Analysis of Temporal Environmental Isotopic Variation in Ground Water of the Georgia Piedmont

Abstract

AbstractTime‐series analyses were made of environmental tritium (3H) concentrations, the stable oxygen isotopic (δ18O) and major‐ion composition of natural waters from the southeastern Piedmont Province of Georgia. The sampling network consisted of four fractured rock production wells (utilized at variable rates), one regolith monitoring well, two streams, and rainfall. Although the isotopic composition of rainfall was highly variable (−11.8‰≤δ18O ≤−l.l‰ and 2.3 ≤ TU ≤ 125), only minor isotopic variation (δl8O generally ≤ 1.0‰ and 3H generally ≤ 2 TU) was observed during the sampling period (∼ 1 year) within ground water at a given well site. The coefficients of isotopic variation for ground water within the bedrock aquifers (δ18Ocv and 3H cv < 10%) were similar to those of ground water within the shallow regolith (δ18Ocv= 5.6% and 3Hcv= 3.3%).Ground water from the most extensively used production well was nearly isotopically and geochemically invariant throughout most of the year. Other than a 0.3–0.5‰δ18O depletion observed during the late spring and early summer, there were few temporal trends and little to suggest the presence of a seasonal component of recharge within ground water extracted from the fractured bedrock aquifers. This is consistent with the 3H‐modeled mean residence time of this ground water which is ∼ 20–40 years. The isotopic composition becomes well‐homogenized by the time ground water infiltrates a few meters below the water table, and ground‐water pumping does little to alter these mixing processes.