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Quantification of Surface-Water Flux into the Biscayne Aquifer in Response to Municipal Well-Field Abstractions

Project Chief: Barclay Shoemaker
Cooperator: Miami-Dade Water and Sewer Department
Period of Project: October 2007 – September 2012

Problem Statement

Figure 1. Canal velocity monitoring station on Snapper Creek (C-2) Canal, at the Snapper Creek Well Field in Miami Dade County (ADVM, Acoustic Doppler Velocity Meter).

The canal system in South Florida was designed primarily to provide drainage of the Biscayne aquifer in flood-prone areas, but it is also used to provide freshwater recharge to the Biscayne aquifer to prevent saltwater intrusion by maintaining elevated groundwater heads, and to provide water supply to agricultural areas. The South Florida Water Management District is concerned that pumping from the Biscayne aquifer may be inducing enough leakage from the canal system to require increasing releases from the Everglades to meet the secondary functions of the canal. As part of a twenty-year, drinking-water permit application, Miami-Dade County is required by the State of Florida to ensure that municipal groundwater withdrawals do not excessively reduce surface-water conveyance through the canal system from the Everglades wetlands to Biscayne Bay. A rigorous, quantitative accounting of potential surface-water depletion from well-field pumping could improve water-management strategies, and provide a more optimal temporal and spatial distribution of groundwater withdrawals.

The unconfined Biscayne aquifer is hydraulically closely connected with the surface-water system in Miami-Dade County because of its high transmissivity. The stresses on the system are spatially and temporally variable; they include flows through canals, groundwater withdrawals, precipitation, evapotranspiration, and regional groundwater flow, making it difficult to discern isolated effects of groundwater pumping on canal leakage. Direct measurements of leakage between the canals and the groundwater system are difficult to make, and it is difficult to distinguish cause and effect relations between the many measurable hydrologic signals. Previous estimates of leakage between canals and the aquifer have been imprecise and made with few data, and do not account for all stresses on the system.

The C-2 canal traverses the middle of the Snapper Creek Municipal Well Field, therefore the effects of well-field withdrawals on canal flows may be easier to isolate there than at other locations in the County. Previous attempts to quantify the effects of well-field withdrawals on canal flux at this location have been unsuccessful because direct measurements of fluxes are within the error of flow measurement devices, and model-derived estimates were based on too few data to sufficiently constrain the local hydrologic budget.

Objectives

1. Quantify the effects of Snapper Creek Municipal Well Field withdrawals on the C-2 canal
2. Estimate the influence of municipal withdrawals from the Biscayne aquifer on the regional canal system
3. Evaluate field-scale hydraulic properties, and compare the value of the different types of data for quantifying groundwater-surface water exchanges.

Figure 2. Selected daily data at the Snapper Creek Well Field, including groundwater levels, canal stage, mean daily pumpage, mean daily canal leakage, evapotranspiration, and rainfall.

Approach

To characterize and measure the multiple stresses on the local hydrologic system, groundwater-, surface water-, and weather-monitoring equipment were installed at the Snapper Creek well field, and data have been collected since January 2009. Data collected include:

• lithologic data from drill cores and borehole geophysical data to define well-field scale hydrogeologic framework;
• continuous groundwater levels and temperature;
• continuous and intermittent canal stage, temperature and flow rates;
• precipitation and evapotranspiration;
• high frequency drawdown and recovery data from aquifer performance tests to estimate hydraulic properties; and
• water-quality parameters to evaluate surface water and groundwater mixing.

The data will be used to develop well-field-scale groundwater-flow and heat-transport models to simulate the flow system and estimate flow budgets under variable conditions. Collection of a variety of types of data at a high spatial density and at high frequency over multiple years will enable a more accurate and precise evaluation of the effects of municipal withdrawals on the canal system.

Results

Not yet available.

Information Product(s)

• U.S. Geological Survey Scientific Investigations Report – Hydrogeology of the Biscayne aquifer at Snapper Creek Well Field, Florida (in preparation)
• Journal Article – Estimation of canal losses at Snapper Creek Well Field, Miami-Dade County, Florida (in preparation)
• Journal Article – Effect of well-field withdrawals on canal leakage, Miami-Dade County, Florida (in preparation)