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Groundwater quality in the St. Johns River Water Management District
Accurate groundwater quality information in the St. Johns River Water Management District (district) increases our understanding of the hydrogeologic, climatic and human factors that affect groundwater resources. The district’s groundwater resource assessment section designs and constructs monitoring network observation wells, collects and evaluates geologic and hydrologic data, and performs detailed hydrogeologic investigations. The purpose of this web report is to describe the district’s groundwater quality monitoring network, to summarize the major ion chemistry of water in the principal aquifers, to describe the geochemical patterns of water in the Floridan aquifer system, and to evaluate long-term trends.
Test wells are drilled so that scientists can monitor water quality and quantity.
The evaluation and interpretation of groundwater quality data requires knowledge of the hydrogeologic system and use of the appropriate methods for data analysis. The lithostratigraphy and structure of the geologic system control the nature and distribution of the hydrostratigraphic units and have distinct influence on the storage, movement and quality of groundwater. The hydrostratigraphic units are the aquifer and confining units of regional extent that are composed of sediments or rocks that have similar geologic characteristics and that subdivide the hydrogeologic system based on permeability. The hydrostratigraphic units identified within the district are the surficial aquifer, the intermediate confining/aquifer unit, and the units within the Floridan aquifer system. The aquifer units within the Floridan aquifer system include the upper permeable zone of the Upper Floridan aquifer, the Avon Park permeable zone (lower permeable zone of the Upper Floridan aquifer), and the Lower Floridan aquifer.
Groundwater quality monitoring network
The major focus of district groundwater quality monitoring is the Floridan aquifer system, which is the primary source of water for public supply and other uses. Regional groundwater monitoring assesses water quality resulting from hydrogeologic features, climatic conditions, groundwater flow patterns and long-term water use trends. Data analysis from a regional perspective allows for network optimization by minimizing monitoring wells and sampling frequency in areas that have shown insignificant variability, allowing for additional monitoring in those areas where significant variability has been documented or is projected to occur.
Subregional monitoring assesses groundwater quality related to localized hydrogeologic features and stresses to the aquifer system, such as water withdrawals. The data are used to describe localized spatial variability, water quality variability with depth and temporal water quality changes. The monitoring well network includes increased well coverage and sampling frequency in areas that have increasing trends in water quality variables, in areas near the potable and nonpotable groundwater interface, in areas of seawater intrusion along parts of the Atlantic coast, and in areas with projected water level drawdowns in the Upper and Lower Floridan aquifers.
Research divers take water samples and gather other data at Florida’s springs at various times throughout the year.
The Floridan aquifer system water quality monitoring network includes 332 monitoring wells (as of September 2011). The network includes 277 Upper Floridan aquifer wells, 23 Avon Park permeable zone wells, and 32 Lower Floridan aquifer wells. The wells are sampled at frequencies of once or twice per year, depending upon the historical water quality trend and the location of the well relative to areas of groundwater quality concern. Surficial and intermediate aquifer wells are sampled twice after well completion to provide baseline data for these aquifers. The district also samples 21 springs four times per year.
Water quality data
Water quality samples from network wells are analyzed for the chemical constituents (major cations and anions) typically used as general indicators of groundwater quality. The data are used to characterize the major ion chemistry of each aquifer and to describe a variable’s spatial variability, temporal trend and water quality changes with depth at a specific location. The major cation data evaluated include calcium, magnesium, sodium and potassium. The major anion data include chloride, sulfate and alkalinity. Phosphate, nitrate + nitrite and fluoride are important minor anions. The concentration of total dissolved solids is also analyzed. Field measurements include temperature, pH and specific conductance.
The data characteristics realized from the exploratory data analysis, trend analysis and an understanding of the hydrogeologic environment are used to guide monitoring network design and the frequency of well sampling. Historical water quality data from active and inactive monitoring wells sampled through October 2009 were compiled and evaluated. A time-series graph of data for each well was used to display a variable’s concentration versus time. The graphs provided information on the completeness of the period of record, the general level of temporal or seasonal variability, and the presence of outlier or extreme values that were obviously higher or lower than the historical values for a well.
Descriptive statistics provide information about the distribution and characteristics of a population. Descriptive statistics were calculated for each water quality variable by well, including the minimum, mean, median and maximum values as well as the range of data values, the range of sample dates and the number of samples. The median is the 50th percentile and is a better estimate of the central tendency of the data set than the mean.
A list of the water quality variables evaluated and the distribution of the concentrations for each variable by aquifer is presented in a box plot summary. Box plots are graphs used to show the distribution, tendencies and extremes in the data. Descriptive statistics were also calculated by variable for the population of all wells grouped by aquifer. These statistics include the minimum, mean, median and maximum values as well as the 10th, 25th, 75th and 90th percentiles.
Trend analysis statistical tests determine if the observed water quality changes that occur over time are due to random variability or statistically significant changes. Nonparametric Mann-Kendall and Sen’s Slope statistical tests from S-Plus scripts were used to determine the significance of a trend and estimate the magnitude of the trend.
Geochemical patterns in the Floridan aquifer system
The chemical composition and the physical properties of water in the Floridan aquifer system represent the net effect of the processes that have dissolved, altered or precipitated the chemical constituents. Rainfall chemistry, land surface features, soil types and recharge, discharge and leakage relationships among aquifers influence groundwater quality. The lithology, structure and porosity of aquifer materials and the residence time of water affects the precipitation and dissolution of minerals as water moves along flow paths. Land uses, groundwater withdrawals and irrigation may also have an impact on groundwater quality.
Mineral solubility controls the chemical character of the water. The mineralogy of the aquifer material through which water flows determines which water type, or hydrochemical facies, will develop. The vertical and horizontal groundwater flow patterns control the distribution of the facies. The groundwater flow pattern is controlled primarily by the distribution of heads, or hydraulic pressure differences, within the hydrologic system. The distribution of heads is controlled by the relative topographic elevations, the location and effectiveness of recharge and discharge areas, and by variations in the transmissivity of the aquifer system.
Stiff diagrams and trilinear diagrams for 306 Upper Floridan aquifer wells that had complete analyses suitable for evaluation are used to portray these relationships between hydrogeology and geochemistry. A stiff diagram is a graphical representation of the proportions of dominant ions from a chemical analysis, with the concentrations in milliequivalents of cations plotted to the left of a vertical axis and anions plotted to the right of the axis. The shape formed by the stiff diagram allows for quick interpretation of the water type for each well. Water type nomenclature is based on the proportion of each cation and anion to the total of their respective groups.
A trilinear diagram provides a convenient method to display and compare water types from the Upper Floridan aquifer wells. Cation and anion concentrations for each well are plotted as percentages of their respective totals in two base triangles. The cation and anion relative percentages in each triangle are then projected into a diamond-shaped grid, showing clustering of wells that have similar chemical compositions. The diagram provides a convenient method to display and compare water types and to depict the evolution of groundwater chemistry as water moves along flow paths.
A trilinear diagram allows the classification of water by both its chemical composition and its source. Geochemical pattern analysis (for details, see the References page for Piper 1944; Frazee 1982; and Toth 1988) of water samples collected in the district suggests four general water types are present. These are: (1) fresh recharge water, (2) interface or transitional water, (3) connate (saline formation) water and (4) seawater. These types portray the chemical characteristics of the water as it flows through the Floridan aquifer system, starting with recharge waters and ending with saline formation waters or seawater.
A well’s water type, when plotted on a map, allows for the interpretation of water quality in relation to geologic features, recharge and discharge areas and flow patterns. The similarity in water type among groups of Upper Floridan wells suggests that similar geochemical processes may be controlling major ion chemistry and that the waters had the same or similar origins and flow paths, revealing geochemical patterns in the flow system.
In areas of high-to-moderate recharge to the Upper Floridan aquifer, such as in the upland ridges or karst areas where the Floridan limestones are at or near land surface, residence times are relatively short and flow paths are generally shallow. The fresh recharge water has not reached equilibrium conditions with aquifer materials, and the water quality typically reflects climatic conditions, soil types, recharge from the surficial aquifer, and possibly human activities. The chemical composition of water in the Upper Floridan aquifer in high-recharge areas is characteristically low in concentrations of calcium, magnesium, chloride, sulfate, total dissolved solids (TDS) and other constituents.
In recharge areas, calcite dissolution reactions in shallow, well-developed flow systems account for the predominant Ca‑HCO3 water type. The Ca‑HCO3 water type is dominant in the Upper Floridan aquifer in the west-central area of the district; the DeLand ridge and areas to the east in Volusia County; the Crescent City ridge in southeastern Putnam County; and the Geneva hill in northeast Seminole County.
As water in the Floridan aquifer system moves along flow paths from recharge to discharge areas, the longer residence times allow more opportunity for the chemical composition of the water to be altered and for chemical reactions between the water and aquifer materials to reach equilibrium, resulting in increased TDS concentrations. The increased residence times and longer flow paths are evidenced by the chemical evolution of groundwater composition from the predominant Ca‑HCO3 water type to Ca,Mg‑HCO3; Ca,Mg‑HCO3,SO4; Ca,Mg‑SO4; Na‑Cl; and mixed water types.
As water in the Floridan aquifer system moves down gradient from recharge areas, such as near Keystone Heights in Clay County, a Ca,Mg‑HCO3 water type becomes prevalent. Dissolution of dolomite in the Floridan with increased residence time adds magnesium ions to the water. Dissolution of magnesium-rich clays and dolomite in the overlying Hawthorn Group sediments also contribute to magnesium ions as this overlying confining unit thickens to the north and east. The Ca,Mg‑HCO3 water type is prevalent in Clay County, western Duval County, Baker County, northwestern Putnam County, eastern Alachua County, eastern Marion County and southern Lake County. Chloride, sulfate, and TDS are typically low in these areas, with a thick lens of freshwater and low recharge occurring through the relatively thick confining units.
Sulfate concentrations are notably high in parts of eastern Duval, northern St. Johns and Nassau counties, where water in the Floridan aquifer has come into contact with gypsum in and at the base of the aquifer system as it travels along deep flow paths. Gypsum dissolution adds sulfate ions to the water, resulting in Ca,Mg‑HCO3,SO4 or Ca,Mg‑SO4 water types.
The chemical composition of water in the Upper Floridan aquifer in and near discharge areas along the St. Johns River and most of the Atlantic coast includes mixed and Na‑Cl water types. Hydraulic pressure conditions in discharge areas result in upward leakage of highly mineralized water into overlying formations or more direct upward movement of water through faults or fractures. This upwelling of highly mineralized water into upper parts of the Floridan aquifer occurs along the St. Johns River in parts of Brevard, Seminole, Orange, Lake, Volusia and Putnam counties. Mixed and Na‑Cl water types occur in the transition zone between the upwelling residual seawater and fresh groundwater to the east and west of the St. Johns River.
In areas of St. Johns County south of St. Augustine, as well as parts of Flagler, Brevard and Indian River counties, fresh recharge waters have not readily flushed residual formation waters still present in the Floridan aquifer, probably related to relatively flat flow gradients. This mixing of fresh groundwater with highly mineralized residual formation water results in mixed and Na‑Cl water types. In addition, lower hydraulic pressures in the Upper Floridan aquifer result in the upward movement of highly mineralized formation water in the tri-county agricultural area of St. Johns, Flagler and Putnam counties.
In coastal areas, a wedge-shaped body of seawater underlies fresh groundwater in the Upper Floridan aquifer. The zone between the fresh groundwater and seawater is the freshwater/saltwater transition zone. The freshwater/saltwater interface is the sloping surface between the freshwater and the underlying saltwater. Factors that influence water quality in this transition zone include the characteristics of the geologic strata, recharge and discharge, groundwater flow and pumping of wells. As water levels in the Floridan aquifer decline, the depth to the interface between fresh groundwater and seawater decreases and the transition zone migrates inland into areas of fresher water. This slow migration of saltwater into freshwater zones is known as seawater intrusion.
The mixing of fresh groundwater in the Upper Floridan aquifer with laterally intruded seawater from the Atlantic Ocean occurs in parts of Volusia and Brevard counties, resulting in a Na-Cl water type, with very high concentrations of TDS, calcium, magnesium, sulfate, and hardness. A transition zone between intruded seawater and fresh groundwater or residual formation water also occurs in other areas along the coast, resulting in a mixed water type. The current position of the transition zone does not reflect the full effect of current pumpage, as seawater intrusion may not become apparent for years following stresses on the aquifer.
The Ca‑HCO3 water type is prevalent in the Avon Park permeable zone in central Orange, southwestern Seminole and central Lake counties, with a mixed water type at increasing depths in the Lower Floridan aquifer. Toward the east, mixed to Na‑Cl water types are present, depending on well depth, as water moves along deeper flow paths with longer residence time. The mixing of fresh groundwater with residual formation seawater or the mixing with upwelling saline water from deeper zones also accounts for the mixed to Na‑Cl water types.
Declines in potentiometric levels of the Upper Floridan aquifer in some areas have increased the potential for the upward movement of highly mineralized water from lower zones to higher zones through leaky confining beds, structural geologic features and improperly constructed wells. Structural geologic features, such as faults or fractures, can provide pathways for the upward movement of water with high chloride concentrations into freshwater zones. In some northeastern areas of the district, structural geologic features in combination with decreasing hydraulic heads in freshwater zones are a likely cause for the mixed to Na‑Cl water types with increasing depth in the Floridan aquifer system.
The water quality conditions described are depicted in maps of the distribution of chloride, sulfate and TDS concentrations in the Upper Floridan aquifer. Drinking water standards have been established for chloride at 250 milligrams per liter (mg/L), sulfate (250 mg/L) and TDS (500 mg/L). These variables are of primary concern in the use of water from the Floridan aquifer for public supply. These variables are also the most useful variables in network design due to the number of wells sampled for these variables, the distribution of the wells and sampling history of the wells.
Posted on 2-22-2013