notebook - Southwest Florida Water Management District

Background
Depending on the characteristics of the site where phosphate matrix is to be mined, the mining process
will usually require some form of dewatering of the SAS. Typically, pre-mining dewatering activities are
conducted to ensure ground stability and to ensure mine cuts are sufficiently free of water to allow the
mining equipment to operate safely and efficiently. Effective dewatering of the mining area also enables
the operator to visually evaluate the depth and extent of the mineral reserve and maximize resource
recovery. Dewatering activities have the potential to result in drawdown of the SAS. The extent of the
drawdown is dependent on a number of factors including the overall depth of mining, the overburden
thickness, the duration of the dewatered open cut, and the hydrogeologic characteristics of the
overburden soils, including but not limited to, the absence or presence of aquicludes and/or aquitards and
the vertical and horizontal hydraulic conductivity of the overburden soils.
For this reason, an approach has been developed to define a Mandatory Mitigation Distance (MMD)
(formerly referred to as the “setback” distance) for each mine site. The MMD will be used to determine
when the implementation of a Site-Specific Drawdown Mitigation Plan (SSDMP) is required. Dewatering
activities outside of the MMD are presumed not to cause impacts.
Since Mine Activities encompass a broad range of hydrogeologic and other site specific characteristics,
the MMD must be, to a certain extent, site specific. Determination of the MMD must also be conservative
to ensure that it represents the full potential extent of SAS drawdown that could occur in the absence of
mitigation measures.
Methodology for Establishment
Mosaic uses site specific field data and a detailed engineering analysis of the potential, unmitigated SAS
drawdown at a site. A transient flow seepage analysis will be accomplished through the use of the
computer application SEEP/W, developed by GEO-SLOPE International LTD, or other mutually agreed
upon method to determine the maximum extent of dewatering drawdown influence in the SAS.
The Permittee shall use a 0.5-foot and greater cumulative drawdown in the SAS from the groundwater
model and dewatering modeling as a guide for determining when the MMD process contained within the
EMP is to be implemented. Numerical modeling is preferred over analytical models for determining the
estimated range of drawdowns to occur from mining and dewatering. However, as updated models and
other techniques are made available, these shall be taken into consideration for use.
A stratigraphic cross section will be developed adjacent to environmental features/property boundaries for
determining the depth of overburden and matrix, and to develop stratigraphic profiles from land surface
down to the bottom of the mineable matrix. Prospecting data is suitable for use in determining depth of
overburden and matrix. Split spoon, continuous coring, or another generally accepted geotechnical
investigation technique shall be the preferred methods for developing the stratigraphic profile from land
surface to the top of the matrix.
Once the stratigraphic profile of the surficial aquifer has been determined, field derived hydraulic
conductivity measurements will be input in the seepage model for the differing SAS stratigraphic units and
cast overburden (if present) in the model. The determination of the MMDs for each mine area will be
based on the results of the approved modeling technique as set forth below or such other appropriate
MosaicFertilizer,LLC–WUP20011400.025–EnvironmentalManagementPlan
Page7
75