stratified. Density differences are 

 so great across the chemocline that 

 the possibility of overturn is nil. 

 Hence, the lower layer will influence 

 the upper only by vertical diffusion. 



and 0.65 years for Units 24 and 30 

 respectively. These values influence 

 lake water quality. 



LAKE NUTRIENT LOADINGS 



LAKE WATER BUDGET 



The lakes receive water from 

 surface runoff from the various land 

 uses; from interflow, (i.e., water 

 that infiltrates to the shallow water 

 table, forms a ground water mound, 

 and thence moves laterally to the 

 nearby lakes) from regional ground 

 water flow in the shallow fresh water 

 layer, and from direct rainfall. 

 Water is lost by surface runoff, 

 ground water outflows and evapora- 

 tion. If the change in storage is 

 zero on an average annual basis, then 

 inflows equal outflows, and the water 

 budget equation may be evaluated for 

 each term. 



Annual precipitation in the 

 area is about 50 inches (1,270 mm). 

 Surface runoff and interflow were 

 evaluated utilizing a land surface 

 evapotranspiration rate for the 

 region of 75 percent of annual rain- 

 fall (Figure 1). Groundwater move- 

 ment was determined by analysis of 

 regional potentiometric contours (see 

 paper by Amy in this proceedings). 

 Lake evaporation was taken as 70 per- 

 cent of pan, and surface outflows 

 were deduced by subtraction, knowing 

 all other terms. The derived water 

 budget is shown in Table 2 in which 

 units are inches over the lake 

 surface area. 



Assuming the depth of the fresh- 

 water layer in the lakes will be 6.5 

 ft (2 m) (Figure 2) residence times 

 for these layers may be computed by 

 dividing the volume above this depth 

 by the sum of inflows (or outflows) 

 in Table 2 expressed as a volumetric 

 flow rate, yielding values of 0.42 



The primary task is to develop 

 loadings for TN and TP that coincide 

 with the various pathways of the 

 water budget inflows to the lake, 

 plus possible diffusion from the 

 lower layer. The latter was cal- 

 culated on the basis of gradients 

 measured across the chemocline in 

 existing Lake Marco Shores. Concen- 

 trations in ground water and rainfall 

 were measured. Loadings for urban 

 stormwater were needed to evaluate 

 this contribution to surface runoff 

 and interflow. 



Fortunately, Broward and Dade 

 Counties (near Fort Lauderdale and 

 Miami) in southeast Florida were the 

 sites of four intensive urban runoff 

 monitoring programs by the U.S. 

 Geological Survey (USGS) in the mid 

 1970' s (Mattraw and Sherwood 1977; 

 Mattraw and Miller 1978; Hardee et 

 al. 1979; Miller et al. 1979). These 

 data were acquired as part of the 

 EPA Urban Rainfall-Runoff-Quality 

 Data Base (Huber et al. 1979) and 

 analyzed statistically to develop 

 flow weighted average concentrations 

 which were then used to develop 

 surface runoff and interflow loadings 

 to the lakes. The USGS data are 

 appropriate for use in the study area 

 because of similar meteorologic, 

 hydrologic, and demographic charac- 

 teristics of the locations. The 

 USGS data also have the unusual 

 advantage of a large number of sam- 

 ples, from 15 to 41 storms at the 

 four sites of differing land uses. 



Incorporating the various 

 fluxes, the nutrient loadings shown 

 in Table 3 are developed. Of interest 

 is the influence of the lower layer 

 on both T-N and T-P and the relative 



244 



