Dissolved Organic and Inorganic ¹⁴C Concentrations and Ages for Coastal Plain Aquifers in Southern Maryland

Abstract

ABSTRACT, The Aquia (Paleocene) and Magothy (Late Cretaceous) Formations of the Atlantic Coastal Plain represent two well-characterized (hydrodynamically and geochemically) aquifers in southern Maryland.14C measurements of the dissolved organic (DOC) and inorganic carbon (DIC) of Aquia and Magothy groundwaters have been made using accelerator mass spectrometry (AMS). Both DI14C and D014C concentrations in the initial flow path are unexpectedly low. As the water progresses farther from the recharge area, the D114C percent modern carbon (pMC) is consistently lower than the D014C pMC; this difference stays constant for all samples. The 14C-derived ages for an Aquia water sample downgradient at Site 4 are 17 ka and 12 ka for D114C and D014C, respectively. Radiocarbon ages have been compared to ages determined by two other independent dating methods: computer-simulated hydrodynamic modeling and age estimates based on changes in C1-,180 and 2H distributions, which are interpreted to be influenced by sea level and climate.

A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)

“…If the He ages are maximum estimates, this result represents a lower limit for the true flow velocity. It is considerably smaller than the estimates discussed by Purdy et al (1992), which range between 2.6 and 4 m yr Ϫ1 . Yet, the even lower flow velocity of 0.2 m yr Ϫ1 inferred from 36 Cl by Purdy et al (1996) for flow distances larger than 50 km is in reasonable agreement with the high apparent He age at well MD4.…”

“…Note that the sampled wells lie on several different flow lines and that flow lines may have been different in the past. The procedure to estimate flow distances is analogous to that used by Purdy (1991) and Purdy et al (1992Purdy et al ( , 1996. Thus, our results can be compared to those of the earlier studies, although different wells were sampled (except MD1 and 2, which are wells no.…”

“…Considering an inland gradient of Ϫ0.6‰/100 km as found in the Floridan aquifer (Atlantic Coastal Plain, Georgia) by Clark et al (1997), and an increase of the distance to the coast by ϳ180 km during the LGM (Purdy et al, 1992), we estimate that the continental effect could have had a similar magnitude as the ice-volume effect.…”

“…According to this hypothesis, the Cl -minimum observed ϳ40 km from the outcrop area corresponds to the last glacial sealevel lowstand (ϳ18 radiocarbon kyr BP), when the distance from the recharge area to the sea was nearly 200 km larger than at present. 14 C data from a site at a distance of 45 km from the outcrop indicated somewhat smaller ages (12 kyr from DIC and 17 kyr from DOC) after corrections for low initial 14 C activity as well as addition of dead carbon within the aquifer had been applied (Purdy et al, 1992). Purdy et al (1996) re-interpreted the records of chloride concentrations and 36 Cl/Cl ratios in terms of a two-regions flow model.…”

“…Age estimates for Aquia water were derived from chloride concentrations (Purdy, 1991), as well as from some stable isotope and 14 C measurements (Purdy et al, 1992). The systematic variation of chloride concentration with distance from the outcrop in the Aquia aquifer was interpreted as a result of changes in meteoric deposition due to sea-level fluctuations.…”