Aqueous Geochemistry of the Sand‐and‐Gravel Aquifer, Northwest Florida

Abstract

The aqueous geochemistry of the sand‐and‐gravel aquifer in northwest Florida was characterized as part of the Florida Ground‐Water Quality Monitoring Network Program, a multiagency cooperative study delineating baseline and/or background water quality for the major aquifer systems throughout the State. The aquifer is the principal source of water in northwest Florida and consists predominantly of quartz sand with smaller amounts of andesine, chlorite, calcite, kaolinite, and illite. Water from 42 wells in the sand‐and‐gravel aquifer sampled during 1986 and 1987 was characteristically low in dissolved solids (median value of 72 milligrams per liter) and of nondistinct water type, although relative concentrations of magnesium and sulfate tended to be lower than those of other major ions. Nonparametric statistical tests of major‐ion concentration variations with depth and relative distance along flow paths indicate that the water chemistry does not change significantly (0.05 significance level) as the water moves vertically and laterally through the aquifer. Mass‐balance calculations indicate that dissolved solids from rainfall and saline‐water sources each account for 13 percent of the concentration of dissolved solids in ground water. Incongruent dissolution of andesine, chlorite, and muscovite to form kaolinite accounts for 94 percent of the neutralization of the total hydrogen ion input from rainfall and from carbonic acid weathering in the subsurface. The remaining neutralization is accounted for by the dissolution of calcite.

Chemical Evolution of Groundwater Near a Sinkhole Lake, Northern Florida: 2. Chemical Patterns, Mass Transfer Modeling, and Rates of Mass Transfer Reactions

“…The processes that influence the chemical evolution of groundwater in regional-scale groundwater systems have been quantified using modeling techniques for unconsolidated silicate aquifers [e.g., Siegel and Pfannkuch, 1984;Katz and Choquette, 1991] and for carbonate aquifers [e.g., Back and Hanshaw, 1970; Deines et al, 1974;Plumrner, 1977]. In areas where carbonate systems are poorly confined, however, relatively few studies have assessed the changes in the composition of groundwater as it moves downward and reacts chemically with silicate material and recharges the underlying carbonate aquifer.…”

Uncertainty and Trend Analysis—Radium in Ground Water and Drinking Water

“…A data base of radium in ground water and drinking water for Escambia County, Florida, was assembled from publicly available information and used as an example. The drinking water supply in Escambia County is predominantly derived from the pumping of the sand-and-gravel aquifer (Katz and Choquette 1991), and total radium activity in the aquifer is naturally close to the MCL, with exceedances reported in several wells and locations within the water supply system. The data set is derived from 407 water samples collected from 137 locations that include municipal supply wells, distribution points, irrigation wells, and monitoring wells in the aquifer.…”

Hydrochemistry of the surficial and intermediate aquifer systems in Florida

“…The study area includes all of Florida, but more detail was given to those areas of principal use of each of the aquifer systems and to the areas where the monitoring wells were located. The sand-and-gravel aquifer of the surficial aquifer system and the Biscayne aquifer have been described in previous publications (Katz and Choquette, 1991;RadellandKatz, 1991).…”

“…In areas where the surficial aquifer system is used almost exclusively as a source of water supply and has been extensively studied, surficial aquifers have been individually named. The baseline water quality of these aquifers, the sand-and-gravel aquifer in northwest Florida and the Biscayne aquifer in Broward, Dade, and part of Palm Beach Counties, have been the focus of previous studies by Katz and Choquette (1991) and Radell and Katz (1991), respectively, and will not be discussed in this report.…”