Geology topics

S. Drenkard

Publications and source records attributed to S. Drenkard.

6 recordsLinked to original sources

USGS1998

Flow of river water into a Karstic limestone aquifer. 1. Tracing the young fraction in groundwater mixtures in the Upper Floridan Aquifer near Valdosta, Georgia

The quality of water in the Upper Floridan aquifer near Valdosta, Georgia is affected locally by discharge of Withlacoochee River water through sinkholes in the river bed. Data on transient tracers and other dissolved substances, including Cl − , 3 H, tritiogenic helium-3 ( 3 He), chlorofluorocarbons (CFC-11, CFC-12, CFC-113), organic C (DOC), O 2 (DO), H 2 S, CH 4 , δ 18 O, δD, and 14 C were investigated as tracers of Withlacoochee River water in the Upper Floridan aquifer. The concentrations of all tracers were affected by dilution and mixing. Dissolved Cl − , δ 18 O, δD, CFC-12, and the quantity ( 3 H+ 3 He) are stable in water from the Upper Floridan aquifer, whereas DOC, DO, H 2 S, CH 4 , 14 C, CFC-11, and CFC-113 are affected by microbial degradation and other geochemical processes occurring within the aquifer. Groundwater mixing fractions were determined by using dissolved Cl − and δ 18 O data, recognizing 3 end-member water types in the groundwater mixtures: (1) Withlacoochee River water (δ 18 O=−2.5±0.3‰, Cl − =12.2±2 mg/l), (2) regional infiltration water (δ 18 O=−4.2±0.1‰, Cl − =2.3±0.1 mg/l), and (3) regional paleowater resident in the Upper Floridan aquifer (δ 18 O=−3.4±0.1‰, Cl − =2.6±0.1 mg/l) (uncertainties are ±1σ). Error simulation procedures were used to define uncertainties in mixing fractions. Fractions of river water in groundwater range from 0 to 72% and average 10%. The influence of river-water discharge on the quality of water in the Upper Floridan aquifer was traced from the sinkhole area on the Withlacoochee River 25 km SE in the direction of regional groundwater flow. Infiltration of water is most significant to the N and NW of Valdosta, but becomes negligible to the S and SE in the direction of general thickening of post-Eocene confining beds overlying the Upper Floridan aquifer.

Georgia↗

USGS1998

Flow of river water into a karstic limestone aquifer: 2. Dating the young fraction in groundwater mixtures in the Upper Floridan aquifer near Valdosta, Georgia

Tritium/helium-3 ( 3 H/ 3 He) and chlorofluorocarbon (CFCs, CFC–11, CFC–12, CFC–113) data are used to date the young fraction in groundwater mixtures from a karstic limestone aquifer near Valdosta, Georgia, where regional paleowater in the Upper Floridan aquifer receives recharge from two young sources—the flow of Withlacoochee River water through sinkholes in the river bed, and leakage of infiltration water through post-Eocene semi-confining beds above the Upper Floridan aquifer. In dating the young fraction of mixtures using CFCs, it is necessary to reconstruct the CFC concentration that was in the young fraction prior to mixing. The 3 H/ 3 He age is independent of the extent of dilution with older ( 3 H-free and 3 He trit -free) water. The groundwater mixtures are designated as Type-1 for mixtures of regional paleowater and regional infiltration water and Type-2 for mixtures containing more than approximately 4% of river water. The fractions of regional paleowater, regional infiltration water, and Withlacoochee River water in the groundwater mixtures were determined from Cl − and δ 18 O data for water from the Upper Floridan aquifer at Valdosta, Georgia. The chlorofluorocarbons CFC–11 and CFC–113 are removed by microbial degradation and/or sorption processes in most anaerobic (Type-2) groundwater at Valdosta, but are present in some aerobic Type-1 water. CFC–12 persists in both SO 4 -reducing and methanogenic water. The very low detection limits for CFCs (approximately 0.3 pg kg −1 ) permitted CFC–11 and CFC–12 dating of the fraction of regional infiltration water in Type-1 mixtures, and CFC-12 dating of the river-water fraction in Type-2 mixtures. Overall, approximately 50% of the 85 water samples obtained from the Upper Floridan aquifer have CFC–12-based ages of the young fraction that are consistent with the 3 H concentration of the groundwater. Because of uncertainties associated with very low 3 H and 3 He content in dilute mixtures, 3 H/ 3 He dating is limited to the river-water fraction in Type-2 mixtures containing more than about 10% river water. Of the 41 water samples measured for 3 H/ 3 He dating, dilution of 3 H and low 3 He concentration limited 3 H/ 3 He dating to 16 mixtures in which 3 H/ 3 He ages are defined with errors ranging from ±2 to ±7.5 a (1 σ). After correction for dilution with (assumed) CFC-free regional infiltration water and regional paleowater in the Upper Floridan aquifer, adjusted CFC–12 ages agree with 3 H/ 3 He ages within 5 a or less in 7 of the 9 co-dated Type-2 mixtures. Tritium data and dating based on both CFC–11 and CFC–12 in Type-1 mixtures indicate that travel times of infiltration water through the overlying Post-Eocene semi-confining beds exceed 35 a. The CFC and 3 H/ 3 He dating indicate that the river fraction in most groundwater entered the groundwater reservoir in the past 20 to 30 a. Few domestic and municipal supply wells sampled intercept water younger than 5 a. Calculated velocities of river water in the Upper Floridan aquifer downgradient of the sinkhole area range from 0.4 to 8.2 m/d. Radiocarbon data indicate that ages of the regional paleowater are on the 10 000-a time scale. An average lag time of approximately 10 to 25 a is determined for discharge of groundwater from the surficial and intermediate aquifers above the Upper Floridan aquifer to the Withlacoochee River.

Georgia↗

USGS1996

Age dating of shallow groundwater with chlorofluorocarbons, tritium/helium: 3, and flow path analysis, southern New Jersey coastal plain

Groundwater age dating through the combination of transient tracer methods (chlorofluorocarbons (CFCs) and tritium/helium 3 ( 3 H/ 3 He)) and groundwater flow path analysis is useful for investigating groundwater travel times, flow patterns, and recharge rates, as demonstrated by this study of the homogeneous shallow, unconfined Kirkwood-Cohansey aquifer system in the southern New Jersey coastal plain. Water samples for age dating were collected from three sets of nested observation wells (10 wells) with 1.5-m-long screens located near groundwater divides. Three steady state finite difference groundwater flow models were calibrated by adjusting horizontal and vertical hydraulic conductivities to match measured heads and head differences (range, 0.002–0.23 m) among the nested wells, with a uniform recharge rate of 0.46 m per year and porosities of 0.35 (sand) and 0.45 (silt) that were assumed constant for all model simulations and travel time calculations. The simulated groundwater travel times increase with depth in the aquifer, ranging from about 1.5 to 6.5 years for the shallow wells (screen bottoms 3–4 m below the water table), from about 10 to 25 years for the medium-depth wells (screen bottoms 8–19 m below the water table), and from about 30 to more than 40 years for the deep wells (screen bottoms 24–26 m below the water table). Apparent groundwater ages based on CFC- and 3 H/ 3 He-dating techniques and model-based travel times could not be statistically differentiated, and all were strongly correlated with depth. Confinement of 3He was high because of the rapid vertical flow velocity (of the order of 1 m/yr), resulting in clear delineation of groundwater travel times based on the 3 H/ 3 He-dating technique. The correspondence between the 3 H/ 3 He and CFC ages indicates that dispersion has had a minimal effect on the tracer-based ages of water in this aquifer. Differences between the tracer-based apparent ages for seven of the 10 samples were smaller than the error values. A slight bias toward older apparent ages, found not to be statistically significant, was noted for the 3 H/ 3 He-dating technique relative to the CFC-dating technique. This result may be caused by enrichment of local air in CFC-Il and CFC-12 from urban and industrial sources in the northeastern United States and minor contamination from sampling equipment. The demonstrated validity of the combined tracer-dating techniques to determine the age of water in the Kirkwood-Cohansey aquifer system indicates that groundwater flow models can be refined when apparent ages based on 3 H/ 3 He- and CFC- dating are used as calibration targets.

New Jersey↗

USGS1995

Applications of the transient tracers tritium/helium-3, and chlorofluorocarbons for tracing and age-dating yound ground water: Field examples from the USA and Germany

The transient tracers tritium/helium-3 ( 3 H/ 3 He) and chlorofluorocarbons (CFC-11, CFC-12, CFC-113) are well suited for tracing and age-dating young ground water. Their detection in ground water indicates waters recharged within the past 30 ( 3 H/ 3 He, CFC-113) to 50 (CFC-11, CFC-12) years, or ground water mixtures that contain at least a portion of young water. The ground water age can be determined independently from measurements of 3 H/ 3 He, CFC-11, CFC-12, and CFC-113, and in each case refers to the time elapsed since the recharge water was isolated from the soil air. Ground water age can be used to define recharge rates and refine numerical models of ground water flow. Transient tracers are particularly useful in characterising ground water flow in hydrologic systems where, because of insufficient geologic and hydro-logic data, numerical simulation may be difficult. Transient tracers are also useful in defining movement of ground water contaminants in studies aimed, for example, at the design of strategies to safeguard drinking water supplies.

Book chapter↗

USGS1995

Mantle helium in ground waters of eastern North America: Time and space constraints on sources

Mantle helium in continental environments is generally considered to be the result of active volcanism and/or active extension. The latest episodes of volcanism in northeastern North America are the track of the New England hotspot (95–190 Ma) and the closure of the Iapetus sea (before 300 Ma). Thus, the identification of mantle helium in young ground waters of central New England is counter to the conventional wisdom. On the basis of evaluation of helium evolution in emplaced magmas, we postulate an “aged” mantle source for the excess helium component in ground waters of central New England that is either (1) a local, near-surface–emplaced, gas-rich magma that has retained significant volatiles (e.g., in fluid inclusions) or (2) a deeply emplaced gas-rich magma with high initial 2 He/ 4 He (10 −5 ) and helium transport (with dispersion) through the crust over time. This gas-rich initial condition may support the concept of a volatile-enriched mantle wedge and thus explain the increased buoyancy flux of the New England hotspot as it traversed eastern North America, as has been suggested by others.

Geology↗

USGS1994

Mantle helium in the groundwater of the Mirror Lake Basin, New Hampshire, USA, 1994

Helium isotope analyses of ground waters from the Mirror Lake drainage basin in central New Hampshire (USA) show helium in excess of air-saturated water by up to 200x. The freon ages of these waters are younger than 50 years, consistent with the local hydrology. This excess helium has an isotope ratio of ^3He/^4He = 1.65 ± 0.10 x 10^(-6). It is shown that this component cannot be the result of cosmogenic production or mixing with young water containing ^3He from the decay of (bomb) tritium. Measurements of the helium isotope ratio of local rocks indicate that they cannot be the source of this excess component. This strongly suggests that the excess helium component is the result of the addition of some external source of mantle helium. The generally accepted view suggests that mantle helium in continental environments is the result of active volcanism and extensional tectonics. The latest episodes of volcanism in this region of New England are related to the New England hot spot track (95-190 Myr) and the closure of the Iapetus (> 300 Myr). Thus, either the timescale for helium transport through the crust is of the order of 100's of Myr or the signature of mantle helium can be preserved in (e.g.) fluid inclusions for significant periods of time.

Book chapter↗