Collection and Analysis of Salinity Data

Because of the relative lack of development the Floridan aquifer system in southeastern Florida, the quality of ground water in the aquifer system is considered to have remained virtually constant during the period 1940-90. Water-quality data used to describe salinity in the Floridan aquifer system in this report were selected from data collected since 1940.

Selected water-quality data collected from known intervals in wells completed in the intermediate confining unit and the Floridan aquifer system are given in tables 2 and 3. In both tables, analyses are listed alphabetically by local well identifier and then in order of increasing depth of the sample interval in a well. If a sample interval included the Upper and Lower Floridan aquifers, these data were not used because of the mixing of waters with large differences in salinity. All of the analyses are of water from either completed intervals or from open-hole intervals sampled by drill-stem packer tools.

Table 2 gives USGS data collected from 36 wells (52 analyses), and table 3 gives non-USGS data collected from 24 wells (65 analyses). Most of the non-USGS data were obtained from the wastewater injection system wells and were collected by engineering consulting firms under contract to municipalities or by County agencies and reported to the FDEP. For two wells (FTL-M1 and MDS-BZ1), USGS and non-USGS data are included. All USGS data are stored in a USGS water-quality data storage and retrieval computer system (QWDATA).

All but four of the USGS analyses in table 2 were made before the beginning of this study. These four analyses are water samples collected from wells G-2296 (for the last sampling date), G-2617, G-2618, and G-2619. Sample collection procedures for USGS-collected data are given by Brown and others (1970).

Water sampling and testing methods for wastewater injection system wells are controlled by FDEP (Florida Department of Environmental Regulation, 1982). According to FDEP rules, the background water quality of the injection zone and monitoring zone(s) shall be established prior to injection. FDEP permits issued to construct and operate injection well systems include specific testing requirements; and for one such requirement, included since about 1988 (possibly earlier), at least three well volumes of fluid should be evacuated from a monitoring system before sampling.

Saltwater slugs used to control artesian pressure during drilling might have invaded the formation in the upper part of the Floridan aquifer system. When a completed interval is contaminated, the initial water samples usually show a change in salinity over time. An attempt was made to select analyses from completed intervals after salinity had stabilized (tables 2 and 3). Most monitoring wells are pumped or allowed to flow only on a periodic basis for sampling purposes, so if the formation were heavily contaminated during drilling, the time required for attaining background water quality could be long. An example of particularly heavy contamination with salty drilling water occurred in the upper monitoring zone (1,193-1,222 ft deep) in well CS-I1 (fig. 1). The chloride concentration of samples from this zone decreased from 9,300 mg/L to a constant value of about 5,600 mg/L, 2 years after the well was completed. For the past several years, barite-weighted bentonite drilling mud has sometimes been used instead of saltwater to control artesian pressure.

Water samples from open-hole packer tests are more often contaminated by drilling fluids than those from completed intervals because the volume of water produced before sampling is relatively small. Water samples from packer tests determined to be contaminated were not used in the study and are not included in tables 2 and 3. This determination was based on comparison with samples from nearby wells at similar depths, samples obtained later from completed intervals, or geophysical log responses.

Wells are commonly drilled in the Floridan aquifer system using the reverse-air rotary method in which air is injected into the drill pipe at a variable depth. This air provides the lift needed to bring fluid and drill cuttings up the drill pipe to the surface (return flow). Water samples of this return flow are collected and analyzed at regular intervals while drilling.

Because there could be mixing with water moving down from upper zones outside the drill pipe, water samples collected by this method commonly do not accurately represent formation water present at the drill bit. The return flow might or might not be recirculated down the annulus, and if the return flow is recirculated, it could be mixed with storage tank water before being recirculated. Additionally, return flow might become contaminated with sodium chloride, resulting from the placement of a slug of dense saltwater (salt pillow) in the upper annular space to control artesian pressure in the Upper Floridan aquifer during drilling. The reverse-air rotary method is commonly used during drilling in the upper to middle parts of the Floridan aquifer system.

Collecting water samples during drilling can show large changes in the salinity of formation water over relatively short-depth intervals. However, even a large increase in the salinity with depth might not be detected if the permeability is so low that little of the return flow is emerging from the formation at or near the drill bit.