Variations in growth rates of speleothem calcite have been hypothesized to reflect changes in a range of paleoenvironmental variables, including atmospheric temperature and precipitation, drip-water composition, and the rate of soil CO 2 delivery to the subsurface. To test these hypotheses, we quantified growth rates of modern speleothem calcite on artificial substrates and monitored concurrent environmental conditions in three caves across the Edwards Plateau in central Texas. Within each of two caves, different drip sites exhibit similar annual cycles in calcite growth rates, even though there are large differences between the mean growth rates at the sites. The growth-rate cycles inversely correlate to seasonal changes in regional air temperature outside the caves, with near-zero growth rates during the warmest summer months, and peak growth rates in fall through spring. Drip sites from caves 130 km apart exhibit similar temporal patterns in calcite growth rate, indicating a controlling mechanism on at least this distance. The seasonal variations in calcite growth rate can be accounted for by a primary control by regional temperature effects on ventilation of cave-air CO 2 concentrations and/or drip-water CO 2 contents. In contrast, site-to-site differences in the magnitude of calcite growth rates within an individual cave appear to be controlled principally by differences in drip rate. A secondary control by drip rate on the growth rate temporal variations is suggested by interannual variations. No calcite growth was observed in the third cave, which has relatively high values of and small seasonal changes in cave-air CO 2. These results indicate that growth-rate variations in ancient speleothems may serve as a paleoenvironmental proxy with seasonal resolution. By applying this approach of monitoring the modern system, speleothem growth rate and geochemical proxies for paleoenvironmental change may be evaluated and calibrated.

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rate from those leading to the other springs under all but stormflow conditions. Main, Eliza, and Old Mill Springs share at least one common flow path that contributes contaminants to the From 2003 to 2005, the U.S. Geological Survey, in coop-three springs. Old Mill Spring, however, is less affected by eration with the Texas Commission on Environmental Quality, anthropogenic contaminants than the other springs and receives collected and analyzed water samples from the four springs a greater component of water from a flow path whose geochem-(orifices) of Barton Springs in Austin, Texas (Upper, Main, istry is influenced by water from the saline zone of the aquifer. Eliza, and Old Mill Springs), with the objective of characteriz-At Main Spring, atrazine, simazine, chloroform, and tetrachloing water quality. Barton Springs is the major discharge point roethene concentrations increased following storms, describing for the Barton Springs segment of the Edwards aquifer. A three-breakthrough curves that peaked 2 days following rainfall; at pronged sampling approach was used: physicochemical proper-Upper Spring, atrazine and simazine concentrations described ties (including specific conductance and turbidity) were mea-breakthrough curves that peaked 1 day following rainfall. At sured continuously; samples were collected from the four both Main and Upper Springs, additional anthropogenic comsprings routinely every 2 weeks (during August-September pounds were detected following storms. The geochemical 2003) to 3 weeks (during June 2004-June 2005) and analyzed response of the springs to recharge indicates that much of the for some or all major ions, nutrients, trace elements, soluble transport occurs through conduits. When there is no flow in the pesticides, and volatile organic compounds; and samples were recharging streams, ground water advects from the aquifer collected from the four springs at more closely spaced intervals matrix into the conduits and is transported to the springs. When during the 2 weeks following two storms and analyzed for the there is flow in the streams, recharge through the streambeds same suite of constituents. Following the two storms, samples directly enters the conduit system and is transported to the also were collected from five of the six major streams that pro-springs. Following storms, surface runoff recharges through vide recharge to Barton Springs. Spring discharge during both both interstream recharge features and streambeds, delivering sample collection periods was above average (60 cubic feet per runoff-related contaminants to Barton Springs. second or greater). Barton Springs was found to be affected by persistent low concentrations of atrazine (an herbicide), chloroform (a drinking-water disinfection by-product), and tetrachloroethene (a solvent). Increased recharge from the major 08158920 Williamson Oak Hill Williamson Creek at Oak Hill, Tex. Creek (upstream) 08158930 Williamson Manchaca Williamson Creek at Manchaca Road, Austin, Tex. Creek (downstream)