Strontium isotopes and major elements as tracers of ground water evolution: Example from the Upper Castle Hayne Aquifer of North Carolina

The Eocene-Oligocene Upper Castle Hayne Aquifer (UCH),a well-indurated lime stone with a very high percentage of secondary moldic porosity, is one of t he most productive and extensively developed aquifers in the North Carolina Coastal Plain. Ground mater from western wells in the UCH is Ca- and HCO3- rich and ground water from easternmost wells is alkali- and Cl-rich, In gen eral, from west to east across the study area, Sr concentrations [Sr] and i sotopic ratios of ground water from the UCH and other aquifers evolve towar d those of the host aquifer. ht the same well site, water from older aquife rs usually has a lower Sr-87/Sr-86 ratio than water from younger aquifers, due to interaction between ground mater and sedimentary material in the hos t aquifers, Comparison of Sr-87/Sr-86 ratios and [Sr] suggests that most UC H water represents mixing of strontium-poor recharge water from the Surfici al Aquifer with varying amounts of strontium from the aquifer rock. For sam ples that deviate from the calculated mixing line, strontium ratios can oft en be used to indicate the source of strontium that did not come from UCH r ock. Surface waters are characterized by high Sr-87/Sr-86 ratios and variab le [Sr] that depend on the proportion of intermixed sea water Water from th e overlying Yorktown and Pungo River aquifers call be recognized by higher Sr-87/Sr-86 ratios than those of the UCH and water from the underlying LCH, Beaufort, and Peedee aquifers can be recognized by lower Sr-87/Sr-86 ratio s and higher [Sr].