Temporal Changes in Water Quality of two Karst Springs in Northern Alabama, 1999-2001

From 1999 to 2001, temporal variability of temperature, discharge, major ions, nutrients, pesticides, and selected pesticide metabolites in ground water was investigated at two springs that represent two types of flow systems in a karst aquifer in northern Alabama. Hourly water temperature data for the smaller spring (discharge between 0.1 and 2 cubic feet per second) suggest this spring represents a shallow flow system with a relatively short average ground-water residence time. Water temperature fluctuated seasonally (16 +/- 2 ° C), with some spikes and dips in temperature occurring during storms. Water issuing from the larger spring (1 to 9 cubic feet per second) maintained a constant temperature (16 +/- 0.2 ° C) throughout the year. The larger spring represents a deep, diffuse flow system having a longer average ground-water residence time than the smaller spring. Monthly water samples analyzed for major inorganic constituents and nutrients from the two springs indicate variability in water quality, both seasonally and in response to storms. Decreases in specific conductance and calcite-saturation indices in samples collected from the smaller spring within several days of moderate rainfall, indicate that spring discharge contains a component of water with short residence times. In contrast, an increase in specific conductance and calcite-saturation indices in samples collected from the larger spring after heavy rainfall indicate a contribution of water to the spring discharge with relatively long residence times. An increase in hydraulic head in the aquifer as a result of precipitation, induces movement of water from the diffuse flow system to spring discharge. In samples from both springs, nitrate concentrations typically are lower in fall and higher in early spring. In addition to seasonal variation, nitrate concentrations in the smaller spring are affected by short-term dilution following storms. An increase in nitrate concentration and calcite-saturation index in a sample collected following a storm during the fall indicates storage of nitrate in the diffuse flow system. Attenuation of nitrate as a result of denitrification in much of the ground water flowing to these springs is probably not substantial; dissolved oxygen concentrations in both springs are generally greater than 6 milligrams per liter. Pesticides and pesticide metabolites were detected in all samples collected from the springs, but concentrations generally were low. The maximum total pesticide residue in a sample was about 1.7 micrograms per liter (μ g/L). The herbicides fluometuron, atrazine, and their metabolites were the most frequently detected pesticides. Fluometuron and demethylfluometuron concentrations were an order of magnitude greater (0.54 to 1.5 μ g/L) than atrazine and its metabolites (0.02 to 0.04 μ g/L) in samples from both springs. Demethylfluometuron typically accounted for about 45 percent of the total fluometuron residue in samples, whereas atrazine metabolites accounted for about 70 percent of the total atrazine residue in samples. In both springs, atrazine concentrations were highest after storms in the early spring. Fluometuron concentrations in samples from the smaller spring were elevated for about 5 months after application (late April-early May) before concentrations declined; whereas, fluometuron concentrations in samples from the larger spring did not fluctuate appreciably. Variation of metabolite concentrations was similar to the parent compounds.