lake (as well as to the south), one 

 of the factors contributing to the 

 relative freshness of water found in 

 upper portions of the lake. Direct 

 rainfall also aids in lowering the 

 salinity of the water in the shallow 

 portion of the lake. 



GROUNDWATER FLOW 



As noted previously, the flow 

 system is dynamic; ground water is 

 in constant motion. As water is 

 added to the system in the form of 

 rainfall recharge, discharge occurs 

 along the coast. On a long-term 

 basis, recharge and discharge are 

 approximately equal. Variations do 

 occur as a result of seasonal and 

 annual differences in rainfall. 



One of the goals of the program 

 was to develop an estimate of ground 

 water underflow to the project area. 

 Contour maps of the water level in 

 the principal water-bearing zone 

 were prepared and used to determine 

 the direction of ground water flow 

 and to compute volumes. A typical 

 example is given in Figure 4. This 

 map is based on data collected on 

 July 16, 1980, after the start of 

 the rainy season. Maps based on 

 data from other times during the 

 study period show essentially the 

 same overall relationship except that 

 the water level elevations are some- 

 what different due to variations in 

 recharge. It should be noted that 

 only data from wells in the northern 

 part of the study area were used in 

 determining the direction of ground 

 water movement and developing esti- 

 mates of the quantity of flow. 



Data from wells south of Lake 

 Marco Shores reflect the influence 

 of man's activities. The data from 

 Wells 1, 3, and 7, located on arti- 

 ficial fill areas, reflect the pres- 



ence of the freshwater lens in an 

 area where only saline ground water 

 formerly existed. This has local 

 influence on the lake, but has no 

 influence on the groundwater system 

 contributing flow to the area from 

 the northwest. Examination of Figure 

 4 shows that the elevation of the 

 water table rises gradually, pro- 

 ceeding in a northeasterly direction 

 from Well 2, indicating that the 

 general direction of groundwater 

 flow is toward the southwest. 



The quantity of groundwater 

 flow can be estimated from a modified 

 version of Darcy's Law that is given 

 by the expression: 



Q = TIL 



in which Q is the discharge in gal- 

 lons per day, T is the transmissivity 

 of the aquifer in gallons per day per 

 foot, I is the hydraulic gradient 

 expressed in feet per foot, and L is 

 the width, in feet, of the cross- 

 section through which flow occurs. 

 Underflow estimates for the area were 

 generated using a transmissivity 

 value of 220,000 gpd/ft (2,732 m / 

 day) and a hydraulic gradient based 

 on the differences between the ob- 

 served water level elevations in 

 Well 2012 and Well 8 (Figure 1) . 

 These wells are located about 12,600 

 feet (3,841 meters) apart in a line 

 that is approximately parallel with 

 the direction of flow. 



Water level measurements were 

 taken at weekly intervals in these 

 wells during the period of March 

 through August 1980. During this 

 period, the hydraulic gradient 

 ranged between 1.3, x 10 ft/ft 



ft/ft, and 



and 2.29 



10, 



averaged 1.79 x 10 ft/ft. The most 

 representative value for the hy- 

 draulic gradient is the average one; 

 the others represent values for the 



259 



