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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
W94613D 3-7
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DRAFT
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.
W94613D 5-1
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