The Crandon Mine Experience

Transcription

1 Determining Indirect Impacts to Wetland Plant Communities resulting from Mine-induced changes to Groundwater Hydrology The Crandon Mine Experience Jim Arndt, Ph.D., Westwood Professional Services John Almendinger, Ph.D., Mn Dept. Natural Resources History: 7 years Scoping, 20 years data collection Exxon, Crandon Mining Company, Nicollet Minerals COE Lead Agency, EPA, WiDNR, Tribes, Public Interveners

2 National Environmental Policy Act Promote informed federal agency decision-making by ensuring that detailed information concerning significant environmental impacts is available to both agency leaders and the public. An EIS is the most detailed NEPA instrument that is reserved for projects that have a substantial potential for adverse environmental impact. Lead and reviewing agencies identify and evaluate the likelihood and magnitude of significant impacts. If potential impacts cannot be identified or their magnitude determined, informed decisions regarding the nature of the impacts, and the potential for avoidance, minimization, and mitigation cannot be made

3 Mine Facilities Mine Infrastructure Facilities (119 acres) Access Roads Power Transmission Pipelines Tailings Management Area (292 acres) Soil Absorption System (SAS) Surface Water Supplementation Rail lines Proposed Action: 3 years construction, 28 years mining, 4 year reclamation

4 Wetland Resources Analysis Plan: Issues Direct and indirect impacts on wetland hydrology, plants, soils, and functions Direct fill impacts Indirect impacts on wetlands and wetland plant communities from mine de-watering, supplementation well(s), and SAS operation, reclamation and recovery of watertables.

5 Significance Criteria (Easy): Hydrology, Vegetation, Soils Wetland hydrology is removed (WT not > 12 inches for greater than 5% of the growing season. Wetland vegetation removed. Hydrophytes less than 50 percent (aerial coverage) using vegetation assessments stipulated in the 1987 Wetlands Delineation Manual. Wetland soils removed. Would no longer have the moisture regime required of hydric soils Result in change in wetland jurisdictional status

6 Significance Criteria (Hard): Wetland Functions Percentage reduction or increase in raw scores of functional assessments performed on wetlands that could be affected by the project Percentage to be determined from distribution of wetland functions in individual wetlands and throughout the impact area of influence Indirect impact, losses and gains in functions

7 Impact Area of Influence Most conservative estimate of 0.25-foot contour of estimated steady state water table drawdown with mine de- watering and supplementation well using USACE model Most conservative estimate of 0.25-foot contour of estimated steady state water table rise in the SAS area using USACE model

8 Methodology: Define Baseline Conditions Existing wetland delineations, wetland hydrogeologic settings, and wetland function assessments Baseline conditions assumed to represent No Action conditions Baseline data entered into a database for GIS analysis

9 Impacts on Wetland Plant Communities Baseline Plant Community Data 46 detailed semi-quantitative samples (PEC) 9 Detailed Species Lists (Foth & Van Dyke) 15 quantitative transects (Normandeau) 158 Functions Assessments (Normandeau) ~ 20 wetland delineations ~100 photo-interpretations to class

10 Site Characteristics Sub-glacially molded till Lots of outwash, complex stratigraphy Underlying sandy unit (layer 2) of concern Wetland at varying elevations Wetlands of varying characteristics Three recharge types with respect to interactions with layer 2 Five condition classes with respect to inlets and outlets, surface water interactions.

11 PEC Functions Database (Demo, Output)

12 Hydrology Determine impacts of groundwater drawdown and mounding on wetland hydrology Compare baseline hydrologic regimes to steady state cone of depression or groundwater mound from USACE groundwater model (controversial) Impact assessment performed in a GIS

13 Groundwater Drawdown Modeling (per EarthTec)

14 Step 1: Assignment of Wetlands to NPC Classes: Reclassification Compare existing plant communities to those adapted to conditions under the Proposed Action Existing plant data from ecological plot studies in Minnesota and Wisconsin were used to rank native plant species and the Crandon wetlands by affinity for specific hydrologic regimes Ordination Example

15 Minnesota Native Plant Classification System WFn53a WFn53a Northern Wet Cedar Forest Class Code WFn53a Ecological System Wet Forest Floristic region Northern Moisture (0 driest 9 wettest) Nutrients (0 poorest-9 richest) Type (a driest/poorest in class) Other Similar Classes FPn63 Northern Cedar Swamp, wetter, peat dominated, different understory plants. WFn64 Very Wet Ash Swamp, more ash, wetter, richer.

16 Portion of Class Description

17 Fieldwork: Crandon Wetlands to Mn NPC Classes

18 PEC Project Field Database (Demo, Output)

19 Air Photo Interpretation and Field Verification Used to Place Crandon Wetlands In Minnesota s NPC Classification

20 Step 2: Ordination (DCA Decorana) Non-metric, multi-dimensional ecological statistics. Similar samples are near each other and dissimilar objects are farther from each other. These relationships are then characterized numerically and/or graphically. John Almendinger (MnDNR) very familiar with DCA ordination and used extensive dataset developed on 1,079 (now over 2000) releve plots in Minnesota to develop classifications on wetlands for which we had detailed Crandon Data MnDNR data with functional understanding of how the vegetation relates to water-table, nutrient status, substrate, and hydrology. Representative wetlands in the Crandon Mine Project Data accumulated, plant community numbers/distribution, soils, hydrology, classification Placed in a relational database

21 MnDNR Data with Wetlands in Crandon Area

22 Ordination FAQs

23 Step 3: Assessment of Indirect Impacts to Plant Communities Estimate new plant community composition assuming the rate of the change agent (e.g., groundwater drawdown) would result in converting one adapted native plant community to another

24 Watertable Data for Wetland Classes in the Crandon Area

25 Hydrologic Impact Sensitivity Groups None-to-Slight HSIG. Monitoring or mitigation is not anticipated. Sufficient habitat variability exists for most plants to persist within a wetland basin subject to falls or rises in WT position and variance. Changes in species abundance may occur, but significant loss of species or invasions of weedy speciescapable of dominating the wetland are unlikely. Moderate HISG. Monitoring is recommended, and mitigation would depend upon the monitoring results. Turnover in species (losses and invasions) or changes in physical site properties are likely to cause slow recovery to the community s initial state. Permanent changes in site properties or the invasion of ecologically dominant plants are possible. Wetlands sensitive to modest changes in mean WT position or altered variance were placed in this category when these modest changes were small compared to the predicted accuracy of the groundwater model. Severe HSIG. Mitigation likely required. Class-level, System-level, or irreversible changes in the vegetation are expected, even after restoration of the groundwater regime. Permanent alteration of physical site properties, high turnover in species and invasion of ecologically dominant plants expected. Not applicable to recharge or supplemented wetlands.

26 Example: WFn53, Northern Wet Cedar Forest Vegetation: Patchy to interrupted canopy (25-75% cover) dominated by white cedar. Black ash may also be abundant. The groundlayer is herbdominated and very rich, with % cover in a typical stand. Soils: Layer of well-decomposed organic material cm deep over mineral soil. Underlying mineral soil can be of almost any texture, but medium and fine textures are common. Hydrology: wet throughout most of the growing season, but dry sufficiently often to prevent further accumulation of peat. Frequently in areas of groundwater discharge evident as seepage areas, spring runs, or cold-water streams. (Mean WL 2.2 feet +/- 2.4 feet)

27 Example: WFn53, Northern Wet Cedar Forest Sensitivity to Groundwater Drawdown None-to-Slight HSIG; < 2.0 foot drawdown; remains in WFn53 Class has sufficient habitat variability for most plants to persist within a wetland basin subject to falls in WT position under 56cm (~2 feet). Moderate HISG; 2-4 foot drawdown, conversion to Northern Wet Ash Swamp (WFn55) WFn55 has similar variance in WT compared to Northern Very Wet Cedar Forest but the mean WT position is significantly deeper. Severe HSIG; > 4 foot drawdown, conversion to Wet-Mesic Hardwood Forest The mean WT position at top of gray (gleyed) horizons at about 150cm with bright iron-rich mottles, which indicate soil aeration, were used to estimate the variance in WT position as about 130cm. This would be sufficient to convert most very wet cedar forests to a community that is essentially terrestrial.

28 Demonstration: Appendix B

29 Step 4: Summarize Data: Hydrologic Impact Sensitivity Classes for Discharge Wetlands

30 Hydrologic Impact Sensitivity Classes for Recharge Type 3 Wetlands

31 Hydrologic Impact Sensitivity Classes for Soil Absorption System Wetlands (Groundwater Rise)

32 Suggestions, Issues Groundwater hydrology: variability, scale, accuracy Function and value assessments: limited categories, resolution, not available for all wetlands Use accepted model, incorporate recent literature, new data, airphoto interpretation, interpolation Plant community assessments: Need for quantitative data Detailed assessment of representative wetlands Monitoring plots