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Spring water quality

Green Springs

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.

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.

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.

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.

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.

Definitions of water quality variables