Occurrence of volatile organic compounds in drinking water from the United States: Results from archived chromatograms and water samples, 1989-2000
No abstract available.
Water-Resources Investigations Report↗
Geology topics
Niel Plummer
Publications and source records attributed to Niel Plummer.
Residence times and nitrate transport in ground water discharging to streams in the Chesapeake Bay Watershed
One of the major water-quality problems in the Chesapeake Bay is an overabundance of nutrients from the streams and rivers that discharge to the Bay. Some of these nutrients are from nonpoint sources such as atmospheric deposition, agricultural manure and fertilizer, and septic systems. The effects of efforts to control nonpoint sources, however, can be difficult to quantify because of the lag time between changes at the land surface and the response in the base-flow (ground water) component of streams. To help resource managers understand the lag time between implementation of management practices and subsequent response in the nutrient concentrations in the base-flow component of streamflow, a study of ground-water discharge, residence time, and nitrate transport in springs throughout the Chesapeake Bay Watershed and in four smaller watersheds in selected hydrogeomorphic regions (HGMRs) was conducted. The four watersheds were in the Coastal Plain Uplands, Piedmont crystalline, Valley and Ridge carbonate, and Valley and Ridge siliciclastic HGMRs. A study of springs to estimate an apparent age of the ground water was based on analyses for concentrations of chlorofluorocarbons in water samples collected from 48 springs in the Chesapeake Bay Watershed. Results of the analysis indicate that median age for all the samples was 10 years, with the 25th percentile having an age of 7 years and the 75th percentile having an age of 13 years. Although the number of samples collected in each HGMR was limited, there did not appear to be distinct differences in the ages between the HGMRs. The ranges were similar between the major HGMRs above the Fall Line (modern to about 50 years), with only two HGMRs of small geographic extent (Piedmont carbonate and Mesozoic Lowland) having ranges of modern to about 10 years. The median values of all the HGMRs ranged from 7 to 11 years. Not enough samples were collected in the Coastal Plain for comparison. Spring samples showed slightly younger water under wet conditions than under dry conditions. The apparent age of water from wells, springs, and other ground-water discharge points in the four targeted watersheds was modern to 60 years, which was similar to the apparent ages from the spring study. In the Pocomoke River Watershed in the Coastal Plain Uplands HGMR, the apparent age of ground-water samples ranged from 0 to 60 years; the ages in the vicinity of the streams ranged from 0 to 23 years. The apparent ages of ground water in the Polecat Creek Watershed in the Piedmont crystalline HGMR ranged from 2 to 30 years. The apparent ages of water from wells in the Muddy Creek Watershed in the Valley and Ridge carbonate HGMR ranged from 10 to 20 years (except for a single sample that was 45 years). The ages in the East Mahantango Creek Watershed in the Valley and Ridge siliciclastic HGMR ranged from 0 to 50 years. The distribution in apparent age of water from wells in the targeted watersheds, however, generally is older than that for water from the springs. The median age of water from wells in the Muddy Creek Watershed, for example, was 15 years, compared to 11 years for the water from the springs in that watershed, and less than 10 years for water from all springs in the spring study. The similarity in the ranges in apparent age of water from the wells and from the springs shows that the samples from the targeted watersheds and springs have bracketed the range of apparent ages that would be expected in the shallow ground-water-flow systems throughout the Chesapeake Bay Watershed. The apparent age of water from individual wells does not necessarily represent the entire distribution of ages of the discharging ground water, and it is this distribution of ages that affects the response of nutrient concentrations in stream base flow. Nutrient-reduction scenarios were modeled for two watersheds for which the distribution of apparent ground-water ages was available, the East Mahantango Creek Watershed in the Valley and Ridge siliciclastic HGMR and the Locust Grove Watershed in the Coastal Plain Uplands HGMR. A nutrient-reduction scenario was created for East Mahantango Creek, where the average residence time was determined to be approximately 10 years on the basis of the output of particle tracking from a ground-water-flow model. This scenario showed decreases of nearly 50 percent in base-flow concentrations of nitrate in streams within the first year after the reduction in nitrogen input; smaller reductions in nitrate concentration occurred in each subsequent year. A second scenario for that same watershed, in which the same 10-year average residence time was assumed and an exponential model was used for analysis, showed that a 50-percent reduction in base-flow concentrations of nitrate could take up to 5 years. For the Locust Grove Watershed, in which an average residence time of 32 years was assumed, simulation with the exponential model showed that it may take more than 20 years to achieve a 50-percent reduction in base-flow concentra-tions of nitrate. Although it was not possible to construct such scenarios for all watersheds, these examples show the range of possible responses to changes in nutrient inputs in two very different types of watersheds. Findings from this study include information on factors that affect ground-water age, spatial distribution of ages, and nitrogen transport. In the East Mahantango Creek Watershed and the Polecat Creek Watershed, the residence time varied spatially depending on the position of the flow path, and temporally depending on the recharge conditions. Generally, ground water in areas near the stream had short residence times and the water in upland areas had longer residence times. Water traveling through deep layers had longer residence times than water traveling through shallow layers, and residence times were faster under high recharge conditions than low recharge conditions. Ground water in the Pocomoke Watershed exhibits a similar pattern: younger water discharges to small order streams in headwater basins and older water discharges to larger streams near the basin outlet. Factors affecting nitrogen transport in ground water include spatial and temporal variation in input sources, ground-water age, and aquifer processes that lead to denitrification. Spatial and temporal variations in nitrogen sources affect all the watersheds. Tributaries with higher inputs of nitrogen have higher concentrations in stream base flow. Areas where nitrogen application rates have increased over time show an age-nitrate relation in ground-water samples. The age-nitrate relation can be affected by denitrification, which occurs in Pocomoke and East Mahantango Creeks but is not evident in Polecat and Muddy Creeks. In East Mahantango Creek, the level of denitrification is significant in water with residence times greater than 20 years, but because this is a small component of overall ground-water discharge to a stream, it may not remove a significant quantity of nitrogen from the system. Denitrification in Pocomoke Creek is significant and appears to affect mostly older water discharging to streams. Therefore, if most of the nitrogen entering these two streams is associated with the discharge of younger ground water, denitrification may not greatly affect the overall nitrogen delivery to these streams. Other findings of this study show that nitrate in ground water discharging along preferential flow paths may not be affected by natural processes, such as denitrification or uptake by riparian vegetation. Seeps to swales and ditches beneath the north uplands at Polecat Creek indicate a shallow water table and discharge of young ground water whereas the absence of such seeps on the south side indicates a deep water table and a lack of young ground water. Similarly, discharge at the base of the slope and to the valley wetland south of the creek but not north of the creek indicates a different role for the riparian forest on the two sides of the creek. In many of the systems where water discharges at the base of slopes to wetlands, ditches have been dug to drain the valley. Such drainage circumvents possible removal of nitrate by riparian vegetation. Because ground-water residence times do not appear directly related to the HGMRs, the targeting of management practices will achieve the most rapid response in water quality if directed at 1) watersheds with large agricultural sources of nitrate, 2) areas with the shortest ground-water-flow paths and 3) areas not affected by significant denitrification. The fastest response in stream base-flow concentrations of nitrogen to implementation of management practices would be to implement practices in those areas with the highest loads rather than attempt to target practices on the basis of HGMR stratification. Overall findings of the study indicate that 1) ground-water contributions to nitrogen in streamflow are significant, 2) some response to management practices should be evident in base-flow concentrations of nitrogen and loads within 1 to 5 years in watersheds with the shortest average residence times, but response time may be closer to 20 years in watersheds with longer average ground-water residence times, 3) the majority of the response in ground-water discharge to any changes in management practices will be distributed over a 10-year time period even in the watersheds with the fastest response times, and 4) given that half the streamflow is from ground-water discharge and the other half is runoff or soil water, about 90 percent of total water being discharged to a stream will be less than about a decade old; therefore, full implementation of nutrient reductions may result in improved streamwater quality in about a decade. In the more-likely scenario of gradual source reduction, the reduction in concentrations of nitrate in streams and aquifers would take longer than the examples shown here.
Chesapeake Bay watershed↗
Radiocarbon dating of dissolved inorganic carbon in groundwater from confined parts of the Upper Floridan aquifer, Florida, USA
Geochemical reaction models were evaluated to improve radiocarbon dating of dissolved inorganic carbon (DIC) in groundwater from confined parts of the Upper Floridan aquifer in central and northeastern Florida, USA. The predominant geochemical reactions affecting the 14 C activity of DIC include (1) dissolution of dolomite and anhydrite with calcite precipitation (dedolomitization), (2) sulfate reduction accompanying microbial degradation of organic carbon, (3) recrystallization of calcite (isotopic exchange), and (4) mixing of fresh water with as much as 7% saline water in some coastal areas. The calculated cumulative net mineral transfers are negligibly small in upgradient parts of the aquifer and increase significantly in downgradient parts of the aquifer, reflecting, at least in part, upward leakage from the Lower Floridan aquifer and circulation that contacted middle confining units in the Floridan aquifer system. The adjusted radiocarbon ages are independent of flow path and represent travel times of water from the recharge area to the sample point in the aquifer. Downgradient from Polk City (adjusted age 1.7 ka) and Keystone Heights (adjusted age 0.4 ka), 14 of the 22 waters have adjusted 14 C ages of 20–30 ka, indicating that most of the fresh-water resource in the Upper Floridan aquifer today was recharged during the last glacial period. All of the paleowaters are enriched in 18 O and 2 H relative to modern infiltration, with maximum enrichment in δ 18 O of approximately 2.0‰.
Hydrogeology Journal↗
Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA: A multi-tracer approach
Chemical and isotopic properties of water discharging from springs and wells in Shenandoah National Park (SNP), near the crest of the Blue Ridge Mountains, VA, USA were monitored to obtain information on groundwater residence times. Investigated time scales included seasonal (wet season, April, 1996; dry season, August–September, 1997), monthly (March through September, 1999) and hourly (30-min interval recording of specific conductance and temperature, March, 1999 through February, 2000). Multiple environmental tracers, including tritium/helium-3 ( 3 H/ 3 He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF 6 ), sulfur-35 ( 35 S), and stable isotopes ( δ 18 O and δ 2 H) of water, were used to estimate the residence times of shallow groundwater discharging from 34 springs and 15 wells. The most reliable ages of water from springs appear to be based on SF 6 and 3 H/ 3 He, with most ages in the range of 0–3 years. This range is consistent with apparent ages estimated from concentrations of CFCs; however, CFC-based ages have large uncertainties owing to the post-1995 leveling-off of the CFC atmospheric growth curves. Somewhat higher apparent ages are indicated by 35 S (>1.5 years) and seasonal variation of δ 18 O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the 35 S and δ 18 O data reflect travel times through the unsaturated zone and, in the case of 35 S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the 3 H/ 3 He data (median age of 0.2 years) are lower than those obtained from SF 6 (median age of 4.3 years), and in contrast to median ages from 3 H/ 3 He (0.3 years) and SF 6 (0.7 years) obtained during the late summer dry season of 1997. Monthly samples from 1999 at four springs in SNP had SF 6 apparent ages of only 1.2 to 2.5±0.8 years, and were consistent with the 1997 SF 6 data. Water from springs has low excess air (0–1 cm 3 kg −1 ) and N 2 –Ar temperatures that vary seasonally. Concentrations of He and Ne in excess of solubility equilibrium indicate that the dissolved gases are not fractionated. The seasonal variations in N 2 –Ar temperatures suggest shallow, seasonal recharge, and the excess He and Ne data suggest waters mostly confined to gas exchange in the shallow, mountain-slope, water-table spring systems. Water from wells in the fractured rock contains up to 8 cm 3 kg −1 of excess air with ages in the range of 0–25 years. Transient responses in specific conductance and temperature were observed in spring discharge within several hours of large precipitation events in September, 1999; both parameters increased initially, then decreased to values below pre-storm base-flow values. The groundwater residence times indicate that flushing rates of mobile atmospheric constituents through groundwater to streams draining the higher elevations in SNP average less than 3 years in base-flow conditions.
Virginia↗
In-situ growth of calcite at Devils Hole, Nevada--Comparison of field and laboratory rates to a 500,000 year record of near-equilibrium calcite growth
Calcite grew continuously for 500,000 years on the submerged walls of an open fault plane (Devils Hole) in southern Nevada, U.S.A. at rates of 0.3 to 1.3 mm/ka, but ceased growing approximately 60,000 years ago, even though the fault plane remained open and was continuously submerged. The maximum initial in-situ growth rate on pre-weighed crystals of Iceland spar placed in Devils Hole (calcite saturation index, SI, is 0.16 to 0.21 at 33.7 °C) for growth periods of 0.75 to 4.5 years was 0.22 mm/ka. Calcite growth on seed crystals slowed or ceased following initial contact with Devils Hole groundwater. Growth rates measured in synthetic Ca-HCO 3 solutions at 34 °C, CO 2 partial pressures of 0.101, 0.0156 (similar to Devils Hole groundwater) and 0.00102 atm, and SI values of 0.2 to 1.9 were nearly independent of P CO 2 , decreased with decreasing saturation state, and extrapolated through the historical Devils Hole rate. The results show that calcite growth rate is highly sensitive to saturation state near equilibrium. A calcite crystal retrieved from Devils Hole, and used without further treatment of its surface, grew in synthetic Devils Hole groundwater when the saturation index was raised nearly 10-fold that of Devils Hole water, but the rate was only 1/4 that of fresh laboratory crystals that had not contacted Devils Hole water. Apparently, inhibiting processes that halted calcite growth in Devils Hole 60,000 years ago continue today.
Aquatic Geochemistry↗
Tritium/3He measurements in young groundwater: Progress in applications to complex hydrogeological systems
Tritium/3He dating has been applied to many problems in groundwater hydrology including, for example, determination of circulation patterns, mean residence times, recharge rates, or bank infiltration. Here, we discuss recent progress in the application of the tritium/3He dating method to sites with complex hydrogeological settings. Specifically, we report on tritium/3He dating at sites with (a) river infiltration into the basaltic fractured rock aquifer of the Eastern Snake River Plain, and (b) river infiltration through sinkholes into the karstic limestone Upper Floridian aquifer near Valdosta, Georgia.Tritium/3He dating has been applied to many problems in groundwater hydrology including, for example, determination of circulation patterns, mean residence times, recharge rates, or bank infiltration. Here, we discuss recent progress in the application of the tritium/3He dating method to sites with complex hydrogeological settings. Specifically, we report on tritium/3He dating at sites with (a) river infiltration into the basaltic fractured rock aquifer of the Eastern Snake River Plain, and (b) river infiltration through sinkholes into the karstic limestone Upper Floridian aquifer near Valdosta, Georgia.
Conference Paper↗
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↗
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↗
Chemistry of unsaturated zone gases sampled in open boreholes at the crest of Yucca Mountain, Nevada: Data and basic concepts of chemical and physical processes in the mountain
Boreholes open to the unsaturated zone at the crest of Yucca Mountain, Nevada, were variously sampled for CO 2 (including 13 C and 14 C), CH 4 , N 2 , O 2 , Ar, CFC-11, CFC-12, and CFC-113 from 1986 to 1993. Air enters the mountain in outcrops, principally on the eastern slope, is enriched in CO 2 by mixing with soil gas, and is advected to the mountain crest, where it returns to the atmosphere. The CFC data indicate that travel times of the advecting gas in the shallow Tiva Canyon hydrogeologic unit are ≤5 years. The 14 C activities are postbomb to depths of 100 m, indicating little retardation of 14 CO 2 in the shallow flow systems. The 14 C activities from 168 to 404 m in the Topopah Spring hydrogeologic unit are 85–90 pMC at borehole USW-UZ6. The CFC data show that the drilling of USW-UZ6 in 1984 has altered the natural system by providing a conduit through the Paintbrush Nonwelded unit, allowing flow from Topopah Spring outcrops in Solitario Canyon on the west to USW-UZ6, upward in the borehole through the Paintbrush, to the shallow Tiva Canyon flow systems, and out of the mountain.
Nevada↗
Evaluating the source and residence times of groundwater seepage to streams, New Jersey Coastal Plain
A conceptual model of the patterns and residence times of groundwater seepage to gaining streams indicates that groundwater seepage originates from sources that are both near and far from the stream. Consequently, the age of groundwater seepage across a stream-channel transect increases from its banks to its center and becomes progressively older with distance downstream. A groundwater flow model and particle-tracking analysis of the Cohansey River Basin in the New Jersey Coastal Plain supports this conceptual model and demonstrates that the orientation of the stream channels with respect to the regional groundwater flow direction, and the heterogeneities of the aquifer and stream-channel patterns, can shift source area locations and distributions of groundwater residence time from those expected. Groundwater samples collected from stream transects were analyzed for nitrogen, representative of widespread agricultural land use in the basin in recent decades, and for chlorofluorocarbons, used to estimate groundwater ages. The patterns of nitrogen concentration and the age of groundwater entering the stream channel corroborate model inferences. The conceptual model of groundwater seepage to streams presented herein is relevant to unconfined aquifer systems with gaining streams and demonstrates how nonpoint-source contaminants are transported to streams by groundwater. Results are useful for the design of programs needed to monitor stream-water quality.
New Jersey↗
Chlorofluorocarbons, sulfur hexafluoride, and dissolved permanent gases in ground water from selected sites in and near the Idaho National Engineering and Environmental Laboratory, Idaho, 1994-97
From July 1994 through May 1997, the U.S. Geological Survey in cooperation with the Department of Energy, sampled 86 wells completed in the Snake River Plain aquifer at and near the Idaho N ationa1 Engineering and Environmental Laboratory (INEEL). The wells were sampled for a variety of constituents including one- and two-carbon halocarbons. Concentrations of dichlorodifluoromethane (CFC-12), trichlorofluoromethane (CFC-11) and trichlorotrifluororoethane (CFC-113) were determined. The samples for halocarbon analysis were collected in 62-milliliter flame sealed borosilicate glass ampoules in the field. The data will be used to evaluate the ages of ground waters at INEEL. The ages of the ground water will be used to determine recharge rates, residence time, and travel time of water in the Snake River Plain aquifer in and near INEEL. The chromatograms of 139 ground waters are presented showing a large number of halomethanes, haloethanes, and haloethenes present in the ground waters underlying the INEEL. The chromatograms can be used to qualitatively evaluate a large number of contaminants at parts per trillion to parts per billion concentrations. The data can be used to study temporal and spatial distribution of contaminants in the Snake River Plain aquifer. Representative compressed chromatograms for all ground waters sampled in this study are available on two 3.5-inch high density computer disks. The data and the program required to decompress the data can be obtained from the U.S. Geological Survey office at Idaho Falls, Idaho. Sulfur hexafluoride (SF6) concentrations were measured in selected wells to determine the feasibility of using this environmental tracer as an age dating tool of ground water. Concentrations of dissolved nitrogen, argon, carbon dioxide, oxygen, and methane were measured in 79 ground waters. Concentrations of dissolved permanent gases are tabulated and will be used to evaluate the temperature of recharge of ground water in and near the INEEL.
Idaho↗
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↗
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↗
Water-resources data for the Valdosta area, south-central Georgia, 1961-93
The Upper Floridan aquifer is the sole source of water supply for the city of Valdosta, Ga., and much of the surrounding area. Users and water-resources managers and developers are concerned about the quality of water in the aquifer. The water quality of a large part of the Upper Floridan aquifer in the Valdosta area is affected by direct recharge of water from the Withlacoochee River to the aquifer through sinkholes in the river channel north of Valdosta. Furthermore, because the Withlacoochee River receives little filtration as it recharges the aquifer in this area, ground water might be vulnerable to contamination as a result of human activities within the Withlacoochee River basin. Stream-discharge and water-quality data from 17 surface-water sites and ground-water-quality dam from 111 wells in the vicinity of Valdosta, Ga., are presented for the period 1961-93. Also, ground- water-level data for the Upper Floridan aquifer in the Valdosta area are presented in a series of potentiometric-surface maps. The water-resources data were collected mostly in Lowndes County in the vicinity of Valdosta and lesser amounts were collected in surrounding Berrien, Brooks, Cook, Echols, and Lanier Counties.
Georgia↗