WATER-ROCK INTERACTIONS NEAR ICHETUCKNEE SPRINGS, COLUMBIA COUNTY, FLORIDA

Water-quality data collected at nearly 100 wells and six major springs in southern Columbia and eastern Suwannee counties were investigated using factor analysis, hierarchical analysis, star diagrams and WATEQ4F. These analyses were used to identify geochemical facies within the Floridan aquifer near Ichetucknee Springs. The factor analysis suggests that six factors help explain more than 75 percent of the variability in the data. These six factors reflect flow path and recharge characteristics, as well as water-rock interactions, in the Floridan aquifer near the springs.

The variables that best represent these six factors are alkalinity, chloride, magnesium, nitrate, ammonium, and phosphorus. Hierarchal analyses of these six factors suggest that the wells and springs can be grouped into three distinct water-quality domains. These domains indicate areas where: 1) clay-rich sediments of the Hawthorn Group influence the water chemistry in the Floridan aquifer, 2) regional flow paths appear to be emerging, and 3) ground-water flow paths are short in the Floridan aquifer.

Star diagrams representing the relative importance of each factor for each well and spring show the three major domains occupy distinct geographic areas. The first domain is located beneath the Northern Highlands, where the Hawthorn Group acts as a confining unit. The second domain is concentrated around the Santa Fe River, where upwelling of deep aquifer water is expected. The third domain occupies most of the study area, where the Floridan aquifer is poorly confined or unconfined.

Supplemental analysis of the water-quality data using WATEQ4F suggests that much of the ground water and spring water is at or near saturation with respect to calcite. However, in areas where Factor 1 is low (<1), such as in the Cody Scarp, the ground water tends to be undersaturated with respect to calcite. These areas tend to be characterized by the presence of disappearing streams, where surface water has not reached chemical equilibrium with the limestone matrix.