Excess nutrients entering Biscayne Bay are producing ecosystem wide problems associated with eutrophication and no data exists on the relative importance of groundwater versus canal discharges and nutrient loads. The primary purpose of this research is to quantify groundwater discharge and nutrient loads into the near-shore waters of Biscayne Bay.

This study is significant for several reasons. First, water quality has declined in response to land use changes (Parker et al., 1955, U.S.G.S. 1255, 965 p.). Second, agriculture lands surround the study area, which may increase nutrient concentration in the ground and surface waters (Johannes, 1980, Mar. Ecol. Prog. Ser. 3 365-373). Third, the South Florida Water Management District (Alleman et al., 1995, Biscayne Bay SWIM, SFWMD) is attempting to increase groundwater flow into Biscayne Bay.

Groundwater flows into estuaries are believed to make up a small part of the total water budget, however, the nutrient concentrations are often significantly higher than local rivers and streams (Johannes, 1980). The only previous work in Biscayne Bay suggests groundwater discharges influence benthic community zonation (Kohout and Koplipinski, 1967, Estuaries 5 488-499).

Two transects of wells were drilled between the outlets Military and Mowry Canals. Nutrient concentrations were determined by sampling groundwater in wells drilled in the near-shore estuary. The wells and surface water were sampled during the dry and wet season. Comparisons were made upon distance from shore, depth of well and seasonality.

Groundwater discharge was determined by seepage meter measurement (Lee, 1977, Limnol Ocean. 22 no 1, 140-147) and traditional computations using Darcy's equation. Seepage meters were installed in the near-shore environment along three transects. The meters were measured during the wet and dry seasons and throughout a full tidal cycle. The traditional Darcy's computation was done using field measured hydraulic gradients.

Seepage meters discharge rates averaged 10 m³ per liner m of shoreline per day. Total discharge for the study area with 2100 m of shoreline is 21,000 m³ d^-1 . Darcy's equation yielded a maximum and minimum discharge depending on hydraulic conductivity. Maximum discharge value is 18,286 m³ m^-1 d^-1 and 38,400,000 m³ d^-1 for the entire study area. Minimum discharge is 7,300 m³ m^-1 d^-1 and 15,400,000 m³ d^-1 for the entire study area.

Total phosphorus concentration in the groundwater is 0.031 ppm. Total nitrogen concentration is 1.11 ppm and total organic carbon concentration is 10.80. This is not significantly different than the nutrient concentrations of Mowry Canal. In Mowry Canal the nutrient concentrations are as follows, total phosphorus 0.032 ppm, total nitrogen 0.920 ppm and total organic carbon is 12.50 ppm (Scheidt et al 1993, Report SFRC-83/06, ENP).

Groundwater nutrient loading ranges from 0.7 to 1,190 g d^-1 for total phosphorus. Total nitrogen ranges from 23 g d^-1 to 43,000 g d^-1 and total organic carbon 227 g d^-1 to 3,840,000 g d^-1 . Due to different estimates of discharge, estimates of loads vary greatly.

Completion of this study will yield a more reliable discharge estimate along with better estimates of nutrient loading. Upon completion, managers should be able to determine the overall importance of groundwater discharge to the estuary. Water managers can than use these data to plan the most effective policies concerning Biscayne Bay revitalization. Water managers currently plan to increase groundwater discharge without knowing the current discharge and nutrient concentrations. This research will yield the baseline information needed to make the best possible decision.

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