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Water bodies, watersheds and storm water

Spring water quality

Green Spring in Volusia County gets its name from its naturally milky green-colored water.

Green Spring in Volusia County gets its name from its naturally milky green-colored water.

Accurate and reliable water quality data are required to characterize groundwater chemistry for use in resource management. Major ion chemistry is used as a general indicator of groundwater quality and provides a means to identify regional differences and changes in water quality with time. A spring’s water quality is determined by several factors. These include the chemical composition of the water entering the aquifer, the composition and solubility of the rocks with which the water comes into contact along flow paths, the length of time the water is in contact with the rocks as it moves from recharge to discharge areas, and the mixing of fresh groundwater with residual formation water or seawater.

Land use activities in a spring’s recharge basin and the upconing of poorer quality water from deeper zones due to groundwater withdrawals may also impact water quality. The most pristine springs in the state occur in the Ocala National Forest, with spring recharge basins that encompass limited land uses. Other springs have recharge basins that encompass urban land uses and well withdrawals that have resulted in spring flow reductions and declining water quality.

The chemical composition and physical properties of a spring sample represent the net effect of all the previous chemical processes that have dissolved, altered, or precipitated the chemical constituents. The major constituents considered are those commonly present in concentrations exceeding 1.0 milligram per liter and that constitute a major part of the total dissolved solids content. The major cations include calcium, magnesium, sodium, and potassium. The major anions include chloride, and sulfate, carbonate alkalinity. Fluoride, nitrate + nitrite, and orthophosphate are important minor anions. Chemically related parameters include total dissolved solids and total organic carbon. Field measurement data include temperature, specific conductance, pH, and spring discharge.

The water quality results are summarized in a table for each spring. Descriptive statistics for each water quality variable include the number of samples, period of record, and the minimum-, mean-, median-, and maximum values. The median is the 50th percentile and is a better estimate of the central tendency of the data set than the mean. Concentrations are reported for the total fraction. Only quality-assured data were used; data below detection limit or data from samples held beyond holding time before analysis were not used. Total dissolved solids, chloride, sulfate, total nitrogen (nitrate + nitrite), and orthophosphate are graphed where sufficient measurement data are available.

Statistical Trend Analysis

A temporal trend is the general increase or decrease in observed values over time. Trend analysis is used to determine the significance of a trend in water quality for that spring and to estimate the magnitude of that trend. The Mann-Kendall test and Sen’s Slope estimator (Gilbert 1987; Helsel and Hirsch 1992) were used to evaluate trends in chloride, sulfate, total nitrogen (nitrate + nitrite), orthophosphate, and total dissolved solids where sufficient measurement data are available. The period-of-record data were used in this evaluation.

The Sen’s Slope estimator is a nonparametric, linear slope estimator that works most effectively on monotonic data. Unlike linear regression, it is not greatly affected by gross data errors, outliers, or missing data. For the chloride and sulfate analyses, a slope of 3 milligrams per liter (mg/L) per year was used as a threshold between a stable trend and an increasing trend; for total dissolved solids (TDS), a slope of 5 mg/L per year was used as a threshold between a stable trend and an increasing trend; for nitrate + nitrite, a slope of 0.01 mg/L per year was used as a threshold; for orthophosphate, a slope of 0.012 mg/L per year was used as a threshold. These thresholds are based on laboratory analysis replication limits. If the Sen’s Slope estimator resulted in an increasing trend and the Sen’s Slope was less than the threshold limit, the increasing trend is considered as insignificant, but these springs would be watched closely in future monitoring.

The Mann-Kendall test is a nonparametric test. The Mann-Kendall test is not greatly affected by missing data, but does require a minimum number of data points. This test is a linear estimator and works most effectively on monotonic data. The 80 percent confidence limit was chosen as an indicator of a trend. Because the test is two-tailed, the p-statistic is compared to an alpha of 0.100. If the p-statistic is less than alpha, the trend is significant.

Trend Results

For the chloride data, 28 springs were evaluated for trend. Of these 28 springs, the Mann-Kendall test resulted in 12 springs with no trend. A decreasing trend was observed in three springs. However, only one spring, Sweetwater Springs, had a significantly decreasing trend for which the slope was greater than 3.0 mg/L per year. An increasing trend was found in 13 springs, but all were insignificant trends because the slopes were below the 3.0 mg/L per year slope threshold limit. These 13 springs are Alexander, Blue (Lake County), Bugg, Holiday, Miami, Palm (Seminole County), Ponce de Leon, Rock, Sanlando, Silver, Starbuck, Wadesboro and Wekiwa. These springs will be watched closely in future monitoring.

Chloride Trend Analysis Table
Chloride Trend Analysis Table

Chloride Trend Analysis

Spring No. of
Samples
Period Mann-
Kendall
p-Statistic
Sen’s Slope
(mg/L per year)
Mann-Kendall Trend
Alexander Springs 120 1956–2010 0.01914 0.593 increasing – insignificant
Apopka Spring 120 1972–2010 0.46347 0.066 stable
Beecher Springs 15 1960–2001 0.11947 –0.952 stable
Blue Spring (Lake) 18 1967–2004 0.00774 0.147 increasing – insignificant
Blue Spring (Volusia) 238 1960–2010 0.19786 –1.367 stable
Bugg Spring 57 1967–2010 6.46E-06 0.428 increasing – insignificant
Croaker Hole Spring 31 1981–2010 0.1926 -1.870 stable
Double Run Spring 18 1997–2005 1.000 0.011 stable
Fern Hammock Springs 94 1972–2010 0.02552 –0.038 decreasing – insignificant
Gemini Springs 56 1972–2010 0.62428 0.596 stable
Green Springs 45 1972–2010 0.5841 4.701 stable
Green Cove Spring 45 1956–2010 1.000 -2.60E-04 stable
Holiday Springs 16 1967–2004 0.0269 0.148 increasing – insignificant
Island Spring 29 1982–2010 0.6022 8.033 stable
Juniper Springs 122 1956–2010 0.7823 –0.004 stable
Miami Springs 60 1960–2010 2.76E-06 0.187 increasing – insignificant
Orange Spring 41 1972–2010 0.1272 -0.025 stable
Palm Springs (Seminole) 60 1956–2010 5.39E-08 0.256 increasing – insignificant
Ponce de Leon Springs 211 1956–2010 3.25E-04 1.315 increasing – insignificant
Rock Springs 138 1956–2010 0.0213 0.041 increasing – insignificant
Salt Springs 109 1956–2010 0.2992 2.694 stable
Sanlando Springs 62 1956–2010 2.65E-09 0.350 increasing – insignificant
Silver Springs 142 1956–2010 4.21E-06 0.043 increasing – insignificant
Silver Glen Springs 97 1972–2010 1.06E-03 –2.461 decreasing – insignificant
Starbuck Spring 62 1944–2010 4.48E-07 0.281 increasing – insignificant
Sweetwater Springs 76 1980–2010 3.09E-05 –9.763 decreasing
Wadesboro Spring 6 1960–2002 8.54E-03 0.286 increasing – insignificant
Wekiwa Springs 139 1956–2010 4.30E-08 0.154 increasing – insignificant

For the sulfate data, 28 springs were evaluated for trend. Of these 28 springs, the Mann-Kendall test resulted in 10 springs with no trend. A decreasing trend was observed in six springs. An increasing trend was found in 12 springs, but all were insignificant trends because the slopes were below the 3.0 mg/L per year slope threshold limit. These 12 springs are Alexander, Apopka, Blue (Lake County), Bugg, Holiday, Miami, Palm (Seminole County), Ponce de Leon, Sanlando, Starbuck, Wadesboro and Wekiwa. These springs will be watched closely in future monitoring.

Sulfate Trend Analysis Table
Sulfate Trend Analysis Table

Sulfate Trend Analysis

Spring No. of
Samples
Period Mann-
Kendall
p-Statistic
Sen’s Slope
(mg/L per year)
Mann-Kendall Trend
Alexander Springs 118 1956–2010 0.00706 0.197 increasing – insignificant
Apopka Spring 118 1972–2010 0.00232 0.105 increasing – insignificant
Beecher Springs 15 1960–2001 0.31004 -0.152 stable
Blue Spring (Lake) 16 1972–2004 0.00236 0.256 increasing – insignificant
Blue Spring (Volusia) 99 1960–2010 0.71078 0.063 stable
Bugg Spring 55 1967–2010 0.00639 0.168 increasing – insignificant
Croaker Hole Spring 30 1981–2010 0.01944 -0.780 decreasing – insignificant
Double Run Spring 16 1998–2005 0.90154 -0.073 stable
Fern Hammock Springs 97 1972–2010 0.70646 0.002 stable
Gemini Springs 58 1972–2010 0.39867 -0.299 stable
Green Springs 45 1972–2010 0.66142 0.772 stable
Green Cove Spring 45 1956–2010 0.00029 -0.102 decreasing – insignificant
Holiday Springs 14 1972–2004 0.00938 0.258 increasing – insignificant
Island Spring 29 1982–2010 0.46551 3.943 stable
Juniper Springs 123 1956–2010 0.01543 -0.053 decreasing – insignificant
Miami Springs 61 1960–2010 3.87E-06 0.425 increasing – insignificant
Orange Spring 41 1972–2010 0.58295 -0.038 stable
Palm Springs (Seminole) 61 1956–2010 2.81E-07 0.322 increasing – insignificant
Ponce de Leon Springs 112 1956–2010 0.00019 0.330 increasing – insignificant
Rock Springs 138 1956–2010 0.14447 0.021 stable
Salt Springs 109 1956–2010 0.55596 0.214 stable
Sanlando Springs 62 1960–2010 0.00412 0.099 increasing – insignificant
Silver Springs 83 1956–2010 0.07633 -0.131 decreasing – insignificant
Silver Glen Springs 97 1972–2010 0.00974 -0.700 decreasing – insignificant
Starbuck Spring 63 1944–2010 2.34E-05 0.239 increasing – insignificant
Sweetwater Springs 78 1980–2010 0.00011 -1.620 decreasing – insignificant
Wadesboro Spring 6 1960–2002 0.06029 0.371 increasing – insignificant
Wekiwa Springs 139 1956–2010 1.13E-11 0.206 increasing – insignificant

For the TDS data, 28 springs were evaluated for trend. Of these 28 springs, the Mann-Kendall test resulted in 14 springs with no trend. A decreasing trend was observed in four springs, but in two springs the trend was significant with a slope of greater than 5.0 mg/L per year. Those springs were Double Run and Sweetwater. An increasing trend was found in 10 springs, but all were insignificant trends because the slopes were below the 5.0 mg/L per year slope threshold limit. These 10 springs are Bugg, Holiday, Miami, Palm (Seminole County), Ponce de Leon, Rock, Sanlando, Starbuck, Wadesboro and Wekiwa. These springs will be watched closely in future monitoring.

Total Dissolved Solids Trend Analysis Table
Total Dissolved Solids Trend Analysis Table

Total Dissolved Solids Trend Analysis

Spring No. of
Samples
Period Mann-
Kendall
p-Statistic
Sen’s Slope
(mg/L per year)
Mann-Kendall Trend
Alexander Springs 112 1967–2010 0.11936 0.990 stable
Apopka Spring 115 1972–2010 1.000 -0.008 stable
Beecher Springs 11 1972–2001 0.25145 -1.067 stable
Blue Spring (Lake) 17 1967–2004 0.16113 1.000 stable
Blue Spring (Volusia) 95 1960–2010 0.70606 -0.591 stable
Bugg Spring 46 1967–2010 7.56E-05 1.969 increasing – insignificant
Croaker Hole Spring 30 1981–2010 0.35020 -4.286 stable
Double Run Spring 18 1997–2005 0.00250 -5.167 decreasing
Fern Hammock Springs 93 1972–2010 0.78634 -0.064 stable
Gemini Springs 54 1993–2010 0.18707 -5.000 stable
Green Springs 43 1972–2010 0.82667 5.833 stable
Green Cove Spring 42 1957–2010 0.04875 -0.464 decreasing – insignificant
Holiday Springs 15 1967–2004 0.00488 1.764 increasing – insignificant
Island Spring 28 1982–2010 0.75445 27.602 stable
Juniper Springs 116 1962–2010 0.28464 -0.103 stable
Miami Springs 56 1972–2010 0.01242 1.091 increasing – insignificant
Orange Spring 39 1972–2010 0.50216 -0.364 stable
Palm Springs (Seminole) 57 1960–2010 0.00708 1.427 increasing – insignificant
Ponce de Leon Springs 105 1960–2010 0.00243 3.973 increasing – insignificant
Rock Springs 129 1960–2010 0.00193 0.315 increasing – insignificant
Salt Springs 100 1956–2010 0.23730 11.063 stable
Sanlando Springs 57 1972–2010 6.59E-05 1.505 increasing – insignificant
Silver Springs 79 1960–2010 0.32120 0.166 stable
Silver Glen Springs 91 1981–2010 0.00820 -4.583 decreasing – insignificant
Starbuck Spring 57 1960–2010 8.65E-05 1.471 increasing – insignificant
Sweetwater Springs 76 1980–2010 1.28E-05 -16.786 decreasing
Wadesboro Spring 6 1960–2002 0.06029 1.881 increasing – insignificant
Wekiwa Springs 130 1959–2010 3.80E-11 1.218 increasing – insignificant

For the nitrate + nitrite data, 26 springs were evaluated for trend. Of these 26 springs, the Mann-Kendall test resulted in 18 springs with no trend. A decreasing trend was observed in three springs, Bugg, Rock and Wekiwa. Only Blue (Volusia), Gemini and Silver Springs showed a significantly increasing trend, with a slope above the 0.01 mg/L per year slope threshold limit, requiring close evaluation in the future. Other springs with an increasing trend were Fern Hammock and Juniper, but these trends were insignificant because the slopes were below the 0.01 mg/L per year slope threshold limit. These springs will be watched closely in future monitoring.

Nitrate + Nitrite Trend Analysis Table
Nitrate + Nitrite Trend Analysis Table

Nitrate + Nitrite Trend Analysis

Spring No. of
Samples
Period Mann-
Kendall
p-Statistic
Sen’s Slope
(mg/L per year)
Mann-Kendall Trend
Alexander Springs 103 1977–2010 0.23372 -3.60E-04 stable
Apopka Spring 104 1986–2010 0.97412 -6.21E-03 stable
Blue Spring (Lake) 12 1996–2004 1.000 3.75E-02 stable
Blue Spring (Volusia) 82 1976–2010 3.64E-05 1.40E-02 increasing
Bugg Spring 54 1985–2010 1.02E-03 -1.57E-02 decreasing
Croaker Hole Spring 23 1995–2010 0.91697 -5.90E-04 stable
Double Run Spring 15 1998–2005 0.26551 -1.87E-01 stable
Fern Hammock Springs 92 1988–2010 1.99E-03 1.36E-03 increasing – insignificant
Gemini Springs 55 1995–2010 1.26E-05 4.38E-02 increasing
Green Springs 24 1996–2009 0.10741 2.46E-03 stable
Green Cove Spring 9 2001–2009 1.000 -2.60E-04 stable
Holiday Springs 10 1996–2004 0.80650 -4.67E-02 stable
Island Spring 6 2002–2009 1.000 -9.12E-03 stable
Juniper Springs 109 1984–2010 1.41E-04 1.06E-03 increasing – insignificant
Miami Springs 53 1993–2010 0.90165 -2.20E-04 stable
Orange Spring 11 2001–2009 0.53619 1.25E-03 stable
Palm Springs (Seminole) 53 1993–2010 0.65046 2.87E-03 stable
Ponce de Leon Springs 95 1984–2010 0.56834 3.62E-03 stable
Rock Springs 100 1984–2010 2.28E-03 -1.25E-02 decreasing
Salt Springs 97 1984–2010 0.53519 2.66E-04 stable
Sanlando Springs 53 1977–2010 0.36332 7.90E-03 stable
Silver Springs 59 1974–2010 2.47E-06 1.59E-02 increasing
Silver Glen Springs 92 1984–2010 0.35513 1.73E-04 stable
Starbuck Spring 54 1993–2010 0.53665 -6.42E-03 stable
Sweetwater Springs 75 1989–2010 0.16690 5.00E-04 stable
Wekiwa Springs 108 1977–2010 1.70E-04 -3.28E-02 decreasing

For orthophosphate, 27 springs were evaluated for trend. Of these 27 springs, the Mann-Kendall test resulted in 22 springs with no trend. A decreasing trend was observed in three springs, Blue (Volusia), Fern Hammock and Juniper, but the decrease was not significant because it was less than the 0.012 mg/L per year threshold value. An increasing trend was observed in two springs, Bugg and Silver Glen, but the increase was not significant because it was less than the threshold value of 0.012 mg/L per year. These springs will be watched closely in future monitoring.

Orthophosphate Trend Analysis Table
Orthophosphate Trend Analysis Table

Orthophosphate Trend Analysis

Spring No. of
Samples
Period Mann-
Kendall
p-Statistic
Sen’s Slope
(mg/L per year)
Mann-Kendall Trend
Alexander Springs 77 1972–2010 0.46225 -1.70E-04 stable
Apopka Spring 83 1986–2010 0.87113 5.33E-05 stable
Beecher Springs 12 1985–2001 0.71052 -3.60E-04 stable
Blue Spring (Lake) 10 1996–2002 0.25966 -7.50E-04 stable
Blue Spring (Volusia) 58 1972–2010 0.00022 -5.20E-04 decreasing – insignificant
Bugg Spring 43 1972–2010 0.08647 1.42E-03 increasing – insignificant
Croaker Hole Spring 24 1995–2010 0.83394 3.99E-04 stable
Double Run Spring 13 1998–2004 0.54801 5.00E-04 stable
Fern Hammock Springs 60 1994–2010 0.05790 -2.00E-04 decreasing – insignificant
Gemini Springs 53 1995–2010 0.26035 7.82E-04 stable
Green Springs 45 1996–2010 0.66933 1.74E-04 stable
Green Cove Spring 7 2001–2008 0.70711 -1.48E-03 stable
Holiday Springs 8 1996–2001 0.80650 -2.30E-04 stable
Island Spring 22 2001–2010 0.13313 1.81E-03 stable
Juniper Springs 73 1994–2010 0.03566 -3.30E-04 decreasing – insignificant
Miami Springs 51 1995–2010 0.39231 -1.51E-03 stable
Orange Spring 40 2000–2010 0.21291 1.72E-03 stable
Palm Springs (Seminole) 53 1972–2010 0.10312 -7.40E-04 stable
Ponce de Leon Springs 62 1972–2010 0.65046 -1.60E-04 stable
Rock Springs 73 1994–2010 0.48301 -3.00E-04 stable
Salt Springs 40 1994–2010 0.17403 3.28E-04 stable
Sanlando Springs 55 1972–2010 0.52887 3.00E-04 stable
Silver Springs 56 1967–2005 0.27872 -5.93E-05 stable
Silver Glen Springs 64 1972–2010 0.09309 1.67E-04 increasing – insignificant
Starbuck Spring 56 1972–2010 0.54449 -2.90E-04 stable
Sweetwater Springs 62 1994–2010 0.43383 -2.10E-04 stable
Wekiwa Springs 80 1972–2010 0.64904 1.60E-04 stable

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