Values

Under either Concept 1 or Concept 2, the restored wetlands would be designed primarily to

maximize their value as wildlife habitat. Because of the relatively small size and narrow width

of the meander scar, it will not be possible to create a large, uniform patch of a single type of

habitat (such as habitat for forest interior dwelling birds or for large sportfish). Instead, the

design will focus upon providing a mosaic of small areas of diverse habitat types with gradual

transitions in vegetation and hydrology (Figures 2-1 and 2-3). The transition from upland forest

(on the side slopes) to marsh (throughout most of the bottom area) will include a narrow fringing

zone of wetland shrubs. The transition from permanently dry land to inundated lands will

include a zone of seasonally saturated land. A similar pattern of zonation likely occurred in the

meander scar prior to lagoon excavation, although the NWI designation of the entire meander

scar as Palustrine scrub-shrub may indicate that marsh (Palustrine emergent) or open water

habitats never existed.

As indicated in Section 1.3, wetlands in the meander scar may be playing a key role in trapping

and sequestering heavy metals and other contaminants from the NHPC Site, reducing the

contamination of underlying groundwater. Although the importance of this role may become

reduced following remediation of the site, complete elimination of all heavy metals and other

contaminants from the site may not be possible. Therefore, the restored wetlands will be nearly

W94613D 3-1

DRAFT

completely vegetated and will include deep deposits of soil high in organic matter. The

vegetation will serve to adsorb contaminants and to assimilate contaminants into their tissues.

More importantly, the organic matter in the soil will serve to permanently bind contaminant

ions. Organic matter is better capable of binding contaminants than most mineral soil colloids.

As indicated in Section 1.3, recreation and aesthetics will be a secondary consideration in the

restoration process. Establishment of a diversity of habitat types and maximal preservation

mature forest on the side slopes and surrounding uplands should result in an aesthetically

pleasing natural landscape. No modifications will be made solely for aesthetic purposes.

Depending upon the extent of remediation called for in the Feasibility Study, it may be possible

to open the meander scar to the public and allow the local community to construct trails and/or

boardwalks through the meander scar and surrounding uplands for hiking, birdwatching, and

public education. Narrow boardwalks constructed at a height just above seasonal high water

should be constructed instead of trails across wetland areas. It will not likely be possible to

establish sportfish populations in the shallow waters created in the meander scar. Hunting will

not be possible due to the urban surroundings.

As indicated in Section 1.3, wetlands in the meander scar may have (and may presently)

contribute to recharging the underlying aquifer and to reducing flood volumes in the Merrimack

River. Neither will be a primary consideration in restoring wetlands in the meander scar. It

is possible that re-excavation of the South Wetland and Lagoon 1 Area (Concept 1) or excavation

of uplands (Concept 2) may slightly increase the capacity for these functions.

3.2 Design Objectives - Hydrology

Available evidence suggests that existing (and historic) wetland conditions in the meander scar

are primarily the result of surface runoff. Even though soils in the meander scar were mapped

in the Rippowam soil series, considered to have a seasonal high water table (SCS, 1981),

W94613D 3-2

DRAFT

groundwater does not appear to be a significant factor in the hydrology of the meander scar.

Recent hydrological data collected in March 1993 indicated that the water table was between 5

and 15 feet below the bottom surface of the meander scar (Goehlert, 1993).

Conceptual finished grading schemes are shown for Concept 1 and Concept 2 in Figures 2-2 and

2-4. In both, wetland hydrology involving at least seasonal saturation would extend up to an

elevation of 112 feet above mean sea level (msl) on the meander scar side slopes. The restored

wetlands would have to continue to rely on surface runoff intercepted by the meander scar for

their hydrology. Their hydrology will resemble that of stormwater detention basins, with

extended dry periods separated by episodes of deep inundation possible following heavy rainfall

events. Parts of the meander scar bottom will remain shallowly inundated throughout wet

seasons such as winter and early spring, becoming saturated or dry in dry seasons such as

summer and early fall. Parts may remain continuously inundated during unusually wet years or

continuously dry during unusually dry years.

Adding to these natural sources of hydrologic variability, the hydrology of wetlands in the

meander scar will be subject to land use changes in the surrounding upland watershed, including

adjacent privately-owned lands. Additional paving and deforestation could rapidly increase

runoff volumes reaching the meander scar, while installation of offsite stormwater management

facilities could deflect expected runoff inputs to other waterways such as Horseshoe Pond or the

Merrimack River.

Revegetation may also influence hydrology. Replacement of the existing sparse vegetation in

the meander scar with dense vegetation could result in more rapid loss of standing water by

transpiration. Cattails and waterhyacinth have been reported to transpire two or three times the

amount of water evaporated by open water. Nevertheless, considerable disagreement exists as

to the relative rates of evaporation from open water versus transpiration by a cover of wetland

vegetation (Adamus et aL, 1991).

W94613D 3-3

DRAFT

Both Concept 1 and Concept 2 call for an area of open water near the former location of Lagoon

2 and seasonally saturated soils throughout the rest of the meander scar. The anticipated extent

of open water (most of the year) would roughly correspond to an elevation of 108 feet msl. This

elevation roughly corresponds to the elevations of open water observed in an April 1993 aerial

photograph. The actual area of open water that may result is difficult to predict and could differ

substantially from that shown in the concept drawings. Topographic changes, land use changes,

substrate changes, and other design elements selected following review of the Feasibility Study

could all greatly influence the ultimate hydrology of the meander scar.

The conceptual grading schemes (Figures 2-2 and 2-4) also call for filling part of the area below

the 108-foot contour. This would help distribute shallow quantities of water over a larger part

of the meander scar during dry periods rather than allowing limited runoff quantities to all

collect in deepwater pools.

The installation of clay, bentonite, or other impervious liners in the meander scar to artificially

create wetter hydrology is not recommended. Recent aerial photographs, ground photographs,

and anecdotal observations over the last several years suggest that extended periods of shallow

inundation and soil saturation already occur without the need for these features (Palermo, 1994).

Addition of a liner could result in the accumulation of deep water that kills vegetation and only

slowly dissipates through evaporation.

If the Feasibility Study supports a decision to cover rather than remove contaminated soils from

the meander scar, it would be necessary to install an impervious substrate beneath the wetlands.

The final elevations shown in Figures 2-2 and 2-4 for the wetlands would likely have to be

upshifted by 2 feet. The anticipated upper elevation of wetlands in the meander scar would then

be approximately 114 feet msl rather than 112 feet msl. Under Concept 1, a berm would have

to be created at the south end of the meander scar to prevent saturation or inundation of lands

off of the NHPC Site.

W94613D 3-4

3.3

DRAFT

Because the contaminated soils would have to be covered with an impermeable cap, the

overlying wetlands could then be subject to much longer periods of much deeper inundation,

increasing the risk that wetland vegetation will be killed by flooding. It may be possible to

install an overflow spillway or other mechanism to carry away excess runoff. This could be

necessary to establish anything other than open water in the meander scar. However, offsite

property would have to be acquired in order to route a spillway to the Merrimack River,

Horseshoe Pond, or other suitable receiving waterway. Costly deep excavation could also be

necessary.

Design Objectives - Soils

If contaminated soils will be removed from the meander scar, both Concept 1and Concept 2

would recommend approximate simulation of the original Rippowam soil series originally present

in the meander scar. Following excavation of contaminated soils, clean fill could be used to

raise the soil elevation to within 12 inches of the desired final elevations shown in Figure 2-2

or 2-4. Although the availability of clean fill may dictate the actual soil texture used, a loamy

sand or sandy loam would best replicate the original Rippowam soils. Actual soil borings taken

in the meander scar in 1993 indicate that the upper 10 feet of soil comprises mostly fine sandy

loam (Halliburton NUS, 1993). Clay or clay loam fills would be best avoided since they would

diminish the groundwater recharge functions of the meander scar and could cause wetland

vegetation to be drowned.

The upper 12 inches could then be Filled with topsoil to create a planting bed at the desired final

grade. Topsoil should be purchased from nurseries or other commercial sources, not excavated

from other intact wetlands. Use of stockpiled topsoil from remnant wetlands presently in the

meander scar would not be possible due to the contamination. Although the use of topsoil in

wetland restorations is no longer recommended by some experts (Garbische, 1993), the high

organic matter of topsoil would be necessary for the restored meander scar to continue to be

W94613D 3-5

DRAFT

capable of sequestering heavy metals and other contaminants. Although not essential, the use

of topsoil would also facilitate the rapid establishment of wetland vegetation.

Commercial topsoil should be of adequate fertility and suitable pH to support the establishment

of wetland vegetation (Hammer, 1992). Amendment with chemical fertilizers or lime is not

recommended. Water soluble fertilizers could result in undesirable algal growth (eutrophication)

of standing water in the meander scar. The anticipated hydrological variability should ensure

that all or most soils in the meander scar experience episodes of dryness where oxidizing

conditions release nutrients immobilized during prolonged wet periods.

If the Feasibility Study supports a decision to cover rather than remove contaminated soils from

the meander scar, it would not be possible to simulate the original Rippowam soils. Instead a

12-inch planting bed would have to be created using topsoil placed over the cover. Many natural

wetlands do comprise shallow layers of topsoil over clay hardpans or bedrock, but establishment

of vegetation in such substrates can be difficult. As indicated in Section 3.2 for hydrology,

installation of a mechanism to carry excess standing water out of the meander scar would be

essential.

3.4 Design Objectives - Vegetation

Because of the uncertain hydrology that would result from restoration of the meander scar,

revegetation efforts under either Concept 1 or Concept 2 would best follow a two-phased

approach. First, the exposed soils in the meander scar would be stabilized using an inexpensive

seed mix of wetland grasses, sedges, and/or rushes. Wetland stabilization mixes are readily

available from a number of wetland nurseries. Only mixes specifically produced for use in the

New England area would be considered. The resulting hydrology would then be monitored over

a 2-year period to determine the actual extent of various hydrological conditions over the

W94613D 3-6

DRAFT

meander scar. A follow-up planting effort would then be undertaken to establish the desired mix

of plant communities as conceptualized in Figures 2-1 and 2-3.

The spatial arrangement of plant communities shown in Figures 2-1 and 2-3 is strictly

conceptual. A conceptual section view of the proposed plant communities in the meander scar

is provided in Figure 3-1. The actual planting arrangement would have to reflect hydrological

observations made over the 2-year period following site remediation. Tables 3-1, 3-2, 3-3, and

3-4 show illustrative planting schedules for permanently inundated water areas, permanently

saturated/seasonally inundated areas, seasonally saturated/rarely inundated areas, and a

seasonally saturated shrub-sapling fringe area. Table 3-1 would correspond to areas shown on

Figures 2-1 and 2-3 as POW, Tables 3-2 and 3-3 would correspond to areas shown as PEM, and

Table 3-4 would correspond to areas shown as PSS. Many suitable combinations of species are

possible for each type of hydrology, and the tables are provided as suggestive guidance only.

The species shown in the tables were selected based on their wildlife habitat value, suitability

for the corresponding hydrological regime, availability from commercial wetland nurseries

serving New England, tolerance of seasonal hydrological fluctuations, and recorded or potential

occurrence on the NHPC Site or surrounding area. Information as to the hydrological tolerances

of plant species was obtained using information in the Wetland Planting Guide for the

Northeastern United States (Thunhorst, 1993), not inferred from the wetland indicator statuses

of the plants. The indicator statuses are not necessarily an actual reflection of the hydrological

requirements for establishing plant material.

No invasive species such as purple loosestrife (Lythrum salicaria), reed canarygrass (Phalaris

arundinaced), phragmites (Phragmites australis), and cattail (Typha sp.) would be recommended.

Purple loosestrife, reed canarygrass, and cattail either occur presently on the site or occurred

prior to the 1992 removal action. These species would likely establish as volunteers, and could

even crowd out the planted species. If this happens, attempts at eradication using herbicides or

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mechanical methods would not be taken. Other wetland species would likely replace the

invasive species over time by successional processes as the restored wetland ecosystem matures.

The value of the wetlands for wildlife habitat will be greatly enhanced by the presence of upland

forested habitat on the side slopes of the meander scar and on the surrounding area. Disturbance

of existing upland forest (or shrub and sapling) vegetation when performing remedial work or

other grading activities in these areas should be minimized. A few areas (designated as upland

reforestation areas in Figures 2-1 and 2-3) would be revegetated with upland trees and shrubs.

An illustrative planting schedule is shown in Table 3-5.

W94613D 3-13

DRAFT

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W94613D 3-14

DRAFT

4.0 CREATION OF ONSITE COMPENSATORY WETLANDS

(OPTION 2 ONLY)

Under Concept 2, the South Wetland and Lagoon 1 Area would be capped to create uplands

following remediation, and a compensatory area of new wetlands would be created elsewhere

on the NHPC Site. Figures 2-3 and 2-4 illustrate the most likely scenario for onsite creation of

compensatory wetlands: excavation of the gently sloping uplands on the east-central side of the

meander scar, effectively widening its bottom area to accommodate additional wetlands. If the

wetlands are created contiguous to the meander scar, as exemplified in Figures 2-3 and 2-4,

these considerations will be similar to those for restoring wetlands within the original area of

the meander scar.

4.1 Alternative Onsite Wetland Creation Scenarios

There are two basic alternative approaches to creating wetlands on the NHPC Site: (1) widening

the meander scar to accommodate a larger area of wetlands, and (2) excavating an isolated

depression elsewhere on the NHPC Site, away from the meander scar. Possible wetland creation

scenarios based on each of these approaches are addressed below.

Lengthening the Meander Scar

The best way to create wetlands similar to the restored wetlands would be to elongate the

meander scar. The created wetlands could be of similar width as the restored wetlands and the

adjoining side slopes could be of similar grade. The hydrological conditions which historically

and presently maintain wetlands within the meander scar would also influence the created

wetlands. However, space is not available to extend the meander scar either to the north or the

south. The north end of the meander scar presently abuts the site boundary and the Boston and

W94613D 4-1

DRAFT

Maine Railroad tracks, and the south end presently abuts Lagoon 1, which would be capped to

create an upland under Concept 1.

Widening the Meander Scar

The other way to create wetlands similar to the restored wetlands would be to excavate back one

of the side slopes of the meander scar, widening the bottom area to accommodate more

wetlands. Although the resulting wetlands would be substantially wider than those which

originally existed in the meander scar, the hydrological conditions which historically and

presently maintain wetlands within the meander scar would also influence the created wetlands.

The central part of the meander scar (Lagoons 3 and 4 Area) could be widened to create roughly

0.6 acres of additional wetland without having to encroach on offsite lands. Because the east

slope is gentler than the west slope, the meander scar could be widened to the east with less

excavation (and therefore at lower cost) than to the west. Furthermore, the reconstructed side

slope east of the created wetlands could be gentler than that constructed if the meander scar were

widened to the west. Gradual slopes are preferred over steep slopes for the edges of created

wetlands (Kentula, 1993). Gradual slopes can be more readily stabilized and revegetated than

a steeper slope.

The northern part of the meander scar can not be widened without having to encroach on

industrially-developed lands adjoining the site.

The chief disadvantage to wetland creation by widening the meander scar is the need to clearcut

mature upland forest prior to the necessary excavation. The reconstructed side slope adjoining

the created wetlands would be immediately reforested with indigenous upland trees and shrubs,

but a restored upland forest could require more than 50 years of growth before it assumes the

appearance of the existing forest.

W94613D 4-2

4.2

DRAFT

Creating Isolated Wetlands

An isolated area of wetlands could be created on part of the NHPC Site outside of the meander

scar. Potential locations could include the former NHPC building and operations area facing

Wright Avenue, lands west of the meander scar (the former pugmill area), and lands east of the

meander scar. Regardless of location, these created wetlands would be different in character to

the original wetlands in the meander scar. Even worse, the likelihood for failure would be

greater, since the isolated excavation may not intercept adequate surface runoff to support

wetland habitat.

Of the potential wetland creation locations, the preferred location would be the former NHPC

building and operations area. No mature forest would have to be cleared, and the disturbed soils

could be regraded to create a naturalistic east-facing slope directing surface runoff to the created

wetlands.

Design Objectives

If the created wetlands are constructed by widening the central part of the meander scar, as

shown in Figures 2-3 and 2-4, then the design objectives would be as described for the restored

wetlands in Section 3.0.

If the created wetlands are constructed elsewhere on the NHPC Site, the design objectives would

be formulated once a location is selected. The principal functional objective of the created

wetlands would always be habitat for terrestrial wildlife. A small area of isolated wetlands

would not be capable of substantially contributing to groundwater recharge or floodflow

abatement. Establishing wetland hydrology in a small depression with little watershed would

be very difficult, if not impossible, and would almost definitely require the installation of an

W94613D 4-3

DRAFT

impervious liner. Any contaminants received by the wetlands would therefore be prevented from

entering the groundwater, whether or not adsorbed to vegetation or organic matter.

Any wetlands created in a small depression with a small watershed would likely be only

seasonally saturated, although they could be subject to episodes of deep inundation following

heavy rainfalls. The illustrative planting schedule outlined in Table 3-2 would likely be the best

option.

W94613D 4-4

5.0

DRAFT

REFERENCES

Adamus, P.A., L.T. Stockwell, E.J. Clairain, Jr., M.E. Morrow, L.P. Rozas, and R.D. Smith.

1991. Wetland Evaluation Technique (WET); Volume I: Literature Review and Evaluation

Rationale. Technical Report WRP-DE-2, US Army Engineer Waterways Experiment Station,

Vicksburg, MS.

FIA (Federal Insurance Administration). 1979. Flood Insurance Rate Map, Town of

Merrimack, Hillsborough County, New Hampshire. Panel 5 of 10. Community-Panel Number

330095 0005 A. Effective Date: July 16, 1979.

Garbisch, E. 1993. The Need to Topsoil with Mineral Loam Soils in Planned Wetland

Projects.

Goehlert, R. 1993. Written correspondence dated March 8, 1993 from R. Goehlert of U.S.

EPA Region 1 to J. Chaconas of Halliburton NUS Corporation.

Halliburton NUS. 1993a. Memorandum dated March 29, 1993 from Jim Chaconas of

Halliburton NUS Corporation to Richard Goehlert and Susan Svirski of EPA Region 1.

Halliburton NUS. 1993b. New Hampshire Plating Company, XRF Analytical Results - Soil

Samples. Unpublished data collected in support of Remedial Investigation for NHPC Site.

Interagency Committee on Wetland Restoration and Creation, 1992. A National Program for

Wetlands Restoration and Creation. Report of the Interagency Committee on Wetland

Restoration and Creation. August 1992.

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DRAFT

Kentula, M.E. 1993. Status of Restoration Science: Wetlands Ecosystems. In: Proceedings

of Symposium on Ecological Restoration, Chicago, IL, March 1993. U.S. EPA, ERL-Corvnslis,

OR. EPA/600/A-93/231.

Kruczynski, W.L. 1990. Options to be Considered in Preparation and Evaluation of Mitigation

Plans. In "Wetland Creation and Restoration - The Status of the Science" by J.A. Kusler and

M.E. Kentula. Island Press, Washington, D.C.

Palermo, R. 1994. Telephone conversation dated August 5, 1994 between R. Palermo of

Badger Engineers, Inc. and Peyton Doub of Halliburton NUS Corporation.

SCS (U.S. Department of Agriculture Soil Conservation Service). 1981. Soil Survey of

Hillsborough County, New Hampshire - Eastern Part. October 1981.

U.S. EPA (U.S. Environmental Protection Agency). 1990a. Wetland Investigation - New

Hampshire Plating, Merrimack, New Hampshire. EPA Work Assignment Number 2-345.

U.S. EPA (U.S. Environmental Protection Agency). 1990b. New Hampshire Plating Site,

Ecological Assessment Final Report. U.S. EPA Environmental Response Branch/Roy F.

Weston, Inc. - REAC.

U.S. FWS (U.S. Fish and Wildlife Service). 1994. Ecological Characterization of New

Hampshire Metal Plating, Merrimack, NH.

W94613D 5-2