notebook - Southwest Florida Water Management District

notebook - Southwest Florida Water Management District


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




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