Numerical Groundwater Flow Model Report. Caloosa Materials, LLC 3323 Gulf City Road Ruskin, Florida 33570

Transcription

1 Numerical Groundwater Flow Model Report Caloosa Materials, LLC 3323 Gulf City Road Ruskin, Florida GHD 2675 Winkler Ave Suite 180 Fort Myers, FL Report July 27, 2017

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3 Table of Contents 1. Introduction Conceptual Model Model Description Model Simulations and Results Conclusions and Recommendations.4 Figure Index Figure 1 Figure 2 Figure 3 Stress Period 2 Hydraulic Heads Pre-development/Mining Conditions Stress Period 3 Hydraulic Heads Stage 1 Excavations active after 180 days of dewatering Stress Period 4 Hydraulic Heads Stage 2 Excavations active after 365 days of dewatering Appendix Index Appendix A Appendix B Groundwater Vistas Model Files Mass Balance/Hydraulic Budget GHD Caloosa Groundwater Flow Model Report (x) Page i

4 1. Introduction Groundwater flow modeling was conducted using DWRM3 (District-Wide Regulation Model) because it accurately represents the aquifers underlying the site including the surficial aquifer, Intermediate aquifer, and Upper Floridan aquifer (UFA); therefore, this model should adequately simulate the proposed mining operation s hydraulic system. DWRM3 is a collaborative effort between James Rumbaugh of Environmental Simulations, Inc. and the Southwest Florida Water Management District (SWFWMD) using the Groundwater Vistas (Environmental Simulations, Inc., 2013, 2009, 2007, and 2004) graphical user interface. This version of the District s regional model includes six layers, one layer which represents the surficial aquifer system, two layers representing flow zones within the Intermediate aquifer (Southwest Florida Water Management District, 2009), two layers representing the UFA, and one layer representing the Lower Floridan aquifer. For the purpose of this study, which is to determine the potential effects of dewatering associated with the operation of the mine, this model will generally require minor revisions to obtain the necessary results. In addition, this model includes the Telescopic Mesh Refinement (TMR) feature that enables more detailed representation and analyses of local hydraulic features with a smaller userdefined grid, within the regional coarse-grid DWRM3 model. 2. Conceptual Model The conceptual model for this model can be found in the reports describing the development of DWRM, DWRM2, and DWRM3 available through the SWFWMD (Environmental Simulations, Inc., 2013, 2009, 2007, and 2004). The conceptual model for this site operation is that drawdown from the well points used to dewater the excavation cells creates drawdown in the surficial aquifer that could potentially cause significant drawdown in the general vicinity of the excavation affecting nearby wetlands, if not abated. The design of the mine excavation includes a recharge trench around the perimeter of the excavation that will be used to maintain the water level, i.e. a constant head, in the trench at levels that will mitigate drawdown. The hydraulic conductivity and the aquifer thickness (transmissivity) of the surficial aquifer strongly control the potential magnitude of the drawdown caused by dewatering. The surficial aquifer composed of sand and shells is underlain by the low permeability sediments of the Intermediate Aquifer System, which makes up part of the upper confining unit in this area. The Floridan aquifer underlies the site and is hydraulically separated from the surficial aquifer by the upper confining unit. Impacts from the proposed dewatering should be restricted to the surficial aquifer. Due to the proximity of the site to Tampa Bay, the water quality of the surficial aquifer underlying most of the site is generally brackish, with the exception of the eastern part, which exhibits salinities consistent with drinking water standards. Total dissolved solids (TDS) determined from measurements of salinity, range from slightly greater than 500 mg/l to approximately 77,000 mg/l in the western deeper part of the aquifer. Seawater has a TDS concentration of approximately 35,000 mg/l; therefore, the deep groundwater is considered hypersaline. These conditions have been documented to occur in association with coastal wetlands and are attributed to storm over wash and high evaporation rates resulting in highly concentrated brine solutions. GHD Caloosa Groundwater Flow Model Report (x) Page 1

5 3. Model Description The DWRM3 model used for this study required minor modifications to simulate steady state premining conditions and transient mining conditions. Boundary conditions for the TMR model are automatically assigned by DWRM3 based on the calibrated regional flow system. The TMR model has dimensions of 20,000 feet by 20,000 feet (ft.) with 243 rows and 279 columns, creating a grid with 406,782 cells. These model dimensions are sufficient to allow for the potential distance that drawdown will occur from the mine dewatering operations, without impacting the model boundaries. The row spacing range from 12.5 to 242 ft. and the column spacing range from to 225 ft. This expanding grid was used to enable more detailed representation of hydraulic features within the immediate area of the mining operation. Although the grid outside of the proposed mining area is coarser, hydraulic features can still be adequately represented. The same hydraulic features and assigned parameters used in DWRM3 were used in the TMR model; however, in order to model the local flow system additional features such as wetlands and shallow ditches were also represented. The shallow ditch network that exists in the immediate vicinity of the site was designed to ensure that groundwater elevations would not rise sufficiently to inundate the crops under cultivation. These diches typically reduce groundwater elevations and overflow towards Cockroach Bay. Overflow is controlled by the invert elevations and these were used to identify the maximum stage elevations associated with the modeled drains. These invert elevations are presented on the permit application CAD drawings as upgradient invert elevations. Therefore, the regional DWRM3 model simulated steady state pre-mining heads would typically be higher in the vicinity of the site, compared to this TMR model. Although, the DWRM3 model representation of the surficial aquifer thickness in the general vicinity of the site is generally similar to the observed from soil borings installed for site characterization, the observed thickness was slightly greater. Therefore, the surficial aquifer thickness in the immediate vicinity of the excavation footprint was increased by adjusting the bottom elevation of layer 1 and the top elevation of layer 2 from -21 ft. to -30 ft. NAVD. In DWRM3, the hydraulic conductivity of the surficial aquifer in the area of the TMR model is assigned a value of 20 ft. /day. For this modeling effort, the hydraulic conductivity of the proposed excavation area was estimated by conducting slug tests at the four initial monitor well clusters installed at the site. The average hydraulic conductivities for the shallow and deep zones were 11.3 and 3.3 ft. /day, respectively, for an equivalent or weighted mean horizontal hydraulic conductivity of 7.5 ft. /day. This value is approximately half of the calibrated value used in DWRM3. Since slug tests represent a small volume of the aquifer, the calibrated value may still be representative of the site. All hydraulic parameters used in DWRM3 are preserved in the TMR model. DWRM3 was constructed using MODFLOW 2005; therefore the TMR model utilized the same code. The TMR model is set up with the two initial steady state simulations that are set up in DWRM3, with stress period 2 representing the pre-development (excavation) conditions. Stress periods 3 and 4 are transient simulations with durations of six months and one year, respectively. The dewatering effects for each stage were represented with the constant head package, whereby the maximum anticipated drawdown during each stage was represented as a constant head of -30 ft. NAVD. The elevation of groundwater in the cell created during stage 1 was maintained at -7 ft. NAVD during the dewatering and excavation of stage 2. During the dewatering and excavation of GHD Caloosa Groundwater Flow Model Report (x) Page 2

6 stage 1 and stage 2, a recharge trench was modelled with stage elevations between 2 and 3 feet NAVD. The matrix of finite difference equations was solved using the Preconditioned Conjugate Gradient Non-linear (PCGN) solver. Model modifications to DWRM3 for the TMR model are included in the Groundwater Vistas (GWV) file provided in Appendix A. 4. Model Simulations and Results The model simulation results include simulated hydraulic heads representing pre-development or mining conditions (Figure 1), post-stage 1 (Figure 2) and, post stage 2 (Figure 3) operational mining periods. The simulated predevelopment heads across the site range from approximately one foot on the northwest side of the site to six feet on the upgradient southeast side of the site. The general groundwater flow direction is to the northwest across the site. At the completion of stage 1 (Figure 2), the hydraulic heads are approximately -10 ft. in the vicinity of the northwest part of the site and approximately six feet along the upgradient southeast part of the site. After stage 2 (Figure 3), the hydraulic heads are approximately -6 ft. in the vicinity of the northwest part of the site and approximately 6.0 feet along the upgradient southeast part of the site. The drawdown that is observed in the vicinity of the downgradient part of the site is in the estuarine areas in hydraulic communication with the brackish waters of Cockroach Bay that bound the site. The temporary drawdown observed in this area should not adversely affect the various wetlands that exist in the vicinity of the site. The upgradient areas of the site that bound the proposed excavation do not exhibit any significant drawdown during either phase of the mining stages, compared to the predevelopment conditions. The proposed recharge trench and the design stage elevations appear to effectively reduce the drawdown created by the dewatering operations. Mass balance or hydraulic budgets were extracted from the model for the mining stages and are provided in Appendix B. According to these mass balance reports, under pre-development conditions the total inflow to the site was 60,961 cubic feet/day (ft 3 /day) with a total outflow of 60,856 ft 3 /day. Under the dewatering and excavating conditions associated with stage 1, the total inflow to the site was 217,286 ft 3 /day, with an outflow of 217,300 ft 3 /day. After the stage 2 dewatering and excavating effort, the total inflow was 1,168,630 ft 3 /day and outflow amounted to 1,168,515 ft 3 /day. Specifically under the simulated stage 1 excavation, approximately 189,644 ft 3 /day ( ft. ) were discharged in association with the dewatering effort. Under stage 2 excavation, approximately 977,960 ft 3 /day (356,955,499 ft. ) of water were discharged in association with dewatering. These volumes discharged during dewatering represent the volumes of water that will be available to introduce into the recharge trench to mitigate the effects of drawdown. Taking into consideration the ditches and the wetlands, which were both simulated with the Drain Package, outflow to the drains amounted to 8,239 ft 3 /day during stage 1 and 4,936 ft 3 /day during stage 2. GHD Caloosa Groundwater Flow Model Report (x) Page 3

7 5. Conclusions and Recommendations The DWRM3 model was used to construct a local flow system using telescopic mesh refinement to evaluate the proposed Caloosa mining operation. Various hydraulic boundaries used in DWRM3 were adjusted to represent the details that became apparent within the local refined grid domain, including surface water bodies associated with the estuarine environment, wetlands, and drainage ditches used in the former agricultural property. These features were represented in the predevelopment and mining stress periods. The modeling results reasonably represent the pre-development and operational conditions. Due to the proposed use of a recharge trench around the perimeter of the entire excavation with stage elevations maintained at 2 and 3 ft NAVD, excessive drawdown is mitigated. The downgradient west side of the site is bound by the shallow brackish waters of the Cockroach Bay estuary; therefore, the temporary effects of dewatering in this part of the site should not present significant impacts. The upgradient part of the site exhibits higher groundwater elevations and, although significant drawdown occurs in close proximity to the proposed excavation, drawdown at the upgradient site boundary is minimal. As a result, it does not appear that the condition of the adjacent wetlands will be jeopardized by the operation of the mine. If the hydraulic properties of the surficial aquifer and operational conditions permit increasing the stage elevations in the trench during operation of the mine, this modification is recommended to ensure that potential impacts to wetlands are minimized. Using TDS to evaluate the water quality of the surficial aquifer underlying the site, the concentrations range from potable in the upgradient part of the site to brackish and hypersaline in the western part of the site. Dewatering and introducing the discharge water into the recharge trench should not adversely affect the quality of the groundwater, considering the mixing and dilution of groundwater concentrations that will take place as the groundwater is pumped from the subsurface. GHD Caloosa Groundwater Flow Model Report (x) Page 4

8 All of Which is Respectfully Submitted, GHD William C. Hutchings, Ph.D., P.G. Professional Geologist WH/cr/1 Encl. GHD Caloosa Groundwater Flow Model Report (x) Page 5

9 Figures GHD Caloosa Model Report (1)

10 Figure 1 Stress Period 2 Hydraulic Heads Pre-development/Mining Conditions GHD Caloosa Model Report (1)

11 Figure 1. Stress period 2 hydraulic headspre-development/mining conditions

12 Figure 2 Stress Period 3 Hydraulic Heads Stage 1 Excavation active after 180 days of Dewatering GHD Caloosa Model Report (1)

13 Figure 2. Stress period 3 hydraulic heads-stage 1 excavation active after 180 days of dewatering Rock Ponds Ecosystem Restoration project Stage 1 Recharge trench Stage 2 Rock Ponds Ecosystem Restoration project

14 Figure 3 Stress Period 3 Hydraulic Heads Stage 2 Excavation active after 365 days of Dewatering GHD Caloosa Model Report (1)

15 Figure 3. Stress period 4 hydraulic heads-stage 2 excavation active after 365 days of dewatering and total time of 545 days Rock Ponds Ecosystem Restoration project Stage 1 Recharge trench Stage 2 Rock Ponds Ecosystem Restoration project

16 Appendices Appendices GHD Caloosa Model Report (1)

17 Appendices Appendices Appendix A Groundwater Vistas Model Files GHD Caloosa Model Report (1)

18 Appendix B Mass Balance/Hydraulic Budget GHD Caloosa Model Report (1)

19 Mass balance/hydraulic budget for Stage 1 Excavation after 180 days

20 Mass balance/hydraulic budget for Stage 2 cell during Stage 1 excavation after 180 days

21 Mass balance/hydraulic budget for Stage 2 Excavation after 365 days

22 Total mass balance/hydraulic budget for proposed excavation area during Stage 1 excavation

23 Total mass balance/hydraulic budget for entire site during Stage 1 excavation

24 Total mass balance/hydraulic budget for entire site during Stage 2 excavation

25 Site mass balance/hydraulic budget over time