Home Biscayne aquifer Hydrologic system Water quality Ground-water withdrawal Vulnerability of Biscayne aquifer Future problems of potable supplies Other drinking water sources Summary & References PDF Version

Figure 18. -- Location of major public supply well fields (Broward County Land Use Plan, 1977, p. 107; Metropolitan Dade County Environmental Resources Management facilities map, 1977). [larger image]

The chemical quality of the ground water in the Biscayne aquifer differs slightly from place to place; most differences in quality are related to the nature of the aquifer and local land use (Pitt and others, 1975, p. 44). In general, the water is hard, a calcium bicarbonate type, and contains different amounts of dissolved iron. Dissolved constituents in the ground water are influenced by rainfall and dry fallout, reaction with soil and aquifer material, application of fertilizers and pesticides, biological processes at the surface and within the aquifer, infiltration of wastes, chemical reactions among constituents, temperature, and pressure.

Ground-water quality in the vicinity of canals is also affected by canal water during dry seasons. The areas of this type of effect probably are small because of the seasonal reversals of hydraulic gradients between the canals and the aquifer. However, where large municipal well fields near canals are pumping continuously, such as the Miami-Springs Hialeah field adjacent to the Miami Canal, the induced infiltration of canal water to the aquifer is virtually constant and a wide zone of ground water is affected by canal water.

Chemical analyses of untreated water from selected large municipal well fields that withdraw water from the Biscayne aquifer (fig. 18), are shown in table 1. Samples were collected June 1975. Also shown are analyses of water from two test wells in interior parts of Dade County (fig. 18); both wells are remote from urban and agricultural development. The test-well samples were collected in October 1973 and October 1974. The analyses from the municipal wells represent ground-water quality affected by several years of pumping, urban runoff, periodic rainfall, canal drainage and canal-water seepage, septic-tank operation, and agricultural activities. The analyses from the two interior wells can be considered base-line water quality, not affected by development. In general the analyses are similar, except for low concentrations of sulfate and potassium in the remote test wells which are distant from urban and agricultural areas where fertilizers are used regularly. For comparison table 2 shows analyses of untreated water from four public supply well fields for 1941-42--during the early stages of urban growth (Parker and others, 1955, table 114).

Table 1. -- Analyses of untreated water from selected municipal well fields and from Everglades sites, 1973-75. [Healy, 1977 and Pitt and others, 1975]
Constituent (Concentrations in milligrams per liter, except as indicated)	Boca Raton well field	Fort Lauderdale Fiveash well field	Miami Preston well field	Miami Orr well field	Keys Aqueduct well field, south Dade County	Northwest Dade test well	Southwest Dade test well
Silica (SiO2)	7.5	9.7	7.0	4.3	3.2	7.1	5.3
Calcium (Ca)	80	100	90	92	100	98	81
Magnesium (Mg)	2.1	2.8	5.4	3.2	3.8	7.0	4.1
Sodium (Na)	18	19	38	16	13	17	14
Potassium (K)	1.4	1.5	3.0	1.7	5.7	0.7	0.3
Strontium (Sr)	0.75	.78	.87	.74	1.10	.89	.61
Bicarbonate (HCO3)	212	299	272	259	236	321	263
Sulfate (S04)	26	26	28	28	62	1.3	0.9
Chloride (Cl)	34	33	57	25	23	29	25
Fluoride (F)	0.3	0.3	0.3	0.2	0.3	0.3	0.2
Nitrate (N03-N)	.01	.01	.01	.25	.00	.00	.00
Nitrite (N02-N)	.00	.00	.00	.00	.00	.00	.00
Nitrogen, organic (N)	-	-	1.1	-	-	.55	.89
Nitrogen (Ammonia total NH4-N)	0.31	.63	1.10	.01	.01	1.20	.46
Iron (Fe)	.08	1.8	.90	.03	.29	2.9	-
Phosphorus, total (P)	-	-	-	-	-	.02	.00
Dissolved solids (residue at 180°C)	320	388	394	322	356	353	285
Total hardness (as CaCO3)	210	260	250	240	270	280	220
Noncarbonate hardness (as CaCO3)	36	15	27	28	76	13	9
Alkalinity (as CaCO3)	174	245	223	212	194	263	215
pH (units)	7.0	7.4	7.6	7.5	7.5	7 3	7.8
Specific conductance (umhos/cm at 25°C)	500	619	663	540	569	561	465
Color (Pt-Co units)	7.5	45	55	5	5	-	-
Temperature (°C)	28	-	28	28	-	24.3	24
Turbidity (JTU)	-	-	1	-	-	9	16
Carbon, organic, total (C)	-	-	-	-	-	15	9.0
Orthophosphate total (PO4-P)	-	-	-	-	-	.06	.00

Table 2. -- Analyses of untreated water from certain public supplies in southeast Florida, 1941-42. [Data from Parker and others, 1955, table 114]
Constituent (Concentrations in milligrams per liter, except as indicated)	Boca Raton	Fort Lauderdale	Miami (Hialeah - Miami Springs)	Homestead
Color (units)	10	110	85	5
pH (units)	7.2	7.3	6.8	--
Specific conductance (K x105)	32.1	45.8	57.7	37.9
silica (SiO2)	9.6	11	7.4	2.8
Iron (Fe)	0.06	1.9	1.3	0.02
Calcium (Ca)	59	88	94	63
Magnesium (Mg)	3.1	3.3	9.6	6.5
Sodium (Na)	3.0	11	22	6.8
Potassium (k)		--	2.2	0.4
Bicarbonate (HCO3)	168	266	266	218
Sulfate (SO4)	20	2.4	34	8.3
Chloride (cl)	18	18	38	10
Fluoride (F)	0.3	0.1	0.1	0.2
Nitrate (NO3)	.7	2.1	1.5	.3
Dissolved solids	195	294	370	212
Total hardness as CaCO3	160	233	274	184

One of the prime problems of water treatment for municipal supplies is the high organic content of the raw ground water. Color of the water in southeast Florida is related to the proximity of the thick organic peat soils of the Everglades. As indicated in table 1, color is high in the raw water from the Miami Preston well field which is a short distance downgradient from the areas of thick, organic soil in the Everglades. In contrast, the color at the Miami-Orr and Key West well fields is much lower because of the greater distance from the organic soils. The organic content in raw water cm become a problem in the removal of color. Chlorine used in the treatment process has been shown to produce halogenated organics (principally trihalomethanes) some of which may be carcinogenic (Rook, 1975).

Treatment of public water varies from chlorination only to a comprehensive, treatment which includes aeration, chlorination, coagulation, filtration, flocculation, pH control, softening, taste and odor control, recarbonation, and fluoridation.